stability of fishing vessels in waves and wind · os parâmetros de onda considerados foram os...
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Stability of fishing vessels in waves and wind
José Luis Mantari Laureano
Dissertation for the degree of Master of Science in
Naval Architecture and Marine Engeniering
Jury
President : Prof. Yordan Ivanov Garbatov
Supervisor : Prof. Carlos António Pancada Guedes Soares
Co-Supervisor : Prof. Sergio Bruno Nogueira Ribeiro e Silva
Member : Prof. Tiago Alexandre Rosado Santos
June 2010
ii
To my wife (Lizbeth), for her understanding.
To my babies (Cielo and Italo), for giving me their time.
To my parents in law (Celina and Melecio), for their help.
To my parents (Aida and Jesus), for giving me the opportunity of be here.
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ACKNOWLEDGMENTS
_____________________________________________________________
This work has been performed within the project “SADEP-Decision support system for the
safety of fishing vessels subjected to waves”, which was financed by the Foundation for
Science and Technology (“Fundação para a Ciência e a Tecnologia”), from the Portuguese
Ministry of Science and Technology, under contract PTDC/EME-MFE/75233/2006. However,
this thesis and the published papers have been done because the opportunity, confidence,
orientation and special support of the Professor: Carlos Guedes Soares.
I want to give special thanks also to my best friends: Eng. Jose Luis Mendoza Carhuamaca,
Eng. Xueqian Zhou and Eng. Julien Melot.
Lisbon, October 2011
Jose Luis, Mantari Laureano
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ABSTRACT
_____________________________________________________________
The stability characteristics of 27 fishing vessels, mainly from the Portuguese and Peruvian fishing
fleet, are studied. Calculations of ship’s stability for longitudinal and beam waves were made. The
variations of dynamic transverse intact stability of the fishing vessels under 3 operational loading
conditions are calculated and analyzed for 2 representative sinusoidal longitudinal waves. The
wave parameters considered were the following: s=1/20 and (a) /Lpp=1, (b) /Lpp=1.6, with wave
crest position along the wave or vessel’s length. Potential for the occurrence of stability failure due
to pure loss of stability and parametric resonance were found. For beam waves, fishing gear and
gusty wind loads were included to evaluate the energy balance between the heeling and righting
moments. Based on these calculations, size, hull form and others particularities of the fishing
vessels, some light on the occurrence of partial or total stability failure were found. An overview of
the International Code on Intact Stability (2008 IS Code) related to fishing vessels, is made. Finally,
conclusions are drawn about 2008 IS Code, and the loss of fishing vessel’s intact stability in
longitudinal and beam waves.
KEYWORDS: Intact Stability, GZ variation, GM variation, fishing vessels, fishing gears,
energy balance.
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RESUMO
_____________________________________________________________
Nesta investigação são estudadas as caracteristicas da estabilidade de 27 navios de pesca,
principalmente da frota de pesca portuguesa e peruana. São realizados cálculos de estabilidade
do navio para ondas longitudinais e para ondas de través. São calculas para duas ondas
longitudinais sinusoidais as variações de estabilidade transversal dinâmica (intacta) dos navios de
pesca em três condições de carga operacional. Os parâmetros de onda considerados foram os
seguintes: s = 1/20 e (a) /Lpp = 1, (b) /Lpp = 1.6, com a posição da crista da onda ao longo do
comprimento entre perpendiculares da embarcação. Foram encontradas potenciais causas que
pode levar à perda da estabilidade pura na crista e ressonância paramétrica. Para avaliar o
balanço de energia entre os momentos inclinantes e momentos de endireitantes para ondas de
través, foram incluídos os aparelhos de pesca e rajadas de vento. Com base nestes cálculos,
foram encontrados algumas razões que levam à perda de estabilidade parcial ou total, que
depende da forma e tamanho do casco e particularidades dos navios de pesca. Foi realizada uma
revisão geral do Código Internacional de Estabilidade Intacta (2008) relativo a navios de pesca.
Por fim, são retiradas conclusões sobre o código de estabilidade intacta (2008), sobre a perda de
estabilidade intacta do navio de pesca em ondas longitudinais e de través.
PALAVRAS-CHAVE: Estabilidade Intacta, variação do braço endireitante, variação da altura
metacêntrica, navios de pesca, aparelhos de pesca, balanço energético.
vi
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CONTENTS
_____________________________________________________________
Acknowledgment...…..…………………………….…………………..……......…… ii
Abstract………………………………………………………………………………… iv
Resumo………………………………………………………………………………… v
Contents.……………………………………………………………………….…….... vi
List of Figures……………………………………………………..……………….….. ix
List of tables…………………………………………………………………………… xi
1. INTRODUCTION………………………………………………………………….... 1
2. FISHING VESSEL STABILITY FAILURES……………………………………...4
2.1. Vessel Casualties………………………………………………………………………………..4
2.1.1. Foundering/flooding……………………………………………………………………..6
2.1.2. Fires and explosions…………………………………………………………………….6
2.1.3. Grounding………………………………………………………………………………...6
2.1.4. Capsizing…………………………………………………………………………………7
2.1.5. Collisions and contacts………………………………………………………………….7
2.1.6. Machinery damage………………………………………………………………………7
2.1.7. Heavy weather damage…………………………………………………………………7
2.1.8. Other………………………………………………………………………………………8
2.2. Fishermen casualties………………………..…………………………………………………...8
3. INTACT STABILITY OF FISHING VESSELS…………………………………...10
3.1. Origin of present stability criteria……………………………………………………………….10
3.2. Background of criteria regarding righting lever curve properties (part A of the 2008 IS
Code) ……………………………………………………………………………………………...11
vii
3.2.1. Results of the Analysis of Intact Stability Casualty Records and Stability
Parameters………………………………………………………………………………….13
3.2.1.1. Analysis of details relevant to the casualties………………………………..13
3.2.1.2. Analysis of stability parameters using Rahola method…………………….17
3.2.2. Discrimination Analysis………………………………………………………………….18
3.2.3. Adoption of the final criteria and checking the criteria against a certain number of
ships………………………………………………………………………………………….20
3.3. Background of the severe wind and rolling criterion (part B of the 2008 IS Code)………..20
3.3.1. Energy Balance Method………………………………………………………………...21
3.3.2. Wind heeling moment…………………………………………………………………...22
3.3.3. Roll angle in waves (Japanese Method)………………………………………………23
3.3.3.1. Wave steepness………………………………………………………………..24
3.3.3.2. Hydrodynamic coefficients…………………………………………………….24
3.3.3.3. Natural roll period………………………………………………………………25
3.3.3.4. Wave randomness……………………………………………………………..27
3.3.3.5. Steady wind velocity…………………………………………………………...27
3.3.4. Rolling in waves (USSR’s method) ……………………………………………………28
3.3.5. Adoption of the final weather criteria…………………………………………………..29
3.4. 2008 IS Code for fishing vessels……………………………………………………………….29
3.4.1. Criteria regarding righting lever curve properties…………………………………….29
3.4.2. Weather criterion…………………………………………………………………………30
3.5. Intact stability of Portuguese and Peruvian fishing vessels………………………………….34
3.6. Overview of IMO 2008 Intact Stability Code...………………………………………………...35
3.6.1. IMO, 2008 intact stability code, part A…………………………………………………36
3.6.2. IMO, 2008 intact stability code, part B…………………………………………………37
4. STABILITY FAILURE IN LONGITUDINAL WAVES……………………………39
4.1. Variations of transverse stability in longitudinal waves………………………………………40
4.1.1. Studies on parametric resonance……………………………………………………...40
4.1.2. Studies on pure loss of stability………………………………………………………...40
4.2. Calculation results of variation of transverse stability in longitudinal waves………………42
4.2.1. Potential for the occurrence of parametric resonance……………………………….49
4.2.2. Potential for the occurrence of Pure loss of stability…………………………………52
4.3. Susceptibility to parametric resonance in head seas…………………………………………54
4.3.1. Vessels stability in following or head waves………………………………………….55
4.3.2. Physical and mathematical understanding of parametric resonance………………56
4.3.3. Roll motion and susceptibility criteria …………………………………………………60
4.3.3.1. Frequency condition of susceptibility criteria………………………………..60
4.3.3.2. Damping threshold condition of susceptibility criteria………………………61
viii
4.3.3.3. Forward speed for susceptibility criteria……………………………………..62
4.3.4. Calculation results……………………………………………………………………….62
4.3.4.1. Critical loading condition evaluation………………………………………….62
4.3.4.2. Roll motion and susceptibility criteria………………………………………...63
5. STABILITY FAILURE IN BEAM WAVES, WIND AND FISHING GEAR
FORCES……………………………………………………………………………..69
5.1. Types of industrial fishing vessels more common in Portugal and Peru. ………………….69
5.1.1. Fishing process of purse seiners………………………………………………………69
5.1.2. Fishing process of trawlers……………………………………………………………..72
5.2. Transverse stability in beam wind and rolling…………………………………………………74
5.3. Transverse stability due to fishing gear loads…………………………………………………76
5.4. Calculation results………………………………………………………………………………..80
6. CONCLUSIONS AND RECOMMENDATIONS………………………………….87
6.1. General conclusion……………………………………………………………………………….87
6.2. Conclusion and recommendations on stability failure in longitudinal waves………………87
6.3. Conclusion and recommendations on stability failure in beam waves, wind and fishing
gear forces………………………………………………………………………………………...88
7. REFERENCES………………………………………………………………………89
ANEXOS…………………………………………………………………………………………………….95
ix
List of Figures
Figure 1. Pre-casualty (Describes how the vessels were being operated prior to the fishing vessels
casualty) Dickey (2008)……………………………………………………………………………………...4
Figure 2. Incidents of fishing vessels directly associated with the fishing vessel casualty (Dickey,
2008)…………………………………………………………………………………………………………..5
Figure 3. Incidents of fishing vessels directly associated with the fishing vessel casualty (Antão and
Guedes Soares 2004)……………………………………………………………………………………….5
Figure 4. Explanation of righting levers and heeling angles……………………………………………12
Figure 5. Distribution of length of capsized ships collated by IMO (1985)……………………………13
Figure 6. Place of casualty (IMO 1985)…………………………………………………………………..13
Figure 7. Season of casualty (IMO 1985)……………………………………………………………......14
Figure 8. Sea and wind condition during casualty (IMO 1985)………………………………………...14
Figure 9. Way of casualty (IMO 1985)……………………………………………………………………14
Figure 10. Condition at time of casualty. Distribution of GM0 (IMO 1985)……………………………15
Figure 11. Condition at time of casualty. Distribution of GZ20 (IMO 1985)……………………………15
Figure 12. Condition at time of casualty. Distribution of GZ30 (IMO 1985)……………………………15
Figure 13. Condition at time of casualty. Distribution of GZm (IMO 1985)……………………………16
Figure 14. Condition at time of casualty. Distribution of m (IMO 1985)……………………………...16
Figure 15. Condition at time of casualty. Distribution of v (IMO 1985)………………………………16
Figure 16. Condition at time of casualty. Distribution of e (IMO 1985)………………………………..17
Figure 17. Plot of righting levers for ships at time of casualty. Cargo vessels only………………….18
Figure 18. Estimation of critical parameter……………………………………………………………….19
Figure 19. Discrimination analysis for parameter GZ30 (IMO 1965)…………………………………...20
Figure 20. Energy balance method used by Pierrottet (1935)………………………………………....22
Figure 21. Energy balance methods in standards of USSR (upper) and Japan (lower)…………….22
Figure 22. Gustiness of measured sea wind (Watanabe et al. 1956)…………………………………23
Figure 23. Relationship between wave age and wave steepness (Sverdrup and Munk 1947)…….24
Figure 24. Relationship between roll period and wave steepness in Japanese criterion (Yamagata
1959)…………………………………………………………………………………………………………25
Figure 25. Effective wave slope coefficient: measurements (circles) and estimation (solid line)
(Yamagata 1959)……………………………………………………………………………………………25
Figure 26. Example of N coefficients measured in model experiments………………………………26
Figure 27. Estimation accuracy for empirical formula for roll period…………………………………..26
Figure 28. Comparison of roll amplitude in regular and irregular waves (Watanabe et al. 1956)….27
Figure 29. Results of test calculations for determining steady wind velocity. Relation between wind
velocity and the b/a factor for various sample ships (Watanabe et al. 1956)………………………...28
Figure 30. Standard roll amplitude in USSR’s criterion (USSR, 1961)………………………………..29
Figure 31. Severe wind and rolling………………………………………………………………………..30
Figure 32. Changes on righting arm curves in longitudinal waves for a Portuguese fishing vessel.39
x
Figure 33. Fishing vessel model (“FV10”)........................................................................................41
Figure 34. Body view of 15 fishing vessels. “FV1”, “FV6”, “FV8”, “FV10”, “FV11”, “FV14” and “FV15”
are from the Portuguese fleet and the rest are from Spain and Japan.............................................44
Figure 35. Body view of 12 fishing vessels. The “FV17” and “FV27” are from Spain and Chile,
respectively. The rest of fishing vessels are from the Peruvian fleet................................................45
Figure 36. Changes of draught and displacement due to operational loading conditions................46
Figure 37. Roll restoring energy variation at three operational loading conditions (%), from 0 to 30º
(left side) and from 0 to 40º (right side), with respect to still water. Conditions (C1, C2 and C3).....48
Figure 38. Fishing vessel model (FV10)...........................................................................................50
Figure 39. GM(t) and GZmax(t) (left side) and the roll-restoring energy (RRE) (right side) as
function of the wave crest position along to wave, including the minimum requirement of..............53
Figure 40. Physics of parametric resonance, development of parametric roll (Shin et al. 2004).....56
Figure 41. Mathematical understanding of parametric resonance. Ince-Strutt diagram (Chang et al.
2008)................................................................................................................................................57
Figure 42. GM as a function of wave crest position along fishing vessel (FV10, see Table 1)........58
Figure 43. Maximum variation of GM at different loading conditions (m).........................................63
Figure 44. Fishing vessel model.......................................................................................................64
Figure 45. Run file of the fishing vessel model “FV26”.....................................................................64
Figure 46. Linear and high order approximation for the boundary of the first instability zone (Ince-
Strutt diagram). /Lpp 1, wave steepness of 1/20……………………………………………………..65
Figure 47. Linear and high order approximation for the boundary of the first instability zone (Ince-
Strutt diagram). /Lpp [0.84, 1.6], wave steepness of 1/20…………………………………………..66
Figure 48. Wall of netting of purse seiners……………………………………………………………….70
Figure 49. Process of fishing of purse seiners…………………………………………………………..71
Figure 50. American purse seiner…………………………………………………………………………72
Figure 51. European purse seiner………………………………………………………………………...72
Figure 52. Beam trawler, modern, large; Holland (FAO)……………………………………………….73
Figure 53. Very large, Factory trawler; Holland (FAO) …………………………………………………73
Figure 54. Balance of energy for the weather criteria including fishing gear pull. …………………..74
Figure 55. Contour of highest 1/1000 value of wave height (left side) and wind speedy (right side)
(annual) (Ogawa 2009). …………………………………………………………………………………...75
Figure 56. Gear loads on a traditional pelagic purse seiner……………………………………………77
Figure 57. General arrangement of two purse seiners fishing vessels, Portuguese (smaller) and
Peruvian (bigger) …………………………………………………………………………………………...81
Figure 58. Inclining arms due to external forces (wind and fishing gear loads) and restoring arms of
the fishing vessel FV6. …………………………………………………………………………………….84
Figure 59. Inclining arms due to external forces (wind and fishing gear loads) and restoring arms of
the fishing vessel FV8. …………………………………………………………………………………….85
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List of Tables
Table 1. Common types of injuries or fatalities due to accidents onboard or vessels casualty……8
Table 2. Percentages of ships below limiting line……………………………………………………..18
Table 3. Values of factor X1……………………………………………………………………………...33
Table 4. Values of factor X2……………………………………………………………………………...33
Table 5. Values of factor k……………………………………………………………………………….33
Table 6. Values of factor s……………………………………………………………………………….33
Table 7. Fishing vessels hull characteristics. MU (Moderate U type), U (U type), EV (Extreme V
type), V (V type), MT (Medium transom), DP (Deep transom) and ITPS (Inclined transom purse
seiner)………………………………………………………………………………………………………43
Table 8. Vessel characteristics at critical operational loading condition of the Peruvian and
Portuguese fleets………………………………………………………………………………………….46
Table 9. Intact stability calculation according to the IMO 2008 IS Code and FAO/ILO/IMO 2005
stability criteria, considering the loss of stability in waves at critical operational loading
condition……………………………………………………………………………………………………47
Table 10. Change of roll-restoring energy variation in waves (respect to still water) at critical
operational loading condition. Wave parameters: Upper (H/=0.05,/Lpp=1.6), lower
(H/=0.05,/Lpp=1)……………………………………………………………………………………….51
Table 11. Susceptibility analysis of parametric rolling………………………………………………...66
Table 12. Susceptibility analysis of parametric rolling (Shin et al. 2004; ABS, 2008; Taylan,
2007)……………………………………………………………………………………………………….67
Table 13. Critical operational loading condition (O.l.c) and sailing condition (vessel speed and wave)
for parametric rolling in head seas………………………………………………………………68
Table 14. Fishing gear forces (Tons) acting on the 7 fishing vessels……………………………….80
Table 15. Balance of energy between the heeling energy due to external forces (wind (15m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14……….83
Table 16. Balance of energy between the heeling energy due to external forces (wind (19m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14……….83
Table 17. Balance of energy between the heeling energy due to external forces (wind (26m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14……….83
1
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CHAPTER I
INTRODUCTION_____________________________________________________________
The marine activity in general and the fishing sector in particular, is one of the most dangerous
activities with a high rate of mortality. An average of 24,000 fishermen looses their lives every year
(Petursdottir et al. 2001). Injury and fatality rates are between 25 and 40 times the national average
in many Europe countries, Australia and USA (Havold 2009). However the fatal accident rate
among fishermen was 115 times greater than in the general British workforce (Roberts 2010).
The safety management systems applied at the international level in merchant ships do not
have their equivalent in the vessels dedicated to fishing, and much less to the most numerous sub-
sector, the so called-artisan fishing (Piniella et al. 2009, Kuo 2003). For example, the annual
property, injury, and other costs of US commercial fishing vessels accidents are estimated to be
over $240 million, more than three times the comparable cost of tanker accidents (Di Jin et al.
2005).
The safety of fishing vessels remains a key concern given the high rates of accidents occurring
worldwide. The IMO, FAO and some Classification Societies have some records or data of the
world fishing vessels, but no representative and trusted database about casualties of fishing
vessels is available.
Although the database of the IMO can be representative, there are many casualties at the sea
that are not reported properly, making difficult definitive studies due to the uncertainties about how
may ships or fishing vessels were actually lost. There are no standardized accident reporting
system in the maritime environment (Havold 2009, Perez-Labajos 2008), so the existing data
should be carefully analyzed. Some representative fisheries do not have even implemented
systems for data acquisition.
It is important to distinguish between casualties (Antão and Soares 2004) and work accidents
onboard (Antão et al. 2008). The casualties can occur because of the stability failure of the vessel,
for many reasons well described by (Umeda et al. 1999, Umeda 2002, Francescutto 2007), mostly
including additional factors suggested by Kobylinski (2003) and others authors. On the other hand,
the accidents onboard occur mainly because the work environment is affected by: the dynamic
stability of the vessels (Piniella et al. 2008), weather condition, vessel location, time of the year,
vessel characteristics (Di Jin et al. 2005).
2
Most of the accidents aboard happen on deck or in holds (Havold 2009) during the trips to and
from the fishing grounds and many authors believe that the main reason of accidents on the fishing
industry is human error, which account for between 75% and 96% of all accidents in the industry
(Rothblum et al. 2000, Umberti et al 2001), which is maybe higher but not too different from what is
found in commercial vessels (Guedes Soares et al. 2001, Antão and Guedes Soares 2008). These
ideas are supported by Kobylinski (2003), in the sense that not only the “environment” basic
element should be considered in the ship stability analysis in general, it should consider the four
basis elements: ship, environment, cargo and operations. A clear example is the Peruvian fishing
fleet that in the last five years has suffered total and partial stability failures, in several decked and
undecked fishing vessels.
At the moment, because it is not easy to model the vessel behavior in an environment
characterized by waves and wind attention should be paid to operational measures (Francescutto
2002). Fortunately the developments of computer technologies and of sensors allow the monitoring
of ship responses with the use of several devices that can be integrated in a bridge decision
support system that allows masters to monitor dynamically the stability of fishing vessels (Kose et
al. 1995). A computer system on board can not only monitor the course of the ship in rough water
but also the load on fishing gears, devise of safety in the hydraulic system, etc. As a parameter of
comparison, for the decision support system, it is necessary to have all the modes of stability
failure due to individual or combined event, so in order to guide the decision for a real situation
there is still a lot of work to do.
The present studies represent an initial effort in that project by studying a set of 27 fishing
vessels mainly from the Portuguese and Peruvian fleets, with different configurations and modes of
operation in order to have a better understanding of:
The range of variation that can be expected in the intact stability characteristics in
longitudinal waves and in this way to guide the design of the decision support system to
cope with pure loss of stability on a crest wave and with the likelihood of parametric roll.
The action of fishing gear forces, wind and combinations of some fishing vessels in a
fishing trip scenario, and their influence in the stability failure.
The second Chapter of this thesis presents the stability failure of fishing vessels. Where fishing
vessels casualties are studied. Fisherman casualties are also pointed out.
In the third Chapter an extended study of the intact stability of fishing vessels is presented.
Where a Background of criteria regarding righting lever curve properties (part A of the 2008 IS
Code), and a Background of the severe wind and rolling criterion (part B of the 2008 IS Code) are
presented. In the end of this Chapter the 2008 IS Code for fishing vessels and the current intact
stability criteria for the Portuguese and Peruvian fishing vessels are also included. Finally an
overview of the International Code on Intact Stability (2008 IS Code, part A and part B), related to
fishing vessels is made.
3
In the fourth Chapter the stability failure in longitudinal waves are studied. The variation of
righting lever that produces changes of stability is determined considering longitudinal waves. The
27 fishing vessels were studied using 2 representative longitudinal waves for each vessel. The
wave parameters considered were the following: s=1/20 and (a) /Lpp=1, (b) /Lpp=1.6, with wave
crest position along the wave or vessel’s length. The wave parameters considered were based on
the studies conducted by Umeda et al. (1999), Neves et al. (1999, 2002), Kuroda et al. (2003),
Bulian (2006), ITTC (2005), Hashimoto (2008).
The calculations were also carried out for 3 different loading conditions: Port departure (0%
Cargo, 100% Consumables (C1)), Fishing ground departure (100% Cargo, 35-50% Consumables
(C2)), and Port arrival (100% Cargo, 10-20% Consumables (C3)). All these calculation were done
for the Portuguese and Peruvian fleets, which are well represented by the fishing vessels
considered in this study.
In the fifth Chapter the stability failure due to beam waves, wind and fishing gear forces are
studied. This Chapter represents an initial effort in the stability failure by studying a set of 7 fishing
vessels mainly from the Portuguese and Peruvian fleets, with different configurations and modes of
operation in order to have a better understanding of the action of fishing gear forces, wind and
combinations of them in a fishing trip scenario.
In the sixth Chapter some conclusion and recommendations on:
2008 IS Code (general conclusion);
Prevention of stability failure in longitudinal waves, such as: pure loss of stability and
parametric resonance;
Stability failure in beam waves, wind and fishing gear forces. Utilization of different kind
of fishing gears, taking into account their influence onto fishing vessel’s transverse
stability, specially for purse seiners, were given.
Finally, references and annexes are described.
4
5
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CHAPTER II
FISHING VESSEL STABILITY FAILURES______________________________________________________________
Analysis of casualty data normally is presented in two parts: (a) Fishing vessels casualties (Antão
and Guedes Soares 2004), and (b) work accidents onboard or fishermen casualties (Antão et al.
2008). In the following sections they are described.
2.1. Vessel Casualties
An interesting article prepared by Dickey (2008) describes how the vessels were being
operated prior to fishing vessel casualty occurrence for the US fishing fleet. He used the following
categories: Transiting, Inbound, Moored, Anchored, Outbound, Drifting, Being towed, Towing,
Fishing trip (Fishing, trawling, hauling gear, setting gear), Unknown and Other, see Figure 1.
Figure 1. Pre-casualty (Describes how the vessels were being operated prior to the fishing vessels
casualty) Dickey (2008)
However, reordering the categories, i.e. considering the following set of categories: Fishing,
trawling, hauling gear, setting gear as part of a unique called “fishing trip category”, then this
become the second more dangerous category prior to the vessel casualty. Most fishing vessels
6
casualties can be described as a series of events that lead to the loss of the vessel, for example, a
hull failure can be followed by flooding, and then sinking (Dickey, 2008). The importance of having
a well description or clear knowledge of the origin of the vessels casualty is fundamental. Figure 2
summarizes the incidents by the type of event most directly associated with the vessel loss for the
US fishing fleet in a period from 1992 to 2007.
Figure 2. Incidents of fishing vessels directly associated with the fishing vessel casualty (Dickey,
2008).
Antão and Guedes Soares (2004) ordered the vessels casualties that occurred with the
Portuguese fishing fleet as shown in Figure 3. This Figure shows that in the deep-sea fishing, 37%
of vessels casualties were related to machinery, 22% of accidents were caused by loss of the
fishing net, 22% due to collisions, 7% due to flooding and 4% due to large waves. In the coastal
fishing 61% of accidents occur due to collisions (with other vessels, with other fishing gear or other
objects) and 26% are due to damage propeller or machinery.
Figure 3. Incidents of fishing vessels directly associated with the fishing vessel casualty (Antão and
Guedes Soares 2004).
7
From the literature survey, it was found that safety assessment of fishing vessels had been
limited to stability consideration and very little work has been carried out on the operational and
equipment safety assessment (Loughran et al. 2002). Hence, it is important to describe all the
vessels and fishermen casualties. In order to direct the attention of the safety assessment on
fishing vessels, the probable causes of each accident category have been investigated by Wang et
al. (2005) and they are summarized below.
2.1.1. Foundering/flooding
Typically, these incidents are caused by burst pipes, fittings working loose, leaking glands and
sprung planks. Flooding is a particular problem with smaller wooden vessels. Smaller vessels are
often of clinker construction, where the strakes are lapped against each other and clenched. They
are reliant upon the swelling nature of the wood when soaked for making a good seal. This method
of construction is particularly vulnerable in heavy sea conditions.
These types of accidents can also happen on vessels that are of metal construction.
Sometimes, incompatible metals become rapidly corroded in a seawater environment; examples
are copper piping adjacent to steel or aluminum structures.
2.1.2. Fires and explosions
The investigation of these accidents has shown that in most cases the fire had originated from
the engine room and was caused by oil or fuel coming into contact with hot exhausts. Other causes
are heating and cooking stoves and electrical faults. There have been several cases, where the fire
had started in the accommodation area due to the crew smoking cigarettes in the sleeping bunk.
The number of accidents caused by fire has been relatively low compared to other categories.
However, due to the limited fire fighting resources on-board fishing vessels, it has the potential to
cause severe damage and even loss of lives.
2.1.3. Grounding
These incidents are associated with all classes of fishing vessels and can be due to various
causes. Engine or gearbox failures and propellers fouled by a rope or fishing net are common
causes. However, many cases have been associated with navigational error.
8
2.1.4. Capsizing
It is evident that the majority of capsizing incidents occurred during the fishing and recovery of
gear operations (Wang et al. 2005). This shows that for the vessels that do capsize, there is a
negative factor of safety in the present stability criteria. This negative factor is introduced by the act
of fishing and the associated moment lever introduced by the gear along with the wind lever in the
dynamic situation at sea (Loughran et al. 2002). The most common cause of capsizing is when the
fishing gear becomes fastened.
2.1.5. Collisions and contacts
The majority of the collision and contact incidents involved a fishing vessel and a merchant
vessel. Large merchant vessels may have a poor line of sight from the wheelhouse and small
fishing vessels are not easily seen under the bow. Apart from that, skippers on fishing vessels are
too involved in the fishing operation to plot the position and movement of other vessels
approaching them. Collisions and contacts could also occur involving two or more fishing vessels.
This is especially true, when pair trawling is in progress. However, the consequences are less
severe and such incidents normally occur due to errors of judgment by one or both parties involved.
2.1.6. Machinery damage
Although most machinery failures do not threaten the vessel or lives of the crew, given other
factors, such as bad weather or being in a tideway, the consequences could be disastrous. Upon
investigation of several fishing vessels in the UK, it was found that maintenance activities on-board,
these vessels were almost non-existent. This is thought to lead to the high number of machinery
failures.
2.1.7. Heavy weather damage
The number of vessels suffering weather damage is comparatively low, as it can be seen in
Figure 2 and in Wang et al. (2005). Small vessels are particularly vulnerable to these accidents.
Heavy weather can weaken the hull structure of the vessel and at the same time, cause deck
fittings to come loose and lead to an accident.
2.1.8. Other
9
The reported incidents which present higher numbers in the official statistics in US, UK, and
Portugal are described above. A minor number of reported incidents are: Allison, Loss of vessel
control, Structural failure, weather, etc.
2.2. Accidents onboard of fishermen casualties
As mentioned above, the accidents onboard occur mainly because the work environment is
affected by: the dynamic stability of the vessels (Piniella et al. 2008), weather condition, vessel
location, time of the year, vessel characteristics, etc. (Di Jin et al. 2005). However, the accidents
onboard, normally, are originated also by the vessel casualty. Table 1 summarizes the most
common types of injuries and casualties due to accidents onboard or vessels casualty.
Table 1. Common types of injuries or fatalities due to accidents onboard or vessels casualty
Item Accidents onboard or vessel casualty
1 Vessel flooding/Sinking/Capsize
2 Fall into water
3 Pulled overboard by equipment
4 Diving Accident
5 Caught in winch
6 Dangerous Atmosphere
7 Unknown Injury Type
8 Struck by Moving Object - Other
9 Crushed by equipment
10 Smoke Inhalation - Vessel Fire
11 Struck by/Caught in lines
12 Drowned - Entered water voluntarily
13 Drowned while attempting to unfold propeller
14 Fall onto surface
15 Struck a Fixed Object
16 Electrical shock
17 Blown Overboard By Explosion
18 Vessel Collision/Grounding
19 Exposure - Other
20 Fell overboard, crushed between dock and vessel
21 Burns
10
Dickey (2008) shows results of fatalities due accidents onboard of fishing vessels for the US
fishing fleet. He shows that just over half (55%) of all fishing vessel deaths are attributed to flooding,
sinking, or capsizing of the vessel. Another 23% of the fatalities were falls overboard. With three
quarters of all fatalities, water exposure is by far the most significant factor in personnel loss. The
next largest group of accident types includes fishermen that were struck by or caught in lines or
other equipment, for 6% of the total. Antão and Guedes Soares (2004), based on studies
performed by Lopes (2000), showed Portuguese fatalities due to accidents onboard.
From all the casualties of fishing vessels and fishermen described above, this Thesis will
discuss vessels casualty due to:
Capsizing (stability failure or vessel casualty) in longitudinal waves.
Capsizing in beam waves (bad weather conditions).
Capsizing due to fishing gear forces.
Capsizing due to fishing gear forces and bad weather conditions.
11
________________________________________________________________________
CHAPTER III
INTACT STABILITY OF FISHING VESSELS ______________________________________________________________
MSC.1/Circ.1281 (2008) describe the explanatory notes to the International Code on Intact Stability,
and in IMO-MSC.267(85) (2008) the adoption of the international code on intact stability (2008 IS
Code). Considering both articles, the most relevant arguments were taken and shown in the
following sections.
3.1. Origin of present stability criteria
The Maritime Safety Committee requested the Sub-Committee on Stability and Load Lines and
on Fishing Vessels Safety (SLF), to develop a range of intact stability requirements to cover all ship
types for eventual incorporation into the 1974 SOLAS Convention. At the thirty-third session of the
Sub-Committee (SLF 33), the Working Group on Intact Stability (IS) considered this matter and
foresaw the procedural problems that would arise by incorporating a wide range of stability criteria
covering different ship types into the Convention, and also recognized that these criteria could not
be developed in a short time. The group recommended that, alternatively, consideration should be
given to developing a comprehensive code to incorporate the then existing stability requirements
contained in all IMO recommendations and codes for various types of ships. Criteria for additional
ship types could be added later as each ship type was considered and a criterion developed. The
proposed Code could be divided into two parts: part A, containing mandatory requirements; and
part B, containing recommendatory requirements.
The final draft of the Code was agreed by SLF 37 and subsequently adopted by resolution
A.749(18) (IMO 1993). It was subsequently amended in 1998 by resolution MSC.75(69). The Code
was considered to be a “living” document under constant review, into which all new requirements
developed by IMO would be incorporated.
The Maritime Safety Committee at its 85th session (MSC 85) adopted the International Code
on Intact Stability (2008 IS Code) and the most significant change, in comparison with the original
version of the code adopted in 1993, is that the Part A of the 2008 IS Code is mandatory under the
1974 SOLAS Convention and the 1988 LL Protocol, which entered into force on 1 July 2010.
12
3.2. Background of criteria regarding righting lever curve properties (part A of the 2008 IS
Code)
The statistical stability criteria were originally included in resolutions A.167(ES.IV) and
A.168(ES.IV). They were developed as a result of discussions conducted at several sessions of the
Sub-Committee on Subdivision and Stability Problems (STAB), a forerunner of the SLF Sub-
Committee and the Working Group on Intact Stability (IS). There was general agreement that the
criteria would have to be developed on the basis of the statistical analysis of stability parameters of
ships that had suffered casualties and of ships that were operating safely.
The IS Working Group agreed to a programme of work that eventually included the following
item:
Collation, analysis and evaluation of existing national rules or recommendations on stability;
Evaluation of stability parameters which could be used as stability criteria;
Collection of stability characteristics of those ships that become casualties or experienced
dangerous heeling under circumstances suggesting insufficient stability;
Collection of stability characteristics of those ships which were operating with safe
experience;
Comparative analysis of stability parameters of ships becoming casualties and of ships
operated safely;
Estimation of critical values of chosen stability parameters; and
Checking formulated criteria against a certain number of existing ships.
The analysis of existing national stability requirements revealed considerable consistency in
the applicability of certain parameters as stability criteria. It was noted also that in many countries
there was a tendency to adopt weather criterion. However, weather criterion was not considered by
the IS Working Group at that time.
The IS Working Group singled out a group of parameters characterizing the curve of righting
levers for the ship at rest in still water. This was done notwithstanding the fact that if a ship sails in
a seaway, the curve of static stability levers changes. However, it was decided that the only
practical solution would be to use the “stipulated” curve of righting levers and this curve could be
characterized using the following set of parameters:
Initial stability: GM0;
Righting levers at angles: GZ10, GZ20, GZ30, GZ40, GZφ, GZm;
Angles: φm, φv, φf, φfd;
Levers of dynamic stability: e20, e30, e40, eφ.
13
The number of stability parameters which could be used as stability criteria should be, however,
limited. Therefore, by analyzing the parameters used in various national stability requirements, the
Working Group on Intact Stability concluded the following eight parameters have to be left for
further consideration: GM0, GZ20, GZ30, GZm, φm, φv, φfd, e.
During the collection of stability characteristics of those ships that become casualties or
experienced dangerous heeling under circumstances suggesting insufficient stability, a special
form of casualty record was prepared and circulated amongst IMO Member States (IMO 1963).
Altogether there were casualty records collected for 68 passenger and cargo ships and for 38
fishing vessels (IMO 1966, 1966a). In a later period, some countries submitted further casualty
records so that, in the second analysis that was performed in 1985, data for 93 passenger and
cargo ships and for 73 fishing vessels were available (IMO 1985). On the basis of the submitted
data, tables of details of casualties were prepared.
Figure 4. Explanation of righting levers and heeling angles.
During the collection of stability characteristics of those ships which were operating with safe
experience, data on stability characteristics for 62 passenger and cargo ships and for 48 fishing
vessels, which were operated safely, were collected and for this purpose a special instruction
containing detailed specifications for the manner how the stability information was to be submitted
was developed. Also, for these ships, tables were prepared of stability parameters.
After IMO resolutions A.167(ES.IV) and A.168(ES.IV) had been adopted and further intact
stability casualty data were collected, it was decided to repeat the analysis in order to find out if
additional data might change conclusions drawn in the first analysis. This second analysis
confirmed, in general, the results achieved in the first analysis (IMO 1985). In the following text, the
results of the second analysis that was based on the larger database are referred to.
The analysis performed consisted of two parts. In the first part, details relevant to casualties
were evaluated, which allowed qualitative conclusions with regard to the circumstances of
casualties to be developed and therefore the specification of general safety precautions. In the
second part, stability parameters of ships reported as casualties were compared with those for
ships which were operated safely (quantitative conclusions). Two methods were adopted in this
analysis. The first was identical with the method adopted by Rahola (Rahola 1939) and the second
was the discrimination analysis. The results of: (a) the analysis of intact stability casualty data, and
14
of the first part of the analysis of stability parameters using Rahola method are included in section
3.2.1.; and (b) the results of the discrimination analysis are referred to in section 3.2.2.
3.2.1. Results of the Analysis of Intact Stability Casualty Records and Stability Parameters
3.2.1.1. Analysis of details relevant to the casualties
The evaluation of details relevant to the casualties is shown in Figures 5 to 9.
Figure 5. Distribution of length of capsized ships collated by IMO (1985).
In all 166 casualties reported, the ships concerned were: 80 cargo ships, 1 cargo and
passenger ship, 1 bulk carrier, 4 off-shore supply ships, 7 special service vessels, and 73 fishing
vessels. Distribution of ship’s length is shown in Figure 5. It is seen that the majority of casualties
occurred in ships of less than 60 m in length.
The result of the analysis of the location of the casualty is shown in Figure 6. It may be seen
that the majority of casualties (72% of all casualties) occurred in restricted water areas, in estuaries
and along the coastline. This is understandable because the majority of ships lost were small ships
of under 60 m in length. From the analysis of the season when the casualty occurred (Figure 7) it
may be seen that the most dangerous season is autumn (41% of all casualties).
Figure 6. Place of casualty (IMO 1985).
15
Figure 7. Season of casualty (IMO 1985).
Figure 8. Sea and wind condition during casualty (IMO 1985).
The result of the analysis of the weather conditions is shown in Figure 8. About 75% of all
casualties occurred in rough seas at a wind force of between Beaufort 4 to 10. Ships were sailing
most often in beam seas, less often in quartering and following seas.
The manner of the casualty was also analyzed (Figure 9). It showed that the most common
casualty was through gradual or sudden capsizing. In about 30% of casualties, ships survived the
casualty and were heeled only.
Figure 9. Way of casualty (IMO 1985).
16
The distributions of stability parameters for ships’ condition at time of loss are shown in Figures
10 to 16.
Figure 10. Condition at time of casualty. Distribution of GM0 (IMO 1985)
Figure 11. Condition at time of casualty. Distribution of GZ20 (IMO 1985).
Figure 12. Condition at time of casualty. Distribution of GZ30 (IMO 1985).
17
Figure 13. Condition at time of casualty. Distribution of GZm (IMO 1985).
Figure 14. Condition at time of casualty. Distribution of m (IMO 1985).
Figure 15. Condition at time of casualty. Distribution of v (IMO 1985).
18
Figure 16. Condition at time of casualty. Distribution of e (IMO 1985).
3.2.1.2 Analysis of stability parameters using Rahola method
The stability parameters for casualty condition were analyzed by plotting in a similar manner,
as was done by Rahola (1939), together with parameters for ships operated safely for comparison.
The parameters chosen for analysis were GM0, GZ20, GZ30, GZ40, GZm, e40, and ϕm. From the
available data, histograms were prepared, where respective values of stability parameters for
casualty condition were entered by starting with the highest value at the left of the vertical line
(ordinate) down to the lowest value, and the values of the same parameter for safe ships were
entered on the right side by starting from the lowest and ending with the highest value. Thus, at the
ordinate, the highest value of the parameter for casualty condition is next to the lowest value of the
parameter for the safe case. In Figure 17 an example diagram for righting levers comprising all
ships analyzed is shown. In the original analysis (IMO 1966, 1966a, 1985) diagrams were prepared
separately for cargo and fishing vessels, but they are not reproduced here.
In Figure 17, the values for casualty condition are shaded, only those that have to be specially
considered due to exceptional circumstances were left blank. On the right side of the ordinate the
areas above the steps were shaded in order to make a distinction between the safe and unsafe
cases easier. The limiting lines or the imaginary static stability lever curves were drawn in an
19
identical way as in the Rahola diagram. Percentages of ships in arrival condition, the respective
stability parameters which are below the limiting lines are shown in Table 2. The lower percentages
mean in general that there is better discrimination between safe and unsafe conditions.
Figure 17. Plot of righting levers for ships at time of casualty. Cargo vessels only. (IMO 1966, 1985)
Table 2. Percentages of ships below limiting line
The type of analysis described above is not entirely rigorous; it was partly based on intuition
and allows arbitrary judgment. Nevertheless, from the point of view of practical application, it
provided acceptable results and finally was adopted as a basis for IMO stability criteria.
3.2.2. Discrimination Analysis
When two populations of data, as in this case, data for capsized ships and for ships considered
safe, are available and the critical values of parameters from these two sets have to be obtained,
the method of discrimination analysis may be applied.
The application of the discrimination analysis in order to estimate critical values of stability
parameters were contained in a joint report by (IMO 1966, 1966a), and constituted the basis for
development of IMO stability criteria along the previously described Rahola method.
20
In this investigation, discrimination analysis was applied independently to nine stability
parameters. Using data from intact stability casualty records (group 1) and from intact stability
calculations for ships considered safe in operation (group 2) the distribution functions were plotted,
where for group 1 the distribution function F1 and for group 2 function (1 − F2) were drawn.
Practically, on the abscissa axis of the diagram, values for the respective stability parameter were
plotted and the ordinates represent the number of ships in per cent of the total number of ships
considered having the respective parameter smaller than the actual value for ships of group 1 and
greater than the actual value for ships of group 2 considered safe.
The point of intersection of both curves in the diagram provides the critical value of the
parameter in question. This value is dividing the parameters of group 1 and of group 2. In an ideal
case, both distribution functions should not intersect and the critical value of the respective
parameter is then at the point between two curves (see Figure 18). In reality, both curves always
intersect and the critical value of the parameter is taken at the point of intersection.
The set of diagrams was prepared in this way for various stability parameters based on IMO
statistics for cargo and passenger ships and for fishing vessels. One of the diagrams is reproduced
in Figure 19. It means that the probability of capsizing of a ship with the considered parameter
higher than the critical value is the same as the probability of survival of a ship with this parameter
lower than the critical value.
In order to increase the probability of survival, the value of the parameter should be increased,
say up to x* (Figure 18), at which the probability of survival (based on the population investigated)
would be 100%. However, this would mean excessive severity of the criterion, which usually is not
possible to adopt in practice because of unrealistic values of parameters. It is possible that ships of
group 2 having values of the parameter in question x < xcrit are unsafe, but they were lucky not to
meet excessive environmental conditions which might cause capsizing. On the other hand, the
conclusion could also be drawn that consideration of only one stability parameter is not sufficient to
judge the stability of a ship.
Figure 18. Estimation of critical parameter
21
As can be seen from Figure 19, the accurate estimation of the critical values of the respective
parameters is difficult because those values are very sensitive to the running of the curves in the
vicinity of the intersection point, especially if the population of ships is small.
Figure 19. Discrimination analysis for parameter GZ30 (IMO 1965)
3.2.3. Adoption of the final criteria and checking the criteria against a certain number of
ships.
The final criteria, as they were evaluated on the basis of the diagrams, are prepared in the form
as shown in Figures 17 and 19. The main set of diagrams did show righting lever curves (Figure
17), but diagrams showing distribution of dynamic stability levers were also included. Diagrams
were prepared jointly for cargo and passenger vessels and for fishing vessels, except vessels
carrying timber deck cargo. Sets of diagrams were also separately prepared for cargo ships and
fishing vessels. Diagrams in the form as shown in Figure 19 were prepared separately for each
stability parameter and separately for cargo and passenger ships and for fishing vessels.
After discussion by the Working Group on Intact Stability and the SLF Sub-Committee, the
stability criteria were rounded off and finally adopted in the form as they appear in the resolutions
A.167(ES.IV) and A.168(ES.IV).
In the original analysis the angle of vanishing stability was also included. However, due to the
wide scatter of values of this parameter, it was not included in the final proposal.
3.3. Background of the severe wind and rolling criterion (weather criterion)
The severe wind and rolling criterion (weather criterion) is one of general provisions of the 2008
IS Code. This criterion was originally developed to guarantee the safety against capsizing for a ship
22
losing all propulsive and steering power in severe wind and waves, which is known as a dead ship.
Because of no forward velocity of ships, this assumes a irregular beam wind and wave condition.
The weather criterion firstly appeared in the IMO instruments as Attachment No.3 to the Final
Act of Torremolinos International Convention for the Safety of Fishing Vessels, 1977. During the
discussion for developing the Torremolinos Convention, the limitation of the GZ curve criterion
based on resolution A.168(ES.IV) was remarked; it is based on experiences of fishing vessels only
in limited water areas and it has no way for extending its applicability to other ship types and other
weather conditions. Thus, other than the GZ curve criterion, the Torremolinos Convention adopted
the severe wind and rolling criterion including a guideline of calculation. This new provision is
based on the Japanese stability standards for passenger ships.
Then, a similar criticism to the GZ curve criterion for passenger and cargo ships, resolution
A.167(ES.IV), was raised at IMCO. At least resolution A.167(ES.IV) was claimed to be applicable
to ships of 100 m in length or below because of the limitation of statistical data source. As a result,
a weather criterion was adopted also for passenger and cargo ships as well as fishing vessels of
45 m in length or over, as given in resolution A.562(14) in 1985. This new criterion keeps the
framework of the Japanese stability standard for passenger ships but includes USSR’s calculation
formula for roll angle. For smaller fishing vessels, resolution A.685(17) in 1991 was passed. Here
the reduction of wind velocity near sea surface is introduced reflecting USSR’s standard. When the
IS Code was established as resolution A.749(18) in 1993, all the above provisions were
superseded.
3.3.1. Energy Balance Method
The basic principle of the weather criteria is energy balance between the beam wind heeling
and righting moments with a roll motion taken into account. One of the pioneering works on such
energy balance methods can be found in Pierrottet (1935) (Figure 20). Here, as shown in Figure
3.1, the energy required for restoring is larger than that required for the wind heeling moment.
Since no roll motion is taken into account, a ship is assumed to suddenly suffer a wind heeling
moment at its upright condition.
In Japan the energy balance method is extended to cover a roll motion and to distinguish
steady and gusty wind as shown in Figure 21. Then it is adopted as the basic principle of Japan’s
national standard (Watanabe et al., 1956). The regulation of the Register of Shipping of the USSR
(1961) also assumes initial windward roll angle as shown in Figure 21. The current IMO weather
criterion of Chapter 2.3 of the IS Code, part A, utilizes the energy balance method adopted in
Japan without major modification.
23
Figure 20. Energy balance method used by Pierrottet (1935)
Figure 21. Energy balance methods in standards of USSR (upper) and Japan (lower).
3.3.2. Wind heeling moment
In the Japanese standard the steady heeling moment, Mw, is expressed as follows:
2
00 )(
2
1WDW V
H
HAHCM (1)
where:
ρ :air density
CD :drag coefficient
24
A :lateral wind age area above water surface
H :heeling lever
H0 :vertical distance from centre of lateral wind age area to a point at one half the mean
draught
VW :wind velocity
Values of CD obtained from experiments of passenger ships and train ferries ranges from 0.95
to 1.28. In addition, wind tunnels test for a domestic passenger ship (Okada 1952) shows that H/H0
is about 1.2. Considering these data, the value of CD(H/H0) was assumed to be 1.22 on average.
These formula and coefficients were adopted also at IMO.
To represent fluctuating wind, gustiness should be determined. Figure 25 shows the ratio of
gustiness measured in various stormy conditions Watanabe et al., 1955). Here the maximum is 1.7
and the average is 5.1 (≈ 1.23). However, these were measured for about 2 hours of duration but
capsize could happen within half the roll natural period, say 3 to 8 seconds. In addition, reaction
force could act on centre of ship mass because of such short duration. Therefore, in place of the
maximum value, the average value of Figure 22 is adopted. This results in 1.5 as heeling lever ratio
for gustiness as shown in the 2008 IS Code.
Figure 22. Gustiness of measured sea wind (Watanabe et al. 1956).
3.3.3 Roll angle in waves (Japanese Method)
In general, ship motion consists of surge, sway, heave, roll, pitch and yaw. In beam seas,
however, only sway, heave and roll are dominant. Furthermore, the effect of heave on roll is
negligibly small and coupling from sway to roll can be cancelled with roll diffraction moment (Tasai
and Takagi 1969). Therefore, the roll motion can be modeled without coupling from other motion
modes if the wave exciting moment is estimated without wave diffraction. Consequently,
considering nonlinear roll damping effect is taken into account, the amplitude of resonant roll in
regular beam waves, φ (degrees), can be obtained as follows:
25
)(2 N
r (2)
where:
Θ(=180s) : maximum wave slope (degrees)
s : wave steepness
r : effective wave slope coefficient
N : Bertin’s roll damping coefficient as a function of roll amplitude.
3.3.3.1 Wave steepness
Based on observations at sea, Sverdrup and Munk (1947) published a relationship between
wave age and wave steepness as shown in Figure 23. Here the wave age is defined with the ratio
of wave phase velocity, u, to wind velocity, v, and wave height, Hw, means significant wave height.
If we use the dispersion relationship of water waves, u =2
gT, this diagram can be converted to
that with wave period, T, as shown in Figure 24. Further, since the ship suffers a resonant roll
motion, the wave period could be assumed to be equal to the ship natural roll period. Here it is
noteworthy that the obtained wave steepness is a function of roll period and wind velocity. In
addition, because of possible spectrum of ocean waves, regions for the maximum and minimum
steepness are modified from the original data.
Figure 23. Relationship between wave age and wave steepness (Sverdrup and Munk 1947)
3.3.3.2 Hydrodynamic coefficients
To use Equation (2), it is necessary to estimate the values of r and N. Since we should
estimate wave exciting moment without wave diffraction due to a ship, it can be obtained by
integrating undisturbed water pressure over the hull under calm water surface. Watanabe (1938)
applied this method to several ships and developed an empirical formula, which is a function of
26
wave length, VCG, GM, breadth, draught, block coefficient and water plane area coefficient. For
simplicity sake, it is further simplified for 60 actual ships only as a function of VCG and draught
shown in Figure 25. The formula used in the IMO weather criterion for r was obtained by this
procedure.
Figure 24. Relationship between roll period and wave steepness in Japanese criterion
(Yamagata 1959)
Figure 25. Effective wave slope coefficient: measurements (circles) and estimation (solid line)
(Yamagata 1959)
For estimating the N coefficient, several empirical formulae were available. However, in the
Japanese stability standards, N=0.02 is recommended for a ship having bilge keels at the roll angle
of 20°. Some evidence of this value can be found in Figure 26 (Motora 1957).
3.3.3.3 Natural roll period
To calculate the wave steepness, it is necessary to estimate the natural roll period for a subject
ship. In the Japanese standard, the value measured with the actual ship is corrected with Kato’s
27
empirical formula (Kato 1956). However, at the STAB Sub-Committee, this procedure was
regarded as tedious and Japan was requested to develop a simple and updated empirical formula
for the roll period. Thus the current formula was statistically developed by Morita, and is based on
data measured from 71 full-scaled ships in 1982. As shown in Figure 27, all sampled data exist
within ± 7.5% of error from Morita’s formula. More precisely, the standard deviation of the error
from the formula is 1.9%. Furthermore, sensitivity analysis of C on required GM indicated that even
20% error of C estimation results in only 0.04 m error of required GM calculation.
Therefore, IMO concluded that this formula can be used for weather criteria.
Figure 26. Example of N coefficients measured in model experiments
Figure 27. Estimation accuracy for empirical formula for roll period.
28
3.3.3.4 Wave randomness
While the wave steepness obtained from Sverdrup-Munk’s diagram is defined by the significant
wave height in irregular waves, the resonant roll amplitude given by Equation (2) is formulated for
regular waves. For filling the gap between two, the roll amplitude in irregular waves whose
significant wave height and mean wave period are equal to height and period of regular waves was
compared with the resonant roll amplitude in the regular waves. As shown in Figure 28, if we focus
the maximum amplitude out of 20 to 50 roll cycles, an obtained reduction factor is 0.7.
Figure 28. Comparison of roll amplitude in regular and irregular waves (Watanabe et al. 1956)
3.3.3.5 Steady wind velocity
As explained above, the Japanese weather criterion introduced probabilistic assumptions for
determining gust and roll in irregular waves. These make final probabilistic safety level unclear.
Possible estimation error for wind heel lever coefficient, roll damping coefficient, effective wave
slope coefficient, natural roll period and wave steepness added uncertainty to the required safety
level. Therefore, Japan carried out test calculations for 50 ships, which include 13 ocean going
ships as shown in Figure 29. Based on these calculated outcomes, the steady wind velocity was
determined to distinguish ships having insufficient stability from other ships. In other words, for
ships having insufficient stability the energy balance should not be obtained with the above
procedure. As a result, the wind velocity for ocean going ships is determined as 26 m/s. Here a
sunken torpedo boat (0-12-I), a sunken destroyer (0-13) and three passenger ships having
insufficient stability (0-3, 7, and 9) are categorized as unsafe and 2 cargo ships, 3 passenger ships
and 3 larger passenger ships are done as safe. It is noteworthy here that 26 m/s of wind velocity is
only obtained from casualty statistics for ships and is not directly obtained from actual wind
29
statistics. IMO also adopted 26 m/s as critical wind velocity. If we substitute Vw=26 m/s to Equation
(1), the wind pressure in the current IS Code is obtained.
Figure 29. Results of test calculations for determining steady wind velocity. Relation between wind
velocity and the b/a factor for various sample ships (Watanabe et al. 1956).
3.3.4. Rolling in waves (USSR’s method)
In the stability standard of USSR (USSR, 1961), the maximum roll amplitude of 50 roll cycles is
estimated as follows:
AR XkX 21 (3)
Here k is a function of bilge keel area, X1 is a function of B/d, X2 is a function of the block
coefficient and φA is roll amplitude of the standard ship, which is shown in Figure 30. This formula
was developed by systematic calculations for a series of ships utilizing the transfer function and
wave spectrum (Kobylinski and Kastner 2003).
30
Figure 30. Standard roll amplitude in USSR’s criterion (USSR, 1961)
3.3.5. Adoption of the final weather criteria
As mentioned earlier, IMO decided to partly use this USSR’s roll formula together with the
Japanese criterion. This is because the USSR’s formula depends on hull forms for estimating roll
damping while the Japanese does not. The proposed formula is as follows:
rsXkXCJR 21.)(deg (4)
Here CJR is a tuning factor for keeping the safety level of the new criterion as the same as the
Japanese domestic standard. To determine this factor, member states of a working group of STAB
Sub-Committee executed test calculations of Japanese and new formulations for many ships. For
example, Japan (1982) executed test calculation for 58 ships out of 8,825 Japanese flagged-ships
larger than 100 gross tonnage in 1980. These include 11 cargo ships, 10 oil tankers, 2 chemical
tankers, 5 liquid gas carriers, 4 container ships, 4 car carriers, 5 tug boats and 17 passenger or
RoPax ships. As a result, IMO concluded that CJR should be 109.
3.4. 2008 IS Code for fishing vessels
3.4.1. Criteria regarding righting lever curve properties
The area under the righting lever curve (GZ curve) shall not be less than 0.055 metre-radians
up to = 30° angle of heel and not less than 0.09 metre-radians up to = 40° or the angle of down-
flooding f if this angle is less than 40°. Additionally, the area under the righting lever curve (GZ
curve) between the angles of heel of 30° and 40° or between 30° and f, if this angle is less than
40°, shall not be less than 0.03 metre-radians.
31
f is an angle of heel at which openings in the hull, superstructures or deckhouses which cannot be
closed weather tight immerse. In applying this criterion, small openings through which progressive
flooding cannot take place need not be considered as open.
The righting lever GZ shall be at least 0.2 m at an angle of heel equal to or greater than 30°.
The maximum righting lever shall occur at an angle of heel not less than 25°. If this is not
practicable, alternative criteria, based on an equivalent level of safety, may be applied subject to
the approval of the Administration.
The initial metacentric height GM0 shall not be less than 0.35 m.
3.4.2. Severe wind and rolling criterion (weather criterion)
The ability of a ship to withstand the combined effects of beam wind and rolling shall be
demonstrated, with reference to Figure 31 as follows:
the ship is subjected to a steady wind pressure acting perpendicular to the ship centreline
which results in a steady wind heeling lever (lw1);
from the resultant angle of equilibrium (0), the ship is assumed to roll owing to wave action
to an angle of roll (1) to windward. The angle of heel under action of steady wind (0)
should not exceed 16° or 80% of the angle of deck edge immersion, whichever is less;
the ship is then subjected to a gust wind pressure which results in a gust wind heeling lever
(lw2); and
under these circumstances, area b shall be equal to or greater than area a, as indicated in
figure 2.3.1 below:
Figure 31. Severe wind and rolling
32
where the angles in Figure 31 are defined as follows:
ϕ0 :angle of heel under action of steady wind
ϕ1 :angle of roll to windward due to wave action (see 2.3.1.2, 2.3.4 and footnote 6)
ϕ2 :angle of down-flooding (ϕf) or 50° or ϕc, whichever is less,
where:
ϕf :angle of heel at which openings in the hull, superstructures or deckhouses which
cannot be closed weather tight immerse. In applying this criterion, small openings
through which progressive flooding cannot take place need not be considered as
open
ϕc :angle of second intercept between wind heeling lever lw2 and GZ curves.
The wind heeling levers lw1 and lw2 are constant values at all angles of inclination and shall be
calculated as follows:
g
PAZlW 10001 (5)
12 5.1 WW ll (6)
where:
P : wind pressure of 504 Pa. The value of P used for ships in restricted service may be
reduced subject to the approval of the Administration
A : projected lateral area of the portion of the ship and deck cargo above the waterline (m2)
Z : vertical distance from the centre of A to the centre of the underwater lateral area or
approximately to a point at one half the mean draught (m)
Δ : displacement (tons)
g : gravitational acceleration of 9.81 m/s2.
Alternative means for determining the wind heeling lever (lw1) may be accepted, to the
satisfaction of the Administration, as an equivalent to calculation of the formula presented before.
When such alternative tests are carried out, reference shall be made based on the Guidelines
developed by the Organization. The wind velocity used in the tests shall be 26 m/s in full scale with
uniform velocity profile. The value of wind velocity used for ships in restricted services may be
reduced to the satisfaction of the Administration.
The angle of roll (ϕ1) shall be calculated as follows:
33
rsXkX 211 109 (7)
where:
X1 : factor as shown in Table 3
X2 : factor as shown in Table 4
k : factor as follows:
1.0 for round-bilged ship having no bilge or bar keels
0.7 for a ship having sharp bilges
as shown in Table 5 for a ship having bilge keels, a bar keel or both
r : 0.73 + 0.6 OG/d (8)
with:
OG :KG . d
d : mean molded draught of the ship (m)
s : factor as shown in Table 6, where T is the ship roll natural period. In absence of
sufficient information, the following approximate formula can be used:
Rolling period : GM
CBT
2 (9)
where:
C :0.373 + 0.023(B/d) - 0.043(Lw1/100) (10)
The symbols in tables the following tables and the formula for the rolling period are defined as
follows:
Lwl :length of the ship at waterline (m)
B :molded breadth of the ship (m)
D :mean molded draught of the ship (m)
CB :block coefficient (nd)
Ak :total overall area of bilge keels, or area of the lateral projection of the bar keel, or
sum of these areas (m²)
GM :metacentric height corrected for free surface effect (m).
34
Table 3. Values of factor X1 Table 4. Values of factor X2
Table 5. Values of factor k Table 6. Values of factor s
Note: Intermediate values in Tables 3-5 shall be obtained by linear interpolation.
The tables and formulae described before are based on data from vessels having:
B/d smaller than 3.5;
(KG/d-1) between - 0.3 and 0.5; and
T smaller than 20 s.
For ships with parameters outside of the above limits the angle of roll (ϕ1) may be determined
with model experiments of a subject ship with the procedure described in MSC.1/Circ.1200 as the
alternative. In addition, the Administration may accept such alternative determinations for any ship,
if deemed appropriate.
35
3.5 Intact stability of Portuguese and Peruvian fishing vessels
The evaluation of the stability of fishing vessels has traditionally been made through the criteria
described in the Torremolinos Convention (1977), which applies to vessels with deck longer than
24m. Strictly speaking, this agreement does not come into force due to their not signing by major
maritime nations. Until June of 2009 only 17 states have ratified the protocol with only 19.78% of
the tonnage, so this also is not in force.
Meanwhile, the International Maritime Organization has developed several recommendations
for safety in the fisheries sector. Most of these regulations, voluntary in nature, adopting the
recommendations of the Torremolinos Convention, but there are some rules and regional
agreements in force for each country or region of the world. In Portugal, the safety requirements for
fishing vessels are found in Decree-Law 155/2003 which transcribes the Torremolinos Convention,
amended by the Protocol of 1993, applicable and vessels over 24m. The basic criterion of stability
used in almost all these rules, are similar to these from the 2008 IS Code (IMO 2008). Peru uses
the Torremolinos convention with additional modification, for example, the minimum GM for fishing
vessels is 0.9m, a value which is much higher than the one established in 2008 IS Code or in the
Torremolinos convention (Mantari et al. 2009).
3.6 Overview of the 2008 IS Code
Since the starting of the IMO (before called: IMCO), interesting comparisons, analysis and
reviews of intact stability were made by Kuo (1981), Umeda et al. (1999), Womack (2002),
Francescutto (2002, 2007), Atua (2003), Kobylinski (2003), just to name some examples. All of
them believe, in different ways, that the intact stability criteria do not necessarily ensure safe
vessels.
Looking at the procedures of how this criteria came up, and comparing the procedures of
calculation of intact stability nowadays, and considering the interesting paper from Nowacki (2003)
(where a interesting historical findings, comparisons and reviews regarding the fathers of the intact
stability: “Euler (1749, related with GZ) and Bouguer’s (1746, related with GM)” has been done), it
seems little change has been made in terms of normative (after the IMO resolution A.167 (ES.IV) in
1968, and the Final Act of Torremolino International Convention in 1977). However, it is generally
accepted that significant works are still being developed at scientific/research level, which might
turn into normative in a future and therefore used by naval architects responsible for ship design.
36
3.6.1 IMO, 2008 intact stability code, part A
The Maritime Safety Committee at its 85th session (MSC 85) adopted the International Code
on Intact Stability (2008 IS Code) and the most significant change, in comparison with the original
version of the code adopted in 1993, is that the Part A of the 2008 IS Code is mandatory under the
1974 SOLAS Convention and the 1988 LL Protocol, which entered into force on 1 July 2010.
Actually, the present stability criteria are practically the same statistical criteria based on
analysis of details relevant to the casualties and Rahola diagram adopted by the IMO resolution
A.167 (ES.IV) in 1968. An overview of MSC.1/Circ.1281 (2008) and IMO-MSC.267(85) (2008)
suggests that the IMO is still trying to keep focus on the analysis of details relevant to casualties,
on the analysis of stability parameters using Rahola’s method to assess the intact stability code for
the “development new-generation intact stability code” currently underway, even when current IMO
criteria were questioned by several authors some years ago (Umeda et al. 1999, Johnson 2001,
Womack 2002, Atua 2003). Their statistical methodology was also questioned by Kobylinski (2003).
The importance of having different stability standards for different types of ships, operating in
various seas states, were pointed out by Kuo (2009).
The most-frequent modes of stability failure, initially pointed out by Umeda et al. (1999) and
Umeda (2002) according to the IMO (IMO-SLF 51/WP.2 2008, Umeda et al. 2009) are:
Restoring arm variation problems such as parametric excitation and pure loss of stability.
Stability under dead ship condition.
Maneuvering-related problems in waves such as broaching-to.
However, statistical classification of individual scenarios of stability failure analyzed by the IMO
did not actually show the real scenarios of failure. Usually there is more than one individual
scenario involved in a capsizing or these casualties often occur due to combined effects. The
cases involving pure stability failure without other factors present are quite rare (Womack 2002).
Indirectly, Pérez-Rojas et al. (2007, 2008) reached also this conclusion in experimental test of
small fishing vessels, at real operational loading condition, that had great difficulties in the
fulfillment of the IMO stability criteria but did not capsize (failure). Similar experimental results were
also given by Maron (2006). This is something that researchers should keep in mind.
Fortunately, remarkable work has been done to prevent, somehow, scenarios of total or partial
stability failure (Hamamoto et al. (1996), Umeda et al. (1999), Neves et al. (1999, 2002), Ribeiro e
Silva (2000, 2004, 2005), Kuroda (2003), Pérez-Rojas (2003, 2006, 2007, 2008), De Juana Gamo
(2005), Neves and Rodrigues (2006, 2007), Maron (2006) , etc.) and fishing vessels accidents
onboard due to seakeeping performance (Fonseca et al. (1996), Pérez-Arribas (2005), Sayli (2007,
2010), Tello (2009)). It is expected that in a near future the contributions of these authors will be
incorporated in the design of new fishing vessels. At the moment, despite this comment was made
almost a decade before, attention should moved to operational measures (Francescutto 2002).
37
The development of a new-generation intact stability criteria is now underway, based on a
series of clarified concepts about stability failures (in relation to the consequences and the modes),
types of criteria (deterministic or probabilistic, parametric or performance-based), vulnerability
criteria, and so on IMO-SLF 51/WP.2 (2008). These criteria will be applicable to all ships on
international voyages which have to comply with SOLAS or the LL Convention (ship greater than
24 meters). Therefore, it is expected that these contributions will lead in the near future to solve, or,
at least alleviate, the problem of intact stability of fishing vessels.
3.6.2 IMO, 2008 intact stability code, part B
This section is focused in the recommendation part of the 2008 IS Code (part B) (IMO-
MSC.267(85) 2008, MSC.1/Circ.1281 2008), were recommendation for the weather criterion and
fishing gear pull are given. Not significant change were done in the 2008 IS Code related to
weather criteria and fishing gear pull, compared with codes published before. The well known
recommendations are given, without almost any significant change.
However interesting critical review were done about the element involved in a capsizing which
are not considered by the current IMO, for example, Atua (2003) mentioned that: “the combined
effect of all probable factors affecting the heeling moment on fishing vessels such as the resulting
from the wind rolling, trapping water on deck, hauling or pursing a fishing gear, direction of the
force when trawling, structural damage due to steep waves, crew mistakes, etc. are involved in a
fishing vessel failure”. These ideas are supported by Womack (2003), Kobylinski (2003), and
several authors, in the sense that not only the “environment” basic element should be considered in
the ship stability analysis in general, it should consider the four basis elements: ship, environment,
cargo and operations.
Few papers have been published considering all these basic elements; most of them include
two basic elements only. For example, a representative study was presented by Umeda et al.
(1999), where an experimental analysis of combined waves and water on deck allowed
reproduction of capsizing of two Japanese fishing vessels. Water on deck and waves were later
included in their calculations by Francescutto et al. (2001) and Laranjinha et al. (2002). Waves and
wind were also studied by Taylan (2003) and Ucer and Odabasi (2008), and the concept of safety
margin was given in Taylan (2003). Numerical simulation results of combined wave, wind and
fishing gear loads were also calculated by Mantari et al. (2009a, 2009b, 2011b).
Hence, most researches have combined only ship and environment, mostly waves. However,
the statistics shows that failure of a ship in general and fishing vessels in particular occurred when
simultaneous combined effects of most significant loads are present (Wang et al., 2005). Actually,
with respect to fishing gear pull and wind forces, several authors believe that fishing gear pull
acting individually or combined with wind forces might explain several vessel stability failures (Atua,
2003; Wang et al., 2005; Gefaell, 2005; Mantari et al., 2009a, 2009b, 2011b).
38
Back to 1965, the IMO subcommittees on safety of fishing vessels discussed about the
possibility of inclusion of a normalized heeling moment due to fishing gear pull. However, after
some limited investigation the subcommittee felt that there was no need for any special criteria for
fishing vessels related to the action of fishing gear pull. Hence, taking into account the existence of
vessels failure due to this type of phenomena, which is not considered by the 2008 IS Code, then
special attention should be drawn to this topic.
Moreover, the purse seiners, studied in here, have fishing gear that can easily put in danger
the vessel, mainly for those vessels which still have a power block installed on a high position. As
reported before (IMO, 1979), the increase of machinery power also increases the heeling moment
up to a level where weather criterion is not the most conservative scenario, and therefore it is not
enough to guaranty the stability of the fishing vessel in operation. For example, on the Peruvian
fleet, there is still an ongoing clear tendency to increase the power of machinery on vessel’s deck
(main engine, winch and power block) due to abundant sources associated with mild weather
conditions. This stresses out the facts that four basic elements of fisheries are variable, and
adopting an optimal ship design, mentioned by Womack (2002) and Kuo (2009), is of paramount
importance.
Clearly, the evaluation of the stability of fishing vessels has traditionally been made through the
criteria described in the Torremolinos Convention (1977), which applies to vessels with deck longer
than 24 m. Strictly speaking, this agreement did not come into force due to lack of signing by major
maritime nations. In Portugal, the stability criteria for fishing vessels are based on the Torremolinos
Convention, amended by the Protocol of 1993, applicable to fishing vessels over 24 m. Peru uses
the Torremolinos convention as well, but with the additional modification (among others) that the
minimum GM for fishing vessels is 0.9 m.
39
_______________________________________________________________________
CHAPTER IV
STABILITY FAILURE IN LONGITUDINAL WAVES
______________________________________________________________
The intact stability in longitudinal waves, rather than in still water, gives a better idea of dynamic
stability of the vessel. Longitudinal, quartering and head seas are some of the most critical wave
conditions that should be analyzed and are applicable to all kinds of ships. The sea waves are
generally irregular. However, regular waves, representing a swell system may be the worst
scenario (Umeda et al. 1999).
Figure 32. Changes on righting arm curves in longitudinal waves for a Portuguese fishing vessel.
This thesis considers the righting arm with respect to 3 relative positions of ship in waves
)(wavetGZ on longitudinal wave profiles. As mentioned by several authors, )(wavetGZ decreases
with the wave crest aligned with amidships and increases with the wave trough aligned with
40
amidships, in comparison with the still water righting arm )(stilltGZ . Moreover, the righting arm for
the vessel in her own wave due to advance speed has also a slight reduction in comparison with
the still water righting arm. For example, Figure 32 shows the righting arm curves of a Portuguese
fishing vessel referring to the ship on the wave crest )(cresttGZ , on her own wave )(owntGZ , wave
trough )(troughtGZ , and in still water.
4.1. Variations of transverse stability in longitudinal waves
In this Thesis, the longitudinal sinusoidal waves are used to study their effect on the intact
stability of fishing vessels. In order to reach large variation in the righting arms, 2 representative
sinusoidal longitudinal waves are used for each vessel. The wave parameters considered were the
following: s=1/20 and (a) /Lpp=1, (b) /Lpp=1.6, with wave crest position along the wave or vessel’s
length. The wave parameters studied were based on the studies conducted by Umeda et al. (1999),
Neves et al. (1999, 2002), Kuroda et al. (2003), Bulian (2006), ITTC (2005), Hashimoto (2008).
There are interesting studies about fishing vessels considering this kind of waves. Experimentally
and numerically the changes of stability in longitudinal waves that induces parametric resonance
and pure loss of stability were studied by several authors:
4.1.1. Studies on parametric resonance
When a vessel is in a critical operational loading condition and unfavorable sailing condition,
which could produce an encountering frequency twice the vessel natural roll frequency, she starts
experiencing significant roll amplitudes because of a periodic change of transverse stability in
waves, characterized by a decrease of stability when the ship is at the wave crest and by an
increase in the wave trough. Studies performed by Hamamoto et al. (1996), Umeda et al. (1999),
Neves et al. (1999, 2002), Ribeiro e Silva (2000, 2004, 2005), Pérez-Rojas (2003, 2006, 2007,
2008), De Juana Gamo (2005), Neves and Rodrigues (2006, 2007), Maron (2006), just to name
some authors, explained and/or demonstrated such phenomena.
4.1.2. Studies on pure loss of stability
According to Kuroda (2003), when a ship sails in following seas, the encounter frequencies
becomes lower and heave and pitch motions can be regarded as being in static balance. Hence, a
vessel operating in following waves can be analyzed as a relatively stationary situation (Vassalos
1986), and ship stability treated as a static stability problem. The stability decreases significantly
41
when the midship stays on the wave crest and this condition is kept for long time in following seas.
If the ship is acted on by a lateral exiting force while there is small stability because it is on the
wave crest then total stability failure occurs by pure loss of stability (Kuroda 2003).
In head, following and quartering seas, Umeda et al. (1999) show similar results, with similar
comments about the pure loss of stability, vessel behavior in surf-riding and broaching-to.
These contributions and arguments motivated the present study, but this time considering real
scenarios in terms of operational loading conditions. Normally, this kind of study is performed with
one or two vessels, but here a representative set of 27 fishing vessels has been studied.
In order to calculate the changes of stability by means of GM, righting arm, and restoring
energy variations, it is convenient to use a commercial software or the equations presented in the
proposal of the new generation intact stability criteria (for the vulnerability purpose analysis),
submitted by Japan (IMO-SLF 51/4/3 2008) or even the formulation proposed by (Vidic-Perunovic
et al. 2009) (where a good background for the GZ variation is given). In this Thesis all the
calculations related to this and next Chapter were carried out using the commercial software
“AutoSHIP”. Figure 33 shows the fishing vessel model of “FV10”, see Table 7.
Figure 33. Fishing vessel model (“FV10”).
42
4.2 Calculation results of variation of transverse stability in longitudinal waves
The changes in Cb, GM and TM/cm at light loading condition for some fishing vessels, for 3
defined wave profiles have been showed in (Santos et al. 2008). For the same wave profiles
Mantari et al. (2009, 2009a) have shown the variation of righting arms at 30 degrees of heel, and
the variation of restoring energy from 0 to 30 degrees of heel for a well representative Portuguese
and Peruvian fleet.
In this Chapter a set of 2 sinusoidal longitudinal waves, which include a subset of 34 waves
profiles for each fishing vessel and each operational loading condition, with crest wave position
along Lpp or , were studied. Additionally, a number more significant of fishing vessels were
considered, and the following calculations were done:
a) Change of draft and displacement at still water, and at different loading conditions;
b) Calculation of the intact stability of fishing vessels in waves for vessels larger than 24m,
these calculations were done using the International Code on Intact Stability (2008 IS Code)
(IMO-MSC.267(85) 2008);
c) Calculation of the intact stability of fishing vessels in waves for vessels smaller than 24m,
these calculations were done using FAO/ILO/IMO2005 intact stability criteria
(FAO/ILO/IMO 2005);
d) Considering the items b and c, variations of GM, variations of righting lever at 30º, and
variations of restoring energy from 0º to 30º and 0º to 40º are presented. These
calculations were performed for all the fishing vessels studied in this Thesis and
considering different loading conditions, when full data were available.
From the 27 vessel studied, see Table 7, 16 fishing vessels present full data, see Table 8, and
they were studied at different loading conditions: (C1) 0% cargo, 100% consumables; (C2) 100%
cargo, 35 – 50% consumables; (C3) 100% cargo, 10 – 20% consumables.
The fishing vessels studied in this paper are shown in Figures 34, 35 and Table 7. They were
classified by hull forms and hull size for their analysis, as shown in Tables 7. The majority of the
fishing vessels, as mentioned above, were analyzed at three different operational loading
conditions. However, some of the vessels were studied as they were analyzed by the
corresponding authors in their respective papers. For example, FV2-FV5, FV9 presented in Table 7
were taken from Pérez-Rojas et al. (2003, 2006) and they were also studied by Santos et al. (2008).
FV 7 which came from Amagai et al. (2000). The fishing vessel FV 27 is a modern Chilean tuna
43
purse seiner. The fishing vessels FV 12 and FV 13 were extensively published by Neves et al.
(1999). The Portuguese fishing vessels are studied in here for the first time (FV1, FV6, FV8, FV10,
FV11, FV14 and FV15). The rest of fishing vessels are Peruvian pelagic purse seiners (FV18-
FV26).
The importance of the hull form parameters onto the stability failure due to parametric
resonance, such as the longitudinal distribution of local breadth and the flare at waterline, were
pointed out by several authors (Neves et al. (1999, 2002), Hashimoto et al. (2008), Taylan (2007),
Vidic-Perunovic (2009), etc.). Moreover, this Chapter shows that the roll restoring energy variation
give potential to the occurrence of stability failure. Figures 34, 35 and Table 7 show the hull form
characteristics of the fishing vessels studied. In these Figures were considered all the fishing
vessels.
Table 7. Fishing vessels hull characteristics. MU (Moderate U type), U (U type), EV (Extreme V
type), V (V type), MT (Medium transom), DP (Deep transom) and ITPS (Inclined transom purse
seiner).
44
Figure 34. Body view of 15 fishing vessels. “FV1”, “FV6”, “FV8”, “FV10”, “FV11”, “FV14” and “FV15”
are from the Portuguese fleet and the rest are from Spain and Japan.
45
Figure 35. Body view of 12 fishing vessels. The “FV17” and “FV27” are from Spain and Chile,
respectively. The rest of fishing vessels are from the Peruvian fleet.
46
In order to understand the impact of changing from one operational loading condition to
another onto variations of flare and breadth distribution along the vessel’s hull, the percentage
changes of draught and displacement changes are presented in Figure 36 (left and right side,
respectively). This figure shows that no significant change of draft and displacement can be noticed
in the Portuguese fleet, with the exception of fishing vessel “FV14”, but significant changes of
draught, almost up to 60%, are noticed in the Peruvian fleet, which may produce large changes of
flare and breadth distribution. This shows the importance of analyzing the dynamic behavior of the
vessels in all possible operational loading condition. Hence, not only GM should be checked when
experimental analysis is performed, but also the change of draft should be considered. The change
of displacement shows that, in fact, these representative fishing vessels which belong to the
Portuguese and Peruvian fleets are two completely different fleets. Therefore, these should be
analyzed with different criteria, as will be addressed in future work.
Figure 36. Changes of draught and displacement due to operational loading conditions.
After all calculations mentioned above were performed, the critical operational loading
conditions were found to be based on the following, ordered by priority: (a) Value of GM in still
water, (b) value of GM in waves, and (c) loss of energy in waves, as shown in Table 8.
Table 8. Vessel characteristics at critical operational loading condition of the Peruvian and
Portuguese fleets.
47
When a wave profile: (a) with wave steepness 1/20 and Lpp=1, and (b) with wave steepness
1/20 and Lpp=1.6 are used, the fishing vessels which do not comply with the 2008 IS Code (IMO-
MSC.267(85) 2008) and the FAO/ILO/IMO 2005 stability criteria (FAO/ILO/IMO 2005), considering
wave effects and critical operational loading conditions (Table 8) are shown in Table 9.
Table 9. Intact stability calculation according to the IMO 2008 IS Code and FAO/ILO/IMO 2005
stability criteria, considering the loss of stability in waves at critical operational loading condition.
48
Table 9 shows that the Portuguese fleet complies with the FAO/ILO/IMO 2005 adapted to a
longitudinal wave scenario at critical operational conditions, for both of the wave parameters
studied. On the other hand, more than half of the Peruvian fleet do not comply with the 2008 IS
Code adapted to a longitudinal wave scenario (“FV16”, “FV18”, “FV22”, “FV23”, “FV25”, “FV26”).
However, all the fishing vessels studied complies with the 2008 IS Code and FAO/ILO/IMO 2005
stability criteria in still water.
Figure 37. Roll restoring energy variation at three operational loading conditions (%), from 0 to 30º
(left side) and from 0 to 40º (right side), with respect to still water. Conditions (C1, C2 and C3).
Figure 37 shows the roll-restoring energy variation, from 0 to 30º (left side) and from 0º to 40º
(right side), at three operational loading conditions for the 16 fishing vessels considered in this
Chapter. Notice that some of these fishing vessels present only one operational loading condition,
because they were presented as they were studied previously. Figure 37 (lower graphics) shows
49
the roll-restoring energy variation of the fishing vessels “FV3” and “FV5” at operational loading
condition C3, which, in addition to the fishing vessel “FV4”, (Figure 37 (upper graphics)), capsized
due to stability problems (Pérez-Rojas et al. 2006). Figure 37 also shows that they have larger roll-
restoring variations compared with other small fishing vessels of similar size and hull form. These
large variations, in addition to some other particularities of stability changes, have influence on the
total or partial failure of a vessel. The causes of partial or total stability failure according to the IMO
criteria are several. This work aims to provide some preliminary guidance to ship designers on the
possibility of occurrence of parametric resonance and pure loss of stability in longitudinal waves.
4.2.1. Potential for the occurrence of parametric resonance
The fishing vessels “FV12” and “FV13”, both with the same GM=0.5 m in the still-water
condition as used in the experiments conducted by Neves et al. (1999) and similar dimensions (see
Table 1 and Figure 37), have different roll-restoring variations in waves. Figure 37 shows that the
fishing vessel “FV13” has a larger variation than “FV12”, which is more susceptible to resonance
phenomena as pointed out by the authors (Neves et al. 1999). The experience at sea
demonstrates that, in some cases, strong parametric resonance in head seas can take place in
quite few cycles, and angles of the order of 38º have been reached for fishing vessels (Neves et al.
2002). It was, therefore, decided to include roll-restoring variations up to 40º.
Studies performed by Ribeiro e Silva et al. (2004), demonstrated that the statically stable
fishing vessel “FV17” (see, Figure 37) encountering waves of 1.7 times her own length and a
frequency twice her natural roll frequency will experience a wave-induced parametric resonance
situation. Therefore, under these conditions, roll angles exceeding 20º to each side can rapidly be
produced, resulting sometimes in cargo loss, ship damage, and eventually in capsize.
Figure 37 (right side) shows that the hard chine hulls with inclined transom purse seiners,
except for the fishing vessels FV20 and FV21 (Peruvian) have greater maximum restoring energy
changes, compared with all the Portuguese purse seiners; this is because the length of these
vessels are different, which could be explained by scalability problems (Johnson and Womack,
2001; Womack, 2002). However, a comparison between FV15 and FV16, which are of similar
length, shows that the most classical Portuguese purse seiner FV15 has much less maximum
variation of roll-restoring energy.
In a previous paper (Mantari et al. 2009a), the average roll-restoring energy variation was
presented for a set of 15 fishing vessels encountering waves with wave steepness 1/10, and the
maximum restoring energy changes with respect to calm water was on average 80%, the average
positive restoring energy variation was 36.6% (on a wave trough) and the negative one was 43.4%
(on a wave crest). These average values were presented in a generic way, and it seems that the
roll-restoring energy variation on a wave crest, with respect to still water, is always larger in
absolute value than the one on a wave trough. However, in this study special attention will be given
to the variations of roll-restoring energies for each vessel, by looking in detail at the ratios between
50
the absolute value of At and Ac, see Figure 38, from a more-favorable and less-favorable wave
crest position along the vessel, respectively, both with respect to still water.
Figure 38. Fishing vessel model (FV10)
The most rapid increase of roll in a parametric resonance scenario could be observed when the
vessel experiences an internal roll disturbance combined with a condition of increasing energy in a
particular sailing condition. This combination of restoring with a larger-than-calm-water and
resisting the roll with less-than-calm-water can cause the roll angle to progressively increase to a
large and possibly dangerous level. However, all the fishing vessels have this combination of roll-
restoring energy. But some of them has the gain of roll-restoring energy larger than the loss of roll-
restoring energy, i.e. At is larger than |Ac|, see Figure 38, in such cases the restoring moment
tends to accelerate the vessel back to equilibrium with a excitation which is even larger than other
situation (At < |Ac|), and potential for resonance phenomena can be shed, see Table 10.
Figure 38 show the model of the FV10 (left side) and the percentage of gain and loss of roll-
restoring energy up to 30º (right side).
In Table 10, it can be seem that some specific fishing vessels have particular restoring energy
variations which, combined with low GMsw and large GM variations in waves (see Table 2) could
lead to the occurrence of partial or total stability failure. The ratios between the absolute value of
the percentage gain and loss of roll-restoring energy up to 30º and 40º, which are less than 1, give
us potential for the occurrence of parametric resonance; see fishing vessels FV10, FV14, FV15,
FV25 and FV26. Attention is called to these particular fishing vessels marked in bold in Table 10.
Similar analyses were performed to the vessels mentioned above (FV12, FV13, FV17), and not
only for the ones which are included in Table 8. As expected, they showed potential for the
occurrence of parametric resonance, as confirmed by other authors which have studied these
same fishing vessels Neves et al. (1999, 2002), Ribeiro e Silva et al. (2004) and De Juana Gamo
et al. (2005).
51
Table 10. Change of roll-restoring energy variation in waves (respect to still water) at critical operational loading condition. Wave parameters: Upper (H/=0.05,/Lpp=1.6), lower (H/=0.05,/Lpp=1).
As shown in Table 10, the ITPS (Inclined Transom Purse Seiner, see Table 7), has a direct
relationship with decreasing restoring energy in the trough of waves for fishing vessels of less than
approximately 40 m in length, despite this kind of waves being the most favorable. This may be
beneficial for the avoidance of parametric resonance, for the reasons mentioned above. Moreover,
according to Ribeiro e Silva (2005), parametric resonance can occur only when low damping
(reduced speed) and large transverse stability changes driven by wave characteristics, coupled
heave and pitch responses, and hullform parameters such as hull flare, end section shapes, and
main deck position, are combined. Hence, hard chine hulls may also have larger roll damping
52
forces due to viscous effects at the hard chine, and turn out to be less sensitive to parametric
resonance than round bilge hullforms. This is a topic that should be considered for further research.
4.2.2. Potential for the occurrence of Pure loss of stability
Reviewing again the last two columns of Table 10, larger values of the ratio between the
absolute value of gain and loss of roll-restoring energy with respect to still water, as shown “FV23”,
give clear indications of how vulnerable they can be to total failure when critical operational loading
and unfavorable sailing condition can allow the occurrence of pure loss of stability on a wave crest.
“FV1” has particular roll-restoring energy variation, because when a wave crest is varied along
the vessel length, the roll-resting energy is less than the roll-restoring energy produced in still water,
see Figure 39. Therefore, in some cases, she is not applicable for our analysis. Due to the
limitations of this preliminary study, it is not possible to determine the minimum value. Moreover, it
is important to mention that “FV23” has much larger maximum roll-restoring variation compared
with “FV1”. Additionally ‘FV23” does not comply with the current stability criteria adapted to a
longitudinal wave scenario, but “FV1” does. In order to reach more conclusions, it will be
interesting to evaluate how actually “FV1” behaves in waves by means of experimental analysis.
However, in a preliminary way, it is possible to see which kinds of fishing vessels with particular
size, hullform, and other vessel particulars are more susceptible to the occurrence of this
phenomenon. Engineers should have this kind of information on hand for the design of new fishing
vessels. Also important was the fact that “FV23” was lost in a condition as presented in the Table 8.
Until now there is not consistent study about the stability failure mode, however, Table 9 and 10
can give a rough idea about the “poor” behavior of “FV23” in longitudinal waves.
Figure 39 shows GMw and GZwmax (left side) and the roll-restoring energy (RRE) (right side) as
a function of the wave crest position along the vessel’s length, including the minimum requirement
of the 2008 IS Code for the most-critical operational loading condition and s=1/20 and Lpp1;
1.6for the fishing vessels “FV1” (upper graphics), “FV10” (middle graphics) and “FV22” (lower
graphics). This figure also shows that the ITPS (Inclined Transom Purse Seiner) fishing vessels
having length of less than approximately 40 m have GM fluctuations along the vessel’s length
which can not be modeled sinusoidally, see Figure 39 (lower graphics). Hence, appropriate
advanced numerical calculations must be used in order to evaluate parametric resonance.
Fortunately, they do not show potential for the occurrence of such phenomena, see Table 10.
As part of the recommendation for the IMO new generation criteria for parametric resonance,
made by Japan, a simplified prediction formula for the metacentric height on wave crest is
desirable (Umeda 2008). Figure 39 shows metacentric heights at different wave profiles at critical
operational loading conditions. As shown, it is very difficult to have a priori a prediction formula of
GMw, even for these so-called “conventional fishing vessels”.
53
Figure 39. GM(t) and GZmax(t) (left side) and the roll-restoring energy (RRE) (right side) as
function of the wave crest position along to wave, including the minimum requirement of (2008 IS
Code).
Being difficult to define the so-called conventional fishing vessels associated with the
knowledge that fishing vessels have the highest rate of accidents occurring worldwide, it is clear
that the intact stability code of fishing vessels is a topic not yet solved.
54
4.3. Susceptibility to parametric resonance
Partial or total stability failure of fishing vessels is one of the biggest risks, mainly due to the
high number of fatalities. Therefore, even when statistics gives low percentages of occurrences
related to capsizing for dynamic stability, safety of fishing vessels, particularly small ones, remains
a subject of great importance, which has been confirmed by the intense regulatory activities carried
out by IMO.
The most dangerous motion to vessel stability failure is large rolling. Wave induced stability
failure of intact vessels may be divided into the following dynamic modes, which are associated
with different physical phenomena (Bulian 2006, Chang 2008):
For beam seas:
Impact excitation due to a steep, possibly breaking, beam wave.
Resonant excitation in beam seas.
Loss of transverse stability in beam seas.
For longitudinal seas:
Pure loss of transverse stability in following seas.
Parametric rolling in head or following seas.
Broaching in following seas.
The studies in this Chapter represents an initial effort in that project by studying a set of 16
fishing vessels mainly from the Portuguese and Peruvian fleets, with different configurations and
modes of operation in order to have a better understanding of the parametric resonance in head
seas, in order to prevent vessel failure.
Previous calculations indicate how sensitive they can be to the changes from one wave profile
to another in longitudinal waves, which can be related with the susceptibility of the vessel to partial
or total stability failure (Mantari et al. 2009, 2009a, 2011a ).
Large variation of roll restoring energy may have influence on the magnitude of the potential for
the total or partial failure of a vessel. Additionally, the analysis of the ratio between the absolute
value of gain and lost of roll restoring energy, from a more favorable and less favorable wave crest
position along the vessel, has strong relation with the stability failure of the fishing vessel.
The ITPS (Inclined Transom Purse Seiner) fishing vessels less than approximately 40m in
length, which have chinned hull, seem to be beneficial to the avoidance of parametric resonance.
The ITPS (Inclined Transom Purse Seiner) fishing vessels less than approximately 40m in
length have GM fluctuations along the vessel’s or wave length that can not be modeled sinusoidally.
55
However, this paper assumes that the vessel’s GM(t) fluctuations have sinusoidal representation.
Fortunately, these fishing vessels do not show potential for the occurrence of the phenomena of
parametric resonance.
In order to complement previous calculations (Chapter 5), the objective in this Chapter is to
evaluate the parametric resonance of 16 fishing vessels using the method developed by the
American Bureau of Shipping (ABS) (Shin et al. 2004). For this purpose the variations of GM that
produce changes of stability is determined using commercial software.
The 16 fishing vessels are studied for the most critical range of ratios of vessel length to wave
length in the interval of [0.84, 1.6] (Umeda, 1999; Bulian, 2006; Neves et al., 1999, 2002; Kuroda
2003; ITTC 2005; Hashimoto, 2008) and a standard wave steepness of 1/20, and considering 3
different operational loading conditions: Port departure (0% Cargo, 100% Consumables (C1)),
Fishing ground departure (100% Cargo, 35-50% Consumables (C2)), Port arrival (100% Cargo, 10-
20% Consumables (C3)). All these calculations are done for the Portuguese and Peruvian fleets,
which are well represented in this study.
4.3.1. Vessels stability in following or head waves
The stability of fishing vessels changes with waves, and so does their analysis, for example,
the way to analyze beam waves is different than longitudinal waves. This section just deals with the
stability in longitudinal waves, where the phenomenon of parametrically excited roll takes place.
This phenomenon is caused by the periodic change of transverse stability in waves, characterized
by a decrease of stability when the ship is at the wave crest, and by an increase in the wave trough
(Paulling et al. 1959, Hua et al. 1992, Hamamoto et al. 1996, Neves et al. (1999, 2002), Ribeiro e
Silva et al. (2000, 2004, 2005), Shin et al. 2004, De Juana Gamo et al. (2005), Bulian et al. 2006,
Neves and Rodriguez (2006, 2007), ABS 2008), which may produce the total stability failure of
small vessels (Chang et al. 2008). This variation of stability is caused mainly by changes on the
hull forms that experience large volumetric changes in the submerged portion during a wave
passage, predominantly seen for ships with large bow and stern, as reported by several authors, as
Neves (1999, 2002) and Hashimoto (2008).
In order to calculate the changes of stability by means of GM, it is convenient to use
commercial software or the equations presented in the proposal of new generation intact stability
criteria (for the vulnerability purpose analysis), developed by Japan (IMO-SLF 51/4/3 2008) or the
formulation proposed by (Vidic-Perunovic et al. 2009), where a good background of the GZ
variation is given, as mentioned before. In this Thesis all calculations of hydrostatics were carried
out using commercial software (Autoship), assuming instantaneous static equilibrium of sinkage
and trim, which was proved to be more accurate than simplified numerical calculation (Shin et al.
2004, Bulian 2006 and ABS 2008).
56
4.3.2. Physical and mathematical understanding of parametric resonance
When a vessel is in a critical operational loading condition and unfavorable sailing condition,
which could produce an encountering frequency twice the vessel natural roll frequency, she starts
experiencing significant roll amplitudes because of a periodic change of transverse stability in
waves (see Figure 40), characterized by a decrease of stability when the ship is at the wave crest
and by an increase in the wave trough. Experimentally and numerically, the phenomenon of
parametrically excited roll, have been studied by (Paulling et al. 1959, Hua et al. 1992, Hamamoto
et al. 1996, Neves et al. (1999, 2002), Ribeiro e Silva et al. (2000, 2004, 2005), Shin et al. 2004,
De Juana Gamo et al. 2005, Pérez-Rojas et al. 2006, Bulian et al. 2006, Neves and Rodriguez
(2006, 2007), ABS 2008, Vidic-Perunovic et al. 2009), and others.
Figure 40. Physics of parametric resonance, development of parametric roll (Shin et al. 2004).
Theoretically, this phenomenon is described as parametric resonance, which can be
explained by the Mathieu equation (equation 22). Consider a single 1 DOF equation for roll motion
in head seas, taking into account the changing GM due to wave encounter.
0)(.
244
2
2
AI
tGMW
dt
d
dt
d
x
(11)
where, is the linear damping coefficient, W is the weight displacement of the vessel, Ix is the
transversal moment of inertia, and A44 is the added mass coefficient in roll.
The value of GM (due to different operational loading condition) and its variation with time (due
to waves) may result in parametric resonance. To check if this phenomenon can occur, the roll
equation (11) must be transformed to the form of a Mathieu equation (22) in order to use the Ince-
Strutt diagram (Figure 41) to examine the properties of the solution. To achieve this aim, a set of
57
assumptions and appropriate procedures have been made to obtain the analytical solution
(equation 23-24) of the equation (22).
Figure 41. Mathematical understanding of parametric resonance. Ince-Strutt diagram (Chang et al.
2008).
The fluctuation of GM(t) as a function of wave position along the fishing vessel is assumed
sinusoidal, see Figure 42.
GM(t)=GMm+GMacos(wt) (12)
where, GMm (equation 13) is the mean value of the GM. GMa (equation 14) is the amplitude of the
GM change in waves.
GMm=0.5(GMmax + GMmin) (13)
GMa=0.5(GMmax - GMmin) (14)
where, GMmax and GMmin are respectively the maximum and minimum value of metacentric height
for a number of wave crest positions along the vessel’s length, determined using commercial
software (Autoship).
58
GM as a function of wave crest position varied along
FV4 length
0.400.450.50
0.550.600.650.70
0.750.80
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 1
1.1
1.2
1.3
1.4
1.5
1.6
GMswGM(t)SinusoidalGMm
Figure 42. GM as a function of wave crest position along fishing vessel (FV10, see Table 1).
Substituting equation (12) into the roll equation (11) yields the equation (15)
0))cos((2 222
2
wtdt
d
dt
dam
(15)
where:
mw =44
.
AI
GMW
x
m
, aw =
44
.
AI
GMW
x
a
In order to transform the equation (15) into the standard form of the Mathieu equation, it is
important to introduce dimensionless time, see equation (16).
t (16)
substituting equation (16) into equation (15) and dividing both parts by the square of wave
frequency, it results in the dimensionless form equation (17)
0))cos((222
2
2
amd
d
d
d (17)
59
Here the coefficients of the equation (17) are the dimensionless quantities, see equation (18-20)
(18)
mm (19)
aa (20)
The next substitution gets rid of damping:
)exp()()( x (21)
This finally expresses roll in the form of a Mathieu equation (22)
0)cos(2
2
xqpd
xd
, (22)
Solution of Mathieu equation may be found in many references and depends strictly on p and q.
Thus, the solution may be periodic, increasing or decreasing in nature. In equation (22), p is the
square ratio of natural frequency and excitation frequency, and q is the parameter that dictates the
amplitude of parametric excitation, see equations (23) and (24), respectively.
)( 22 mp (23)
2aq (24)
In a clear explanation on the bounded and unbounded periodic solution to equation (22), Shin
et al. (2004) reached the following conclusion: “There is a threshold value for roll damping for each
pair of Mathieu parameters p and q. If roll damping is less than the threshold value, it becomes
60
unbounded as the solution to the Mathieu equation. If the roll damping is larger than the threshold,
roll is still bounded, even if the Mathieu equation is unbounded. The increment of the Mathieu
solution is not enough to overcome decrement of roll damping. In addition, it also means that with
linear damping, the instability zone is narrower and requires some finite value of GM variations
even at p=1/4; i.e., it does not touch the axis”.
To calculate this threshold, it is necessary to find a way to express the increment of the
unbounded solution of the Mathieu equation. The problem is that solution to the Mathieu equation
cannot be expressed in terms of elementary functions (Shin et al. 2004). Based on the well-know
expansions for periodical solutions of the Mathieu equation (corresponding to the boundary
between “stable” and “unstable” zones), and following (Hayashi 1953), the threshold value can be
presented as the equation 25.
231 15.0 kqkT (25)
where, k1 and k3 are coefficients, calculated with the following formulae:
21 1875.01 qk (26)
q
pqqqk
16
102435216 242
3
(27)
4.3.3. Roll motion and susceptibility criteria
Susceptibility criterion is utilized to check whether the situation indicates any vulnerability. If
any susceptibility is detected the severity of the parametrically excited roll is calculated with
numerical procedure. The severity of this susceptibility analysis is a warning to designer and
operators, for more accurate and conclusive findings and data, model tests and supporting
computing simulations need to be considered (Taylan 2007).
The susceptibility criteria for parametrically excited roll must include two conditions (Shin et al.
2004): (a) frequency condition, formulated in terms of the Mathieu parameters p and q, and (b)
damping threshold condition
4.3.3.1. Frequency condition of susceptibility criteria
61
The vessels are said susceptible to parametrically excited roll if “p” is between the left (LB) and
the right (RB) boundary of the following inequality:
LB<p<RB (28)
LB=38432824
1 432 qqqq (29)
RB=24
1 q (30)
To calculate the natural period or natural frequency the well known relationship are used, as
defined equation (31)
B
GM
Tnn 8.0
22 (31)
where, GM is the metacentric height in still water, and B the beam of the vessel.
4.3.3.2. Damping threshold condition of susceptibility criteria
Based on the calculation of containerships, Shin et al. (2004) concluded that the threshold
damping is underestimated by formula (25). it is said that the error is probably caused by the
assumption that q is small, so formula (25) needs calibration for values of the Mathieu parameter q
that are typical for containerships. They, initially, set the damping equal to the threshold according
to equation (25) and then it is increased until the solution becomes bounded.
Similar procedure could be done for fishing vessels, in order to make the solution become
bounded. This Thesis in this Chapter assumes, two damping thresholds, as proposed by Hayashi
(1953) (equation (25)) and (Shin et al. 2004, ABS 2008) (equation (32)), in order to evaluate the
parametric rolling of fishing vessels at operational loading conditions and check if the damping
threshold condition requires a calibration procedure.
62
2321 15.0 kkqkT (32)
where,e
nT
; k1, k2 and k3 are coefficients. To calculate k1 and k3 see formula (26) and (27),
respectively, while k2 is calculated by
759.016.0002.12 qpk (33)
4.3.3.3. Forward speed for susceptibility criteria
Appropriately defined wave steepness (which is related to wave frequency depending on water
depth), ratio of wave length and length between perpendiculars, and encounter frequency are the
other factors in development of parametric roll. The encounter frequency is calculated by equation
(34).
Vg
wwe
2 (34)
Using: (a) the wave dispersion relationship in deep water, (b) the average value of the
metacentric height (GMm) instead of the metacentric height in still water (GM) to deal with natural
frequency of roll motion in waves, and (c) encounter frequency twice the natural roll frequency in
waves, the vessel’s forward speed that induces parametric rolling is found, see equation 35.
2
)2(
w
wmpr
gV
(35)
4.3.4. Calculation results
4.3.4.1. Critical loading condition evaluation
The critical loading conditions were found based on the following criteria, ordered by priority: (a)
Minimum value of GM in still water, (b) maximum variation of GM in waves, (c) maximum lost of
energy in waves. The operational loading conditions considered are: Port departure, Fishing
ground departure, and Port arrival.
63
In previous studies, the changes in Cb, GM and TM/cm at light loading condition for some
fishing vessels were studied by Santos et al. (2008) . Variations of the righting arms at 30 degrees
of heel, and the variation of restoring energy from 0-30 and 0-40 degrees of heel, and calculations
of intact stability in waves, were done by Mantari et al. (2009, 2009a, 2011a).
In this section, the maximum variations of GM in waves at different loading condition were
calculated, see Figure 43. In this Figure can be seen that GM variations change with the
operational loading condition and is different from vessel to vessel. Only for the “FV1” and “FV6”
these have almost the same variation, which is opposite to other conclusions (Bulian et al. 2006).
Hence, it is important to study a set of fishing vessels and not only one.
Figure 43. Maximum variation of GM at different loading conditions (m).
In Table 8 some hull characteristics at critical loading condition, based on the procedure
mentioned above, were found. “FV25” and “FV26” have Port departure as a critical operational
loading condition. These vessels which are belong to the Peruvian fishing fleet have RSW system
for cooling and have the lowest GM’s among all the vessels studied, see Table 8, which finally
plays a important role in the evaluation of parametric rolling of these particular fishing vessels, see
Figure 46.
4.3.4.2. Roll motion and susceptibility criteria
Figure 44 shows the fishing vessel “FV26” modeled in Autoship, and Figure 45 shows one of
the runs files of this vessel. Macros were implemented in the above-mentioned commercial
software, and after a total of 144 runs, critical waves and vessel speed were found for each fishing
64
vessels, this vessel parameters were not included in this Thesis for lack of space, they are: (a) in
the range of /Lpp [0.84, 1.6] for the wave steepness selected, as expected, and (b) the speed of
parametric rolling are in the acceptable interval as well, see Figures 46 and 47. The authors
pretend to use, both the critical operational loading condition and the critical sea condition
discovered in this section in future experimental analysis.
Figure 44. Fishing vessel model.
Figure 45. Run file of the fishing vessel model “FV26”.
65
Figure 46 shows the Ince-Strutt diagram of the fishing vessels studied in this paper for /Lpp
1 and wave steepness of 1/20. The left and right boundary, which corresponds to each vessel,
shown in Figure 46 is presented in the second and third column of Table 11. In fact, Table 11
presents the susceptibility analysis of parametric rolling of the 16 fishing vessels studied in this
paper. This Table presents results for /Lpp 1 and wave steepness of 1/20. The threshold values
were calculated by using Hayashi (1953) and Shin et al. (2004) method.
Ince-Strutt diagram (=0.1)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45p
q
q1 q2 FV1 FV6 FV8FV10 FV14 FV15 FV16 FV18FV19 FV20 FV21 FV22 FV23FV24 FV25 FV26 q1h q2h
Figure 46. Linear and high order approximation for the boundary of the first instability zone (Ince-
Strutt diagram). /Lpp 1, wave steepness of 1/20.
From Table 11 it can be inferred that FV8, FV10, FV14, and FV15 (Portuguese), do not present
vulnerability to parametric resonance in head seas for the defined wave parameters at its
determined forward speed for susceptibility criteria (Vpr). Moreover, they do the pass the check of
2008 IS Code adapted to longitudinal wave scenarios.
Opposite occur for the rest of fishing vessels, because they have different wave scenarios. In
addition, FV25 and FV26: (a) do not the pass the check of 2008 IS Code adapted to longitudinal
wave scenarios, and (b) present potential for stability failure (Mantari et al. 2011a).
Figure 47 shows the Ince-Strutt diagram for /Lpp [0.84, 1.6] and wave steepness of 1/20.
Table 12 presents the susceptibility analysis of parametric rolling of the 16 fishing vessels studied
in this paper. This Table presents results for /Lpp [0.84, 1.6] and wave steepness of 1/20.
Similarly to Table 12, the threshold values were calculated by using Hayashi (1953) and ABS (Shin
et al. 2004) method. From Table 12 it can be inferred that FV1 (Portuguese), FV16 and FV20
(Peruvian), do not present vulnerability to parametric resonance in head seas for the defined wave
parameters and its determined forward speed for susceptibility criteria (Vpr). Moreover, FV1 and
FV20 do pass the check of 2008 IS Code adapted to longitudinal wave scenarios (Mantari et al.
66
2011a). Opposite occur to the rest of fishing vessels at COLC and sailing condition (vessel speed
and wave parameters), see Table 13, because they do not pass the frequency and/or damping
threshold condition for parametric resonance, i.e. they have vulnerability to parametric resonance
in head seas. Therefore, their severity should be also determined.
Table 11. Susceptibility analysis of parametric rolling.
Ince-Strutt diagram (=0.03)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45p
q
q1 q2 FV1 FV6 FV8FV10 FV14 FV15 FV16 FV18FV19 FV20 FV21 FV22 FV23FV24 FV25 FV26 q1h q2h
Figure 47. Linear and high order approximation for the boundary of the first instability zone (Ince-
Strutt diagram). /Lpp [0.84, 1.6], wave steepness of 1/20
67
The FV18, FV22 and FV23 do not pass: (a) the frequency and damping threshold condition; (b)
the check of 2008 IS Code adapted to longitudinal wave scenarios (Mantari et al. 2011a), for the
same abovementioned wave parameters, and for these reasons her severity should be also
determined.
Table 12. Susceptibility analysis of parametric rolling (Shin et al. 2004; ABS, 2008; Taylan, 2007).
However, the results that came up from this study, related to these vessles, should be treated
carefully, because: (a) the present method used to calculate parametric resonance of this particular
vessel is not suitable (the GM variation can no be modeled as a sinusoidal function); (b) The ITPS
fishing vessels less than approximately 40m in length, which have chinned hull, seem to be
beneficial to the avoidance of parametric resonance (Mantari et al. 2011a). Then further studies are
needed.
FV25 and FV26 do not pass: (a) the frequency and damping threshold condition; (b) the check
of 2008 IS Code adapted to longitudinal wave scenarios; and (c) present potential for stability
failure. Therefore, for more reason, their severity should be determined as well.
In Tables 11 and 12 can be shown that the calibration procedure may not be necessary to
assess the damping threshold for fishing vessels, because the calculations results of the
susceptibility analysis by using the ABS (2004) damping threshold are similar to the results
obtained by using Hayashi (1953) damping threshold. However, it could be interesting to prove it in
future research work.
In fact, in Table 11 and mainly in Table 12, it can be noticed that the vulnerability to parametric
rolling were found for all the vessels studied in this paper, i.e. the unfavorable sailing condition at
critical operational condition were found. The ship and waves parameters involved in such
unfavorable sailing condition are presented in Table 13.
68
Table 13. Critical operational loading condition (O.l.c) and sailing condition (vessel speed and wave)
for parametric rolling in head seas.
Critical sailing scenario for parametric resonance in head seas Fishing
Vessel
COLC w(rad/s) (m) /Lpp Vpr (Kn)
FV1 C3 2.27 11.94 1.00 5.95
FV6 C3 2.03 15.00 1.00 5.42
FV8 C2 2.12 13.67 0.91 8.67
FV10 C3 1.39 32.00 1.60 0.45
FV14 C3 1.83 18.38 0.84 10.47
FV15 C3 1.66 22.50 0.92 11.05
FV16 C2 1.48 28.27 1.00 3.75
FV18 C3 1.44 29.90 1.00 4.52
FV19 C3 1.19 43.57 1.34 12.31
FV20 C2 1.33 35.08 1.00 7.62
FV21 C2 1.33 35.11 1.00 7.96
FV22 C2 1.28 37.90 1.00 6.27
FV23 C2 1.10 51.40 1.35 13.15
FV24 C2 1.19 43.42 1.00 4.35
FV25 C1 0.94 69.45 1.60 8.85
FV26 C1 0.91 74.53 1.60 10.18
69
________________________________________________________________________
CHAPTER V
STABILITY FAILURE DUE TO BEAM WAVES, WIND AND
FISHING GEAR FORCES
______________________________________________________________
The study in this Chapter represents an initial effort in that “SADEP” project by studying a set of 7
fishing vessels mainly from the Portuguese and Peruvian fleets, with different configurations and
modes of operation in order to have a better understanding of the action of fishing gear forces,
wind and combinations of them in a fishing trip scenario.
The heeling moment due to fishing gear loads, gusty wind and combination of them is
calculated and compared with each other, for these purpose two different criteria’s are used: (a) the
submission of the Soviet Union delegation to the Subcommittee on Safety of Fishing Vessel in
1979 (IMO 1979), (b) the paper published by Machii et al. (1989) where experimental results are
also given. Also, in this Chapter, a clear distinguishing about sea characteristics (Atlantic and
Pacific Ocean), hull parameters, vessel machinery onboard, and fishing gears of these two
fisheries, is made.
5.1. Types of industrial fishing vessels more common in Portugal and Peru.
Two different types of industrial fishing are used in majority in Portugal and Peru, these are the
purse seiner and the trawler fishing vessels, the first being the most widely used type. The
following will describe both types of vessels.
5.1.1. Fishing process of purse seiners
The purse seine is used mainly for catching dense, mobile schools of pelagic fish and includes
all the elements of searching, hunting down and capture. The schools of fish are surrounded and
impounded by means of large pursed surround nets called either ring nets or purse seines
according to design. A purse seine is a wall of netting with a mesh size to suit the target species
and a head rope carrying numerous floats to keep the net on the surface, see Figure 48.
70
Figure 48. Wall of netting of purse seiners
The net is equipped with rings (purse rings) along its lower edge through which a special cable
(purse wire) is passed to enable the fisherman to close off the space surrounded by the purse
seine from below, preventing the fish from escaping downwards and forming a bowl-like shape of
net in the water containing the fish.
Setting. The net is set from the after deck, purse rings being stowed on the bar forward of
the stowed net and sliding off as the net is shot, see Figure 49.
Pursing. The dhan has been retrieved, the purse wire is being hauled using the winch on
the foredeck, see Figure 49.
Hauling. Pursing complete, the net is led over the powerblock, onto the transport roller and
into the net bin, stowed ready for the next shot. The purse rings are passed aft from the
pursing davit via a wire to be stowed alongside the net on a bar, see Figure 49
Pumping. The fish pump is lifted into the net by a crane and the fish are pumped into a
water separator before being channeled into the RSW tanks below deck, see Figure 49.
71
Figure 49. Process of fishing of purse seiners
The origins of the purse can be traced back to one of the most basic types of fishing gear, the
beach seine, which has been used through the ages almost all over the world. A deep beach seine
operated offshore could be regarded as an early ring net, made deeper still and fitted with primitive
purse rings and purse line it could be regarded as an early purse seine.
Purse seines are operated throughout the world by vessels of almost any size, from large
canoes (6m) in Israel and Africa right up to ocean going tuna seines (100m) with the size of net
adapted to suit the vessel size, degree of mechanization and target species. Both nets and vessels
have evolved to suit local conditions.
It is important to differentiate between the American and European purse seiners, because
they have different fishing gears, see Figure 50 and 51, respectively. A classical American purse
seiner has mainly two devices for fishing (the winch and powerblock).
72
Figure 50. American purse seiner.
Figure 51. European purse seiner
5.1.2. Fishing process of trawlers
Bottom trawling, for example, (known in the scientific community as Benthic trawling) is a
fishing method which involves towing trawl nets along the sea floor, as opposed to pelagic trawling,
where a net is towed higher in the water column. Bottom trawling can be carried out from one
vessel or two vessels fishing cooperatively. It is practiced from a very wide range of fishing vessels,
starting with small motor vessels powered by engines of several tens of horsepower and up to
large ocean-going trawlers, up to 100 m of length, and powered by engines of several thousand
horsepower.
73
The idea that fish are passively "scooped up" is commonly held, and has been since trawling
was first developed, but has been revealed to be erroneous. Since the development of scuba
diving equipment and cheap video cameras it has been possible to directly observe the processes
that occur when a trawl is towed along the seabed. The trawl doors disturb the sea bed, create a
cloud of muddy water which hides the oncoming trawl net and generates a noise which attracts fish.
The fish begin to swim in front of the net mouth, but do not seem to be distressed by it. As the trawl
continues along the seabed, fish begin to tire and slip backwards into the net. Finally, the fish
become exhausted and drop back, into the "cod end" and are caught.
The speed that the trawl is towed at depends on the swimming speed of the species which is
being targeted, but for most demersal species, a speed of around 4 knots (7 km/h) is appropriate.
Figure 52. Beam trawler, modern, large; Holland (FAO)
Figure 53. Very large, Factory trawler; Holland (FAO)
74
5.2 Transverse stability in beam wind and rolling
The basic principle of the IMO rough weather criterion is the balance between restoring and
inclining energy in beam waves and wind, assuming certain roll amplitude that takes into account
the excitation moment due to waves. The criteria are used to determine fishing vessel performance
in beam seas and strong winds. These criteria assume that a fishing vessel has taken a large roll to
windward from a passing beam wave. After the wave crest passes the vessel quickly rolls to the
upright position due to both the wind pressure on the lateral plane and the backside of the passing
wave.
In Figure 54, the Area “A” represents the amount of energy associated with the inclining
moment that acts to snap roll the fishing vessel back upright, after the beam wave passes. Area “B”
is the restoring energy available to counter the fishing vessel rollback. 1 is the angle of roll to
windward due to wave action, and 2 is the angle of downflooding or the angle of second intersect
between steady wind lever l1 and righting arms curve c or 50 deg, whichever is less, as described
in the Chapter 3.
-10 0 10 20 30 40 50 60 70 80 90-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6Balance of energy of the intact stability at fishing operation
Heel Angle (deg)
Arm
(m
)
Righting arm in still water condition
Heeling arm due to wind and fishing gear loads
O
A
B
Figure 54. Balance of energy for the weather criteria including fishing gear pull.
The intent of the weather criterion was that the vessel’s stability should be adequate to survive
full ocean storms, even if the vessel is limited to work near coastal areas. This may not appear to
be a problem at first glance; the vessels would just have excess stability, but the design is not
75
optimal, so the consequences behind this excess (Womack 2002) (fishing vessels design with
excessive stability make the crew a wrong or false interpretation of safety of their ship). In other
cases, the criteria are not sufficient to reflect individual stability weaknesses in a vessel (like the
problems of scalability for small boats compared with those more than 100m or the overloading of
fishing gear).
Taking into account the importance of optimal designs of fishing vessels, this Chapter
considers such suggestions and uses the appropriate wind speed to evaluate the intact stability in
critical conditions. Figure 55 shows the highest 1/1000 value of significant wave height and wind
speed (data computed every 6 hours, for a period of 10 years, January 1997 to December 2006).
The hindcasting data was computed by means of third generation wave hindcasting model of
Global Climate by Japan Weather Association (JWA3G) (Ogawa 2009). Both graphics are
important in this study because they show the characteristics of two different fishing areas.
Portugal exclusive fishing area has highest value of wave and wind speed, approximately: 12m and
26m/s, respectively, compared with Peru (6m and 15m/s), as shown in Figure 55.
Figure 55. Contour of highest 1/1000 value of wave height (left side) and wind speedy (right side)
(annual) (Ogawa 2009).
The Japanese criterion is applied to ships engaged in restricted services with the values of
wind speed, wind pressure “P” and calculation formulae of wave steepness “s” for Coasting-I,
Coasting-II and ocean ongoing of ships as shown in (IMO-SLF 51/4/1 2008). The Peruvian fleet
operate mainly inside his Exclusive Economic Zone (5 to 200 miles from shore), but the
Portuguese fleet is still international (Santos et al. 2008), which means that the values of wave
height and wind speed found in the paragraph before are again supported by Japanese criterion.
76
5.3 Transverse stability due to fishing gear loads
Pelagic trawling and purse seining require considerable skills to precisely control a fishing
vessel and her gear. It is important to note that it is very rare, except in special cases, that the heel
caused by wind only would endanger a fishing vessel (Gefaell 2005). Usually what happens is that
stability is lost when there is an unusual combination of wind, waves and fishing loads. An
interesting research even highlighted that there are more fishing vessel accidents during the fishing
and recovery of gear operation (Wang 2005) than in heavy weather. These two arguments; the
tendency to increase the power of machineries onboard as showed, for example, in Townsend
(2005) and Oliveira (2006); and those criteria developed by IMO Subcommittee on safety of fishing
vessels (IMO 1979) motivated this work.
In 1965 the IMO subcommittees on safety of fishing vessels discussed about the inclusion of a
normalized heeling moments due to fishing gear pull. After some brief investigation the
subcommittee felt that there was no need for any special criteria for fishing vessels related to the
action of fishing gear pull.
The current purse seiners, for example, have fishing gears that put in danger a fishing vessel
quite easily, mainly for those which still use the power block in a high position. As reported before
(IMO 1979) the increase of power of the machinery, in general, increases the heeling moment until
a point that weather criterion is not the only way to guaranty the stability of the fishing vessel in
operation. However, nowadays in some countries, for example Peru, there is still a tendency to
increase the power of the machinery like the main engine, winch and power block; see also
Townsend (2005) and Oliveira (2006).
Two interesting research works were a basis for the main contribution in this Chapter, the first
published by the Soviet Union delegation and submitted to the Subcommittee on Safety of Fishing
Vessel in 1979 (IMO 1979) and second by Machii et al. (1989), but here they are applied to a
traditional pelagic pure seiner (Figures 56).
As mentioned above, there is a difference between the American and European purse seiners.
A classical American purse seiner has mainly two devices for fishing. The winch which has the
function of hauling the purse wire (process called Pursing), produces a load in the purse gallows
that is represented as P2 in Figure 56. The power block has the function of fish leaning toward the
side of the vessels, collect the buoys and the rest of the upper net (process called Hauling); the
power block produces a load that is represented as P1 in Figure 56.
77
Figure 56. Gear loads on a traditional pelagic purse seiner.
Two different simplified scenarios for the calculation of the heeling moment due to fishing loads
are shown in Figure 56. The equations 36-40 describe the procedure to calculate the heeling
moment due to fishing gear pull using the research work done by Machii et al. (1989) and the
equations from 41 to 48 describe the same procedure but using, as reference (IMO 1979).
Considering the first case, Figure 56 (right side) will be a representative scenario to calculate the
heeling moment due to fishing gear pull. In this case, it can be understood as a normal fishing trip
scenario and suddenly the gusty wind appears, see Machii et al. (1989). To find the static angle of
equilibrium of a fishing vessel that is in the range of initial stability (as the angle is indefinitely
diminished, M tends to a limiting position termed the metacentre), it is possible to derive the
following expression after the equilibrium between heeling moment and static stability, as shown
below.
GZFlF )cos()cos(cos (36)
where F could be P1 or P2 in accordance with the fishing trip scenario, in this case hauling and
pursing, respectively. The angle “” is use when the fishing vessels is pursing, and it is replaced by
when the fishing vessel is hauling. The angle “” is 1 when pursing, and 2 when hauling,
because normally the hauling (described before) is after the pursing. However when the net is
fastened in the sea bottom, both winch and powerblock could get tensed, which is the most critical
fishing trip scenario.
After a simple modification, this expression can be represented as:
78
)sincos
sincos)cos((
lF
lFGMFarcctgS
(37)
which is the static equilibrium angle of heel, where =s and sin GMGZ
The dynamic angle of heel is based on the equilibrium between the work done by the heeling
moment and dynamical stability, similar to equation 36, as expressed below.
d d
dGZFdlF
0 0
)cos()cos(cos (38)
After a simple simplification and modification, this expression can also be expressed as:
)sincos
sincos)cos((2
lF
lFGMFarcctgS
(39)
Finally, relating the expressions 37 and 39,
d2s (40)
With equations 36-40, it is possible to find the static and dynamic angle, and indirectly the
heeling moment due to fishing gear forces and finally to evaluate the energy balance. However, to
calculate the heeling moment due to fishing gear, the procedure shown in the following equations
(41-48), is more reasonable. This second case can be understood as a fishing trip with wind, and
suddenly the fishing gear forces and gusty wind are included as worst condition. This consideration
uses a coordinate system xyz fixed in space, and a coordinate system attached to the vessel x’y’z’.
As illustrated in Figure 56 (left side), after an angular displacement the following orthogonal
coordinate transformation matrix [T] is applied to describe the position of points A’, B’ and Q’ fixed
to a fishing vessel:
AZ
AY
TA
ZA
Y
'
' ,
BZ
BY
TB
ZB
Y
'
' ,
QZQ
YT
QZQ
Y
'
'
where:
cossin
sincosT (41)
79
The moment due to fishing net load P1 at the power block is given by:
cos)''
(1
sin)''
(11 Q
YA
YPQ
ZA
ZPM (42)
)]cos()sin()[(11
A
YA
QA
ZPM (43)
)1
cos(111
DPM (44)
The moment due to the pull of winch P2 is given by:
)''
(sin2
)''
(22 Q
YB
YPQ
ZB
ZCosPM (45)
)2
cos(222
DPM (46)
where:
1 = P1 angle as shown in Figure 56 (left side);
2 = P2 angle as shown in Figure 56(left side);
D1 = distance from point Q to point A (Powerblock);
D2 = distance from point Q to point B (Purse Gallows).
The total heeling moment due to both fishing gear loads is therefore given by:
)2
cos(22
)1
cos(11
DPDPt
M (47)
The maximum heeling moment, with “=0” and “” variable, can also be found using basic
trigonometric theory, see equation (48)
2211maxDPDP
tM )
2cos( (48)
80
Despite the simplifications used in this static analysis, both methods are still conservative
because they consider the maximum pull of the fishing gear as static and therefore dynamic effects
due to waves and current acting on the ship and fishing gear are ignored.
5.4 Calculation results
Figure 57 shows two classical purse seiners fishing vessels. Table 14 shows the general
characteristics of the fishing vessels studied in this Chapter, considering the numbering used also
in previous publications (Mantari et al. 2011a, 2011b), which are the same as in the last Chapter.
This table also shows the fishing gears loads acting when a particular fishing trip scenario is in
course, for more details see Figure 56.
These Figures (representing results for the fishing vessels: “FV6”, “FV8”, “FV11”, “FV16”,
“FV22”, “FV24” and “FV25”) show the righting and heeling arms for different wind speeds and 5
different fishing trip scenarios of loading over the fishing gear (C1, C2, C3, C4 and C5), in light
operational loading condition, when starting the first fishing trip. If a combination of fishing gear
forces and wind is done, it produces 5 additional fishing trip scenarios, together with the fishing trip
scenario when only wind is acting (weather criterion), they are in total 11 different fishing trip
scenarios.
Table 14. Fishing gear forces (Tons) acting on the 7 fishing vessels.
Figure 56 shows the positions of the fishing gear forces, Table 14 contain the values of these
forces, considering partial and maximum pull of the winch and powerblock (indicated in
percentages), without considering dynamic effect, neither additional external forces in the fishing
trip scenario.
Figures 58 and 59 show the energy balance between the heeling energy due to external forces
(wind and fishing gear) and righting restoring energy of the fishing vessel “FV6” and “FV8. These
fishing vessel have similar general characteristic and machinery on deck, except for their
81
displacements (the fishing vessel “FV8” is made of GRP, and that is why it has lower displacement,
see Table 15).
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POR肙 (104.8 m^3)
MESSE ALOJAMENTOSCASA
DE BANHO
PAIOL DA
AMARRARUFO
PA
IOL D
O P
RO
PU
LSO
R D
E
PR
OA
E D
O S
ON
AR
CASA DAS M罳 UINASCOMBUST蚔EL
(7.1 M^3)
COMBUST蚔EL(2.6 M^3)
COMBUST蚔EL(1.2 M^3)
COMBUST蚔EL(2.0 M^3)
罣 UAS NEGRAS(2.4 M^3)
COMBUST蚔EL(0.5 M^3) COMBUST蚔EL
(1.6 M^3)
罣 UA DOCE
(2.5 M^3)
CO
MB
US
T蚔
EL
(3.5 M^3)
Figure 57. General arrangement of two purse seiners fishing vessels, Portuguese (smaller) and
Peruvian (bigger).
The fishing vessel “FV6”, when the fishing gear loads, as presented in Table 14, are
considered, comply successfully with the energy balance analysis, but they become dangerous and
lead to total failure when the gusty wind is also included, see Figure 58, Table 15, 16 and 17.
The fishing vessel “FV8”, when only the fishing gear loads are considered, comply successfully
with the energy balance analysis, except by fastened condition, but they become dangerous and
lead to the total failure when the gusty wind is also included, see Figure 59, Table 15, 16 and 17. It
is important to remark that only partial winch pull were used for this fishing vessel, which is not the
case for the fishing vessel “FV6”, see Table 14. The unfavorable hydrostatics and her low
displacement play against her intact stability, in a critical fishing trip scenario. Additionally, due to
the difference between the Atlantic and Pacific Ocean, for this Portuguese fishing vessels is
82
reasonable use wind speed of 26 m/s, as mentioned above. Then it is even more unfavorable when
this wind is considered, see Table 17.
After the several calculations made on the fishing vessels “FV8”, with different fishing gear
forces and scenarios of fishing trip scenarios (not include in this Thesis for lack of space), it can be
concluded that this particular fishing vessel has over dimensioned machinery on deck. Then,
preventive action should be taken to asses her intact stability during a critical fishing trip scenario.
Fortunately, the Portuguese fishing fleet, particularly this fishing vessel (as fishing vessel “FV8”),
fish around 13.5 tons of fish (Mantari et al. 2011a). Then it seems that is very difficult to reach such
tension on the cables or net as presented in Table 14. This is maybe why there is not casualties
reported due to fishing gears forces (Antão and Guedes Soares 2004). However, in each of the
fishing trip scenarios, for example pursing, the cable can get fastened and then the tension on this
cable can reaches these values (see Table 14), then special attention need to be taken to such
situations.
In the Peruvian fleet, for a fishing vessel of similar size as “FV8”, she normally could fish more
than 100 tons (Mantari et al. 2011a). Then, fish recovering (fishing trip) is dangerous, and it is
possible to reach these tension values or forces as presented in the Table 14. Moreover, it is
believed that casualties due to fishing gear forces are frequently (Mantari et al. 2009a).
Similar comments are valid for the Portuguese Fishing vessel “FV11” and the Peruvian fishing
vessel “FV16”, see Table 14, 15, 16, and 17.
The Peruvian fishing vessels “FV24” and “FV25”, operating in the Pacific Ocean, comply with
the energy balance for all the fishing trip scenarios, except for the last one, see Table 15. It is
important to remark that the forces considered here are the maximum ones, see Table 14. Then,
after a comparison analysis, it seems that these particular fishing have better intact stability than
“FV22”, when critical fishing trip scenarios are presented. The “FV22”, has quite good intact
stability in fishing trip scenarios as well, see Table 15. However, it will be better if the power of the
machinery on deck is reduced. As Table 14 shows, this particular fishing vessel has less length but
bigger machinery on deck compared with “FV24” and “FV25”.
However, if the Peruvian vessels were working on the Atlantic Ocean (analysis of the intact
stability at different seas, a procedure recommended by Kuo (2009)), then the intact stability in
critical fishing trip scenarios becomes dangerous, and action should be taken, see Table 15 and 17.
The sixth rows of the Tables 15, 16 and 17 show the results of energy balance as the weather
criterion recommend, and these results show that all these fishing vessels complies very well.
The critical fishing gear forces acting in the fishing trip scenarios are considered as realistic
and, when only the fishing gear loads are considered, these particular fishing vessels comply
successfully with the energy balance analysis, except for the fastened scenarios or when the
existence of over dimensioned machinery on deck. However, they become dangerous and lead to
83
the total stability failure of the vessels when the gusty wind is also included, see Tables 15, 16 and
17.
Table 15. Balance of energy between the heeling energy due to external forces (wind (15m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14.
Table 16. Balance of energy between the heeling energy due to external forces (wind (19m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14.
Table 17. Balance of energy between the heeling energy due to external forces (wind (26m/s) and
fishing gear) and righting restoring energy of a set of 7 fishing vessel, see Table 14.
This study also shows that in particular fishing trip scenario (see Table 15, 16 and 17) the IMO
rough weather criterion is unconservative.
Figures 58-59, and Tables 15, 16 and 17, show that the combined effect of the fishing gear and
moderate to whole gale wind description can be larger than the requirements of the weather
criterion as pointed out also in IMO (1979), Machii et al. (1989), Gefaell (2005), Mantari et al. (2009,
2009a, 2011b).
Finally, with respect to the fishing gear heeling moment, it can be said that few or limited
number of research articles are available in the literature. A literature survey shows few articles
dealing with fishing gear forces, and any article dealing with fishing gear forces combined with
waves or winds. It is perhaps due to the fact that nowadays there is not abundance of natural
resources (fish), which may cause danger in a fishing trip scenario. However, exist the possibility
that the net may be fastened at the bottom and generate a stability failure. Moreover, some
countries have still abundance of fish and the tendency to increase machinery onboard still exist,
as mentioned above. Therefore, the fishing gear forces acting individually of in combined action
with waves and winds should be considerer for further research.
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3Intact stability at fishing operation, U=15m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3Intact stability at fishing operation, U=19m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3Intact stability at fishing operation, U=26m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
GZC1, haulingC2, pursingC3, haulingC4, pursingC5, fastenedH.WindUm/s ArmH.W.Um/s + C1, haulingH.W.Um/s + C2, pursingH.W.Um/s + C3, haulingH.W.Um/s + C4, pursingH.W.Um/s + C5, fastened
Figure 58. Inclining arms due to external forces (wind and fishing gear loads) and restoring arms of
the fishing vessel FV6.
85
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3
0.4Intact stability at fishing operation, U=15m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3
0.4Intact stability at fishing operation, U=19m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
-10 0 10 20 30 40 50 60-0.1
0
0.1
0.2
0.3
0.4Intact stability at fishing operation, U=26m/s
Heel angle (deg)
Rig
htin
g ar
m (
m)
GZC1, haulingC2, pursingC3, haulingC4, pursingC5, fastenedH.WindUm/s ArmH.W.Um/s + C1, haulingH.W.Um/s + C2, pursingH.W.Um/s + C3, haulingH.W.Um/s + C4, pursingH.W.Um/s + C5, fastened
Figure 59. Inclining arms due to external forces (wind and fishing gear loads) and restoring arms of
the fishing vessel FV8.
86
87
6 CONCLUSIONS AND RECOMMENDATIONS
6.1 General conclusion
An overview of the International Code on Intact Stability (2008 IS Code) related to fishing
vessels, is made. The stability characteristics of 27 fishing vessels, mainly from the Portuguese
and Peruvian fishing fleet, are studied. Calculations for longitudinal and beam waves are made.
The variations of dynamic transverse intact stability of the fishing vessels are studied. Potential for
the occurrence of stability failure due to pure loss of stability and parametric resonance are found.
For beam waves, fishing gear and gusty wind loads are included to evaluate the energy balance
between the heeling and righting moments; based on these calculations, size, hull form and others
particularities of the fishing vessels, some light on the occurrence of partial or total stability failure
due to fishing gear forces are also found.
The (a) different kinds of size, hull form and fishing vessels particularities, and (b) change of
draft and displacement, from one operational loading condition to another show that these
representatives fleets belong to the Portuguese and Peruvian fleets are two completely different
fishing fleets, which should be analyzed with different criteria.
For general understanding and acceptance, the IMO IS Code 2008 should be completed, it
should consider all aspects related to stability (as well as the combined effects presents in the
causalities), should conduce for the optimal design (develop intact stability code by ship typologies)
and cover all the variants that can exist (as example in fishing vessels: ocean, power machinery,
fishing capacity, etc. should be considered). At the moment the IMO stability criteria does not
require safety checks neither, in longitudinal nor in beam waves, making a decision support system
very useful for the safety of certain fishing vessels in waves.
Several of the main results of this thesis have been already published in Mantari et al. (2009a,b;
2011a,b).
6.2 Conclusion on stability failure in longitudinal waves
In this Thesis the GM, righting lever and restoring energy variation in waves were studied for
27 fishing vessels, identifying the effect of fishing vessels size and hull form. These calculations
indicate how the stability is sensitive to the changes from one wave profile to another in longitudinal
waves, which can be related with the susceptibility of the vessel to partial or total failure.
The results show that the GM variations are not constant in different operational loading
conditions, so additional analyses need to be carried out to find out the critical operational loading
condition.
88
Large variation of roll restoring energy may have influence on the magnitude of the potential for
the total or partial failure of a vessel. Additionally, the analysis of the ratio between the absolute
value of gain and loss of roll restoring energy, from a more favorable and less favorable wave crest
position along the vessel, has relation with the stability failure of the fishing vessel, which can give
potential for the occurrence of (a) parametric rolling and (b) pure loss of stability on a wave crest. It
is a simple analysis that can be done in the conceptual design.
Critical loading condition and unfavorable sailing condition in head seas have been determined
for 16 of the fishing vessels studied in this Thesis; ratios of wave length to vessels length between
perpendicular around 1 are more critical than larger ratios, with a wave steepness of 1/20 and head
seas; however for some particular fishing vessels, as the trawler fishing vessel “FV10”, it occurred
around 1.6.
The boundary of the first instability zone of the fishing vessels show some agreements with the
preliminary stability failure analysis, in the sense that the fishing vessels that are with of the
boundary (unsafe) do not comply with the International Code on Intact Stability (2008 IS Code) in
waves.
6.3 Conclusion on stability failure in beam waves, wind and fishing gear forces
The reduction of dynamic transverse stability of these fishing vessels due to the fishing gear
pull moment, wind and combination of them, operating in the Pacific and Atlantic Oceans, have
been compared with the International Intact Stability Code (2008 IS Code). The calculations
presented in this Thesis, show that:
fishing gear heeling moments, in occasions, are more important than the heeling moments
produced by the weather criterion, even when the real scenarios of operations (oceans)
are consider and, effectively, a combination of them may leads to capsizing even when the
fishing trip scenario is normal.
“FV8”, “FV11” and “FV16” have over dimensioned fishing gear machinery aboard, and
then preventive action should be taken.
This Thesis shows that it is possible, individually or comparatively to calculate the fishing
vessel stability in a fishing trip scenario and select adequate fishing machinery for fishing, as
winches and powerblocks. To do that, a simple and practical numerical calculation can be
developed in MATLAB, which can be supported by commercial software, as AUTOSHIP.
89
7. REFERENCES
Amagai K., Ueno K. and Kimura N. 2000. Characteristics of roll motion for small fishing boats,
Contemporary Ideas on ship stability, Vassalos D. et al. (Editors), Elsevier Science, 137-148
Antão, P. and Guedes Soares, C., 2008. Causal Factors in Accidents of High Speed Craft and
Conventional Ocean Going Vessels. Reliability Engineering and System Safety . 2008;
93(9):1292-1304.
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ANNEX 1 TECHNICAL INFORMATION OF POWERBLOCK
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ANNEX 2 TECHNICAL INFORMATION OF WINCHS
97
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ANNEX 3 TECHNICAL INFORMATION OF FISHING GEAR
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ANNEX 4 Technical information of fishing gears for a purse seiner of 210 tons. Upper
graphics (standard), lower graphics (with increased power in the winch) (Townsend 2005).