ABSTRACT

The advance in offshore Remotely Operated Vehicle (ROV) technology during the last few years has resulted in proof of their capability and economic advantage in installation and maintenance tasks, pipeline surveys and general inspection So that ROV's may be fully integrated into offshore hydrocarbon production by facilitating the replacement of divers for Non-Destructive Testing (NDT) of structural members and nodes in steel jacket platforms, a joint development program has been funded by the Offshore Supplies Office of the Department of Trade and Industry (OSO), Mob11 North Sea Limited, Slingsby Engineering Limited, Technical Software Consultants Limited and Camera Alive Subsea Offshore Limited The objective of this program was to design and manufacture a manipulator and associated vision and control system that could perform all functions required to inspect welds using state-of-the-art subsea NDT equipment, including Magnetic Particle, Visual, Eddy Current, AC-FM and Flooded Member Detectors Emphasis has been placed on the use of supervisory feedback control systems to assist In the movement of the NDT end effectors along any conceivable weld trajectories with minimum human intervention This paper describes work carried out within the above Project aimed at providing a vision sensory / measurement system for environment acquisition

THE SPECIFICATION

In the same way in which human eyes communicate with the brain, which communicates with the hands and arms, so the vision system communicates with the "bran" which controls the manipulator arm The benefit of this is that the working environment for the manipulator can be modeled from what is actually there, rather than what 1s thought to be there, the motion of the arm/ROV relative to the worksite can be measured, and hence a compensation made, and the actual movement of the arm relative to the worksite can be monitored independently of the arm control system

The main differences in the analogy are the form of the interfaces The vision system is required to see the environment as a three-dimensional mathematical model, and communicate this to the controlling computer in a form which it can interpret easily and quickly This dictates a requirement for both digital vision (vision which can be interpreted by a computer) and measurement

ENVIRONMENT ACQUISITION

The success of any automated or supervised manipulator operation is dependent on the accuracy with which it is possible to plan the intervention It is well known that confidence in as-built drawings in the Offshore Oil and Gas Industry is low If the working environment of the manipulator is modelled from as-built drawings it is possible that incorrect dimensions, additional features or omissions (for simplification) could result in mission failure, or an aborted mission This would be very expensive in terms of operational time, re-planning and wasted resource It is hence a desirable feature of any automated system to be able to model the working environment as it is found

The standard non-contact measurement system extracts three-dimensional information from the images on the basis of point selection The points may be linked Into an entity, for example two points for a distance, three for an angle.

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