Research On Mechanical Properties Of Riser Based On Internal And External Flow Coupling

Deep water oil and gas resources will become the main battlefield of hydrocarbon exploration and exploitation in the near future. Higher requirements on offshore drilling equipment have been expressed under the complex deep water environment. Therefore, as the only channel connecting drilling platform and underwater wellhead, marine riser is of crucial importance to deep water oil and gas exploitation. Because marine riser is under complex ocean current environment, fatigue damage tends to happen due to vortex-induced vibration, which has a strong impact on the safety issue of marine riser. As a consequence, the study on the static characteristics and mechanisms methods of marine riser vortex-induced vibration has great theoretical meaning and practical engineering significance. The main contents and results of this paper are as follows.(1) The calculation method for both of the shallow and deep sea marine environment load considering the wind, wave and current based on the causes of the formation of wind, wave and flow as well as the characteristic parameter related to marine environment are created. Based on the analysis of marine environmental loads and distribution of flow velocity, the research of regularity of drilling riser static characteristics is carried out with the consideration of top tension, drilling fluid density, flow rate and platform drift location. The equations are solved with the use of finite difference method. The research results indicate that drilling riser's displacement in flow direction increase with the increasing of surface flow rate and the platform drift location. And the larger top tension strongly inhibits drilling riser's displacement in flow direction, which can be used in actual operation.(2) Natural frequency analysis model is established with the consideration of common drilling riser configuration, with the use of variation principle and functional extremum method. The accuracy of this model is verified by the comparison between theoretical calculating value and simulation value of ADINA. The impact of top tension, buoyancy modules, length of riser, internal and external diameter of riser are carried out. The results shows that the natural frequency is extremely sensitive to the top tension and buoyancy modules.(3) Based on the distribution of flow velocity, the CFD model is established to study the flow field around a deep water riser body. Lift coefficient, drag coefficient and vortex shedding frequency of different riser sized in different uniform flow rate are studied. The results reveal the relationship between lift coefficient, drag coefficient, vortex shedding frequency and flow rate, riser sizes.(4) Based on the research of vortex shedding frequency and natural frequency of marine riser, the marine riser's fluid-solid coupling model with consideration of internal drilling fluid flow and deep water current is established using FEA software ADINA. The actual size of the model is 1500m (length-diameter ratio is 3000). And the accuracy of this model is verified by the comparison of simulation value and experimental value. The results indicated that the lock-in phenomenon occurs; the increasing of drilling fluid density increases the amplitude of vortex-induced vibration, while it decreases its response frequency; the flow rate of drilling fluid has no obvious impact on vortex-induced vibration of riser. The influence of different surface flow rate is carried out. The result show that the axial stress on marine riser increases with the growth of surface flow rate, and the method of enhancing top tension will endanger the whole system for the exaggerated axial stress when extreme weather comes.The study is of guiding significance to strength design, life prediction and field application of deep water riser.