The paper describes the main dynamic positioning system used on the "Terebel" and the alternate equipments, concepts and hardware tried - The results of five years of operations are stated and discussed. They cover the efficiency of the various systems, their freedom from trouble, the position and heading accuracies under various sea and wind conditions, the power requirements and the economics of dynamic positioning - Examples of field logs are included.
In 1963, the Institut Francais du Petrole decided to outfit a research ship which would serve as a floating platform for testing new equipment and methods being developed by its Drilling and Production Division for subsea coring, drilling and oil production.
This ship is the "Terebel", a former landing craft of 900 metric tons, which was extensively modified in order to enable it to carry out its new mission.
A new anchoring system was needed, as it had to keep the ship on station safely, accurately and, if possible, economically, in shallow as well as deep water .. Furthermore, holding the ship on station had to take as little time as possible, in order not to penalize short and repetitive experiments such as coring shallow holes or introducing wireline tools in underwater wells.
A dynamic positioning system was selected as the most likely to fulfill all, or most, of these requirements.
Since 1964, the "Terebel" has been engaged, almost continuously, in various campaigns, either on behalf of its owner, the Institut Francais du Petrole, or under charter to various companies and organizations. It has drilled and taken cores in water depths ranging from 45 to 8,500 feet. It has drilled, completed and worked-over underwater test wells, laid small diameter pipelines, set and retrieved bulky and heavy equipment with pinpoint accuracy in 2,000 feet of water, etc.
All this work was carried while the "Terebel" was positioned dynamically. Further-more, all the significant dynamic positioning parameters were continuously recorded during operations, and alternate positioning hardware and control loops were tested.
So far, the performances of this unconventional anchoring method have been quite satifactory.
Dynamic positioning is essentially a method for keeping a ship or a floating platform above a pre-selected position on the surface of the sea, by the exclusive means of various combinations of thrusters.
The ship's position and heading are given by an array of sensors. The readings of these sensors are continuously fed to a computer which controls the output of the thrusters in such a way that the external sea and wind forces acting on the ship are exactly compensated for and the ship is kept in its desired position and heading.