A Spectral Fatigue Analysis for Offshore Structures

Abstract

ABSTRACT Deepwater platforms may experience a metal fatigue problem that governs the design of many joints even for relatively mild environments such as the Gulf of Mexico. The need to consider the fatigue characteristics arises from the dynamic amplification of wave-induced response. The fatigue model used in this paper is the well-known Palmgren-Miner rule which requires a stress history curve and a cycles to failure (S-N) curve for implementation. The concern of this paper is with the stress-history curve. The dynamic response of the platform is determined using a lumped-mass mathematical model with viscous damping, linearized soil springs, and a consistent forcing function based on a generalized Morison's equation. The equations of motion are solved in the frequency domain using a power spectral density approach for one standard deviation values of various response parameters. Members in the wave zone are assumed to have component stresses due to lateral wave loads and inertial loads. An example problem using this fatigue algorithm is worked for a location on a typical joint where there is a rapid change in curvature of the material, and results are presented in graphical form. The platform environment is assumed to be represented by a number of sea states each of which is assumed to be a stationary, ergodic, narrow-banded, Gaussian process. The platform is assumed to vibrate at its fundamental period, and the cycles of stress for any particular point on the platform during each sea state are assumed to be Rayleigh distributed. Introduction Offshore oil platforms are normally composed of tubular members made up of steel plate, cold rolled and welded. These members are in turn cut to fit and welded to form the overall platform. Wave loads are primarily resisted by bracing members, and these members transfer the applied forces through the joints into the legs. Designers make a concerted effort to design the joints so that the load transfer will be as efficient as possible. Nevertheless, stress concentrations will occur in the vicinity of the member intersection.