Fig 2 Offshore self-elevating drilling platform (a)----Underwater design( b)-----self-elevating drilling platform location, where the legs are lowered to the sea floor, and the platform is jacked up above wave height. These self-contained platforms are especially suited to wildcat and delineation drilling They are best in firmer sea bottoms with a depth limit out to 300 ft(90m)of water The submersible platforms have been developed from earlier submersible barges which were used in shallow inlet drilling along the United States Gulf Coast. The platforms are towed location and then submerged to the sea bottom. They are very stable and can operate in areas with soft sea floors. Difficulty in the rapidity with which they can be raised or lowered, once on location Semisubmersibles(Fig 3)are a version of submersibles. They can work as bottom-supported units or in deep water as floaters. Their key virtue is the wide range of water depths in which they can operate, plus the fact that, when working as floaters, their primary buoyancy lies below the action of the waves, thus providing great stability. The"semis"are the most recent of the rig-type Floating drill ships(Fig 4) are capable of drilling in 60-ft(18-m)to abyssal depths. The built as self-propelled ships or with a ship configuration that requires towing Several twin-hulled versions have been constructed to give a stable catamaran design. Floating drill ships use anchoring or ingenious dynamic positioning systems to stabilize their position, the latter being necessary in deeper waters. Floaters cannot be used in waters much shallower than 70 ft because Fig 3 Offshore semisubmersible drilling platformFig.2 Offshore self-elevating drilling platform. (a)----Underwater design (b)-----self-elevating drilling platform location, where the legs are lowered to the sea floor, and the platform is jacked up above wave height. These self-contained platforms are especially suited to wildcat and delineation drilling. They are best in firmer sea bottoms with a depth limit out to 300 ft (90m) of water. The submersible platforms have been developed from earlier submersible barges which were used in shallow inlet drilling along the United States Gulf Coast. The platforms are towed to location and then submerged to the sea bottom. They are very stable and can operate in areas with soft sea floors. Difficulty in the rapidity with which they can be raised or lowered, once on location. Semisubmersibles (Fig.3) are a version of submersibles. They can work as bottom-supported units or in deep water as floaters. Their key virtue is the wide range of water depths in which they can operate, plus the fact that, when working as floaters, their primary buoyancy lies below the action of the waves, thus providing great stability. The “semis” are the most recent of the rig-type platforms. Floating drill ships (Fig.4) are capable of drilling in 60-ft (18-m)to abyssal depths. They are built as self-propelled ships or with a ship configuration that requires towing. Several twin-hulled versions have been constructed to give a stable catamaran design. Floating drill ships use anchoring or ingenious dynamic positioning systems to stabilize their position, the latter being necessary in deeper waters. Floaters cannot be used in waters much shallower than 70 ft because Fig. 3 Offshore semisubmersible drilling platform