Fatigue Analysis of Offshore Structures

Abstract

This paper presents a procedure for fatigue analysis where the dynamic response of the structure is analyzed through a spectral approach. The sea waves, which constitute the forcing function acting on the structure, are represented as energy spectra; the response is obtained in spectral terms and subsequently is interpreted according to probabilistic concepts. Introduction The critical loading conditions for design of an offshore platform are primarily dependent on water depth and prevailing meteorological conditions such as waves, currents, and winds. If dynamic amplification is not a major consideration, the static applied force of the largest single wave traditionally is used as the critical or design load condition for fixed offshore structures. In the corresponding fatigue analysis the sea surface is represented by a discrete set of waves characterized by period, amplitude, and percentage of occurrence. The expected fatigue life then is obtained by applying each wave to the structure and totaling the individual effects.Due to the dynamic amplification of wave-induced response, this approach is not adequate for deepwater structures, where fatigue may be a controlling factor in design. In this case, distribution of the frequencies of wave loadings is needed over a much larger range to give a more realistic description of the sea surface. In practice, this is accomplished by statistically modeling the sea-surface elevation. It has been shown that the time history of sea-surface elevation over a period of several hours (i.e., a sea state) can be represented as a stationary random process with Gaussian probability distribution and process with Gaussian probability distribution and narrow-band power spectrum. This work discusses a fatigue analysis procedure based on these considerations.The first three sections of this paper describe a fatigue analysis procedure in which the spectral density of member end stresses is obtained by a frequency domain solution using existing dynamic analysis and space-frame static analysis programs. The fourth section discusses narrow-band approximations for the stress-range distribution and their effect on fatigue analysis. The fifth section presents fatigue results for a caisson structure. The presents fatigue results for a caisson structure. The sixth section gives conclusions and observations. Fatigue Analysis Procedure The first step in this fatigue analysis technique involves a static analysis. The structural model uses a space-frame, full-geometry simulation of the jacket with three-dimensional beam elements. Every member of the structure contributing to the overall stiffness is modeled by one or more elements. Unit loads are applied at selected levels of the three-dimensional model, and the resulting deflections for each unit load are noted. For each preselected member, the stresses at the member ends also are computed and saved for use later in the analysis.Next, the wave-induced dynamic response is calculated using a modified version of the TOWER spectral analysis program developed by Penzien et al. JPT P. 1614