OPERATIONS SYNOPSIS (continued)Return to Hole 1150D
In exceptionally calm seas, we reentered Hole 1150D. Using a 9-7/8-in tricone drill bit, we drilled through the cementing shoe at a depth of 1031.8 mbsf and began drilling the 9-7/8-in diameter hole. Drilling in Hole 1150D was completed to a depth of 1140.0 mbsf. The final 95.2 m of hole took 5.5 hr to drill at a rate of 17.3 m/hr. The hole was circulated with a 50-bbl bentonite gel mud sweep and the wiper trip commenced back to the 10 3/4-in casing shoe. We completed the trip back to the casing shoe without incident, though on the return back down experienced 20-30 kips of drag almost from the start. Torquing and elevated pump pressure was also apparent. After reaming the hole to bottom twice and circulating two additional sepiolite pills, the hole seemed to be clean.
Cementing Hole 1150C
At 1315 hr on 23 July, we began pulling out of the hole to the seafloor. We cleared the reentry cone at 1530 hr and offset the ship back to Hole 1150C to plug the open hole with cement. During the move between holes we identified a long string of 10 3/4-in casing resting on the seafloor near Hole 1150C. As discussed above, the 10 3/4-in casing string at Hole 1150C apparently failed in more than one place, which allowed some casing to fall into the reentry cone and the rest to fall outside onto the seafloor. We were not able to accurately determine the quantity of casing outside the reentry cone; however, it is possible that only one or two joints actually fell into the hole.
At 1630 hr, we reentered Hole 1150C and, as anticipated, we had to rotate the drill bit with the top drive to pass through the cone transition area. Because we had made five flawless reentries into Hole 1150D, we can now definitively rule out environmental causes for the reentry problems experienced in Hole 1150C. Therefore, there must be a major structural problem or failure in the transition area of the Hole 1150C reentry cone. The pipe was advanced to a depth of 263.0 mbsf and a 15-bbl plug of cement was pumped into the 16-in casing. This was to prevent flow between Hole 1150C and Hole 1150D, where the instrument string was to be emplaced. At 1845 hr, after displacing the cement plug, the pipe was pulled clear of the Hole 1150C reentry cone. The pipe cleared the rotary table at 2230 hr, and preparations began for deployment of the instrument package in Hole 1150D.
Installation of the Borehole Instruments at 1150D
The ship was offset back to Hole 1150D as the rig floor was prepared for handling the instruments. The instrument string was assembled by its various parts, of which the lowest is a stinger pipe, a tube that allows cement to be pumped below the instruments (Fig. 8). The stinger is bolted to a overlying strain meter. Above this is a deployment frame with two seismometers and a tilt meter. This part of the instrument package was lowered into the moonpool area below the rig floor where the four 3/4-in diameter instrument cables were connected to the respective instruments. The rest of the instrument string consisted of 4 1/2-in casing, a circulating sub, and a riser/casing hanger. Ninety-five joints (~1107.1 m) of 4 1/2-in casing were run while strapping and taping the instrument cables with tie wraps and duct tape. In addition, casing centralizers were installed ~7.0 m apart along the casing. After deploying 40 joints of casing, the circulating sub, which allows the drill string to be flushed after the instruments are cemented in place, was installed. Then the other 55 joints of 4 1/2-in casing were assembled with cables being attached in the same manner. The last 2 joints were not taped because this portion of cable was to be removed later to allow the watertight connectors to be installed in the subsea shop.
After running all of the 4 1/2-in casing, the J-slot running tool was assembled, and the riser and casing hanger were picked up and connected to the top joint of 4 1/2-in casing. It was then lowered into the moonpool area, where the instrument cables were measured and cut to length. The riser was pulled back up to the rig floor while the cable ends were fed into the subsea shop for installation of the watertight connectors, a process referred to as cable termination.
The cable termination began at 1945 hr on 24 July and extended until 1030 hr on 25 July. By 1200 hr, MEG had been installed into the specially designed riser carrier, and the four cables and their new connectors were plugged into the MEG. (MEG is composed of combiner/repeater module, analog-digital converter modules, strainmeter interface module, and power conditioning/distribution module, all of which acquire signals from the sensors, convert analog signals to digital data, and send out all the converted digital data to recorders via a single serial link.) By 1300 hr the final instrument checks had been completed. Once the riser/hanger was lowered beneath the ship, the VIT camera was deployed over the running tool/riser/casing hanger assembly to ensure that there was no interference. The VIT was recovered after the successful test and we commenced tripping the drill string to seafloor.
After positioning the ship for 1 hr, we reentered Hole 1150D at 1730 hr on 25 July. The reentry drill string was spaced out so as to allow the entire 16-m instrument package to be inserted completely into the throat of the reentry cone with the top of the instruments below the 10 3/4-in casing hanger. The bottom of the instrument string was then lowered to a depth of 1032 mbsf where the top drive was picked up in preparation for inserting the instrument package into the borehole. Slow circulation was maintained as the package was lowered into the open 9-7/8-in hole so we would know immediately if the end of the stinger began to plug. Resistance was met almost immediately after entering the open hole. Washing continued for 3 hr as the package was advanced to a depth of 1101.8 mbsf or 23.4 m short of landing the riser/casing hanger in the reentry cone. Throughout the insertion period we experienced 10-20 kips overpull or drag and pressures of up to 1500 psi. When making connections with each new pipe joint, the hole would close around (pack off) the stinger or the stinger would become plugged. Casing advance proved impossible until pump pressures returned to normal. Several times the casing had to be picked up and worked back down again. This frequently failed to lower the pressure, indicating the stinger pipe was probably still plugged. Our efforts to emplace the instrument package were further aggravated by a leak path that was apparently coming from the J-type running tool. This made it difficult to determine how much circulation was going out the stinger and how much was leaking past the running tool. Mud sweeps did not improve the situation. At 2400 hr on 25 July we elected to pull the instrument string back inside the casing shoe. At this point, the stinger was completely plugged, and increasing the pump pressure did not unplug it.
It was obvious to all that we needed to reenter the hole with a drilling assembly and ream out the ~100 m of open hole before we had any chance of successfully inserting the instrument package. Fortunately the stinger unplugged itself during the pipe trip eliminating another potentially serious problem. The final step was to check the status of the instruments once the riser was pulled into the moonpool area. To our relief, all of the instruments were functioning. Given their multiple trips up and down the tight part of the open hole and their trip through the casing and reentry cone, the instruments and cables proved to be quite robust.
We decided to attempt hang off the intact instrument string from the ceiling above the moonpool rather than undertake the enormous job of re-spooling the cables, removing the tape, tie wraps, and centralizers, breaking down the 4 1/2-in casing string, and disassembling the stinger/strain meter/seismometer/tilt meter instrument package. We were able to attempt this nonstandard operation because of very calm weather and sea conditions and because the hang-off loads were relatively low at ~20 tons. To allow hanging off the instrument string we had to make some modifications to the riser/hanger stabilizer fins. These were cut back ~18 cm (~7-in) to install the 10 3/4-in casing elevators with the 10 3/4-in casing slips installed below. We then disengaged the running tool and set it aside for inspection and repair. Four 1-1/8-in diameter wire slings were attached to the 10 3/4-in elevators and connected to the blocks. As the blocks were lowered, the load was slowly transferred from the lowering slings to the hang-off slings. This allowed the ~1130-m-long instrument string to swing slightly forward in the moonpool where it was secured with a tugger line. The transfer was completed in 5 hr (including riser/hanger modifications), and the rig floor was then clear for pipe handling/drilling operations to continue. In addition to the hang-off activities, the wash pipe in the running tool was welded at the top end to eliminate a circulation leak path within the J-type running tool.
A 9-7/8-in tricone drill bit was assembled with two stands of drill collars and the normal 5 1/2 in transition pipe. We lowered the pipe to bottom and reentered Hole 1150D at 1730 hr on 26 July. We reamed out the hole making several passes through tight spots and circulated several 30-bbl sepiolite mud sweeps. This was followed by a wiper trip, circulating 5 m of fill from the hole, and then displacing the hole with 40 bbl of sepiolite mud.
We pulled out of the hole clearing the reentry cone at 0515 hr on 27 July and proceeded pulling pipe until clearing the rotary table at 0900 hr. Within 2 hr, the instrument string was transferred back to the running tool and a final instrument check was again performed. The pipe was once again run to bottom, and Hole 1150D was reentered for the eighth and final time at 1630 hr on 27 July.
This time the string entered the open hole with little resistance; however, after three pipe joint connections the drag and elevated pump pressure once again became our nemesis. As before, when a connection was made the string would pack off and high pump pressure was required to initiate circulation. Despite the repairs made to the running tool, we were once again faced with a circulation leak. The pipe was worked, the stinger unplugged multiple times, and after 3 hr we successfully landed the riser and casing hanger in the reentry cone placing the end of the instrument string stinger at 1125.2 mbsf.
We pumped 80 bbl of 15.8 lb/gal cement, displacing it with the rig pumps. The cementing dart landed in the circulating sub located ~455 m above the instrument package and the sleeve sheared as designed at ~1500 psi. This opened up circulation ports to the annulus and allowed the drill string to be thoroughly circulated (two times capacity) to remove any remaining cement. The VIT camera was recovered, and at 0045 hr on 28 July we began to rig up for assembling the battery frame halves on the moonpool doors.
Battery Pack Installation
The battery frame halves were bolted together around the pipe on the moonpool doors. Final deployment rigging and release bridles with dual acoustic release systems were attached. We waited until daylight to deploy the assembly through the moonpool because of the intricate rigging required and the poor visibility at night within the moonpool. We did not want to risk snagging a deployment cable on the guide horn. After waiting until 0430 hr (~45 min) we began to lower the platform through the moonpool. This intricate operation was completed quite well, partly because of the relatively calm sea state, although there was some surge within the moonpool.
The battery platform was lowered at ~3500 ft/hr using the logging line, and, at 0830 hr, the assembly landed in the reentry cone. Within 10 min, a portable command unit was used to send a 10-kHz acoustic signal to the release system. After recovering the logging line and the remaining deployment bridle equipped with acoustic releases, the VIT camera was deployed. At 1145 hr we verified that the battery platform had indeed been placed exactly in position as planned. All cables and surface gear appeared to be in good condition, undamaged by the deployment process.
It took about 1 hr to actually release the J-type running tool from the riser, keeping the entire drill crew and science party on edge. At 1245 hr on 28 July, while working the tool through the neutral point and with ~200 A torque, the running tool abruptly released from the riser/hanger. This completed the instrument installation for Hole 1150D.
During the pipe trip to the rig floor, both positioning beacons were released and recovered aboard, the rig floor was secured for transit, and at 1900 hr on 28 July the ship got under way for Site 1151.
The weather was quite variable during operations in Hole 1150B. At no time was the operation halted because of weather, though conditions did deteriorate markedly during the passage of a low pressure cell northwest of the drilling location. We began to experience the effects of one low pressure cell on the afternoon of 30 June, which continued through midday on 1 July. At its worst this gale brought sustained winds of 42 kt gusting to 56 kt, 28-ft seas on 7-s periods, and 30-ft swells on 7- to 8-s periods. The maximum roll, pitch, and heave experienced was 5°, 4°, and 9 ft, respectively.
Weather while drilling Hole 1150C was moderate and did not influence operations. The climate was characterized by frequent fog, overcast skies, frequent rain showers, and cool temperatures.
Weather and sea state were both excellent during operations at Site 1150D. The concern that a tropical depression was going to cross the site before installation of the instrument string influenced us to head to port earlier than would have been ideal. The storm appeared to dissipate as it neared the site, though we have no idea what its final effect on the site was since we were in Yokohama then.
Transit to Site 1151
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 BHA was assembled, core barrels spaced out for RCB coring, and the pipe was tripped to the seafloor. The depth estimated from the precision depth recorder (PDR) for this site was 2202.4 m; however, we determined our actual rig floor adjusted seafloor depth by using the 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 mbrf). Because this was to be a reentry site, we conducted a jet-in test to 78.0 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 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 1). Recovery was highly variable ranging from 0% to 102%. The average 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 began pulling 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 displacing 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. 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 6 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 3/4-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 3/4-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 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 pumping 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 3/4-in Casing String
The casing shoe joint, 79 joints (1065.8 m) of 10 3/4-in casing, and casing hanger were assembled in just over 7 hr. We lowered the 10 3/4-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 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 3/4-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 back flow 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 wiping the drill string and expelling the wiper plug assembly at the seafloor.The drill string was then tripped back to the drill ship clearing the rotary table by 1500 hr on 6 August.
The 9-7/8-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 9-7/8-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 3/4-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 1/2-in casing, making up the riser/hanger, and installing 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 MEG, we isolated the problem in MEG rather than the downhole instrument package or cabling. Within 3.75 hr the problem had been identified, corrected, and MEG 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 forth 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 and 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. The logging line was recovered and the VIT deployed 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 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 APC/XCB coring BHA was assembled while the ship was offset 200 m to the south of Hole 1151A. Bathymetric data indicated the seafloor was about 10 m shallower than at Holes 1151A and 1151B, and a PDR reading that indicated 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 so as not to risk 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. The following personnel came aboard for the day. 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 (TAMU). 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 FMS and the Digital Sonic (FMS-DSI) tools. This string was deployed to TD of 874.0 mbsf. After logging up to the bit, a second pass with the FMS-sonic was made, again reaching TD. The second tool string consisted of the 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.
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. This officially ended 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 this rarely exceeded 0.6 m.
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