Page 268 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Background 121245
with personnel performing platform activities or working on (prevent flotation) and to protect the corrosion coating. This
other pipelines. Anchoring and dropped objects are examples concrete coating provides a measure of protection against
of damage causes related to nearby work activities. Even impacts and can be considered as a type of cover protection and
though the offending personnel may be employed by the scored as suggested.
owner/operator company and hence not be ‘third-party dam-
age’ technically, this threat may be more efficiently addressed B. Activity level (weighting: 25%)
in this index.
Although not the cause of the majority of offshore pipeline In this variable, the evaluator assesses the probability of poten-
accidents, third-party damages appear to the cause of most of the tially damaging activities occurring near the pipeline. For sim-
deaths, injuries, damages, and pollution [71]. Consequently, this plicity and consistency, a list of activities or conditions can be
is a critical aspect of the risk picture. generated to guide the assessment. Indications of high activity
levels may include high vessel traffic, high density of other off-
A. Depth of cover (weighting: 20%) shore structures (including other pipelines), and shoreline
development activities. Any of these might increase the oppor-
Cover, as a means to reduce third-party damages, actually has tunity for pipeline damage. More specific activities that could
two components in most offshore cases: water cover (depth) be assessed include fishing, dredging, anchoring, construction,
and sea bottom burial depth. Each can provide a measure of platform activities, excavation, underwater detonations, diving,
protection from third-party damage since increasing water salvage operations, and recreational boat traffic.
depth usually limits the number of activities that could be Potential damage depends on characteristics of the striking
harmfd to the pipeline, and sea bottom cover provides a physi- object. Force, contact area, angle of attack, velocity, momen-
cal barrier against damage. When depth is sufficient to pre- tum, and rate of loading are among these characteristics.
clude anchoring, dredging, fishing, and other third-party Potential consequences include damages to coating, weights,
activities as possible damage sources, failure potential is anodes, and pipe walls, possibly leading to rupture immediately
reduced. When a pipeline poses a known threat to navigation, or after some other contributing event.
there is effectively no cover and the threat of impact is usually To better estimate possible loadings that could be placed on
high. Note that submerged pipelines also have a threat of dam- the pipeline, fishing and anchoring can be assessed based on
age from dropped objects (see discussion of activity level next), the types of vessels, engine power, and type of anchors or fish-
which is minimized by protective barriers. ing equipment. Although anchoring is usually forbidden
Accurate knowledge of the amount of cover is sometimes directly over a pipeline, the setting of an anchor is imprecise.
difficult to obtain. Profile surveys are necessary to monitor Anchoring areas near the pipeline should be considered to be
constantly changing seabeds. The frequency of surveys should threats. Fishing equipment and anchors that dig deep into the
be dependent on water conditions such as wave and current sea bottom or which can concentrate stress loadings (high force
action, and on seabed and bank stability, as is evidenced by his- and sharp protrusions) present greater threats. Analyzing the
torical observation. In scoring the depth of cover, the evaluator nature of the threat will allow distinctions to be made involving
must also judge the uncertainty of the knowledge. This uncer- types of anchored vessels or certain fishing techniques. Such
tainty is dependent on the timing and accuracy of survey data. distinctions, however, may not be necessary for a simple risk
See the design index (Chapter 5) for a further discussion of model that uses conservative assumptions.
survey techniques. As another threat from third-party activities, dropped objects
Especially susceptible areas for damage are shore can strike the pipeline with sufficient force to cause damage.
approaches and, to a lesser degree, platform approaches. A Objects can be dropped from some surface activity (construc-
common practice is to protect the pipelines by trenching to a tion, fishing, platform operations, mooring close to platforms,
depth of 3 ft out to a distance of 200 to 500 ft from a platform. cargo shipping, pleasure boating, etc.) and, depending on con-
However, shore approach protection is inconsistent. Shore ditions such as the object’s weight in water, its shape, and water
approaches are often the most hazardous section ofthe offshore currents the object will reach a terminal velocity. The impact
pipeline. Long-term seabed stability is best when the shoreline stress on the pipe is partly dependent on this velocity.
is minimally disrupted. Use of riprap, twin jetties, directional Shore approaches and harbors are often areas ofhigher activ-
dnlling, dredging, and backfilling are common techniques ities. Beach activities, shoreline construction, and higher vessel
used near shorelines. In many modem installations, a shore traffic all contribute to the threat in an often unstable sea bot-
approach is directionally drilled and placed well below any tom area.
depth where normal activities or wave actions can affect the External overpressure can occur from subsea detonations.
pipeline. The historical performance of a certain technique in a An example is the common practice of clearing structural ele-
certain environment would be of value in future design efforts ments from abandoned platforms down to 15 A below the mud-
and in assessing the stability of the cover. line by detonating an explosive charge inside each of the hollow
Other types of barrier protection can serve the same purpose supporting members that penetrate the sea bottom (platform
as depth of cover, and should be scored based on their effective- legs and well conductors). Possible unintended damage to
ness in preventing third-party damages. Certain barriers may nearby structures can result from the shock wave, specific
also receive risk mitigation credit in reducing the threat from impulse, and energy flux density associated with the event.
floating debris and current forces (see design index discus- The evaluator can create qualitative classifications by which
sion). Examples of barriers include rock cover, concrete struc- the activity level can be scored. In concert with the categories
tures, and metal cages. Many offshore pipelines will have a shown in Chapter 3, a classification guide specifically for
‘weight coating’ such as concrete to ensure negative buoyancy offshore lines could be similar to the following: