Numerical simulation on dynamic behaviour of deepwater J-lay systems

Abstract

The J-lay method is a comparatively feasible way to install offshore pipelines with larger diameter and heavier submerged weight in deepwater and ultra-deepwater area. In this paper, a full finite element model (FEM) for deepwater J-lay systems was established using the software OrcaFlex to study the actual dynamic pipelay effects, which are mainly caused by hydrodynamic forces and pipelay vessel motions. Meanwhile, a nonlinear hysteretic soil model was employed to characterise the vertical pipe-seabed interactions, and the modified Coulomb friction model was applied to simulate the lateral pipe-seabed interactions. An explanatory example about a steel pipeline being installed to the seabed of 1500 m water depth in the LW3-1 gas field of South China Sea was introduced to carry out the numerical analysis. The dynamic behaviour of the J-lay pipeline in terms of top effective tension, bending moment, seabed resistance and pipeline embedment in the touchdown zone (TDZ) was quantitatively evaluated. The comparisons between static and dynamic results of J-lay pipelines and between J-lay and S-lay dynamic results with identical pipelay conditions except for the departure points were carried out, respectively, in which the more noticeable dynamic effects are observed for deepwater J-lay pipelines.