OPERATIONS

The JOIDES Resolution arrived at Pier 6 in Balboa, Republic of Panama, with the first line ashore at 1030 hr on 6 November 2002, marking the end of Leg 205 and the start of Leg 206. All times are reported in local ship time, which is Universal Time Coordinated (UTC) (a list of operation acronyms is given in Table T1) minus 5 hr in Panama and UTC minus 6 hr at Site 1256. A summary of operations completed during Leg 206 is given in Table T2.

Extensive repairs to the guide horn, along with the large amount of freight that had to be unloaded from Leg 205 and loaded for Leg 206, resulted in a full 5-day-long port call. Items of note were the removal of 80 joints of 4-in casing, two CORK assemblies, and seven joints of 10-in casing, the latter of which were damaged during Leg 205. Hardware loaded on the vessel for Leg 206 included 36 joints of 10-in, 37 joints of 13-in (two of which were rejected owing to damage and then discarded in port), 23 joints of 16-in, and 7 joints of 20-in casing. Also loaded via the bulk hopper were 60 metric tons of sepiolite in 66 standard 2000-lb bags and 420 sacks (20 metric tons) of 94 lb/sack Class G cement. Two new pieces of drilling hardware arrived for Leg 206. These were two Downhole Design, Inc. (DDI) B145x185 and B182x215 bi-centered reamers (BCRs). In addition, four each of Rock Bit International (RBI) CC-4 and CC-7 coring bits and one 18-in Atlas 5JS hard-formation tricone drilling bit were loaded. After welding on the guide horn was completed, 1284 metric tons of fuel were loaded.

The pilot boarded the vessel at 1404 hr on 11 November, which corresponded to low tide. This time was selected to ensure safe clearance of the derrick under the Bridge of Americas. The last line was released at 1500 hr, and the vessel maneuvered into the channel and cleared channel marker buoys "G" and "R" at 1518 hr. At 1534 hr, the pilot disembarked and the vessel headed to Site 1256 (Site GUATB-03C in the Leg 206 Scientific Prospectus) in the Guatemala Basin.

The 821-nmi transit to Site 1256 was accomplished at an average speed of 11.0 kt and was without incident. In the mid-afternoon of 14 November 2002, the captain reduced speed as the vessel slowly approached the coordinates of the location using the Global Positioning System (GPS) interface to the dynamic positioning (DP) system. The thrusters were then lowered and the DP system activated. The vessel was on station using the GPS data by 1630 hr. The hydrophones were lowered concurrent with the making up of the bottom-hole assembly (BHA). A beacon was deployed at 1707 hr.

In preparation for coring with the APC in Hole 1256A and with the APC and extended core barrel (XCB) in Hole 1256B, the BHA was assembled. The BHA consisted of a 11-in APC/XCB rotary bit, a bit sub, a seal bore drill collar, a landing saver sub, a modified top sub, a modified head sub, a nonmagnetic drill collar, five 8-in drill collars, a tapered drill collar (TDC), six joints of 5-in drill pipe, and one crossover sub. The total BHA length was 136.2 m.

In accordance with routine, all tubulars were measured (strapped) and cleared of obstructions (rabbited) prior to deployment. After the drill string was deployed to a depth of 3634.5 meters below rig floor (mbrf, which is a drill pipe measurement made from the top of the dual elevator stool on the rig floor), the top drive was picked up and spaced out in preparation for obtaining the initial piston core of the site. To increase the chances of sensing weak magnetic signatures in the laboratory, both piston coring assemblies were constructed with nonmagnetic components to reduce the magnitude of the overprint caused by the magnetization of the corer.

Hole 1256A was spudded with the APC at 0640 hr on 15 November. The seafloor depth was estimated at 3634.5 m (3645.2 mbrf) based on core recovery. This value was 0.2 m shallower than the corrected precision depth recorder (PDR) depth. The single mudline core from this hole was obtained to ensure that a good mudline was recovered and to provide additional material for microbiological and geochemical sampling (Tables T3, T4).

Before starting Hole 1256B, the APC temperature (APCT; formerly known as the Adara temperature tool) shoe was held just above seafloor from 0753 to 0813 hr to obtain a temperature measurement of the bottom water (see "Temperature and Heat Flow" in "Downhole Measurements" in "The Sedimentary Overburden (Holes 1256A, 1256B, and 1256C)"). Hole 1256B was then spudded with the APC at 0815 hr on 15 November. The seafloor depth inferred from the recovery was 3634.7 m (3645.4 mbrf), which agreed with the depth estimated from the PDR.

Piston coring deepened the hole until Core 206-1256B-18H failed to achieve a full stroke of the corer at a depth of 160.1 mbsf. The recovery for the piston-cored interval was 163.91 m, representing an average recovery of 102.4%. Cores were oriented using the Tensor tool starting with Core 206-1256B-3H. In addition to the bottom water temperature measurement, downhole temperature measurements were attempted with the APCT at 34.6 mbsf (Core 206-1256B-4H), 53.6 mbsf (Core 6H), 82.1 mbsf (Core 9H), 120.1 mbsf (Core 13H), and 158.1 mbsf (Core 17H). The first attempt (Core 206-1256B-4H) did not record a valid temperature because of battery failure in the instrument, but the other measurements were successful.

Coring in Hole 1256B continued with the XCB system to 251.7 mbsf, where the last core penetrated basaltic basement and recovered 6 cm of basalt. The XCB cored 91.6 m and recovered 60.23 m, with an average recovery of 65.8%. Recovery below 210 mbsf dropped considerably because chert nodules were getting jammed in the throat of the cutting shoe.

The drill string was pulled free of the seafloor at 1635 hr on 16 November, ending Hole 1256B. The total results for the hole were 251.7 m cored and 224.14 recovered for an average recovery of 89.1% (Tables T3, T4).

The vessel offset 10 m south of Hole 1256B to conduct a jet-in test. This test was performed in order to establish the length of 20-in surface casing to deploy with the reentry cone in Hole 1256D. The test penetrated 100 m with a maximum pump of 34 strokes/min (spm) and pump pressure of 200 psi. The drill string was then recovered to change the BHA for rotary core barrel (RCB) coring in Hole 1256C. The APC/XCB bit was at the rotary table at 0630 hr on 17 November.

The RCB BHA was assembled with a new RBI CC-4 bit, a mechanical bit release, a modified head sub, an 8-in controlled-length drill collar, a modified top sub, 10 more 8-in drill collars, a TDC, two stands of 5-in drill pipe, and a crossover to 5-in drill pipe. The total length of the BHA was 112.9 m.

The pipe was tripped to the seafloor, and Hole 1256C was spudded at 1515 hr on 17 November. The seafloor depth was assumed to be the same as that at Hole 1256B (3634.7 m; 3645.4 mbrf). Before coring began, the hole was drilled with a center bit to a depth of 220.1 mbsf at an average rate of penetration (ROP) of 84 m/hr. The first core (206-1256C-1R) contained only a few small pieces of chert in the core catcher. Chert plagued recovery on the next two sedimentary cores, although 4.67 m of nannofossil ooze was recovered in Core 206-1256C-3R. Basement was contacted in Core 206-1256C-4R at a depth of 251.8 mbsf, and a few pieces of chert and ~34 cm of basalt were recovered.

Subsequent coring into basement resulted in some spectacular recovery percentages for RCB coring in hard rock, with 10.07 m recovered (recovery = 109.5%) in Core 206-1256C-9R and 10.24 m recovered (recovery = 107.6%) in Core 206-1256C-11R. Recovery from Cores 206-1256C-5R through 11R (252.4-312.8 mbsf) averaged 81.8%. The last two cores (206-1256C-13R and 14R) penetrated 18.3 m and recovered 0.16 m, or 0.9% of the cored interval. This reduced the average recovery in basement to 61.3%. The average ROP for RCB coring in basement was 1.4 m/hr. Recovery for the hole, including sediment and basement, averaged 50.8% (Tables T3, T4). The total depth of Hole 1256C is 340.3 mbsf, with the lower 88.5 m in basement.

Microbiological studies were conducted with the deployment of Whirl-Pak dispenser packets containing fluorescent microspheres and the injection of perfluorocarbon tracer (PFT) during the coring of Cores 206-1256C-7R (266.4-275.5 mbsf), 9R (284.7-293.9 mbsf), and 11R (303.3-312.8 mbsf).

Logging

At the conclusion of coring, Hole 1256C was prepared for logging with a wiper trip to 83 mbsf. After the bit was released, the hole was displaced with 92 bbl of sepiolite mud and the end of the pipe (EOP) was set at the logging depth of 125.8 mbsf.

The triple combination (triple combo) tool string was deployed first but was unable to pass a bridge at ~203 mbsf (only 77 m below the EOP). Following recovery of the logging tool and de-rigging the Schlumberger equipment, the drill string was lowered to within 20 m of the bottom of the hole to clear obstructions. A second attempt to deploy the triple combo was made with the EOP placed at 231 mbsf. This time the tool successfully logged across the sediment/basement interface. The FMS-sonic tool string was then deployed but was unable to advance deeper than ~257 mbsf (~5 m into basement). Given the time it would have taken to clear the obstruction and the relatively short interval of basement that could have been logged, we decided to terminate the logging program. The drill string was recovered, with the EOP clearing the rotary table at 0345 hr on 23 November.

Reentry Cone and 20-in Casing

To prepare for the deployment of the reentry cone and 20-in casing, the upper guide horn was removed and the Dril Quip Cam-Actuated-Drill-Ahead (CADA) tool was made up to the 20-in casing hanger. The CADA tool and hanger were then connected with a crossover sub to a stand of 5-in drill pipe and set back in the derrick. The orientation of hanger to the running tool was clearly marked.

The assembled reentry cone was positioned over the moonpool doors with the mud skirts oriented fore and aft of the vessel (Fig. F10). The 0.17-m transition pipe was set inside the lower guide horn, and the moonpool doors were secured together. Six full joints and a shoe joint of 20-in casing (K-55, 94 lb/ft) were assembled and lowered into the throat of the reentry cone (Fig. F11). The first three joints were Baker-locked (epoxy), and the remaining tubulars were tack-welded at each connection. The CADA tool and casing hanger were picked up and affixed to the top joint of casing, and the assembly was carefully lowered into the throat of the reentry cone and latched into the lowest snap-ring groove (Fig. F11). The connection was visually confirmed, and the reentry cone and casing were then picked up a small distance off the moonpool doors to verify a proper latch-in of the hanger. The hanging weight of the assembly was measured at 39,000 lb. The reentry cone was then gently set back on the moonpool doors. The CADA tool was released from the latched-in hanger by rotating 3.5 turns clockwise and set down on the rig floor.

A jet-in BHA was assembled from a used Smith 2JS 18-in tricone drilling bit with three 0.5-in (-in) nozzles, a bit sub, an 8-in controlled-length drill collar, a modified top sub, a modified head sub, another controlled-length drill collar, the top section of the CADA tool, the bottom section of the CADA tool, two more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to 5-in drill pipe connections. The length of the BHA from the bit to the top of the TDC was 122.9 m. The measured weight of the BHA was 50,000 lb. The section of the BHA below the landing shoulder of the CADA tool to the tip of the bit was 94.95 m. The length of the casing string from the base of the mudskirt to the end of the casing shoe was 95.12 m. A white vertical strip was painted on the 5-in transition drill pipe 3 m above the CADA tool as a means of verifying the amount of rotation.

The BHA was lowered and the CADA tool landed into the hanger of the reentry cone and engaged by rotating the drill pipe 3.5 turns counterclockwise. The entire cone, casing, and BHA was then picked up and the moonpool doors opened. The reentry cone was in the water at 1606 hr on 23 November. After the drill string was lowered to 2694 mbrf, the subsea vibration-isolated television (VIT) camera was deployed and lowered with the drill string to 3622 mbrf. The top drive was picked up and the drill string spaced out in preparation for jetting in the reentry cone and casing.

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. F12). It required 10.2 hr to jet the 95 m of casing into the sediment. The mudskirt contacted the seafloor at 1035 hr. The CADA tool was released, and the VIT camera and drill string were recovered. The bit was at the rotary table at 2015 hr on 24 November.

Opening the Hole for 16-in Casing

The next phase of operations required the drilling out of a 21-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 21-in hole (rathole) below the cement shoe to accept rubble that may accumulate during the casing deployment procedure. In order to drill this large hole, the Drilling Services Department of ODP acquired two DDI B182x215 BCRs (Fig. F13). This hardware is designed to be utilized in conjunction with a 9-in-diameter rock bit. This combination may be used to drill and ream simultaneously or to open a previously drilled pilot hole. The B182x215 BCR has an 18-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 21 in. This was the first time such a device was used in scientific ocean drilling.

The BCR was affixed to a 9-in hard-formation tricone drilling bit (Smith F7). To acquire the recommended pressure drop across the drilling hardware of 500-1000 psi, the bit was fitted with three -in nozzles. This size nozzle mated with the single -in nozzle on the reamer would provide a 590- to 730-psi drop at flow rates ranging from 400 to 450 gallons/min (gpm), which is equivalent to 80 to 90 spm. The BCR BHA consisted of a tricone bit, a BCR, a bit sub, an 8-in controlled-length drill collar, a modified top sub, a modified head sub, 13 more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to the 5-in drill pipe. The length from the bit to the top of the TDC was 140.7 m.

The BCR 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 21-in hole was drilled 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. The drilling parameters were weight on bit = 5000 lb, rotary speed = 60 rpm, and rotary current = 150 A (2000 ft·lb of torque).

It was surmised that a thick chert layer was encountered at 161 mbsf because of erratic torque and variations in rotary speed. Basement contact was made at 250 mbsf as the ROP slowed and the rotary current smoothed out. To keep basalt rubble from accumulating on top of the BCR, the driller picked the drilling assembly off bottom after every 1 to 2 m of advance and then backreamed the previously drilled interval.

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 (~17 m into basalt) 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 obvious. One of the three cones on the pilot bit was almost completely destroyed and was 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 BCR 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 (Fig. F14) was made up with a slightly used hard-formation Smith F7 tricone bit. The assembly was made up of the bit, two junk baskets, a controlled-length drill collar, a modified top sub, a modified head sub, 13 more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to 5-in drill pipe. The length from the bit to the top of the TDC was 140.3 m.

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 9-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 (curated as Core 206-1256D-1M) consisted of about a half a bucket of basaltic debris, some chert fragments, and ground-up metal filings, which were removed from the debris with magnets (Fig. F15).

The second B182x215 BCR was then connected to a new 9-in Smith F9 tricone bit fitted with three -in nozzles. The rest of the BCR BHA was assembled as the first BCR BHA described above, and the length from the bit to the top of the TDC was 140.7 m.

The BCR was lowered and reentered the hole at 1755 hr on 28 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. The drilling parameters were similar to those of the first BCR.

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 the 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 from the remaining cones. There was also 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 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 if it was 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 drill out the cement shoe with a C-4 RCB core bit mated to a junk basket. The thought process was that if the cone was 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 then be able to resume coring. If there was instead high erratic torque once the bit had penetrated to the basalt, suggesting that the cone was more or less intact, then we would need to make a round trip and go back in and clean out the hole.

Installing the 16-in Casing

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. It contained 20 joints of 16-in (K-55, 75 lb/ft, range III) buttress casing, a cement float shoe, and a 16-in hanger. The poppet valve in the cement float shoe was inspected, and, in accordance with routine, the standoff lugs were tucked under the sealing lip of the poppet to allow seawater to flow into the casing while running in the hole.

Each of the threaded connections was made up with the rig tongs to the recommended 7520 ft·lb of torque. In addition to this torque, the bottom three joints of casing were Baker-locked (epoxy) and tack-welded in 3-in strips at 90° increments around the periphery of the connection (top and bottom of the casing collar). The casing joint directly above was not Baker-locked, but it was tack-welded in the same fashion as the bottom three joints. The Halliburton single-collet subsurface release equalizer swivel and wiper plug were assembled below the CADA tool. The top of the casing string was topped off with seawater, the 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 VIT camera captured the grim vision 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 one additional casing collar was damaged in 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 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 was down 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 top drive was on line during the reentry and maintained circulation rates of 100-150 gpm (20-30 spm) though the sediment section. 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.

In preparation for landing the casing, the cement manifold and an additional 20-ft knobby were picked up. 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 3.75 hr, this string came free and the hanger successfully landed in the reentry cone. Confirmation of the hanger engaging 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 Halliburton subsurface release plug was sheared at 2800 psi after 1708 strokes. The cement plug was "bumped" after an additional 1616 strokes. The CADA running tool was released by clockwise rotation of the drill pipe at 1955 hr.

Cleaning Hole 1256D

The RCB BHA was made up of a new RBI CC-4 bit (the first of six RCB bits used in Hole 1256D) affixed to a bit sub/boot basket also known as a junk basket, an 8-in controlled-length drill collar, a modified top sub, a modified head sub, 13 more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to a 5-in drill pipe connection. The length of the BHA from bit to the top of the TDC was 140.4 m.

The drilling assembly reentered the cone at 1500 hr on 4 December, and by 1815 hr drilling on the cement plug was initiated. The cement, subsurface release dart, and wiper plug were drilled out by 0115 hr the following morning. After a fresh core barrel was dropped, the bit was carefully lowered to the bottom of the hole and rotation was 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 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 (Fig. F15). A fresh barrel was dropped in attempt to "core" some more fragments, but this was not successful. The drill string was then recovered. An additional couple of metal fragments were found lodged between the cones and cutting guides.

A fishing assembly was made up of a 9-in Bowen fishing magnet, two junk baskets, a bit sub, an 8-in controlled-length drill collar, a modified top sub, a modified head sub, 13 more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to a 5-in drill pipe connection. The length of the BHA from bit to the top of the TDC was 141.8 m.

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 that resides inside the fishing guide, which was a mill guide on this run.

The fishing BHA reentered Hole 1256D at 2358 hr on 5 December. At 0230 hr the next morning, the driller began "working" (circulating water at the bottom of the hole) 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 80-100 spm (400-500 gpm) to stir up the material from the bottom for capture in the junk baskets. The top drive rotary current limit was set at 200 A, and a rotation of 20-30 rpm was established as 5000 lb of weight was applied to the fishing magnet. This was repeated two to three times. 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 fragments of cone were removed from the magnet (Fig. F15). 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.

RCB Coring in Basement

Basement coring operations consist of coring with the RCB system for the life of a bit, which is ~60-65 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) affixed to a bit sub/boot basket (junk basket), an 8-in controlled-length drill collar, a modified top sub, a modified head sub, 13 more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and a crossover sub to a 5-in drill pipe connection. The length of the BHA from the bit to the top of the TDC was 140.4 m.

The drill string entered the 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 206-1256D-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 206-1256D-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. As soon as the sinker bars were removed from the drill string following the recovery of the last core barrel (Core 206-1256D-21R), the driller observed an increase in torque and pump pressure. After pulling up to 388 mbsf, the drill string 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 good condition with no missing inserts and no excessive wear 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 to 244.0 m with an average recovery of 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 ~-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 60-65 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 bit but possesses more inserts on the surface of each bit cone. Because the CC-9 bit has a less invasive cutting surface than the CC-7 bit, it should have a smoother cutting action in the very hard basalts 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 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 206-1256D-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 ROP of 1.3 m/hr. The used bit was found to be ~-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, 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, with an average recovery of 40% and had amassed 59.4 rotating hours. The average ROP for this bit was 1.4 m/hr, and the overall average ROP 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 on 23 December. It exhibited the same characteristics regarding cone wear and erosion as the previous CC-9 bit. The bit body was ~-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 DP 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 was 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 was 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 -in under gauge.

Coring Summary for Hole 1256D

Coring in Hole 1256D began into a massive flow (igneous Unit 1256D-1) that extended from Core 206-1256D-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 ROP slowed, sometimes with rates as low as 0.5 m/hr. The reason for the drop-off in 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 206-1256D-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 502.0 mbsf (Fig. F16; Tables T3, T4). The overall recovery in Hole 1256D was 47.8%, with 227.3 m of core recovered from 475.9 m cored. The average ROP for all coring bits was 1.6 m/hr.

Logging

The logging BHA was made up of a drag-type open-throat bit without a float valve and consisted of the bit, one controlled length drill collar, a modified top sub, a modified head sub, four more controlled-length drill collars, a TDC, two stands of 5-in drill pipe, and one crossover sub. The length from the bit to the top of the TDC was 56.6 m.

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 the 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 to 529 mbsf, at which point the wireline heave compensator (WHC) failed and could not be restarted. The tool string 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 9 in and the largest breakouts reaching 17 in (full range of the tool).

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. Sonic velocities measured by the Dipole Sonic Imager (DSI) tool appear to be of variable quality; S-wave velocities appear to be good in quality, and P-wave velocities are of poor quality. After removing the sonic tool, the FMS tool functioned properly. Three passes were made with the FMS to try and get better hole coverage. Even though part of the hole was round, the tool tended to track fairly close to the same path on all three runs. The FMS data were very good, as expected in basement, with large hole size being a problem in only a few short intervals.

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. In the first two runs, operations ceased at 1000 and 2500 mbrf, respectively. A first analysis of the BGR data shows that the data flow ceased because of a malfunction caused by the power requirements of the gyro unit in the tool. All data sets indicate that a large rotation rate (>40°/s) coupled with acceleration in the z-direction of the tool preceded tool failure. The gyro circuitry consumes around double the nominal current when such large rotations and accelerations occur. Because of the resistance in the long cable, the voltage at the tool dropped below the minimum operation voltage of the gyro circuitry, resulting in termination of data acquisition by the tool. The tool could not be restarted downhole because the gyro requires high and variable currents for startup.

Two passes were made with the 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 ODP that the UBI was used in hard rocks. The UBI is designed for a smaller borehole size than is typically drilled during ODP legs, and to be deployed in optimal conditions, the transducer should be as close as possible to the formation. Overall, the UBI data were very good considering the sub being used was designed to work in a hole diameter of 7 in. When the hole was between 9 and 11 in, the data were very good, whereas the data were marginal when the hole was >12 in. 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.