OPERATIONS SYNOPSIS (continued)Transit to Site 1170
After settling on location, an APC/XCB BHA was made up and run close to the estimated depth of 2716.4 mbrf calculated from the PDR. Hole 1170A was spudded with the APC at 2030 hr on 31 March. The seafloor depth of 2715.5 mbrf, or 2704.7 mbsl, was calculated from the amount of recovery of the first core. Piston coring advanced to 163.2 mbsf with an average recovery of 98%. Piston cores were oriented starting with Core 3H, continuing through to Core 18H. Because of the 11-ft vessel heave, no heat flow measurements were attempted.
The XCB system was used to deepen Hole 1170A from 163.2 to 464 mbsf with an average recovery of 73%. The rate of penetration (ROP) slowed to nearly 1 m/hr in a hard nannofossil limestone interval, which clearly signaled that this was the refusal depth of the XCB system. As XCB coring was concluding, the growing swell was now pushing the vessel heave over 6 m, which was outside the operating limits of the JOIDES Resolution. After the bit was pulled clear of the seafloor at 0545 hr on 3 April, coring operations were suspended for 4.3 hr while we waited for the swell to abate.
At 1000 hr on 2 April, the vessel was offset in dynamic positioning mode 20 m north of Hole 1170A. To obtain a stratigraphic overlap with the initial hole of the site, the bit was positioned 3 m shallower before starting Hole 1170B, which was spudded with the APC at 1145 hr on 2 April. The estimated seafloor depth was calculated to be 2715.7 mbrf, or 2704.7 mbsl, from the recovery of the first core. Piston coring advanced to 175.8 mbsf with an average recovery of 102%. Cores were oriented starting with Core 3H. Because of a growing swell that pushed the vessel heave over 4.5 m, the Tensor orientation tool was removed subsequent to the recovery of Core 14H to prevent damage to critical hardware. The bit was pulled clear of the seafloor at 0800 hr on 4 April and the vessel was offset 20 m north of Hole 1170B.
At 0945 hr, Hole 1170C was spudded with the APC. The stratigraphic overlap required lowering the bit 6 m from the spud-in depth of Hole 1170B. The seafloor depth estimated from recovery of the first core was 2714.4 mbrf, or 2703.3 mbsl. Piston coring advanced without incident to the depth objective of 180.1 mbsf, with an average recovery of 99.7%. Cores were oriented beginning with Core 3H, and one Adara deployment was made on Core 19H.
The ship was offset 20 m north of Hole 1170C, and a rotary core barrel (RCB) BHA was made up and deployed to 2668 mbrf. Hole 1170D was spudded with the RCB at 2000 hr on 4 April and drilled ahead with a center bit to 425 mbsf at an average rate of 55 m/hr. At 0745 hr on 6 April, RCB coring was initiated in Hole 1170D and advanced with increasing recovery (mean = 81%) to the depth objective of 779.8 mbsf. The recovery below 529 mbsf to the bottom of the hole averaged nearly 100%. There were no stability problems encountered while drilling and coring in this hole.
At the conclusion of coring, the contrast in hole conditions between the upper and lower
section of the hole was discussed among the relevant personnel. Logging difficulties were expected
at a depth of 474-529 mbsf, where indurated limestone horizons were present above a zone of
partially consolidated glauconitic sandstones and siltstones. It was agreed that it would be difficult
for a logging tool to easily make the transition from the upper 425 m of the hole into the bottom
355 mbsf because of the likelihood that there would be washed out sections overlying a relatively
narrow 10- to 12-in-diameter hole into the limestone capped lower section. Hence, it was decided
to log the hole in two stages, with the end of the pipe placed initially at 529 mbsf to log the base of
the hole then raised to ~200 mbsf to log the top of the hole.
Following a routine wiper trip and displacement of the hole with a 200-bbl sepiolite solution,
the bit was dropped and the end of the drill string was placed at 529 mbsf. At 1730 hr on April 9,
the first logging tool was deployed. Three tool-string runs were made with the pipe at 529 mbsf;
the triple combo, the GHMT-sonic, and the FMS. The borehole was smooth with a relatively
uniform diameter (~11-12 inches), and high-quality data were obtained. All the logs came within
6 m of the bottom of Hole 1170D.
At 1700 hr on April 10, the first part of the logging program was completed and the drill crew
made preparations to reposition the end of the pipe at 200 mbsf. After pulling the drill string back
from 529 mbsf while encountering increasing drag, the drill string stuck firmly at 468 mbsf. The
top drive was picked up and the drill string was subjected to overpulls as large as 200 kilopounds
(Kips) in an attempt to free the pipe. After attempts to move the pipe from 1715 to 2130 hr met
with no progress, it was decided to sever the drill string just above the BHA.
An explosive charge was made up and deployed on the logging line to 3033 mbrf, which corresponded to the position of the first 5-in drill pipe joint above the BHA. At 0415 hr on April 10, the charge was detonated and severed the pipe. The drill string was then recovered, as were the two beacons. The vessel departed for Site 1171 at noon on 11 April 2000.
Operations Synopsis-Site 1171 | Table of Contents