Leg 198 officially began at 1100 hr on 27 August 2001 with passage of the first line ashore in Yokohama, Japan, ending Leg 197 approximately 19 hr early. At 0930 hr on 31 August, after four days of port call activities, the last line was released and the ship maneuvered into the harbor.
The transit to the first site was initially slowed by the presence of Typhoon Wutip, which was forecast to cross over Site 1207 about the same time as our full-speed arrival projection. To maintain a safe distance from the storm, we kept the correct course but at reduced speed for the first several days of the transit. As the typhoon increased speed and began moving to the northeast of the site, we increased our speed accordingly. Overall, the 1158-nmi, 5.2-day transit was accomplished at an average speed of 9.2 knots (kt). Upon arriving at the coordinates for the site, the thrusters and hydrophones were lowered at 1730 hr on 5 September, followed by deployment of a positioning beacon.
Once the vessel settled on location, an APC/XCB/motor-driven core barrel (MDCB) outer barrel assembly was assembled and the MDCB system was deck tested prior to deployment of the APC. After two unsuccessful attempts at obtaining a mudline core, a 4.82-m core was retrieved at 1225 hr on 6 September, initiating Hole 1207A. With the bit positioned at a depth of 3107 meters below rig floor (mbrf), the mudline core indicated a seafloor depth of 3111.7 mbrf, or 3000.7 meters below sea level (mbsl).
Piston coring advanced to 181.8 mbsf, with recovery averaging 103%. Cores 3H-18H were oriented. The advance-by-recovery technique was used for Cores 19H and 20H. Piston coring was terminated when these two successive cores failed to fully stroke, impeded by chert layers. We then switched to the XCB system.
The first XCB core (Core 21X) was advanced 6.7 m, with good recovery (101%) until encountering a chert layer, which destroyed a soft formation XCB cutting shoe. A second XCB core barrel was deployed, equipped with a hard formation cutting shoe; however, only minimal advancement (0.1 m) was achieved after 1 hr of rotating time, no recovery, and the destruction of the hard formation shoe. An XCB center bit was then deployed to drill through the chert, advancing to a depth of 197.9 mbsf. Three successive cores were then cut (Cores 23X through 25X); the first two recovered 100% and 99%, respectively. Core 25X advanced only 2.0 m in 75 min rotating time with no recovery. The XCB center bit was deployed for a second time to drill through the chert. Another XCB core was attempted; however, Core 26X advanced only 2.0 m, recovering only a few chert fragments (5% recovery). The center bit was deployed for a third time to drill through a chert layer to a depth of 237.3 mbsf. Core 27X advanced 6.4 m recovering a few pieces of chert (1% recovery). The center bit was deployed for a fourth time to clean up the hole and hopefully clear any remnant carbide cutters and debris from the previously used XCB cutting shoe. Prior to recovering the center bit, a 30-barrel (bbl) mud sweep was pumped to aid in flushing any debris from the hole.
We then switched to the MDCB system in attempt to increase recovery. Upon retrieval of MDCB Core 28N (5% recovery), drilling personnel observed that the core barrel had been heavily scoured. In addition to the scouring on the core barrel, the crown of the diamond bit was severely damaged. These observations suggested that either the MDCB bit had been in contact with a primary bit cone or that there was debris in the hole. Because of the doubt about the condition of the primary bit and hole, additional MDCB coring was suspended. A final XCB core was attempted with only 0.2 m advanced in 35 min. With no recovery in this core and concerns about bit and hole conditions, we decided to terminate coring in Hole 1207A at 256.6 mbsf in favor of RCB coring in Hole 1207B. Overall, the XCB was deployed for eight cores, with an average recovery of 71.1%. The drill string was retrieved, clearing the rotary table at 0015 hr on 9 September, ending Hole 1207A.Hole 1207B
After retrieving the drill string, an RCB bottom-hole assembly (BHA) with a center bit assembly was deployed, and Hole 1207B was spudded at 1115 hr on 9 September. Approximately 4.25 hr was spent drilling the hole down to a depth of 157.0 mbsf and recovering the RCB center bit assembly. An RCB core barrel was deployed, and RCB coring advanced to a depth of 622.8 mbsf, taking 49 RCB cores. As anticipated, recovery was poor through an interval of closely spaced chert layers interbedded with ooze. Coring parameters were varied continuously in an attempt to boost recovery. In addition, several core catcher configurations were also used, including flapper core catchers. Beginning with Core 40R, a combination of flapper core catcher and no core liner was used to minimize or eliminate sources of potential jamming. Although somewhat unorthodox for coring in sedimentary strata, recovery did improve over the last 96.3 m. Improved recovery also coincided with increased lithification of the interbedded carbonate layers, so it is difficult to attribute improved recovery solely to coring without liners. Overall, recovery in Hole 1207B averaged 12.9%.
Coring was terminated when the depth objective was attained, and the hole was swept with 40 bbl of drilling mud, followed by a wiper trip to 159.9 mbsf. No overpull or drag was noted. During the return trip to bottom, 40 m of soft fill was noted and circulated out of the hole. Another 40-bbl mud sweep was conducted, and the bit was released. The hole was then displaced with 217 bbl of logging mud, and the pipe was tripped back and positioned at 126.4 mbsf in preparation for logging.
The first logging run was made with the triple combo tool suite (see "Physical Properties, Downhole Measurements, and Core Logging" in "Specialty Syntheses" for details on the individual sensors on the logging tools), which was deployed to a depth of ~621 mbsf, or within 1.5 m of total depth. A second pass with this tool suite failed to pass ~379 mbsf.
The second logging run included two passes with the FMS-sonic tool string. Both passes with this tool suite were also halted at a depth of ~379 mbsf. The third and final logging run was made with the geologic high-resolution magnetic tool (GHMT) tool suite and reached a depth of ~367 mbsf.
After completion of the logging operations, the drill string was retrieved, clearing the seafloor at 1715 hr on 13 September, ending Site 1207. Upon recovery of the beacon and retraction of the thrusters and hydrophones, the ship began the transit to Site 1208.
The 242-nmi transit to Site 1208 was made in 22.5 hr at an average speed of 10.8 kt. Upon arrival at the site coordinates, the ship's crew commenced lowering thrusters and hydrophones, and the ship was switched over to dynamic positioning (DP) mode, initiating operations at Site 1208 at 2315 hr on 14 September.
After deploying an operational beacon, an APC/XCB was assembled and lowered close to the seafloor. After one unsuccessful attempt at obtaining a mudline core, a 4.75-m core was retrieved at 0920 hr on 15 September, initiating Hole 1208A. The recovery of the mudline core indicates a seafloor depth of 3356.8 mbrf, or 3345.7 mbsl.
Piston coring advanced to 185.2 mbsf, with recovery averaging 105.3%. Cores 3H through 20H were oriented. Attempted retrieval of Core 20H resulted in overpull of ~75 kilopounds (kips), which required drilling over with the primary bit to free the core. We continued coring with the XCB, taking 22 cores (Cores 21X through 42X), extending the hole to 392.3 mbsf. Average recovery for the XCB interval was 73.5%. At Core 42X, only 0.25 m of chert was recovered; hence, the hole was terminated. The drill string was retrieved, clearing the rotary table at 1330 hr on 17 on September, ending drilling at Site 1208. After recovery of the beacon and retraction of the thrusters and hydrophones, the ship began the transit to Site 1209.
The 210-nmi transit to Site 1209 was made in 18.4 hr at an average speed of 11.4 kt. Upon arriving on site, the thrusters and hydrophones were lowered, and the ship was switched over to DP mode, initiating Site 1209 at 0755 hr on 18 September.
An APC/XCB BHA was assembled, and the bit was positioned above the seafloor at 2397.0 mbrf. Hole 1209A was officially spudded at 1405 hr on 18 September, recovering 8.21 m, which established a seafloor depth of 2398.3 mbrf, or 2387.2 mbsl. APC coring continued through Core 26H to a depth of 245.7 mbsf. An incomplete stroke on Core 26H led to abandoning APC coring in favor of using the XCB. Recovery with the APC averaged 101.9%. Cores 4H through 26H were oriented. Coring with the XCB continued through Core 198-12089-28X to a depth of 259.6 mbsf. Coring was terminated after 30 min of rotating time on a hard layer, presumably chert, with no advancement. The drill string was tripped up, clearing the seafloor at 1335 hr on 19 September, officially ending drilling at Hole 1209A.Hole 1209B
After offsetting the drill ship 15 m north, the drill string was spaced out with the bit placed 3.0 m higher than the spudding depth of Hole 1209A. Hole 1209B was spudded at 1454 hr on 19 September. Recovery on the first core measured 5.16 m, yielding a seafloor depth of 2398.4 mbrf, or 2387.4 mbsl. APC coring advanced to a depth of 297.6 mbsf through Core 32H with 98.2% recovery. Cores 4H through 32H were oriented, and Adara temperature shoe deployments were made on Cores 6H, 8H, 10H, and 12H. Three of the Adara tool measurements were good quality, but the tool deployed with Core 6H failed. Core 32H was not a complete piston stroke; hence, an XCB center bit was deployed to drill through the presumed chert layer. With 1.5 hr of rotating time and only 0.4 m advancement, we decided to terminate coring in Hole 1209B. While attempting to recover the XCB center bit, we discovered that a sub on the pulling tool had backed off. In addition, the forward wireline was damaged, which required 150 m of line to be removed. While effecting repairs, the aft wireline was used in an attempt to fish the core stub of the pulling tool. However, after two unsuccessful attempts, we had to trip the drill string back to the ship, clearing the rig floor at 0710 hr on 21 September and ending drilling at Hole 1209B.Hole 1209C
The ship was offset 15 m east of Hole 1209A, and Hole 1209C was spudded at 1155 hr on 21 September. The hole was drilled to a depth of 98.0 mbsf, and then APC coring was initiated, continuing to a depth of 251.5 mbsf. At this depth, Core 17H contacted chert after stroking ahead only 1.5 m. An XCB center bit was deployed, drilling ahead to 252.5 mbsf. APC coring resumed (Cores 198-1209C-18H and 19H), progressing to 264.9 mbsf before encountering another chert layer. The XCB center bit was used to drill ahead to a depth of 268.4 mbsf. The next APC core barrel (Core 20H) failed to fully stroke, and shear pins in the overshot connecting the nonmagnetic sinker bars failed. This barrel was recovered after a second wireline run, and core orientation activities were suspended to avoid further complications with the nonmagnetic components. APC coring continued with Cores 21H and 22H to 296.9 mbsf, where a chert layer impeded progress. The XCB center bit was deployed and used to advance the hole through chert to 299.7 mbsf. While attempting to shoot APC Core 198-1209C-23H, the pins failed to shear and the barrel did not penetrate into the formation. Upon recovery, this barrel was inspected and seals were changed to ensure that the barrel was functioning correctly. It was redeployed, but the pins did not shear again. We then switched to the XCB to determine if the problem was related to the APC barrel or the BHA. XCB Core 23X was advanced to 307.5 mbsf; however, despite latching properly, the barrel was recovered empty. A final APC core was attempted, but the APC barrel could not be landed correctly. Convinced that there was a mechanical problem with the bit and/or lockable float valve (LFV), coring was suspended, and the drill string was retrieved. Total recovery for Hole 1209C was 99.2%, with APC recovery averaging 103%.
At 0425 hr on 23 September, the pipe cleared the rotary table on the rig floor, ending operations at Site 1209. A subsequent inspection of the coring assembly showed that the primary bit was heavily damaged on the inner row cutters and the flapper hinge pin in the LFV was broken. Chert was found behind the flapper, likely preventing it from fully opening when the core barrels attempted to pass through. This resulted in overloading the flapper hinge, leading to the failure.
The 29-nmi transit to Site 1210 required just 3.25 hr, at an average speed of 8.9 kt. At 0750 hr on 23 September, the ship was switched over to DP mode, initiating operations at Site 1210.
An APC/XCB BHA was assembled and run to near the seafloor. With the bit positioned at 2581.0 mbrf, a mudline core was recovered at 1358 hr on 23 September 5.90 m of sediment, indicating a seafloor depth of 2584.6 mbrf, or 2573.5 mbsl. APC coring continued to a depth of 233.9 mbsf without incident. Cores 4H through 26H were oriented. An incomplete stroke on Core 26H required deploying a center bit to drill through the ~1-m chert layer. APC Core 27H was deployed, but was also an incomplete stroke, resulting in a decision to terminate the hole at a depth of 242.4 mbsf. Recovery for APC coring in Hole 1210A averaged 103.3%. The drill string was tripped up to the seafloor at the pipe trip and then continued with the bit clearing the seafloor at 1415 hr on 24 September, ending drilling at Hole 1210A.Hole 1210B
The ship was offset 15 m to the north, and the bit was positioned 3 m deeper to provide stratigraphic overlap with Hole 1210A. Hole 1210B was spudded at 1645 hr on 24 September, recovering a 9.2-m APC core, which indicated a seafloor depth of 2584.3 mbrf, or 2573.2 mbsl. APC coring advanced the hole to 267.9 mbsf. Cores 4H through 27H were oriented. An incomplete stroke on Core 29H resulted in deployment of the XCB center bit to drill through a chert layer, followed by the taking of another piston core. This technique of drilling through chert layers when encountered (a total of 11.5 m was drilled) and then redeploying the APC to core between layers continued for 13 cores to a depth of 377.0 mbsf. Because of the diminishing core quality of a few of the last cores and the time being expended to drill through the chert layers, we terminated coring after Core 42H. APC coring recovery averaged 103.0% in Hole 1210B. The drill string was retrieved and the ship secured for transit to Site 1211 by 0000 hr on 27 September.
The 24-nmi transit to Site 1211 was completed without incident in 2.5 hr at an average speed of 9.6 kt. After arriving at the site coordinates, the thrusters and hydrophones were lowered and the ship was switched over to DP mode at 0225 hr on 27 September, initiating operations at Site 1211.
An APC/XCB BHA was assembled and run to near the seafloor. The bit was positioned at 2912.0 mbrf, and Hole 1211A was spudded with the APC at 0915 hr on 27 September. The mudline core contained 2.85 m of sediment, establishing a seafloor depth of 2918.7 mbrf, or 2907.5 mbsl. APC coring continued through Core 11H to 97.8 mbsf. While deploying Core 12H, the barrel dropped off the overshot and fell to the bottom. Two unsuccessful retrieval attempts and wireline trips were made before the core barrel was returned during the third run. The cause of the problem was identified as the safety tube located in the overshot, which had dislodged and resulted in the core barrel becoming jammed in the pipe. We resumed APC coring through Core 18H, which did not achieve full stroke, impeded by a chert layer. Coring was terminated at 158.9 mbsf, with an average recovery of 102.4%. The drill string was tripped up, clearing the seafloor at 0610 hr on 28 September, ending Hole 1211A.Hole 1211B
The ship was offset 15 m to the north and the bit positioned 2 m higher than at Hole 1211A to achieve stratigraphic overlap and position critical intervals away from core breaks. Hole 1211B was spudded at 0700 hr on 28 September, recovering 6.15 m of sediment in Core 1H, indicating a seafloor depth of 2917.4 mbrf, or 2906.2 mbsl. Coring with the APC continued through Core 17H at 151.6 mbsf. An incomplete stroke on Core 17H resulted in deployment of the XCB center bit to drill 1.6 m through a chert layer. The APC was redeployed for Cores 18H and 19H. When Core 19H did not stroke completely, coring operations were suspended at 169.9 mbsf. The drill string was retrieved, with the bit clearing the seafloor at 0625 hr on 29 September, ending drilling at Hole 1211B.
The short transit (28 nmi) to Site 1212 was completed in 3.2 hr at an average speed of 8.8 kt. At 0955 hr on 29 September, the ship was switched over to DP mode, initiating operations at Site 1212.
An APC/XCB BHA was run to 2689.0 mbrf, and Hole 1212A was spudded with the APC at 1540 hr on 29 September. The mudline core recovered 4.96 m of sediment, indicating a seafloor depth of 2693.6 mbrf, or 2682.5 mbsl. APC coring advanced the hole to 101.6 mbsf with 115.1% recovery. Cores 4H through 13H were oriented. After an incomplete stroke on Core 13H, we terminated coring in Hole 1212A. The drill string was recovered to the seafloor at 0255 hr on 30 September, ending operations at Hole 1212A.Hole 1212B
The ship was offset 15 m to the north and the bit was positioned at a depth of 2689.0 mbrf. The same bit position was used in an attempt to keep critical intervals from falling at core breaks. Hole 1212B was spudded at 0345 hr on 30 September, recovering 6.74 m and establishing a seafloor depth of 2691.8 mbrf, or 2680.6 mbsl. Continuous APC coring progressed to 101.2 mbsf. Cores 4H through 12H were oriented. After an incomplete stroke on Core 12H, we adopted a strategy of deploying the XCB center bit to drill through chert layers, followed by redeployment of the APC. Ten deployments of the XCB center bit were required, including three to penetrate a layer at ~136 mbsf, drilling through 14.8 m to obtain 13 additional APC cores. Total APC recovery for Hole 1212B was 93.6%. Because of the increasing frequency of chert layers and diminishing core recovery on the last three APC cores, we terminated coring at Hole 1212B at 207.6 mbsf. The drill string was retrieved, clearing the rig floor at 2330 hr on 1 October, which concluded operations at Site 1212.
The 56-nmi transit to Site 1213 took 6.0 hr at an average speed of 8.6 kt. Upon arriving on site, the thrusters and hydrophones were lowered, and the ship was switched over to DP mode, initiating operations at Site 1213 at 0606 hr on 2 October.
An RCB BHA was assembled and run to 3902.4 mbrf, and Hole 1213A was spudded at 1425 hr on 2 October. The first core recovered 8.43 m of sediment, indicating a seafloor depth of 3894.0 mbrf, or 3882.8 mbsl. RCB coring continued to 198.9 mbsf, with an average recovery for the 21 RCB cores of 21.7%. After deploying core barrel 21R, the sinker bars on the forward core line were being run in the hole when the hole packed off without warning. Before the driller could shut down the circulating pumps, the pressure built up enough to blow the sinker bars back up into the line wiper, unseating this assembly. During this event, the core line failed just above the rope socket, and the sinker bars fell to the bottom of the drill string on top of the previously deployed core barrel. The core line was repaired, and the second set of sinker bars was attached. While the repair was being made to the wireline, Core 21R was cut. The first attempt to engage the rope socket with the overshot failed, and two more wireline runs were made using a core barrelcore catcher assembly. All attempts failed to engage the rope socket, and coring had to be terminated. The drill string was retrieved after a brief loss of circulation and rotation, ending drilling at Hole 1213A at 0300 hr on 4 October.Hole 1213B
The ship was offset 30 m to the north, and Hole 1213B was spudded with the RCB at 1155 hr on 4 October. We drilled ahead to 189.7 mbsf to provide approximately a one core overlap with Hole 1213A. Below 189.7 mbsf, RCB coring advanced the hole to 447.8 mbsf through largely a chert/chalk interval, with an average recovery of 10.5%. From 447.8 to 494.4 mbsf, RCB coring continued through diabase and basalt with good recovery (71.7%) at a slow rate ranging from 1.5 to 2.0 m/hr. Coring operations were terminated when the depth objectives were achieved (~50 m into igneous rock).
Following coring operations, the hole was prepared for logging, including a wiper trip to 90.6 mbsf and circulation of 40 bbl of drilling mud before and after the wiper trip. After releasing the bit, 165 bbl of logging mud was circulated, and the drill string tripped back and positioned at 105.4 mbsf. The first logging run with the triple combo tool string was only able to reach ~431 mbsf. While logging up from that depth several tight spots were noted, including one at ~190 mbsf that required working the tool for ~30 min. The tool had to be turned off during this process, so no logs were obtained between 190 mbsf and the base of the pipe.
We did not deploy the second tool string, the FMS-sonic, because of the questionable hole conditions and the approach of severe tropical storm Krosa into the operational area. With the storm forecast to come within 150 nmi of the location, it was necessary that the drill string be recovered and the drill collars stowed before conditions worsened. After rigging down the logging equipment, the drill string was retrieved, clearing the rig floor at 1900 hr on 9 October, concluding operations at Site 1213. After securing the ship for transit, we departed on a southerly heading to avoid the approaching storm.
After departing Site 1213, we proceeded due south to distance ourselves from the forecast center of severe tropical depression Krosa. During the night, the storm accelerated to 40 kt and moved rapidly to the northeast, passing north of the operational location. After passage of the storm, we reversed course and proceeded back to Site 1211. A total of 116 nmi were covered during the 22.7-hr transit at an average speed of 5.1 kt. At 1742 hr on 10 October, the ship was switched over to DP mode, recommencing operations at Site 1211.
An APC/XCB BHA was assembled and run to 2916.0 mbrf, near the seafloor. Hole 1211C was spudded with the APC at 0545 hr on 11 October. The mudline recovered 7.30 m of sediment, indicating a seafloor depth of 2918.2 mbrf, or 2906.9 mbsl. APC coring continued to a depth of 138.3 mbsf, after which coring was terminated when the objective was achieved. Cores 4H through 15H were oriented, and APC coring averaged 101.9% recovery. The drill string was retrieved clearing the rig floor at 0125 hr on 12 October, concluding operations at Site 1121.
The 22-nmi transit to Site 1214 took 2.5 hr at an average speed of 8.8 kt. At 0420 hr on 12 October, the ship was switched over to DP mode, initiating operations at Site 1214.
An RCB BHA was tripped to the seafloor, and the bit tagged bottom at 3413.0 mbrf, or 3402 mbsl. RCB coring advanced the hole from the seafloor to 235.9 mbsf before coring time expired for the leg. Chert was encountered in drilling the first core and continued throughout the hole, which is reflected in the poor overall recovery (7%). The drill string was retrieved while the usual end-of-leg maintenance and inspections were performed. At 0025 hr on 15 October, the bit cleared the rig floor, ending operations at Site 1214.
After securing the ship for transit, we departed Site 1214 for port at 0030 hr on 15 October. The 2500 nmi transit to Honolulu, Hawaii took 9.85 days at an average speed of 10.75 kt. At 2100 hr on 23 October, the first line was ashore, officially ending Leg 198.
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