Leg 173 Operations Synopsis

To Site 1070

SITE 1069
(New Site Iberia-7B)

Once the location for Site 1069 (Iberia-7B; near Prospectus Site Iberia-7A) was reached, a positioning beacon was deployed at 0845 hr on 22 May 1997. A brief TV survey was conducted, although the closest subsea cable was estimated to be at least 11.5 nmi away. Hole 1069A was spudded at 1715 hr on 22 May 1997 at 5086.0 mbrf with the RCB center bit assembly. The center bit was recovered for inspection at ~350 mbsf and again at ~700 mbsf. Drilling proceeded until 0830 hr on 25 May when a depth of 5804.9 m (718.9 mbsf) was reached. Originally the plan had been to drill to a depth of 810.0 mbsf before initiating coring; however, the steadily decreasing penetration rate (3.4 m/hr) made coring a more attractive option at that point.

Continuous RCB coring was begun after the center bit was recovered and a core barrel was deployed. Metasedimentary basement rocks were encountered in Core 17R. Penetration proceeded with extremely poor recovery (0%-10%) at an average ROP of 9.7 m/hr. The ROP was too slow for coring through loose sand and too fast for basement material. To explain the erratic torque ranging from 200-350 amps, lack of fill between connections, and coring times ranging from 35 80 minutes (16.5 to 7.2 m/hr ROP), we speculated that the unrecovered formation consisted of loosely cemented sands interlayered with harder, more indurated rock. Coring ended at 959.3 mbsf. Because of the importance of logging the basement section of the hole, extra time was spent making wiper trips and hole conditioning. The first wiper trip (902-593 mbsf) was an abbreviated trip made during the coring operation to short trip 10 stands of wear knotted 5-1/2-in pipe. While running the pipe back to bottom, the top drive had to be picked up and a center bit dropped at a depth of 795 mbsf. High torque during reaming was noted from 887 to the bottom of the hole at 902 mbsf.

After coring operations were completed at a TD of 959 mbsf a second complete wiper trip was made to ~100 mbsf. Then the pipe was run back to 773.0 mbsf where the top drive was picked up and a center bit deployed. It took 4.0 hours to ream back to bottom. A third wiper trip was made up to a depth of 767.0 mbsf. On the return trip an obstruction was encountered at 800 mbsf. The hole was reamed to bottom from this point using the top drive and a center bit. Elevated pump pressure was experienced during the last 15 m. As on the previous wiper trip, 30 bbl of sepiolite mud were circulated once TD was reached. In total, 23 hr was spent in making the 3 wiper trips, including the mud sweeps. Upon recovering the center bit after the last wiper trip, 0.41 m of silty sand and a gray clay ball were found inside the core barrel that had been extruded through the 1 cm diameter center bit jet. This material was curated as Core 26G, a "ghost" core, and came from an interval extending from 767 to 959 mbsf.

Circulation was immediately lost when the bit was released, and the pipe became plugged with back-flow material. The pipe was pulled back to a depth of 862 mbsf, which was the originally intended pipe placement for logging. While attempting to set back the top drive, the pipe was taking 20,000-lb weight forcing the driller to pull additional pipe back to 800 mbsf. The pipe was then pulled farther up to 776 mbsf to enable logging of the deepest sedimentary section.

Wireline logging proceeded with the triple combo, consisting of NGS/DIT/HLDT/APS and the Lamont temperature tool. This tool string was unable to pass a point only 1-2 m beyond the open ended pipe. After working the tools for a while without success, the effort was abandoned and the tools were recovered. The top drive was then picked up and the drill string was advanced 11 m to a depth of 788 mbsf without seeing any indication of an obstruction in the hole. The top drive was set back and the pipe was then pulled to a depth of 102 mbsf. While pulling pipe to this position, back flow was observed coming from the drill pipe at a depth of ~651 mbsf. A total of 30 bbl of 10.2 ppg barite weighted gel mud was spotted in the pipe at a depth of ~564 mbsf and this solved the back-flow problem. The hole was displaced with sepiolite mud and rig up for logging began.The triple combo logging suite was again run, this time reaching a depth of 764 mbsf. Logging was conducted up from that point. The hole was in good shape, and the caliper logs showed a hole only slightly out of gauge (12-14 in). The second logging run consisted of the NGT, SDT, and FMS. These tools were able to reach to the same depth (780 mbsf), however, a tool fault made the FMS inoperable and the tools had to be pulled out of the hole. At 0600 hr 31 May, rig down was completed and wireline logging operations for the hole were concluded. The pipe was pulled clear of the seafloor at 0615 hr and the positioning beacon was released and recovered aboard at 0845 hr. The pipe trip was completed, and the ship was secured for transit to Site 1070. Departure from the site began at 1445 hr 31 May 1997.

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SITE 1070
(Proposed Site Iberia-10A)

The 44-nmi distance from Site 1069 to Site 1070 was covered in 4.25 hr at an average speed of 10.4 kt. At 1900 hr on 31 May 1997, a positioning beacon was deployed, thrusters and hydrophones were lowered, and by 1930 hr the vessel was stabilized over the drilling location. A new C-7 core bit and mechanical bit release were made up, and the drill string was tripped to the seafloor. At 2045 hr on 31 May, a backup positioning beacon was deployed. Hole 1070A was spudded with the RCB center bit assembly at 0445 hr on 1 June at a seafloor depth of 5333.0 mbrf. At 20 mbsf, the driller noted higher than normal pump pressure, which continued sporadically throughout the drilling operations at this site. Intermittent plugging of the bit jets was suspected as the cause. The center bit was recovered for inspection at ~100 mbsf, ~200 mbsf, and again at ~400 mbsf. At a depth of ~500 mbsf, the center bit was recovered and a core barrel, without a check ball installed, was deployed to see if the rate of advancement would improve. Drilling ahead continued at exactly the same rate (19.2 m/hr) as with the center bit. Drilling proceeded until 2230 hr on 2 June at 599.0 mbsf. The wash barrel was then recovered and a core barrel was deployed. Continuous RCB coring continued to a total depth of 718.8 mbsf. Crystalline basement, made up of serpentinized peridotite, was identified beginning with Core 9R at ~676.0 mbsf. The last two cores (13R and 14R) achieved an ROP of only 1.1 m/hr, but recovery was an impressive 55% and 71%, respectively.

At 0945 hr on 5 June, coring operations were ended and a pre-logging wiper trip was begun. While pulling the first stand of drill pipe off the bottom, a 20,000-lb overpull was noted. During the remaining pipe trip to 100 mbsf there were no indications of hole problems. The trip back to bottom was equally uneventful until a hard obstruction was encountered at 673.0 mbsf, or about 46 m above the bottom. Once the top drive was in place, the pipe was able to be rotated and circulated but could only be worked between the depths of 666.0 to 655.0 mbsf. A 20-bbl sepiolite mud pill was circulated but did nothing to improve the situation. The ability to rotate the pipe was lost on several occasions and then regained, although high and erratic torque was present throughout the exercise. After working the pipe for nearly 3 hr without any measurable success, it was decided that logs in the lower part of the hole were unlikely to be obtained even if the pipe could be freed. The shifting tool was deployed in the hope that the pipe was stuck at the bit and would come free once the bit was released. At 1630 hr, the bit was released but the drill string remained stuck. Another 2 hr were spent circulating sepiolite mud and working the pipe with overpulls of up to 150,000 lb over the 630-kip hanging weight.

At 1930 hr on 5 June, while continuing to work the stuck drill string, an unusual noise was heard from the drawworks when the low drum clutch was engaged. Further investigation revealed that the hub had spun on the motor shaft of the forward traction motor, separating the drive sprocket from the armature shaft. At the time of the incident, no overpull was being applied and only a static string weight of 630,000 lb was being supported. As a result of the failure, the drawworks were electrically isolated and the input chain was removed from the motor and chain case. No progress had been made up to that point in freeing the drill string and a single drawworks motor did not provide enough hoisting capacity to continue the attempt. Therefore, at 2000 hr on 5 June, drill string severing operations were initiated. The Schlumberger logging line was rigged up and an explosive charge was assembled to sever the stuck drill string in the 5-1/2-in transition pipe above the BHA. The first charge was detonated at 0215 hr on 6 June at a depth of 536.0 mbsf. Approximately 30,000 lb overpull and 4,000 ft-lb of torque were applied to the drill string at the time. There was no indication that the charge had fired, and the drill string remained stuck. From all indications, the hole had collapsed above the point of detonation. A second charge was detonated at 0900 hr on 6 June 97 at 30 mbsf. This time the drill string came free. The severing tool was retrieved and Schlumberger logging line and sheaves were rigged down.

The drill string trip commenced at 1130 hr on 6 June using the single remaining (aft) drawworks motor. Once repairs were completed, 1.5 hr were taken to reinstall the forward drawworks motor. At 0130 hr on 7 June, with 1982 m of pipe remaining to be recovered, the trip resumed. The pipe trip was completed at 0415 hr and the rig was secured for transit. Aside from losing a complete BHA, including 10 each 8-1/4-in. drill collars, a tapered drill collar, and 2 stands of 5-1/2-in. transition pipe, an additional 16 stands of 5-in. drill pipe were left in the hole plus a single severed joint. As a result of the stuck pipe and the resultant severing, no wireline logging was accomplished.

Weather conditions during the last 2 days of operations at this site were the worst experienced during the leg. Force 9-10 conditions continued for the last 24-36 hrs eventually moderating to force 8. Sustained winds averaged 33 kt with gusts to 42 kt. Seas and swells averaged 25 ft, and 18 ft respectively. The vessel began the voyage to Halifax, Nova Scotia at 0415 hr on 7 June, departing the last drill site 7.25 hr behind schedule in force 8 weather conditions.

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