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.
In mooring of floating bodies, spread system is suitable when the variation of environmental forces such as wave, wind or current are small about heading angles, while, turret system is better in case of a ship type body such as weathervaned FPSO (Hong et al, 2007). Previous studies are shown for the turret system. Kim et al (2005) numerically investigated coupled behavior for turret moored ship with mooring lines. Milne et al (2016) proposed numerical prediction of heading angle of turret system. Model test investigation for a turret- moored FPSO was done by Sanchez-Mondragon et al (2017). The present study also investigated the behavior of turret-moored ship and presented some findings regarding comparison of turret-moored and spread-moored systems especially focusing on differences of linear and nonlinear results for both systems. The two systems will be different in mechanical behaviors as well as application types. In spread system, yaw angle of ship is usually small because mooring lines are attached at many points. So, linear analysis without coordinate transformation is applicable in motion analysis. In turret system, yaw angle will be large if environmental load comes from unwanted direction. In that case, coordinate transformation is needed in motion analysis and the governing equation will be nonlinear. If the analysis include the coupled effect of mooring lines, the differences will be more complicated.