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doi:10.2204/iodp.proc.311.104.2006

Operations

Hole U1326A

After transiting in dynamic positioning (DP) mode from Hole U1325A and downloading the data from the LWD/MWD tools, the LWD/MWD bottom-hole assembly (BHA) was tripped to the seafloor. Hole U1326A (proposed Site CAS-03C; Collett et al., 2005) was spudded at 1445 h on 22 September 2005 at an estimated seafloor depth of 1828.1 meters below sea level (mbsl; 1839.0 meters below rig floor [mbrf]). To avoid blowing out the top of the hole, we changed the spud-in parameters to 100 gallons per minute (gpm) circulation with 10–15 rpm rotation. At 10 meters below seafloor (mbsf), top drive rotation was increased to 40 rpm. At 29 mbsf, parameters were increased to maintain a 50 m/h rate of penetration (ROP) with a minimum circulation of 270 gpm. The LWD/MWD safety protocol was followed without incident and required no corrective action (see "Downhole logging" in the "Methods" chapter). The hole was drilled to a total depth (TD) of 300 mbsf. After displacing the hole with 10.5 ppg sepiolite mud, the drill string was pulled clear of the seafloor at 0600 h on 23 September, ending operations in Hole U1326A. The pipe was pulled to a safe distance above the seafloor in preparation for the DP transit to Site U1327.

Hole U1326B

After completing Hole U1325D, we pulled the drill pipe back to ~1600 mbrf and transited ~2.8 nmi in DP mode to Site U1326. We spudded Hole U1326B at 1205 h on 23 October 2005, but the core contained only 1.55 m of sediment, which was not ideal for the planned shallow microbiological and geochemical studies. Based on recovery in Core 1H, the estimated seafloor depth is 1828.4 mbsl (1840.0 mbrf).

Hole U1326C

The bit was set at 1834 mbrf (2 m lower than in Hole U1326B), and without offsetting, Hole U1326C was spudded at 1245 h on 23 October 2005. The first core recovered 3.93 m of sediment, indicating a seafloor depth of 1828.0 mbsl (1839.6 mbrf). On the fourth advanced piston corer (APC) run (~30 mbsf), we unexpectedly hit APC refusal and switched to XCB coring. XCB coring advanced the hole to a depth of 82.7 mbsf, which was followed by three consecutive pressure core deployments within a high electrical resistivity zone identified on LWD/MWD downhole logs. The first pressure core system deployed was the FPC, which recovered a partial core (15 cm) at less than full pressure (see "Pressure coring"). The second system to be deployed was the PCS, which recovered a partial core under pressure. The third pressure core system deployed was the HRC, which was damaged at the bottom of the hole because of excessive ship heave (>4 m) and packing of formation sands around the outer barrel. The HRC cutting shoe and a spacer section came unscrewed and were left in the hole, which resulted in the termination of Hole U1326C at a TD of 86.7 mbsf.

Hole U1326D

After tripping the BHA back to the seafloor, the ship was moved 30 m southwest (15 m southwest of Hole U1326A) (Fig. F6). Hole U1326D was spudded at 1130 h on 24 October 2005 and drilled to a depth of 78.8 mbsf in preparation for continued coring operations. Because of problems associated with heave, which worsened throughout the day, we decided to suspend all pressure coring operations for the remainder of the expedition to avoid the possibility of damaging the tools and losing the last hole with little operational time remaining. XCB coring deepened the hole to 271.4 mbsf with an average recovery of 63.3% (Table T1). With forecasts of deteriorating weather conditions on the morning of 26 October, it was decided to stop coring at a depth of 271.4 mbsf and deepen the hole to a TD of 300 mbsf by drilling, which allowed us to gain time and complete logging, rigging down, and securing the rig floor before the expected arrival of the forecasted storm. Despite the marginal conditions, we did attempt three deployments with the Davis-Villinger Temperature Probe (DVTP) at 252.2, 271.4, and 300 mbsf, which yielded marginally useful data.

After completing coring operations, the hole was prepared for logging with a wiper trip followed by displacing the hole with weighted mud. We decided to conduct a single downhole logging run with a nonstandard IODP tool string, which included the Scintillation Gamma Ray Tool (SGT), Phasor Dual Induction Tool (DIT), and the Dipole Sonic Imager (DSI). At 2315 h on 26 October, the tool string was deployed in the hole and lowered to 298.4 mbsf. Two successful passes were made from this depth and the tool was back on deck at 0345 h. The drill string was pulled clear of the seafloor at 0530 h on 27 October, ending operations in Hole U1326D.

Transit to Victoria, British Columbia

After tripping the drill string to the rig floor, recovering two beacons, and securing the ship for transit, we departed under deteriorating sea states (with 40–45 kt sustained winds gusting to >50 kt) at 1320 h on 27 October 2005. We made the short 157 nmi transit to Victoria in 22 h at an average speed of 7.1 kt. Expedition 311 officially concluded with the first line ashore at 1225 h on 28 October.

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