Computer-Controlled Underwater Robot Manipulator
Authors: Sagatun S.I. , Fossen T.I. 
Published: October 2008

An advanced Supervisory Computer Control System (SCCS) for underwater robot manipulators is being developed. The control system allows extensively use of automatic modes in operations of underwater robot manipulators. We describe a system which performs semiautomatically Eddy current testing of underwater structures. Nondestructive testing (e.g. Eddy current, MPI) of nodal welds involves following a three-dimensional curved surface with an accuracy in the millimeter range. This is an extremely difficult, if not impossible, task to accomplish with a master/slave controlled manipulator. We have simulated the supervisory computer control system for an existing 6 link underwater robot manipulator. The simulations are based on a complex dynamic and kinematic model of the manipulator. The computer simulations show good performance for all degrees of freedom. The concept allows extremely accurate control of an underwater robot manipulators. Nondestructive testing of underwater structures is used through this paper to illustrate the principles behind our concept. The concept is general enough to be used wherever finer control of an underwater robot manipulator is needed.

1 Introduction

Oil and gas exploration and production offshore moves towards increasingly deeper waters. The cost of building, maintaining, inspecting and installing jackets and tension leg platforms, increases drastically as the depth of operation of increases. IT some of the equipment can be placed "wet", that is on the seabed, substantial costs can be saved. This is due to the reduced steel weight of the structures. For that reason, more production equipment is moved to the seabed. These underwater installations need annual inspection and maintenance. As an example, the Norwegian Petroleum Directorate's regulation states that primary and secondary structures have to be inspected annually [1]. The annual inspection includes visual inspection of selected areas, potential measurements, determine the extent and type of corrosion attack, dimensional measurement of selected anodes, and visual inspection for mapping type of marine growth. In addition, there is a requirement to perform non-destructive testing of critical stee1members and areas where cracks or corrosion may be expected. The conventional method of inspecting underwater installations, e.g. bottom templates, pipe-lines and risers, is by using divers or remotely operated vehicles (ROVs). Divers are much more flexible than ROVs with respect to the various work tasks which they can be set to do. Divers are also inherently more adaptive to unexpected changes of the surrounding conditions. This is due to the fact that the humans has flexible manipulators (i.e. human arms and legs), and sensor system, (e.g. eyes, touch, hearing and sense of distance, motion and orientation). The state of the art ROV IS usually equipped with two manipulators with 4 to 8 degree of freedoms. The human being on the other hand is equipped with two arms with approximately 30 degrees of freedom each [2]. There is however a desire to limit the use of divers in water depths more than 300 m (900 ft). This is due to physiological, economical and political reasons. As earlier mentioned, there is a number of possible application areas for ROVs. In the North Sea and in other areas, we have for a number of years used ROVs for visual inspection.