Page 49 - 6634

four, six or eight legs. Jackets with three legs are known as tripods. Jackets

with a single caisson type leg also exist. These are also known as

monopods.

* Piles made of tubular steel are installed through the legs of the jacket or

through the pile sleeves connected to the jacket legs at its base. The piles

installed inside the jacket legs normally extend to the top of the legs.

Through leg piles are connected to the jacket legs at the top using shim

plates, known as “crown shims”, that are installed in the annulus between

the leg and the pile and are welded to both. In some structures, the annulus

between the jacket and the pile is grouted, although this is no longer a

common practice. Piles installed through sleeves on the outside of the leg

structure are connected to the sleeve by grouting the pile-sleeve annulus.

* Regardless of the size or the type of jacket installation, once the jacket is on
the seabed, its weight is temporarily supported by mudmats. Mudmats are

added to the bottom of the jacket legs to provide the required bearing area to
support the jacket weight and resist environmental loading during

installation and until the strength of the piles has sufficiently developed.
This phase is known as the “unpiled stability” phase.

* Lift and Lower in Water

* This method is used for small jackets, in very shallow water, which are

transported on barges in the upright position already pre-rigged for offshore

lift and installation by a crane vessel. Once offshore, the jacket is lifted off

the deck of the barge and lowered down to the seabed. Jackets installed in

such a configuration are typically less than 50 m tall.

* The foundation piles for this size of jacket structure are typically transported

together with the jacket on the same cargo barge. Once the jacket is set on

the seabed, the piles are installed using the same crane vessel and a pile

hammer of an adequate size.

* Lift and Upend
49