Investigation On Hydrodynamics And Motion Performance Of An Innovative Deep Draft Multi-Spar Platform

Recently, the exploration of oil and gas in deepwater expands quickly to meet global development and energy needs. The problems related to deepwater floating platform are becoming the research hotspot due to the significant position of deepwater floating platform serving as drilling and production facility in deepwater oil and gas exploitation. At present, three main floating platform types, Semi, TLP and Spar platform, are widely applied in the worldwide deepwater oil and gas exploitation. However some problems are still existing e.g. poor hydrodynamic performance of conventional Semi, water depth sensitivity of TLP platform and the fabrication difficulty of Spar platform. Therefore, developing innovative deepwater floating platform and improving the behavior are significant and necessary. Meanwhile, being one of the essential research issues for deepwater floating platform, the hydrodynamics and motion performance of the platform still need further study. Based on review, analysis and comparison of various floating platform types for deepwater, a novel concept i.e. Deep Draft Multi-spar (DDMS) is proposed to be an alternative solution for deepwater oil&gas exploration. In this dissertation, the conceptual design of DDMS platform, hydrodynamics and motion of DDMS in frequency domain, the motion analysis of DDMS based on the hull/mooring/riser coupled model in time domain, the analysis of the platform Mathieu instability based on heave and pitch coupled equation, and the physical scale model test of DDMS platform are carried out. The main works of this dissertation are as follow:1. In this dissertation, a conceptual design of DDMS platform with 70,000t displacement working in 1500m water area is conducted. The conceptual design procedure includes the general arrangement of the platform, estimation of hull dimensions, arrangement of the mooring system and riser system, platform weight control, stability check, motion performance check and so on. Furthermore, the correlation between the characteristic structure parameters of the platform with the stability, hydrodynamics of the platform are acquired and analyzed.2. In frequency domain analysis, the first-order hydrodynamics of the DDMS platform and Truss Spar are computed and the seakeeping ability of the two platforms is compared. The interactive numerical method is applied in the computation of structure response to handle the nonlinear factors such as viscous damping. Furthermore, the influence of the heave plates and the viscous damping generated by the hull to the motion performance of the floating platform is especially analyzed. The results show that the heave plate can reduce the heave response effectively and the viscous damping contributed by the hull can suppress the low frequency response for surge obviously. The DDMS platform could be applied in more ocean environments than Spar platform due to its much better heave motion performance than the one of Spar platform.3. In time domain simulation, a hull/mooring/riser coupled model is established. The three different numerical models, uncoupled, hull/mooring coupled and hull/mooring/riser coupled model, are subjected to free decay test as well as the wind, wave and current joint action test respectively. Based on the above two tests, the effect of mooring line and riser on the global motion of the floating platform as well as the coupled effect are compared and analyzed. The results clearly show the damping from the heave plates and mooring line is able to reduce the dynamic response for heave and especially the low frequency motion for surge. The mooring line tension of wave frequency is the major contributor in dynamic analysis while the low frequency component is dominator in static analysis.4. Mathieu instability is carefully analyzed upon the establishment of coupled vertical and rotational motion equations which rigorously consider the varieties of water area, metacentric height and displaced volume with floater's motion. The Mathieu instability of DDMS is studied in regular and random waves, and the damping effects which suppress the occurrence of Mathieu instability are investigated especially. For the Mathieu instability, damping of heave plate and mooring line play an important role in restraining the instability. The results also obviously indicate Mathieu instability is owing to the energy transfer and exchange between the coupled motions.5. The scale model test is executed in wave flume and relevant hydrodynamics as well as global motion are acquired. Six different model cases are employed in the model test to investigate the effect of various horizontal thin plates attached to the model such as heave plate and guide plate at keel with hole on the hydrodynamics and motion performance of platform. The recording heave responses significantly reveal that the low frequency component is comparable with the wave frequency component and even higher when the controlled wave period is far away from the heave natural period. For 6 different extreme environments in model test, even for the extreme environment of South China Sea with 1000-r return period, the DDMS platform exhibits favorable seakeeping ability whose maximal heave response does not exceed 2.5m.