Coupled Nonlinear Motion Analysis Of FPSO In Time Domain

The ocean platforms used to drill for oil and natural gas in the deep ocean have become central issues that academics and engineers in many countries are focused on.FPSO(Floating Production Storage and Offloading unit,FPSO)as a floating platform with good economy has a wide application prospect in our country.The motion performance of FPSO determines the design of FPSO's oil production equipment,which is one of the key technologies in FPSO design.The two key problems to determine the motion performance of FPSO include the calculation of nonlinear wave and wind current forces and the coupling effect of mooring system on FPSO.The surge and sway of FPSO are low frequency motions,so it is necessary to accurately calculate the second-order difference frequency wave forces.Carry out the above research is necessary to accurately predict the nonlinear motion performance of FPSO.In this paper,the nonlinear motion of FPSO is taken as the research object,and the second-order wave force is calculated by the potential flow theory.At the same time,the coupling effect of mooring line on FPSO motion is considered in the simulation of mooring line with three-dimensional beam element.Finally,the nonlinear coupling time-domain motion analysis of FPSO under mooring condition is carried out.The nonlinear wave force calculation of FPSO is studied firstly.Based on a three-dimensional linear frequency-domain potential flow theory program,the hydrodynamic coefficient,wave excitation force and linear motion response of the example FPSO are calculated.The accuracy of the calculation results of the program is verified by the comparison between the relevant calculation results and AQWA software results.Based on the second order wave forces from first order potential method in the theory of 3D potential flow,and also the second order differential frequency forces of incident and diffraction wave are approximately considered,the second-order drift force,second order differential frequency force quadratic transfer function and second order sum frequency force quadratic transfer function are derived and the relevant calculation programs are compiled.The calculation results in this paper are verified by the calculation of AQWA software.The influence of the components of the first-order potential second-order force on the steady force and the second-order differential frequency incident force and diffraction force on the second-order diffractive force transfer function are analyzed.Then,the three-dimensional beam element is used to simulate the mooring line segment,and the solution equation of the stress and deformation of the beam element is established,and the solution of the related nonlinear equation is derived.The calculation program is compiled with FORTRAN programming language,and the tension and deformation of the mooring line is calculated and analyzed,and compared with the calculation results of the relevant paper.On this basis,the analysis is carried out and the changes of tension and deformation of mooring lines at different velocity are studied.Finally,on the basis of the first two parts,combining the time-domain motion equation of the free floating body with the tension deformation equation of the mooring line,the time-domain coupled motion equation of the mooring floating body in waves is constructed.Combined with the previous contents,the calculation methods of various forces in the motion equation and the time-domain solution methods of the motion equation are given.On this basis,the nonlinear time-domain coupled motion of the FPSO is compiled the nonlinear motion of an example FPSO is solved,and the accuracy of the coupled program is verified by comparing with the results of AQWA software.Finally,the influence of wave and current on the coupled motion of FPSO is analyzed.