The Influence of Current Profile on On-Bottom Stability of Subsea Cable

With the development of offshore wind industry towards deeper water depth, the demands for transferring electricity through subsea power cables over long distance across uneven seabed have increased. An important design criterion in subsea cables is to ensure that the cable can remain stable on the seabed when exposed to complex hydrodynamic loads induced by waves and currents. Accurate current profile is critical in assessing the hydrodynamic loads property on the cable. The current profile near seabed is significantly affected by the boundary layer effect above the sea bottom. According to common practice, the boundary layer related to the wave kinematics at the seabed, is usually neglected. This is known to have a marginal effect for large diameter subsea pipelines. However, the diameter of the subsea cable may be in the same order as the boundary layer thickness and its impact on the on-bottom stability can therefore not be ignored. DNV-RP-F109 includes the ability to take the effect from waves onto the current profile into account. In the present study, time-domain dynamic on-bottom stability analysis of a subsea cable is performed by using a tailor-made finite element program SIMLA to investigate the influence of the boundary layer. The case study is conducted on a 300-mm-diameter cable, which is placed on a sandy soil in shallow water, exposed to irregular waves and two different current profiles, however, by respectively excluding and including the wave effect on the current profile. Sensitivity study regarding different diameter and submerged weight of soil has been carried out and validated against published paper.