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API REPORT 26 Document Information:
Title
Stage II - Time History Analyses Earthquake Analytical Studies for Offshore Structures
American Petroleum Institute
Publication Date:
Oct 1, 1981
Scope:
SUMMARY
The overall objective of this Earthquake Analytical Studies for
Offshore Structures project is to provide a basis for evaluating and
improving current API RP-2A design guidelines for earthquake
resistance, with particular emphasis on platform capacity and
ductility requirements.
In the project, an example steel template-type drilling platform was
simulated mathematically and analyzed through computer program INTRA,
a recently-developed nonlinear finite element program. The structure's
response to earthquake ground motion was calculated by several
different methods consistent with current API RP-2A recommendations.
The baseline analyses done in Stage I of the project comprised a
typical strength requirement response spectrum analysis and an
equivalent static pushover analysis to check ductility. In Stage II,
the analytical model was subjected to three-dimensional ground motion
histories from three real earthquakes, scaled first to roughly
correspond to the API strength requirement and then doubled to roughly
correspond to the ductility requirement.
In all of the strength requirement analyses, all of the superstructure
and pile elements remained within their proportional limits, thus
satisfying the API strength requirement. However, the responses
calculated in each analysis differed substantially. The variations in
some key structural forces, displacements and baseline strain energies
were as high as 78, 42, and 142 percent respectively. These rather
large variations were attributed to a combination of several factors:
1. Differences in the intensities of the input shaking, even though
the records were nominally scaled to match the API design spectrum.
2. Differences in the characteristics of ground shaking, which led to
differences in the distribution of forces in the structure.
3. Direct combination of time history responses versus SRSS
combination of the spatial and modal responses in the response
spectrum analysis.
In the ductility analyses, the analytical model included a detailed
nonlinear inelastic model of the supporting soil. It appears that the
soil limited the amount of earthquake force transmitted into the
superstructure in the time history analyses, and further, that the
soil accumulated substantial energy if the duration of shaking was
sufficiently long. Of course this load limiting effect was not
displayed in the static ductility check, where "equivalent" static
loads were applied directly to the superstructure. The structure
showed approximately 50 percent more mudline shear capacity in one of
the time histories than was displayed in the static ductility check;
this may have been due to soil damping. Limited damage (inelastic
action) was displayed by the superstructure and piles in all but one
of the ductility analyses, but the platform remained stable in all
cases at energies at least six times the baseline strength requirement
energy computed in the response spectrum analysis. However, in one
time history, doubling the input ground accelerations was insufficient
to demonstrate four times the baseline energy if the corresponding
strength requirement time history was chosen as the baseline case.
It is concluded that the example analyses provide a sound data base
for evaluating and improving current design guidelines. The wide
variations in computed responses for different ground motion inputs
and analytical methods emphasize the need for consistency between
analytical procedures used in strength and ductility analyses.
Furthermore, the observed importance of the soil in absorbing energy
warrants efforts to standardize procedures for modeling soils.
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