1.0 INTRODUCTION 



The extraction and exploration of offshore oil, gas, and other important 

 materials during the past decade has encouraged the development of a wide 

 variety of underwater support services. In 1977, according to the Exxon 

 Company, 16 percent of worldwide crude-oil production came from offshore 

 bore-holes. Proven crude-oil reserves are estimated at 26 percent of the 

 world's total . 



As a result of current economic and political conditions, the nations of 

 the world have been turning to the oceans to fill their ever-increasing 

 demands for energy and food, even though extraction may have to take place in 

 cold and hostile ocean regions with water depths of 3000 feet and more. In US 

 waters alone, about 7000 miles of pipeline have been laid. More than 3000 

 structures have been erected in the Gulf of Mexico alone in the last 10 years 

 (ref 1). All these structures require routine inspection, maintenance, and 

 rectification of defects. The scale of the task is enormous. 



Divers play a vital role in the difficult task of establishing a physical 

 link between surface and seabed. Advanced diving technology will place a 

 diver down to 1000 feet. However, as man progresses into deeper water, into 

 areas under arctic conditions, or into exposed locations having wave heights 

 of 95 feet, effective substitutes for the unsaturated diver must be developed. 



Recently, the greatest surge in developmental activity and operational 

 deployment has been in the area of remotely operated vehicles (ROVs). In 

 unmanned submersible operations, three vehicle configurations are in use: (1) 

 towed vehicles; (2) tethered vehicles; and (3) untethered, free-swimming 

 vehicles. 



(1) Towed vehicle systems are operated by means of an umbilical cable 

 which conveys mechanical propulsion, electrical power, and communica- 

 tions to the vehicle from a relatively large surface support craft. 

 The submersible is fairly uncomplicated in nature, but is capable of 

 maneuvering only forward on its own and up/down by means of a cable 

 winch. 



(2) The tethered vehicle, which appeared a few years later, incorporated 

 an umbilical cable that provided only electrical power and communica- 

 tion. Propulsion was provided by thrusters located on the vehicle. 

 Because the cable no longer conveyed mechanical power for the 

 vehicle, the cable was made lighter and more flexible, and support- 

 ship size requirements were reduced. Since these vehicles were 

 capable of three-dimensional maneuvering, undersea inspections and 

 work operations were possible. 



(3) Free-swimming submersibles, or untethered unmanned submersibles, have 

 recently made an appearance as a result of emerging technology in 

 this field. Eliminating the tether affords a free-swimming submer- 

 sible the performance advantages of higher speed (due to reduced 

 cable drag) and entanglement-free operation. Support-ship require- 

 ments are minimized because there is no need for a cable-handling 



