Simulation and analysis of wave-structure interactions

Abstract

Today, it is widely recognized that Computational Fluid Dynamics (CFD) methodologies should be used for the analysis of Engineering systems and that design and method of simulation must be practical and realistic to provide cost effective solutions. The recent development in CFD of Engineering Structures has led to the adoption of the Eulerian and Lagrangian concepts of numerical analysis. Although the importance of these concepts cannot be overemphasised in offshore structures hydrodynamics, thus the choice of a concept to define the flow field surrounding a structure is one of the fundamental problems identifiable with marine hydrodynamics, especially where non linear effects become paramount. A narrow focus and use of a concept not adaptable to the ship hydrodynamic problem cannot guarantee the development of a good CFD code. Traditional approaches using the Eulerian and Lagrangian concepts have progressed relatively in the last three decades with the continuous rise in computer development. Smooth Particle Hydrodynamics (SPH), Volume of Fluid Method (VOF), Boundary Integral Method are but a few methods that have been widely used in recent applications. However when a detailed description, evaluation and analysis of the flow field is required in a ship hydrodynamics problem, some of these methods fall short of expectation when they have to strictly adhere to some given assumptions to make the computational analysis stable and provide results. In the use of Eulerian and Lagrangian concepts in fluid dynamics, mathematical skill is an essential requirement for solving flow problems. Modeling methods require discretization of the flow field equations which can be linear and non linear, depending on the parameters for simulation. The understanding of mathematical principles such as Partial differential equations, Fourier transforms and integrals, Integral calculus, Complex Analysis, Matrices, Vectors, Greens Function, Bessel Function, etc are necessary for solving the various equations of fluid motion when calculating the properties of the flow field associated with the hydrodynamic problem. Solution techniques which have the attributes of providing stability, consistency, convergence and accuracy are part of the mathematical requirements for a valid algorithm. Moving Particle Semi - Implicit Method is a computational method for incompressible fluid flow problems. MPS is a lagrangian particle method with robust capability for numerical representation. Particle interaction models representing differential operators in the Navier Stokes equation are proposed for divergence, gradient and laplacian. Boundary interfaces are transformed to interactions between particles. Computational difficulties associated with Eulerian methods such as numerical diffusion and regriding due to fragmentation and large deformations can be overcome with MPS method. This study addresses in detail the MPS method as a computational tool for wave - structure interactions. Investigation of both laboratory and numerical experiments associated with Wave - Structure Interactions were the primary focus of this study with the Development of a 2-dimensional MPS Simulation and Analysis of Wedge Water entries code, and Green Water flow simulation and effects on FPSO deck structure. Computational codes were developed for the prediction of deck flow and wave loads on the deck structure using the Navier Stokes equations. An experimental study was carried out at the Newcastle University Marine Laboratory in order to understand the detailed nature of green sea physics. Green Sea effects were measured on a model FPSO. Empirical relations and data obtained from the experiment were used in the numerical prediction code to obtain the deck flow pattern and validate the wave loads on the deck structure using the MPS method. The experiment on merit was necessary to test the capability of the hydrodynamics facility and provide understanding of the underlying principles surrounding the green water phenomena.