Dynamic Stability of Ships in Waves

A method is proposed for evaluating the overall dynamic stability of an intact vessel in a seaway. We use an existing ship motions program to study the motion of a vessel with a certain loading condition, speed and heading, in given wave conditions. A deterministic method is discussed for looking at the stability of a vessel over a wide range of these parameters. This is done with a view to giv ing operators advice on the safest headings and speeds to adopt in extreme conditions, as well as gauging the overall safety of a particular vessel. It is hoped that eventually such dynamic stability analysis can be used to modify the present IMO stability criteria for ships. INTRODUCTION At present, safety regulations with regard to ship capsize are based primarily on static stability concepts. The efficient calculation of GZ curves that is possible nowadays permits quick and accurate determination of a ship’s static stability. Although it is well accepted that capsizing of ships is a dynamic phenomenon, static stability considerations are still used almost exclusively to gauge the propensity of a ship to capsize in waves. This means, for example, that some vessels are at risk of dynamic capsize despite having favourable GZ curves, whereas others are perhaps being over-penalized due to the nature of their GZ curve, despite having good dynamic stability. Seakeeping is now a well-developed field, and basic techniques for simulating the motion of a ship in both regular and irregular waves are well-known (see e.g. [1,2]). Newton’s 2 Law is used to predict the time rate of change of each of the six degrees of freedom (roll, pitch, yaw, heave, surge, sway), using the nett forces and moments acting on the vessel at each point in time. The hydrodynamic forces and moments are caused by the accelerated motion of the vessel through the water, as well as the action of the wave, and may involve significant cross-coupling. Still, if the time-varying forces and moments on the vessel can be accurately modelled, the computational solution of the resulting ship motions is straightforward. Such time-domain simulations may be used to predict the “capsize” of a vessel, and thereby used to assess ship safety. The capsize of a vessel is normally defined by a predetermined roll angle, such as the angle at which downflooding occurs, or the angle at which the GZ-curve becomes negative. By definition, capsize involves large roll angles and hence highly nonlinear behaviour. Therefore, basic linear seakeeping theory has given way to more accurate modelling of the hydrodynamic forces for predicting capsize, involving significant cross-coupling and nonlinear terms (see e.g. [3,4,5]) Gourlay, T.P. & Lilienthal, T. 2002 Dynamic stability of ships in waves. Proc. Pacific 2002 International Maritime Conference, Sydney, Jan 2002.