Page 274 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Design index 121251
Span correction techniques include concrete mattresses, grout severe enough to cause oscillations or impact loads from
bags, mechanical supports, antiscour mats, and rock dumping. floating or rolling debris. Rare occurrence events have a high
Different techniques are found to be effective in different probability of damage if they should occur. This includes
regions. Some stabilization using the above methods is often hurricanes, severe storms, and rare ice movements.
done as part of initial construction. Low
Naturally occurring external forces may need to be more Evidence of soil movements or unsupported spanning is rare.
fully investigated in the offshore environment. Uncertainty is The area is stable in terms of potentially damaging events
usually high. Often bottom conditions such as current and and/or the pipeline is so well isolated from such events as to
seabed morphology must be estimated from more available sur- make the potential almost nonexistent. Rigid pipes may fall
face wind- and wave-induced current models. Even when more into this category even if the potential threat is seen as
definitive surveys are done, actual conditions can often vary “none.”
dramatically over time. This plays a critical role in the stress sit- None
uation ofthe pipeline. No evidence of any potentially threatening soil, ice, earth, or
Floating debris and material being moved along the seabed water event is found.
are potential sources of damage to an exposed pipeline. Such Seabed profile surveys are a powerful method to gauge the sta-
external forces can damage coatings, both concrete and anti- bility of an area. (The effectiveness of the survey technique
corrosion types, and even damage the pipe steel with dents, should be considered as discussed below.) When surveys are
gouges, or punctures. unavailable and anecdotal evidence (personal observations
Special considerations for instability events also include over the years) is minimal, the evaluator may score the area as
hurricanes, tsunamis, and associated storm-related damages to relatively unstable in order to reflect the uncertainty ofthe situ-
platforms, changes in bottom topography, temporary currents, ation. Of course, previous episodes of pipeline damages are a
tidal effects, and iceipermafrost challenges. Potential damages very strong indicator of potential.
can be caused by the presence and movements of ice including To the above scores, ‘credits’ can be awarded if actions such
ice scour (ice gouging), subscour soil deformation (even when as the following are taken to reduce the potential damage:
the pipeline is below the maximum scour depth, a danger
exists), icebergs, ice keels of pressure ridges, and ice islands. 0 Regular monitoring and corrective actions, if needed,
Note that there can be extensive differences in the presence of are done at least annually and in accordance with a well-
icebergs in a given region from season to season [71]. designed survey program.
The stubilig variable can be scored as detailed on pages 0 Continuous monitoring and corrective actions are taken.
110-1 15 with the additional considerations noted for offshore Stress relieving.
conditions. Points are awarded based on the potential for dam-
aging stability events and mitigating measures. Potential can be Note that the use ofmitigating measures should not increase the
scored as high, medium, low, or none, as discussed next. point score to the highest level-the level at which no threat
Interpolation between these categories is appropriate and, as exists (20 points). This is in keeping with the philosophy used
always, higher uncertainty should cause the risk model to show throughout this book. Note also that credit for extra strong pipe
higher risk. They can be scored as follows: to withstand instability events is awarded in the safety factor
item and should not earn credit here.
High
Any of the following conditions is sufficient to score the poten- Regular monitoring
tial as high: Areas where damaging soil movements and/or
water effects are common or can be quite severe; where a Monitoring is achieved by a variety of survey methods for sub-
high-energy water zone-wave-induced currents, steady sea pipelines. As an indirect preventive measure, an accurate
currents, scouring-is causing continuous, significant survey will alert the operator to pipe sections more susceptible
seabed morphology changes; where unsupported pipeline to external damage. Regular, appropriately scheduled surveys
spans are present and changing relatively quickly; where that yield verifiable information on pipeline location, depth of
water current action is sufficient to cause oscillations on cover, and water depth should score the most points. Common
free-spanning pipelines-fatigue loading potential is high- survey techniques range from hands-on, where the divers use
or impacts of floating or rolling materials; regular fault their hands and probing rods to locate and record pipe location,
movements, landslides, subsidence, creep, or other earth to the use of manned or unmanned subsea vehicles (ROVs), to
movements are seen; ice movements are common and poten- sophisticated instrumented surveys (sonar and/or signals
tially damaging; the pipeline is or can easily be exposed to impressed onto the pipe) that measure both seabed profiles
any of these conditions. Rigid pipelines, under less severe and pipeline profiles. Side-scan sonar is one such instru-
conditions should be included in this high potential category, mented survey that can detect free spans, debris on or near the
because of their diminished capacity to withstand certain pipeline, and seabed marks caused by anchors, fishing equip-
external stresses. ment, etc., and in general record the position of the pipeline.
Medium The evaluator should award points partly based on the reliabil-
Damaging soil movements are possible but unlikely to rou- ity and accuracy of the technique. Repeatability-where mul-
tinely affect the pipeline due to its depth or position. tiple surveys of the same area with the same technique yield
Unsupported pipeline spans might exist, but are relatively the same result-is often a good indicator of the usefulness of
stable. Water energy is sometimes (but not continuously) the technique.