Pipe–Seabed Interaction

Abstract

Subsea pipelines rest on or in the seabed, and therefore their behavior and condition depends on pipe–seabed interactions. Pipelines expand and contract during operation and can also be loaded by hydrodynamic action, turbidity flows, or debris flows. These actions are resisted by seabed reaction forces in the vertical, axial, and lateral directions. In some situations, the pipeline may be deliberately buried, for example, to allow overtrawling, provide thermal insulation, or prevent expansion‐induced buckling. However, in many cases, the pipeline may be laid directly onto the seabed. It may self‐bury through the action of sediment transport, remain only partially embedded into the soil, or some intermittent combination of these effects along its route. Assessments of pipe–seabed interaction forces for all the abovementioned burial conditions begin with estimation of the as‐laid (or as‐constructed) embedment. Classical methods such as limit plasticity underpin solutions for embedment and the limiting pipe–soil resistance in each direction. The critical uncertainties are usually the operative soil strength—which is affected by the seabed disturbance during the lay (or backfilling) process and throughout the operating life—and the pipeline embedment (or cover depth). Specific geotechnical testing technologies have evolved to reduce uncertainties in pipe–seabed assessments, including tools for determining the near‐seabed (and disturbed) soil strength and the pipe–soil interface strength at very low stresses. The associated analysis methods also aim to quantify the variability of the seabed reaction forces along the pipeline. The abovementioned predictions of pipe–seabed interaction behavior are performed to feed into structural analyses of the pipeline, which often use a reliability‐based framework. Pipe–soil interaction assessments require close collaboration between the pipeline and geotechnical experts due to the inherent influence of soil–structure interaction, as well as the aim of tailoring the geotechnical assessment to address the particular design criticalities, which vary depending on the seabed and pipeline conditions.

An Overview of Flow Assurance Heat Management Systems in Subsea Flowlines

“…The subsea flowline burial is either fully or partially buried [51,96]. Comparing both methods, a partially buried flowline offers less insulation effect to the fully buried flowline [51,97,98]. Additionally, the partially buried flowline (trenched flowline) encounters lower heat loss than the bared flowline but greater than the buried flowline [51,99,100].…”