We then switched to XCB coring, which provided excellent core recovery, albeit with slow and ever decreasing rates of penetration (ROP). The high clay content of the sediments was apparently adhering to the cutting structure of the bit, reducing the penetrating ability and making it difficult to advance into the formation. The weight on bit, pump pressure, and rotary speed were varied in an attempt to increase the ROP, with negative results. The average rate of penetration on 19 January had decreased to 4.7 m/hr.
We considered the possibility of dropping a free fall funnel (FFF) so that we could trip the pipe to change the rotary cone bit to one more suited for this formation (e.g., 10 1/8-in PDC fixed cutter bit). However, after reviewing the condition of the microfossil preservation, the slow ROP, and the time/risk to round trip for a PDC XCB bit, we decided to terminate Hole 1050A at 319.9 mbsf. We decided to increase the penetration at Hole 1050B to ~240 mbsf and to not drill a third hole at this site as originally planned. The time saved would be used to achieve primary scientific objectives at another site. We pulled the drill string up to 100 mbsf, displaced the hole with 40 bbl of 10.5 ppg mud. Hole 1050A ended at 0015 hr on 20 January when the drill string was pulled above the seafloor.
We offset the vessel 10 m to the east and spent 1-hr to accomplish the routine task of cutting and slipping 115 ft of drilling line. After the driller verified that the seafloor depth was the same as at Hole 1050A (2311.0 mbrf; based on an observed reduction in drill string weight), we spudded Hole 1050B with the XCB at 0200 hr on 20 January. XCB Core 1X was cut from 0 to 7.0 mbsf. Then we took APC Cores 2H through 9H to 83.0 mbsf. We wanted to switch to XCB at this depth because this was just above the depth where we had to drill over the stuck APC core barrel at Hole 1050A. XCB coring continued to a total depth of 240.0 mbsf. After the last core was retrieved (1325 hr, 21 January) we started pulling the drill string out of the hole. As soon as the drill bit was a safe distance above the seafloor, the beacon was recovered, and the vessel was slowly offset toward the next site while the drill pipe was retrieved.
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