Dry Tree Semisubmersible Application With Low Heave Motion and VIM Response

Abstract

The oil and gas industry has made substantial efforts to find a semisubmersible design with low hull motion response suitable for a dry tree application in deep water. The top tensioned riser (TTR) makes possible both the drilling and completion of production wells, and the support of production risers for vertical access to the producing well over the life of the field, without need for dis-connecting due to environmental events. The application of TTRs requires a hull with both minimal heave response and lateral motion due to the limitation of tensioner stroke in extreme conditions. Conventional semisubmersible designs exhibit motion responses to extreme environmental design events for the central Gulf of Mexico that exceed current generation of riser tensioners stroke limits. To satisfy the industry demand on the dry tree application, the Heave and VIM Suppressed (HVS) class semisubmersible has been adapted to provide a longer heave natural period while maintaining reduced heave motion and VIM (Vortex Induced Motion) response. The longer heave natural period of the hull accommodates the natural period reduction effect of TTRs. This new adaption of the HVS class of semisubmersible maintains the characteristic features of a column step and tall narrow pontoon. The column step breaks the coherence of vortex shedding along the length of columns. The design transfers the water plane area from the narrower pontoons to the larger diameter columns, which provides greater shallow draft stability for quayside integration of topsides and for ballasting down of the hull at site. The new adaptation increases the heave natural period by adding hydrodynamic mass (added mass) with the inclusion of upper and lower "paired" pontoon plates at the junction of the pontoons and column. These "corner" paired pontoon plates also increase the stiffness of the lower h ull. Model tests for the adapted HVS class of semisubmersible were performed to investigate the in-place hull motion for the Gulf of Mexico environmental conditions, and the measured data were correlated with the numerical tool MLTSIM, which is the in-house time-domain nonlinear 6-Degree of Freedom (DOF) motion analysis program. Also, extensive Computational Fluid Dynamic (CFD) work was done with Technip's proprietary Numerical Wave Basin (NWB) to optimize the design. With the addition of pontoon plates, the HVS class of semisubmersible hull can now be designed to serve either wet or dry tree completions by adjusting hull geometry parameters in order to achieve the desired heave natural period. Note also that the dry tree version of the HVS class of semisubmersible does not use keel joints on the risers making the hull layout and structure similar for wet and dry applications.