container handbook sect_02-03-03 - marine transport g loads

7/25/2019 Container Handbook Sect_02-03-03 - Marine Transport g Loads

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2.3.3 Mechanical stresses in maritimetransport

Section 1 "General conditions" in the CTU packing guidelinesclearly states, for examplein point 1.1:

Voyages are made in a variety of weather conditions which are likely to exert acombination of forces upon the ship and its cargo over a prolonged period. Suchforces may arise from pitching, rolling, heaving, surging, yawing or swaying or acombination of any two or more.

Point 1.2 continues:

Packing and securing of cargo into/onto a CTU should be carried out with this inmind. It should never be assumed that the weather will be calm and the seasmooth or that securing methods used for land transport will always be adequateat sea.

The acceleration values to be anticipated in maritime transport depend on the shape ofthe surface or sub-surface vessel, its beam, the position of the center of gravity andcenter of buoyancy and similar parameters which determine the behavior of ships atsea.

Ship movement

at sea

All kinds of ship movement may be divided into three types of linear motion and threetypes of rotational motion.

Linear motion Rotational motion

Surging is motion along the longitudinalaxis.

Rolling is motion around the longitudinalaxis.

Swaying is motion along the transverse

axis.

Pitching is motion around the transverse

axis.

Heaving is motion along the vertical axis. Yawing is motion around the vertical axis.

Summary of ship movement

Page 1 of 12Container Handbook - Section 2.3.3 Mechanical stresses in maritime transport

28/04/2016https://www.containerhandbuch.de/chb_e/stra/stra_02_03_03.html


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It can in general be stated that the outwardly directed centrifugal accelerations broughtabout by any rotational motion are not significant. This accordingly applies to yawing,pitchingand rolling.

Yawing is motion around theship's vertical axis.

Yawinginvolves rotation of the ship around its vertical axis. This occurs due to theimpossibility of steering a ship on an absolutely straight course. Depending upon seaconditions and rudder deflection, the ship will swing around its projected course. Yawingis not a cause of shipping damage.

Heaving is motion along the ship's vertical axis.

Heavinginvolves upward and downward acceleration of ships along their vertical axis.Only in an absolute calm are upward and downward motion at equilibrium and the shipfloats at rest. Buoyancy varies as a ship travels through wave crests and troughs. If thewave troughs predominate, buoyancy falls and the ship "sinks" (top picture), while if thewave crests predominate, the ship "rises" (bottom picture). Such constant oscillation hasa marked effect on the containers and their contents.




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Surging is motion alongthe ship's longitudinal axis.

Swaying is motion along theship's transverse axis.

In surgingand swaying, the sea's motion accelerates and decelerates the ship forwardand backward and side to side. Depending upon the lie of the vessel, these movementsmay occur in all possible axes, not merely, for example, horizontally. If a vessel'sforebody is on one side of a wave crest and the afterbody on the other side, the hullmay be subjected to considerable torsion forces.

Pitching is the movement of a ship around its transverse axis.

In pitchinga ship is lifted at the bow and lowered at the stern and vice versa. Pitchingangles vary with the length of vessel. In relatively short vessels they are 5 - 8 andsometimes more, while in very long vessels they are usually less than 5. In a containership 300 m in length with a pitching angle of 3, a container stowed in the bay closest tothe bow or stern at a distance of approx. 140 m from the pitching axis will cover adistance of 29 m within a pitching cycle, being raised 7.33 m upwards from thehorizontal before descending 14.66 m downwards and finally being raised 7.33 m againand then restarting the process. During upward motion, stack pressures rise, while theyfall during downward motion.

Rolling is the movement of a ship around its longitudinal axis, therolling angle in this case being 10.




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Rollinginvolves side-to-side movement of the vessel. The rolling period is defined asthe time taken for a full rolling oscillation from the horizontal to the left, back tohorizontal then to the right and then back to horizontal. In vessels with a high rightingcapacity, i.e. stiff ships, rolling periods of ten seconds and below are entirely usual.Rolling angle is measured relative to the horizontal. Just in moderate seas, even verylarge vessels roll to an angle of 10.

Rolling angles of 30 are not unusual in heavy weather.

In bad weather, angles of 30 are not unusual. Even the largest container ships must beexpected to roll to such angles. Stabilizers and other anti-heeling systems may help todamp ship movements. However, not all systems are usable or sufficiently effective in

bad weather.

Rolling angle of 45

On rare occasions, rolling anglesmay reach 45 and above. It is easy to imagine whatthat means for inadequately secured container cargoes.

Rollingand pitchingof a vessel generate upward and downward acceleration forcesdirected tangentially to the direction of rotation, the values of which increase withdistance from the rolling or pitching axis and are inversely proportional to the square ofthe rolling or pitching periods. At an identical distance from the axis, if the rolling orpitching period is halved, acceleration forces are quadrupled, while if the rolling orpitching period is doubled, acceleration forces are quartered. Rolling or pitching anglesgenerate downslope forces. Steeper tilting, as occurs during rolling, promote cargoslippage. As already mentioned, the outwardly acting centrifugal accelerations generatedby rotational motion are of no significance in rolling and pitching.

Overall, containers and packages may be exposed to such accelerations for very longperiods when at sea. Moreover, the oscillations may be superimposed one on the otherand be intensified.




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Damage to containers in rolling motion, caused by inadequatelysecured cargo:

left: in a container stowed

athwartships

right: in a fore and aftstowed

container

It must be emphasized that it was not the "hazards of the sea" which caused thedamage, but instead inadequate securing inside the container. While such damage hasindeed occurred in association with the rolling motion of the ship, the root cause is the"home-grown" acceleration forces arising from shortcomings in packing and securing.

Slamming describes the hydrodynamic impacts undergoneby a ship.

Slamming is the term used to describe the hydrodynamic impacts which a shipencounters due to the up and down motion of the hull, entry into wave crests and theconsequent abrupt immersion of the ship into the sea.

Vibrationfrom the hull can be transferred to the cargo. Goods are exposed to stressesfrom the extremely low frequency oscillations generated by sea conditions and by higher

frequency machinery and propeller vibration. Such risks can and must be avoided byusing seaworthy shipping packages which are fit for purpose.

The absolute acceleration valuesencountered on board ship are not excessively high.In favorable stowage spaces, they may even be considerably lower than thoseencountered in land or air transport. In many cases, not even the values stated in thefollowing Table occur. However, the frequency with which the motion occurs mustdefinitely be borne in mind. At a rolling period of 10 s, a ship moves side to side 8640times daily. Over several days' bad weather, the cargo will thus be exposed toalternating loads tens of thousands of times.

Mode of transport:ocean-going vessel

Forward actingforces

Backward actingforces

Sideways actingforces

Baltic Sea 0.3 g (b) 0.3 g (b) 0.5 gNorth Sea 0.3 g (c) 0.3 g (c) 0.7 g

Unrestricted 0.4 g (d) 0.4 g (d) 0.8 g




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1 g = 9.81 m/sec The above values should be combined with static gravity force of1.0 g acting downwards and a dynamic variation of:

(b)= 0.5g (c)= 0.7g (d)= 0.8g

Extract from a Table in the CTU packing guidelines

In relation to the Table, it is stated in point 1.7 of the CTU packing guidelines thatexamples of accelerations are given which could arise during transport operations:

However, national legislation or recommendations may require the use of othervalues.

The values stated in footnotes (a), (b) and (c) in principle describe accelerations in thevertical direction. Such accelerations are particularly high in pitching and rollingmovements and, in exposed positions in very bad weather, can easily reach 1 g. TheCTU packing guidelines here state the maximum at 0.8 g. Vertical acceleration reducesfriction forces and increases stack pressure.

Overview ofaccelerationforces prevailingon board a ship

Annex 13 of the CSS Code contains tables for determining acceleration forces as afunction of stowage space on board, the ship's length and speed. However, these tablesare not suitable for use when packing CTUs and securing cargoes in or on CTUs,specifically for the following reasons:

If containers, road vehicles, rail cars or the like and road trailers, roll trailers andsemitrailers are loaded inland for maritime transport, their ultimate stowage space onboard is unknown. The least favorable conditions should thus always be taken intoaccount. As a rule of thumb, loads of 1 g in the vertical direction and 0.8 g in thehorizontal direction should be anticipated for worldwide transport. The shipowner will notaccept any attempt by the shipper to specify a particular container slot in advance. Evennotes on the bill of ladingrequiring loading below deck are ineffective. All shippingpackages must accordingly be constructed so as to be able to withstand 0.8 times theweight of all adjacently stowed cargo and twice the mass of the cargo loaded on top. Ifthis is not the case, appropriate protective measures must be taken. Additional rigidreceptacles, frames, false decks and similar measures may be helpful.

Modern cargo handling procedures and the ships developed for this purpose have mademaritime transport faster and cheaper and, in particular, have reduced cargo handlingstresses in port. In order to ensure great flexibility in terms of loading and unloading,modern ships, in particular ro/rofreighters and ferries, inevitably have poorer

seakeeping ability than conventional general cargo or heavy-lift vessels.

For reasons of stability during loading, they require a high righting capacity. As "stiff"vessels they initially oppose heeling movements with a very high righting moment. Thehigh roll moment of inertia of these vessels entails shorter rolling periods and hightransverse acceleration forces. Due to the particular nature of ro/ro shipping, the ship'scommand is not generally able to influence the stability behavior of these ships byadjusting the weight distribution. The risk of accidents is particularly high because, giventhe large free surfaces in the ship, overturning cargo and the possible consequentingress of water may result in an abrupt capsize. Most readers will remember majoraccidents of this kind. Inadequate cargo securing in transport receptacles such ascontainers, swap-bodiesetc. may consequently have a very significant impact on shipsafety.

Ro/ro freighterlisting as a

result of wateringress




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Free surfaces on board always increase the risk of capsize.

"Home-grown" acceleration forces in maritime transportare the commonestcause of cargo damage on board ship.

Damage caused

by "home-

grown"accelerationforces

Because container packers do not have the appropriate knowledge and skills, theyunderestimate the effect of gaps in the stow. The consequent motion has a devastatingeffect on the cargo.

Damage causedby "home-

grown"acceleration

forces




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"Home-grown" acceleration can readily be identified on board ship if the stowage spaceshave been subjected to similar forces, but only some of the goods have suffereddamage. It is even clearer when goods stowed in an exposed location remainundamaged, while other goods suffer damage despite being exposed to loweracceleration forces. The cases on the platform at the top left were exposed to higheracceleration forces than the cargoes in the containers, which were stowed beneath orfurther inwards. Although the cases were only secured with a single belt each, they haveonly shifted slightly, while the containers and their contents have been completelydestroyed.

Effects of "home-grown"acceleration forces

In the lower container (1), the poorly secured machine has been set in motion and hasforced the container doors open. The tank on the platform (2) stowed above is securedwith only two belts and thus also inadequately. Nevertheless, it has withstood theacceleration forces and has slipped only a little. This is a clear indication that theacceleration forces were still relatively slight.

The following pictures clearly show the results of home-grown acceleration forces. Itshould be noted that almost all of the containers have been exposed to stresses fromthe inside outwards, i.e. they have bulges rather than dents.




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Bulges in containers as a result of home-grown acceleration forces

Container "rippedapart" as a result ofhome-grownacceleration forces

The fiber structure of the plywood walls of the container in the picture above clearlyreveals that the forces were acting from the inside outwards. The container wasdestroyed by gaps in the stow. These gaps resulted in extremely high acceleration forcesand shocks.




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Damage as a result of home-grown acceleration forces

Annex 13 of the CSS Code provides tools to assist in calculating wind pressure andthe effects of spray. The details provided in this publication may possibly be ofassistance in dimensioning cargo securing on open containers such as flatracks,

platforms etc., but they are otherwise of interest only to the ship's command, but not tocontainer packers working inland. As a rough guide, wind pressure may be estimated,for example for fastening tarpaulins etc., at 100 daN/m. Closed containers are spray-tight provided that they have no technical defects.

Effects ofbreaking-waveimpact

Cargoes stowed on deck may be exposed to breaking-wave impact. Even for experts,the magnitude of these forces and their effects are difficult to estimate. Additionalsecuring measures cannot prevent such effects or only to a very limited extent. Whilesecuring can never withstand breaking-wave impact, cargoes on open containers should,as a precaution, be secured against floating away.

In conventional shipping, damage prevention is the responsibility of the ship's

command. Responsible ship's commands will accordingly use any means available tothem to keep the effects of rough seas and breaking-wave impact as small as possible.Cargo officers will stow cargoes which are particularly sensitive or require particularlyextensive securing in locations which are subjected to less acceleration. In containertrade, no consideration can be given to special requirements with regard to stowagespace for particular containers. Moreover, the central stowage planning offices, whichprepare initial plans, and the ship's command have no knowledge as to what is loaded inthe containers. Dangerous cargo containers are an exception. In this case, the contentsare known and the containers receive special stowage spaces.

Breaking-wave impact means that"green water" has come onto thedeck.

Damage caused by breaking-

wave impact




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Container ripped

open bybreaking-wave

impact

Summary of mechanical stresses arising during maritime transport

In very general terms, it can be stated that cargo transport units may be exposed tovery different stresses during maritime transport than they are in road, rail or inlandwaterway transport. Unless the voyage proceeds very calmly in good weather, thecontainers and their cargoes will be exposed to oscillation/vibration which is primarilycaused by rolling and pitching. It is almost exclusively during rolling, due to thetilt/rolling angleswhich arise, that shipping packages are pressed against the containerwalls and are squashed against the walls or the surfaces of adjacent shipping packages.

The same occurs in "open" containers if parts of the cargo are pressed against thelashingsor bracing. The oscillations of rolling and pitching alternately increase andreduce stack pressure. These changes peak at the moment the motion is reversed.Assuming a vertical acceleration of 1 g, a package can thus alternate between "twice itsweight" and "weightless". Appropriate deductions or additions may be made for otheracceleration values. When containers are incorrectly packed or the cargoes inadequatelysecured, the packages may shift, be dented, squashed, jumbled up etc.

Jumbled cartonsin a container



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