Pipeline Walking: Accurate Estimation of Mitigation Force for Axial Movements due to Global Seabed Slope and Steel Catenary Riser Tensions
- Emmanuel Chigozie Elendu (The Shell Petroleum Development Company of Nigeria Limited)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 4-7 May, Houston, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2020. Offshore Technology Conference
- Steel Catenary Riser Tensions, Pipeline Walking, Seabed Slope, Abaqus, Soil Friction
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- 49 since 2007
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Deep water pipelines operating under high pressures and temperatures are susceptible to gradual global axial movements over a number of heating and cooling cycles – A phenomenon known as axial walking. These cyclic axial movements that occur in high pressure-high temperature(HPHT) offshore pipelines are usually induced by four mechanisms:
Seabed slopes along the pipeline route
Riser base tension induced by a steel catenary riser at the end of a pipeline
Thermal transients during startup/shutdown cycles
Multiphase flow behaviour like slug-induced flows, start up, shut down and ramp up operations
Several mitigation measures for pipeline walking as a results of these mechanisms have been developed and studied over the years with the most common being the use of hold-back anchors installed either at the middle or the end of a pipeline, the latter being the preferred method. Historically, estimation of the required anchor force to restrict pipeline axial movement has been premised on the amount of soil frictional resistance required to equal the driving force due to temperature and pressure. This methodology can lead to erroneously high estimates of the required anchor force for walking mitigation, leading to large-sized pipeline anchors with an attendant increase in capital expenditure associated with pipeline projects. This paper estimates, with a higher degree of accuracy, the required anchor force to mitigate axial walking due to seabed slope or steel catenary riser(SCR) tension.
The methodology described in this work involves the use of a mathematical proof to show that the magnitude of the walking mitigation force for a pipeline susceptible to walking is directly proportional to the pipeline submerged weight. The solution was also validated by a finite element analysis(FEA). The finite element analysis was carried out using the multipurpose industry FEA tool, Abaqus with the pipeline modelled using beam elements and the soil modelled using three dimensional analytically rigid elements. The coulomb friction model was used with frictional forces defined in the axial direction only.
Several cases were analysed in Abaqus using several pipeline sizes and soil friction coefficients in order to validate the analytical results. The analytical and FEA results agree quite well and show that the restraining force required to mitigate pipeline walking due to global seabed slope and SCR tension is solely dependent on the pipeline submerged weight and SCR tension respectively.
This paper proffers a cost-effective mitigation to seabed slope and SCR induced walking by showing that pipeline walking mitigation is not dependent on the magnitude of the axial friction resistance which usually requires large mitigation forces to counter the large frictional forces that may result particularly when we have a fairly long pipeline.
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