An Improved Discrete Vortex Method (DVM) And Its Application In The Vortex Induced Motion (VIM) Of The Offshore Platform

Offshore platforms are important marine equipment for petrol resources exploitation in the deep water, and are usually distinguished by large characterized scale and deep operating waters. As a sequence of the high speed ocean current, platforms are vulnerable to the vortex induced motion(VIM), thus making anchor chain and risers suffer severe fatigue damage, and shortening their fatigue life. The study on the vortex induced motion has been a hot and difficult point for the ocean engineering area since 1996.Discrete vortex method based on the stream function is a famous computational fluid dynamics (CFD) tool for its simple and clear fundament, unconstrained by the element or mesh, programmed easily, high resolution in the concentration of vortices, as well as big time-step to simulate high Reynolds number flow field, which is competitive when investigating VIM of platforms. This paper applied the DVM to simulate the VIM of a Spar platform with low mass ratio and a semi-submerged platform with deep draft, aiming at analyze the VIM response features of the platforms, and trying to give a mechanism explanation.To validate the DVM model adapted in paper, a code was programmed using Fortran to compare the effect of different new vortex position, the results of two formulas calculating the force, and the differences of various core functions when inducing the vortex velocities. Also, this paper presented a method of geometry image to prevent vortex blobs moving into an object, discussed the ways to handle multi-cylinders other than a single cylinder. Numerical simulations for the vortex induced motion of a fixed circle cylinder and a elastically supported cylinder are carried out using the DVM, which indicated the model be efficient for solving cross flow problem at high Reynolds number.Subjected to current algorithm, the Spar platform and the semi-submerged platform are simplified as two dimensional models. The response characteristics, wake vortex structure, the force curves, sway and surge displacement and trajectory are researched. Simulation on the VIM for the typical offshore platforms using DVM was conducted in hope for deepening the understanding of the features and mechanism.