Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 198

All members and connections of a lifted component must be designed for
the forces resulting from static equilibrium of the lifted weight and the sling ten-
sions. Moreover, API RP2A recommends that, in order to compensate for any
side movements, lifting eyes and the connections to the supporting structural
members should be designed for the combined action of the static sling load
and a horizontal force equal to 5% of this load, applied perpendicular to the
padeye at the center of the pin hole.
It is worth mentioning that, all these design forces are applied as static loads
if the lifts are performed in the fabrication yard. On the other hand, if the lifting
derrick or the structure to be lifted is on a floating vessel, then dynamic load
factors should be applied to the static lifting forces.
In particular, for lifts made offshore, API RP2A recommends two minimum
values of dynamic load factors: 2.0 and 1.35. The first factor is for designing the
padeyes as well as all members and their end connections framing the joint
where the padeye is attached, while the second is for all other members trans-
mitting lifting forces. For load-out at sheltered locations, the corresponding
minimum load factors for the two groups of structural components become,
according to API RP2A, 1.5 and 1.15, respectively.
Jacket, topside and living quarters lift analyses (onshore and offshore) should
be performed based on the requirements of DNV rules. All members and connec-
tions should be checked to API RP2A or AISC basic allowable stresses.
It is worth mentioning that the weight contingency factor is a factor that
allows for lift weight inaccuracies. For jacket structures, a minimum factor of
1.1 should be used unless the jacket is weighed at the end of the construction
by using load cells, in which case this factor may be reduced to 1.03. The
weight contingency factor should be applied to the

“ ” “

net weight and rigging

weight. ”

Dynamic amplification factors allow for the dynamic effects experienced
during the lift. For typical jacket structures, the dynamic amplification factors
are as presented in Table 3.16 .
Note that the dynamic amplification factors for the offshore lift presented in
Table 3.16 should be used for calm sea conditions (Hs 2.5 m). If, for any rea-

son, the lift is carried out in an adverse condition, the factor should be recalcu-
lated based on the expected accelerations associated with the sea state.

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TABLE 3.16 Dynamic Amplification Factors

Gross Weight (tons)

0 - 100 100 - 1000 1000 - 2500 > 2500

Offshore Lift 1.3 1.2 1.15 1.10

Onshore Lift 1.15 1.10 1.05 1.05