The influence of sea state on dynamic behaviour of offshore pipelines for deepwater S-lay

The dynamic response of offshore pipelines induced by deepwater S-lay is highly noticeable, and directly dominates the design of such structures and the installation feasibility in practice. A comprehensive finite element model for deepwater S-lay systems using the software OrcaFlex is specially developed to explore the influences of sea state on the pipeline dynamic responses, considering surface waves, ocean currents, pipelay vessel motions, clashing contact between the pipeline and rollers. Meanwhile, a non-linear hysteretic soil model is applied to simulate the vertical pipe-seabed interaction, and the modified Coulomb friction model is used to model the lateral pipe-seabed interaction. The numerical model is then used to carry out an illustrative example analysis of a 12-in. gas export pipeline being laid onto the seabed with a water depth of 1500 m using the HYSY 201 pipelay vessel in the LW3-1 gas field in the South China Sea under various sea states. The influences of sea state on the pipeline dynamic behaviour on aspects of configuration, lateral displacement, axial tension, bending moment, stress and strain as well as pipeline embedment are estimated quantitatively. The findings show that a strong relevance exists between surface wave and resulting pipelay vessel motions, and pipeline dynamic responses.