The seismic response of articulated offshore tower has been investigated by the spectral analysis method which is based on the principle of random vibration, where seismic excitation is assumed to be a broadband stationary process. The nonlinear dynamic eqliation of motion is derived using Lagrangian approgch and the solution is obtained by Newmark's P integration scheme. The present study includes nonlinearities associated due to variable submergence, drag force, Coulomb damping, variable buoyancy, and added m s along with the geonmrical nonlinearities of the system. The study includes the joint occurrence of waves and seismic fofces together with the current under random sea state. A parametric study has been conducted to investigate the relative importance of the seismic response in comparison to the response due to wave forces.
A B S T R A C T The paper describes a methodology for computation of fatigue reliability of universal joint in an articulated offshore tower. Failure criteria were formulated using the conventional Palmgren-Miner rule (S-N curve approach) and the fracture mechanics (F-M) principle. The dynamic analysis of double hinged articulated tower under wind and waves is carried out in time domain. The response histories of hinge shear stresses are employed for the reliability analysis. Advanced first-order reliability method and Monte Carlo simulation method were used to estimate the reliability. Various parametric studies were carried out, which yield important information for the reliability based design. The S-N curve approach yields a significantly conservative estimate of probability of failure when compared to the F-M approach. A = fatigue strength coefficient a = crack size a • = initial crack size a c r = critical crack size B = stress modelling error C = Paris coefficient [C] = damping matrix C • , m = crack propagation parameters C d = drag coefficient D = accumulated damage da/dN = crack growth rate E[.] = expectation f i = zero crossing frequency f x (X) = probability density function of the random variable X g(X) = limit state function H s = significant wave height [I] = mass matrix K = intercept of the S-N curve at σ R equals to one [K] = stiffness matrix L = load effect {M θ } = forcing function m = slope of the S-N curve m = fatigue exponent m o = nth moment of the stress spectrum N = number of cycles to fatigue failure Correspondence: M. M. Zaheer.
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