VIM Model Test of Deep Draft Semisubmersibles Including Effects of Damping

Abstract

Abstract Semisubmersible platforms are good candidates for hydrocarbon exploration and production in the Gulf of Mexico (GoM) and elsewhere. These platforms are a preferred choice for deep-water applications involving higher design throughput. A dry tree application of semisubmersible provides the benefit of having improved well control, direct vertical well access, easy access to production equipment, reduced capital expenditures and other dry tree benefits. These are in addition to the benefits that a semisubmersible platform has over spars and tension leg platforms (TLPs). A public-private partnership has sponsored multiple projects since 2009 with the aim of maturing a dry tree semisubmersible design that is cost-effective and safe like spars and TLPs. Due to the deeper draft of the semisubmersibles proposed for the dry tree applications, vortex induced motion (VIM) is an area of concern that needs to be addressed during the design stage. The research work presented here is a part of the "Vortex Induced Motion Study for Deep Draft Column Stabilized Floaters." The effect of additional damping due to mooring lines and risers on the VIM response of a deep draft semisubmersible is an area of focus of the ongoing project. CFD-based predictions show a significant reduction in the VIM response when additional damping is considered. With the intent of validating the CFD models and to further understand the effect of additional damping on the VIM response, a model test campaign was conducted with different levels of applied additional damping. The paired column semisubmersible and conventional semisubmersible platforms were tested, with and without damping and the results are discussed here. In addition, during model testing, the conventional semisubmersible hull was equipped with a column force measurement system to measure the hydrodynamic forces on the individual columns. The results of the study show that damping plays a significant role in reducing the platform VIM which directly impacts the estimated fatigue damage of mooring lines and risers, and which in turn can reduce the overall cost of the system.