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doi:10.2204/iodp.proc.339.106.2013

Operations

The 55 nmi transit from Site U1387 to Site U1388 (proposed Site GC-04D) was accomplished in ~5 h at an average speed of 11 kt. At 0745 h, 18 December 2011, the vessel was positioned on the new location.

The operations plan for Site U1388 called for advanced piston corer (APC)/extended core barrel (XCB)/rotary core barrel (RCB) coring in three holes and conducting downhole logging operations in Holes U1388B and U1388C. The first two holes were to be cored with the APC to refusal and then with the XCB to ~350 mbsf. The third hole was to be cored with the RCB from 350 to 1300 meters below seafloor (mbsf). Ultimately, three holes were drilled (Table T1), but the coring objectives were not achieved because of hardware failure caused by a difficult sandy formation. In Hole U1388A, APC Core 339-U1388A-1H advanced only 3.4 m (107.6% recovery) because a thick sand layer at the sediment surface prevented the core from penetrating any further. Hole U1388B was cored using the XCB to 225.7 mbsf with 47.2% recovery. Hole U1388C was drilled without coring to 205 mbsf and then cored using the RCB to 229 mbsf. Overall recovery at Site U1388 was 120.54 m (47.6%): 3.64 m (107.6% recovery) with the APC, 106.54 m (47.2% recovery) with the XCB, and 10.36 m (43.17% recovery) with the RCB. The site was terminated earlier than planned because of formation instability.

Hole U1388A

An APC/XCB bottom-hole assembly (BHA) with a 9⅞ inch polycrystalline diamond bit, lockable float valve, seal bore drill collar, and nonmagnetic drill collar was assembled and deployed. Following a 1.3 h underwater camera survey of the seafloor over a 30 m grid pattern, Hole U1388A was spudded with the APC at 1500 h. The calculated seafloor depth from the recovery of the first core was 675.1 meters below rig floor (mbrf; 663.6 mbsl), which was 0.7 m deeper than the corrected depth given by the precision depth recorder. The second attempt at a piston core appeared to be an incomplete stroke. The core winch operator spent ~3 h attempting to recover the stuck core barrel. The Kinley cutter was deployed to cut the coring line above the sinker bars when it was not possible to jar off the core barrel. The only course of action remaining was to recover the drill string and extricate the core barrel on the surface. The drill string was retrieved by 2400 h, 18 December. The reason why the core barrel couldn’t be recovered was obvious; most of the BHA was full of sand that apparently “U-tubed” when the driller advanced into the formation to position the bit for the next piston core (2H).

Hole U1388B

The BHA was cleared of sand and deployed to 669.9 mbrf. The vessel offset 20 m east of the previous hole. The underwater camera displayed what appeared to be man-made debris on the seafloor and precipitated another 10 m offset to the east. The strategy for the second hole of the site was to core an XCB hole to depth and use the information acquired from that pilot hole to assess if and where the APC could be deployed in subsequent holes. The driller tagged seafloor at 674.4 mbrf (662.9 mbsl) at 0645 h. The camera system was recovered, and the top drive was picked up. Hole U1388B was spudded with the XCB at 0835 h, 19 December, and advanced to 225.7 mbsf by 0745 h, 20 December. While coring this interval, extensive cutting shoe damage suggested that the distribution of fluid flow between the cutting shoe and the polycrystalline diamond bit was not optimum for this formation. Rather than continue coring in this fashion and considering that the objectives of this site were deeper in the formation, XCB coring was terminated at 225.7 mbsf. The average recovery for this hole was 47.2%. The driller pumped a 5 bbl slug of heavy mud prior to making a connection for Cores 1X through 12X (110.9 mbsf) to avoid a recurrence of the sand backflowing into the BHA. A 20 bbl sepiolite flush was made at 216.1 mbsf. The drill string was pulled clear of the seafloor at 0925 h and on deck at 1155 h, 20 December.

Hole U1388C

A four-stand RCB BHA with a new CC-4 Rock Bit International bit and mechanical bit release was made up and run in to 668.0 mbrf. After a short underwater camera survey confirmed that the location of the hole was clear of debris, the driller tagged the seafloor at 674.0 mbrf (662.5 mbsl). The underwater camera was recovered, the top drive was picked up, and Hole U1388C was spudded with the RCB at 1915 h, 20 December. The hole was washed to 205.0 mbsf, where rotary coring was initiated. At 119.8 and 167.6 mbsf, 20 bbl Hi-Vis mud sweeps were circulated. Rotary coring advanced from 205.0 to 229.0 mbsf, where the drill sting became firmly stuck in the formation with no rotation and limited circulation. The drill string required 1.5 h to free the pipe utilizing overpulls of as much as 120,000 lb and adjusting rotary current limits as high as 1050 A. Once the pipe came free, more mud (both Hi-Vis and 10.5 ppg) was pumped to clear out some of the sand that had collapsed on the BHA. The drill string was pulled free of the seafloor at 1330 h and on deck by 1725 h, 21 December. The formation was considered too unstable to safely core, and the decision was made to move to the next planned site of the expedition. Total time at Site U1388 was 3.2 days, which was 10 days less than the allotted time. The vessel departed for Site U1389 at 1730 h, 21 December.

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