Samra, Balwinder Singh; (1994) Prediction and control of the motions of marine structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis reports the application of real time prediction and control techniques to mitigate problems caused by the motions of compliant marine structures. Simulations and real ship motion data are used to assess an adaptive predictor. Practical points such as the appropriate choice and modification of forgetting factors, selection of sampling interval, effectiveness of concatenation and recursion of the Diophantine equation to generate multistep predictions are highlighted. Such aspects are vital to applications but hard to derive from theory. Ship motion data is used to assess the predictor as an operator guide. The predictions are shown to be useful in reducing waveoffs and crashes in VTOL operations at sea. A predictor/controller system for crane barge loading operations in rough seas with constraints on control is developed using optimal control and a novel arrangement of model based predictive control (MBPC) techniques. The importance of constraints is emphasized and the MBPC techniques are shown to offer an efficient method of dealing with constraints. Frequency domain techniques are used to design a motion suppression controller for a pneumatic semisubmersible - the exorbitant demanded control inputs shows the engineering impracticality of such a scheme. Significant nonlinearities in the dynamical equations of offshore structures can under regular excitation give rise to disturbingly large subharmonic or chaotic motions. The prediction of incipient bifurcations is shown to be equivalent to very accurately identifying evolving eigenvalues migrating towards an instability boundary. The practical feasibility of detecting the precursors to a bifurcation is illustrated with simulations and experimental data from a fishtailing tanker. The non robustness of subharmonic behaviour in the presence of random inputs is vividly demonstrated using simulation.

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