Mechanical lifting failures - OGP

Risk Assessment Data Directory
Report No. 434 – 8
March 2010
Mechanical
lifting
failures
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

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RADD – Mechanical lifting failures
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
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RADD – Mechanical lifting failures
Abbreviations:
BOP
DNV
HSE
QRA
UKCS
WOAD
Blowout Preventer
Det Norske Veritas
(UK) Health and Safety Executive
Quantitative Risk Assessment (sometimes Analysis)
United Kingdom Continental Shelf
World Offshore Accident Databank
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RADD – Mechanical lifting failures
1.0 Introduction
1.1 Application
This datasheet presents information on the frequency of dropped objects resulting from
the 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 period
1980 to 1999. The data are intended to be applied in quantifying the risks from lifting
operations worldwide. Consideration should be given to factoring the data up or down
where there is reasonable justification that the management of lifting operations is
significantly poorer or safer that UKCS operations.
1.2 Definitions
• Dropped loads Refers to loads (objects) either unintentionally released from
a 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 lifting
devices (see below).
• Other lifting
devices
• Mobile
Installations
• Fixed
Installations
BOP cranes, gantry cranes, tuggers, and a range of portable
devices, e.g winches, sling blocks, wirelines.
The data for mobile installations are gathered almost entirely
from experience in the operation of mobile offshore drilling
units (MODUs). These include semi-submersibles, jackups,
and drill ships.
The data for fixed installations are gathered from a range of
types of production installation ranging from integrated
platforms to wellhead platforms. The data also include
experience from FPSOs (floating production, storage and
offloading 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 lifting
device (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 the
number 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 is
not 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
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RADD – Mechanical lifting failures
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”).
Load
Weight
100 te
All
Dropped Object Probabilities for Mobile Units (per lift)
Lifting
device
Installatio
n
Drop Onto:
Sea
Vessel
Total
Main crane 3.2 × 10 -5 8.8 × 10 -6 1.1 × 10 -5 4.1 × 10 -5
Drilling
Derrick
1.7 × 10 -5 7.3 × 10 -7 6.1 × 10 -8 1.8 × 10 -5
Other Device 8.6 × 10 -5 1.1 × 10 -5 0* 9.7 × 10 -5
Main crane 3.1 × 10 -6 2.0 × 10 -6 3.0 × 10 -6 5.4 × 10 -6
Drilling
Derrick
3.6 × 10 -6 4.6 × 10 -7 0* 4.0 × 10 -6
Other Device 7.6 × 10 -6 2.9 × 10 -6 0* 1.1 × 10 -5
Main crane 1.2 × 10 -5 7.1 × 10 -6 9.5 × 10 -6 2.0 × 10 -5
Drilling
Derrick
1.8 × 10 -6 0* 0* 1.8 × 10 -6
Other Device 1.9 × 10 -6 0* 0* 1.9 × 10 -6
Main crane 2.8 × 10 -4 0* 0* 2.8 × 10 -4
Drilling
Derrick
4.7 × 10 -3 1.4 × 10 -3 0* 6.1 × 10 -3
Other Device 4.9 × 10 -4 2.4 × 10 -4 0* 7.3 × 10 -4
Main crane 8.5 × 10 -6 3.3 × 10 -6 4.6 × 10 -6 1.2 × 10 -5
Drilling
Derrick
1.1 × 10 -5 6.7 × 10 -7 3.0 × 10 -8 1.1 × 10 -5
Other Device 4.5 × 10 -5 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
2
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RADD – Mechanical lifting failures
Load
Weight
100 te
All
Dropped Object Probabilities for Fixed Installations (per lift)
Lifting
device
Installatio
n
Drop Onto:
Sea
Vessel
Total
Main crane 3.8 × 10 -5 6.9 × 10 -6 1.1 × 10 -5 4.5 × 10 -5
Drilling
Derrick
1.7 × 10 -5 1.2 × 10 -7 1.2 × 10 -7 1.7 × 10 -5
Other Device 1.0 × 10 -4 4.2 × 10 -6 6.1 × 10 -7 1.0 × 10 -4
Main crane 4.7 × 10 -6 1.7 × 10 -6 5.1 × 10 -6 7.9 × 10 -6
Drilling
Derrick
2.7 × 10 -6 1.5 × 10 -7 0* 2.9 × 10 -6
Other Device 1.4 × 10 -5 0* 7.4 × 10 -7 1.5 × 10 -5
Main crane 1.0 × 10 -5 6.2 × 10 -6 1.6 × 10 -5 2.0 × 10 -5
Drilling
Derrick
1.2 × 10 -6 0* 0* 1.2 × 10 -6
Other Device 2.6 × 10 -5 0* 0* 2.6 × 10 -5
Main crane 9.3 × 10 -5 0* 0* 9.3 × 10 -5
Drilling
Derrick
0* 0* 0* 0
Other Device 6.1 × 10 -4 0* 0* 6.1 × 10 -4
Main crane 1.0 × 10 -5 2.8 × 10 -6 6.4 × 10 -6 1.5 × 10 -5
Drilling
Derrick
9.6 × 10 -6 1.2 × 10 -7 6.1 × 10 -8 9.7 × 10 -6
Other Device 5.7 × 10 -5 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 be
obtained 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 UK
continental 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. The
data for dropped objects from main cranes and other lifting devices are not applicable
to 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 similarity
between the operations and equipment under analysis and those represented by the
database.
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RADD – Mechanical lifting failures
The calculated frequencies are derived from 1637 dropped object events in a total
experience of 3063 installation-years. This implies a total of about 111 million lifting
operations. For fixed platforms there were 690 dropped objects in 1857 platform years,
for mobile installations 947 events in 1206 installation-years experience. Therefore the
statistical uncertainty in the overall frequencies is relatively small. Some of the specific
risks are calculated from the experience of a small number of representative dropped
object 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 in
Section 2.0.
The data in the database reflect lifting equipment in operation in the UKCS. While there
is a degree of variation in the equipment used in the UKCS, it is similar in that the vast
majority is maintained and operated in accordance with international certification and
UK legal requirements. Competence requirements for operations and maintenance
personnel are generally enforced, and all operations are conducted in accordance with
documented procedures reflecting good industry practice. Where operations outside
the 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 the
frequency of the initiating event for dropped object risks. The consequence of dropped
objects depends on the impact energy and the people, equipment and structures
impacted by the objects dropped.
For an object falling through air, the impact energy is calculated as the product of the
mass of the object, the height and acceleration due to gravity (≈ 10 m/s 2 ). Generally,
people struck by falling objects can be assumed to be fatally injured, and objects
striking hydrocarbon equipment will cause a hydrocarbon release. Damage to
structures or other equipment struck by dropped objects may require a specific
assessment of the resistance of the object impacted and/or the potential for a release
from live equipment struck. However, incidents involving hydrocarbon releases are
already included in the hydrocarbon release frequencies, so such an assessment is only
recommended 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 to
people from dropped objects may also be included in the statistical data on
occupational accidents. Where this is the case, it is appropriate to disregard the
calculated dropped object risk for immediate fatalities.
In the event of a dropped object, the lifting equipment will be out of service until the
incident can be investigated and any repair can be implemented. An operational risk
assessment should take account of this. Even for minor dropped objects with no
apparent damage, equipment downtime will be of the order of several days. In the event
of 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 are
not carried out over vulnerable subsea equipment. Thus care is required in assessing
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RADD – Mechanical lifting failures
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 can
be followed to calculate locations at risk from dropped object impact.
Heavy, dense objects (such as BOPs) can be assumed to fall vertically and will damage
any infrastructure immediately beneath the drop site. Some other objects, such as pipe
sections and scaffolding poles, may travel a significant horizontal distance through the
water as they descend. The following model is taken from a DNV Recommended
Practice [4].
The analysis assumes that the excursion made by a dropped object can be represented
by a normal distribution:
where x is the horizontal excursion and δ the standard deviation. The standard
deviation is sensitive to the weight and shape of the object, and the water depth (d). The
derivation of δ is given by:
Here α is the spread in the descent angle given in Table 3.1.
Case
Object Shape
Description
Table 3.1 Calculation of Descent Angles
Weight
(tonnes)
Descent
Angle
Spread
(deg)
1 < 2 15
2 Flat/long shaped
2 – 8 9
3
> 8 5
4 < 2 10
5 Box/round shaped
2 – 8 5
6
> 8 3
7 Box/round shaped >> 8 2
The probability that the object lands within a horizontal distance, r, of the drop point is
given 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 depth
spreading 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 horizontal
section(s)), and
2. Calculate the hit probability of these sections at the respective water depths. The
final probability is then found as the sum of all the probabilities for the different
sections.
The effect of currents will become more pronounced in deep water. The time for an
object to hit the seabed will increase as the depth increases. This means that any
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RADD – Mechanical lifting failures
current may increase the excursion (in one direction). At 1000 m depth the excursion is
found to increase 10 to 25 metres for an average current velocity of 0.25 m/s and up to
200 m for a current of 1.0 m/s.
The effect of currents may be included if one dominant current direction can be
identified. 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 current
directions may be difficult to incorporate.
When establishing a "safe distance" away from activities the effect of currents should
be 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/s 2 )
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 gravitation
forces, 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 by
the following equation:
where: m = mass of the object (kg)
g = gravitation acceleration (9.81 m/s 2 )
V = volume of the object (the volume of the displaced water) (m 3 )
ρ water = density of water (typically 1025 kg/m 3 for the North Sea)
C D = drag coefficient of the object
A = projected area of the object in the flow-direction (m 2 )
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:
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RADD – Mechanical lifting failures
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 become
significant for large volume objects such as containers. The effective impact energy
becomes:
where m a is the added mass (kg).
Tubulars are assumed to be waterfilled unless it is documented that the closure is
sufficiently effective during the initial impact with the surface, and that it will continue to
stay 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 has
influence as the object hit something and is brought to a stop. Table 3.2 gives typical
values of these coefficients.
Table 3.2 Drag and Added Mass Coefficients
Object Case Description C d C m
(as Table 3.1)
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 to
complex)
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 a
revised drag coefficient should be evaluated.
Small equipment items (fittings, scaffolding clamps, etc.) are unlikely to do any damage
to 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] and
the 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 observed
data collected for DNV by a drilling contractor. The number of lifts per year on fixed
installations (Table 4.2) are estimated by interpretation of the data on mobile
installations combined with reasonable assumptions and consequently should be
treated with more caution. The numbers of “installation years” represented by the
ORION 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, baskets
and tanks; the remainder were casing, drillpipe, collars, etc.
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RADD – Mechanical lifting failures
Table 4.1 Observed Frequencies of Lifting Operations on Mobile
Installations
Lifting Device
Lifts per Year
Main Crane 24,480
Drilling Derrick 28,670
Other Lifting Device 3,650
Total 56,800
Table 4.2 Calculated Frequencies of Lifts using Main Crane on Fixed
Installations (per year)
Type of installation
Lifts to / from
Vessels
Internal Lifts
Fixed (no drilling) 5520 8,674
Fixed (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 detailed
statistical analysis as the data presented in this report and for this reason the more
recent experience is not included here. However a review of the data over the period
1980 to 2005 shows that although there is considerable variation from year to year, the
average frequency of dropped objects per installation-year remains approximately
constant. This is consistent with the observation that the technology and lifting
procedures 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 in
1980-98, 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 and
Safety 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 Safety
Executive.
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©OGP

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