ssc-367 - Ship Structure Committee

Offshore structures such as a semisubmersibledrilling vessel is a
continuous system,typicallyhaving orthogonallystiffenedmembers.
While a simplified method, such as Munse’s, may be an applicable
screening method, such structures have very specialized
configurations, response characteristics and structural details.
Thus, each structure should be considered unique, requiring a
detailed fatigue analyses.
An offshore platform is made up discrete members and joints. Since
each structure is unique, a detailed fatigue analysis is
recommended. However, asimplified method may beappl icable if such
a method can be developed based on a large number of similar
structures in a given geographic region. Such a method was
developed for the Gulf of Mexico by American Petroleum Institute
(Reference 1.5) and discussed further.
The simplifiedAPI method (Section5.1.1 of Reference 1.5) is based
on defining the allowable peak stresses as a function of water
depth, design fatigue life, member location and the applicable S-N
curve. Although the approach can be modified to apply to other
geographic areas, it was developed by calibrating previously
completed fatigue analyses of fixed offshore platforms. The
maximum allowable stress method is applicable to typical Gulf of
Mexico platforms with structural redundancy, natural periods less
than three seconds, and the water depths of 400 feet or less.
ThisAPI allowablestressmethod i$ intendedfor use as a simplified
fatigue assessmentprocedurefor Gulf of Mexico platforms subjected
to long-term cyclic stresses considered small relative to the
extreme environment stresses. The method attempts to predict
fatigue behavior as a function of the design wave event for a
generalized platform. It should be noted that the applied force
levels can vary substantiallywith platform geometry. The relative
importanceof extremedesignwaves and operatingenvironment fatigue
waves changes with both the water depth and the actual member/joint
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