SECOND-ORDER LOAD AND RESPONSE MODELS FOR FLOATING OFFSHORE STRUCTURES: PROBABILISTIC ANALYSIS AND SYSTEM IDENTIFICATION

Floating structures moored to the sea bottom provide a practical solution to the problem of producing oil and gas in deep water. Unlike more common fixed structures, the mooring systems of floaters can be designed such that the system's natural periods fall outside the range of common wave periods. Due to nonlinearity in the wave to force transformation, however, oscillating forces can still arise in period ranges that will cause resonance in the structure. A second-order model of wave forces can be derived from a perturbation expansion of the linear form of the wave-structure interaction problem . These second-order, diffraction-analysis models have been used increasingly in recent years for modeling the linear and nonlinear wave forces on large volume structures. The first objective of t h.is work is the development of general methodologies that can be used for the efficient probabilistic analysis of these second-order models. The focus is on the prediction of response statistics of a deterministic system subject to a stationary, random wave input (a seastate). For the study of this short-term uncertainty we combine the method of Kac and Siegert for finding statistical moments of a second-order system with the Hermite model for analysis of non-Gaussian extremes and fatigue damage. (Note that the Hermite model is a general, moment-based model that is in no way dependent upon the use of the Kac and Siegert method.) Combined with the power spectrum of output to reflect dynamic character, the moment-level description provides an accurate and flexible characterization of the random output.