-in drill-pipe transition
joint cleared the rotary table.The 2880-km transit from Site 1140 on the northern Kerguelen Plateau to the starting point of the seismic survey on Broken Ridge was made in 135.7 hr at an average speed of 11.5 kt. At 1915 hr on 2 February 1999, we slowed to deploy the seismic equipment, and, at 2015 hr, we crossed over the first way point of the desired seismic line. Before selecting the Site 1141 drill site, a 85-km (7 hr) seismic survey was conducted using the ship's single-channel seismic streamer. By 0215 hr on 3 February 1999, the survey was completed. The ship continued on course until the seismic gear was recovered and then resumed full speed for the selected drilling location. Within 1.3 hr, the ship arrived at the new location. At 0330 hr on 3 February 1999, we deployed a beacon on the precise Global Positioning System (GPS) coordinates for Site 1141.
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 below rig floor (mbrf) based on a hard "tag" indication by the driller. This yields a seafloor depth of 1196.9 m below sea level (mbsl). The 3.5-kHz precision depth recorder (PDR) had indicated an adjusted seafloor depth of 1212.4 mbrf.
The first two cores contained foraminifer nannofossil ooze largely composed of sand-sized foraminifers. Recovery was low in this material. Continuous wireline coring proceeded without incident through Core 183-1141A-10R to a depth of 94.2 meters below seafloor (mbsf). After the first two cores, recovery averaged 72% but was quite variable, ranging from 0% to 104%. The formation then graded into foraminifer chalk with abundant scattered chert through Core 183-1141A-11R. Core 183-1141A-12R contained no recovery. Core 183-1141A-13R contained only a few centimeter-sized pieces of gabbro. The next interval (Core 183-1141A-14R) cored slowly and marked the first contact with the volcanic basement rocks at a depth of 113.5 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 rate of penetration (ROP) averaged 2.9 m/hr. At this point, hole conditions were excellent. Summaries of core numbers, depths, and recovery are given in Tables T1 and T2.
While drilling Core 183-1141A-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 was 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 of the hole, 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 of torque. After numerous unsuccessful attempts to break the string free, overpulls were applied in gradual 20,000-lb increments up to 100,000 lb of overpull with 700 A of 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 was pulled up with 300,000 lb overpull before the pipe became totally stuck at 166.9 mbsf. While we attempted to free the drill string, 180 bbl of bentonite gel mud was swept at 140 strokes per minute at 2500 psi. Circulating pressures were higher than normal, indicating that 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 sever the pipe. This ended coring operations at a total depth 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 was 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-in drill-pipe transition
joint cleared the rotary table.
The positioning beacon was deployed on this site while the pipe was still being tripped back to the drill ship. The beacon was deployed at 1500 hr on 5 February 1999 on the precise GPS coordinates for Site 1142.
We spudded Hole 1142A at 2330 hr on 5 February 1999. The seafloor depth adjusted to the rig floor was estimated at 1213.0 mbrf based, once again, on a hard "tag" indication by the driller. This yields a seafloor depth of 1200.8 mbsl. The 3.5-kHz PDR had indicated an adjusted seafloor depth of 1216.4 mbrf.
Because the objective was basalt and the sedimentary sequence had already been cored in Hole 1141A, we were given permission from ODP headquarters at Texas A&M University to drill through the overlying sediments. The driller washed/drilled with a wash barrel in place for 119 min rotating time (45.9 m/hr) until the formation became firm at a depth of 91.0 mbsf. The wash barrel (1W) was recovered and Core 183-1142A-2R was cut in aphyric to moderately plagioclase-phyric basalt. Coring continued in this formation, with much of the recovered basalt totally altered to brown clay. Recovery was highly variable and generally low (13.0% to 84.4%). Even cutting half cores only resulted in an overall recovery of 34.4% for the section. The ROP was also highly variable, ranging from 2.3 to 7.7 m/hr, and averaging 3.7 m/hr overall.
Before completion of Core 183-1142A-10R, 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, the drill string was immediately made up and circulation was established. As in the last hole, upon picking the drill string up off the elevators, the pipe became stuck. This time rotation was possible but upward travel of the drill string was not possible. The drill bit was at ~6.0 m above the bottom of the hole when the drill string became stuck.
The stuck pipe was worked from 125.6 to 106.3 mbsf. Circulation and rotation were possible with high, erratic torque. We were unable to come up without stalling the top drive. Maximum amperage on the top drive was 1000 and maximum overpull used was 80,000 lb. The pipe was eventually worked free by going gradually down with the bit. At that point, <3 hr of coring time remained in the schedule, so further coring operations were suspended. Total depth of the hole was 141.9 mbsf, or 50.9 m into basement. A total of 17.54 m of core was recovered from the hole for an overall average of 34.4%. Summaries of core numbers, depths, and recovery are given in Tables T1 and T2.
The pipe was pulled out of the hole using the top drive, clearing the seafloor at 0400 hr on 7 February 1999. The forward core line was subsequently run in the hole and coated with compound L corrosion inhibitor. The core barrels were stored away, and the drill string was tripped the remaining distance out of the hole while corrosion inhibitor was applied. During the pipe trip, the positioning beacon was released and recovered at 0615 hr. The bottom-hole assembly was broken down and all drill collars racked and secured in the forward collar rack. With the bit at the rig floor at 1055 hr, the remaining thrusters were pulled and the rig was secured for transit.
At 1100 hr on 7 February 1999, we were under way for Fremantle, Australia.
