Certain limitations are inherent in the design of semi- 

 submersibles. The addition or loss of weight on these vessels must be 

 carefully compensated for by altering ballast. Semi-submersibles are 

 usually towed to a drilling position, while newer semi-submersibles are 

 often selfpropelled. They require large facilities for construction and 

 servicing. As with jack-ups. they have a towing draft of 20 to 30 feet. 



A semi -submersible can be anchored like a drilling barge, or it can 

 be dynamically positioned like a drill ship. Figure 12 is a diagrammatic 

 illustration of a semi-submersible. Some of the recently built semi- 

 submersibles are rather large; one vessel, for example, has a square 

 working platform some 200 feet on a side mounted on six hollow steel 

 columns 26 feet in diameter which in turn are mounted on two pontoons, 

 each 355 feet long, 36 feet wide, and 22 feet deep. A restricted area 

 of at least 1/4 mile and as much as 2 miles surrounding a rig may be 

 required as a buffer/safety zone to prevent fishing and other boating 

 accidents with the rig. 



Drill Ship : A drill ship is self-propelled. The drilling platform 

 is situated in the deck; various internal compartments provide crew 

 quarters and storage space for equipment and supplies. The drill is 

 worked from a derrick through a hole in the center of the ship. 



Modern drill ships such as the one illustrated in Figure 13 provide 

 greater stability than earlier predecessors. For example, the Glomar 

 40, a 450-foot ship displacing 14,500 tons, is designed for operations 

 in water depths ranning vrom 100 feet to 3,000 feet; it has the 

 capability of maintaining onerations in winds of fiO miles per hour 

 and waves of 5n feet 



The modern technological response to the problems of surge and sway 

 came with the development of a sophisticated technique known as "dynamic 

 positioning." This technology involves the use of electronic devices to 

 take constant readings of a platform's precise geographic position with 

 relation to the ocean floor. The processed data is used to automatically 

 activate one or more of the steering propel lors or "thrusters" to keep 

 the platform in proper position over the well. Drill ships incorporating 

 these and other technological features offer the advantaaes of considerable 

 mobility and deep water drilling capability. 



It is not possible to predict precisely which type of drilling rig 

 will be used in each OCS area; but the selection will depend upon a 

 tradeoff of factors including water depth, sea state, and the condition 

 of the sea floor. For anticipated United States OCS work the bottom- 

 supported submersible platform and the drilling barge can be eliminated 

 from consideration since the depth of most areas exceeds their capabilities. 

 Moreover, rough seas could easily capsize drilling barges. 



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