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Submersibles have application in a limited number situations. There are only seven submersibles left in existence, all located in the Gulf of Mexico.
The water depth range for submersibles is between 9 and 85 ft, with a lesser depth rating during hurricane season. Despite their narrow water depth range, they still serve an important, although limited, segment of the market. Most jackup rigs cannot operate in less than 18 to 25 ft of water, although a very few can move into as little as 14 ft. of water. However, when they operate in very shallow water, their hull often needs to be placed on the ocean bottom so that their legs can be pulled. Jackup hulls are not designed for this type of service but they can be used if there are no obstructions such as:
- Rock outcrops
- Wellhead stubs
When the spud cans come out of the mud, the mud spills over onto the deck, making a huge mess. Cleaning the deck usually requires high-pressure wash-down pumps.
Submersible unit advantage
Submersibles are attractive in shallow water of less than 14 to 20 ft and/or where the ocean bottom is very soft (less than 60-psf shear strength). These soil conditions are common in river delta areas such as around the Mississippi River delta. In these areas, independent-leg jackups may drive their legs well beyond 100 ft, and the legs may not be retrievable. Even if a mat-type jackup is used, the mat may be submerged, resulting in a loss of mat stability. In these conditions, the submersible becomes attractive.
Submersibles also have other advantages in that their variable deck load (VDL) or well-consumable load-carrying ability is usually much higher than for comparable shallow-water jackups. They also do not leave a “footprint” like an independent-leg jackup does with its spud can holes. These footprints can cause significant structural leg problems when another jackup with different leg spacing is jacked up in the same area. Even if the second rig jacks up, it may slide into the previous spud can holes and lose its position over the platform, possibly causing significant leg damage.
Submersible unit disadvantage
The biggest disadvantage of submersible units in the past has been their susceptibility to sliding off location in even mild storms. However, one of the seven units, the Atwood Richmond, installed a patented station-keeping system in 2000 consisting of four 10-ft-diameter suction piles that are easily self-installable and retrievable. In 2002, the system held the unit on location in a hurricane with more than 142-mile/hr winds and 30-ft seas.