Internal waves near the ocean surface have been observed in many parts of the world including the Andaman Sea, Sulu Sea and South China Sea among others. The factors that cause and propagate these large amplitude waves include bathymetry, density stratification and ocean currents. Although their effects on floating drilling platforms and its riser systems have not been extensively studied, these waves have in the past seriously disrupted offshore exploration and drilling operations. In particular a drill pipe was ripped from the BOP and lost during drilling operations in the Andaman sea. Drilling riser damages were also reported from the south China Sea among other places. The purpose of this paper is to present a valid numerical model conforming to the physics of weakly nonlinear internal waves and to study the effects on offshore drilling semisubmersibles and riser systems. The pertinent differential equation that captures the physics is the Korteweg-de Vries (KdV) equation which has a general solution involving Jacobian elliptical functions. The solution of the Taylor Goldstein equation captures the effects of the pycnocline. Internal wave packets with decayed oscillations as observed from satellite pictures are specifically modeled. The nonlinear internal waves are characterized by wave amplitudes that can exceed 50 ms and the present of shearing currents near the layer of pycnocline. The offshore drilling system is exposed to these current shears and the associated movements of large volumes of water. The effect of internal waves on drilling systems is studied through nonlinear fully coupled time domain analysis. The numerical model is implemented in a coupled analysis program where the hull, moorings and riser are considered as an integrated system. The program is then utilized to study the effects of the internal wave on the platform global motions and drilling system integrity. The study could be useful for future guidance on offshore exploration and drilling operations in areas where the internal wave phenomenon is prominent.
One key decision operators make when planning field developments is whether the production system will use only subsea equipment or will also include dry trees. Although dry tree floating platforms have primarily utilized SPAR and TLP hull forms, semisubmersible options are being developed and qualified for the Gulf of Mexico. A key component of the dry tree semisubmersible is the riser tensioning system, which must be carefully matched to hull motion characteristics and arrangements. The SPAR platform utilizes self-supporting buoyancy cans or long stroke riser tensioners in a protected location within the SPAR centerwell and in combination with a keel guide. The TLP utilizes short stroke riser tensioners with or without a keel guide and in a location open to interaction with waves and currents. A dry tree semisubmersible is likely to use long stroke riser tensioners in an open wellbay configuration either with or without a keel guide. This paper uses computer analysis and motions calibrated to model test data to evaluate the strength and operational performance of a riser tensioning system for a specific dry tree semisubmersible configuration. The system uses existing riser tensioning equipment in an arrangement that is open to wave and current interaction. The paper compares arrangements with and without a keel guide and presents differences in strength, stroke range and cost. The semisubmersible hull form includes columns arranged in pairs at each corner of the platform and the paper investigates the influence of the paired-column arrangement on wave loading on the risers and supporting structures. The results of the paper indicate what modifications are necessary, if any, for riser tensioning equipment integration. The information and results presented in this paper are applicable to operators evaluating dry tree interfaces on proposed new floater concepts in addition to equipment suppliers planning to provide or qualify riser tensioning equipment for semisubmersibles. The results will also contribute to further development of a dry tree semisubmersible option for Gulf of Mexico projects, which will provide cost and execution plan improvements compared to existing options for deepwater.
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