Investigation on long and short period responses of spread and turret systems in ship-mooring coupled behavior

Abstract

Yaw angle is usually small in spread mooring ship because mooring lines are attached at multi-point fairleads. So, linear analysis without coordinate transformation is applicable in motion analysis. While, yaw angle will be large in weathervaned turret ship if environmental load is applied in unwanted direction (different direction from predicted one). In that case, coordinate transformation due to large yaw angle is needed in motion analysis and the governing equation will take a nonlinear form. If the effect of mooring lines is included in the analysis, the differences of two systems will be more complicated. The objective of this study is to compare ship-mooring coupled behaviors for spread and turret systems. A 300 m long ship with twelve catenary mooring chains is analyzed in the comparison study. HOBEM (Higher Order Boundary Element Method) and convolution is applied for ship motion analysis. FEM (Finite Element Method) is applied for mooring line analysis. Coupled responses such as ship motions and mooring line tensions are calculated and the long & short period responses are compared for spread and turret systems. Numerical results show that long period responses are larger in turret mooring system and short period responses are larger in spread mooring system. Results of linear and nonlinear analyses were also discussed. Copyright ？ 2018 by the International Society of Offshore and Polar Engineers