OPERATIONS SYNOPSIS (continued)Transit to Site 1172
An APC/XCB BHA was made up and run to near the depth indicated by the PDR (2637.4 mbrf). Hole 1172A was spudded with the APC at 1630 hr on 23 April. The seafloor depth was calculated from the recovery of the first core as 2633.2 mbrf, or 2621.9 mbsl. Piston coring advanced to 224.8 mbsf where Core 24H could not be retrieved from the sediment with 80,000 lb of overpull, requiring a drillover to release the core barrel. Piston cores were oriented starting with Core 3H and continued through Core 24H. Because of the ~4 m heave, heat-flow measurements were deferred until the next hole in anticipation of improving conditions. The average recovery in Hole 1172A was 102%; however, the excessive heave resulted in many of the shallower piston cores being dominated by flow-in, in particular Cores 5H through 10H and Cores 15H and 19H.
The XCB system was used to deepen the hole to 522.6 mbsf, with 85% average recovery. XCB coring was terminated because of the slow ROP (7 m/hr) and poor recovery (8%) of the last two cores. The overall average recovery for Hole 1172A was 93%.
The ship offset 20 m north of Hole 1172A, and Hole 1172B was spudded with the APC at 1050 hr on 26 April. To obtain an appropriate stratigraphic overlap with the Hole 1172A, the bit was positioned 3 m shallower at spud-in. The estimated seafloor depth of 2633.6 mbrf (2622 mbsl) was calculated from the recovery of the first core. Piston coring advanced to 202.4 mbsf (average recovery = 102%), where 75,000 lb was required to free the core barrel (Core 22H) from the sediment. Cores were oriented starting with Core 3H and three Adara heat-flow measurements were obtained at 40.9 (Core 5H), 59.9 (Core 7H), and 88.4 mbsf (Core 10H).
The vessel was offset 20 m north of Hole 1172B, and Hole 1172C was spudded with the APC at 1130 hr on 27 April. To maintain stratigraphic overlap with the previous two holes, the bit was positioned 6 m deeper than at Hole 1172B for spud-in. The seafloor depth was estimated to be 2633.0 mbrf (2621.7 mbsl).
Piston coring advanced to 171.0 mbsf, where APC refusal was encountered. The core barrel for Core 18H required drilling over when 75,000 lb of overpull could not free the core barrel from the sediment. The average recovery was 101% and cores were oriented beginning with Core 3H.
Following the recovery of the last core from Hole 1172C, the drill string was recovered and a RCB BHA was assembled. Before spudding the fourth hole of the site, the vessel was offset 20 m north of Hole 1172C. Hole 1172D was spudded with the RCB at 1630 hr on 28 April and drilled ahead with a center bit in place to 343.6 mbsf with an average penetration rate of 57 m/hr. The interval from 343.6 to 372.4 mbsf was rotary cored (Cores 1R through 3R) with an average recovery of 68%; however, the first core did not recovery any material, probably because of excessive hydraulics. The average recovery for the second and third core was 103%.
Following the recovery of the third core barrel, a wash barrel was dropped and was drilled
ahead from 372.4 to 497.4 mbsf with an average ROP of 27 m/hr. Continuous rotary coring was
initiated at 1500 hr on 29 April in Hole 1172D and extended until 2215 hr on 1 May, when
operational coring time expired. The final depth of Hole 1172D was 766.5 mbsf, with an average
recovery of 80%. Following a 12-hr wiper trip and hole preparation, which included the release of
the bit at the bottom of the hole, the end of the pipe was placed at 160 mbsf for logging. In the first
logging run, the triple combo tool string reached a depth of 762 mbsf and the pipe was pulled to
150 mbsf while logging up to accomplish overlap with the shipboard splice. However, excessive
heave caused the wireline heave compensator (WHC) to stroke out repeatedly (six times) during
the logging pass. Despite the heave, the data obtained were of good quality, noting some borehole
rugosity but no extensive areas of washout. Sea conditions continued to deteriorate as logging
progressed (heave of ~6 m), and the WHC was abandoned during the GHMT-sonic run after
stroking out six times within 10 min of activation. The GHMT-sonic also reached target depth, and
good sonic and magnetic data were obtained up to the base of the pipe. The FMS was then rigged
up in hopes that heave would subside to within the effective range of the WHC. The conditions did
not improve within the allotted time, and the FMS run was abandoned.
After the logging equipment was rigged down, the drill string was recovered and the BHA dismantled in preparation for the voyage to Sydney, Australia. Both beacons were successfully recalled and recovered. After the drilling equipment was secured, the vessel departed for Sydney at 2100 hr on 3 May.
Operations Synopsis-Transit from Site 1172 to Sydney | Table of Contents