Hamilton's Argyll Semisubmersible/Production Riser Concept

Abstract

A concept is presented for producing a 40,000-BOPD offshore field in 260 ft of water in the North Sea. The system uses a semisubmersible drilling rig converted to a floating production platform. Design considerations of the production riser system and subsea installations are described. Introduction Hamilton Brothers Oil and Gas Ltd. is currently engaged in producing the Argyll field in the United Kingdom sector of the North Sea. The field lies in Block 30/24, 190 miles southeast of Aberdeen, Scotland, and 25 miles southeast of Phillips Petroleum Co.'s Ekofisk field. The field was discovered in Aug. 1971 and was confirmed in 1972. It has been developed using a unique offshore production facility based on a semisubmersible drilling rig converted to a production platform. This is connected to sea-bed installations by a platform. This is connected to sea-bed installations by a production riser developed by Hamilton Brothers for production riser developed by Hamilton Brothers for this application. Field Data The Argyll field is one of the few Permian fields so far discovered in the North Sea. The main productive zone is a Zechstein dolomite varying in thickness from 30 to 100 ft. It is a highly fractured, vugular formation directly underlain by a Rotliegendes sandstone similar to those encountered in the Southern gas fields. The Zechstein crude is 38 degrees API gravity, and the Rotliegendes is slightly lower at 34 degrees. Sulphur content in the field is around 0.25 percent and wax content averages 6 percent. Depending on the particular well, GOR's range between 150 and 300 scf/bbl The Argyll field appears to be a relatively small field that may be drained by a few wells. From evidence of the logs, it is considered probable that good communication exists between the probable that good communication exists between the sandstone and dolomite. Drillstem testing using restricted chokes has indicated prolific flow rates and good pressure recovery on shut-in. It was planned to produce four wells, perforated in the dolomite interval produce four wells, perforated in the dolomite interval only, and to monitor well behavior closely. Average well productivity of at least 10,000 BOPD was anticipated. Because of the relative complexity of the Argyll geology it has not been possible to predict how the field will produce. Nor, in fact, has the full reserve potential of the area been established. These considerations led Hamilton Brothers to choose a test production facility for the initial phase of development of the Argyll field. The program was designed to place the field in production with minimum investment and to yield reservoir information required to determine future development policy. At the same time, this production test will yield sufficient revenue to assure profitable initial operation of the field. Test Production Facility The corrupt of using a semisubmersible drilling vessel as a production platform evolved logically. The successful operation by Phillips at the Ekofisk field set the pattern of development in the Argyll field. JPT P. 475