Ship Vibrations in Random Seas

Abstract

A general analysis of ship dynamics in random seas is presented. The analysis covers the steady-state wave-induced response and the transient-state slam-induced (whipping) response. Wave-induced response includes both the rigid-body modes (seakeeping) and the hull flexural modes (springing). The strip theory by Salvesen, Tuck, and Faltinsen is used to determine the hydrodynamic forces. The ship structure is idealized by finite beam elements with the lumped-parameter system. The normal-mode approach is used to calculate the vibration characteristics and dynamic responses. Statistics of both wave-induced loads and responses are characterized by zero-mean Gaussian processes. By spectral analysis, the wave-induced responses can be predicted for a ship moving in random seas. The slamming impacts as the input to the ship are treated as a nonstationary filtered Poisson process. The output slam-induced (whipping) response process can be determined by passing such a nonstationary process through the time-invariant ship system or by using a Markov process. Two ship examples are studied. The 1000-ft (305 m) Great Lakes ore carrier Stewart J. Cort is used for the calculation of the wave-induced response, and the 525-ft (160 m) SS Gopher Mariner is used for the calculation of the slam-induced (whipping) response.