NONSTATIONARY RESPONSE ANALYSIS OF OFFSHORE GUYED TOWER SUBJECTED TO EARTHQUAKE LOADING

A method for nonstationary seismic response analysis of an offshore gu yed tower is presented. The equation of motion is formulated for the h orizontal motion of the guyed tower using a multidegrees-of-freedom mo del. The nonlinear hydrodamping and the nonlinear restoring force of t he guyline system are included in analysis with the aid of the equival ent linearizations. The nonstationarity of the earthquake excitation i s modelled by imposing a time varying envelope function onto a station ary random component, The nonstationary responses are obtained in term s of time dependent variance functions. By taking the autocorrelation function of the nonstationary ground acceleration in terms of sinusoid al and exponential functions of time, an analytical procedure is devel oped for computing time varying variances of the tower responses. Expe cted maximum responses are evaluated thereafter. Example analysis indi cates that the nonstationary method gives consistent estimates for the maximum responses, if the earthquake duration is taken to be longer t han the duration of strong excitation, Comparison is made with the res ults obtained by the conventional spectral method based on the station ary assumption. Copyright (C) 1996 Elsevier Science Ltd.