Wave‐induced displacements in seafloor sands

Abstract

AbstractThe stability and deformation of the seafloor in response to storm wave loading is an important consideration in the design of pipelines, anchors, and offshore structures such as gravity‐and pile‐supported platforms. Two quite different approaches have been proposed for investigating the effects of storm waves on seafloor sands. These approaches derive from quite different considerations of soil‐water interaction.In one of these approaches (exemplified by Yamamoto and Madsen) the seafloor is visualized to be a porous elastic or Coulomb‐damped porous nonlinear medium. In this approach, the response of the seafloor, i.e. the displacements, stresses and porewater pressures, are computed for a given wave. The response is transient and is, therefore, related to the largest wave in the storm wave group.On the other hand, in the approach proposed by Seed and Rahman, only the development of residual porewater pressures (for estimating liquefaction potential) due to the storm wave group are considered. The effects of transient porewater pressures and stresses on the seafloor sands are ignored. The residual porewater pressures are computed using properties based on initial effective stresses through a porewater generation‐dissipation model.The application of these techniques to problems of offshore structures such as pile‐supported platforms has led to considerable uncertainty and confusion. In this paper, both of these approaches are combined to investigate seafloor response to a storm wave group. For typical storm wave conditions that exist in the North Sea, it appears that the inclusion of damping, inertia and anisotropic permeabilities in the study is not important relative to ocean sands. Parametric studies show that, for a given wave, the thickness and the stiffness properties of the soil deposit dictate the response of the deposit. The displacements in the deposit are, therefore, affected by the generation of residual porewater pressures.