Mooring system stiffness: A six-degree-of-freedom closed-form analytical formulation

Abstract

A mooring system project is an iterative process, involving design and analysis with numerous parameters. In this context, analytical tools can help to reduce the use of expensive and time consuming full nonlinear numerical simulations based on high-order hierarchical models such as Finite Element models, mainly during early stages of the design. By employing Analytical Mechanics approaches, the present article brings an analytical and closed-mathematical-form formulation to determine the six-degree-of-freedom (DOF) mooring system stiffness matrix at any generic position of the floating body, provided mooring line forces are modeled as conservative ones. In order to demonstrate the use of these tools, the international benchmark of the OC4-DeepCwind platform is considered as a case study. The influence of the vessel mean position and design parameters in the stiffness, natural periods and oscillation modes is studied, revealing interesting coupling effects. Results are benchmarked against the commercial code OrcaFlexTM showing good agreement, observing that the analytical result requires much less computational cost.