Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 627

If there is a wave-in-deck loading trigger, then the procedures provided
should be followed. Alternative wave-in-deck loading methods can be used as
long as they are justifiable. Even though there may not be a wave-in-deck loading
trigger, wave loads may act on other deck areas, such as sump and spider decks,
and the wave loading should be determined in the appropriate manner.
In most cases, dynamic effects should be considered for platforms in water
depths greater than 400 ft. Dynamic effects should also be considered for
damaged platforms that may sustain higher motions in the damaged condition
than in the intact condition. This can occur for any water depth.
The global structural model should be three-dimensional. Special attention
should be given to defensible representation of the actual stiffness of damaged
or corroded members and joints. The following guidelines should be considered
for the USM.

Damage Modeling
The ultimate strength of undamaged members, joints, and piles can be established
using the formulas from API RP2A with all safety factors taken as equal to 1.0.
Nonlinear interactions may also be utilized where justified. The ultimate strength
of joints may also be determined using a mean

versus the
lower-bound formulas for joints design. Alternatively, the ultimate strength of
damaged or repaired elements of the structure may be evaluated using a rational,
defensible engineering approach, including special procedures developed for
that purpose.

“ formula or equation ”

Actual Yield Stress
Actual yield stress or expected mean yield stresses may be used instead of
nominal yield stresses. However, mean yield strength should not be used
when it is greater than actual laboratory test or mill certificate strength.
Increased strength due to strain hardening may also be acknowledged if the
section is sufficiently compact, but not rate effects beyond the normal
(fast) mill tension tests.

Effective Length Factors
Studies and tests have indicated that effective length (K) factors are substan-
tially lower for elements of a frame subjected to overload than those specified
in API RP2A 3.3.1d. Lower values may be used if it can be demonstrated that
they are both applicable and substantiated.

Soil Strength
For USM, it is usually appropriate to use best-estimate soil properties as opposed
to conservative interpretations. This is particularly true for dynamic analyses,
where it is not always clear what constitutes a conservative interpretation.