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Operational strategy

To achieve the overall scientific objectives of the NanTroSEIZE project, drilling has been planned for six primary sites and two contingency sites, targeting the incoming plate section, the frontal thrust of the accretionary wedge, the megasplay fault system at different depths, and the plate interface in the seismogenic zone. See Tobin and Kinoshita (2006a) for details of the four-stage plan to complete these objectives. NanTroSEIZE Stage 1 plans called for drilling and sampling in riserless mode at six of the sites during Expeditions 314–316, including

  • The incoming sediments of Shikoku Basin (two sites),
  • The frontal thrust system at the toe of the accretionary wedge,
  • The midwedge megasplay fault system, and
  • The uppermost 1000–1400 m at the two sites planned for later deep penetrations of the seismogenic zone faults by riser drilling.

Primary drill sites

During Expedition 314 the planned order of drilling at the six sites was determined by scientific priority and operational difficulty. The original plan was to drill at proposed Site NT1-01A to the top of basement at ~600 meters below seafloor (mbsf). Next we would continue to proposed Site NT2-03B to drill at ~1000 mbsf into the splay fault thrust sheet, drill to 1400 mbsf at the future deep riser hole at proposed Site NT3-01B, drill to 1000 mbsf to pass through the splay fault reflector and into the footwall within accretionary prism at proposed Site NT2-01B, drill to 950 mbsf at the frontal thrust site at proposed Site NT1-03A, and finish with 1200 m of drilling to reach the top of the oceanic crust at proposed Site NT1-07A (Kinoshita et al., 2007) (Figs. F7, F8).

Because of concerns regarding geological hazards, operations to drill two pilot holes at proposed Sites NT2-03B and NT1-03A were added to the operational plan prior to drilling with LWD-MWD tools at the six primary sites. In addition, geotechnical coring of the top section of the future Stage 2 riser hole at proposed Site NT2-03A was added to the plan, owing to urgency to obtain strength data for riser wellhead deployment.

During the course of drilling, delays occurred during many of the early operations; therefore, the original plan was modified to drill LWD sites right after the first pilot Hole C0001A. Holes C0001B, C0001C, and C0001D were subsequently drilled at Site C0001, and Holes C0002A and C0003A followed. After the loss of the LWD-MWD bottom-hole assembly (BHA) while drilling at Site C0003 (see “Site C0003”), the drilling plan was modified again because it was deemed operationally necessary to drill pilot holes with either ROV monitoring of the drill site at the seafloor and/or MWD-only measurements to ensure hole safety. Three pilot holes without any LWD-MWD tools (Holes C0004A, C0005A, and C0005B) and one LWD hole (C0004B) were drilled in the vicinity of proposed Site NT2-01. After this, the ship moved to the proposed Site NT1-03B area and drilled one pilot hole with MWD tools (Hole C0006A) and one LWD hole (C0006B) to complete the expedition.

During the planning stage, 22.5 contingency days were factored into the expedition for typhoons, mechanical downtime, and casing operations at proposed Sites NT2-03B and NT2-01D. However, actual days used for each site and operations during the expedition were generally quite different from the original plan. Failure and repair of the dynamic positioning system, fishing for the stuck BHA and cementing operations in Hole C0003A, and numerous other operational delays such as ROV maintenance; Kuroshio current–related operations; and checking and maintaining rig-floor, guidehorn, and derrick mechanical systems at intervals were responsible for the differences.

Logging strategy

Difficulties in attempted wireline logging during previous legs at Nankai and other accretionary prisms and, conversely, the successful experience of ODP Leg 196 (Mikada, Becker, Moore, Klaus, et al., 2002) at Nankai and other previous LWD expeditions (Shipley, Ogawa, Blum, et al., 1995; Kimura, Silver, Blum, et al., 1997; Moore, Klaus, et al., 1998; Mikada, Becker, Moore, Klaus, et al., 2002; Austin, Christie-Blick, Malone, et al., 1998; O’Brien, Cooper, Richter, et al., 2001; Binns, Barriga, Miller, et al., 2002; Tréhu, Bohrmann, Rack, Torres, et al., 2003) clearly indicated that LWD was best suited to acquire in situ physical property data in the Nankai accretionary prism.

During Expedition 314 we employed both LWD and MWD technologies. LWD and MWD tools measure different parameters. LWD tools are formation evaluation tools used while drilling that measure formation properties with instruments located in the drill collars above the drill bit. MWD tools are also located in the drill collars but measure downhole engineering drilling parameters (e.g., WOB, torque, etc.). Another function of the MWD tool is transmitting real-time MWD data and a limited set of LWD data through drilling fluid inside the drill pipe by means of a modulated pressure wave (“mud pulsing”), which is then monitored in real time at the rig floor and by the scientific party. Most LWD data (at least those of highest resolution and thus larger data volume) are recorded in memory at the downhole tool and retrieved when the tool returns to the surface. For safety monitoring purposes, the MWD tool was coupled with APWD during this expedition. MWD tools enable both APWD and MWD data to be transmitted uphole when the tools are used together, along with a subsample of the LWD data.