To prepare for the deployment of the reentry cone and 20-in casing, the assembled reentry cone was positioned over the moonpool doors. The jet-in BHA was lowered and landed into the hanger of the reentry cone. The entire cone, casing, and BHA was then lowered through the moonpool doors at 1606 hr on 23 November. Hole 1256D was spudded at 0025 hr on 24 November when the tip of the 20-in casing shoe contacted the seafloor. Hole 1256D is located ~30 m south of Hole 1256C and ~50 m south of Holes 1256A and 1256D (Fig. F17). It required 10.2 hr to jet the 95 m of casing into the sediment. The mudskirt contacted the seafloor at 1035 hr.
The next phase of operations required the drilling out of a 211/2-in hole below the 20-in casing into basement in order to accept the 16-in casing. Based upon the experience acquired drilling the pilot hole (Hole 1256C), it was planned to set the 16-in casing 20 m into basement with an additional 10 m of 211/2-in hole (rathole). In order to drill this large hole, the Drilling Services Department of ODP acquired two Downhole Design, Inc. (DDI) B182x215 bi-centered reamers (BCRs). This hardware is designed to be operated with a 97/8-in-diameter pilot rock bit. This combination may be used to drill and ream simultaneously or to open a previously drilled hole. The B182x215 BCR has an 181/4-in pass-through diameter that allows this hardware to fit through the 20-in casing. When drilling, the actual diameter of the cut hole is 211/2-in. This was the first time such a device was used in scientific ocean drilling.
The BCR, affixed to a 97/8-in hard-formation tricone drilling bit, reentered the cone at 0620 hr on 25 November and was then lowered to the bottom of the 20-in casing (95 mbsf) to begin drilling a 211/2-in hole through the sediment. Because the upper 190 m of the formation was very soft, the bit was advanced at a controlled rate of 35 m/hr.
Basement was entered and reaming progressed to ~17 m into basement when, starting in the late evening of 26 November and continuing into the early morning of the following day, the torque steadily increased. Because of crew unfamiliarity with the operating characteristics of the BCR, it was considered prudent to pull this hardware a little over the halfway point with only 38 rotating hours and inspect the condition of the pilot bit and BCR.
When the BCR was on the surface, the reason for the high torque was apparent. One of the three cones on the pilot bit was almost completely destroyed and missing all of the ~30 ball bearings. The tungsten carbide inserts were also severely ground down. Interestingly enough, the other two cones were relatively undamaged and the bi-centered reamer was in excellent shape and gauge with only two teeth missing. There was some junk damage to the sides of the bit and reamer.
Because of the potential that metal fragments remained in the hole, a special hole cleaning assembly was put together. A double junk basket was made up with a slightly used hard-formation tricone bit. The clean-out assembly reentered the cone at 2140 hr on 27 November and by midnight was on the bottom of the hole. The junk baskets were "worked" by running the mud pumps at a combined rate of 100 spm (500 gpm) at 800 psi for very short durations to stir up the loose material at the bottom of the hole. When the pumps were shut off, the cloud of debris would settle back to the bottom and into the open tops of the junk baskets. The junk baskets were worked for 1.5 hr, after which 1 m was drilled (267.1–268.1 mbsf) with the 97/8-in bit. The bit penetrated 1 m in 2 hr (ROP = 0.5 m/hr) with uniform torque and no suggestion that large junk remained at the bottom of the hole. The clean-out assembly was recovered and on deck by 1140 hr on 28 November. The contents of the junk basket consisted of about a half a bucket of basaltic debris, minor chert fragments, and ground-up metal filings.
The second B182x215 BCR was then connected to a new 97/8-in tricone bit fitted with three 20/32-in nozzles. The BCR was lowered and reentered the hole at 1755 hr on 29 November. The BCR had reached the bottom of the hole (267.1 mbsf) and was opening new hole by 2000 hr that evening. Drilling was terminated at midnight on 29 November at a depth of 276.1 mbsf. A total of 9.0 m of basement was drilled at an average ROP of 0.4 m/hr. Following the conclusion of the drilling, the hole was flushed with a 40-bbl sepiolite mud sweep. The drilling assembly was picked up above basement to 234 mbsf and lowered to the bottom of the hole. Following a wiper trip to 91 mbsf (inside the 20-in casing) and a return to 276.1 mbsf, the hole was once more flushed with a 50-bbl sepiolite treatment. There was no fill found at the bottom of the hole and no ledges or obstructions detected during the wiper trip. Subsequent to the last mud flush, the hole was displaced with a combination of 150 bbl of sepiolite mud and 100 bbl of heavy mud (10.5 ppg). The bit cleared the reentry cone at 0700 hr on 30 November.
At 1330 hr on 30 November, the bit cleared the rotary table. Inspection revealed that one of the cones from the tricone drilling bit was missing as well as many of the tungsten carbide inserts on the remaining cones. There was noticeable junk damage to the bit body. The BCR appeared to be in gauge with two inserts missing and some junk damage to the bottom sub. During the drilling there was no strong indication of cone loss and, in fact, the torque values for the last 5 hr of rotation were low with very little variation. It was difficult to assess if the cone came off early in the drilling and was ground up or dropped when the bit was picked up off bottom at the end of the hole. The missing cone had the same corresponding orientation relative to the location of the reamer cutting shell as the failed bearing of the tricone pilot bit used in the first BCR.
We decided to deploy the 16-in casing and then drill out the cement shoe with a C-4 RCB core bit mated to a junk basket. If the cone had been ground down in the drilling process, the junk basket would provide a means to clean some of the filings from the hole and we would have been able to resume coring. If there was high erratic torque once the bit was lowered to the basalt, suggesting that the cone was more or less intact, then we would have needed to make a round trip and go back in with a milling assembly.
At 1330 hr on 30 November, the drill crew began to assemble the 16-in casing string. The total casing string length was 269.5 m. The top of the casing string was topped off with seawater, the Cam-Actuated Drill-Ahead (CADA) running tool was connected to the hanger, and the casing hanger was lowered through the moonpool at 0417 hr on 1 December.
As the casing was being deployed, the drill string was filled with seawater every 20 stands. At 1040 hr on 1 December, with the end of the casing string at 3092 mbrf, the vibration-isolated television camera captured the grim vestige of a collapsed joint of casing (the fifth joint from the bottom of the string). The deployment of the casing string was aborted and the string was recovered.
Ultimately, four joints of casing were replaced, as was an additional casing collar damaged during the disassembly of the casing string. The joints above and below the collapsed casing had egg-shaped collars as a result of being directly mated to the collapsed joint and were replaced. Damage to a fourth joint of casing occurred while trying to unscrew a Baker-locked (epoxy) connection. The collapsed joint of casing was cut off in 5-ft sections and removed from the rig floor.
At 1135 hr on 2 December, the 16-in casing once more disappeared beneath the waters of the moonpool. This time, the drill string was filled with water every 10 stands to ensure that there would be no reprise of collapsed casing. At 2008 hr the same day, the casing began to be lowered into the reentry cone. By this juncture, the wind had shifted and vessel motion became pronounced with heave approaching 3 m. At 2215 hr and with heave hovering around 4 m, the reentry was terminated with the cement float shoe at 122 mbsf. If operations continued there was a very real possibility of damaging the casing and/or the reentry cone. The casing was pulled out of the cone and raised to 36 m above the seafloor to wait for calmer weather in which to proceed.
At 0545 hr the next day, the heave had reduced to 1.8 m, and it was deemed safe to resume operations after waiting on weather for 5.8 hr. Hole 1256D was reentered at 0650 hr. The bottom of the casing passed the sediment/basement interface without any drag, and by 1200 hr on 3 December the casing shoe was at 257 mbsf or ~7 m into basement. As the driller gently lowered the casing string, the casing stuck at 263 mbsf or ~6 m above casing landing depth. After the casing was worked for 3.5 hr, the string came free and the hanger was successfully landed in the reentry cone. Confirmation of the hanger engagement with the throat of the reentry cone was made by the application of 10,000 lb of overpull. The casing was cemented with 38 bbl of cement followed by 10 bbl of drill (fresh) water and chased by 213 bbl of seawater. The CADA running tool was released by clockwise rotation of the drill pipe at 1955 hr.
The RCB BHA was assembled, lowered, and then reentered the cone at 1500 hr on 4 December. After the cement plug was drilled out, a core barrel was dropped and the bit was carefully lowered to the bottom of the hole and rotation established. The hole was worked for a couple of hours, but erratic and fairly high torque suggested that metal fragments of the tricone bit cone that had broken off during reaming operation (see above) remained at the bottom of the hole. This was confirmed when the core barrel was recovered with a nose section of the drilling bit cone captured in the core catcher. A fresh barrel was dropped in attempt to recover some more fragments, but this was not successful. The drill string was then tripped to the rig floor, where a couple of metal fragments were found lodged between the cones and cutting guides.
A fishing assembly was made up of a 9-in fishing magnet and two junk baskets. The fishing magnet has been in inventory for many years, but this was the first time that it was used to "fish" metal junk from the bottom of the hole. The design of this tool incorporates a permanent magnet in which the magnetic flux is concentrated in a controlled field around the bottom pole plate of the unit. The pole plate is highly magnetic, with a strong field extending completely across the bottom of the tool. Because no magnetism emanates to any other part of the tool, the outside case is not magnetized and the tool can be deployed inside cased holes without losing its effectiveness. The tool has circulation holes that terminate at the bottom of the magnet, which resides inside the fishing (mill) guide.
The fishing assembly reentered Hole 1256D at 2358 hr on 5 December. At 0230 hr the next morning, the driller began working the junk baskets and the fishing magnet. The magnet was lowered to ~1 m from the bottom of the hole, and the pumps were cycled on and off in brief intervals at 80 to 100 spm (400 to 500 gpm) to stir up the material from the bottom for capture in the junk baskets. After 1 hr of working the fishing assembly, the drill string was recovered.
Once on deck, several pounds of metal ranging from fine cuttings to moderate-sized (~5 cm) fragments of cone were removed from the magnet. Significantly, there was very little material in the boot baskets, suggesting that the bottom of the hole was clean. When all the metal recovered from this exercise was combined with material retrieved previously from the bottom of the hole was weighed, it totaled 6 lb. The bottom of the hole was considered clean enough to run in with a core barrel.
Basement drilling operations consist of coring with the RCB system for the life of a bit, which is ~60 to 65 hr of coring time. After each bit is recovered, a new bit is installed on the end of the BHA, the drill string is tripped to the reentry cone, the hole is reentered, and coring proceeds. We used six RCB coring bits, which includes five for coring in addition to the one already deployed for drilling out the cement plug and debris at the bottom of the hole, as discussed above.
An RCB BHA was assembled with a new RBI CC-7 bit (the second of six RCB bits used in Hole 1256D) and a junk basket and then was lowered into the reenty cone at 2130 hr on 6 December. Just before coring was started, the driller worked the junk basket to clean any small fragments that may have remained in the hole. Coring was initiated in Hole 1256D at 0100 hr on 7 December with generally very good results. The average recovery from 276.1 to 368.9 mbsf (Cores 2R through 15R) was an excellent 84.4%, with an average ROP of 2.3 m/hr. Some of the core barrels from this cored interval recovered >100%. The recovery from 368.9 to 406.0 mbsf (Cores 16R through 21R) averaged 21.7%, with an average ROP of 3.8 m/hr. A drift measurement (deviation of the hole from vertical) made with the Tensor tool at 368.9 mbsf (119 m into basement) recorded an angle of 1.8°.
By the time the hole was deepened to 406.0 mbsf (156 m into basement), the bit had accumulated 49.6 hr. We decided to perform a round trip of the drill string to inspect the condition of the bit and use that as a basis for establishing the rotating hours to core before the next bit change. After pulling the bit up to 388 mbsf, the drill string became stuck. The driller could not lower or raise the drill pipe, although circulation was maintained. The drill string was worked free after 3 hr, following the application of up to 250,000 lb of overpull. The hole was backreamed from 387.6 to 254.0 mbsf and flushed with a 40-bbl sepiolite treatment on the way out of the hole. The bit was pulled free of the cone at 1435 hr on 10 December and was on deck at 2040 hr the same day. The bit was in excellent condition with no missing inserts, and no excessive wear was observed. The average recovery for the second bit run (the first bit was the one damaged by junk in the hole) was 67% with a cored interval of 129.9 m. The average ROP was 2.6 m/hr.
After attaching a new CC-7 RCB bit (the third bit) to the BHA, the ninth reentry of the leg was made at 0410 hr on 11 December. By 0845 hr, rotary coring in Hole 1256D resumed. The hole was deepened from 406.0 to 494.0 mbsf at an average ROP of 1.4 m/hr. The ROP values with this bit ranged from 3.2 m/hr to a lethargic 0.4 m/hr at the bottom of the interval. Rotary coring with this bit increased the penetration into basement from 156.0 m to 244.0 m (average recovery = 47%). A hole angle measurement was made at 451 mbsf (201 m into basement) and recorded a value of 0.8°.
After accumulating 62.9 rotating hours, the CC-7 bit was recovered. It was in good condition, considering the rotating hours in basement, and had sustained a few chipped teeth, some erosional wear, and was ~1/8 in under gauge. The tungsten carbide inserts exhibited normal dulling characteristics, but the loss of gauge and unknown remaining life in the bearings suggested that 60 to 65 rotating hours would be a prudent choice for the next bit.
The fourth bit, a new CC-9 RBI RCB bit, was attached to the BHA. The cutting structure of the CC-9 bit is only slightly less aggressive than the CC-7 but possesses more inserts on the surface of each bit cone. Because the CC-9 has a less invasive cutting surface than the CC-7, it should have a smoother cutting action in the very hard basalt at the bottom of the hole (ROP = ~0.5 m/hr). It was also felt that the smoother action of this core bit should result in improved recovery in the rubble zones between massive flows.
At 1435 hr on 15 December, the drill string entered the reentry cone, and by 1800 hr coring was resumed in Hole 1256D. At the request of the Co-Chief Scientists, a nonmagnetic core barrel was made up and deployed on odd-numbered cores starting with Core 39R (513.5 mbsf). By the afternoon of 18 December, the hole had been deepened to 552.5 mbsf, or 302.5 m into oceanic basement. After accumulating 57.8 rotating hours, the pipe was tripped and the bit was at the rotary table at 0740 hr on 19 December. The average recovery for this bit was 37% with an average rate of penetration of 1.3 m/hr. The used bit was found to be ~1/8 in under gauge and contained the expected dulling characteristics of a bit with 58 rotating hours in basement. The tungsten carbide inserts had sustained some chipping in the nose and heel of the cones. There was minor erosional wear on the cone face, and the cone bearings were effective.
The fifth bit, a new CC-9 RBI RCB bit CC-9 bit, was attached to the BHA, the pipe was tripped down, and Hole 1256D was reentered for the eleventh time at 1550 hr on 19 December. After the subsea camera was retrieved, coring was resumed at 1915 hr the same day. Routine coring continued until 2000 hr on 22 December. By this time, we had cored an additional 84.0 m into oceanic basement (average recovery = 40%) and amassed 59.4 rotating hours. The average rate of penetration for this bit was 1.4 m/hr, and the overall average rate of penetration stood at 1.7 m/hr. The average recovery for the hole had dropped to 50%. The depth of the hole was 655.0 mbsf, or 405.0 m below the sediment/basement interface. The bit was at rotary table at 0625 hr 23 December. It exhibited the same characteristics regarding cone wear and erosion as the previous CC-9 bit. The bit body was ~1/16 in under gauge.
The sixth and final CC-9 bit was attached to the BHA, and the pipe was tripped down to the reentry cone. The drill string reentered the cone at 1540 hr on 23 December after 1 hr of maneuvering the vessel in dynamic positioning mode. Coring in the hole resumed at 1845 hr that same day. Routine rotary coring deepened the hole to a total depth of 752.0 mbsf, or 502 m into basement, by noon on 27 December. The average recovery for the 97.0-m cored interval was 39% and accomplished at an average ROP of 1.5 m/hr, or a total of 64.9 rotating hours. The overall average recovery for the hole was 47.8%. The average ROP for all coring bits was 1.6 m/hr.
There was insufficient time to perform another bit change, so it was decided to recover the drill string and change the BHA to a logging configuration. The bit cleared the seafloor at 1515 hr on 27 December and was at the rotary table at 2300 hr that evening. The bit was inspected and found to have lost teeth in the nose and heel area of the cones. There were also some chipped teeth and missing tungsten carbide inserts on the shank body, which was consistent with the experience of the previously recovered two CC-9 bits. The bit body was 1/16 in under gauge.
Coring in Hole 1256D began into a massive flow (igneous Unit 1) that extended from Core 2R (276.1 mbsf) to near the base of Core 12R (350.3 mbsf), for a thickness of ~74.2 m. The recovery in this unit was 93%. Below this, the recovery dropped off and the penetration rate slowed, sometimes with rates as low as 0.5 m/hr. The low recovery can be attributed to a variety of reasons, the most important of which are related to the formation. The thin and fractured flows along with minor amounts of pillow basalt have a tendency to fragment when cored. These fragments would occasionally jam in the core catcher, core liner sleeve, and even the core liner and prevent additional core from being recovered. We removed the liner starting with Core 52R (600.7 mbsf) to reduce the prospects for rubble jamming associated with the core liner and liner sleeve. In an attempt to further improve recovery, we also frequently recovered a core barrel after a 4- to 5-m advance (half cores).
Hole 1256D was cased to 269 m, which is 19 m into basement, and was cored from 276.1 to 752.0 mbsf (Fig. F18; Tables T4, T5). The overall recovery in Hole 1256D was 47.8%, with 227.3 m of core recovered from 475.9 m cored. The average penetration rate for all coring bits was 1.6 m/hr.
The logging BHA was made up of a drag-type open-throat bit without a float valve. The logging bit reentered the hole at 0622 hr on 28 December for the thirteenth and final time. The pipe was set to 60 m below the reentry cone. Five tool strings were deployed, starting with triple combo tool string. The borehole proved to be in very good shape and gave no problems on any of the logging runs. Very little debris had fallen into the hole between coring and logging operations, and, as a result, all tool strings could be lowered to ~750 mbsf, within ~2 m of the total hole depth.
The first pass of the triple combo tool string went smoothly from 750 up to 529 mbsf, at which point the wireline heave compensator (WHC) failed and could not be restarted. The triple combo was run up to the pipe, and then a short repeat pass was made to overlap the interval from 529 mbsf to the pipe. The WHC was fixed after the triple combo run and was used without further difficulty on all subsequent runs. The caliper showed the average hole size to be ~12 in, with some of the hole being 97/8 in and the largest breakouts approaching 17 in.
The FMS/sonic tool string was deployed next. After lowering the tool string to the bottom of the hole, the FMS would not function, owing to a power problem. The sonic tool was functional, so we decided to log with it and then come out of the hole to troubleshoot the FMS problem. After removing the sonic tool, the FMS tool functioned properly and three passes were made.
The Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) magnetometer was deployed on the next two logging runs, with a third attempt on the last run of the logging operations (see below). In all attempts, the tool failed because the gyro unit was consuming more power than could be supplied due to large rotations and vertical accelerations of the tool.
Two passes were made with the Ultrasonic Borehole Imager (UBI) on the next logging run, with the first pass being run slower (100 m/hr) for higher resolution (0.2-in resolution). This was the first time in the history of the ODP that the UBI was used in hard rocks. A repeat pass was made with the vertical resolution of 0.4 in.
The next tool to be deployed was the Well Seismic Tool (WST). The air gun was used, and 12 stations located along the igneous section were recorded. WST operations were delayed for 45 min because of the proximity of a pod of pilot whales to the experiment.
Finally, as time was available, the BGR magnetometer was redeployed and the speed to run the tool to pipe depth was slowed (600 m/hr, compared with the 3000 m/hr usually used). Unfortunately, at 3680 mbrf, operation of the tool ceased again, and logging was abandoned. Logging operations ended at 2030 hr on 30 December.
Following logging, the pipe was tripped out of the hole, the beacon was recovered, and the ship was offset away from the reentry cone. The final pipe trip took 7 hr, with the logging bit clearing the rotary table at 0325 hr on 31 December 2002.
We got under way for Balboa, Panama, at 0330 hr on 31 December. The 824-nmi transit from Site 1256 took 75.5 hr at an average speed 10.9 kt, with the ship arriving at the Merchant Ship Anchorage at 0829 hr on 3 January 2003. After anchoring overnight, the pilot boarded the ship at 0834 hours and the ship got under way, passing under the Bridge of Americas at ~1045 hr. Leg 206 officially ended with the first line to shore in Balboa, Panama, at 1100 hr on 4 January 2003.