Research On Hydrodynamic Performance And Random Response Of Deepwater FPSO

With the development of industries, the demand for oil and gas is increasing. Due to the consumption of land resources, the development of oil and gas resources expands to the deep ocean gradually. The import dependence of crude oil is high in China, yet the oil and gas reserves in China is relatively large, therefore China must strengthen the capacity of independent research and development of marine equipment. Over the past 20 years, FPSO has gradually become the mainstream for the development of deep water oilfield. Since FPSO is different from ordinary ships, it cannot avoid the sever sea conditions in time. Therefore, the study of wave-induced loads, coupled dynamic response of vessel/mooring/riser and its randomness in the service period of FPSO is a very meaningful topic.A FPSO with 300000 tons in the South China Sea is used as the research object in this article. The impact of moonpool on the hydrodynamic performance of FPSO is studied in the first time. Then, the distribution of the wave-induced loads in the cross-section of the FPSO hull along the longitudinal direction of the ship with the moonpool is studied. In addition, dynamic response of the vessel/mooring/riser coupled is studied; At last, the influence of the wave spectrum on the random of the wave-induced loads, as well as the influence of different environmental parameters on the random of the dynamic response of FPSO is studied. The specific contents of research are described as follows:(1) In order to study the influence of moonpool on the hydrodynamic performance of FPSO, the trend of the additional mass and additional damping for different sizes of moonpool and different longitudinal position of moon pool and different draft of hull are analyzed. This shows that the vertical additional mass and additional damping of FPSO hull with moonpool will show a peak. At the same time, the long term response of the wave-induced loads of FPSO with moonpool is researched also. The distribution of the vertical shear force and bending moment in the cross section along the longitudinal direction of the ship is obtained. At the same time, the effect of moonpool size on wave_induced loads is studied too, the results show that the vertical shear and vertical bending moment at the moonpool decrease slightly with the moonpool size increasing.(2) In order to study the vessel/mooring coupled dynamic response of deepwater FPSO, the influence of different mooring methods on the dynamic response of FPSO such as taut mooring, catenary mooring, single point mooring, spread mooring, single point mooring with group and spread layout are studied from the aspects of motion responses and mooring tension of the FPSO respectively. The results show that the single point taut mooring for the FPSO working in the severe conditions is more reasonable. Second, the influence of tension angle and the combination of wind/wave/current in different directions on the mooring performance of single point taut mooring is studied, and the result shows that with the tension angle between 30° to 45°, the motion displacement and tension of mooring are within the scope of security, and the horizontal movement of FPSO is more sensitive to wind load, and the mooring tension is sensitive to the incident direction of current.(3) The influence of the riser system is uaually ignored when the coupled response of FPSO being studied. In order to study the influence of the riser system on the dynamic response, vessel/mooring/riser coupled analysis in time_doamin is studied in this article. The influence of unit mass and the axial stiffness of the riser on the dynamic response of FPSO is studied, respectively. For the tension of the top of the riser is relatively large, buoy is installed on the riser to reduce the tension of the riser, and the influence of the buoy position, mass and volume on the dynamic response of FPSO is studied. Moreover, the influence of different types of risers on the dynamic response of FPSO is studied also, the results show that the riser types have a little effect on motion and line tension, and have a large effect on riser tension.(4) Due to the random of wind/wave/current, the wave_induced loads, motion of hull and mooring tension will have randomness. The influence of wave spectrum on vertical bending moment random is studied, and the Rayleigh distribution of the vertical bending moment is obtained. In addition, the influence of significant wave height, spectrum peak period, wind speed, current velocity on the randomness of the dynamic response of FPSO is studied respectively, and the Weibull distribution of motion and mooring tension is obtained.