The Upgrade and Performance of the Discoverer 534 Dynamic Positioning System

ABSTRACT The Discoverer 534 was upgraded in 1990 from a turret moored/Dynamically Positioned (OP) drillship capable of operating in 3,500 ft. of water, to a DP drillship capable of operating in 6,000 ft. water depth. The vessel has operated continuously on DP since the upgrade was completed in October 1990, with excellent reliability and station keeping performance. This paper discusses the original DP control equipment, the design of the DP system for the upgrade, and DP drilling operations. INTRODUCTION The drillship was originally intended to operate either moored, moored with DP assist or in a full DP mode. All necessary equipment for these operations was installed during vessel construction. Prior to the upgrade, the vessel completed moored operations in water depths up to 3,500 ft. During heavy weather the DP system was often used to change or maintain heading; the turret mooring system allowed the possibility of heading changes which could be used to minimize environmental loads on the vessel. Only one short, non-drilling DP operation was undertaken. Sonat Offshore had previously performed a deepwater upgrade project on the Discoverer Seven Seas, a sister vessel of the Discoverer 534. That project, so far as the DP system was concerned, was to increase water depth capability and to improve system performance. The upgrade retained the original DP control system but added equipment and features to improve performance and reliability. The upgrade proved to be successful as the vessel later set a new world water depth record of 7,500 ft., and enjoyed much improved station keeping performance and improved reliability of DP controls. Compared to the earlier Discoverer Seven Seas upgrade, the Discoverer 534 project had similar reliability and station keeping goals. It also started with a similar DP control system. The equipment included a Honeywell Automatic Station Keeping (ASK) system with dual H-316 computers, a power plant management feature embedded in the DP controls, data logging, and dual RS-5 short baseline acoustic position reference systems with acoustic riser angle. Initially, it was hoped to duplicate the Discoverer Seven Seas upgrade because of its proven success. This included addition of Starfix. and Long Base line (LBL) acoustics, waterdepth upgrade of Short Base Line (SBL) acoustics, addition of Kalman filter preprocessing of position references, replacement of the vertical gyro units, and equipment reliability enhancement modifications. 1 An important factor in the success of the Discoverer Seven Seas upgrade was the thorough involvement of the DP system manufacturer, Honeywell, in all parts of the upgrade. Upgrade of the existing system on the Discoverer 534 necessarily depended on the ability of Honeywell to provide similar support. Because of an impending transfer of ownership of various Honeywell businesses that would supply equipment and support for this upgrade, it was concluded that the project schedule might be delayed if these businesses were used. Consequently, the nature and amount of work required for the upgrade changed dramatically.