OPERATIONS SYNOPSIS (continued)Transit, Hobart to Site 1168
After the hydrophones and thrusters were extended and the vessel settled on location, the corrected precision depth recorder (PDR) depth referenced to the dual elevator stool (DES) was obtained and indicated 2479.4 m below the rig floor (mbrf). Hole 1168A was spudded with the advanced hydraulic piston corer (APC) at 1715 hr on 17 March. The seafloor depth was calculated from the recovery of the first core and determined to be 2474.2 mbrf, or 2463 mbsl.
Piston coring advanced to 111.8 mbsf. Piston cores were oriented starting with Core
3H, and heat-flow measurements were obtained at 45.3, 64.3, 92.8, and 111.8 mbsf. The average
recovery of the piston-cored region was 103%. There were two minor incidents involving the
wireline while piston coring in Hole 1168A. When vessel heave caused the overshot shear pin to
part, it required an extra wireline trip to recover Core 5H. While attempting to recover Core 10H,
the wireline came apart at the rope socket. This required one trip in with a wireline fishing tool,
which succeeded in recovering the Tensor tool carrying case and core barrel on the first attempt.
Subsequent to a full stroke, the core barrel for Core 12H required drilling over to free the hardware
from the sediments.
Consequently, we switched to the extended core barrel (XCB) system and the hole was
deepened from 111.8 mbsf to refusal depth (883.5 mbsf), which was the depth objective
of the previously planned rotary hole. The average recovery of the XCB portion was 94%. During
XCB coring, coring operations almost had to be stopped on 19 and 20 March when a complex
long period (13 s) swell generated vessel-heave values of 6 m or more. Because of the long period
of the swell, the passive heave compensator was able to handle the vertical excursions and coring
Throughout most of the XCB coring, the average rate of penetration (ROP) was 31 m/hr. The
ROP slowed dramatically below 848 mbsf. From a depth of 848 mbsf to the bottom of the hole at
884 mbsf, the ROP gradually decreased from 6.5 to a lethargic 2.9 m/hr. Because of the
diminishing returns of continued coring, the hole was terminated and preparations were made for
Hole 1168A was logged with the triple combination (triple combo) tool and the geological
high-sensitivity magnetic (GHMT)-sonic tool strings. During the triple-combo tool string run, the
bit was positioned at 108 mbsf and was raised to 101 mbsf during logging. The first pass covered
877 to 101 mbsf, and a repeat run was made from 301 to 201 mbsf. The data indicated that hole
conditions were poor with an uneven and rugged borehole wall. The borehole width commonly
exceeded 19 in, but in places was as narrow as 4 in. Because there were many narrow spots, a
wiper trip was conducted before the deployment of the second logging tool suite.
During the GHMT-sonic run, the bit was placed at 110 mbsf and was pulled up to 100 mbsf. The main pass of the logging tool covered 730 to 102 mbsf, and a repeat run was conducted from 376 to 245 mbsf. It was not possible to get the tool string past a bridge at 730 mbsf, and, as a consequence, the bottom 150 m of the hole could not be logged. In view of the scientific importance of the base of the hole and the poor hole conditions, it was decided that a second attempt to log the lower portion of the hole with the GHMT-sonic was more important than running the Formation MicroScanner (FMS). Accordingly, another wiper trip was made and the bit was set at 666 mbsf. Unfortunately, it was still not possible to get any deeper and logging was discontinued. After the logging equipment was demobilized, the bit was pulled back and cleared the seafloor at 2215 hr on 24 March, ending Hole 1168A.
The vessel was offset 20 m north of Hole 1168A, and Hole 1168B was spudded with the APC at 0130 hr on 25 March. Based on recovery of the mudline core, the seafloor depth was estimated to be 2463.7 mbsl, or within 0.2 m of the calculated depth of Hole 1168A. Piston coring advanced to 108.4 mbsf with an average recovery of 98%. Cores were oriented beginning with Core 3H. After Core 12H was obtained, the bit was pulled clear of the seafloor at 1215 hr on 25 March, ending Hole 1168B.
After the vessel was offset 20 m north of Hole 1168B, Hole 1168C was spudded with the APC at 1330 hr. Piston coring was initiated and advanced to 114.0 mbsf with an average recovery of 96%. Cores were oriented starting with Core 3H. The hole was deepened with the XCB to 290.5 mbsf when the available operational time expired. The average recovery for Hole 1168C was 85%. The average recovery for the entire site was 93%, representing a total of 1192 m of core.
Following the retrieval of the last core barrel, the drill string was recovered and the bottom-hole assembly (BHA) was dismantled in anticipation of the transit to Hobart to transfer two personnel and some light cargo. Concurrent with the recovery of the drill string, the beacon was successfully retrieved. After the hydrophones and thrusters were retracted and drilling equipment secured, the vessel was under way at 2100 hr on 26 March.
Operations Synopsis-Site 1169 | Table of Contents