RESPONSE ANALYSIS OF OFFSHORE SYSTEMS TO NONLINEAR RANDOM
WAVES: RESPONSE STATISTICS
Ahsan Kareem, Professor, Dept. of Civ. Eng. &Geo. Sd., Univ. of Notre Dame, Notre Dame,IN 46556-0767
Chang C. Hsieh, Formerly, Grad. Res. Asst., Univ. of Houston, Houston, TX77204-4791
Michael A. Tognarelli, Grad. Res. Asst., Dept. of Civ. Eng. &Geo. Sci., Univ. of Notre Dame, Notre Dame, IN 46556-0767
ABSTRACT
This paper details an investigation of the probabilistic response characteristics of jacket-type platforms in deep water which are subjected to both linear and nonlinear random wave loadings. Unlike earlier analytical treatments of this class of system, a statistical description of the wave forces according to the Morison equation is first developed to reflect nonlinearities and non-Gaussianity in the wave field kinematics. Subsequently, the deck response resulting particularly due to the effects of the second-order contribution to the loads imparted on an idealized platform up to the mean water level is studied via frequency domain analysis. Further, consideration is given to the importance of the spacing of the legs to the response of a typical jacket-type structure. Finally, numerical examples for the case of an idealized jacket platform in deep water amidst three distinct sea states compare responses computed with and in the absence of wave field nonlineanties for a range of leg spacings and illustrate the importance of including the effects of nonlineanties for making more accurate predictions of response statistics in design analyses. Two of the sea states are characterized by locally wind-generated waves modelled by singly-moded JONSWAP spectra. The third is represented by a bi-modal Ochi-Hubble spectrum which additionally incorporates the effects of swell. Findings indicate that ignoring the nonlinearity of the waves in the forcing process results in underestimation of the response level for all sea states.
