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API REPORT 79-14 Document Information:
Title
Cyclic Inelastic Bahavior of Steel Offshore Structures
American Petroleum Institute
Publication Date:
Aug 1, 1980
Scope:
General
Tubular steel towers resting on piles driven into the sea floor are
used to provide support for
offshore oil drilling and production facilities. As the search for and
development of oil resources
continues, it becomes increasingly necessary to build such structures
in locations susceptible to
seismic disturbances. Safety and environmental considerations require
that these structures be safe
from collapse under the action of anticipated earthquake ground
motions.
The 1977 design recommendations of the American Petroleum Institute
(API RP 2A) [1] stipulate that
both strength and ductility be considered when designing a tower to be
located in an area of
seismic risk. That is, a structure must be sufficiently strong so that
its members will not yield
or buckle during a "Strength Level" earthquake for which the
probability of occurrence is
comparable to that of the design wave. This level of seismic input is
consistent with that used to
design important onshore structures. In addition, the 1977
recommendations stipulate that offshore
structures be sufficiently ductile to remain stable under rare and
unusually intense earthquake
ground motions. To satisfy this "Ductility Level" requirement under
the 1977 API recommendations, a
designer had to demonstrate that a fixed platform was capable of
remaining stable through
displacements twice those corresponding to the "Strength Level" design
criterion. This ductility
requirement was modified somewhat in the 1979 API recommendations [1]
so that an offshore structure
must be capable of absorbing at least four times the amount of energy
absorbed under the strength
level earthquake design criterion with the structure remaining stable.
Under static loads in the
linear elastic range, this is equivalent to doubling the displacements
corresponding to the
strength level criterion.
If either of these ductility requirements were designed for on an
elastic basis, the increased
seismic force levels would substantially increase the cost of a
structure. Fortunately, these
increases in seismic design criteria do not necessarily require a
corresponding increase in member
sizes, if it is possible to take advantage of the structure's
inelastic ductility capabilities, and
the ability of its members to absorb and dissipate energy through
inelastic deformation.
Properly designed and constructed steel structures have performed well
in past earthquakes.
Stresses beyond the elastic range may cause permanent inelastic
deformations and localized damage,
but a properly designed structure will not collapse. This concept is
commonly used in the building
industry. However, the applicability of such design concepts to fixed
offshore structures must be
thoroughly investigated since these structures incorporate a number of
unique features not found in
the building industry.
This report presents results of experimental research on steel
offshore-type braced frames
subjected to large cyclic inelastic loadings, applied in a
quasi-static manner, simulating severe
earthquake excitations. Two one-sixth scale models of a complete bent
of the Southern California
Example Structure (Fig. 1.1) were tested. This prototype structure is
an example four leg, X-braced
production facilities platform designed for 100 ft. water depth in
accordance with API wind and
earthquake criteria (8th edition) [1] appropriate for Southern
California.
A photograph of one of the test Frames is shown in Fig. 1.2. The
planar models were approximately
30 ft. high and 11 ft. wide. The general framing scheme in both test
frames was alike. Tube
diameter-to-wall thickness (D/t) ratios and other details used are
representative of current
practice. Bracing members were heat treated to obtain material
properties similar to mild
structural steel commonly used in offshore construction.
Both frames were tested to destruction under progressively increasing
cyclically applied lateral
displacements simulating earthquake effects. In each test the frames
were extensively instrumented.
The data obtained provided information for generating numerous
force-deformation relationships for
each frame as a whole as well as for the individual members and
joints. These relationships provide
some of the first quantitative data on the cyclic inelastic behavior
of tubular steel braced
frames. These data are useful in evaluating the performance of such
structures as well as in
verifying the reliability of analytical models.
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