Study On Hydrodynamic Performance Of S-Spar And Fatigue Behaviors Of Its Riser System

As the offshore oil exploitation activities expending to deep water and even ultra-deep water, many new types of floating structures suitable for this depth are being developed concomitantly. Compared with other floating structures, Spar platform has excellent stability, benign motion behavior and adaptation to wide range of water depth. Spar platform is then regarded as an attractive design solution for regions of deep water. Since the first generation of Classic Spar has come to use, the Spar platform has been evolved into the second generation of Truss Spar and the latest Cell Spar. At present, the Chinese research in this area is still in the starting phase. However, the special environments, such as ultra-deep water and internal waves, pose a new challenge to the design of Spar platforms. Therefore, the concept design and hydrodynamic analysis of new type of Spar platform are urgently needed.Based on the advantages of Classic Spar and Truss Spar, A new type of Spar platform, named S-Spar, is presented in this paper. This Spar aims at applying to the especial areas with internal wave and ultra-deep water depth. Its midsection is a cylinder with same diameter as the centre well, and the centre well and midsection is designed as an integrated structure. Three heave plates are attached appropriately along the connect section. With the unique closed midsection, S-Spar shields the risers in the centre well from the majority of environment loads near the midsection, then can avoid the high frequency vibration of risers due to current and internal waves. S-Spar can provide sufficient top tension for ultra-deep water application but no buoyancy can made of lightweight alloy is need. Besides, S-Spar has been optimized to carry large payloads due to the decrease of steel using of midsection.Based on the 3D potential flow theory, the hydrodynamic analysis of S-Spar is conducted in frequency domain. The wave exciting force, added mass coefficient, damping coefficient and motion Response Amplitude Operators (RAO) are obtained. The calculation results show that added mass and damping of S-Spar platform has reach good level, and S-Spar is considered to have good motion characteristics. The results from frequency-domain analysis will be further used in time-domain coupling analysis of S-Spar platform.The coupling analysis model of S-Spar platform and'Horn Mountain'Truss Spar are set up using the software DeepC. Then the hydrodynamic performance of two Spars is analyzed through nonlinear time-domain coupling method with the wind, wave, current loads and the nonlinear effects of mooring system. The wave force (including first order wave exciting force and second order wave drift force), time history of global motions, response spectrum, tension time history of mooring lines and maximum tension are obtained. Then the paper compares the motion characteristics of S-Spar and the typical Truss Spar in this paper. The results show that wave force on S-Spar platform is at the same level of that of Truss Spar which indicates the closed midsection doesn't cause obvious increase of wave loads. S-Spar platform shows better motion response than the typical Truss Spar under the extreme wind, wave and flow condition. Besides, the maximum tension and tension fluctuation of mooring lines for S-Spar platform are also well controlled. Therefore, the S-Spar is considered to have excellent motion characteristics and good adaptation to exploitation of deep water and ultra-deep water.Top tensioned riser is usually used to connect sea surface and wellhead in seabed in Spar platform. However, most researches adopt single riser model which can not take the influence of Spar platform on TTR into consideration, especially for the vortex induce vibration (VIV) study. Based on the S-Spar platform, the vortex induced vibration and fatigue behaviors of TTR hosted in Spar platform are investigated in this paper at the last part. The researches mainly focus on protective effects of S-Spar on TTRs in centre well, influence of riser guides on natural frequency and VIV fatigue damage of TTRs, influence of Spar VIM on fatigue damage of TTRs. The following conclusions can be drawn:1) The S-Spar platform can not only protect TTR in the range of midsection, but also decrease fatigue damage of whole riser effectively. The protective effect is more obvious in the range of Spar hull and riser guides, and increases with current velocity. 2) Riser guides in the centre well of Spar platform will make natural frequency of TTR higher than single riser model. For the TTR mentioned in this paper, tolerance of natural frequency can reach to 11.2%.3) Using single riser model to predict the fatigue life of TTR hosted in Spar platform is more dangerous. The fatigue damage of TTR with Spar platform is 2.598 times as that of single riser model under the current condition in South China Sea.4) When VIV fatigue damage and VIM fatigue calculated together, the VIM of Spar platform has great influence on the fatigue damage of TTR. VIM of Spar can restrain the vibration and fatigue damage of TTR It is conservative to predict the fatigue life of TTR without taking the VIM of Spar into consideration.The innovation of this thesis can be concluded as follows: Taking the ultra-deep water condition and internal wave load into consideration, a new type of Spar named S-Spar is presented in this paper by integrating the advantages of Classic Spar and Truss Spar. And the hydrodynamic performance is obtained using frequency-domain and time-domain method, and compared with a typical Truss Spar. Based on the S-Spar, protective effects of S-Spar on TTRs in centre well, influence of Spar platform on VIV fatigue damage of TTRs, influence of Spar VIM on fatigue damage of TTRs are investigate, and beneficial conclusions are obtained.