Numerical Study On Hydrodynamic Performance And Motion Response Of Deepwater Spar Platform

As the rapid development of modern industry, there is less oil resource on earth and offshore. To satisfy the increasing demand of oil resource, people are exploring the oil and gas resource in deepwater area. The Spar platform has been regarded as one of the most popular platforms which has the integrated ability of exploration, exploitation and production, and it has been widely used in the deepwater oil fields. The environment is wicked and the physiognomy is complicated in deepwater. So, the deepwater industry is acknowledged as a high-technique, high-investment and high-risk one. Therefore, it is very important to precisely predict the hydrodynamic performance of Spar platform under harsh environmental conditions. The results are possessed of important value for preliminary design of deepwater Spar platform.In this paper, the truss Spar is designed based on the prototype of a serving Spar platform in Gulf of Mexico, considering two different mooring systems of grouped mooring method and equally distributed mooring method. The hydrodynamic performance and motion response of Spar platform have been studied both in frequency domain and time domain using the way of theoretical analysis and numerical simulation.The 3D hydrodynamic finite element model is built and the effects of the mooring system are taken into account by giving the specified pre-tension, angle and stiffness of the mooring lines on the fairleads. And hydrodynamic analysis of Spar platform is performed utilizing potential flow theory in order to calculate the hydrodynamic coefficients, such as wave force and the motion Response Amplitude Operators (RAO). Then, frequency domain analysis is applied to predict short term motion response for motion performance of Spar platform under the given environmental conditions. The result shows that the Spar platform has excellent hydrodynamic behaviour and it could be used for the analysis of mooring system. The influence of heave plates to the added mass effect and heave motion of the spar platform has been investigated using the numerical model with different amount of heave plates on the presupposition of nonexistence of wave viscous effect.The static analysis of two mooring systems has been conducted utilizing catenary theory and finite element method, acquiring the restoring force and most loaded line tension against vessel excursion. The static characteristics of mooring systems, including the global motions of the truss Spar and tensions in mooring lines, have been studied under different environmental conditions for both intact case and damaged case of one mooring line.Nonlinear time-domain coupling numerical analysis is conducted under the influence of winds, waves and currents considering the notable coupling phenomena between the hull and the mooring system, obtaining the results of the global motions of the truss spar and tensions in mooring lines. During the numerical study, various parameters which are sensitive to the global motions of the truss spar and tensions in mooring lines are scrutinized, such as mooring methods, angle between two mooring lines, riser systems and methods of calculating second wave drift force.The conclusions of this dissertation are mainly the following items:1. As the increasing of the amount of heave plates, the total added mass and natural heave period of the spar platform increase, and the heave motion response decreases. However, there will be less space between two plates if there are more heave plates and the shading effect of the plates will be stronger, which makes the added mass on each plate minimize and the efficiency of each heave plate decrease.2. The static results show that the tension in mooring lines reaches the peak value at the top of the line, and it gets smaller as water goes deeper. The most loaded line appears different when the direction of loads changes for these two mooring systems. Usually tensions in the lines which are closer to the direction of waves in the head-on wave zones are bigger. Different mooring systems have different restoring force under different directions against the same environmental load. Grouped mooring system provides enough remaining force and has a higher safety factor. So, the static motion response characters of grouped mooring system are better than equally distributed mooring system. The results of static analysis could be regarded as a basis for dynamic analysis.3. The dynamic results show that the low frequency loads play an important role in some of the spar platform's motions, such as surge, sway, pitch and roll. However, heave motion and tensions in mooring lines have shown strong wave-frequency characteristics.4. According to the results of the global motions of spar and tensions in mooring lines under extreme ocean environment, the two mooring systems satisfy the requirements of the rules. Under intact and damaged conditions, the extreme tensions in mooring lines of grouped mooring system are lower than equally distributed mooring system, which shows that the grouped mooring system has better motion characters.5. Research on grouped mooring systems with different angle between two lines in one group has been conducted too. The results show that as the angle gets bigger, the motions of the spar platform get smaller and the extreme tensions in lines get bigger. According to the motions and tensions, the conclusion can be made that the system with the smallest angle between two lines has the best motion character.6. The existence of riser system enhances the total mass and the viscous drag damping, so the motion response of spar platform becomes smaller and the tensions in mooring lines minimize too. Meanwhile, the dynamic response strength of tensions in mooring lines becomes smaller. The tensions in the lines facing the waves are much bigger than the others and the dynamic response strength of tensions is stronger too.7. The influence of methods of calculating drift force to the response of Truss Spar has been studied. Results. show that the accuracy of Newman's approximation,method is very close to the method of calculating QTF directly on the calculation of wave drift force.Studies on platform hydrodynamic performance computation, motion performance of heave plates, short term motion response prediction, static analysis of mooring systems, and coupled analysis of mooring Spar in time domain are carried out in this dissertation. Meanwhile, the influence of mooring methods, angle between two lines, riser systems and methods of calculating second wave drift force, which are sensitive to the motion performance of the truss spar are studied. The research results can be served as a reference for the performance study of preliminary design.