Page 314 - Pipelines and Risers

Risk Analysis applied to Subsea Pipeline Engineering 287

Further quantification of human reliability has been comborated for a number of tasks
relating specifically to structural design, the necessary information is investigated by Bea
(1994).

-1
new or rarely performed task
extreme stress, very little time
severe distractions & impairments
- 10'
highly complex task
considerable stress, little time
moderate distractions &
- 10"
complex or unfamiliar task
moderate stress, moderate time
little distractions & impairments
- IO-'
difficult but familiar task
little stress, sufficient time
very little distractions or
- 104
simple, frequently, skilled task
no stress, no time limits
no diswaction or impairments
- 105

Figure 163 Human Error Rates.

16.6 Causes of Risks

16.6.1 General
This section will outline. some common causes for the four different risk scenarios that were
outlined in the introduction.

16.6.2 1'' Party Individual Risk
The scope of this type of risk is limited to a consideration of the potential for ignited releases
as a result of dropped object impact associated with maintenance/workover activities taking
place after commissioning or random failure of the pipeline (discussed in next section).

The sources of the potential dropped objects are assumed to be the vessels employed for
maintenance/workover. The assumptions made in order to determine the probability of loss of
containment is as follows:

Objects are assumed to fall in a 30' cone centered at a point directly above the pipeline;
Objects are assumed to fall with equal probability at any point within the circle on the
seabed defined by the drop cone. It is assumed that all dropped objects enter the sea,
rather than landing on part of the vessel.
The probability that the hazard zone, resulting from a loss of containment, coincides with
the dropping vessel, is assumed to be 0.5.