DRILLING BENEATH THE SEA — BENSON 399 



engines, and capable of delivering thrust in any direction. These 

 units, known as "Harbormasters," were mounted fore and aft and on 

 both sides of the ship. The amount and direction of thrust of each 

 engine were controlled at the pilot's console with a single lever re- 

 sembling an aircraft joystick. The engines and steering system 

 proved capable of holding the ship withm its own length of a position 

 in 12- foot waves and a 25-knot wmd. 



The second problem, that of determinmg the sliip's position relative 

 to the hole, was overcome by using a unique taut-line, deep-moored 

 buoy system, together with sonar and radar, and a Sperry Mark 14 

 Gyro-Compass. 



The AMSOC staff evolved the design of the 6-foot aluminum buoys 

 in the shape of an oblate spheroid (elliptical viewed from the side, 

 round viewed from above). These were placed about the drill site, 

 anchored to the bottom, with, the buoys about 100 feet below the sur- 

 face of the water. Their special shape reduced their resistance to 

 ocean currents and enabled them to remain moored almost directly 

 over their anchors. Mounted on the buoys were sonar transponders 

 that, when triggered on another frequency from the ship, sent back a 

 signal, giving a sonar screen picture of the buoy pattern relative to 

 the ship. Eadar also scanned surface buoys that were secured to the 

 deep-moored buoys. These devices, together with visual sightings of 

 the surface buoys, gave the pilot his relative bearings. The ship's 

 heading was maintained by reference to the Sperry Gyro-Compass 

 repeaters on the bridge. 



The third problem, that of the forces at work on the drill pipe, was 

 worked out on paper for a variety of conditions by using a model. 

 On the basis of the calculations, two sets of specially tapered drill 

 pipes were ordered. One was a spare in the event that the other was 

 dropped, or, if the doubters proved to be right, the pipe wound itself 

 up like a corkscrew and snapped. In the course of the actual opera- 

 tion, the drill string proved to be the least troublesome component of 

 the system. 



WHY MOHOLE? 



With all the attendant problems, why drill a Mohole at sea? The 

 answer is simple. 



Beneath continents, the average depth to the Moho — ^the thickness 

 of the crust — is about 20 miles ; beneath the ocean, only about 5 miles.^ 

 A hole drilled from land, therefore, w^ould have to extend about 20 

 miles in order to reach the Moho, and the deepest hole yet drilled goes 

 down only about 5 miles. But in some places in the ocean the crust is 



° Geologists regard the continents as thick blocks of relatively light, or granitic, rock, 

 "floating" upon the far denser supporting rock of the mantle. The rock of the oceanic 

 crust, however, is relatively thin and dense. 



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