Thermo-mechanical walking of straight and continuous-snake-laid pipelines on sloping seabeds

Abstract

Subsea pipeline design is very much affected by high temperature and pressure loading. Repeated start-up/shut-down cycles throughout the system lifetime impose longitudinal movements that is known as walking. The walking phenomenon as a significant criterion for thermo-mechanical design of subsea pipelines is addressed in this paper. The continuing axial movement of a pipeline may lead to structural failure. Susceptibility to walking of a typical short length flowline is assessed during pipeline design; for example, pipelines installed on diverse seabed topographic conditions such as different angles and lengths of sloping seabed. FE analyses are commonly carried out accounting for pipe-soil interaction (PSI) response to ensure acceptable pipeline performance in a realistic operational situation. An analytical solution method is available to provide suitable estimation for robust design.The present study highlights downhill walking induced by installing a pipeline on a partially sloping seabed. The continuous-snake-lay (CSL) pipeline condition is proposed as a mitigation methodology due to avoiding any over-stresses and excessive displacements. Results are presented for a straight pipeline to act as a comparison for new results for pipelines with seabed slopes along part of their route. The analyses of the straight partially sloped pipelines show a high rate of movement that may result in many practical challenges. High rates of walking might become crucial in that costly mitigation techniques may be required to eliminate the effects of walking, especially for deepwater projects. The CSL system is introduced in this paper with the intent of modifying typical pipeline response to applied cycle of loading to thus provide a cost-effective mitigation method.