Low Motion Semisubmersible Hull for Dry Tree Application

摘要

For three different marginal fields with different payloads, two variations of Technip’s wet tree HVSsemisubmersible are designed for dry tree application and evaluated on the basis of riser tensioner strokeand deck acceleration. These dry tree suitable hulls also have reduced vertical heave motion at the SCRporch 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 verticalproduction risers from a subsea wellhead to a topside production tree. These riser tensioners provideadditional 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 intensioner stroke ranges that are beyond the stroke ranges of field proven conventional riser tensionerequipment.The HVS class semisubmersible with reduced heave and VIM response was chosen as the basis for thedry tree semisubmersible designs in order to achieve riser tensioner stroke ranges within the capability offield proven riser tensioners. The main characteristic of the HVS class of semisubmersible is theredistribution of displacement from the pontoons to the lower part of the column. This is accomplishedwith a column step, which has the appearance of a blister, located partially around the lower part ofcolumn. This redistribution reduces the vertical hydrodynamic excitation, and the heave response. Thecolumn step breaks also the coherence of the vortex shedding along the length of column and consequentlysuppresses the vortex induced motion. The dry tree adaptations of the HVS class semisubmersible includepontoon plates that increase the heave natural period through added mass, and the outcome is reducedheave motion for seastates with high peak periods. The pontoon plates are simple structures to fabricateand have additional benefit of enhancing structural rigidity. The contribution of the pontoon plates to thehull steel weight is minimal. With optimal design of the pontoon plates, the resulting dry tree hulls supportthe 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.