OPERATIONS SYNOPSIS

Hole 1183A
The planned operational scenario for Site 1183 was for two holes. The first hole was to be a pilot hole drilled and cored to bit destruction; the second was to be a double-cased reentry hole. The purpose of the pilot hole was to determine the amount of 16-in and 10.75-in casing to be deployed in the second hole. The bit tagged the seafloor at 1804.7 meters below sea level (mbsl) and was observed with the vibration-isolated television (VIT) system. We began a jet-in test at 1700 hr on 18 September and concluded it by 1900 hr. After completing the jet-in test, we washed ahead through the sediment with a core barrel in place to a depth of 328.0 meters below seafloor (mbsf). After recovering the wash barrel, we dropped another core barrel and initiated coring in Hole 1183A at 0300 hr on 19 September. Coring advanced from 328.0 to 452.7 mbsf with 58.6% average recovery. The material recovered from this interval suggested that ~430 m of 10.75-in casing would be appropriate in the cased reentry hole.

We drilled the interval from 452.7 to 752.0 mbsf without coring using a center bit fitted in the rotary bit. The 299.3-m interval was drilled at an average rate of penetration of 35.4 m/hr with no drilling problems. We then rotary cored Hole 1183A continuously from 752.0 to 1160.6 mbsf. At ~1131 mbsf we contacted basaltic basement. The average recovery in basement from 1136.5 to 1160.8 mbsf was 60% with an average rate of penetration of 1.2 m/hr.

After the bit had accumulated 59.2 rotating hr, we had to decide whether to deepen the pilot hole using a free-fall funnel (FFF) or terminate Hole 1183A and start the reentry site emplacement in Hole 1183B. After considering the risks entailed by using a FFF, the considerable time and resources needed to set a reentry cone and ~430 m of casing, and, particularly, the excellent hole conditions thus far in Hole 1183A, we decided to attempt to achieve the depth objectives with the pilot hole and forego the reentry hole. We deployed a FFF with three floats, and we used the VIT to observe the FFF during extraction of the drill string from the hole. The bit was smoothly withdrawn from the FFF, after which we recovered the VIT and retrieved the drill string. The worn bit, an RBI C-4 coring bit, was replaced with a harder-formation C-7 bit, and we deployed the drill string and positioned it over the FFF. After a 30-min search, we reentered Hole 1183A at 1900 hr on 24 September. The operation was complicated because the FFF had settled into the sediment, and effluent emanating from the hole obscured the funnel and the glass floats marking the position of the hole. Following the recovery of the VIT, we lowered the drill string to 519.7 mbsf, where it encountered a constriction, suggesting that this part of the hole was closing. We picked up the top drive, and we washed and reamed the hole from 442.5 to 577.4 mbsf. After clearing the tight spots in the hole, we set back the top drive and lowered the drill string to bottom. There was ~2 m of soft fill at the bottom of the hole.

We resumed rotary coring in Hole 1183A at 0530 hr on 25 September, and we advanced from 1160.6 to 1211.1 mbsf at an average rate of penetration of 1.2 m/hr. The average recovery for this interval was 54%. To reduce the chances of core jamming in the liner, we used no liners and employed a sleeveless core catcher. After the bit had accumulated 44.1 rotating hours, we decided to make another bit change. After circulating a 50-bbl sepiolite mud flush, we pulled back the drill string to 318 mbsf, where we spotted a 50-bbl solution of 10.5 lb/gal mud. The heavy mud was to serve as a density cap to prevent effluent exiting the FFF and complicating reentry operations.
We recovered the drill string and replaced the worn bit with a new, harder-formation RBI C-9 coring bit. While lowering the drill string, we also launched the VIT and ran it down for the reentry. At 0558 hr on 28 September we reentered Hole 1183A after a 20-min search for the FFF. The FFF was not visible, but we could clearly see the circular sediment pond formed from hole cuttings. The spotting of heavy mud at the top of the hole appeared to have prevented material from the hole from venting to the water column and obscuring the visibility, as happened during the first reentry.

We lowered the drill pipe to 202 mbsf, where unassisted progress was prevented by increasing hole drag. We recovered the VIT and picked up the top drive. After dropping a wash barrel, we washed and reamed the hole in an attempt to work past the tight section. However, the tight hole conditions did not improve. From 0815 hr on 28 September to 0530 hr on 29 September, we advanced the bit to 953 mbsf, where the penetration rate dramatically slowed. We recovered the wash barrel and dropped a center bit to drill ahead through the obstruction. We made no further progress and terminated operations in the hole. We deployed the wireline to recover the center bit, but a failed pin connection on the bottom inner barrel prevented the retrieval of the center bit, inner barrel sub, and landing sub, which remained in the bottom-hole assembly (BHA).

During the washing and reaming of the hole, the relatively rapid rate of advance compared with the rate of coring over the same depth interval strongly suggested that the bit was in the original hole and not drilling a new hole. The drillers commented that it felt as if there was something being pushed ahead of the bit as they tried to advance to 1211 mbsf. We terminated the attempts at getting to the bottom 258 m short of the bottom of the hole. We retrieved the drill string with no noticeable resistance experienced on the way out of the hole. Inspection of the bit showed an unusual amount of wear on the tungsten carbide inserts on the nose of each of the cones. After recovering the beacon and retracting the thrusters and hydrophones, we began the transit to Site 1184 at 1430 hr on 30 September.

During operations at Hole 1183A, we cored 583.8 m of sediment and basement with 260.7 m recovered (44.7% average recovery). An additional 627.3 m of sediment was washed and drilled. We cored ~80.7 m of basaltic basement and recovered a total of 44.2 m of basalt (54.8%).

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¹The Operations and Engineering personnel aboard the JOIDES Resolutionfor Leg 192 were ODP Operations Manager Ron Grout and Schlumberger Engineer Steve Kittredge.