RESPONSE OF FLEXIBLE STRUCTURES IN RANDOM SEAS
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abstract
A general stochastic formulation which addresses the relative motion effects of flexible structures in a random seaway is presented. The formulation utilizes a least-squares cubic polynomial approximation to the nonlinear drag force contribution. This influences the process of developing the stochastic equations of motion in two ways. First, by introducing higher order moments in the covariance form of the equations. Through the use of Gaussian closure these moments are replaced by equivalent expressions involving only second order moments. Secondly, the cubic drag force approximation results in the appearance of a three-fold convolution of the velocity spectrum in the final form of the spectral equations. Although, an iterative solution technique is required to solve the resulting spectral equations, this formulation provides the engineer with a means to directly evaluate the structural displacement spectrum at selected elevations. Once the response spectra are known extremal response behaviour can be predicted. A single-degree-of-freedom model of a monotower platform and a multi-degree-of-freedom model of a marine riser are used to illustrate the influence of the various approximations on the response predictions. Issues addressed include the impact on the viscous damping in the structural model as a result of first and third order approximations to the viscous drag force and the need to include higher modes for very flexible structures. 1990.
