Three-Dimensional Model for Seabed Instability around Offshore Pipelines under Combined Wave and Current Loadings

Seabed stability near offshore pipelines is one of the main concerns in engineering practice, being potentially affected by waves and ocean currents. The traditional model used to analyse soil behaviour near the pipeline assumes a two-dimensional interaction between the seabed and the marine structure. In other words, it is generally believed that the waves travel in the direction of the pipe. However, the actual marine environment is three-dimensional, with waves and currents approaching the structure from all directions. Based on a wide review of the literature, it may be claimed that the simplified 2D model no longer simulates the complex layouts of environment where offshore pipelines can be built, which should be represented as an integrated system. Therefore, the main objective of this project is to study the mechanism of soil response and liquefaction caused by waves and currents in the porous seabed near the offshore pipeline from a three-dimensional perspective. A three-dimensional numerical model is developed based on the Finite Volume Method (FVM) to analyse the instantaneous soil behaviour under the combined loads from both ocean waves and currents. In this integrated model, the hydrodynamic model is governed by the VARANS (Volume-Averaged Reynods Averaged Navier-Stokes) equation for simulating the two-phase incompressible flow motion outside and inside the porous media. The Biot’s consolidation equations are then solved for the soil responses by linking the dynamic wave pressure on the interface between the wave and seabed. The seabed behaviour is considered to be linear elastic with inversely small deformations. Overall good agreement with laboratory experimental measurements validates this newly proposed 3-D model. The numerical results reveal that the flow obliquity between the incident waves and the ocean currents has a non-negligible effect on the instantaneous pore-water pressure around the submarine pipeline, a phenomenon that cannot be observed in two dimensional numerical model. Further, a parametric study is conducted to show that the instantaneous pore-water pressure around the pipeline increases with decreasing flow obliquity; such influence can significantly increase with the increasing current velocity. Moreover, the liquefaction zone is more easily observed near the inlet of the ocean currents. By adopting the established FVM model, a numerical study on the soil response caused by waves and ocean currents near the trench structure has been conducted. The numerical results show that an offshore pipeline positioned in a trench layer is more stable than one directly laid on the seafloor. The following ocean currents can increase the liquefaction depth below the pipeline, while the opposing ocean currents can reduce the liquefaction depth near the pipeline. Moreover, the lee-wake vortex can be avoided with enough backfill thickness, which also decreases the occurrence of the onset of scour around the pipeline. Also, the nonlinear wave-current-induced seabed response around a pipe-protective cover system was investigated using the 3-D integrated model developed in OpenFOAM®. It was shown that, with sufficient quantity of stone covers and protective mattresses, the stability of the system can be maintained even with large current velocities. At this point, valuable suggestions can be drawn from the numerical results and then applied to engineering applications: (i) different backfill materials can be used to maintain the stability of a trenched pipeline with critical backfill thickness;(ii) pipelines laid directly on the surface of the seabed can be protected by a full stone cover or protective mattress under the environmental loadings from both ocean waves and currents with different directions; (iii) the protective mattress can be economically constructed over the pipeline with critical spacing to avoid an increase in the engineering budget.