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API REPORT PRAC 15 Document Information:
Title
Probability-Based Fatigue Design Criteria for Offshore Structures
American Petroleum Institute
Publication Date:
Nov 1, 1979
Scope:
EXECUTIVE SUMMARY
Metal fatigue in the welded joints of offshore structures subjected to
oscillatory wave loading is a principal mode of structural failure.
The overall goal of this project is to critically examine fatigue
design procedures for steel platforms.
In offshore construction where so many design factors are subject to
considerable variability, a probabilistic approach seems particularly
relevant. Therefore a specific goal is to develop probability-based
procedures which could be used for a) safety checking, and b) a basis
for recommending code requirements.
The following components of the fatigue problem were studied in the
first year project.
1. The rainflow method of measuring and counting stress cycles of a
random process for fatigue analysis is generally regarded (by fatigue
experts) as the most accurate. This method was used as a basis for
developing a general form, which is easy to use, for predicting
fatigue in a joint of an offshore structure. The study demonstrated
that a 25% to 30% increase in fatigue life is typically calculated
when this approach is employed as compared to the conventional "narrow
band" approach.
2. A method of statistical analysis of cycles to failure data (S-N
data) was proposed. Fatigue data on welded tubular joints of various
sizes and loading conditions from several investigators were analyzed.
Summary statistics are presented herein.
3. Various approaches to fatigue reliability analysis were reviewed.
The second year study will produce a recommended algorithm for safety
checking and design code development.
4. An example problem of a joint, provided by a member of the advisory
committee, was studied. Deterministic fatigue analysis procedures
commonly used by the industry were compared with a probabilistic
fatigue analysis. The purpose of such an exercise was to a) identify
those factors in the fatigue design process which are most important,
and b) attempt to define a measure of acceptable risk as a basic
requirement in the probabilistic approach.
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