Linear and nonlinear dynamic coupled analysis for floating body with mooring lines and risers with or without bending & torsion effect of riser

Abstract

Since riser includes both catenary and bending & torsion effects, it is a nonlinear system and needs many computations in dynamic analysis. In some cases, the analysis is simplified without bending & torsion effect assuming catenary behavior is dominant. A previous study by Kim and Kim (2019) presented a linearised beam analysis for riser. The method finds static position and pre-tension at t=0 with nonlinear analysis. With those initial configurations, linear beam analysis is done for t>0. The method provides approximate solutions but saves computing time. The previous study applied the method to riser alone. This study extended the method to coupled analysis of floating body, mooring lines and risers and compared three calculation cases such as nonlinear analysis with bending & torsion effect of riser, nonlinear analysis without bending & torsion effect of riser and linearised beam analysis. As a numerical example, a ship with twelve mooring chains and four risers was analyzed in waves. Ship motions, chain tensions, riser tensions and bending & torsion moments or risers were calculated for the three calculation cases to compare their numerical accuracy. Computing times of the three methods were also compared.