PEAK RESPONSE OF OFFSHORE STRUCTURES TO WAVE AND CURRENT FORCES (MARINE, RANDOM, UPCROSSING RATES, DESCRIPTORS, PROBABLISTIC WAVES)

The differential equations governing the motion of an offshore structure subjected to Morison wave loads are nonlinear. The solution of these equations is generally based on approximate methods, such as statistical linearization. This work develops methods for estimating the quasi-static and dynamic peak responses of offshore structures subjected to wave forces.; Response descriptors such as mean upcrossing rates and average peak responses are obtained for the quasi-static response by point-crossing formula and vector crossing analysis. The peak descriptors for the dynamic response are obtained by Monte-Carlo simulation and time discretization methods. The Monte-Carlo simulation developed here uses the FFT technique and is computationally efficient. It can account for the spatial correlation of the wave forces. In the method of time discretization, the wave load is discretized in time and represented as an independent sequence. The marginal probability distribution function of the response to this sequence can be obtained by an FFT inversion of its characteristic function. Mean upcrossing rates of the response are obtained approximately if the response is represented by a translation process. Translation processes can be obtained from Gaussian processes by memoryless transformations.; The methods are applied to the analysis of two offshore platforms; a Tension Leg Platform and a Tower Platform. Approximate methods of analysis such as the statistical linearization are found to be inadequate in prediction of peak response. On the other hand, approximations based on the time discretization provide reliable and accurate results.