Seven deepwater exploration wells have been successfully drilled in water depths ranging from 1,914 to 3,461 ft in the Andaman Sea and the Red Sea by Esso Exploration Inc. using The Offshore Company's DISCOVERER 534. The challenges of deepwater drilling required a concentrated effort to assure that all equipment involved in the operation met design and performance criteria.
A description of the rig and overall operational procedures developed for these record setting water procedures developed for these record setting water depths is presented. Included are discussions regarding deepwater mooring, the use of double "J" wellhead and guidebase running tools, jetting 30 in. casing, and the use of a mooring recovery bridle for anchors without pendant buoys. Additionally, the successful recovery of a BOP stack dropped in 1,914 ft of water is described. Procedures developed to ensure safe operations in the presence of internal waves are also reviewed.
During the twenty-month period, December 1975 to August 1977, Esso Exploration Inc. drilled seven successive wells in an average water depth of 2,694 ft using the drillship, DISCOVERER 534 (D-534). The shallowest water depth was 1,914 ft and the deepest was 3,461 ft.
The first deepwater well drilled by the D-534 was spudded in December 1975 in 1,914 ft of water. This was in Esso's W-9 block offshore West Thailand. The following year, 1976, was the record setting year. Four additional deepwater wells were drilled in block W-9. These were drilled in water depths of 2,632 ft, 2,959 ft, 2,652 ft and finally, 3,461 ft. The locations of these wells are shown in Figure 1. The last Thailand well in 3,461 ft of water set the present water depth record for offshore exploratory present water depth record for offshore exploratory drilling.
Prior water depth records for the years 1965 through 1976 are shown in Figure 2.
Following completion of the five wells offshore Thailand, the D-534 drilled two deepwater wells offshore Egypt in the Red Sea. The first of these was in 2,737 feet of water and the second in 2,506. The locations of these wells are shown in Figure 3.
A listing of the deepwater exploratory wells drilled by the D-534 is given in Table 1.
As would be expected, a learning curve type improvement resulted in drilling these seven deepwater wells. Persistence and use of good technology paid off with significant increases in operating efficiency as equipment problems were solved and procedures improved. Esso now has demonstrated capability and confidence in deepwater drilling.
II. DESCRIPTION OF THE DISCOVERER 534
The DISCOVERER 534, owned by an affiliate of The Offshore Company, is a self-propelled drillship equipped to drill to well depths of 20,000 ft and in water depths to 3,700 ft. The name of the vessel is derived from its overall length of 534 feet. A detailed description of the rig's capabilities was recently published by Mr. Parlas of The Offshore Company.
A profile of the D-534 is shown in Figure 4. The major characteristics of the vessel and related marine equipment are given in Table 2. Table 3 lists the major drilling equipment.
The rig has an 80 ft beam and displaces 20,700 long tons at its drilling draft of 24 ft. The vessel is propelled by two 15-1/2 ft screws, each driven by four 2,000 hp DC electric motors, and has a design cruising speed of 14 knots. The D-534 is equipped to maintain location either with conventional mooring or with a dynamic positioning (DP) system, or with a combination of both. The DP system utilizes six 2,500 hp, fixed azimuth, constant speed, variable pitch, retractable thrusters. It should be pointed pitch, retractable thrusters. It should be pointed out that design and construction of the D-534 commenced before dynamic positioning was proven as a means of station keeping; hence, both mooring and DP capabilities were installed.