Expedition 316 began at 0000 h on 20 December 2007 following the arrival by helicopter of the science party and initial science and prespud meetings. See Table T1 for a breakdown of general coring and Table T2 for downhole measurement operations.

Site C0004

Positioning on Site C0004

The Chikyu was on site on 19 December 2007. The final site drilled during Expedition 315 was <5 nmi from Site C0004, requiring minimal transit time prior to the start of operations for Expedition 316. Additionally, since the site was occupied during Expedition 314 for both pilot hole (Hole C0004A) and LWD (Hole C0004B) operations, the acoustic beacons required for dynamic positioning (DP) operations were already in place. The full science party arrived on the Chikyu on the afternoon of 19 December, and Hole C0004C was tagged at 0535 h on 20 December at 2632 m drillers depth below rig floor (DRF) (2603.5 m mud depth below sea level [MSL]).

Hole C0004C

Coring with hydraulic piston coring system (HPCS) commenced at that time and continued to 89.22 m CSF. Three APCT3 measurements were taken at the same time as collection of Cores 316-C0004C-3H, 6H, and 9H. ESCS coring commenced at 2213 h on 20 December in Hole C0004C and continued to 127.22 m CSF. Because of poor ESCS core quality, HPCS coring was resumed at 0447 h on 21 December, and three cores (Cores 316-C0004C-16H through 18H) were collected. The last HPCS core (Core 316-C0004C-18H, 133.18–135 m CSF) also included an APCT3 run. At 0732 h on 21 December, because of failure to achieve full penetration using the HPCS, the bottom-hole assembly (BHA) and drill pipe was run out of the hole, and the hole was killed with high-density mud.

Hole C0004D

Repositioning to Hole C0004D began at 1300 h on 21 December 2007, in parallel with making up the RCB assembly and running pipe to the seafloor.

Hole C0004D was tagged at 0030 h on 22 December at 2659 m DRF (2630.5 m MSL) and washing down to 100 mbsf commenced. RCB coring commenced at 1225 h on 22 December. During RCB coring, several instances of plugged bit throat and plugged bit nozzles were inferred from coring results and pressure readings, and bit deplugging was conducted to clear the problem. Variations in revolutions per minute (rpm) and drilling fluid pressure/flow rate were used to attempt to improve recovery, and cleaning the bit by increasing flow after each core was implemented. Additionally, because of recovery problems, short cores (4.5 m) were taken on every coring run below 238 mbsf CSF (Core 316-C0004D-21R).

Coring was delayed several times because of weather conditions for a total of ~6 h of standby. An additional 3 h of downtime was caused by a fault in the integrated automation system that provides power to one of the DP modules.

TD of 400 m CSF was reached at 2334 h on 25 December 2007 and pulling out of the hole and killing the hole with high-density mud was carried out. Tripping the drill string and breaking down the bit assembly commenced.

Seafloor acoustic navigation beacons were recovered, the vessel was repositioned to Site C0006, and preparations for testing of the underwater television camera (for drilling at depths >3000 m MSL) commenced.

Site C0006

Positioning on Site C0006

The Chikyu arrived on site on 27 December 2007 and began testing the underwater television system. Electronics problems forced the cancellation of that test, and positioning over Hole C0006C began at 1200 h on 28 December. The HPCS BHA was rigged up and run to the seafloor, and Hole C0006C was tagged at 1915 h on 28 December.

Hole C0006C

Because of the lack of a camera system or ROV observation, collection of a mudline core had to be accomplished by trial and error. One HPCS core (316-C0006C-1H) was taken at 1915 h on 28 December (33°1.6458就, 136°47.6282媧, seafloor: 3880.5 m MSL), resulting in full recovery, requiring repositioning and reshooting in order to collect the mudline.

Hole C0006D

One HPCS core (316-C0006D-1H) was taken in Hole C0006D (33°1.6431就, 136°47.6282媧, seafloor: 3877.5 m MSL) at 2245 h on 28 December, resulting in full recovery with no mudline. The vessel was again repositioned.

Hole C0006E

Core 316-C0006E-1H was collected at 0000 h on 29 December, resulting in 5.19 m of recovery, including the mudline in Hole C0006E (33°1.6444就, 136°47.6282媧, seafloor: 3875.81 m MSL). HPCS coring continued from 5.19 m CSF. Fourteen HPCS cores were collected with total recovery of 79.33 m and average recovery of 96.47%. APCT3 measurements were taken every two cores starting at the third core (Cores 316-C0006E-3H, 5H, 8H, 10H, 12H, and 14H). Refusal (no penetration) occurred at Core 316-C0006E-14H and HPCS coring was halted.

ESCS coring commenced at that time, resulting in the recovery of 35 cores (Cores 316-C0006E-15X through 49X), with total recovery of 330.11 m at a rate of 74.61%. Six DVTP measurements were made following Cores 316-C0006E-23X, 26X, 28X, 29X, 35X, and 42X. Three of the DVTP runs were unsuccessful because of battery failure, thermistor failure, and failure to penetrate formation following Cores 316-C0006E-23X, 28X, and 42X, respectively.

After observing a drastically reduced rate of penetration during the cutting of Core 316-C0006E-49X, we determined that refusal had been reached, and Hole C0006E was completed at 409.4 m CSF at 1930 h on 6 January 2008. Hole C0006E was killed with high-density mud, the BHA was pulled out of the hole, the drill pipe was tripped, and the HPCS/ESCS BHA was racked back. During this time, there was an electrical problem with the wireline winch motor, requiring that Core 316-C0006E-49X be recovered after recovering the BHA on the rig floor. This occurred at 0330 h on 7 January.

Hole C0006F

The Chikyu then relocated to Hole C0006F and the RCB BHA was run to the seafloor. Hole C0006F was tagged at 1730 h on 7 January 2008 (33°1.6242就, 136°47.6282媧, water depth: 3875.5 m MSL). We conducted jet-in to 50 mbsf and washed down to 395 mbsf. RCB coring operations began, and 23 RCB cores were recovered (Cores 316-C0006F-1R through 23R) from 395 to 603 m CSF with total recovery of 56.48 m at a rate of 28.26%.

Following the collection of Core 316-C0006F-23R, it was determined based on pump rates, pressure, torque, and rpm that the hole condition had deteriorated to such a degree as to make further coring too risky. As pumping rate increased, the torque increased to the point of stalling the top drive (0 rpm); as pump rates were dropped, torque simultaneously dropped and rpm increased. The only way to get the bit back to the bottom of the hole was to run in the hole with <10 spm, which was considered unacceptably risky for either drilling or coring. The decision was made at ~0900 h on 13 January to abandon Site C0006 and relocate to proposed Site NT1-03A as an alternative means to achieve the science goals for the NT1-03 family of sites. Hole C0006F was killed with high-density mud, the BHA was pulled out of the hole, the drill pipe was tripped, and the RCB BHA was racked back at 2100 h on 13 January 2008.

Site C0007

Positioning on Site C0007

The Chikyu relocated to Site C0007 (proposed site NT1-03A) on 13 January 2008. The ROV was launched in order to deploy seafloor transponders below the Kuroshio Current (running at 2.0–2.5 kt at the time). The ROV was recovered, and the HPCS/ESCS BHA was rigged up and readied while the vessel was calibrating for DP navigation and positioning over the site. The BHA was run to the seafloor, and Hole C0007A was tagged at 2230 h on 14 January (33°01.23258就, 136°47.94852媧, depth: 4081 m MSL).

Holes C0007A, C0007B, and C0007C

The first core (316-C0007A-1H) was a successful mudline core that penetrated to 3.14 m CSF with 100% recovery. Because of operational and curatorial issues, the following core was labeled 316-C0007B-1H. This core was taken from 3.14 m CSF and it recovered 10.06 m of sediment with recovery of 105.9%. Because the bit came out of the borehole during recovery of this core and no set standard for hole naming dealt with this issue, the following core was labeled 316-C0007C-1H. This core was taken from 12.64 m CSF and recovered 7.95 m of sediment with 100% recovery. All further coring was conducted continuously in Hole C0007C from this point. Cores 316-C0007C-2H to 4H, 5X to 15X, and 16H to 17H were collected with total recovery of 51.26 m at a recovery rate of 33.82% in interlayered silt, sand, and coarse pebbles. Jamming of the piston core barrel in the BHA following the shooting of Core 316-C0007C-18H at ~1500 h on 17 January required several hours of operational down time and resulted in the abandonment of Hole C0007C. The inner core barrel was lost during the attempts to recover Core 316-C0007C-18H.

Pulling out of the hole and recovery of BHA took place between 1200 and 2400 h on 18 January 2008. During this operation, request for permission to drill an alternate hole 185 m northwest of Hole C0007C was made, and permission was granted via Center for Deep Earth Exploration (CDEX) and Environmental Protection and Safety Panel (EPSP). Because of consideration of both science target and time requirements, the decision was made to run the RCB coring system.

Hole C0007D

After permission to core in Hole C0007D was granted, the vessel was repositioned 185 m northwest of Hole C0007C. The RCB BHA was made up and run in the hole, and Hole C0007D was spudded on 19 January 2008 at 0330 h (33°1.3167就, 136°47.8872媧, water depth: 4049 m MSL). Jet-in and wash down to 18 mbsf and drilling to 175 m CSF were conducted down to the first coring point, which was reached at 1330 h on 19 January. Coring operations in Hole C0007D resulted in the collection of Cores 316-C0007D-1R through 35R (175–493.5 m CSF) with total recovery of 87.9 m at a rate of 27.61%. A combination of

  1. Determination that the primary scientific target had been reached,

  2. Deteriorating hole conditions,

  3. Repeated zero-recovery cores, and

  4. Recovery of small amounts of loose fine sand in the final core (316-C0007D-35R)

resulted in the determination (made at 1500 h on 24 January) to pull out of the hole, kill the hole with dense mud, and recover the BHA to the surface. The BHA was recovered at 0130 h on 25 January. All sea bottom acoustic navigation beacons were recovered from Sites C0006 and C0007, and the vessel moved to contingency Site NT2-10 (Site C0008) on 26 January 2008.

Site C0008

Positioning on Site C0008

The Chikyu relocated to Site C0008 (proposed Site NT2-10A) on 26 January 2008. The 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 HPCS/ESCS BHA was rigged up and readied while the vessel was calibrating for DP navigation and positioning over the site. The BHA was run to the seafloor, the ROV was used to check position, and Hole C0008A was tagged at 0130 h on 27 January (33°12.8229就, 136°43.5997媧, depth: 2751 m MSL). The ROV was then recovered.

Hole C0008A

The first core (316-C0008A-1H) was a successful mudline core that penetrated to 6.93 m CSF with 100% recovery. Cores 316-C0008A-2H to 28H, 29X to 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 CDEX and EPSP to shift 215 m southeast and core Hole C0008B. This request was approved.

Pulling out of the hole and the killing of 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 2008.

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就, 136°43.6727媧, water depth: 2797 m MSL). The BHA was run in the hole to the seafloor, and the seafloor was tagged at 2828 m 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 2008. Coring operations in Hole C0008C resulted in the collection of Cores 316-C0008C-1H to 21H, 22X, and 23X* to 25X* (the * for Cores 316-C0008C-23X* to 25X* denote the test of the newly designed "punch" system, which is a modification of the ESCS system) (0–176.2 m CSF) with total recovery of 189.7 m of core at a rate of 108%.

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 to 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 CT imagery and visual core description have confirmed that the drillers' advance data are generally reliable as a 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 to 21H, ESCS coring 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, TD was declared, and the newly developed ESCS punch coring system was deployed for testing. Three cores (316-C0008C-23X* to 25X*) were collected with this system with excellent results. A postexpedition report of the results of this coring system test, including X-ray CT scan, visual core description, and chemistry data (for contamination analysis) will be produced to supplement this report.

Following collection of the final ESCS punch core at 2035 h on 2 February 2008, 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 2008. 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 2008.