Manoeuvring behaviour of ships in extreme astern seas
In an attempt to contribute the efforts for the robust and effective numerical tools concerning ship motions in astern seas, this thesis presents the development of a coupled non-linear 6-DOF model with frequency dependent coefficients, incorporating memory effects in random waves with a new axis system that allows straightforward combination between seakeeping and manoeuvring model whilst accounting for extreme motions. A combination of seakeeping and manoeuvring is achieved through the adoption of relatively new "horizontal body axis system" which accounts for large vertical motions as well. Furthermore, the frequency dependent terms are incorporated in order to improve the accuracy of the numerical model for non-zero encounter frequencies which are experienced especially when the ship has large heading angle. The effect of encounter frequency and so called "memory effects" are calculated in terms of radiation forces using convolution integrals. Equations of motions and external forces are described in terms of a new axis system. The wave forces are calculated through incident and diffraction wave forces. The incident wave forces are calculated using the instantaneous wave surface while low encounter frequency model is adopted for the calculation of diffraction forces. Finally, the whole numerical model is expressed in random sea environment including the convolution terms to carry out the simulations in more realistic sea environments. The validation of the numerical model with the results of benchmark tests commissioned by ITTC Specialist Group on Stability, showed reasonably satisfactory agreement while the inclusion of frequency dependent terms affected the accuracy of the numerical model. Parametrical studies were carried out to investigate the effect of different environmental and operational parameters to ship motions in extreme astern seas along with the effects of degrees of freedom and encounter frequency. In order to enhance the numerical model and to obtain further information about the coupling of the motions and the adequacy of the numerical model to carry out further simulations regarding dangerous situations during ship motions in random following and quartering seas, extensive captive and free running model tests were carried out. The numerical model provided good agreement with the experiments. The terms resulting from the coupling of vertical motions and large heeling angle to wave forces are obtained. It is believed that the numerical model has a good potential for providing a more rational basis for predicting the dangerous conditions which a ship could face in extreme astern seas, and for offering insights about the link of behaviour with the design parameters of a ship in the light of the validation with the experiment results and parametrical studies.