INTRODUCTION 



During the past decade, there has been a vast amount of undersea drilling 

 for tapping oil and natural gas reserves; for example, more than 3000 struc- 

 tures were erected in the Gulf of Mexico alone within the last 10 years (ref- 

 erence 1). The requirement for underwater inspection of these structures and 

 their pipelines is a growing concern. The cost of inspection is high, and it 

 is expected to increase as drilling platforms move into deeper and more hos- 

 tile waters and the complexity of the structure increases. 



The diver is presently the primary means of inspecting these structures; 

 his/her primary tools are visual inspection, photography, and television docu- 

 mentation. Although manned submersibles and tethered, unmanned submersibles 

 are being used, there are problems concerning their use, e.g., entanglement 

 and ship-support costs. The availability of relatively inexpensive, free- 

 swimming (tetherless) robot vehicles may make routine underwater inspections 

 and surveys for many pipelines and structures both economically feasible and 

 practical. Although the free-swimming robot submersible may never truly 

 replace the tethered submersible, just as the manned submersible has never 

 truly replaced the diver, there are applications in which the vehicle has 

 its advantages. 



BACKGROUND 



There are three ways to extend the capability of the human being to 

 perform work in a hostile environment (reference 2). They are protection, 

 projection, and replacement. 



Protection refers to enclosing man in a protective environment while al- 

 lowing him reasonable access to the desired environment through sensors and 

 effectors, e.g., diving suits, one-atmospheric diving chambers, and manned 

 submersibles. Projection involves placing the sensors and effectors in a re- 

 mote hostile environment while locating man in a nonhostile environment, e.g., 

 towed, tethered, and acoustically controlled underwater vehicles and tele- 

 operators. Replacement, the most sophisticated means of dealing with a hos- 

 tile environment, involves replacement of both mind and body by another entity 

 designed to accomplish the desired task, e.g., free-swimming autonomous ve- 

 hicles. Thus, the human accomplishes tasks in hostile environment by protect- 

 ing himself against the environment, using long arms with remote sensors, or 

 devising a machine to perform the task by itself. 



In the ocean environment the use of projection and replacement systems, 

 rather than protection devices, can be justified in terms of danger to human 

 life, payload cost of carrying life-support systems, and additional operating 

 costs. Although there have been arguments for several years over the use of 

 divers, manned vehicles, and unmanned vehicles (references 3, 4 and 5), it is 

 safe to predict that all three will continue in use, each with its specific 

 application. 



The -overall trend of unmanned vehicle systems toward autonomous operation 

 can be seen in figure 1, which shows four generally accepted classifications 

 of unmanned underwater vehicle systems. Towed vehicle systems (figure 1A) are 

 operated by means of an umbilical cable which supplies mechanical propulsion, 



