A tolerance of ±9 inches is possible in positioning equipment 

 utilizing the method illustrated in Figure 16. Most equipment placed through 

 the use of guidelines have built-in guidance devices of simple design, the most 

 common being a male-female cone arrangement. By careful design, practically 

 any number of components could be assembled vertically using this approach. 



The limitations of the guideline approach have been discussed with 

 cognizant personnel. At present, an installation in water 1,000 feet deep is 

 considered "ambitious." However, there is at least one guideline system in 

 water 1,300 feet deep. Tangling has not yet been a problem in guideline 

 installation, being avoided by ensuring that during the lowering operation 

 a large tensile load is subjected to the lines to prevent slack which leads to 

 line entanglement. 



The use of guidelines has been limited almost exclusively to the 

 deep-sea drilling industry. NCEL has plans to investigate a taut, wire 

 guideline system to a depth of 1 ,000 feet, which is at the outer fringes of 

 the state-of-the-art. As depths increase, the potential for tangling may 

 become greater, thereby making extensions of the guideline systems to 

 greater depths possible only if a system of spacers is devised to keep the 

 lines separated. Obviously, these spacers would be fairly complicated if they 

 could keep the lines from tangling while simultaneously allowing passage of 

 the load. 



An important point to consider is that guidelines dictate the form of 

 the underwater unit more than any other positioning-guiding concept. If 

 guidelines are to be used tb help assemble a load or position it, a way to 

 accommodate the guidelines must be designed into the load. This may or 

 may not be convenient. Wellheads, for example, are suited for guideline 

 assembly since they are invariably vertical structures consisting of components 

 stacked onto each other. For other structures, a system of guidelines may be 

 too complicated for efficient assembly, especially for underwater installations 

 which may be built both vertically and horizontally. Thus, one must be 

 cautious in recommending guidelines for assembly and positioning, since such 

 a concept may be too restrictive to be used for all conceivable underwater 

 construction tasks. 



Acoustic Devices 



As is true of virtually all electronic systems, the advances being made 

 in sonar are rapid. The resulting short time to obsolescence, combined with 

 the secretive nature of many development programs, make the state-of-the-art 

 in sonar relatively difficult to assess. 



66 



