Unmanned submersibles are being used in increasing numbers by oil companies to carry out platform inspections. This paper describes a shipborne system and a platform based system. It outlines the style of work undertaken and the difficulties encountered. It introduces a new vehicle which has been specially designed to carry out platform inspection work and overcome many of the problems which have occured in the past. This new vehicle is due to enter service early in 1981.
The tasks which these vehicles can perform are listed below in the order in which they will be discussed:
Manipulative tasks
Video inspection
Still photography
Marine growth clearance
CP probing
Ultrasonic measurement
Radiography
Debris clearance
This system is operated from vessels which have good station keeping ability if anchored, held by mooring lines, or held manually or automatically by a position keeping system. This is of prime importance when operating around structures.
Components of the system are:
the remotely controlled unmanned vehicle
the umbilical winch and umbilical
the launch and recovery system
the control module which houses the vehicle operating console and associated systems
a marinised power generation unit
Vehicle dimensions, length 120 inches, width 60 inches, height 60 inches.
Weight 3500 lbs (in air)
Operating depth 1200 feet
Can be launched/recovered in up to sea state 6
Operates in currents up to l½ KTS
Fixed floatation blocks of syntactic foam are fitted to the vehicle in order to counter the vehicle's weight in water. In addition to this, extra blocks are added to compensate for an additional 50 lbs of equipment which could be fitted for a particular mission. Further additional buoyancy can be added to the vehicle to increase the payload as required.
In addition to the above, two soft tanks, each of 2 cubic feet providing lift when blown or ballast when vented, are positioned over the manipulator shoulder joints in order to compensate for loads which the manipulators may lift, thus counteracting any undesirable pitch angles. The vehicle carries 2 scuba air bottles which give 144 cubic feet of air for use during normal operations.
Four squirrel cage induction motors, each producing 7HP, drive variable pitch propellors which are housed in Kort nozzles. Two mounted on the horizontal axis giving fore and aft movements, one mounted on the vertical axis giving vertical movements and the fourth mounted athwart-ships to give lateral movements.
In order to navigate from the launch position to the work site (or sites) and then return to the recovery position several instruments are installed.
