Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 682

these hazards are well known. In other cases, hazards can be identifi ed
through formal methods that include hazard and operability studies, failure
mode and effect analysis, fault tree analysis, and event tree analysis. Each
of these methods has advantages and disadvantages associated with the
ability to identify all relevant hazards, the level of complexity, and the nec-
essary input data requirements. In many cases, a simple method that is well
performed and meets the objectives and scope of the risk analysis can
provide better results than that from a more sophisticated analysis. Ordinar-
ily, the effort put into the risk analysis should be consistent with the poten-
tial level of risk being assessed. For seismic risk assessment, earthquake
hazards are directly linked to a particular earthquake size, location, and
style of faulting. They are either a direct hazard such as ground shaking and
surface fault displacement or an indirect hazard such as triggered slope
movement, liquefaction, lateral spread displacement, and post-cyclic con-
solidation settlement.
The third step is to estimate the level of pipeline vulnerability for each
hazard event based upon an analytical assessment of the amount of stress
or strain developed in the pipeline. A distinction is typically made between
modest damage modes, such as leaks or holes in the pipe wall, and more
catastrophic damage modes, such as a full line break. Determining the
potential for unacceptable pipeline performance requires relating the likeli-
hood of an earthquake to the severity of the earthquake hazard and then
relating the severity of the earthquake hazard to pipeline response. There
are several sources of signifi cant uncertainty in determining the potential
for earthquake induced pipeline damage:

• estimated earthquake-induced ground displacements which are related

to:
• earthquake recurrence rate,
• earthquake location,
• triggering of indirect earthquake hazards;

• subsurface soil characteristics for determining soil restraint on buried

pipelines;

• pipeline strains produced by ground displacements;

• pipeline strain capacity for a particular performance level (e.g., contin-

ued operation and pressure integrity).

A qualitative comparison of the relative uncertainty in each of these param-
eters is illustrated in Fig. 25.1.
The fourth step in the risk assessment process involves estimating the
likelihood of various consequences of pipeline damage. For natural gas
pipelines, the effects of gas release typically include toxicity of released
materials, such as hydrogen sulfi de, thermal radiation due to ignited gas jets
emanating from small holes or tears in the pipe wall, and external pressure