OPERATIONS

Site 1149 (see Fig. F1), the second drill site of Leg 185, was located east of the Bonin Trench, ~1040 nmi northwest of Site 801 and 300 nmi south-southeast of Japan's Boso Peninsula. Site 1149 was tentatively located on an east-west reference profile (Conrad 2005, Line 39) but a site survey was required to determine the best location for the site. The JOIDES Resolution approached the Conrad reference seismic line from the southeast, slowed to 6 kt, and streamed seismic gear ~12 nmi before crossing the reference line (see "Seismic Stratigraphy"). The survey continued until a location with favorable basement characteristics was found (see "Seismic Stratigraphy"). After a triangular seismic survey pattern was completed, the profiling gear was recovered and the ship returned to the Global Positioning System coordinates of the site. A positioning beacon was launched at 1500 hr on 23 May. The transit from Site 801 to Site 1149, including the 7-hr seismic survey was accomplished in 4.05 days, at an average speed of 11.4 kt.

Hole 1149A was spudded with a "mudline" core at 0200 hr on 24 May. The core barrel contained 4.2 m of core, which was interpreted as the sediment/water interface, fixing the seafloor depth at 5829.3 m from driller's datum. Continuous advanced piston cores (APC) cores were taken with >100% recovery to 164 mbsf, when an abrupt increase in the stiffness of the sediment resulted in an incomplete APC stroke on Core 18H (Tables T1, T2).

APC cores were oriented in azimuth beginning with Core 4H. Temperature recording shoes were also run on Cores 4H, 6H, and 8H, and the Lamont-Doherty Earth Observatory (LDEO) drill string acceleration tool (DSA) was run on Cores 2H and 10H. Tracer experiments, to determine potential core contamination for microbiology studies, were carried out by fixing a bag of fluorescent microbeads into the core catchers of Cores 3H, 6H, 9H, and 12H and by pumping perfluorocarbon tracer into the drilling fluid on Cores 6H, 9H, and 11H (see "Methods for Quantifying Potential Microbial Contamination during Deep Ocean Coring," Smith et al., 2000).

The APC system was replaced by the extended core barrel (XCB), which cored 24.3 m (i.e., Cores 19X-21X) with an average recovery of 21% in stiff clay with interbedded porcelanite (Tables T1, T2). After 2.2 m penetration when cutting Core 22X, the XCB encountered a hard streak that halted penetration.

Further progress with the XCB system was highly unlikely because of the inability to penetrate the chert or porcelanite horizon. The situation had been anticipated, and the plan called for using the motor-driven core barrel (MDCB) system to attempt to core the chert layers. Although conditions were not favorable for the MDCB because of the presence of steel and tungsten carbide junk in the hole from the XCB shoe, it was nevertheless decided to attempt a MDCB core as a "last chance" to core the chert with an alternative system before tripping for the rotary core barrel (RCB) system. When the MDCB was recovered, the corehead was noted to have all the diamonds worn off the crown and to show signs of junk damage. The core barrel also showed grooving from junk. When the core catcher was removed, only 30 cm of chert and clay debris from the hole was found in the split and collapsed plastic liner. There was no indication that any core had been cut or that any new hole had been made by the corer.

Coring attempts in Hole 1149A were abandoned at that point, because there appeared to be no remaining alternatives to tripping for an RCB coring assembly. The MDCB system was rigged down, the top drive was racked, and the pipe trip began. Hole 1149A ended at 0725 hr on 26 May, when the bit arrived on deck.

Hole 1149B was 30 m northwest of Hole 1149A. No mudline core was planned, so the water depth was assumed to be the same as at Hole 1149A. The new hole was spudded at 2000 hr on 26 May and drilled ahead to core point at 161 mbsf in 3.5 hr. The "wash barrel," which contained ~3.5 m of cored material and drilling rubble, was retrieved.

Continuous RCB coring then commenced (Tables T1, T2). At ~190 mbsf, interbedded chert caused core recovery to drop sharply, and drilling conditions deteriorated. As much as 4 m of fill accumulated between cores, causing high torque, and the bit hammered on the chert ledges when the motion compensator was locked out. Only chert "rollers" were recovered in the core barrels, with traces of soft brown clay (probably representing unrecovered material). At ~261 mbsf, the ROP slowed considerably, and hole conditions improved markedly with depth. Recovered cores contained calcareous interbedded material with interbedded chert. Only a small amount of fill was present by the time basaltic basement was encountered at 410 mbsf while drilling Core 29R. Average core recovery in the cherty interval from 190 to 410 mbsf was only ~6%. Although it had penetrated >200 m of cherty sediments, the core bit had accrued only 21 rotating hr when it reached basement. The condition of the cutting structure was unknown, but the plan for the hole called for running the bit to destruction. Penetration of the highly altered basalt continued at >2 m/hr through Core 32R with a recovery rate of ~20%. At the time the core barrel for Core 33R had been dropped, however, without warning the drill string became partially stuck. Indications were that chert or basalt fragments were wedging the bottom-hole assembly (BHA), either at the top or at the bit. The string could be moved up or down in irregular intervals before it would torque up and bind vertically. A mud pill was circulated while the string was worked free, and the inner core barrel landed during the process. Eventually three "knobby" joints were removed from the string as the bit was pulled upward. When the bit passed the approximate depth of the sediment/basalt contact, the resistance ceased. A joint of drill pipe then was added to the drill string, but circulation was nearly plugged off when the pump was restarted. Because the inner barrel had held the float valve open while the pipe was being freed, cuttings had been able to flow back into the jets of the bit. It was then necessary to make a wireline trip to retrieve the inner barrel and regain normal circulation. A total of 4 hr was spent on the consecutive hole problems.

Although additional basement penetration was planned, the highest remaining scientific priority of Leg 185 was a good set of logs of the sediment section at Site 1149. The onset of difficulties with the hole was a reminder that coring to bit destruction could result in loss of the hole and preclude the opportunity for downhole logging. Because there was ~35 m of "rathole" for the logs to record the base of the sediment section, plans were changed to terminate coring in Hole 1149B and proceed with logging operations. When logging operations were completed at 0900 hr on 2 June, the drill string was tripped, and Hole 1149B was completed when the mechanical bit release top connector arrived on deck at 2000 hr on 2 June.

Hole 1149C was spudded at 0820 hr on 3 June and was drilled without coring to 283.6 mbsf. Two "wash" core barrels were pulled at 237 and 283.6 mbsf as a precaution because of the accumulation of loose chert fragments in Hole 1149B. After the wash core barrel at 283.6 mbsf was retrieved, preparations began for retrieving four spot cores requested by the science party. A short trip back to 188 mbsf to replace the "knobby" drilling joints was slowed by the torquing and sticking of the drill string. Although a considerable amount of soft fill was found in the hole, the string was free as it returned to total depth, and coring began.

Mud was circulated while the first core (3R) was being cut (Tables T1, T2), but torquing and sticking tendencies returned during the wireline trip to retrieve it. It was necessary to "work" the pipe for 3 hr and to pull back to 245 mbsf before Core 4R could be cut to 303 mbsf and retrieved. Almost immediately after the barrel for Core 5R was dropped, the drill string began to torque and stick again. While the string was being worked up the hole with restricted circulation, the inner barrel landed, opening the float valve. When the next joint of pipe was removed from the string, backflow plugged the pipe. Rotation and vertical movement were possible, so the bit was pulled to 235 mbsf before a wireline trip was made to retrieve the inner barrel and re-establish normal circulation.

The cause of the hole cleaning problems was believed to be the interval of dark brown clay with lower porosity and shear strength between 150 and 180 mbsf that had stopped logging tools in Hole 1149B (see "Shear Strength" and "Abstract"). Therefore, a "wiper trip" was made to 130 mbsf to ream out any clay restriction. No resistance was noted in either direction until the bit returned to ~235 mbsf, where a solid ledge was encountered. Several other ledges were noted as the top drive was used to clean the hole back to total depth, and ~4 m of fill was found on bottom. After the inner barrel was pumped into place, Cores 5R and 6R were cut and retrieved without incident, but the combined recovery was only 78 cm of chert and chalk core pieces.

With a wash barrel in place, the hole was drilled ahead to 388 mbsf, the next requested core point to sample the sediment/basement interface. Because of the history of hole problems, the knobby joints were laid out and a precautionary short trip was made back up to 130 mbsf, above the unstable clay zone. No resistance registered on the weight indicator, but the top drive was picked up to ream through the clay zone and to clean the hole back to total depth with circulation and rotation. The hole was clean, with only 3 m of soft fill, but an additional 50 bbl of mud was pumped to sweep out any debris. Core 8R then was cut with all parameters normal, and 1 m of chert/chalk core was recovered.

Before the inner barrel was dropped to cut Core 9R, the pipe began to torque and stick. Additional working of the pipe resulted in more sticking. For 4 hr, various circulation rates, amounts of overpull, and stuck-pipe techniques were tried. Success was slowly achieved by "drilling up" or backreaming with tension just short of the amount that would produce stalling of the top drive. Progress up the hole of ~1 m/hr was achieved with that technique. Circulation had been held to moderate rates to avoid excessive hole erosion, but a desperation move involving use of both mud pumps at 90 strokes per minute (spm) produced enough improvement in the backreaming progress that a joint of pipe could be removed. The pipe was set on the elevators with 80 kips overpull to break out the joint. When it was reconnected to the top drive and lifted off the elevators, it was free. The string then was pulled to ~60 m of total depth with no resistance and run back to bottom, where ~4 m of fill was found. The cause of the sticking was believed to be gravel-sized chert fragments that had settled around the BHA.

Hard basement drilling was encountered 2 m into drilling Core 9R (at 401 mbsf), allowing a higher coring circulation rate to be used for the remainder of the core. The pattern of stuck pipe following core retrieval continued after both Cores 9R and 10R, with average core recovery of only ~15% (Tables T1, T2). It became apparent that chronic sticking problems were leading to excessively slow recovery of material and that continuing to operate in Hole 1149C eventually would result in loss of the hole and the BHA. While Core 11R was being cut, a decision to relocate had been made, and the trip out of the hole began as soon as the core had been recovered. The bit arrived on deck at 0215 hr on 7 June. The vessel got under way at 0300 hr, ending Hole 1149C.

The 3.5-kHz echosounder was used to refine the desired offset position on a nearby basement high. A new beacon was dropped at 0515 hr on 7 June ~3.1 nmi east-southeast of Hole 1149A, and Hole 1149D was spudded at 1530 the same day. Hole 1149D was drilled to basement without coring and without pulling the wash barrel.

Drilling parameters indicated chert stringers beginning at ~155 mbsf. No hole problems were encountered except for an incident of sticking during the rereaming of the interval to 263 mbsf. At 272 mbsf, a wash core barrel was pulled to initiate continuous coring in anticipation of encountering basement (Tables T1, T2). Three consecutive cores then produced a combined total of 1.2 m of chert fragments. Basement was encountered at 307 mbsf while coring Core 5R. During coring of Hole 1149D, some problems were encountered with sticking and torquing of the pipe, which caused some delays. However, coring continued until through Core 19R, when the time allotted for coring had expired. Hole 1149D was deepened to a total depth of 6319.4 m (440.4 mbsf). Departure from Site 1149 was at 1615 hr on 13 June.