This paper reviews the characteristics which enabled the SEDCO 445 to drill without anchors in water depths of 1,350 feet and with the objective water depth of 6,000 feet. Specifically, it reviews the design and operating experience gained with the unit's dynamic stationing equipment, the BOP control equipment, the re-entry equipment and the marine riser buoyancy material. Further, it reviews the steps that were taken in developing the vessel and its equipment and what has been achieved since it began operations in late 1971.
Wellhead Re-entry Without Guidelines
BOP Controls for over 1,500 Foot Water Depths
Marine Riser Systems
In early 1970, SEDCO, in conjunction with Shell International Petroleum, began to develop a new generation drilling vessel that could operate without anchors-- the SEDCO 445 (Figs. 1-A, 1-B, 1-C, and 1-D) . During the ensuing four-year period, this concept has become a reality and today the technology and equipment is available to conduct exploration drilling operations with complete well control at seabed in water depths beyond 2,000 feet without anchors or guidelines. Prior to 1970, several coring vessels, including the Eureka, Caldrill and Glomar Challenger, were using an elementary form of dynamic positioning by holding the ship over a designated area. Propulsion equipment linked to a master control center provided the stationing capability to counter the wind, wave and current actions acting on the ship. In contrast, the SEDCO 445 made a major technological step in stationing systems by providing a much more reliable and accurate system to safely conduct exploration drilling activities. Reliability plus accuracy were achieved via rigorous engineering and the provision of 100 per cent back up on all station keeping equipment. In addition to the dynamic stationing capabilities, SEDCO also had to provide a blow-out prevention system that would operate in these increased water depths. This meant providing a re-entry system so guidelines could be eliminated, an advanced BOP control system and a marine riser system which could operate in excess of 1,500 feet of water. In the following portions of this paper, four major technological problems and their solutions are presented. DYNAMIC STATIONING
Dynamic stationing is the technology of maintaining a vessel's position by means of thrust. The SEDCO 445 achieves this with eleven thrusters and the ship's main propulsion system which are linked to a highly advanced computer network. The dynamic stationing system is made up of two major parts--first, the automatic station keeping (ASK) system which accurately detects the vessel's position relative to a fixed point on the ocean floor, commanding the second part of the system, a series of thrusters and main propellers, that provide the thrust necessary to counteract the effects of wind and seas. Both parts of the system are fully redundant and are composed of thoroughly tested and reliable components.
The SEDCO 445, when dynamically stationed without anchors, has complete , freedom to head into prevailing wind, waves or currents thereby reducing the size thrust necessary to hold the vessel on station and also improving the ship's motion characteristics.