A self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit and all attached machinery to the desired location. Once on location, the hull is raised to the required elevation above the sea surface supported by the seabed. The legs of such units may be designed to penetrate the seabed, may be fitted with enlarged sections or footings, or may be attached to a bottom cover. Generally, jack up tackles are not self-propelled and rely on tags or heavy lift ships for transportation. Jack-up barges being used to dismantle a ship that was wrecked at sea.
Working of jack-up barge
A self-elevating jack-up barge is a floating barge fitted with long support legs that can be raised or lowered. The jack-up is manoeuvred (self-propelled or by towing) into the location with its legs up and the hull floating on the water. Upon arrival at the work location, the legs are jacked down onto the seafloor. Then "preloading" takes place, where the weight of the barge and additional ballast water are used to drive the legs securely into the sea bottom so they will not penetrate further while operations are carried out. After preloading, the jacking system is used to raise the entire barge above the water to a predetermined height or "air gap", so that wave, tidal and current loading acts only on the relatively slender legs and not on the barge hull.
Types of vessels
· Mobile offshore Drilling Units (MODU)
This type of rig is commonly used in connection with oil and/or natural gas drilling. There are more jack-up rigs in the worldwide offshore rig fleet than another type of mobile offshore drilling rig. Other types of offshore rigs include semi-submersibles (which float on pontoon-like structures) and drill rig, which are ship-shaped vessels with rigs mounted in their centre. These rigs drill holes in the drillship hulls, known as moon pools.
· Turbine Installation Vessel (TIV)
This type of rig is commonly used in connection with offshore wind turbine installation.
Jack up rigs also refers to specialised barges that are similar to an oil and gas platform but are used as a base for servicing other structures such as offshore wind turbines, long bridges, and drilling platforms.
Jacking up system
In the traditional self-elevating jack-up barge, the jacks are strong enough to elevate the hull, but cannot bear any bending moment. The moments are resisted at guides above and below the rack and lower each leg. This design requires some distance between upper and lower guides and slender heavily constructed legs. In the trident, the jacking units are more robust and the bending moment is sustained mainly at the rack and pinion mechanisms mounted on the three vertical elements of the each leg called chords, only a small part of the moment is borne by guides
To sustain bending moment in this way required increasing the distance between chords to 60 feet. This leads to large but light legs, which turns leads to a less expensive barge.