Low Motion Semisubmersible Hull for Dry Tree Application

Abstract For three different marginal fields with different payloads, two variations of Technip's wet tree HVS semisubmersible are designed for dry tree application and evaluated on the basis of riser tensioner stroke and deck acceleration. These dry tree suitable hulls also have reduced vertical heave motion at the SCR porch and improve quayside integration and commissioning of topsides to the hull. A key element for the dry tree platform is the riser tensioning system which supports the direct vertical production risers from a subsea wellhead to a topside production tree. These riser tensioners provide additional hull heave stiffness, and effectively reduce the overall natural heave period of the hull. Conventional semisubmersible designs have excessive heave response in harsh environments, resulting in tensioner stroke ranges that are beyond the stroke ranges of field proven conventional riser tensioner equipment. The HVS class semisubmersible with reduced heave and VIM response was chosen as the basis for the dry tree semisubmersible designs in order to achieve riser tensioner stroke ranges within the capability of field proven riser tensioners. The main characteristic of the HVS class of semisubmersible is the redistribution of displacement from the pontoons to the lower part of the column. This is accomplished with a column step, which has the appearance of a blister, located partially around the lower part of column. This redistribution reduces the vertical hydrodynamic excitation, and the heave response. The column step breaks also the coherence of the vortex shedding along the length of column and consequently suppresses the vortex induced motion. The dry tree adaptations of the HVS class semisubmersible include pontoon plates that increase the heave natural period through added mass, and the outcome is reduced heave motion for seastates with high peak periods. The pontoon plates are simple structures to fabricate and have additional benefit of enhancing structural rigidity. The contribution of the pontoon plates to the hull steel weight is minimal. With optimal design of the pontoon plates, the resulting dry tree hulls support the top tensioned risers without the need of a keel guide. The dry tree hull forms have been designed using Computational Fluid Dynamic (CFD) analysis. Extensive CFD work was performed in order to finalize the dry tree designs.