Time-domain simulation of nonlinear motions of moored floating body in irregular waves using rankine source method

Abstract

This study introduces a three-dimensional time-domain Indirect Boundary Integral Equation Method (IBIEM) for simulating the motion of the floating body in both regular and irregular waves. The method systematically addresses second-order wave loads, focusing on low-frequency motions induced by low-frequency wave loads, crucial for the stability of offshore structures. Unlike traditional methods, the time-domain IBIEM simplifies the analysis by simultaneously considering all wave components of irregular waves, making the calculation of nonlinear wave loads more straightforward and intuitive. An approximate second-order equation of motion is proposed, simplifying the computational process by eliminating the need for solving boundary value problems for second-order radiation forces. The IBIEM is coupled with a quasi-static mooring model based on the elastic catenary equation, accurately capturing the influence of mooring systems on the response of the floating body. Validation against model tests and established numerical methods demonstrates the accuracy of the developed code in predicting the dynamic behavior of offshore structures. Applied to various scenarios, including a truncated cylinder, the barge, H2FPSO, and K-Semi, the results show strong agreement with experimental data, confirming the reliability and effectiveness of the method in simulating complex wave-structure interactions, making it a valuable tool for offshore structure design and analysis.