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API REPORT 80-15 Document Information:
Title
Probability-Based Fatigue Design Criteria for Offshore Structures
American Petroleum Institute
Publication Date:
Feb 1, 1981
Scope:
EXECUTIVE SUMMARY
Metal fatigue in the welded joints of offshore structures subjected to
oscillatory wave loading is a possible 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 will be 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 (1979).
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 current analysis technique in common use by the petroleum
industry produces fatigue life estimates which are conservative by
approximately 25 to 30%.
2. A method of statistical analysis of fatigue (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.
4. An example fatigue analysis of a joint, data of which were 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.
Results of the second year project (1980) reported herein include the
following:
1. A fatigue reliability model suitable for safety checking of joints
or as a mechanism for code review was proposed. Demonstrations of the
performance of the model showed that its form is compatible with
analysis procedures currently employed in the petroleum industry.
2. The process of computing stresses in joints of platforms from
oceanographic data was identified as a major source of uncertainty in
fatigue life evaluation. An attempt was made to quantify the
uncertainties associated with various ways a company might perform its
analysis.
3. A demonstration of how the level of reliability implied by RP 2A
could be evaluated using the proposed model was provided. The third
year (1981) study will focus on code evaluation and implementation of
reliability based fatigue criteria.
4. A study of the consequences of fatigue failure of members of a
redundant structure was initiated. The goal of such a study is to
relate joint reliability to system reliability. Preliminary results
showed that structural redundancy in offshore platforms can be very
effective in reducing the risk of platform failure due to fatigue.
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