Page 275 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

12/252 Offshore Pipeline Systems
Where movements of icebergs, ice keels, and ice islands are a recorded. The line was reburied using waterjetting 8 years ago.
threat, well-defined programs of monitoring and recording ice With the strong inspection program and a history of corrective
events can be awarded points, based on the program’s effective- actions being taken, the evaluator adjusts the score upward to
ness in reducing pipeline risk. show less threat. This yields a score for the stability variable
Scores should also be awarded based on timeliness of detec- approximately equivalent to a “low” potential for damages due
tion. Frequency of surveying should be based on historical to stability problems.
seabed and bank stability, wave and current action, and per-
haps risk factors of the pipeline section. The evaluator can
review the basis for survey frequency-ideally, a written Alternative scoring approach
report with backup documentation justifying the frequency-
to determine if adequate attention has been given to the issue One of the largest differences between the risk assessments for
of timeliness. offshore and onshore environments appears in this variable of
stability. This reflects the very dynamic nature of most offshore
Continuous monitoring environments even under normal conditions and more so with
storm events. Instead of evaluating this potential failure mecha-
This implies the existence of devices that will alert an operator nism using the general, qualitative categories of threat dis-
of a significant change in stability conditions. Such devices cussed above, the evaluator might choose to use many
might be direct indicators, such as strain gauges on the pipe subvariables that can be independently assessed and then com-
wall itself, or indirect indicators, such as seabed or current bined into a stability score.
monitors. In the case of indirect indicators, some follow-up Support and stability issues consider potentially damaging
inspection would be warranted. The advantage of continuous ground or water effects, primarily from a support andor fatigue-
monitoring is, of course, that corrective actions can be applied loading viewpoint, and conservatively assume that increased
immediately after the event-the uncertainty of scheduling sur- instability of sea bottom conditions leads to increased potential
veys is removed. The evaluator should award maximum credit for pipeline over-stressing and failure. Subsurface features that
for mitigation only if the monitoring is extensive enough to reli- might indicate future instabilities are considered as part of the
ably detect all damaging or potentially damaging conditions. threat assessment. A segment of pipe is “penalized when poten-
tially damaging conditions are present, and then “rewarded” as
Stress relieving mitigating actions or design considerations are employed.
However, in keeping with the overall philosophy ofthe suggested
Corrective actions, as a follow-up to monitoring, include pipe risk assessment, the sum of all mitigating actions should never
burial (or reburial), use of protective coverings, and the place- completely erase the penalty due to the conditions. Only the
ment of support under a free-spanning pipe. These can be absence of any “potentially damaging conditions” results in the
considered to be stress relieving actions since they are designed lowest risk.
to ‘unload’ the pipe, reducing the stress levels. This is often For new or proposed construction, the more threatening
accomplished by using concrete mattresses, grout bags, areas along the pipeline route are normally identified in the
mechanical supports, antiscour mats, rock dumping, etc., to preconstruction phase design studies. Identified threats are
offset natural forces would otherwise add stresses to the usually fully addressed in the design process and that process is
pipeline. in fact a risk management process. Therefore, the risk assess-
Maximum credit can be awarded when the stress relieving is ment of a new pipeline will generally reflect the mitigated
a proactive action or a design feature specifically put in place to threat. However, as evidence of past instabilities and/or indica-
mitigate the effects on a possible instability. An example would tions of possible future instabilities, the potentially damaging
be supports beneath a pipeline where scour-induced free spans themselves can still be captured in the assessment, regardless of
are a possibility but have not yet occurred. Another example is mitigation measures designed to offset their presence.
the excavation of a trench to prevent transmittal of soil move- In general, situations or variables that contribute to a higher
ment forces onto the pipeline (perhaps only temporarily). threat include regions of potential instability as indicated by
Points are awarded when actions have been taken to substan-
tially reduce the possibility of damages due to soil, ice, seismic, Slope
or water forces. Sand ripples and waves
Nearby depressions/slumping potential
Example 12. I: Offore earth movements Liquefaction potential
Highest water current actions
An offshore pipeline makes landfall in a sandy bay. The line Scour, erosion, or washout potential
was originally installed by trenching. While wave action is Known or suspected seismic activity or faults
slight, tidal action has gradually uncovered portions of the line Mobile bedfonns.
and left other portions with minimal cover. With no weight cov-
ering, calculations show that negative buoyancy (floating) is Loading and potential over-stressing situations more unique to
possible if more than about 20 ft of pipe is uncovered. The the offshore environment include
potential for stability problems is therefore scored as somewhat
worse than the “medium” potential classification. This shore Pipe buckling potential (including both initiation and propa-
approach is visually inspected at low-tide conditions at least gation points)
weekly. Measurements are taken and observations are formally Current forces (steady current, storm currents, etc.)