Mechanical lifting failures - OGP

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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