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Lift, Lower, & Upend

The subjects we will cover here are lifting, lowering and upending. In particular, we will discuss lifting and lowering decks and lifting, lowering and upending jackets. In all cases, we will assume that the body being moved is connected to the boom of a crane vessel. As an example, here is a picture of a basic deck lift.

One simply lifts the deck off of the cargo barge and sets it down on the jacket. A classic upending looks quite different:

Here, the jacket starts the upending floating with one face in the water surface. One end of it is lifted and the jacket rotated. After lifting a while, valves are opened and the jacket rotates more until it is in the upright position. It is then lowered until it rests on the sea floor. For non launched jackets, another scenario is possible.

Here, the jacket is lifted off of the cargo barge with the main hook and lowered into the water. A second sling arrangement is connected to the top of the jacket and the auxiliary block, and as the jacket is lowered the relative lengths of the two lines rotate the jacket and lower it to the sea floor.

Deck Lifts:

Of the things described above, deck lifts are the most straight forward. Unless one is interested in the impact loads as the deck is either being lifted off of the cargo barge or as it is being set on the jacket, there is little to analyze. One simply finds an equilibrium position of the deck and performs a stress analysis to ensure that nothing breaks. Sometimes one may consider the dynamics of the deck on the hook due to a seastate, but normally these operations are performed in essentially calm seas.

Standard Upend:

As with a deck lift, one normally considers only the statics of an upending operation. In other words, an upending simulation is normally nothing more than a sequence of equilibria. What is important is that:
  • The jacket remain stable,
  • The jacket roll remains small,
  • The jacket not impact either the bottom or any pre-installed equipment on the bottom,
  • The hook load not exceed the allowable for the operation, and
  • The sling loads not exceed their allowable.
To get some feel for how these thing behave during the process, consider:

which shows that for the upending being considered, the roll is always fairly small and we do not have bottom clearance problems. As to stability, we can look at:

to see that the stability is always greater than 10 (meters in this case). The following:

is fairly typical behavior for the hook load. Finally, one can check the allowable sling tensions against:

Two Block Upend:

Of all the things considered here, analyzing a two block upend is by far the hardest. Each one of them is a special case and we really cannot give you any general advice.