The Analysis of Mooring Systems of a Drillship

Abstract

In this paper the mooring system of a drill ship is analyzed which is designed for South China Sea in 1500 meters depth. The analyses of the mooring lines have been developed based on the theories dealing with slender structure and cables, so the elastic rod theory is used. Elastic rod theory is developed for the analysis of line dynamics. The governing equations of mooring lines and risers are treated in the global coordinate system without transforming the coordinate system. The hydrodynamic forces on the lines together with the strain and the stress of the structures caused by geometric nonlinearity are considered. The model of the rods allows for a small elongation, and permits large deflections and finite rotations. The rods are of elasticity and arbitrary configuration, with kind of loads and tension variation along its length acting on it, including the motion of rod, hydrodynamic force resulted from the external fluid and gravity. The fluid in the riser is considered for riser analysis, and the support of the sea bottom is presented for mooring line analysis. Finite element method is used to discrete mooring lines and risers, and lines dynamic analysis is executed with time integration method. A program is developed, and its validation is checked by comparison of numerical results to exact solutions for a nonlinear, static problem. Both static analysis and dynamic analysis of the whole system are done to ensure the mooring system of the drilling ship on 1500m depth can successfully applied.