Manoeuvring of a ship in a seaway

Abstract

Relations between a traditional manoeuvring theory formulated with respect to a body fixed axis system and a traditional seakeeping theory formulated with respect to an equilibrium axis system are presented and used to compare their experimental and theoretical hydrodynamic data sets. Experimental data are obtained using a Planar Motion Mechanism, modified so that both seakeeping and manoeuvring data sets can be determined by similar means. Theoretical data are obtained using pulsating and translating, pulsating source three dimensional seakeeping methods. The work helps confirm the feasibility of transferring data between theories and details the effects of known deficiencies in the theoretical methods, particularly at low frequencies of oscillation.
The particular case of a ship at zero encounter frequency in following seas is examined by using expansions of the wave excitation to form course stability criteria. The influence of wave characteristics and suitable methods for determining equilibrium conditions are also detailed.
To investigate the manoeuvring of a ship in a seaway, a unified mathematical model is developed which encompasses traditional seakeeping and calm water manoeuvring theories. The fluid forces and moments in this model are represented using convolution integrals so that it is applicable to arbitrary forms of linear motion and the influences of the fluid memory effect are properly accounted for. Experimental and theoretical methods of obtaining data for the unified theory are examined, with special care devoted to ensuring that the impulse response functions model all the necessary system characteristics. During this work, relations between the traditional and unified mathematical models are detailed and a practical hybrid approach presented which entails the use of low frequency experimental manoeuvring data with higher frequency theoretical seakeeping data. Numerical methods for solving the unified equations of motion are developed and results obtained using a time simulation based on a fourth order Runge Kutta scheme. Particular cases are examined to validate theoretically the implementation of the time simulation and to demonstrate that the unified model does encompass the traditional theories. Finally, preliminary results for a Mariner type ship performing manoeuvres in a seaway are presented.

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ORCID for Pandeli Temarel: orcid.org/0000-0003-2921-1242

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