Evaluation of the effect of contact between risers and guide frames on offshore spar platform motions

Abstract

A computer program is developed for the dynamic analysis of a spar platform coupled with mooring lines and risers in waves, winds, and currents. The new multi-contact analysis program is developed for the nonlinear multi-contact coupling between vertical risers and guide frames inside of the spar moon-pool. The program extends capability of the current coupled dynamic analysis program, WINPOST, by adding the capability of analyzing riser effects caused by the contact forces and moments from buoyancy-cans inside the spar moon-pool on the global spar motions. The gap between the buoyancy can and riser guide frames are modeled using three different types of nonlinear gap springs. The new riser model also considers the Coulomb damping between the buoyancy-cans and riser guide frames, and it also calculates the impact force on risers for use in fatigue analysis. The spar platform generally uses vertical risers with dry trees. However, as the water depth increases, the size of the buoyancy-can increases, and it makes installation more difficult. The pneumatic riser support system does not use buoyancy-cans and is an alternative solution to the buoyancy-can approach. The dynamic characteristics of pneumatic riser support system are studied by using the newly developed numerical analysis program. The damped Mathieu instability diagram for the damped Mathieu's equation is developed. Due to spar heave and pitch coupling, Mathieu's instability may become excited in long period waves. In the numerical analysis program, pitch and roll hydrostatic stiffness are recalculated for heave motion in every time step to check Mathieu's instability for the spar platform. Simplified vortex-induced vibration effects on the spar platform are considered in newly developed numerical analysis program, and the results are systematically compared with those of the original program WINPOST. The results in this paper show that the buoyancy-can effect significantly reduces the spar pitch motion, and the Coulomb damping effect also significantly reduces the spar heave motion. The buoyancy-can effect also plays an important role in Mathieu instability. The results also show that a pneumatic riser support system increases the spar heave motion and payload.