Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 64

to underground motions imposed by the rare, intense earthquake; and the
following conditions are adhered to in configuring the structure and proportioning
members:

Jacket legs, including any enclosed piles, are designed using twice the SLE
seismic loads.

●

Diagonal bracing in the vertical frames is configured such that shear forces
between horizontal frames or in vertical runs between legs are distributed
approximately equally to both tension and compression diagonal braces, and
“

●

bracing is not used where the ability of a panel to transmit shear forces is
lost if the compression brace buckles. Where these conditions are not met,
including areas such as the portal frame between the jacket and the deck, the
structural components should be designed using twice the SLE seismic loads.

K ”

Horizontal members are provided between all adjacent legs at horizontal
framing levels in vertical frames and these members have sufficient com-
pression capacity to support the redistribution of loads resulting from the
buckling of adjacent diagonal braces.

●

The slenderness ratio (Kl/r) of primary diagonal bracing in vertical frames is
limited to 80% and their ratio of diameter to thickness is limited to 1900/Fy,
where Fy is in ksi (13,100/Fy for Fy in MPa).

●

All nontubular members at connections in vertical frames are designed as
compact sections in accordance with the AISC specifications or using twice
the SLE seismic loads.
Structure-foundation systems should be analyzed to demonstrate their ability
to withstand the rare, intense earthquake without collapsing. The characteristics
of the rare, intense earthquake should be developed from site-specific studies of
local seismicity. Demonstration of the stability of the structure-foundation
system should be by analytical procedures that are rational and reasonably
representative of the expected response of the structural and soil components
of the system to intense ground shaking.
Models of the structural and soil elements should include their characteristic
degradation of strength and stiffness under extreme load reversals and the interac-
tion of axial forces and bending moments, hydrostatic pressures and local inertial
forces, as appropriate. The P-delta effect of loads acting through elastic and
inelastic deflections of the structure and foundation should be considered.

Topside Structure, Appurtenances and Equipment
Topside structure design for earthquake loads should include the effects of the
global dynamic response of the structure and, if appropriate, the effects of local
dynamic response of the topside and appurtenances.
It is recommended that topside response spectra or other in-structure response
spectra be obtained from time-history analyses of the complete structure. The
topside response spectra are recommended to be the average values from at
least four time-history analyses. Direct spectra-to-spectra generation techniques