Modeling offshore ropes for deepwater lifting applications

Deepwater lifting applications are becoming increasingly important for the oil and gas industry. The dynamic response of the lifting system, i.e. the rope and the attached payload, to the wave-induced motion of the crane tip is of great interest for subsea lifts in order to guarantee safety. In this paper, a nonlinear discrete model of the lifting system is presented, where nonlinear drag forces, structural rope damping and snap loads are considered. The linearized version of this model is validated against a linear continuous model with respect to its static elongation, the natural frequencies and the motion transfer function, where the linear models are obtained by equivalent linearization. Furthermore, motion transfer functions for different rope lengths are used for frequency-domain analysis. Also, time-domain simulations are performed to analyze the lifting system in case of wave excitation and slack rope situations.