Maneuvering and Seakeeping of a Single Ship and of Two Ships in Interaction

Abstract

The main objective of this thesis has been to study the maneuvering and seakeeping of two interacting ships. A systematic approach is adopted. First, the maneuvering behaviour of a single ship in regular waves was considered. Then the seakeeping and maneuvering of two ships involved in close proximity maneuvers such as the overtaking and replenishment maneuvers are studied.

The combined seakeeping and maneuvering analysis for a single ship has been achieved by a new theoretical model based on the two-time scales; one rapidly varying time scale associated with the linear wave induced motions and one slowly varying time scale associated with the maneuvering motion. The slowly-varying maneuvering analysis affects the seakeeping problem when the advancing ship executes a maneuver in a seaway. Linear wave induced motions and loads are determined by the generalized 5 degrees of freedom Salvesen et al.’s (1970) (STF) strip theory. This method is also used for the estimation of the maneuvering derivatives required by the maneuvering analysis based on the nonlinear Söding’s (1982) generalized slender-body theory.

The two-time scale model is developed on a modular concept. The detailed analysis of the propulsion system, the steering system, and the nonlinear viscous effects has been provided by the resistance and propulsion module, rudder module and nonlinear viscous loads module, respectively. The regular wave field effects upon the manuevering ship are determined by a module which gives the estimates of the mean second order wave loads. The mean second order wave loads dependent on the linear unsteady flow field and thereby on the wave-induced ship motions are part of the seakeeping analysis. The prediction of the mean wave loads is carried out according to four different potential flow theories, namely the theories by Salvesen (1974), by Loukakis and Sclavounos (1978), by Faltinsen et al. (1980) and the short-wavelength asymptotic theory by Faltinsen et al. (1980). Comparisons between the different theories are given and extensively discussed on selected types of ships advancing on a straight line course at different mean forward speeds.

The application of the two-time scale model is demonstrated on the numerical examples of combined seakeeping and maneuvering of chosen ships involved in typical Zig-Zag and Circle maneuvers. Wave conditions are chosen for realistic maneuvering cases in sheltered areas. The influence of the incident waves on the maneuvering behaviour of a particular ship is investigated.

As a counterpart to a two-time scale model, the development of the unified seakeeping and maneuvering model based on the convolution integral approach given by Bailey et al. (1997) is presented. The main features of the Bailey et al. (1997) model are outlined and compared with a two-time scale model. The adequacy of the convolution integral based model is investigated and discussed in relation to the maneuvering simulation of the highy nonlinear maneuvers of a single ship. Due to the importance of the convolution integral based model, a two-dimensional analog of the Bailey et al. (1997) model has been applied to study the linear transient behaviour of two dimesional symmetric bodies of selected shapes. In particular, two transient problems are analysed. First, the transient analysis without the presence of the external regular wave excitation is considered. In the second problem, the external regular wave excitation is accounted for.

Finally, the hydrodynamic interaction between two ships advancing in either calm water or regular waves is numerically studied for a typical overtaking maneuver and an UNderway/VERTical REPlenishment (UNREP/VERTREP) ship-to-ship operation. A collision scenario between the two ships is demonstrated in case of overtaking maneuver in calm water. The two-time scale model of a single ship is generalized in order to account for the seakeeping and maneuvering analysis of two interacting ships. Calm water interaction loads are estimated by using the Newman-Tuck’s (1974) theory. The seakeeping analysis is performed by using 5 degrees of freedom Salvesen et al.’s (1970) (STF) strip theory. It is assumed that there is no hydrodynamic interaction between the two ships. The mean second order wave loads are estimated by Faltinsen et al.’s (1980) direct pressure integration method. The motion control ‘autopilot’ module is incorporated in the model.