An investigation into the hydrodynamic analysis of vessels with a zero or forward speed

Abstract

The occurrence of irregular frequencies is known to affect the prediction of hydrodynamic coefficients, whether a rigid body or hydroelasticity analysis is used. For conventional ship-like structures, these irregular frequencies lie outside the range of practical interest for rigid body motions. On the other hand, for a large ship-like offshore structure, such as a Floating Production, Storage and Offloading unit (FPSO), the irregular frequencies are in the range of practical interest. Furthermore, they can create difficulties for the analysis of vessels treated as flexible bodies and of multi-hulled vessels. In these cases, distinguishing between genuine physical effects (e.g. wave interaction between hulls) and that of irregular frequencies is important. The aim of this paper is twofold: first, elimination of irregular frequencies for the zero-forward-speed case and, second, improvement in predictions for the forward-speed case through a more accurate evaluation of the waterline integral term. Although the investigations are illustrated for rigid hulls, namely a rectangular box and a Series 60 hull, the methodology and its effects are equally valid for flexible hulls. For the zero-forward-speed case an extended boundary integral method (the lid method) is used, with the imaginary interior free surface placed approximately 0.1% below the mean free surface. It is confirmed that the lid boundary approach is an efficient and robust method for treating irregular frequencies for stationary rigid floating bodies experiencing low-frequency oscillations. For the forward-speed case a treatment of waterline integral terms is introduced, with reference to sources lying on the free surface. The applications highlight the importance of waterline integral terms which are shown to introduce oscillatory behaviour in the predicted hydrodynamic data. It is also shown that, for the forward-speed case, irregular frequencies do not occur; however, as the forward speed tends to smaller values, the hydrodynamic data exhibit oscillations which depend on the treatment of the waterline integral terms and the use of the direct potential or source distribution method. These methods are also suitable for hydroelasticity analysis with a forward speed.