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based, unified approach to achieving optimum levels of structural
security and safety for offshore structures. Of significant value
to the industry is that ABS may now classify FPSOs as offshore structures
rather than as steel vessels, says Ernesto Valenzuela, ABS Offshore
Technology Manager.
“Because FPSOs are moored, they do not have to weather the
same conditions as vessels, streamlining the classification process,”
said Valenzuela. He adds that when used in conjunction with the
Guide for the Fatigue Assessment of Offshore Structures,
released in April 2003, this new ABS guide provides a set of stand-alone
structural criteria.
“Together, the two guides create a comprehensive and effective
design tool that will help engineers and surveyors with the process
of certifying and classing offshore structures,” he said.
Valenzuela advises that the updated criteria presented in the new
guide also may be used in association with other previously issued
ABS rules and
guides for the classification of offshore structures,
including:
- Rules for Building and Classing Offshore Installations
- Rules for Building and Classing Mobile Offshore Drilling
Units (MODUs)
- Rules for Building and Classing Single Point Moorings
(SPMs)
- Guide for Building and Classing Floating Production Installations
(FPIs)
The safety factors given in these rules and guides are the basis
for determining the maximum allowable strength utilization factors
of structural components, with consideration for component type,
loading condition and failure consequence.
The buckling limits and ultimate strength of structural components
depend upon the shape and amplitude of imperfections sustained during
manufacture, storage, transportation and installation. The guide
clarifies different buckling failure modes and provides the guidelines
to prevent high consequence failure by introducing buckling control
concepts.
The guide provides criteria for assessing these factors for the
following structural steel components or assemblages:
- Individual structural members
- Plates, stiffened panels and corrugated panels
- Cylindrical shells
- Tubular joints
Typical imperfections causing strength deterioration in these components
include initial distortion due to welding or other fabrication processes
and misalignment of joined pieces. These imperfections must be detected,
monitored and repaired, if necessary, during construction and when
the completed structure is in use. For example, the new guide specifies
valid degrees of tolerance gauged against the ABS Guide for
Shipbuilding and Repair Quality Standard for Hull Structures during
Construction.
In addition to the structural integrity of individual components,
the guide addresses loading conditions for offshore structures,
including static loadings and combined loadings for MODUs, as well
as normal operations and severe storm conditions for offshore installations,
FPIs and SPMs.
The guide references the largest existing test database of component
failure as a reference point to benchmark the new buckling formulas.
An appendix to the guide reviews the analytical concepts of the
finite element method (FEM) to establish buckling capacities.
For free download, please go to ABS publication #126:
Guide for Buckling and Ultimate Strength Assessment of Offshore
Structures.
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