NONLINEAR CABLE DYNAMICS

Increasing interest in exploration and production of oil in deep water makes a dynamic analysis of the mooring system very important. Semisubmersibles operating at greater depths are normally positioned with a multi-leg mooring system. The deployment and adequate control of the mooring lines is one of the most critical factors in the overall success of the operation. The dynamics of a mooring line are studied in the nonlinear regime. In particular, the effect of nonlinearities on the dynamic tension amplification predicted by linear theory is investigated. The nonlinearities studied are the fluid drag and the geometric nonlinearity. It is found that the motions are significantly reduced by the drag forces, while the qualitative and quantitative behaviour of the dynamic tension amplification is altered substantially by both nonlinearities studied.For large imposed motions, in particular, the geometric nonlinearity causes a substantial nonlinear dynamic tension at twice the frequency of excitation, superimposed on a linear dynamic tension component. Examples illustrate these findings.