Analysis Of Shallow Water Wave Induced Pipeline-Seabed Interaction Considering Interface Effects

Pipelines are one of the common structures constructed both onshore and offshore regions,which widely employed as a mode of transport to carry oil and natural gas from the drilling platforms to onshore terminals as well as for the transportation of freshwater from mainland areas to the nearby islands.The route of these pipelines transverse through the coastal regions.In the coastal water environment,the condition of the seabed and pipeline are susceptible to wave,current,and tides.When a wave propagates over the seabed,the pore water pressure within the seabed changes and eventually decreases the seabed's strength,thus endangering the stability of the marine pipeline.In the shallow water environment,the waves behaves nonlinearly and might affect the characteristics of the surrounding seabed and buried pipeline.Thus,the analysis of seabed responses around an embedded pipeline is necessary.A two-dimensional numerical simulation is conducted in the present research to analyze the responses of a flat surface seabed and a buried pipeline under various shallow water wave loadings,for instance,wave,current,and tides.Additionally,the analysis of contact effects at the soil-pipe interface is also conducted based on the numerical simulation.The two-dimensional numerical model,consisting of wave,seabed and pipeline,is established in the research study.The governing equations for the numerical model have been described along with the appropriate boundary conditions.Both models are validated against existing analytical data and available experimental outcomes.The main research outcomes can be concluded as follows:1.An integrated numerical model consisting of two sub-models,the wave model and the seabed-pipeline model,is established in the computational software of FLOW-3D and COMSOL Multiphysics,respectively.Both models are one-way coupled by the wave pressure induced on the seabed surface.The wave model is governed by the Reynolds' averaged Navier Stokes(RANS)equations together with a standard k-?turbulence model to generate the wave pressure and propagation pattern induced on the seabed surface.For the wave model,two types of wave generating methods were adopted in the present study;namely,the velocity-inlet boundary wave generating method and the internal mass source wave-maker.As for the seabed-structure model,the seabed and pipeline are modelled based on the Biot's consolidation theory and the linear elasticity theory to evaluate the seabed responses and contact pressures under the action of various shallow water wave loading.2.In this research paper,the nonlinearity of the waves in the shallow water environment are highly focused;thus,various wave loading such as linear wave,current,Cnoidal wave,and tides are considered in the simulation.In the analysis of seabed responses around the buried pipeline subjected to a combined wave and current loading,the presence of current influences the wave outcomes,i.e.,wave pressure and the surface elevation pattern.The results show that as the current velocity increases,the amplitude of the wave outcomes decreases,whereas the wavelength shortens.Therefore,it resulted in a decrease of the pore pressure generation within the seabed around the buried pipeline.The vertical distribution of soil displacement decreases gradually along the seabed depth.Similar vertical soil displacement distribution trend is seen under Cnoidal wave loading.Parametric studies are conducted to analyze the distribution of oscillatory and residual pore pressures under the influence of soil permeability,degree of saturation and relative density.Based on the numerical result,oscillatory pore pressure does not exhibit apparent changes under the variation of soil permeability,and relative density.However,a decrease in the amplitude of oscillatory pore pressure is observed when the saturation degree decreases.On the other hand,residual pore pressure shows an increasing trend when the value of soil permeability,saturation degree,and relative density decreases.3.In the existing wave-seabed-pipeline numerical simulation,the interface between soil and pipe is assumed to be no-slip boundary and the contact effects are not evaluated.Thus,in the current numerical simulation,the soil-pipe interaction is modelled based on the static Coulomb friction model to evaluate the contact effects and friction forces under the action of shallow water wave loadings.Based on the results of the twodimensional integrated numerical model,the distribution of contact pressures at the soilpipe interface highly correlates with the wave propagation patterns and the soil's vertical displacement.When a wave trough is observed above the pipeline,the seabed displaces upward and maximum contact pressures are observed at the lateral sides of the pipeline.Contrarily,maximum contact pressures are notices concentrated mainly at the top and bottom surface of the pipeline.A similar trend is observed under the influence of tides,i.e.,the pipeline can easily move away from its original position during low water whereas relatively stable during high water.