Page 276 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Incorrect operations index 121253
Other hydrodynamic forces (debris impact and loading, An example weighting scheme for pertinent variables and sub-
oscillations, mobile bedforms, inertia, etc.) variables is shown in Table 12.1. In this scheme, each subvari-
Sea ice scour potential. able is to be scored and then combined with scores for other
subvariables according to the following algorithm:
A full evaluation of any of these issues requires an evaluation of
many subvariables such as soil type, seismic event types, storm Potential for damaging ground movements = (erosiodsupport threats) +
conditions, cover condition, water depth, etc. So, stability (seismic movements) + (liquefaction) + (slope stability) + (loadings) +
issues generally fall into one of two types: support and load- (mitigations)
ings. For purposes of risk understanding and this risk model
design, some subcategories of stability variables can be cre-
ated. Support or stability issues are perhaps most efficiently V. Incorrect operations index
examined in four categories:
More than 80% of high-consequence offshore platform acci-
1. Fault movement dents can be attributed to human error, according to one source
2. Liquefaction movement [78]. Whereas platforms normally have a high density of com-
3. Slope stability ponents and a more complex design compared to pipelines, this
4. Erosion potential. statistic can also serve as a warning for the potential for human
5. Loadings error in pipeline operations.
As is the case for the basic risk assessment model, the incor-
These threats all impact the support condition and potentially, rect operations index score will sometimes apply to a whole
the stress level of the pipeline. They are combined to arrive at a pipeline system. Many of the human error prevention factors
relative score for stability. In algorithm form, the relationships represent a company-wide approach to work practices and
can be shown as follows: operating discipline. Only a few risk items such as MOP poten-
tial, safety systems, and SCADA are more location specific.
Potential for damaging instabilities = fifault movement; liquefaction;
slope stability: erosion, loadings} A. Design (weighting: 30%)
where
Fault movement damage potential = flfault type; slip angle; pipeline The design considerations for offshore pipelines are sometimes
angle; seismic event; pipe strength} radically different from onshore pipelines. Special design
Liquefaction damage potential = f{ seismic event; soil type; cover aspects must be included just for the installation process. From
condition; pipe strength}
Slope stability = f{slope angle; soil type; rock falls; initiating event; a human error potential, however, the same items can be scored
angle of attack; landslide potential; pipe strength) for their roles in the risk picture. Score the design items as
Erosion potential = f(current speed bottom stability; pipe strength; describedonpages 119-124.
coating strength}
Loadings = f( hydrodynamic forces; debris transport; current speed; B. Construction (weighting: 20%)
water depth]
Although the risk items to be scored here are identical to the
Most of the subvariables are also comprised of several factors. onshore model, the evaluator should consider the unique off-
For instance, bottom stability, a subvariable under the erosion shore construction challenges. Installation of the pipeline usu-
threat, can be evaluated in terms of several factors that are com- ally occurs from the water surface. The pipe is welded on the
monly found in design documents or recent inspection surveys. construction barge and lowered into the water into a pre-
excavated trench or directly on the sea bottom in a predeter-
Bottom stability = f{observed mobile bedforms; meganpples; sand mined area. Sometimes, the pipeline lying on the seabed is later
dunes; bottom current conditions} buried using pressure jetting or some other trenching tech-
nique. Handling of the pipe (which is already coated with a cor-
These, in turn, can also be further subdivided. For example, rosion-barrier coating as well as a concrete weight coating) is
critical during all phases ofthe process because certain config-
Bottom current conditions = fjspeed direction, duration, tsunami urations can overstress the coating or the pipe itself. A high
potential, tidal effects, storm conditions, river flow} amount of tensile stress is often placed on heavy pipe during
installation, even when handling is done correctly. Buoyancy
One possible mitigation to land movement threats is increased and external pressure effects (before and after filling of the
pipe strength, specifically the ability to resist external loads line) must also be considered.
considering both stress and strain issues. Other mitigation The exact placement of the pipe on the seabed is also impor-
measures include tant. The seabed will rarely be uniform. Unsupported pipe
spans are usually avoided altogether, but the pipe is often
0 Inspection type and frequency designed to safely handle some length of free span under cer-
0 Time since last inspection (linked to storms and seismic tain wave loading conditions. A surveyed route that provides a
events) correct pipeline profile is the target installation location.
Pipeline stabilization (cover condition, anchors, piles, artic- One of the challenges in the offshore environment is
ulated mattresses, various support types, etc.) the inability to directly observe the pipeline being installed.
0 Frequency of sea bottom survey. This is sometimes overcome through the use of divers, cameras,