1.0 INTRODUCTION 



Intimate relationships exist among the components of any undersea search, inspection, 

 or work system. In general, such a system consists of a vehicle which performs the intended 

 mission (ie, sensor suit, manipulators, etc), a support platform which provides all other required 

 functions (power generation, data interpretation, command/control, etc), and a cable inter- 

 facing the above. A complex relationship exists between the functions of the vehicle, platform, 

 and cable in that the respective requirements of these components are highly interactive. For 

 example, the thrust of a tethered vehicle is proportional to the quantity of electrical power 

 shipped to the vehicle but the cable diameter (hence the viscous drag force) is often propor- 

 tional to the power transfer capabihty of the cable and thus necessitates greater thrust to 

 propel the vehicle. This type of "vicious circle" can cause undersea systems to take on larger- 

 than-expected physical dimensions and/or degrade considerably their intended performance 

 expectations. For example, with traditional technologies, several tons of vehicle, cable, and 

 deck and cable-handling systems may be required to transport a television camera weighing 

 only a few pounds into the deep ocean environment for a simple inspection operation. Such 

 an approach obviously carries with it inherent cost and logistical disadvantages. 



1 . 1 UNDERSEA PIPELINES AND STRUCTURES INSPECTION 



During the last decade, a vast amount of undersea drilhng has taken place to tap our 

 nation's oil and natural-gas reserves. More than 3000 structures were erected in the Gulf of 

 Mexico alone within the last 10 years (ref 1). The eventual requirement for underwater 

 inspection of these and the many more planned in the near future, together with their associ- 

 ated pipelines, is a growing concern. The cost of inspection is high now and is expected to get 

 higher as drilhng platforms move into deeper and more hazardous waters and as the complexity 

 of the structures increases. 



At present, the diver is the primary means of inspection in the offshore community. 

 His principal tools have been visual inspection, photographs, and TV documentation. Manned 

 submersibles have come into play and more recently tethered, unmanned submersibles have 

 been used. However, there is a growing concern with problems of entanglement and ship 

 support costs with these platforms. The availability of relatively inexpensive free-swimming 

 (tetherless) robot vehicles may make routine underwater inspections and surveys for a vast 

 number of pipelines and structures both economically feasible and practical but, without a 

 communications link to the support platform, such a free-swimming system might be greatly 

 hampered in capability in the near term. It is believed that the advantage of deployed fiber- 

 optic communication Hnks may hasten the practicality of such submersibles for two major 

 reasons. First, the large bandwidth available with such a link allows the use of the same sensor 

 systems and control-system technology currently used in tethered submersibles employed in 

 such inspection missions but which require a much-larger-diameter cable. Secondly, the 

 use of an inexpensive, deployed fiber-optic link promises to provide the same high-speed, 

 entanglement-free operation as would be possible with an entirely autonomous vehicle. 



1.2 GENERIC TYPES OF UNDERSEA VEHICLES 



Figure 1 depicts several generic types of undersea vehicles. The most basic, the 

 towed vehicle (fig 1 A), consists of a towed sensor platform. The cable serves to impart 



