Response Analysis and Operating Limits of Crane Ships

Results are presented of a recent research project with the objective of quantitatively predicting the dynamic response of crane ships during lifting operations offshore. For this purpose, a mathematical simulation model was developed. This model considers the elasticity and damping of the hoisting rope assembly and includes bi-angular swing of the hook load coupled with hull motions. The nonlinear, time-domain motion equations comprise effects of gravity, buoyancy, hydrodynamic reactions, viscous drag, wave excitation, and the mooring system. The mathematical model was verified by comparing calculated system response with corresponding measurements from scale-model tests. To derive transfer functions of system response, the dynamic motion equations were linearized about the system's equilibrium state. The dynamic behavior of a shear-leg crane barge and a semisubmersible crane vessel was investigated and operational limits were specified. Based on these limits, obtaining the crane ships' probability of successfully performing a heavy lift was demonstrated.