Page 273 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

12/250 Offshore Pipeline Systems
A. Safety factor (weighting: 25%) and fracture mechanics in such dynamic, fatigue-inducing
environments. Higher fracture toughness materials might even
The safety factor is a risk “credit” for extra pipe wall thickness be warranted.
when this thickness is available for protection against impacts, Scoring the potential for this type of fatigue requires evaluat-
corrosion, and other integrity threats. Required wall thickness ing the potential for spans to exist and for water-current condi-
must account for all anticipated internal and external loadings. tions to be of sufficient magnitude. Because both of these
Wall thickness in excess of this requirement is a risk ‘credit.’ factors are covered in an evaluation of land movements (Le,,
From a cost of material and installation viewpoint, higher stability) (see page 1 10). wave-induced fatigue potential is also
strength materials are often attractive. This is especially true in at least partially addressed in that variable.
the challenging offshore environment. However, special weld- Score fatigue potential as described in Chapter 5, with the
ing considerations and strict quality control are often needed in additional considerations discussed here.
the higher strength materials. Other desirable material proper-
ties such as ductility are sometimes sacrificed for the higher C. Surge potential (weighting: loo/)
strength. Pipe installation procedures (techniques such as S-lay,
J-lay, etc.) are another consideration. Anticipated stresses on Score this item as detailed on pages 104-105 and also see
the pipe during installation may be higher than operational Appendix D.
stresses. The evaluator should seek evidence that installation
stresses and potential for pipe damage during construction have D. Integrity verifications (weighting: 25%)
been adequately addressed.
Offshore pipelines can have a high external loading due to This variable normally includes an evaluation of pressure test-
water pressure. This leads to increased chances of collapse ing and in-line inspection (ILI) as methods to verify system
from external forcebuckle. Calculations can be done to esti- integrity. The considerations to the offshore environment are
mate buckle initiation and buckle propagation pressures. It is the same but can also include inspection by side-scan sonar,
usually appropriate to evaluate buckle potential when the ROC: or diver inspection, for partial assurances of integrity
pipeline is depressured and thereby most susceptible to a uni- (‘partial’ since visual inspections should not generate the same
formly applied external force. This is the worst-case scenario level of confidence as more robust integrity verifications).
and reasonable since a depressured state is certainly plausible if Score this variable as described on pages 105-1 10.
not routine. In cases of larger diameter, thin-walled pipe,
buckle arrestors are sometimes used to prevent propagation of E. Stability (weighting: 25%)
buckle. Buoyancy effects must also be considered in the load-
ing scenario. If the weight coating is partially lost for any rea- The interaction between the pipeline and the seabed will fre-
son, the pipe must be able to withstand the new stress situation quently set the stage for external loadings. If a previously
including possible negative buoyancy. buried line is uncovered because of scour or erosion of the
Additional considerations for the offshore environment seabed, it becomes exposed to current loadings and impact
might include hydrodynamic forces (inertia, oscillations, lat- loadings from floating debris and material being moved along
eral forces, debris loadings, etc.) caused by water movements the seabed. Upon hrther scour or erosion, the pipeline can
and an often higher potential for pipe spans and/or partial become an unsupported span. As such, it is subjected to addi-
support scenarios. tional stresses due to gravity and wave/current action. If
With these considerations, variable can be assessed as stresses become severe enough, possible consequences include
described on pages 94-102. damage to coatings and buckling or rupture of the pipe. On a
longer term basis, cycling and fatigue loadings may eventually
B. Fatigue (weighting: 15%) weaken the pipe to the point of yield. Fatigue and overstressing
are amplified by larger span lengths. Such fatigue loadings can
As a very common cause of material failure, fatigue should be be caused by movements of a free-spanning pipeline which,
considered as part of any risk analysis. Fatigue, as discussed on given the right conditions, will reach a natural frequency of
pages 102-1 04, should therefore become a part of the offshore oscillations as previously discussed.
pipeline evaluation. In addition to fatigue initiators discussed in Changes in bottom conditions also impact corrosion poten-
Chapter 5, an additional fatigue phenomenon is seen in tial. As pipelines move from covered to uncovered states, the
submerged pipelines. A free-spanning (unsupported) length of galvanic corrosion cell changes as the electrolyte changes from
pipe exposed to current flows can oscillate as vortex shedding soil to seawater and back. CP currents must be sufficient for
creates alternating zones of high and low pressure. The extent either electrolytic condition.
of the oscillations depends on many factors including pipe The presence of “high-energy” areas, evidenced by condi-
diameter and weight, current velocity, seabed velocity, and tions such as strong currents and tides, is a prime indication of
span length. The pipeline will tend to move in certain patterns instability. Sometimes, seabed morphology is constantly
of amplitude and speed according to its natural frequency. Such changing due to naturally occurring conditions (waves, cur-
movements cause a fatigue loading on the pipe. rents, soil types, etc.). The wave zones and high steady current
There is evidence that fatigue loading conditions may be environments promote scour and vortex shedding. At other
more critical than once thought, including “ripple loading” times, the pipeline itself causes seabed changes because of the
phenomena where relatively small amplitude load perturba- current obstruction that has been introduced into the system.
tions (ripple loads) cause fracture at lower stress intensity lev- Periodic bottom-condition surveys and installation of span-
els. This in turn requires more emphasis on crack propagation correcting measures are common threat-reducing measures.