A kind of new method for solving the random response of the offshore platform, in which the parameters of structure or environment can be modified, is suggested in the paper. The perturbation method, combined with the direct spectral analysis method, has been developed to calculate the dynamic responses of the modified structure. The approach was successfully used for the random wave response of an offshore platform located in the Bohai Bay of China. Comparing with the result obtained by a traditional repeated analysis, its accuracy satisfies the engineering requirements, yet the computer time can be greatly saved. This newly developed method also is powerful for the dynamic analysis of random response of the offshore structure subjected to earthquake or ice forces.
When the random wave response analysis for an offshore structure is performed, it is noted that the exciting forces are caused by the random process of the same source, although they act on most region of the structure. Thus, it is reasonable to consider the random excitation force vector as a definite load distribution vector multiplied by a single random scalar quantity. According to the consideration, a direct spectral analysis method for random wave response has been developed (Zheng, 1985). In general, the calculation of input power spectral density function matrix begins with the wave spectrum via the fluid particle velocity and acceleration spectra and obtains the input load spectrum finally (Borman, 1967). The direct spectral method avoids this complicated calculation process, but the root mean square(rms) of both displacement and reaction force or the stresses in elements can be calculated easily. Among total calculation time for random wave response analysis of a system with higher number of degrees of freedom roof), a large portion is spent on the calculation for natural frequencies "and modes.