Preliminary scientific assessment

Expedition 338 was planned to extend riser Hole C0002F from 856 to 3600 mbsf beginning in early October 2012 and ending in early January 2013. The primary goals for Site C0002 were riser drilling, analyses of cuttings, mud gas, LWD data, and limited cores (2300–2400 mbsf). The hole was to be suspended after setting casing at 3600 mbsf. However, riser drilling operations were suspended at 2005.5 mbsf because of riser damage, so the following riserless contingency operations were conducted instead: coring in Holes C0002H (1100.5–1120 mbsf), C0002J (902–940 mbsf), C0002K (0–286.5 mbsf), C0002L (277–505 mbsf), C0021B (0–194.5 mbsf), and C0022B (0–419.5 mbsf) and LWD in Holes C0012H (0–709 mbsf), C0018B (0–350 mbsf), C0021A (0–294 mbsf), and C0022A (0–420 mbsf).

Overall success

Riser drilling was conducted in Hole C0002F to 2005.5 mbsf and suspended for future reoccupation and completion of the NanTroSEIZE project (i.e., drilling through and sampling the seismogenic part of the megasplay fault [~5000 mbsf] and installing a long-term observatory). LWD data, mud-gas analyses, and cuttings samples in Hole C0002F provided constraints on the lithologic and structural features, physical properties, and geochemistry of the previously unaccessed deeper part of the Nankai accretionary prism. Riserless coring in Holes C0002H, C0002J, C0002K, and C0002L provided core samples (1) across the gas hydrate zone including the BSR of the Kumano Basin, which was not cored during Expedition 315, (2) across the preliminary unconformity boundary between the Kumano Basin sediment and the underlying accretionary prism sediment, and (3) in the uppermost accretionary prism, which allowed constraints on the lithologic and structural features, physical properties, and fluid and gas chemistries of sediment in those intervals. Thus, these operations enabled not only exploration of the accretionary prism to ~2005 mbsf but also complemented current knowledge of Site C0002.

LWD at Sites C0012 and C0018 provided petrophysical data where coring had already been conducted during Expeditions 322 and 333. Integration of the LWD data with the core and the 3-D seismic data already obtained at or across these sites enabled us to comprehensively understand the nature of oceanic basement and its overlying sediment at a subduction input site (C0012) as well submarine landslide dynamics and MTD emplacement processes at a Nankai Trough Submarine Landslide History (NanTroSLIDE) site (C0018).

Site C0021 is located ~2 km northwest of Site C0018 and at a more proximal site for MTDs observed at Site C0018. LWD and coring at Site C0021 provided further information on the nature, provenance, and kinematics of the MTDs observed at Site C0018 and provided data on the lateral heterogeneity of MTDs.

Site C0022 is located ~350 m southeast of Site C0004, where the shallow portion of the megasplay fault was cored during Expedition 316. LWD and coring at Site C0022 provided logging data and samples across the tip of the megasplay fault, which provided additional information on the activity of the megasplay fault and its bearing on earthquakes and tsunamis.

Problems and challenges

On 17 November 2012, the passage of a cold front caused sudden changes in wind direction and strong winds (~27 m/s), which together with the strong Kuroshio Current (~4.8 kt) made it difficult for the Chikyu to maintain position. In order to ensure the safety of the ship and drilling equipment, an emergency disconnect of the blowout preventer (BOP) riser pipe was completed. This left the lower portion of the BOP on the seafloor, safely sealing off Hole C0002F, whereas the riser pipe and upper portion of the BOP remained connected to the Chikyu. After the disconnect, the strong Kuroshio Current caused the Chikyu to drift, creating drag on the riser pipe such that it was not vertical. This tilt in the riser pipe was accommodated primarily by the intermediate flex joint, which in turn suffered damage. Because of this damage, riser drilling was suspended and will resume with IODP Expedition 348. This series of events demonstrated the difficulty of riser operations under such bad weather and rough sea conditions; however, since such weather and sea conditions will also be encountered during future NanTroSEIZE expeditions, this is a major operational issue that needs to be properly addressed for future riser operations.

Riser drilling in Hole C0002F used RWD technology, which allowed LWD/MWD analysis behind a 12¼ inch drill bit while simultaneously opening the hole above the LWD/MWD tool assembly with a 20 inch reamer bit. The distance between the drill bit and the reamer bit was 42.8 m; therefore, RWD results in mixing of cuttings from an interval of at least 42.8 m. However, it was realized that unit boundaries defined by LWD data and those defined by cuttings observations and analyses varied by as much as ~100 m. Cuttings samples also limited lithology and structural observations, physical properties measurements, and pore fluid chemistry analyses. Mud gas was monitored during riser drilling in Hole C0002F. However, for unknown reasons, air contamination occurred in the shipboard gas monitoring system, which affected measurements of gas chemistry analyses.

During contingency riserless drilling, the drill pipe got stuck in the hole several times during drilling and coring the accretionary prism. This occurred because borehole cavings from the accretionary prism were circulated uphole to the base of the Kumano Basin sediment. This narrowed the annulus and choked the drill pipe. The drill pipe was freed after several hours of efforts in Hole C0002H but remained stuck for ~40 h until it was cut off at ~850 mbsf in Hole C0002I. Although similar drill pipe issues were experienced during IODP Expeditions 314 and 315, the extreme difficulty of drilling into the accretionary prism was again underlined during this expedition.

This expedition had one science party split into two groups. We had a week-long crossover time with the entire science party, four Co-Chief Scientists, and Specialty Coordinators in order to hand over the shipboard work and data analyses smoothly. However, because of the riser drilling cancellation, half of the science party worked exclusively on LWD and cuttings and mud-gas samples in Hole C0002F, whereas the other half worked on LWD and core samples at several different sites and holes. This aspect of Expedition 338 was extremely challenging for the Co-Chief Scientists, Expedition Project Managers, and science party. Issues that were particularly difficult were the unification of the two groups as well as integration of the scientific results and reports.