ON THE DEVELOPMENT OF SEAKEEPING CRITERIA

In order to determine the effects of ship motion on operational performance, quantitative seakeeping criteria are required. In this paper, it is proposed that seakeeping criteria should be presented in terms of limits on the incidence of whatever degrading event is determined to curtail the operation of interest, rather than limits on the underlying ship motions. For personnel operating on deck, the degrading events of most interest are the onset of loss-of-balance incidents, termed motion-induced interruptions, due to tipping or sliding. For unsecured helicopter operations, the most important degrading event is the helicopter sliding. The authors present a frequency-domain method of predicting the incidence of personnel or of equipment sliding which includes the forces due to roll, pitch, longitudinal, lateral and vertical accelerations, and also the effects of non-zero mean heel angles. They then present a method for predicting the incidence of personnel experiencing motion-induced interruptions due to loss of balance which includes the above forces and also the effects of steady and unsteady winds. In order to work in the frequency-domain, all of the forces are linearised and we consider only the linear turbulent wind force, and neglect the nonlinear term. Two main applications of the methods are anticipated. The first application is to design. The seakeeping criteria presented here are closely linked with the physical phenomena that are causing performance to be degraded, and are equally applicable to all ship types. The second application is to active operational guidance systems which provide real time information to the operators of naval vessels on the effects of changes in speed or heading on operations. Developing criteria in terms of the degrading events of most interest to the operator should result in an immediate improvement in the quality of the guidance.