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API REPORT 81-15 Document Information:
Title
Probability-Based Fatigue Design Criteria for Offshore Structures
American Petroleum Institute
Publication Date:
Jan 1, 1983
Scope:
EXECUTIVE SUMMARY
This is the final report of a four year study entitled "Probability
Based Fatigue Design Criteria for Offshore Structures," and supported
by the American Petroleum Institute. The overall goal of this program
was to critically examine the process of fatigue safety checking for
the welded joints of fixed offshore structures. 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.
During the project the following issues were addressed,
1. The performance of Miner's rule, a method for using constant
amplitude data to predict fatigue under random stresses, was
quantified.
2. Ocean wave loading produces random stresses in the joints. Various
methods for extracting fatigue stress cycles from the random process
for use with Miner's rule were studied. The rainflow method 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 current analysis techniques
used by some petroleum companies produce fatigue life estimates which
are conservative by approximately 25 to 30% based on cycle counting
methods alone.
3. Methods of fatigue damage assessment used by the petroleum industry
world wide were reviewed and are summarized herein.
4. A method of providing a statistical summary of fatigue (S-N) data
for design purposes was proposed. Fatigue data on welded tubular
joints of various sizes and loading conditions from several
investigators were analyzed. Summary statistics are presented herein.
5. 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.
6. Various reliability analysis methods and reliability formats were
reviewed for specific application to the fatigue problem.
7. A fatigue reliability model based on the lognormal distribution
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.
8. The reliability model was employed to review the current fatigue
requirements of RP 2A. This exercise demonstrated that the current
(1982) rules specified in Paragraph 2.5.3a (peak hot spot stress at
joint due to design environment should be less than 60 ksi) produce
inconsistent levels of risk. Recommendations for a more discriminating
safety check requirement was provided.
9. A method of calibrating the reliability model using observed
fatigue failures in existing structures has been developed. The median
rank concept and theory of suspended data is employed. At present,
data is being solicited, and reliability calibration exercises will be
conducted in a future study.
10. A study 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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