Page 138 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 511 15
used to monitor the movements of the pipeline, but must be ing the acts of monitoring, site evaluations, or other informa-
placed to detect the areas of greatest pipe strain (largest deflec- tion gathering. Monitoring implies that corrective actions are
tions). This requires knowledge of the most sensitive areas of taken as needed. Continuous monitoring offers the benefit of
the pipe wall and the most likely movement scenarios. Use of immediate indication of potential problems and should proba-
these gauges provides a direct measure of pipeline strain that bly reflect lowered risk compared with occasional monitoring.
can be used to calculate increased stress levels. Continuous monitoring can be accomplished by transmitting a
Corrective actions can be sometimes performed to the point signal from a soil movement indicator or from strain gauges
where the potential for significant movements is “none”. placed on the pipeline. Proper interpretation of and response to
Examples include dewatering of the soil using surface and sub- these signals is implied in awarding the point values. Periodic
surface drainage systems and permanently moving the surveys are also commonly used to detect movements.
pipeline. While changing the moisture content of the soil does However, surveying cannot be relied on to detect sudden move-
indeed change the soil movement picture, the evaluator should ments in a timely fashion.
assure herself that the potential has in fact been eliminated and In the case of landslide potential, especially a slow-acting
not merely reduced, before she assigns the “none” classifica- movement, stress relieving is apotential situation-specific rem-
tion. Moving the pipeline includes burial at a depth below the edy and can be accomplished by opening a trench parallel to or
movement depth (determined by geotechnical study; usually over the pipeline. This effectively unloads the line from soil
applies to slope movements), moving the line out of the area movement pressures that may have been applied. Another
where the potential exists, and placing the line aboveground method is to excavate the pipeline and leave it aboveground.
(may not be effective if the pipe supports are subject to soil Either ofthese is normally only a short-term solution. Installing
movement damage). the pipeline aboveground on supports can be a permanent solu-
Earthquake monitoring systems tell the user when and where tion, but as already pointed out. may not be a good solution if
an earthquake has occurred and what its magnitude is often the supports are susceptible to soil movement damage. The use
only moments from the time of occurrence. This is very useful of barriers to prevent landslide damage, for example, can also
information because areas that are likely to be damaged can be be scored as stress relieving.
immediately investigated. Specific pipeline designs to with-
stand seismic loadings is another mitigation measure.
Scour and erosion threats can be reduced through armoring Example 5.9: Scoringpotentinl for earth movements
of the pipeline and/or reducing the potential through diver-
sions or stabilizations. These can range from placements of In the section being evaluated, a brine pipeline traverses a
gravel or sandbags over the pipeline to installations of full relatively unstable slope. There is substantial evidence of slow
scale river diversion or sediment deposition structures to deep downslope movements along this slope although sudden,
pipeline installation via horizontal directional drill. The eval- severe movements have not been observed. The line is thor-
uator must evaluate such mitigations carefully, given the rela- oughly surveyed annually, with special attention paid to poten-
tively high rate of failure of scour and erosion prevention tial movements. The evaluator scores the hazard as somewhere
schemes. between “high” and “medium” because potentially damaging
Where a land movement potential exists and the operator has movements can occur but have not yet been seen. This equates
taken steps to reduce the threat, point values may be adjusted by to a point score of 3 points. The annual monitoring increases the
judging the effectiveness of threat-mitigation actions. includ- point score by 3 points, so the final score is 6 points.