An Aid to Operating Decisions Based on Nonlinear Response of a Crane Barge in Waves

Abstract

A nine-degree-of-freedom time domain mathematical model was used to numerically simulate crane ship dynamics. This model considers the elasticity and damping of the hoisting rope assembly and includes arbitrary, biangular swing of the suspended hook load coupled with surge, sway, heave, roll, pitch, and yaw motions of the hull. The linear frequency-dependent hydrodynamic response to the hull’s own motions accounted for memory effects that were approximated in the time domain by a finite state space model. The nonlinear hydrodynamic drag force acting on the oscillating hull was quadratically approximated using an empirical drag coefficient. The incident wave force on the hull consisted of the first-order force oscillating at wave frequency and the second-order slowly varying wave drift force. The nonlinear horizontal mooring system restoring force was approximated by a third-order polynomial. To specify operating limits for a shear-leg crane barge in a heavy lift offshore operation, a stochastic analysis was performed based on the system’s simulated dynamic response in an ensemble of natural seaways.