Root-Cause Analysis of Offshore Pipeline Failures
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This paper evaluates potential causes of failure for nine pipelines operating in shallow waters (8 to 14 m) in the Gulf of Guinea. The authors develop an analytical method to identify root causes and provide recommendations for pipeline design and placement.
Once the subject pipelines had been identified, field visits and failed-section analyses were conducted. Pipeline properties and conditions are provided in Tables 1 through 5 of the complete paper. Mechanical-failure, hydrodynamic, and third-party-interference assessments were performed. Design-stability assessment was reviewed and gaps identified. A further stability assessment of the pipelines was performed in accordance with Det Norske Veritas-Germanischer Lloyd Recommended Practice (DNVGL-RP) F109. The authors then developed the best remedial option by use of a scoring matrix based on weighting with respect to business drivers.
For a new tie-in spool, a subsea flexible spool can be used. This approach has been used safely and successfully to accommodate instability. This approach may be adopted for pipelines, with the following caveats:
- An unbonded subsea flexible spool verified for use in a multiphase system should be used.
- A flexible spool should be combined with a rigid riser.
- The flexible spool does not represent a solution to pipeline instability but rather a method to prevent failure of the riser caused by a mobile pipeline. The pipeline may continue to move over time, and the flexible spool may eventually enter tension and fail. An unstable pipeline is also at significant risk of fatigue if instability continues.
A flexible spool could be used with a concrete mattress. This will allow a degree of movement (e.g., if the mattresses take some time to settle into the seabed). The position of the flexible tie-in can also be monitored and, in the event that the pipeline continues to move after mattresses have been installed, will give warning that the pipeline is still unstable.
The majority of the studied failures occurred between the months of May and October, the rainy season in the Gulf of Guinea. This season features the region’s most extreme weather, likely contributing to the failures. Most failures involved pipeline movement or riser kinks.
Several pipelines were found to be unstable and unable to achieve self-burial. Environmental conditions (fluidized topsoil and current) and pipeline weight contributed to the failures. Fig. 1 shows the typical mode of failure of most of the pipelines. Pipelines having a specific gravity (SG) of greater than 1.5 should self-bury, but, despite the fact that the majority of the studied pipelines met this criterion, they remained unstable. Hydrodynamic analysis using a finite-element analysis (FEA) tool also indicated that the pipelines were unstable on the basis of environmental conditions. FEA confirmed that in 1-, 10-, and 100-year-storm conditions, the pipelines will exhibit significant instability. In some cases, the pipelines lost contact with the seabed in several places. High levels of movement and bending were observed within the pipeline during FEA.
Recommendations for Pipeline Tests
For offshore pipelines, especially those in shallow waters, pipeline global analysis (static and hydrodynamic) should be performed using an FEA tool, taking into consideration relevant soil characteristics.
Buried pipelines should be checked for flotation by comparing the SG of the pipe with that of the surrounding soil. If the SG of the pipeline is greater, the pipelines should sink to the bottom of the liquefied layer, but this does not always happen. To perform an assessment of the thickness of the liquefied layer, the following important soil properties would be required from a soil survey:
- Coefficient of consolidation
- Relative density
- Rate of pore-pressure generation for given cyclic shear-stress values
Knowing the depth of the liquefied layer is not critical. However, because the profile of the mobile-seabed interface with the stable seabed might be different from that included in the pipeline as-laid profile (azimuth), sections of the pipelines can remain in the fluidized zone. This was the case for most of the pipelines studied.
The depth of pipeline burial cover should be equal to the depth of the mobile seabed layer plus the burial depth required for protection against fishing gear and dropped objects. Although surveys and alignment sheets suggested that megaripples and sand waves were not present, the mobile seabed depth was 0 to 6 m with an undrained shear strength of 0 to 2.5 kPa. The required depth of cover for the pipelines within this shear strength is at least 2.92 m below the mobile seabed. This includes a safety factor of 1.25.
A stability analysis should be performed considering not only hydrostatic loads (pipeline self-weight and buoyancy) but also hydrodynamic loads (loads caused by the action of 100-year waves and current). Self-burial of a pipeline should not be relied upon, even if the pipeline has a high SG. The line may fail because of instability during a single large weather event before it has become buried.
Before burial, a seabed bathymetric survey should be performed to identify any mobile features (megaripples or sand waves) and their depth. A prelay survey should also be performed to characterize the seabed soil type along the pipeline route. This will identify areas of rock or hard clay, which will need to be considered during trenching operations.
Recommendations for Remediation
The authors provide a strategy for pipelines in shallow waters with mobile topsoils. Pipeline instability in shallow waters can be remediated by increasing SG, using flexible loops, or trenching/burial. This strategy should be considered on a case-by-case basis depending on the soil survey for that pipeline and the pipeline specification after an on-bottom stability assessment has been performed in accordance with DNVGL-RP-F109. For pipelines that have moved a significant distance from their original position, assessment of cracks, dents, fractures, and fatigue should be performed before deploying these solutions.
Increasing SG. In conditions of shallow waters with loose topsoil, pipeline design must involve higher SGs. On the basis of the authors’ study, the pipelines require SG of greater than 1.5. To increase SG, designers can increase wall thickness or, preferably, use a concrete weight coating to save cost.
Using Subsea Flexible Loops. Where the pipeline remains unstable on the basis of the global hydrodynamic assessment and static assessment, operators should consider the use of a subsea flexible loop. This can be done by inserting the loop with slack to accommodate instability. This solution will require monitoring.
Trenching and Burial. The authors conclude that burial generally is the most suitable and effective method of remediation.
The line must be buried below the mobile seabed depth if a mobile seabed is present. Otherwise, a mobile seabed could reveal sections of the line, leading to future failure. However, the authors state that the soil survey for the shallow-water region they studied in the Gulf of Guinea indicates the topsoil (1 to 5 m) along most sections of the right-of-way is fluidized. Pipeline burial by trenching is recommended to attain stability in line with the stable seabed gradient.
Because many of the lines studied showed evidence of movement caused by instability, depth-of-burial surveys should be performed along the length of the right-of-way. A bathymetric survey should be performed along the pipeline route to identify whether a mobile sediment layer is present. This will be apparent because of the presence of megaripples and sand waves.
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Root-Cause Analysis of Offshore Pipeline Failures
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