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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 ЉVortex Induced Motion Study for Deep Draft Column Stabilized Floaters.Љ 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.

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Vortex Induced Motions (VIM) of semi-submersibles occur when the vortex shedding frequency is close to the natural frequency of the semi-submersible in the direction transverse to the current. Recent studies suggest that the magnitude of VIM predicted during model tests is higher than that observed in the field. Among others, the damping effect provided by the risers and mooring lines is regarded as one of the reasons for this difference. In this paper the setup and results are presented for model tests to investigate the influence of damping on VIM. For these model tests an active damping system was developed, which introduces an actively controlled external force mimicking a damping force. This applied damping force is based on the floater sway motion and sway velocity. With this system the introduced damping level can easily be controlled and verified without changing the stiffness of the system. In this paper the advantages and disadvantages of this active damping system are presented. The VIM tests were conducted for two semi-submersibles: a paired-column semi with eight columns and a four column semi. Reduced velocities ranged from Ur=3 to Ur=10 and different levels of additional linear damping were applied to the floaters in the direction transverse to the current direction. Damping was found to reduce the VIM motions significantly: reductions of more than 60% were observed in the nominal A/D response for 25% equivalent linear damping. This indicates that damping has a significant effect on the global VIM response and thus should be considered in the design phase of the risers and mooring lines of the semi-submersibles. To improve the understanding of the driving mechanism of VIM and also to provide validation data for CFD analyses, forces were measured on each column of the four column semi. Column force measurements indicate that for the four column semi for 45 degrees heading, i.e. the heading with largest VIM responses, the upstream, the portside and the starboard side columns are exciting the VIM motions. For 22.5 degrees, the downstream, the portside and the starboard side columns excite the VIM motions. For all tested headings the pontoon always damps the VIM response.