PROJECT MOHOLE DRILLING PLATFORM: 



MANEUVERING, PROPULSION, 



AND STATION-KEEPING 



A. C. McClure and A. S. Hove 



Project Mohole - Brown and Root, Inc. 



Houston, Texas 



THE DRILLING PLATFORM 



Project Mohole is this nation's contribution to the International Upper Man- 

 tle Study and is the most ambitious part of this program. The major scientific 

 objective of Project Mohole is to obtain a core record of the layers of the 

 earth's crust and the upper part of the underlying mantle. In addition to core 

 samples, valuable information will be acquired from measurements made in the 

 hole both during drilling and after completion. 



The boundary between the crust of the earth and the mantle is defined in 

 terms of seismic measurements, indicated by a sharp increase in acoustic ve- 

 locity at the interface known as the Mohorovicic Discontinuity. This phenome- 

 non occurs at much shallower depths under the oceans than under the continents; 

 hence, the decision was made for the drilling to be carried out at sea. At the 

 selected drilling site, northeast of the island of Maui, Hawaii, the water is 

 14,700 ft deep and the mantle is another 16,000 to 18,000 ft beneath the ocean 

 floor. 



The drilling operation at the selected site requires an accurately positioned 

 stable platform. The steadiness of the working vessel is of critical importance 

 in every phase of activity. The most critical items regarding steadiness are 

 the riser system and the drill pipe which connect the vessel to the ocean floor. 

 The manipulations for reentry demand minimum surface motions and precise 

 maneuvering capabilities. 



Positioning of the vessel must be accomplished by propulsive means rather 

 than by anchoring because of the great depth of water at the proposed drilling 

 site. The platform cannot be allowed to drift out of the prescribed area with the 

 riser pipe attached or the drill pipe in the hole. Precise and dependable posi- 

 tioning control is required in the performance of all drilling and reentry 

 operations. 



A high degree of steadiness is designed into the platform by developing a 

 configuration which has very weak coupling with surface waves, and consequently 

 minimal response to the sea. Low response to wave action is inherent in the 



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