Page 51 - 6634

jacket is prerigged with two sets of four slings. The first set of slings - the lifting

slings - are attached somewhere along the top jacket frame, while in the horizontal

position. The second set of slings - the upending slings - are attached to padeyes at

the top of the legs when the jacket is in the upright position. The jacket is lifted off

the cargo barge with the lifting slings and lowered into the water until its buoyancy

balances its weight. The lifting slings are then disconnected from the crane hook

and the upending slings are connected to the hook. The jacket is then ballasted in a

controlled manner until it is upended a few meters above the seabed. Further

ballasting is then carried out until the jacket is positioned on the seabed. This

method only requires one crane albeit it has to be capable of lifting the full jacket

weight without assistance. The jacket legs need to be made buoyant by installing

rubber diaphragms at the bottom of the legs and steel caps at the tops. Figure 4.3

shows a single block upend operation.

Figure 4.3 – Single block upend operation

51