Improvements in the conduct and interpretation of ship seakeeping trials

Abstract

Although the complex nature of sea waves is often cited by authors as a reason for poor results, the published papers show little in the way of explanation of these complexities when treating full-scale trials data. In this thesis it is argued that measurement of the directional wave spectrum is essential for any seakeeping trial. Ways are demonstrated in which the wave complexity, particularly wave directionality, may be dealt with.

The consequences of assuming the seaway is long crested are explored with the aim of establishing guidelines for a generally applicable minimum wave height and maximum wave spreading for ship trials. The guidelines derived show that this assumption is unrealistic for a 5% uncertainty in the ship response.

Accepting that sea waves on a trial are likely to be multidirectional in nature, a case is made for a specific 'star' type ship evolution for trials so that the ship samples waves from all directions. The beneficial effects of this evolution are illustrated with trials results for five different ships of various types.

The direct validation of ship motion codes against trials data is not considered per se, however some guidance is given on using the sensitivity of ship motion codes to uncertainty in the exact ship condition during the trials. An efficient technique to include these error bands is proposed.

Validation of ship motion codes usually includes a great deal of simplification of the characteristics of the seaway, both in its directional nature and its spectral form. This thesis demonstrates that meaningful transfer function results can be achieved where the trials have been made with the ship in a strongly directional seaway. The theory of Fryer (1991) is used to calculate the transfer functions of a frigate ship operating in a clearly bimodal seaway.

Where sea trials currently take place, seakeeping performance assessment is rather a subjective process. Application of these methods shown here would assist in the process of demonstrating the seakeeping ability of new designs with sea trials, and hence allowing seakeeping to take a more accountable place in the procurement and acceptance cycle.

Furthermore two approaches are made to the 'reverse calculation' of the characteristics of the seaway based upon the ship motions, as if it were a moving wave buoy. In the first, a matrix solution at discrete frequencies of a set of simultaneous equations involving the transfer functions and ship motion results proved unsuccessful. It is suggested that making independent calculations at each frequency step is a fundamental source of difficulty with this method. A further complication arises as it is shown that derivation of wave slope encounter spectra from wave height encounter spectra requires knowledge of the stationary frequency, and is thus subject to the 'following seas' problem.

The second approach used multiple linear regression to derive simple formulae for the ship relative heading and significant wave height to an accuracy of the order 10% based purely on the RMS ship motions. This could find application in a shipboard guidance system.

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