Coupled Dynamic Analysis Of FPSO Tandem Offloading System In Time Domain

It is a hotspot to research FPSO’s oil transmission problem in working sea condition.With the offshore oil and gas resources increasingly depleted,the working area of FPSO is gradually moving to the deep sea.If the submarine pipelines are adopted to transport crude oil,the length of pipelines will be very long and plenty of money will be cost.So the shuttle tanker is widely used in offshore oil transmission nowadays.Two forms of tanker transmission are adopted: side-by-side and tandem.Considering the latter one has less demand in matching tonnage,loading conditions,sea conditions,and it can be used in a harsh environment,quick release and fast escape could be provided in tandem system,the latter one is used more widely.In this paper,the FPSO tandem offloading system in working sea condition is in-depth studied.Based on three dimensional potential flow theory,the general methods of solving zero speed floating body’s motion in waves and calculating hydrodynamic parameters in frequency domain are given.Floating body’s hydrodynamic coefficients in time domain are obtained by means of fast fourier transform method(FFT)from the results of frequency domain.Nonlinear finite element method is adopted to simulate the mooring cables.Mooring floating body’s coupled equations of motion in time domain is established,given the numerical method for solving equations of motion,and the Fortran program is compiled to solve the equation.Compared the numerical results with the experimental results,the correctness of the program is verified.Coupled motion system of equations of FPSO tandem offloading system in time domain is established.Based on coupled motion program of single floating body,the computational program of tandem offloading system is compiled.The theoretical calculation is carried out for a FPSO tandem offloading system consisting of a FPSO and a shuttle tanker.And the two floating bodies’ motion response time history curves,mooring cables’ top point tension time history curves and mooring line’s tension time history curves are obtained.Also,by means of spectrum analysis,the influence of low frequency wave forces and high frequency wave forces for the whole system is analyzed.FPSO tandem system is a complex system that is influenced by many parameters.In this paper,the numerical value and direction of wind and current force,the mooring pattern of FPSO and shuttle tanker,the stiffness of mooring line between two ships and the loading conditions are considered to study their influence on system’s motion and cables’ tension.And reliable safety recommendations are put forward by computational results.This numerical model can be used in coupled dynamic calculation of multi-body moored tandem system,and the results can provide with reliable security reference and guidance for tandem offloading system.