Page 136 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 5/113
done in terms of probubilib of e..;ceedunce. For instance, a the earth. Rather, they can be zones of intensely sheared
common building code requirement in the U.S. is to design fsr ground. In the Houston, Texas, area, such zones exist, measure
an earthquake event with a probability of exceedance of 10% in a few tens of feet wide, and are oriented in a horizontal direc-
50 years: tion perpendicular to the trend of the fault [86]. Evidence of
aseismic faulting includes visible damages to streets (often
Probability of exceedance = 1 - [ 1 - 1 /t.~]’ with sharp, faultlike displacements) and foundations, although
where all such damage is not the result of this phenomenon.
t = design life Aseismic faulting threatens pipe and pipe coatings because
is = return period. soil mass is moving in a manner that can produce shear, bend-
ing, and buckling stresses on the pipeline. A monitoring pro-
For example, a 10?’0 probability of exceedance in 50 years gram and stress calculations would be expected where a
equates to an annual probability of I in 475 of a certain ground pipeline is threatened by this phenomenon. The risk evaluator
motion being exceeded each year. A ground motion noted as can seek evidence that the operator is aware of the potential and
having a 10% probability ofexceedance in 50 years means that has either determined that there is no threat or is taking prudent
the level of ground motions has a low chance of being exceeded steps to protect the system.
in the next 50 years. In fact, there is a 90% chance that these
ground motions will nor be exceeded. This probability level Tsunamis
requires engineers to design structures for larger, rarer ground
motions than those expected to occur during a 50-year interval. Tsunamis are high-velocity waves, often triggered by offshore
Fault displacement is another potential threat to a pipeline. seismic events or landslides. A seiche is a similar event that
The relative displacement of the ground on opposite sides of an occurs in a deep lake [70b]. These events are of less concern in
assumed fault rupture will produce strains in a pipeline that deep water, but have the potential to cause rapid erosion and
crosses the rupture. scour in shallow areas. Most tsunamis are caused by a major
Several types offault movements are possible. Each produces abrupt displacement of the seafloor. This hazard can be evalu-
a different load scenario on the pipeline crossing the fault. ated by considering the potential for seismic events, and the
Generally, normal fault displacement leads to bending and elon- beach geometry, pipeline depth, and other site-specific factors.
gation of the pipeline (tension dominant loading), whereas Often a history ofsuch events is used to assess the threat.
reverse fault displacement leads to bending and compression of
the pipeline (compression dominant loading). Strike-slip fault Scour and erosion
displacement will either stretch or compress the pipeline
depending on the angle at which the pipeline crosses the fault. Erosion is a common threat for shallow or above-grade pipelines,
Oblique raulting is a combination of normal or reverse move- especially when near stream banks or areas subject to high-
ment combined with strike-slip movement. Oblique faulting velocity flood flows. Even buried pipelines are exposed to threats
will result in either tension-dominant loading or compression- from scour in certain situations. A potential is for the depth of
dominant loading of the pipeline depending on the pipeline’s cover to erode during flood flows, exposing the pipeline. If a lat-
fault crossing angle and the direction ofthe fault movements. eral force were sufficiently large. the pipeline could become
Fault displacement resulting in axial compression of the overstressed. Overstressing can also occur through loss of
pipeline is generally a more critical condition because it can support if the pipeline is undermined.
result in upheaval buckling. Upheaval buckling causes the At pipeline crossings where the streambed is composed of
pipeline to bend or bow in an upward direction. rock, the pipeline will often have been placed within a trench
In typical settlement‘flotation analyses, the pipeline is sub- cut into the rock. During floods at crossings where flow veloci-
jected to bending where it passes through the liquefied soil sec- ties are extremely high, the potential exists for pressure differ-
tion and the bending is maximum at the transition of liquefied ences across the top of the pipeline to raise an exposed length
and nonliquefied soil zones. When bending occurs, axial strains of pipeline into the flow, unless a concrete cap has been
are compressive in the inner fibers of the bend and tensile in the installed or the overburden is otherwise sufficient to prevent
outer fibers of the bend relative to the neutral axis of the pipeline. this. Calculations can be performed to estimate the lengths of
Calculations of maximum tensile and compressive strains pipeline that could potentially be uplifted from a rock trench
for known faults can be made and incorporated into the assess- into flows of varying velocities.
ment. Similar calculations can also be made for maximum Fairly detailed scour studies have been performed on
strains in areas of seismic-induced soil liquefaction. These cal- some pipelines. These studies can be based on procedures
culations require the use of assumptions such as maximum dis- commonly used for highway structure evaluations such as
placement, maximum slip angle, amount of pipeline cover, and “Stream Stability at Highway Structures.” A scour and bank
intensity of the seismic event. Ideally, such assumptions are stability study might involve the following steps:
also captured in the risk assessment since they indicate the
amount of conservatism in the calculations. Review the history of scour-related leaks and repairs for the
pipeline.
Aseismic faulting 0 Perform hydraulic calculations to identify crossings with
potentially excessive flood flow velocities.
Aseismic faulting refers to shearing-type ground movements that Obtain current and historic aerial photographs for each ofthe
are too small and too frequent to cause measurable earth tremors. crossings of potential concern to identify crossings that show
Aseismic faults can be of a type that are not discrete fractures in evidence of channel instability.