Stability analysis of spar platform with four mooring cables in consideration of cable dynamics

Abstract

The stability of floating platforms is closely related to the sea states, fairlead positions, and center of mass (CM) of the platform. To investigate the influences of these interference factors, numerical modeling of a spar platform with four mooring cables is performed using an in-house MATLAB code. To analyze the influences of the nonlinear hydrodynamic forces of cables, cable dynamics are considered using the lumped mass method with respect to a relative-velocity-element frame. The relation between the motions of the spar and the propagating waves is revealed in both the time and frequency domains. Additionally, the contribution of the hydrodynamic forces to the oscillating amplitude of the mooring forces is approximately 20% in the X- and Y-directions, and 40% in the Z-direction. The hydrodynamic forces also generate hysteresis between the peak displacements and the peak forces. Finally, the influences of the CM and fairleads are studied using four and five virtual maneuvers, respectively. The analysis results indicate that both the CM and fairleads have a slight influence on the translational motions, whereas they have a significant influence on the rotational motions. The optimal positions for setting the CM and fairleads are found with respect to the spar frame.