Stochastic Seismic Analysis of Yielding Offshore Towers

An analytical procedure to determine the stochastic response of inelastic offshore towers to a nonstationary ground motion in a semiclosed form is described. The method is approximate but the solution is obtained without recourse to time-consuming Monte Carlo solution generation techniques. The offshore tower is modeled as a lump mass system and the resulting forces are obtained as a combination of linear and hysteretic forces. A filtered shot noise process is used to represent earthquake ground motion. Nonlinear hydrodynamic drag forces and the structure resisting hysteretic forces are replaced by appropriate linear forms by using equivalent linearization technique. The system response is then immediately obtained from the Kolmogorov-Fokker-Planck equation for the linearized structure system as the solutions of a nonlinear first-order matrix differential equation. To confirm the validity of the method an example problem is solved using this method and the Monte Carlo solution technique.