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doi:10.2204/iodp.proc.340.110.2013

Operations

Transit to Site U1400

After pulling thrusters at Site U1399, the vessel switched to cruise mode and began the 17 nmi move to Site U1400. The vessel stabilized over Hole U1400A at 1625 h on 4 April 2012. All times reported in this volume are given in ship local time, which was Universal Time Coordinated (UTC) – 4 h. The position reference was a combination of GPS signals and one of two acoustic beacons. The first positioning beacon was deployed at 1650 h on 4 April ~250 m from the primary site coordinates along the seismic line running to the northeast in anticipation of a move in that direction for Hole U1400B. The second beacon was deployed 250 m to the southeast along the crossing seismic line at 1410 h on 5 April. At the end of operations at Site U1400, both beacons were sent acoustic commands to release. The first beacon was retrieved at 0500 h on 13 April, but the second beacon apparently did not release and was not recovered. The vessel began a dynamic positioning move to the next site at 0630 h on 13 April.

Site U1400

Site U1400 consists of three holes (Table T1). The original plan called for two holes to be cored to ~510 mbsf. The first hole was terminated at 51.3 mbsf because of unstable hole conditions. After offsetting the vessel 750 m in a direction 160° from Hole U1400A, the second hole, U1400B, was piston cored to 212.5 mbsf. After the core barrel became stuck, the drill string had to be tripped to the surface and the core barrel freed from the upper landing sub. A shear pin had become wedged between the removable landing seat and the core barrel assembly. After offsetting 20 m further along the 160° seismic line, the third hole of the site began with two failed attempts at spudding Hole U1400C. The first failed attempt broke off the lower section of a nonmagnetic core barrel with the cutting shoe. After the core barrel was retrieved and the breakage was discovered, a steel core barrel was installed and run to bottom. The second attempt was even less successful. This time the steel barrel bent in such a fashion that it was unable to be pulled back through the bottom-hole assembly (BHA). The drill string was again pulled to surface, and the bent core barrel was cut away and the BHA was reassembled for another attempt. This time, the hole was advanced to 15 mbsf without coring before running in with the wireline to begin coring. Coring with the advanced piston corer (APC) continued through Core 340-U1400C-22H at 191.1 mbsf. APC coring was terminated when the formation became so stiff that the core liner folded up inside the core barrel and had to be pumped out with a 10,000 psi pump. The extended core barrel (XCB) was then deployed and used to successfully complete coring operations to 436 mbsf. A logging program was planned for Hole U1400C; however, hole problems, resulting in a severed BHA, forced the abandonment of the hole. The vessel was then moved 7 nmi to Site U1401 using the dynamic positioning system. The APC was deployed 58 times. The cored interval with the APC was 439.9 m with a recovery of 447.34 m of core (102%). The XCB coring system was deployed 27 times. The cored interval with the XCB system was 244.9 m with a recovery of 124.14 m of core (51%). Overall recovery for Site U1400 was 83%. Total time spent on Site U1400 was 207.00 h.

Hole U1400A

The vessel arrived at Site U1400 and was in position at 1630 h on 4 April 2012. The pipe trip to bottom was uneventful. The top drive was then picked up, and the bit was spaced out to spud Hole U1400A. The initial corrected precision depth recorder (PDR) reading was 2755.4 meters below rig floor (mbrf). The bit was set at 2750 mbrf, and the hole was spudded at 0030 h on 5 April. The seafloor depth was calculated from the length of the first core (3.5 m) to be 2756.0 mbrf (2744.4 mbsl). The APC was advanced to Core 340-U1400A-9H at 51.3 mbsf. Apart from the first core, all piston strokes were partial strokes. Eight partial strokes were recorded. The hole was advanced by recovery. At 51.3 mbsf, the drill string became stuck without circulation and rotation. Keeping within the overpull limits, the string was worked and was eventually freed from the formation. The decision was made to abandon Hole U1400A and offset the vessel as far as beacon deployment would allow. Nine piston cores were taken over the 51.3 m interval with a total recovery of 51.8 m of core (101%). The drill string was pulled clear of the seafloor at 1445 h on 5 April, ending Hole U1400A. Total time spent on Hole U1400A was 22.25 h.

Hole U1400B

Because of challenging hole conditions, we decided to move Hole U1400B as far away from Hole U1400A as possible. A second beacon was deployed 250 m on a heading of 160° from Hole U1400A. We decided to maximize beacon-offset capabilities, and the final distance between Holes U1400A and U1400B was 750 m at a heading of 160°. Hole U1400B was spudded at 1605 h on 5 April 2012. Seafloor depth was calculated from the length of the first core (6.8 m) to be 2754.7 mbrf (2743.0 mbsl). APC coring advanced to Core 340-U1400B-28H at 213 mbsf. Twelve partial APC strokes were recorded—most near the top section of the hole. The hole was advanced by recovery. After shooting Core 28H, we were unable to pull the core barrel to surface. Two wireline runs were made to attempt to free the core barrel. After the second attempt, we abandoned Hole U1400B and retrieved the BHA to free the stuck core barrel. Twenty-eight APC cores were taken from 0 to 212.5, recovering 215.19 m of core (101%). The drill string was pulled back to the rig floor, and the core barrel was extricated from the BHA. A shear pin had become lodged between the landing seat and the core barrel assembly. The landing seat had to be removed from the landing sub and cut off with an acetylene torch. After removing all the coring equipment from the BHA, Hole U1400B ended at 1030 h on 7 April. The total time spent on Hole U1400B was 43.75 h.

Hole U1400C

We decided a third hole was necessary to fully achieve the science objectives of Site U1400. The vessel was offset another 20 m at 160° from Hole U1400B. After tripping to bottom, the bit was set at 2751 mbrf. A mudline core was attempted, but the bottom section of the nonmagnetic core barrel broke off. Because the uppermost 20–25 mbsf of Hole U1400B had been hard sand, we switched to steel core barrels to make another attempt at a mudline core. Despite successfully coring the mudline on Hole U1400B, the second mudline core attempt in Hole U1400C resulted in a bent core barrel. Because we could not pull the core barrel out of the BHA, the entire drill string had to be tripped back to surface. After the BHA reached the rig floor, it was necessary to remove the bit and bit sub and to cut the steel barrel so that it could be pulled back through the bottom of the BHA.

The BHA was reassembled and tripped back to just above the seafloor. We then picked up the top drive to spud Hole U1400C at 0950 h on 8 April 2012. The seafloor depth of Hole U1400C was assumed to be the same as at Hole U1400B: 2754.7 mbrf (2743.0 mbsl). To penetrate through the problematic seafloor surface, we drilled without coring from 0 to 15 mbsf. APC coring started with Core 340-U1400C-2H at 15 mbsf. Advanced piston corer temperature tool (APCT-3) measurements were taken on Cores 5H, 8H, and 12H. Core orientation started on Core 5H, and nonmagnetic core barrels were deployed starting with Core 2H. Because of poor hole conditions, the core orientation tool and the nonmagnetic core barrels were not used after Core 10H. Ten out of twenty-two piston cores were partial strokes, and the hole was therefore advanced by recovery. After reaching APC refusal on Core 22H, the XCB was deployed and XCB coring continued to 436.0 mbsf. Coring was terminated after Core 49X. Twenty-one piston cores were taken over a 176.1 m interval with a total recovery of 180.35 m of core (102%). Twenty-seven XCB cores were taken over a 244.9 m interval. A total of 124.14 m of core were recovered (51%), and the hole was terminated at 436.0 mbsf. Overall core recovery for Hole U1400C was 72%.

At the conclusion of coring, Hole U1400C was conditioned with a 25 bbl high-viscosity mud sweep and displaced with 201 bbl of 10.5 ppg mud. While attempting to pull out of the hole to logging depth, the drill string started to pick up overpull between 400 and 320 mbsf. By the time the bit had reached 318 mbsf, upward motion of the drill string was almost impossible. For almost 17 h, we attempted to backream the BHA upward. At 0045 h on 12 April, we effectively lost rotation but not circulation. After trying to free the stuck pipe, the Schlumberger Wireline Engineer was called out to rig up the severing charges. At 0945 h, the first severing charge was used to sever the drill string at the top of the tapered drill collar. After the charge was detonated, we were still unable to establish rotation or upward movement of the drill string. A second severing charge was prepared and run in the hole with the Schlumberger wireline to just below the seafloor. At 1845 h we attempted to detonate and were still unable to free the pipe. On removal of the tool from the pipe, the charge was found to be intact. No detonation had occurred. Several hours were spent troubleshooting the tool at the surface and, after finding a faulty connection, the tool was reassembled and run back to bottom. Shortly after midnight on 13 April, the drill string was severed and rotation and upward movement were reestablished. The entire BHA, including two stands of 5½ inch drill pipe and 15 joints of 5 inch drill pipe, was abandoned in Hole U1400C. The remainder of the pipe was tripped to the surface, and the severed joint of drill pipe cleared the rotary table at 0730 h on 13 April, ending Hole U1400C and Site U1400. Total time spent on Hole U1400C was 141.0 h.

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