Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 174

In lieu of a more accurate procedure for analysis, ring-stiffened joints may
be checked using the same procedure as for simple joints, but using modified
chord thickness.

3.8 TOPSIDE DESIGN
In general, major rolled shapes for offshore structure design should be compact
sections, as defined by AISC. The minimum thickness of a structural plate or
section should be 6 mm. The minimum diameter to thickness ratio of tubular
members should not be less than 20, where the diameter is based on the average
of the tubular outside and inside diameters.
Connections design to comply with the codes must fulfill the following
minimum requirements.

● The minimum fillet weld should be 6 mm.
● Wherever possible, joints should be designed as simple joints with no overlap.
● Tubular joints should be designed in accordance with API RP2A.
The deflection (discussed in Chapter 2 ) should be matched with the codes and
defined in the owner ' s specification.

Deflections should be checked for the actual equipment live loads and
casual area live loads. Pattern loading should be considered.

●

Deflection of members supporting deflection-sensitive equipment should be
not greater than L/500 for beams and L/250 for cantilevers under live loads.

●

Deflection of beams in the workrooms and living quarters should be not
greater than L/360 for beams and L/180 for cantilevers under live loads.

●

Deflection limits for other structures should be L/250 for beams and L/125
for cantilevers under live loads.

●

In performing structural analysis using software (such as SACS, for example), it
is better to define the direction of the model. In most cases, the directions can be:

+x axis aligned with platform east ●

+y axis aligned with platform north

●

+z axis aligned vertically upward

●

The datum for the axes should be at chart datum.

●

In general, only the primary structural steel should be modeled. However,
secondary members should be modeled where they are necessary for the struc-
tural integrity or to facilitate load input. Deck plates should be modeled as shear
panel elements. Joint eccentricities should be modeled using discrete elements
rather than using the

“ ”

facility of SACS. When individual elements are
used, joint forces can be more easily extracted from the output.
All the differing analyses (in-place, lift, loadout, etc.) should utilize the same
base model. That is, the in-place model should form the basis for all the other
analyses to be performed.

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