On the wave-excited ship vibrations in regular waves and in confused seas

Abstract

This paper is concerned with the theoretical investigations of wave-excited ship vibrations (springing) in regular waves and in confused seas.

For deterministic analysis, a step-by-step approach is linked with the transfer matrix technique for determining the springing response in regular waves. The main advantage of the transfer matrix technique is its systematic formulation, and one of the major features of the step-by-step approach is its generality. This approach can be used to tackle the sinusoidal or nonperiodic vibration problems. Through the application of the present approach the effects on springing of vibratory deflection, ship motions and the time rate of change of sectional added mass (dm’/dt) are also studied.

For statistical analysis, the ship behaviour in confused seas was generally predicted with “short-term” springing response in most of the existing information. This “short-term” value, which is obtained by combining the deterministic analysis results and wave spectra, cannot be the basis of ship design since it cannot predict the probabilistic maximum springing response over the ship’s life-time. For practical application, the “short-term” value needs to be extrapolated into the “long-term” value. This further step has been completed in this paper.

For a ship in regular waves, the influence on springing of the key parameters such as draft, ship speed, and heading angle are also examined, and the theoretical explanation for the trend of springing response to each influencing factor is presented.