Study Of Internal Waves Impact On The Behavior Of Deepwater Floating Systems

Internal waves 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 platforms and its mooring/riser systems have not been extensively studied, these waves have in the past seriously disrupted offshore exploration and drilling operations. Several China's new deepwater oil and gas discoveries are also located in the internal wave affected areas. Recent offshore developments in South China Sea have reported excessive floating platform motions and riser system damages, which are expected to be related to internal waves.The purpose of this paper is to develop a reasonable internal wave numerical model based on the observations, and targeted to study its impact on deepwater floating platforms such as SPARs, SEMIs and TLPs. The internal wave model is derived based on the combination of nonlinear soliton Korteweg and de Varies (KdV) solutions for the larger. leading waves and the smaller, slower trailing linear waves in the internal pycnocline. The pertinent differential equation that captures the physics is the KdV equation which has a general solution involving Jacobian elliptical functions. The solution of the Taylor Goldstein equation captures the effects of the pycnocline. Therefore, the ocean internal wave can be modeled as soliton packets containing decayed oscillations, which has the similar presentation as the satellite pictures.The nonlinear internal waves are characterized by wave heithts that can exceed 50 ms and the present of horizontal shearing currents near the layer of pycnocline. The effect of internal waves on deepwater floating platforms is studied through nonlinear fully coupled time domain analysis. The numerical model is implemented in a coupled analysis program HARP where the hull, moorings and risers 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 mooring/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.