STUDIES OF DYNAMIC RESPONSE OF OFFSHORE GUYED TOWERS

A critical examination is made of the dynamic response of a realistic guyed-tower structure to wave-induced excitations. The tower is modeled as a cantilever, discrete, stick-like system, and due consideration is given in the analysis to the nonlinear behavior of the guylines and foundation. The wave forces are defined by Morison's equation in combination with a Pierson-Moskowitz wave spectrum and linear wave theory. The response of the system is evaluated in the time domain by application of the concept of pseudo-forces and the modal method of analysis. The effects of numerous factors on the dynamic response of the structure are investigated. The factors considered include: (1) the relative contribution of the modes of vibration of the system to its response; (2) the effects of damping and fluid-structure interaction; (3) the nonlinear effects of the guying and foundation systems; (4) the P-Delta effects of the vertical loading; (5) the effects of variations in the values of the drag and inertia coefficients; and (6) the influence of the random phase angles used in the generation of the random sea. The response quantities examined are the lateral displacements of the structure, the associated shear forces and bending moments, and the guyline reaction. Particular emphasis is placed on the maximum values of these response quantities. The results are presented in graphs and tables and they are interpreted in a manner that is believed to provide valuable insight into the dynamic action of this type of structure. An exploratory study of the applicability of the response spectrum approach to design for waves is also reported. The results of this study demonstrate the potential of the approach as a practical design tool for offshore structures in general