Page 315 - Pipelines and Risers

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Details of such operations are unlikely to be known during design, thus judgements are often
required (based on previous experience) and the analysis updated later. During design this
analysis necessary since decisions about protective requirements need to be considered.

16.63 Societal, Environmental and Material Loss Risk
Risks associated with construction, installation and commissioning of the pipeline do not
impact on members of the general public. Only incidents that occur during the operation of
the pipeline are therefore considered to be initiating with respect to Societal Risk.

The hazards giving rise to societal risks will also contribute to the environmental and material
loss risks. These hazards include the following:

1. Fishing Interaction
Movement of fishing vessels around the location of subsea pipelines pose a risk. The
frequency of such an event can be derived from existing databases (PARLOC).
2. Merchant Vessels
Incidents caused by passing merchant ships include emergency anchoring, dropped containers
and sinking ships. Databases can again be used to determine the density of merchant vessels
and the probability of the above incidents occurring.
3. Construction Vessels
Loss of containment incident frequencies as a result of construction vessel activities may be
estimated based on databases. However, while it is accepted that construction activities
contribute to the overall loss of containment frequency for pipelines, it is not considered to be
appropriate to treat such incidents as initiating for Societal Risk calculations. This is because
the presence of construction vessels will in itself exclude the presence of merchant shipping.
4. Random Failures
This may be due to any material failure of the pipeline and can usually be determined using
reliability analysis.

16.7 Consequence Analysis

16.7.1 Consequence Modeling
The consequence model attempts to model the sequence of events that occur after a failure
event. The sequence for consequence Modeling is shown in Figure 16.4. It should be noted
that this method of consequence Modeling is only suitable for failures relating to the pipeline
releasing some type of fluid or gas. The following steps for the Modeling of a release event
gives only a general outline of the sequence of events that ultimately leads to a calculation of
the various losses. Many different models exist for modeling these release characteristics
(from simple to sophisticatedcomplex). However, there has not been extensive
researchtexperimentation into Modeling of subsea releases so generally there is a high degree
of uncertainty in this Modeling and conservatism is often used. One specific suite of computer
Modeling programs available is the HGSystem written by Thomton Research Center.