Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 503

be expected to have sufficient reserve strength to survive a 10,000-year regional
environmental event with an RSR of approximately 2.3 being required. On the
other hand, a typical platform structure in region 2, with a steeper hazard curve,
would not be expected to survive the 10,000-year environmental event, since it
would require an RSR of approximately 3.4, which is greater than that explicit
in API RP2A.
This example illustrates that, where regions have different hazard curves,
the same RSR will be associated with significantly different probabilities of
failure or structural reliability. Therefore, in order to meet the objective of
consistent minimum reliability against environmental overload for offshore
platforms around the world, it is apparent that acceptance criteria need to
be region-specific, because they should take account of the slope of the regio-
nal environmental hazard curve.
The probability of structural failure of offshore platforms that exist in the
region in question should be understood very well. This is achieved by carrying
out a structural reliability analysis of the platforms selected as representative of
the fleet. Statistical sensitivity studies may be required to establish whether cer-
tain characteristics are significant.
The probability of failure is estimated using reliability analysis to account
for uncertainties in the derivation of both the loading and the response.

7.9.1 Nonlinear Structural Analysis in Ultimate Strength Design
Conventional structural analysis practice relies on an idealized linear-elastic
model to determine forces in the structure members. The components

'

adequacy
is then determined by comparing the applied element forces with parametric
code-check capacity equations that are based on checking every element

' s

failure data.
In ultimate strength analysis, nonlinearities associated with the plasticity
and large deformations of components under extreme load are included expli-
citly in the element modeling. The analysis tracks the interaction between
components as member end restraints are modified and forces are redistribu-
ted in response to local stiffness changes. The sequence of nonlinear events
leading to a global collapse mechanism and the associated system capacity
are determined.
Thus, while the typical linear design process checks for the adequacy of
each individual component, nonlinear ultimate strength analysis models the per-
formance of the system as a whole.
Elastic structural analysis and nonlinear structural analysis are shown in
Figures 7.20 and 7.21 .
In general, nonlinear analysis has four basic techniques:

General-purpose nonlinear beam column models ●

Plastic hinge beam column models

●