Numerical Studies for Seabed Response around Marine Structures

Abstract

Seabed instability induced by flow fluctuations is of particular significance for coastal engineers involved in design the soil foundation of offshore infrastructures, such as pipelines, breakwaters, and anchors and piles. Soil liquefaction generally occurs under cyclic loading when the excess pore water pressure overcomes the effective stress. The liquefied foundation will be incapable of supporting marine constructions and consequently structure damage and destruction will be induced because the soil will behave like heavy fluid and the shear resistance will vanish. The present study provides a new numerical model to simulate the soil behaviour for the water-structure-soil interactions. The conventional approaches for the wave-induced soil response, such as finite element method, finite volume method and finite difference method, have been reported in the literature. These techniques have been evolved and matured over the past decades and reliable results can be accessed. However, meshing is a time-consuming procedure during establishing a model. Moreover, mesh singularity is a challenging issue although it appears easily for the computational domain with a large deformation. In contrast to that, meshless methods attracted growing attentions from researchers because of its strong features, such as handling cases with a complex boundary or large deformation precisely. Furthermore, the continuity problem of interpolation can be improved. The adoption of meshless methods will reduce data storage and computational time. In the present study, a meshless approach is applied to establish fluid-seabed-structure models for marine pipelines or breakwaters. The present numerical modelling framework involves two sub-models associated with the fluid and soil field. The first is developed within OpenFOAM, in which the VARANS equations are solved. Finite difference two-step projection method and the forward time difference method are used for the space and temporal discretization, respectively. Biot’s equations are used to governing the behaviour of soil sediments, meanwhile local radial basis function collocation method and Crank-Nickson method are employed for space and temporal discretization, respectively. A one-way coupling algorithm is implemented on the interface between the flow water and solid domain.The dynamic wave pressure simulated in the hydrodynamic process will be exerted on the seabed and structure surface as a boundary condition based on the grid nodes of geotechnical model. The geotechnical model including marine structures in this study is a new meshfree model. Numerical simulations are implemented and validated with a series of analytical solutions and experimental results. After the accuracy and capability of the integrated model is confirmed, it is used to obtain the dynamic soil behaviour around marine structures, especially evaluating the potential failure risks for structures induced by liquefaction occurring in loosely deposited sand foundations. Computational outcomes illustrate that the newly proposed meshless geotechnical model is reliable to estimate wave-induced dynamic soil response, such as pore water pressure, effective stresses and shear stress, as well as the development of liquefaction depth from the seafloor or under the bottom of structures. It can be found that wave characteristics, soil properties and configuration of offshore pipeline or breakwater affect the distribution of liquefaction zone in the vicinity of marine structures considerably. The influence of wave randomness on the soil behaviour is presented, which demonstrates that random wave is a significant component during the design phase of offshore structure foundation.