Finite Element Calculation of Mooring Stiffness of Floating Body Considering Catenary and Elastic Behavior of Mooring System

Abstract

Horizontal stiffness of floating body due to mooring system is evaluated by finding tension & excursion relation at moored point of floating body considering catenary and elastic behavior of mooring line. If mooring line has a simple form such as pure catenary, mooring stiffness can be easily found because tension is analytically calculated. However, various types of mooring line such as chain, wire and rope are combined in real layout of mooring system. In that case, the behavior of mooring system will be very complex because the effect of material elasticity as well as catenary deflection is included. Therefore, numerical method is preferable for such complex mooring systems. In this paper, horizontal stiffness of a mooring system with catenary and elastic material is calculated using FEM. Minimum potential energy principle and the Lagrange multiplier method are used to derive equilibrium equation with constraint for given length of mooring line. Layout of mooring line at sea bottom is described by using contact spring element. Using the FEM solver, top tensions for selected range of horizontal offsets are calculated and the horizontal mooring stiffness is evaluated from the tension & excursion relation. The results are also compared with those of simple mooring system with pure catenary.