Vortex-Induced Motion Evaluation on Semi-Submersible Platforms Using a Time Domain Methodology

Abstract

As exploitation activities moves into fields located in deep water, the industry has been addressing studies aiming at concepts of platforms that reduce the influence of environmental loads on risers. The deep draft semi-submersible platform is one of these concepts [1]. The semi-submersible deep draft platform is described in a simplified way as a deck supported by columns connected by pontoons. This particular geometry can make this structure susceptible to a significant low frequency vibration movement caused by vortex shedding. This phenomenon is usually known as VIM (Vortex Induced Motions). The amplitude of these motions depends on some parameters such as current velocity, hydrodynamic diameter of the column, natural frequency of the platform due to the mooring system and structural damping (hull, mooring lines and risers). One of the consequences of these motions is the increase of the fatigue damage on the risers connected to the platform. In a previous work presented by the authors in 2009[2], a time domain methodology based on a Van der Pol equation was described. This methodology was developed in order to represent the vortex shedding phenomenon. In this paper the Van der Pol methodology is improved by considering the VIM phenomenon acting on a multicolumn structure.