The Terebel Dynamic Positioning System Results of Six Years of Field Work and Experiments

Abstract

The Terebel has shown that after close to 8,000 hours of operation, dynamic positioning is a safe, accurate, and fast way of keeping a ship on station in waters as shallow as 45 feet and as deep as 8,500 feet. It has proved very handy when precise, fast handling ship is required, and equally effective when the need is for accurate maneuvering at low speeds. Introduction Dynamic positioning bas come of age. Following the pioneering work of Shell Oil Co. and of the Baylor pioneering work of Shell Oil Co. and of the Baylor Co., more than half a dozen ships actively engaged in coring, drilling, and miscellaneous petroleum or other activities are now equipped with this station-holding system. The Glomar Challenger by its round-the-world, deep-ocean-coring cruises has greatly contributed to the renown of the system. Bigger drilling ships and even semisubmersibles are being planned or outfitted with dynamic positioning in various parts of the world. It is time to take stock and review the information available and decide if dynamic positioning is just an interesting curiosity or a valid tool for working at sea. At the Institut Francais du Petrole, we have had the advantage of operating, almost continuously for 6 years, a dynamically positioned vessel, the Terebel, engaged in activities ranging from coring to pipe laying. At the same time, we have helped design and build the dynamic positioning systems for two other vessels, and we are in the process of providing an important portion of the system for a new, large drilling ship. Furthermore, all the significant dynamic positioning factors were continuously recorded positioning factors were continuously recorded during the Terebel operations, and alternative positioning hardware and control loop were tested. positioning hardware and control loop were tested. This has provided a wealth of data and experience, which we shall discuss briefly. The Terebel Dynamic Positioning System Dynamic positioning is essentially a method for keeping a ship or a floating platform above a preselected position on the surface of the sea exclusively by means position on the surface of the sea exclusively by means of various combinations of thrusters. The ship's position and heading are given by an array of sensors. 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. The only ship motions that are countermanded by dynamic positioning are yaw, surge, and sway. No attempt is made to eliminate the roil, pitch, and heave of the vessel. Heading and Position Sensors Heading and yaw can be measured by any good gyroscopic compass. Excellent and well proven models are readily available. Their characteristics are well known, and it is sufficient to say here that a relatively inexpensive one was installed aboard the Terebel and performed quite well. performed quite well. Position sensors are another matter. They have been developed recently and are not entirely free of trouble. Three types of sensors have been tried and used on the Terbel: a taut-wire inclinometer, and acoustic system called the Bremius, and the Raydist radiolocating equipment. JPT P. 329