We spudded Hole 1141A at 0815 hr on 3 February 1999. The seafloor depth adjusted to the rig floor was estimated at 1209.0 m based on a hard "tag" indication by the driller. The 3.5-kHz PDR had indicated an adjusted seafloor depth of 1212.4 m.
The first two cores contained foraminifer nannofossil ooze that was composed largely of sand-sized foraminifers. Recovery was low in this material. Continuous wireline coring proceeded through Core 10R to a depth of 94.2 mbsf without incident. After the first two cores, recovery averaged 72% but was quite variable, ranging from 0% to 104%. The formation then graded into a foraminifer chalk with abundant scattered chert through Core 11R. Core 12R contained no recovery. Core 13R contained only a few centimeter-sized pieces of gabbro. The next interval (Core 14R) cored slowly and marked the first contact with the volcanic basement rocks at a depth of 114.5 mbsf. Coring operations were temporarily interrupted at 0500 hr on 4 February 1999, when the core winch electrically shut down. After repairing the electrical problem, coring proceeded through five separate flows of aphyric to sparsely plagioclase-phyric basalt to a depth of 185.6 mbsf. Recovery in basement averaged 55% (cutting half cores), and the ROP averaged 2.9 m/hr. At this point, hole conditions were excellent.
While coring Core 24R, a 20-bbl sweep of sepiolite mud was circulated and displaced out of the hole. The sinker bars were installed, and the core was retrieved without incident. After laying out the core, a fresh barrel was dropped, the drill string was made up, and circulation was established. Immediately upon picking the drill string up off the elevators, the pipe became stuck. Attempts to gain rotation were unsuccessful, and circulating pressure was ~500 psi higher than normal. The drill bit was at 179.0 mbsf, or 6.6 m above the bottom when the drill string became stuck.
A 60-bbl sepiolite sweep was circulated while we attempted to work the drill string free. Initial attempts to free the drill string were made by setting the pipe in compression and applying 700 A torque. After numerous unsuccessful attempts to break the string free, overpulls were applied in gradual 20,000 lb increments up to 100,000 lb overpull with 700 A torque. Torque was released and overpull was gradually increased to 300,000 lb. A single joint of drill pipe was slowly worked out of the hole and laid out. As we were able to move the drill string slowly upward, we decided not to expend the wireline time required to retrieve the empty core barrel, as this would have resulted in minimal circulation and could have jeopardized further drill string movement. Another 2.5 m were pulled up with 300,000 overpull before the pipe became totally stuck at 166.9 mbsf. While attempting to free the drill string, 180 bbl of bentonite gel mud were swept at 140 spm and 2500 psi. Circulating pressures were higher than normal, indicating there was some packing off of the drill string. While attempting to free the pipe, 1000 A of rotary torque was applied. The maximum overpull used, with no torque in the drill string, was 300,000 lb. With all efforts at freeing the drill string unsuccessful, there was no other alternative left but to explosively severe the pipe. This ended coring operations at a TD of 185.6 mbsf, or 71.1 m into basement. A total of 96.79 m of core was recovered from the hole for an overall average of 52.2%.
The severing charge was run in and fired at the box end of the transition drill collar, which was at 53.6 mbsf. Drill string overpull was 100,000 lb above the new calculated string weight of 240,000 lb when the severing charge was fired. A loss of 100,000 lb was immediately noticed upon detonation, and the remaining string came free. The wireline was then retrieved and rigged down, and the drill string was pulled clear of the seafloor at 1330 hr on 5 February 1999.
While the drill string was being tripped, the positioning beacon was released and recovered. A new drill site was identified by the co-chief scientists, and the vessel was moved using the dynamic positioning system thrusters ~800 m to Site 1142. This move took ~30 min and was completed before the drill string trip was completed. Hole 1141A officially ended at 1615 hr on 5 February 1999, when the severed end of the lower 5 1/2-in drill pipe transition joint cleared the rotary table.
Leg 183 Operations - Site 1142
Leg 183 Table of Contents