Page 322 - Pipelines and Risers

Risk Analysis applied to Subsea Pipeline Engineering 295

Assumptions
The following assumptions are made in addition to those stated earlier in the chapter.

1. Water depth has been assumed to be 300111.
2. The probability that the hazard zone resulting from a loss of containment coincides with
the dropping vessel is assumed to be 0.5.
3. The probability of ignition has been take as 0.3.
4. It is assumed that 50% of the persons on the vessel are working at any one time.

Consequence analysis
It is assumed that all persons on the vessel are at risk, the FAR is then a function of the
proportion of persons on the vessel who are working, not of the total number of persons on
the vessel.

Risk Estimation
The number of ignited releases per working location is given by:
the
~~x~,,~~x~~~~,,x~,~~ =2~2.56.10-~~0.01&r0.5~0.3=1.23.1~’If vessel remains on location for 48
hours and has n persons on board then this would result in x fatalities, as a result of 24n hours
worked. The FAR is therefore equal to 0.51 x (1.23 xlO-’ divided by 24). This is far less
than the acceptance criteria established.

16.8.4 Societal Risk
Acceptance Criteria
The acceptance criteria is 10” deaths per year.

Initiating Incidents
Fishing Interaction
Damage frequencies due to trawl gear interaction have been extracted from the PARLOC
database. These are considered to be conservative, since the failure frequencies given in the
PARLOC report are where no failures have been experienced. This is based on a theoretical
analysis that does not take into account the robustness of the pipeline.

Merchant Vessels
Because the minimum water depth for the pipeline is approximately 275m, emergency
anchoring has not been considered. Incidents initiated by passing merchant vessels have
therefore been restricted to dropped containers and sinking vessels. The initiating incident
frequency data adopted is given in Table 16.5.