mechanical lifting failures 434-08

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Risk Assessment Data Directory

Report No. 434 – 8

March 2010

I n t e r n a t i o n a l A s s o c i a t i o n o f O i l & G a s P r o d u c e r s

Mechanicallifting

failures



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RADD – Mechanical lifting failures

©OGP

Contents:

1.0 Introduction .......................................................................... 1

1.1 Application ...................................................................................................... 1

1.2 Definitions ....................................................................................................... 1

2.0 Summary of Recommended Data............................................ 1

3.0 Guidance on use of data ........................................................ 3

3.1 General validity ............................................................................................... 3

3.2 Uncertainties ...................................................................................................3

3.3 Use of the Data................................................................................................ 4

3.4 Consequence Analysis of Objects Dropped Into the Sea........................... 4

3.5 Kinetic energy ................................................................................................. 6

4.0 Review of data sources ......................................................... 7

5.0 References ............................................................................ 8




©OGP

Abbreviations:

BOP Blowout Preventer

DNV Det Norske VeritasHSE (UK) Health and Safety Executive

QRA Quantitative Risk Assessment (sometimes Analysis)UKCS United Kingdom Continental Shelf

WOAD World Offshore Accident Databank




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1.0 Introduction

1.1 Application

This datasheet presents information on the frequency of dropped objects resulting fromthe failure of lifting devices on offshore installations. Specifically it includes dropped

load frequencies for the following types of lifting equipment:

1. Main cranes

2. Drilling derrick

3. Other devices

The data are derived from offshore operating experience in the UKCS over the period1980 to 1999. The data are intended to be applied in quantifying the risks from liftingoperations worldwide. Consideration should be given to factoring the data up or down

where there is reasonable justification that the management of lifting operations issignificantly poorer or safer that UKCS operations.

1.2 Definitions

• Dropped loads Refers to loads (objects) either unintentionally released froma lifting device or else swinging and impacting some part of the installation structure (or vessel, if the lift is to/from a

vessel).

• Lifting devices Main crane, derrick main hoisting assembly, and other liftingdevices (see below).

• Other liftingdevices

BOP cranes, gantry cranes, tuggers, and a range of portabledevices, e.g winches, sling blocks, wirelines.

• MobileInstallations

The data for mobile installations are gathered almost entirely

from experience in the operation of mobile offshore drillingunits (MODUs). These include semi-submersibles, jackups,and drill ships.

• FixedInstallations

The data for fixed installations are gathered from a range of

types of production installation ranging from integratedplatforms to wellhead platforms. The data also include

experience from FPSOs (floating production, storage andoffloading vessels) and FSUs (floating storage units).

“Main cranes” and “drilling derrick” referred to in Section 1.0 are considered self

explanatory.

2.0 Summary of Recommended Data

Dropped object probabilities per lift on offshore installations are tabulated below for

mobile installations and fixed installations, for different load weights and by liftingdevice (main crane, drilling derrick, or other device).

The data represent the probability of a dropped object per lift. Estimation of the

dropped object frequency combines the probability of a dropped object per lift with thenumber of lifts carried out (for example, per year if the annual risk is required).

Note that, for drops from the main crane, in general the frequency in the Total column isnot the sum of the Installation, Sea and Vessel drop frequencies in the same row

because not all main crane lifts are between vessel and installation (some are across




©OGP2

the installation). Each frequency in the Total column is calculated from the total number of lifts, whereas the Sea and Vessel frequencies are calculated from the number of external lifts (between installation and vessel) only.

Of the reported events on which the probabilities tabulated below are based, 10% of dropped objects on mobile installations and 20% of dropped objects on fixed

installations resulted in all or part of the lifting device falling (see Section 1.2 above for the definition of “lifting device”).

Dropped Object Probabi l i t ies for Mobi le Un i ts (per l i f t )

Drop Onto:oad

Weight

Lift ing

device Installatio

n

Sea Vessel

Total

Main crane 3.2 × 10-5

8.8 × 10-6

1.1 × 10-5

4.1 × 10-5

DrillingDerrick

1.7 × 10-5

7.3 × 10-7

6.1 × 10-8

1.8 × 10-5

<1 te

Other Device 8.6 × 10-5 1.1 × 10-5 0* 9.7 × 10-5


2.0 × 10-6

3.0 × 10-6

5.4 × 10-6

DrillingDerrick

3.6 × 10-6

4.6 × 10-7

0* 4.0 × 10-6

1 – 20 te

Other Device 7.6 × 10-6

2.9 × 10-6

0* 1.1 × 10-5


7.1 × 10-6

9.5 × 10-6

2.0 × 10-5

DrillingDerrick

1.8 × 10-6

0*

0* 1.8 × 10-6

20 – 100 te


0*

0* 1.9 × 10-6


0* 0* 2.8 × 10-4

DrillingDerrick

4.7 × 10-3 1.4 × 10-3 0* 6.1 × 10-3

>100 te


2.4 × 10-4

0* 7.3 × 10-4


3.3 × 10-6

4.6 × 10-6

1.2 × 10-5

DrillingDerrick

1.1 × 10-5

6.7 × 10-7

3.0 × 10-8

1.1 × 10-5

All


6.5 × 10-6

0* 5.2 × 10-5

Total All 1.2 × 10-5

1.4 × 10-6

9.4 × 10-7

1.4 × 10-5




©OGP 3

Dropped Object Probabi l i t ies for Fixed Instal lat ions (per l i f t )

Drop Onto:oadWeight

Lift ingdevice Installatio

nSea Vessel

Total


6.9 × 10-6

1.1 × 10-5

4.5 × 10-5

DrillingDerrick

1.7 × 10-5 1.2 × 10-7 1.2 × 10-7 1.7 × 10-5

<1 te


4.2 × 10-6

6.1 × 10-7

1.0 × 10-4


1.7 × 10-6

5.1 × 10-6

7.9 × 10-6

DrillingDerrick

2.7 × 10-6

1.5 × 10-7

0* 2.9 × 10-6

1 – 20 te


0* 7.4 × 10-7

1.5 × 10-5


6.2 × 10-6

1.6 × 10-5

2.0 × 10-5

DrillingDerrick

1.2 × 10-6

0* 0* 1.2 × 10-6

20 – 100 te


0* 0* 2.6 × 10-5

Main crane 9.3 × 10

-50* 0* 9.3 × 10

-5

DrillingDerrick

0* 0* 0* 0

>100 te


0* 0* 6.1 × 10-4


2.8 × 10-6

6.4 × 10-6

1.5 × 10-5

DrillingDerrick

9.6 × 10-6

1.2 × 10-7

6.1 × 10-8

9.7 × 10-6

All


2.0 × 10-6

5.8 × 10-7

6.0 × 10-5

Total All 1.4 × 10-5

8.8 × 10-7

1.6 × 10-6

1.6 × 10-5

• In both of the above tables, either there are no recorded incidents, or the incident is

not credible. If the analyst believes it is credible, then a suitable frequency could beobtained by pro rating a non-zero frequency, e.g. using the “All” frequencies.

3.0 Guidance on use of data

3.1 General validity

The frequencies given are based on analysis of offshore lifting operations on the UKcontinental shelf (see Section 4.0). They may be applied to lifting operations in other

offshore regions which comply with recognised industry good practice, as it is applied

in the UKCS.The data for dropped objects from derricks may be applied to onshore drilling

operations where these are similar to offshore drilling activities and equipment. Thedata for dropped objects from main cranes and other lifting devices are not applicableto onshore lifting operations because the equipment used is unlikely to be similar to

that used offshore.

3.2 Uncertainties

Sources of uncertainties in the data include statistical variation and the similaritybetween the operations and equipment under analysis and those represented by the

database.




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The calculated frequencies are derived from 1637 dropped object events in a totalexperience of 3063 installation-years. This implies a total of about 111 million liftingoperations. For fixed platforms there were 690 dropped objects in 1857 platform years,

for mobile installations 947 events in 1206 installation-years experience. Therefore thestatistical uncertainty in the overall frequencies is relatively small. Some of the specific

risks are calculated from the experience of a small number of representative droppedobject accidents and correspondingly the uncertainty in the risk will be more significant.The risks with the higher uncertainty are those with the lower likelihoods shown inSection 2.0.

The data in the database reflect lifting equipment in operation in the UKCS. While thereis a degree of variation in the equipment used in the UKCS, it is similar in that the vastmajority is maintained and operated in accordance with international certification andUK legal requirements. Competence requirements for operations and maintenancepersonnel are generally enforced, and all operations are conducted in accordance withdocumented procedures reflecting good industry practice. Where operations outsidethe UK can be assumed to follow a similar standard of operation and maintenance, it is

reasonable to assume the data are valid for assessment of the dropped object risks.

3.3 Use of the Data

The dropped object probability values are an input to QRA and are used to calculate thefrequency of the initiating event for dropped object risks. The consequence of dropped

objects depends on the impact energy and the people, equipment and structuresimpacted by the objects dropped.

For an object falling through air, the impact energy is calculated as the product of themass of the object, the height and acceleration due to gravity (≈ 10 m/s2). Generally,

people struck by falling objects can be assumed to be fatally injured, and objectsstriking hydrocarbon equipment will cause a hydrocarbon release. Damage tostructures or other equipment struck by dropped objects may require a specificassessment of the resistance of the object impacted and/or the potential for a releasefrom live equipment struck. However, incidents involving hydrocarbon releases arealready included in the hydrocarbon release frequencies, so such an assessment is onlyrecommended where the analyst identifies a particular vulnerability to dropped objects,

or a stand-alone dropped objects study is being carried out.

When using dropped object risks in a total risk assessment for a facility, the risks topeople from dropped objects may also be included in the statistical data onoccupational accidents. Where this is the case, it is appropriate to disregard thecalculated dropped object risk for immediate fatalities.

In the event of a dropped object, the lifting equipment will be out of service until theincident can be investigated and any repair can be implemented. An operational risk

assessment should take account of this. Even for minor dropped objects with noapparent damage, equipment downtime will be of the order of several days. In the eventof a fatality or major equipment damage, the equipment is likely to be out of service for

several weeks.

3.4 Consequence Analysis of Objects Dropped Into the Sea

The calculation of the consequences of objects dropped into the sea is more complex.

For heavy lifts (e.g. BOP or xmas tree) over the sea it is standard practice that these arenot carried out over vulnerable subsea equipment. Thus care is required in assessing




©OGP 5

whether a dropped BOP or other heavy load can cause damage to subsea equipment or if the precautions carried out are adequate. For other lifts, the following approach canbe followed to calculate locations at risk from dropped object impact.

Heavy, dense objects (such as BOPs) can be assumed to fall vertically and will damageany infrastructure immediately beneath the drop site. Some other objects, such as pipe

sections and scaffolding poles, may travel a significant horizontal distance through thewater as they descend. The following model is taken from a DNV RecommendedPractice [4].

The analysis assumes that the excursion made by a dropped object can be representedby a normal distribution:

where x is the horizontal excursion and δ the standard deviation. The standarddeviation is sensitive to the weight and shape of the object, and the water depth (d ). Thederivation of δ is given by:

Here α is the spread in the descent angle given in Table 3.1.

Table 3.1 Calculation of Descent Angles

Case Object ShapeDescription

Weight

(tonnes)

DescentAngle

Spread(deg)

1 < 2 15

2 2 – 8 93

Flat/long shaped> 8 5

4 < 2 10

5 2 – 8 5

6

Box/round shaped

> 8 3

7 Box/round shaped >> 8 2

The probability that the object lands within a horizontal distance, r , of the drop point isgiven by the equation:

When considering object excursion in deep water the spreading of long/flat objects,cases no. 1 to 3, will increase down to a depth of approximately 180 m. Below this depthspreading does not increase significantly and may conservatively be set to be vertical.

For a riser, any vertical sections will complicate the hit calculations. One way of calculating the probability of hit to a riser is to:

1. Split the riser into different sections (normally into vertical section(s) and horizontalsection(s)), and

2. Calculate the hit probability of these sections at the respective water depths. Thefinal probability is then found as the sum of all the probabilities for the differentsections.

The effect of currents will become more pronounced in deep water. The time for anobject to hit the seabed will increase as the depth increases. This means that any




©OGP6

current may increase the excursion (in one direction). At 1000 m depth the excursion isfound to increase 10 to 25 metres for an average current velocity of 0.25 m/s and up to200 m for a current of 1.0 m/s.

The effect of currents may be included if one dominant current direction can beidentified. This may be applicable for rig operations for shorter periods, for example

during drilling, completion and intervention/construction above subsea wells. However,for a dropped object assessment on a fixed platform, seasonal changes in currentdirections may be difficult to incorporate.

When establishing a "safe distance" away from activities the effect of currents shouldbe included. A conservative object excursion should be determined, including

consideration of the drift of the objects before sinking, uncertainties in the navigation of anchor handling vessel, etc.

3.5 Kinetic energy

A dropped object from a crane and hitting the topsides will have a kinetic (impact)

energy E k given by:

E k = m.g.h

where: E k = kinetic energy at impact (J)

m = mass of the object (kg)g = gravitation acceleration (9.81 m/s2)h = height from release point to point of impact (m)

The maximum impact force depends on the object itself and the orientation when

hitting, and can be found from structural collapse calculations. The impact resistance of structures can be found from deterministic structural strength calculations.

The kinetic energy of a dropped object on subsea installations depends on the velocity

through the water, the shape of the object and the mass in water. After approximately 50- 100 metres, a sinking object will usually have reached its terminal velocity.

The terminal velocity is found when the object is in balance with respect to gravitationforces, displaced volume and flow resistance. When the object has reached this

balance, it falls with a constant velocity, its terminal velocity. This can be expressed bythe following equation:

where: m = mass of the object (kg)g = gravitation acceleration (9.81 m/s2)

V = volume of the object (the volume of the displaced water) (m3) ρwater = density of water (typically 1025 kg/m3 for the North Sea)

C D = drag coefficient of the object A = projected area of the object in the flow-direction (m2)

v T = terminal velocity through the water (m/s)

The kinetic energy of the object, E T , at the terminal velocity is:

Combining these to equations gives the following expression for the terminal energy:




©OGP 7

In addition to the terminal energy, the kinetic energy that is effective in an impact, E E ,includes the energy of added hydrodynamic mass, E A. The added mass may becomesignificant for large volume objects such as containers. The effective impact energy

becomes:

where ma is the added mass (kg).

Tubulars are assumed to be waterfilled unless it is documented that the closure issufficiently effective during the initial impact with the surface, and that it will continue tostay close in the sea.

Intact, sealed containers may not sink at all.

The drag and added mass coefficients are dependent of the geometry of the object. The

drag coefficients will affect the objects terminal velocity, while the added mass only hasinfluence as the object hit something and is brought to a stop. Table 3.2 gives typicalvalues of these coefficients.

Table 3.2 Drag and Added Mass Coefficients

Object Case(as Table 3.1)

Description C d C m

1,2,3 Slender shape 0.7 - 1.5 0.1 - 1.0

4,5,6,7 Box shaped 1.2 - 1.3 0.6 - 1.5

All Misc. shapes (spherical tocomplex)

0.6 - 2.0 1.0 - 2.0

It is recommended that a value of 1.0 is initially used for C d , after which the effect of arevised drag coefficient should be evaluated.

Small equipment items (fittings, scaffolding clamps, etc.) are unlikely to do any damageto subsea equipment if they fall into the sea.

4.0 Review of data sources

The recommended probabilities of dropped objects presented in Section 2.0 have been

calculated by combining recorded incidents of dropped objects from the WOAD [1] andthe UK HSE’s ORION databases with data on the number of lifts carried out.

The incidents have been analysed by DNV and full reports are available in HSE research

reports [2] and [3].

The numbers of lifts per year for mobile installations (Table 4.1) are based on observeddata collected for DNV by a drilling contractor. The number of lifts per year on fixedinstallations (Table 4.2) are estimated by interpretation of the data on mobileinstallations combined with reasonable assumptions and consequently should be

treated with more caution. The numbers of “installation years” represented by theORION and WOAD data are provided by the HSE from primary records.

The experience data for mobile installations were collected over the period 1980 to 1998;

those for fixed installations were collected over the period 1991 to 1999.

Of the main crane lifts, 46% were to or from a supply vessel and 54% were across the

installation. Of the lifts to and from supply vessels, 75% were of containers, basketsand tanks; the remainder were casing, drillpipe, collars, etc.




©OGP8

Table 4.1 Observed Frequencies of Lift ing Operations on Mobile

Installations

Lifting Device Lifts per Year

Main Crane 24,480Drilling Derrick 28,670

Other Lifting Device 3,650

Total 56,800

Table 4.2 Calculated Frequencies of Lifts using Main Crane on FixedInstallations (per year)

Type of instal lation Lifts to / from

Vessels

Internal Lifts

Fixed (no drilling) 5520 8,674Fixed (drilling for 6 months /year)

8400 10,937

Wellhead platform 552 867

The UK HSE has also published accident data for more recent period up to and

including 2004/2005 [5, 6. 7] These data have not been subjected to the same detailedstatistical analysis as the data presented in this report and for this reason the morerecent experience is not included here. However a review of the data over the period1980 to 2005 shows that although there is considerable variation from year to year, the

average frequency of dropped objects per installation-year remains approximatelyconstant. This is consistent with the observation that the technology and liftingprocedures used on offshore installations have not changed to any great extent over the

period the data were collected.

5.0 References

1. DNV, 2006. WOAD, Worldwide Offshore Accident Databank , version 5.0.1.

2. DNV, 1999. Accident statistics for mobile offshore units on the UK continental shelf in1980-98 , HSE Offshore Technology Report OTO 2000/091 / DNV Report No. 99-2490.

3. DNV, 2002. Accident statistics for fixed offshore units on the UK Continental Shelf 1991-1999, HSE Offshore Technology Report OTO 2002/012.

4. DNV, 2002. Risk Assessment of Pipeline Protection, Recommended Practice No. DNV-

RP-F107 (amended).

5. HSE, 2006. Offshore Injury, Ill Health and Incident Statistics 2004/2005 (provisional data),HID Statistical Report HSR 2005 001.

6. HSE, 2005. Accident statistics for Floating Offshore Units on the UK Continental Shelf 1980-2003, Research Report 353, prepared by Det Norske Veritas for the Health andSafety Executive.

7. HSE, 2005. Accident statistics for Fixed Offshore Units on the UK Continental Shelf 1980 –2003, Research Report 349, prepared by Det Norske Veritas for the Health and SafetyExecutive.



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