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doi:10.2204/iodp.proc.314315316.136.2009

Operations

Positioning on Site C0008

The D/V Chikyu relocated to Site C0008 (proposed Site NT2-10A) on 26 January 2008. The remotely operated vehicle (ROV) was launched in order to deploy seafloor transponders below the Kuroshio Current (running at 2.0–2.5 kt at the time), and the hydraulic piston coring system (HPCS)/​extended shoe coring system (ESCS) bottom-hole assembly (BHA) was rigged up and readied while the vessel was calibrating for dynamic positioning system navigation and positioning over the site. The BHA was run to the seafloor, the ROV launched, position checked, and Hole C0008A (33°12.8229′N, 136°43.5997′E; water depth = 2751 m mud depth below sea level [MSL]) was tagged at 0130 h on 27 January.

Hole C0008A

The first core (316-C0008A-1H) was a successful mudline core that penetrated to 6.93 m core depth below seafloor (CSF) with 100% recovery. Cores 316-C0008A-2H through 28H, 29X through 42X, and 43H were collected with a total recovery of 271.2 m at a rate of 75.8% in interlayered mud, silt, and sand. Near the bottom of the hole, starting from Core 316-C0008A-34X, the drilling rate of penetration dramatically increased and recovery dropped to near 0%. After several repeated attempts to improve recovery by changing drilling and pumping parameters and a final attempt to successfully HPCS core the inferred unconsolidated sand at the bottom of Hole C0008A, the hole was abandoned. During this time, a request was made to the Center for Deep Earth Exploration (CDEX) and the Environmental Protection and Safety Panel (EPSP) to shift 215 m southeast and core Hole C0008B. This request was approved.

Pulling out and killing Hole C0008A took place between 0900 and 1815 h on 31 January 2008. The BHA was pulled out of the hole to 200 m above the seafloor and the rig was shifted to Hole C0008B between 1815 and 1930 h on 31 January. Table T1 shows the cores collected, meters recovered, and recovery percentage for Hole C0008A.

Holes C0008B and C0008C

The vessel was positioned above Hole C0008B (Hole C0008C is at the same location) at 2130 h on 31 January 2008 (33°12.7313′N, 136°43.6727′E; water depth = 2797 m MSL). The BHA was run in to the seafloor, and the seafloor was tagged at 2828 m drillers depth below rig floor (DRF). The first HPCS core (316-C0008B-1H) was shot at 2135 h but did not successfully capture the mudline. Hole C0008B was abandoned and Core 316-C0008C-1H (5.51 m of recovery, successful mudline) was shot in Hole C0008C at 0005 h on 1 February. Coring operations in Hole C0008C resulted in the collection of Cores 316-C0008C-1H through 21H, 22X, and 23X* through 25X* (0.0–176.2 m CSF) with total recovery of 189.7 m of core at a rate of 108% (Table T2).

During this operation, observations from previous Hole C0008A were used to adjust the determination of the advance of the piston core based on the drillers information and the observation of large amounts of exotic material sucked into the bottoms of HPCS cores deeper in the section (Cores 316-C0008C-7H through 21H). This phenomenon resulted in initial recovery rates that in some cases exceeded 200% (i.e., with an advance of 5 m and recovery of 10.2 m). However, X-ray computed tomography (CT) imagery and visual core description (VCD) have confirmed that the drillers advance data is a generally reliable measure of actual HPCS penetration. The intervals of each core occupied by exotic material pulled into the core bottom have been recorded, and use of the J-CORES depth processing and “D-Tunes” functionality will allow postprocessing of the coring recovery and data registration with little difficulty.

After HPCS refusal was determined based on low penetration, severe core liner deformation, and severe core expansion problems in Cores 316-C0008C-17H through 21H, the ESCS coring system was employed. However, the first ESCS core (316-C0008C-22X) was not of sufficient quality to justify further coring (preservation of the unconsolidated portions of the lithologic section and gas hydrates were particularly poor). The scientific goals of this site were deemed to have been accomplished, total depth (TD) was declared, and the newly developed ESCS “punch” coring system was deployed for testing. Three cores (316-C0008C-23X* through 25X*) were collected using this system with excellent results. A postexpedition report of the results of this coring system test is available through the CDEX website.

Following collection of the final ESCS punch core at 2035 h on 2 February, Hole C0008C was abandoned, the hole killed with dense mud, and the BHA pulled out of the hole to the surface. The BHA was recovered at 0530 h on 3 February. The BHA was rigged down, excess drill pipe laid out, and the guidehorn rigged down. Transponders were recovered on the morning of 4 February and the Chikyu began transit to Shingu, Wakayama Prefecture, Japan. Expedition 316 ended with first line ashore at 0930 h on 5 February.

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