Filter approaches to stochastic dynamic analysis of compliant offshore platforms
Spanos, Pol D.
Doctor of Philosophy
The dynamics of offshore structures subjected to random waves is investigated. The load modeling and response analysis are conducted by using a new approach. The problems of the wave kinematics simulation and of the dynamic response determination are treated in an unified approach using filter techniques and the state space analysis. Some interesting results on water depth characterization in connection with the two parameter spectra are derived. These permit the application of the linear wave theory from monochromatic waves to random seas. Throughout this investigation filters which can generate the relevant wave kinematics or represent the random wave forces are sought. This is achieved by relying on developments in the field of signal processing and filter design. Novel platform concepts which hold promise for offshore oil production in deep waters are adopted for the dynamic analysis. In this context, attention is focussed on the efficacy of the method of state space analysis in the time domain, using the well known Liapunov equation. The advantages of this technique, particularly when motion control is of interest, is emphasized. The dynamic response of an idealized guyed tower is obtained in a closed form by using the Liapunov equation. A complete numerical example is included for the guyed tower. An alternate solution in the frequency domain is also obtained for comparison. Finally, the applicability of the state space method for modeling the wave excitation on an idealized 3 degree of freedom tension leg platform is investigated. The tension leg platform dynamics is examined conceptually, and further research recommended. Some key problems in the use of this powerful method are identified.
Civil engineering; Ocean engineering; Mechanical engineering