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Service Interruption
1. Background Note that ensuring an uninterruptible supply often conflicts
with ensuring a failure-proof system. The conflicts occur when
A service interruption is defined here as a deviation from prod- erroneous valve closures or equipment failures cannot be toler-
uct andor delivery specifications for a sufficient duration to ated and steps are taken to make shutdowns more difficult. In so
cause an impact on a customer. The definition implies the exis- doing, necessary, desirable shutdowns are also made more dif-
tence of a specification (an agreement as to what and how ficult. This often presents a desigdphilosophy challenge, espe-
delivery is to occur), a time variable (duration of the deviation), cially when dealing with pipeline sections close to the customer
and a customer. These will be discussed in more detail later. where reaction times are minimal.
Terms and phrases such as speczjkation violations, excursions, This module is a parallel version of the overall risk assess-
violations of delivery parameters, upsets, specification non- ment methodology. In fact, the basic risk assessment model is a
compliances, and offspec will be used interchangeably with part of the risk of service interruption, Because a pipeline fail-
service interruption. ure as modeled by this technique almost certainly leads to a
Assessing the risk of service interruption is more compli- service interruption, the probability ofpipeline failure itself is a
cated than assessing the risk of pipeline failure. This is because component of the risk of service interruption. Added to this
pipeline failure is only one of the ways in which a service inter- potential is the potential for events that cause a service inter-
ruption can occur. Service interruptions also have a time vari- ruption but do not cause a pipeline failure. Therefore, the sam-
able not present in the risk of pipeline failure. An event may or ple point scale for the potential of service interruption (the
may not lead to a service interruption depending on how long equivalent of the index sum, MOO points, in the basic risk
the event lasts. assessment model) is 540 points. An additional 140 points of
