Civil Engineering Reference
In-Depth Information
Rocker
arm
FIGURE 5.44
Launching and installing a jacket.
Skew load factor (SKL) accounts for sling fabrication tolerance or any other
inaccuracy in the sling length. SKL should be calculated based on the DNV
recommendations. In the absence of exact information, SKL should be set to
1.25 for a typical indeterminate four-point single-hook lift.
As an alternative to the SKL, the lift weight (hook weight) might be distrib-
uted in a 75%
25% split between each pair of slings in turn. All structural mem-
bers, padeyes, shackles and rigging components should be designed or checked
for both load distributions.
For padeye design, an additional lateral force equivalent to 5% of the sling
force should be applied at the eye of the padeye, in conjunction with the design
sling load.
The criterion of 75%
-
-
25% split or SKL of 1.25 is based on the variation in
fabrication tolerance
0.25%. If, for any reason, this cannot be achieved, then
the SKL must be modified.
±
5.12 LAUNCHING AND UPENDING FORCES
Launching is the most critical process in platform construction, because in
launching, the jacket is affected by different stresses during transfer from the
barge to the sea and during the subsequent upending into its proper vertical
position to rest on the seabed. A schematic view of these operations is shown
in
Figure 5.44
.
There are six stages in a launching and upending operation (
Figures 5.45
to 5.48
):
Jacket is put in a stable position on the barge
●
Jacket slides along the skid beams
●
Jacket rotates on the rocker arms
●
Jacket rotates and slides simultaneously
●
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