Wave-induced motions on high Froude number slender twin-hull vessels
thesisposted on 2023-05-27, 12:58 authored by Watson, NL
A study of the wave-induced motions experienced by slender twin-hull vessels was conducted through experimental measurements and numerical computation. Particular attention was required in the analysis of measured data for comparison with the predicted motions. The difficulties of analysing measured data from a vessel that had encountered sea waves of a random nature were addressed by grouping the data into subsets based on primary wave direction, vessel speed, and hull configuration. The primary wave direction was determined by considering the motions of the vessel as for a directional wave buoy. The data acquisition system consisted of hardware and software that was developed and assembled on two twin-hull full-scale vessels for periods of up to 12 months each. Designed to operate mostly unsupervised whilst collecting data in remote locations, a data recording sequence was initiated through a preset trigger level from a motion sensor. In the subsequent analysis, transfer functions were obtained from the measured data based on the sorted data subsets. Numerical modelling of the vessels in head sea waves used an existing strip theory code that defined the hull as sectional boundary elements that contributed to solving a Green function problem for the free surface and sectional motion in the time domain. A modification to the code allowed the effects of motion control surfaces on the motion transfer functions to be determined through selection of appropriate gain setting that minimised the average vertical hull accelerations. With the damping effect of control surfaces not scaling linearly with wave height, the controls were modelled to minimise the hull average vertical acceleration in a 2.5 metre Bretschneider wave spectrum of 7 second average period. The inclusion of control surfaces in the motion computation combined with non-linear wave height effects in some instances greatly improved the correlation between the numerical and measured motion transfer functions by reducing the frequency of maximum response. The accelerations and motion sickness incidence (MSI) distributions derived through computations on an 86 metre vessel indicate the presence of high accelerations that are not fully counteracted by a motion control system of practicable size. Future advances in numerical prediction codes may require verification with detailed full-scale measurements to ensure the scale effects are adequately considered. Fullscale measurements should therefore constitute part of a complete testing and analysis program for motions, but due to the high cost of these trials the measurements required for analysis may best be obtained from data supplied from on board monitoring systems during regular service operations.
Rights statementCopyright 2004 the author. This thesis contains a confidentiality statement, permission to release this thesis was obtained from the identified parties in May 2018. The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).