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OPERATIONS (continued)

Site 1221
Transit to Site 1221 (PAT-10B)

The 144 nmi transit to proposed Site PAT-10B was accomplished in 17.0 hr at an average speed of 8.5 kt. The vessel did not proceed directly to PAT-10B because of strong winds (27–31 kt) and wind-driven seas (combined sea and swell >6 m). To avoid heavy pounding, the vessel pursued a course at a reduced speed that exposed the starboard quarter to the elements for most of the transit. The second half of the dogleg track required the vessel to steer almost directly into the seas. The vessel motion was moderate with maximum pitch and heave values measured at 6°. The modified track added 33 nmi to the transit.

At 0230 hr on 5 December, the vessel slowed to 4 kt, and a short 3.5-kHz survey was made from east to west across the site. Upon conclusion of the 42-min survey, the vessel came about and returned to site as the thrusters were lowered and the DPS was activated. The GPS receiver interface to the DPS was employed to situate the vessel over the coordinates of the new location by 0315 hr. A beacon was deployed at 0425 hr. The corrected PDR depth was 5197.4 mbrf.

Hole 1221A

After the drill string was deployed to a depth of 5157 mbrf, the driller lowered the bit until the heave compensator appeared to activate, suggesting contact with a firm seafloor. Hole 1221A was spudded with the APC at 1425 hr on 5 December. The seafloor depth that was indicated by the recovery of the initial core was 5186.6 mbrf. This depth was 10.8 m shallower than the PDR depth.

Piston coring in Hole 1221A advanced to the APC refusal depth of 110.6 mbsf with an average recovery of 102.8% (Table T1). Core orientation began with Core 3H. During piston coring, the heave of the vessel frequently exceeded 5 m and compromised the quality and recovery of the core. It was also difficult to land the APC assembly in the BHA without parting the overshot shear pin. Because of the excessive vessel motion combined with the deep water, it was also difficult to ascertain whether the APC actually fired or was hydraulically pushed into the sediment after the shear pin parted during landing.

XCB operations began with Core 13X, and the interval from 110.5 to 115.6 mbsf was cored in 15 min. Most of the rotating time was consumed penetrating the first 2 m of this interval. Upon retrieval of the core, the core barrel became stuck in the BHA. Efforts to recover the core barrel by alternately applying overpull and upward impacts with the wireline jars were given up after 40 min. The overshot shear pin was intentionally parted, and the wireline recovered. The drill string was pulled out of the hole and tripped to a depth of 3572 m, where another attempt was made to recover the stuck core barrel. This effort was unsuccessful. The BHA reached the rig floor at 0215 hr on 7 December, and the core barrel was removed. Only 0.14 m of chert fragments were salvaged from the core catcher.

A fresh bit seal and lockable float valve were installed, and the BHA was redeployed. Concurrently, the vessel was offset 20 m south of Hole 1221A in preparation for spudding Hole 1221B.

Hole 1221B

Hole 1221B operations were designed to retrieve two cores near the mudline in an interval that contained the E/O boundary. Hole 1221B was spudded with the APC at 1400 hr on 7 December and advanced to 14.2 mbsf (two cores) with 102.3% recovery. The estimated seafloor depth that was calculated from the recovery of the first core was 5186.4 mbrf. The bit cleared the seafloor at 1505 hr on 7 December.

Hole 1221C

Hole 1221C was spudded with the APC at 1640 hr on 7 December, and the estimated seafloor depth that was calculated from the recovery of the first core was 5185.7 mbrf. This hole was spot cored (five cores) with the APC to a depth of 102.5 mbsf to recover material missed in Hole 1221A. The cored interval was 45.4 m with an average recovery of 96.3% (Table T1). The last three cores were oriented.

XCB coring (Cores 6X through 12X) deepened the hole from 102.5 mbsf to a basement contact at 157.3 mbsf. Abundant chert stringers adversely affected recovery. The average recovery over the 45.4 m cored interval was 43.7%. Upon completion of Hole 1221C, the bit was pulled to the seafloor at 1530 hr on 8 December. The vessel was then offset 20 m south in order to spud the last hole.

Hole 1221D

Hole 1221D plans were to drill ahead (in the interest of time) to 132.0 mbsf and then core to basement in the hope of recovery a second P/E boundary at this site. Hole 1221D was spudded with the XCB and a center bit at 1715 hr. After drilling ahead to 132 mbsf, the center bit was recovered and an XCB barrel was deployed. XCB coring advanced from 132.0 to 156.0 mbsf (Table T1). The interval from 144.8 to 151.0 mbsf was drilled ahead with a center bit to reduce penetration time. Basement contact was confirmed by the recovery of the basalt in the last core. The cored interval was 17.8 mbsf with 2.28% recovery. The drilled interval was 138.2 m.

After recovering the drill string, the vessel departed for the last site of the leg at 2100 hr on 9 December.

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