The Single Steel Drilling Caisson: A New Arctic Drilling Unit

Abstract

Summary Dome Petroleum Ltd.'s experience with a new mobile offshore drilling unit - the single steel drilling caisson (SSDC) - is described. The SSDC's design enables offshore exploratory drilling operations in the Beaufort Sea to continue beyond the short open-water season. The SSDC proved to be well-suited to offshore arctic operations. The operator's requirements for storage facilities and rig equipment are discussed and the drilling and testing of the first well are described to illustrate the success of this innovative drilling unit. Problems associated with Beaufort Sea operations are discussed with specific reference to ice management and drilling problems. A brief update describes the relocation of the SSDC to a second wellsite and its successful operation through a second winter. Introduction Dome has been drilling offshore wells in the Canadian Beaufort Sea with drilling units owned by its subsidiary Canadian Marine Drilling Ltd. (Canmar) since 1976. Initially, drillships modified for arctic conditions were used for offshore operations in this area during the short open-water season. Ice breakup typically occurs between mid-June and early July. During the summer, drilling operations are interrupted occasionally by incursions of multiyear ice. New ice usually begins to form in October. As freeze-up progresses, ice breakers are used to prevent interference with drilling operations. Drilling operations may be suspended at any time from October to late November, depending on government regulations, drilling program, anticipated ice conditions, and well progress. Occasionally, operations have continued to the limit of operational capability in severe ice conditions. For the last 9 years, season length, from first to last operating day, has ranged from 85 to 150 days. The average number of operating days on location is 100. An average of 13 days per year waiting on ice or weather reduces the effective number of operating days to 87. Restriction of floating drilling operations to this brief period means that it is not possible to drill and to test a typical deep Beaufort well in one summer season when a drillship is used. Because of this limitation, we considered a variety of alternative drilling systems. Modifications to the most recently acquired drillship, Explorer 4, extended its operating ability beyond that of the other three drillships. It was evident, however, that the drilling season could not be extended throughout the winter with existing floating rigs. The limiting factor in the case of a floating rig was station-keeping ability. Existing mooring or dynamic-positioning systems could not resist the forces generated by moving first-year ice. A much heavier mooring system and higher-class icebreaker support would be required to operate a floating rig beyond the existing season. The alternative was a bottom-founded structure that was designed to operate in ice conditions. Artificial, dredged islands have been built for drilling in the Beaufort Sea. These islands are built with "sacrificial" beaches and sandbag protection. The berm slope is relatively low, and berm volume is high. These islands are restricted to relatively shallow water on economic grounds and are susceptible to summer storm damage. Generally, the accepted economical water-depth limit for these islands is about 20 m [66 ft]. To provide a platform for winter drilling operations in deeper water outside the landfast ice zone, we determined that a bottom-founded structure that rested on a subsea berm would be most appropriate and cost-effective. Canmar's first arctic winter drilling system was a sandfilled, concrete multiple-caisson structure placed on a subsea berm - Tarsiut artificial island. Tarsiut island was built in 1981. Gulf Canada Resources Inc. operated Tarsiut during the winter of 1981–82 and the spring and summer of 1982.1 This caisson-retained island provided a stable drilling platform throughout, and two wells were drilled. The system had disadvantages, however, including limited mobility and "deck" space. Also, relocation of the structure would require considerable time because of the need to rig down completely, to excavate the caissons, and then to reconstruct the structure and rig up completely at the new location. Canmar's second-generation system, the SSDC, overcame many of the disadvantages of the Tarsiut system and has proved to be an excellent solutions to the problems of arctic drilling operations.