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Unfortunately, the Mat-Type Jackup unit also has several disadvantages: Note size of mat and why workboats and tugs may damage it in shallow water. 2-Typical Bethlehem mat-type jackup under construction.

Their key advantages are that they were relatively inexpensive to build and leave no footprint at the drilling location.įig. built most of these units from the 1950s through the 1980s. When the unit comes onto location, it jacks the mat down to the ocean bottom, and because of its low bearing pressure, usually under 500 to 600 psf, the unit jacks the hull out of the water without going through the preload sequence required for independent-leg units. The mat is carried just under the hull during mobilization, usually with ≈ 5-ft gap. The mat-type jackup also usually consists of three legs that are cylindrical and are from 8 to 12 ft in diameter ( Fig. Jackups are most susceptible to major damage or loss when they are floating. This is a significant advantage in that a much smaller “weather window” can be acceptable to move the unit.

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The newer, enhanced premium units do a single preload in which the jacking system is strong enough to jack the unit with all the preload water onboard, the basic weight of the hull, and the full transit VDL. The concept is to load the legs to a level above that which the unit will encounter in the harshest predicted environment. This process occurs until the legs no longer penetrate the ocean bottom. After the legs are driven and the hull goes into the water, the seawater is dumped overboard and the sequence is begun again. The unit’s pumps seawater into its preload tanks, adding weight to the hull and driving the legs. If the ocean bottom is soft and consists of clay, it is not uncommon to take 7 or more sequences, with each sequence taking 7 to 12 hours. The preload sequence is usually done in stages, with the hull never rising more than 5 ft out of the water to safeguard against having a leg punch through. If the hull should roll, pitch, and heave to an extent that the legs come into contact with the ocean bottom, particularly if it is hard, the legs can be severely damaged. Generally, 5-ft swells and/or a combined sea of 8 ft are the maximum seas in which these units can jack out of the water. During this procedure, the jackup MODU is at risk from weather and leg “punch through” i.e., one leg breaks through a hard crust, putting the other legs in a large bending movement. The legs are attached to a very large mat that rests on the ocean bottomīoth types of jackups have a hull, float onto location, jack the legs to the ocean bottom, and then jack the hull out of the water.įor the independent-leg units, “preloading” is required to drive the legs into the ocean bottom before the hull is completely jacked out of the water. Usually three legs with lattice construction 1-Maersk’s giant jackup (largest in the world) designed for deepwater use (550 ft in the GOM) and harsh North Sea environment.

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