Structural Analysis And Dynamic Characteristics Study Of A Novel MCPSO Platform

In China,there are a large number of offshore marginal oilfield with abundant oil and gas reserves,it is a major trend to develop offshore marginal oilfield in the future.The common features of these oilfields are higher investment risk,more difficult than conventional oilfield development,higher economic performance requirements for mining facilities and management methods,etc.Therefore,it is high investment risk to develop marginal oil field by traditional methods.The conclusion is that the research of novel offshore marginal oil field exploitation equipment is of great significance to the exploitation of marginal oil field.Aiming at the exploitation of marginal oil field,a novel Multi-Cylinder Concrete Production Storage and Offloading(MCPSO)platform is proposed.Which has the functions of drilling,workover,production storage,transportation and so on.The single tank is constructed by two separate multiple materials,including concrete and steel.The suction piles are adopted as the anchor foundation,which make the platform easy to install and reuse.Based on the three-dimensional potential flow theory and the discrete method,the wave force of MCPSO platform is studied.Results show that the wave load of MCPSO platform is not sensitive to wave direction in both low frequency and 100-year return period conditions.However,due to the influence of the special shape,the wave force is sensitive to the wave direction in high frequency condition.The motion and dynamic response of the fluid in moon pool of MCPSO platform are studied.It is found that when the wave frequency is far away from the natural frequency of fluid piston motion,the wave force is effectively shielded by the moon pool and the working environment in the moon pool is stability.Furthermore,by contrary,when the incident wave frequency is close to the natural period of fluid piston motion,the heave and pitch wave forces of the MCPSO platform are obviously affected.The influence of multi cylindrical shape on the wave force are studied.Results show that the wave force of this structure is significantly lower than that of the single cylindrical structure,and by the shielding effect of extension bar,the local wave force on the structure is reduced to a certain extent.The finite element model of MCPSO platform is established,and the stress distribution of different positions under different wave loads and loading conditions are analyzed.Results show that the conceptual design of double layer tank ensures that the stress transmission of internal and external tanks is clear.According to the results of structural strength analysis,the high stress area of concrete tank under different working conditions is analyzed.The influence of wave direction and sea condition on stress in high stress area is studied.It is found that due to the direct influence of wave force,the tensile stress of concrete structure near the joint of tank is always large along the direction of wave incidence.Hydrodynamic pressure owing to water-structure dynamic interaction under seismic excitation is studied via CFD approach.It is found that the inertial force plays a dominant role in the total hydrodynamic pressure,therefore,the Maccamy-Fuchs diffraction algorithm is adopted to calculate the hydrodynamic pressure of multi cylindrical structure,and the accuracy in verified.Moreover,a structural model under the seismic wave is further established.And the effects of hydrodynamic pressure and different load cases on the natural frequency and seismic response are analyzed.It indicated that the simplified method of hydrodynamic pressure in this paper has a high accuracy for the seismic response analysis of the platform.Modal analysis results show that the stiffness of MCPSO platform along the horizontal direction is basically the same,and the horizontal response is the main response of the platform under threedimensional seismic load.Based on the analysis of transient dynamic response,the seismic influence of MCPSO platform are studied.Results show that the concrete structure at the bottom of the platform is prone to tensile cracking.And the top and bottom of the extension bar under the seismic load are high stress areas.In conclusion,a novel MCPSO platform is proposed in this paper,the dynamic features of MCPSO platform are analyzed,and then the distribution of wave force and stress of the structure are obtained.Hydrodynamic pressure owing to seismic excitation is studied,then the distribution of stress in different positions of the platform under the earthquake is obtained,and the high stress area where the structure is prone to damage is pointed out.In a word,this work has established the important foundation for the subsequent studies of the platform.