Finite element assessment of influence of ratcheting effect on subsea pipeline design

Ratcheting effect exists widely in subsea pipelines with frequent operation for startup and shutdown during the service period, and it seriously impacts the in-place statement and structural safety of subsea pipelines. Hence, the triggering mechanism of ratcheting effect on subsea pipelines was analyzed with consideration to the requirements of flow assurance and the impact of pipe-soil interaction. Meanwhile, a screening and identification method of ratcheting effect was developed for the subsea pipelines, and a finite element assessment method of ratcheting effect was proposed during the engineering design of subsea pipelines. As a case study, the finite element assessment on ratcheting effect of pipeline was performed based on an export subsea pipeline in a single-point mooring system. According to the results, the ratio of constant friction to soil friction of a pipeline under constant circulation is 1.91, which indicates a relatively obvious ratcheting effect. So, compared with the conventional calculation on thermal expansion, the impact of another 0.09 m ratcheting displacement should be considered in the engineering design. In general, the research results could provide theoretical support for the engineering design of subsea pipelines, expansion bends and underwater manifolds, so that the design results would be in better agreement with the actual service status of subsea pipelines.