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doi:10.2204/iodp.proc.345.111.2014 Holes U1415K, U1415L, U1415M, and U1415N1K.M. Gillis, J.E. Snow, A. Klaus, G. Guerin, N. Abe, N. Akizawa, G. Ceuleneer, M.J. Cheadle, Á. Adrião, K. Faak, T.J. Falloon, S.A. Friedman, M.M. Godard, Y. Harigane, A.J. Horst, T. Hoshide, B. Ildefonse, M.M. Jean, B.E. John, J.H. Koepke, S. Machi, J. Maeda, N.E. Marks, A.M. McCaig, R. Meyer, A. Morris, T. Nozaka, M. Python, A. Saha, and R.P. Wintsch2OperationsIntegrated Ocean Drilling Program (IODP) Holes U1415K–U1415N were sited on a small bench ~200 m shallower than the main target area (see Fig. F8 in the “Expedition 345 summary” chapter [Gillis et al., 2014b]). This was done in order to exploit and test a geologic model whereby the small bench was the top of a headwall scarp and would expose a more competent formation. Hole U1415K attempted to establish a hole with reentry capability penetrating to 35.3 meters below seafloor (mbsf) without any coring except for a single ghost core (345-U1415K-2G) that recovered 4.72 m of surficial mud, gravel, and rocks. Hole U1415L consisted of a jet-in to 4 mbsf; no cores were recovered. Hole U1415M was a failed attempt to establish reentry capability that penetrated to 25.9 mbsf without any coring except for a single ghost core (345-U1415M-2G) containing 5.87 m of surface gravel and mud. Hole U1415N consisted of rotary core barrel (RCB) coring that penetrated to 37.0 mbsf and recovered a total of 1.56 m (4%) of olivine-phyric basalt and gabbro. Hole operations are summarized in Table T1 and outlined below. All times are ship local time (UTC – 7 h). Hole U1415K drilling operationsAfter moving to the small bench site ~400 m northwest of Hole U1415J, we lowered a new bottom-hole assembly (BHA) with a 14¾ inch tri-cone bit to the seafloor and prepared to conduct a short camera survey prior to spudding the hole. However, the camera developed problems during the trip in and had to be recovered. A spare camera was installed before the BHA was redeployed. We conducted an expanding box survey extending out 10 m, and a location to start Hole U1415K was chosen. We verified the seafloor depth (4698.4 meters below rig floor [mbrf]) by tagging the seafloor with the bit, retrieved the camera system, and started Hole U1415K at 2155 h on 16 January 2013. The 14¾ inch hole reached a total depth of 35.3 mbsf by 0400 h on 17 January. A wiper trip was performed, and the bit was pulled back to 6.6 mbsf. A 16 inch free-fall funnel (FFF) without any stinger was deployed to facilitate subsequent reentries. The driller waited 1.25 h before pulling out of the hole to allow time for the FFF to reach the seafloor. The bit was then pulled clear of the seafloor at 2310 h on 17 January. We deployed the camera to observe the FFF cone; the cone was not upright and appeared to be nearly on its side. We felt that the base of the FFF cone was still in or immediately adjacent to the hole. We decided to retrieve the bit, change to an RCB BHA, and attempt to reenter the hole (either through the FFF cone or by a bare rock reentry). The ultimate goal was to deploy a second FFF with 10¾ inch casing to stabilize the upper ~35 m of hole and allow deeper coring. The 14¾ inch bit arrived back on the rig floor at 1110 h on 18 January. A 36 m length of 10¾ inch casing string was assembled and hung off in the moonpool doors using C-plates and casing elevators. The RCB BHA was assembled and lowered through the casing and back to the seafloor. The camera was deployed, and a seafloor survey was conducted from 2333 h on 18 January until 0051 h on 19 January in an attempt to locate Hole U1415K. No discernible hole or cuttings mound could be identified during the survey, and it was decided that the hole was located at the base of the leaning FFF cone. A reentry was attempted by placing the bit at the base of the cone. This was accomplished relatively quickly (~45 min) and the bit was lowered into the seafloor, penetrating 1–2 m before taking weight. We decided to retrieve the camera and to attempt to walk the bit into the hole using light bit weight and low rpm. A perceived reentry into Hole U1415K occurred at 0140 h. Rotation was applied for ~2 h without any luck in penetrating further into the hole. We decided to abandon our attempts to continue operations in Hole U1415K, retrieve the RCB BHA, and start a new hole with a 14¾ inch tri-cone bit. We recovered the core barrel that was in place during the attempt to reenter Hole U1415K (Core 2G; 0 to 0.5 mbsf; 4.72 m recovered) and then retrieved the RCB BHA. The RCB BHA and coring bit was back on the rig floor at 1510 h on 19 January. The seafloor positioning beacon deployed at the start of our Hess Deep operations on 22 December 2012 was nearing the expected lifetime of its batteries, so we commanded it to release at 1503 h and it was back aboard at 1639 h on 19 January. Hole U1415L drilling operationsWe assembled a 14¾ inch tri-cone bit and drilling BHA with three stands of drill collars and lowered it to the seafloor. While the BHA was being lowered, we also deployed the camera system with the same positioning beacon (with new batteries) and the 3.5 kHz pinger. The seafloor positioning beacon was commanded to release at the Hole U1415K coordinates, but it did not immediately fall to the seafloor. After 40 mines of working the camera system up and down, the beacon eventually came loose. At 0515 h on 20 January 2013, we started a visual and 3.5 kHz subbottom seafloor survey by moving the ship 95 m east of Hole U1415K (see Table T1 and Fig. F3 in the “Bench site survey” chapter [Gillis et al., 2014a]). Unfortunately, the 3.5 kHz pinger ceased working before the survey commenced, but we continued with the visual inspection of the seafloor. After we arrived at the target location, we conducted a box survey extending out 10 m to ensure that no large boulders or rubble were in close proximity. We also tagged the seafloor with the bit, revealing an approximate seafloor slope of ~14° to the south. Hole U1415L consisted of a 1 h jet-in test that penetrated to 4 mbsf through sediment and soft rubble. Hole U1415L was spudded at 0845 h on 20 January, and the bit was pulled clear of the seafloor at 0900 h. Hole U1415M drilling operationsAfter recovering the camera system, Hole U1415M was started at 1240 h on 20 January 2013. The initial penetration from seafloor to 5 mbsf went relatively quickly, indicating that the formation was sediment/soft rubble. From ~5 to 9 mbsf, the formation became hard with a significantly slower rate of penetration (ROP; 0.5–1.0 m/h) and smooth torque, as is typical of massive hard rock. The ROP increased again to ~4.0 m/h from 9.0 to 11.0 mbsf. A hard rock reentry system–style FFF with a 26 inch opening was assembled around the drill string in the moonpool. We did not attach a stinger or base plate. The funnel was deployed at 2100 h on 20 January. We then deepened the hole to 19.6 mbsf, where the hole began to pack off. We spent the next 4 h washing and reaming the hole and circulating high-viscosity mud. Eventually the hole was stabilized, and we continued drilling to 25.9 mbsf. At that depth, the bottom of the hole became problematic and the bit was unable to penetrate deeper than 24 mbsf, so we conducted a wiper trip by raising the bit to 8 mbsf and then lowering it to the bottom of the hole. However, the bit still could not penetrate past 24 mbsf. Another high-viscosity mud sweep was pumped. The camera system was lowered to observe the orientation of the FFF cone, but it was obscured by clouds of mud and cuttings coming from the hole. Portions of the rim of the FFF cone, however, were discernible at times buried in cuttings. We pulled the bit out of the hole at 1602 h on 21 January and then waited for 1 h to let cuttings settle for better visibility. The top of the FFF cone was clearly visible in the cuttings mound. The camera was retrieved, the drill string raised to 4660 mbrf, the top drive set back, and we performed a slip and cut of 115 ft of drill line. After the slip and cut, the drill string was recovered, and the bit arrived back on the rig floor at 0330 h on 22 January. Our next step was to assemble 25 m of 10¾ inch casing and hang it off on the moonpool doors. We assembled a RCB BHA with a used C-7 core bit (2 h rotating time in Hole U1415K) and lowered it and the camera system to the seafloor. The top drive was installed and the drill string spaced out for reentry into Hole U1415M. An attempt was made to maneuver the vessel for reentry; however, because of the drill string space out, the bit was positioned nearly at the seafloor, and the driller was unable to raise the bit any higher. This resulted in the bit repeatedly tagging the cuttings mound and the FFF, creating clouds of debris in the water column and completely obscuring the FFF cone. We offset the ship south to slightly deeper water while a 30 ft knobby was laid out and a 20 ft knobby was picked up, giving an additional 3 m of room in the derrick. We moved back over Hole U1415M and reentered the FFF cone at 1645 h on 22 January. The cone appeared to tip/shift as the bit made contact with the inner surface. Circulation along with slow rotation was used in an unsuccessful attempt to find the 14¾ inch hole. No progress was made, and we retrieved the camera system so we could use more rotation. This also was not successful, so we redeployed the camera system and offset the ship in an attempt to drag the funnel out of the way. This also did not work, and the FFF cone appeared to be solidly in place in the cuttings mound, wedged sideways into a large boulder that had been hidden beneath the seafloor sediment. We reentered the FFF cone for one last failed attempt to find the hole below the FFF cone. We then decided to abandon Hole U1415M at 0000 h on 23 January. Ghost Core 2G (assumed to be from 0 to 3.5 mbsf) was recovered at 0230 h on 23 January containing 5.87 m of surface gravel and mud from the multiple failed reentry attempts. Hole U1415N drilling operationsBecause we had the RCB coring system in place, we decided to offset the ship to immediately spud a single-bit unsupported hole. The vessel was moved 15 m west of Hole U1415M. We recovered the camera system, dropped a new core barrel, and started Hole U1415N at 0340 h on 23 January 2013. Core 1R was cut from 0 to 14.9 mbsf and was recovered at 1315 h on 23 January with 0.45 m of roller rocks. After retrieving Core 2R (14.9–18.9 mbsf; 0.15 m recovery), 4.75 h were spent to work the bit down to bottom before beginning to cut the next core at 2330 h on 23 January. Core 3R was a full 9 m advance (18.9–27.9 mbsf; 0.50 m recovered), and then the ROP for Core 4R (29.7–37.0 mbsf; 0.46 m recovered) was very high. After recovering Core 4R, the drillers were never able to work the bit back down to bottom. The hole continued to pack off (high pump pressures) and had high/erratic torque, as seen previously during this expedition associated with the unstable/rapidly drilled intervals. We decided to abandon Hole U1415N at 1700 h on 24 January. The bit cleared the seafloor at 1740 h and was back on the rig floor at 0145 h on 25 January. The core barrel that was in place during the final attempts to clean out the hole back to bottom (Core 5G) was recovered at 0125 h on 25 January. The four cores recovered in Hole U1415N extended from 0 to 37.0 mbsf and recovered 1.56 m (4%) of basement rock. |
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