Knoop, J. M. M.; (1993) Real time estimation of directional waves and drift forces on floating structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis aims to establish the feasibility of using on-board measurements to determine the nature of the incident wave and the slowly varying drift forces whilst they occur in directional or short crested seas. This capability could have widespread applications in offshore engineering, such as environmental data gathering, active mooring systems and dynamic position keeping. If measurements of surrounding wave properties were to be used to determine the incident wave, they would be distorted relative to the undisturbed wave properties by scattered and radiated waves generated by the presence of the body. Alternatively, if the motions of the body itself were to be used to infer the undisturbed wave, the information would be filtered and even lost at some frequencies. In either case the problem is essentially one of linear inverse filtering or deconvolution, given that it is possible to determine the linear transfer functions relating the undisturbed wave height to any first order body motion or wave property, by means of modern numerical diffraction analysis. This work demonstrates how information about the undisturbed wave at all frequencies of interest can be retrieved by constrained deconvolution of the outputs of a number of sensors with different response characteristics, placed on or around the vessel. It is shown how this approach is readily extended so that the array of sensors can be made to act as either a conventional or an adaptive beamformer which filters out noise or waves from different directions, and the means of evaluating the performance of a given system is described. The problems associated with implementing the inverse filter and the beamformer in real time are addressed and results are presented for real time wave estimation from simulated, experimental and full scale data. The real time model of a directional sea which retains information about the amplitude, phase and direction of the waves can be used to calculate the resulting non-linear drift force. The quadratic transfer function for a given degree of freedom describes how the different frequencies in the incident wave interact to produce a low frequency force component and it is shown here how, with certain limitations, the drift force calculation can be implemented in real time. This would provide a time history which could be used as feed forward information by the controller of a dynamic positioning system, to improve performance and efficiency. Finally it is shown by using auto regressive model fitting on both simulated and experimentally measured data, that it is possible to predict the variation in the drift force to a greater extent than the wave itself can be predicted, thereby increasing the benefit of the information to a ship control system.

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