Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 678

than steel. This scope is similar to that defi ned for liquid hydrocarbon and
other pipelines in ASME B31.8 (ASME, 2007), with the only difference
being the extension to the fi rst pipeline connection within a facility in the
scope of ASME B31.8.

25.2 Purpose of performing a risk assessment

Risk is formally defi ned as the product of the probability of an event occur-
ring and the consequences of the event. This defi nition implies that risk be
stated in a quantitative manner. For example, a 10% chance per year of an
event causing the death of one person has a risk of 0.1 deaths per year. In
the context of this chapter, a distinction is made between risk assessment
and risk management or, as commonly referred to in the oil and gas pipeline
industry, integrity management. The term risk assessment is appropriate for
quantitative estimates of both the potential for damage and potential con-
sequences. The basis for determining the quantitative value of risk is typi-
cally highly dependent upon qualitative descriptions that can be translated
into numerical values. For example, the likelihood of earthquake-triggered
slope movement suffi cient to cause pipeline failure given a peak ground
acceleration of 0.4 g might be ranked as low, moderate, or high and subse-
quently assigned numerical values of 5%, 50%, and 85% for use in a risk
calculation.
The goal of any risk assessment is to provide information to answer some
specifi c questions that necessarily includes a scenario, a performance metric,
and an acceptance criterion. Examples of questions with reliability sce-
narios include the following:

• What is the annual probability that earthquake damage (scenario) will

interrupt service to customer XYZ (performance metric) for more than
24 hours (acceptance criterion)?

• What is the probability that costs (performance metric) to repair earth-

quake damage (scenario) will exceed $2 000 000 over the next 15 years
(acceptance criterion)?

Other categories of system performance metrics and acceptance criteria are
provided in Table 25.1.
As of 2012, there are very few regulations for oil and gas pipelines that
explicitly defi ne risk acceptance criteria and require a quantitative seismic
risk assessment to demonstrate compliance. Integrity management regula-
tions in North America (PHMSA, 2011) are directed at long-term measure-
ments of performance and gradual system upgrades and do not specify
specifi c performance requirements. Some local agencies in the United
States rely upon risk assessments for land use planning (e.g., County of
Santa Barbara, 2000). Annex O of the Canadian pipeline code (CSA, 2007)