OPERATIONS

The 26-nmi distance between Site 1150 and Site 1151 (JT-2L) was covered in 2.8 hr at an average speed of 9.3 kt. The primary positioning beacon was deployed at 2200 hr on 28 July. The bottom-hole assembly (BHA) was assembled, core barrels were spaced out for rotary core barrel (RCB) coring, and the pipe was tripped to the seafloor. The depth estimated for this site from the precision depth recorder (PDR) was 2202.4 m; however, we determined our actual rig floor adjusted seafloor depth by using the vibration-isolated television (VIT) subsea camera to visually observe the bit tag the seafloor.

Hole 1151A was spudded at 0445 hr on 29 June 1999 at a depth of 2182.2 m (2193.7 meters below rig floor [mbrf]). Because this was to be a reentry site, we conducted a jet-in test to 78.0 meters below seafloor (mbsf) as our first operation. Because the RCB coring system was unlikely to recover much quantity or quality core in the upper sediments, we decided to conserve valuable time by initiating continuous RCB coring at the base of the jet-in hole rather than pulling back to the seafloor and starting a new hole. The first barrel recovered was considered a wash barrel (1W) because of the extended interval of advance.

The wash barrel was checked with the handheld hydrogen sulfide (H₂S) monitor, and it registered 4 ppm H₂S gas. This core was left on the catwalk after splitting until it had degassed. H₂S gas was not detected in any of the subsequent cores. Methane varied from 3711 to 61,526 ppmv and ethane from 1.4 to 13.4 ppmv; both were similar to levels observed at Site 1150. The lowest C₁/C₂ ratio was 3374, and no higher molecular weight hydrocarbons were detected.

RCB coring in Hole 1151A began with Core 2R at a depth of 78.0 mbsf and continued through Core 109R to a depth of 1113.6 mbsf. Of the 1035.6 m cored, 707.57 m, or 68.3%, was recovered (Table T1, also available in ASCII format). Recovery was highly variable ranging from 0% to 102%. The average rate of penetration (ROP) for the cored interval was 61.3 m/hr, which is exceptionally high. Even at depth, the ROP was quite rapid with the last 48.3 m of hole advancing at a rate of 32.2 m/hr.

Coring was halted at that point because the formation was considered hard enough for a successful instrument emplacement, which was planned for Hole 1151B. Wireline logging was not conducted in this hole because all remaining time was to be used for the primary leg objective of installing the second instrument string.

At 1130 hr on 2 August we pulled the pipe to a depth of 600.0 mbsf and rigged the circulating head for setting a cement plug. The hole plugging operation was finished at 1345 hr on 2 August after we displaced a 15-bbl plug of 15.8 lb/gal cement into the hole. With the cementing operation completed, we continued to pull out of the hole, clearing the seafloor at 1430 hr. The bit was back at the rig floor by 1745 hr on 2 August, officially ending Hole 1151A.

Reentry Cone and 16-in Casing String Installation

Preparations began immediately for making up the 16-in casing string/reentry cone assembly (Fig. F4). While this was happening, we offset the ship 60 m to the west. At 1900 hr, the reentry cone was moved onto the moonpool doors and preparations began for running the six joints (73.7 m) of 16-in casing. Within 4.75 hr, the entire casing operation was completed. This included welding all casing couplings, making up the 16-in casing hanger, engaging the Dril-Quip running tool, lowering the casing into the reentry cone and verifying latch-in, disengaging the running tool, making up the stinger assembly and remaining BHA components, and reengaging the running tool. At 2345 hr on 2 August, we began tripping the drill string to the seafloor with its 30,000-lb load.

At 0300 hr on 3 August, we spudded Hole 1151B as the casing shoe tagged the seafloor at 2181.6 m (2193.7 mbrf). The jetting process took 10.5 hr (at an average rate of 7.3 m/hr), and at 1330 hr that same day we landed the reentry cone at the seafloor with the casing shoe positioned at 76.3 mbsf.

While recovering the VIT camera, we began to trip the drill string back to the rig floor to change out the jet nozzles in preparation for drilling the 14-in hole. By 2045 hr on 3 August, the ship and drill string were positioned for reentry. After only 15 min of maneuvering time, Hole 1151B was reentered for the first time at 2100 hr.

It took 29.5 hr to drill the 14-in diameter hole to a depth of 1081.3 mbsf, completing the drilling at 0315 hr on 5 August. A 34.1 m/hr net ROP was achieved. There were no mud sweeps pumped while drilling the hole. At total depth (TD) we circulated a 30-bbl sepiolite mud pill out of the hole and then made a wiper trip to 76.3 mbsf. The trip up and back did not identify any trouble spots in the hole. On bottom we found ~12 m of fill that was circulated out of the hole while we pumped a 50-bbl sepiolite mud sweep. The hole was displaced with an additional 400 bbl of sepiolite, and the drill string was recovered back to the ship. The reentry cone was cleared at 1215 hr on 5 August, and the bit was back at the rig floor by 1500 hr.

The 10-in Casing String

The casing shoe joint, 79 joints (1065.8 m) of 10-in casing, and casing hanger were assembled in just over 7 hr. We lowered the 10-in casing to just above the seafloor, and after 30 min of positioning the ship Hole 1151B was reentered for the second time. The casing was run to 1023.3 m without resistance. The top drive was picked up and the remaining casing was lowered into the hole, placing the bottom of the casing string at 1068.2 mbsf. This is the longest casing string set in open hole (991.9 m of open hole) in the operation history of DSDP and the Ocean Drilling Program (ODP). Because of the excellent hole conditions, the casing slid into the hole with ease, making it also one of the fastest casing operations. After making up the cementing swivel we landed the 10-in casing hanger at 0615 hr on 6 August. Latch-in was verified with 15,000 lb of overpull.

We displaced 50 bbl of 15.8 lb/gal cement to bottom using the rig pumps, but there was no indication that the cement dart had properly landed or that the wiper plug sleeve had sheared. We overdisplaced the cement by a few barrels and then checked for backflow verifying that the check valve in the cementing shoe was holding. After releasing the running tool from the reentry cone, it was clear that the casing wiper plug was still attached to the launching sub. Further investigation of the cementing manifold verified that the dart had hung up in the sub and not released. We offset the ship 50 m and then pumped the cementing dart out, wiping the drill string and expelling the wiper plug assembly at the seafloor. The drill string was then tripped back to the drillship, clearing the rotary table by 1500 hr on 6 August.

The 9-in drilling assembly was assembled and run to bottom. Within 45 min of positioning the ship, the third reentry of Hole 1151B was made at 1930 hr on 6 August. The bit was run to bottom, and we tagged the cementing shoe at 1068.3 mbsf. Without the rubber wiper plug and dart in the hole, it only took an hour to drill out the cementing shoe and cement. By 0145 hr on 7 August, we had drilled the 9-in hole to a TD of 1113.0 mbsf. The hole was swept with 20 bbl of sepiolite mud, and a wiper trip was made to the 10-in casing shoe and back. No problems were experienced with the wiper trip and, once back on bottom, another 20-bbl sepiolite mud sweep was circulated out of the hole. After displacing the hole with an additional 30 bbl of sepiolite, the drill string was recovered back to the ship. The reentry cone was cleared at 0715 hr, and at 1030 hr on 7 August the bit was back at the rig floor.

Installation of the Borehole Instruments

Deployment of the instrumentation string went smoothly and even more efficiently than in Hole 1150D. After having run the instrument package, strapping/taping the cables, making up 93 joints (1082.4 m) of 4-in casing, making up the riser/hanger, and installing the multiple- access expandable gateway (MEG), it was time to conduct the final instrument checks. Much to our dismay, only three of the four instruments responded correctly. One of the seismometers was not functioning properly. After removing the MEG, we isolated the problem in the MEG rather than the downhole instrument package or cabling. Within 3.75 hr the problem had been identified and corrected, and the MEG was mounted once again on the riser/hanger.

The trip to bottom was interrupted only to deploy the VIT sleeve and test it over the riser/hanger. The VIT camera was then positioned above the hanger and followed the drill string to bottom. By 2130 hr on 8 August, after maneuvering the ship for 30 min, Hole 1151B was reentered for the fourth and final time. The hanger landed at 0100 hr on 9 August, placing the end of the cementing stinger at a depth of 1098.6 mbsf. The instrument string was cemented into the hole with 80 bbl of 15.8 lb/gal cement.

Battery Pack Installation

The battery frame halves were moved onto the moonpool doors, and preparations began on the assembly and rigging of the battery platform at 0315 hr on 9 August. At 0645 hr, the battery frame was deployed through the moonpool on the logging line and run to bottom at ~3500 ft/hr. The frame landed in the reentry cone at 0900 hr and was immediately released, using the same acoustic release system as was used at Site 1150. We recovered the logging line and deployed the VIT to inspect the installation. At 1145 hr, we determined that the battery frame was resting in the correct position around the riser/hanger. The J-type running tool (J-tool) was released, and the drill string and VIT camera were both pulled out of the hole. The J-tool cleared the rig floor at 1745 hr on 9 August, completing Hole 1151B and ending the second successful instrument emplacement operation.

The running tool and associated subs were disassembled, and the advanced hydraulic piston corer/extended core barrel (APC/XCB) coring BHA was assembled while the ship was offset 200 m to the south of Hole 1151A. Bathymetric data indicated that the seafloor was about 10 m shallower than at Holes 1151A and 1151B, and a PDR reading indicated that the seafloor depth was about 6 m shallower. Based on this information, the bit was placed at a depth of 2179.0 m. Hole 1151C was spudded with the APC coring system at 0045 hr on 10 August, establishing a seafloor depth of 2174.2 m (2186.3 mbrf), about 8 m shallower than at Holes 1151A and 1151B.

APC coring continued through Core 11H to a depth of 97.2 mbsf. Nonmagnetic core barrels were run intermittently with the standard alloy steel barrels to evaluate the effectiveness of nonmagnetic components relative to the magnetic overprint seen in the cores. Coring was halted after experiencing an overpull of 70,000 lb on Core 11H. A single Adara temperature measurement was taken on Core 4H at a depth 30.7 mbsf. Because overpull on that core was 40,000 lb, we decided to abandon further Adara measurements to avoid possible loss of the hole or equipment. Core orientation was also begun with Core 4H. The drill string was pulled clear of the seafloor, and Hole 1151C was officially ended at 0945 hr. Of the 97.2 m penetrated with the APC, 101.75 m (104.7%) of core was recovered.

There were no hydrocarbon problems associated with this hole, though H₂S gas was measured in Core 4H at 20 ppm. Readings for all other cores never exceeded 4 ppm.

While coring operations were under way, a helicopter chartered through the Royal Japanese Helicopter Service landed at 0815 hr on 10 August. Kim Cheh (U.S. congressional staff), James Henry Hall and Edward Kloth (American Embassy, Tokyo), Timothy Clancy (National Science Foundation), Michael Woods (Pittsburgh Post Gazette), and Jeff Fox (ODP/Texas A&M University [TAMU]) came aboard for the day. The helicopter departed at 1500 hr with all passengers except for Michael Woods, who stayed aboard until the Yokohama port call.

With the drill string clear of the seafloor, the ship was offset 15 m further to the south, and Hole 1151D was spudded with the APC at 1030 hr on 10 August. We continued the nonmagnetic core barrel component evaluation on this hole. A calculated seafloor depth of 2179.9 m (2184.0 mbrf) was established with the first APC barrel, which was shot 3 m lower to stagger the core breaks. APC coring continued through Core 10H to a depth of 93.0 mbsf. Cores 4H through 10H were oriented with the Tensor tool. Overall, APC coring in this hole netted a recovery of 96.19 m, or 103.4%, of the formation cored.

Drilling resumed from 93.0 mbsf in Hole 1151D using an XCB center bit assembly. At 1800 hr on 11 August, we terminated the drilling operation at a depth of 3058.4 m, or 874.4 mbsf. After circulating a 40-bbl bentonite gel mud sweep out of the hole, the top drive was set back and we pulled the drill string to a logging depth of 100.4 mbsf.

At 2100 hr on 11 August, we installed the logging sheaves and began rigging up for wireline logging. The first tool string consisted of the Formation MicroScanner (FMS) and the dipole sheer sonic imager (DSI) tools. This string was deployed to TD of 874.0 mbsf. After logging up to the bit, a second pass with the FMS-DSI tool was made, again reaching TD. The second tool string consisted of the triple combination logging tool (triple combo), which also reached TD. By 1515 hr, after 18.25 hr of logging, all tools were out of the hole and the sheaves were rigged down.

The drill string was lowered to TD, and the hole was displaced with 260 bbl of barite-weighted (10.5 lb/gal) mud. The string was then recovered back to the drillship. The bit cleared the seafloor at 1830 hr, and the positioning beacon was released and recovered aboard at 1915 hr on 12 August. At 2330 hr on 12 August, the last thruster was secured, and we were under way for the transit to Yokohama, Japan.

Almost immediately after departing Site 1151, the transit propulsion motor overheated because of a water cooling problem. Because the propulsion motors are coupled, we had to take a second propulsion motor off line. An opposing current further slowed our overall speed for a good portion of the trip, though we still managed to maintain an average speed of 8.6 kt over the 346-nmi distance. The bay pilot came aboard at 1400 hr, the harbor pilot came aboard at 1540 hr, and the vessel was tied up with the first line ashore at 1615 hr on 14 August 1999, officially ending Leg 186.

Weather on site and during transits was exceptionally good. Calm seas, scattered clouds, and a gentle wind typified each day. A southerly swell generated some heave, but it rarely exceeded 0.6 m.