Modeling of soil-water-structure interaction : A Finite Volume Method (FVM) approach to fully coupled soil analysis and interactions between wave, seabed and offshore structure

Abstract

The trend towards the installation of more offshore constructions for the production and transmission of marine oil, gas and wind power is expected to continue over the coming years. An important process in the offshore construction design is the assessment of seabed soil stability exposed to dynamic ocean waves. The goal of this research project is to develop numerical soil models for computing realistic seabed response in the interacting offshore environment, where ocean waves, seabed and offshore structure highly interact with each other. The seabed soil models developed are based on the ’modified’ Biot’s consolidation equations, in which the soil-pore fluid coupling is extended to account for the various nonlinear soil stress-strain relations included. The Finite volume method (FVM) together with a segregated solution strategy has been used to numerically solve the governing equations. In the FVM segregated scheme, the conventional linear and uncoupled terms are discretized implicitly, whereas the nonlinear and coupled terms are discretized explicitly by using available values from previous time level or iteration step. The implicit-explicit discretisation approach leads to linearized and decoupled algebraic systems, which are solved using the fixed-point iteration method. Upon the convergence of the iterative method, fully nonlinear coupled solutions are obtained. The developed nonlinear coupled soil models are capable of predicting the transient and gradual pore pressure variations as well as the developed nonlinear soil displacements and stresses under monotonic and cyclic loading. With the FVM nonlinear coupled soil models as a basis, multiphysics modeling of wave-seabed-structure interaction is carried out. The computations are done in an open source code environment, OpenFOAM, where FVM models of Computational Fluid Dynamics (CFD) and structural mechanics are available. The interaction in the system is modeled in a 1-way manner: First detailed free surface CFD calculations are executed to obtain a realistic wave field around a given structure. Then the dynamic structural response, due to the motions in the surrounding water, are calculated using a linear elastic solver. Finally, the direct wave loads on the seabed and the indirect wave loads on the seabed-structure interface through the structure are provided as input for a dynamic soil response calculation. Simulation results in general demonstrate that, the interaction modeling provides improved wave loading environments for geotechnical assessment of the seabed soil.