Calculation Second Order Wave Loads Based On Multi-Domain Hybrid Method

The accurate simulation of the motion and wave-induced load is point out for the evaluation of structural strength for surface vessels and the design of mooring system and dynamic positioning system for offshore platforms.The ship or large scale offshore structures suffer from various nonlinear mechanical phenomena,because of the coupled interaction of wave-body and the nonlinearities of motions of floating body.The second order wave force is considered to be the main source of excitation.Therefore,it is necessary to improve the accuracy of the existing hydrodynamic calculation methods and find a more reasonable and practical prediction method for second order wave loads.In this paper,the responses of offshore structures are predicted through the multi-domain hybrid method by use of a meshless control surface,and a method for calculating the second order wave loads is developed based on the middle-field theory.The specific research contents are as follows:Based on the three dimensional potential flow theory,the boundary conditions of the external domain of the multi-domain hybrid method are given.By using series expansion method,the velocity potential and its normal derivatives on the control surface are expressed as the form of two series.Thus,the relationship between the velocity potential and its normal derivative can be obtained.The correctness of this relationship is verified by comparing the numerical results with theoretical results of the diffraction on an infinite cylinder.Combined the coefficients of inner domain with those of external domain,the radiation potential and diffraction potential are calculated by finding the solution of the integral equation.The hydrodynamic coefficients and first order motions of a sphere is calculated.And the numerical solutions are compared with the analytical solutions and results of other hydrodynamic calculation software to verify the accuracy of the program.Moreover,the influences of the number of grid elements and the radius of the control surface on the numerical accuracy are discussed.Based on the velocity potential and its normal derivative expressed as the form of two series,the second order wave loads formulations of middle-field method can be deduced,and the calculation program for second order wave loads is developed.The second order wave loads on a hemisphere,a cylinder and a floating production storage and offloading(FPSO)are calculated.In order to verify the correctness of the proposed method,the numerical results are compared with those of other hydrodynamic calculation software.Based on indirect time-domain theory,the time-domain results of hydrodynamic coefficients of the platform are obtained.Using the nonlinear finite element method to calculate the mooring force of mooring system,taking into account both the wind load and the flow load acting on the platform,the time-domain coupled equations of motion of the offshore platform are established.The hydrodynamic coefficient and motion responses of a FPSO are solved to verify the correctness of the time-domain coupled method.