On the simulation of ship motions induced by extreme waves

Abstract

In rough sea conditions, ships or offshore structures may experience highly nonlinear phenomena such as slamming and green water on deck. Impact loads due to slamming and green water shipping are associated with highly nonlinear free surface flows. These impact loads can cause serious structural damages, and are of considerable concern to the stability and survivability of ships. There is a great need for calculation methods to simulate ship dynamics in extreme waves. In this dissertation, a numerical seakeeping tank is developed to investigate ship motions induced by extreme waves. The numerical seakeeping tank is based on the coupling of a parallel incompressible flow solver, a Volume of Fluid (VOF) technique, and a simple mooring cable model. Specifically, an Arbitrary Lagrangian-Eulerian (ALE) frame of reference is used to handle the freely moving body. The incompressible Euler/Navier-Stokes equations in this frame are solved using projection schemes and a Finite Element Method (FEM) on unstructured grids. The VOF technique is used to capture the violent free surface motion. The simple mooring cable model is used to account for the mooring effects. The position of the ship is obtained based upon the solution of the general equations of rigid body motion with six degrees-of-freedom (6-DOF) in terms of the hydrodynamic forces and the mooring cable tension forces acting on the ship. Validations of the numerical seakeeping tank developed are first performed for the following problems: green water overtopping a fixed deck; green water on the deck of a ship model; and two side-by-side moored boxes in dam-breaking waves. Numerical results predicted by the present numerical seakeeping tank show fairly good agreement with the experimental measurements. The numerical seakeeping tanks is then used to investigate a single ship in extreme waves with or without mooring cable constraints, and multiple ships interactions in extreme waves with or with mooring cable constraints. Highly nonlinear wave-ship interactions and mooring effects have been modeled successfully.