Site summary

Site C0002 ultra deep riser top hole

  • C0002F = 33°18.0507′N, 136°38. 2029′E (Fig. F5)
  • Water depth = 1968 m
  • Drilling depth = 872.5 mbsf

The D/V Chikyu left Shingu, Japan, at 0700 h on Monday, 19 July 2010 and started deploying transponders on the night of the same day until early morning on the next day. Meanwhile, the watch boat was surveying the Kuroshio Current speeds around the location. A seafloor survey was conducted early on 20 July and found no obstructions around the location. The 36 inch conductor pipe and jetting BHA were made up and run, and by 21 July the guide horn was installed and the vessel started drifting to site. However, ROV winch trouble on 22 July held up the operation until spare parts could be loaded. This happened on the night of 24 July, and repair of the ROV winch continued until 25 July. Hole C0002F was finally spudded at 38°18.0507′N, 136°38.2029′E on 25 July, jetting in the 36 inch conductor pipe to 54 mbsf. Releasing the drill-ahead tool went smoothly, followed by drilling with 26 inch bit to a target depth of 856.5 mbsf on 27 July.

The subsequent wiper trip encountered a few tight spots, including one too close to the target depth that could endanger the cement job for the casing operation. We decided to deepen the hole slightly to reach a more stable interval at 868.5 mbsf. Issues with mismatch in positioning software delayed the following reaming operations of the several tight spots, but by 30 July we were able to clear the hole and move upstream to prepare for casing operations.

Lowering of the 20 inch casing began on 31 July, and by 1 August the Chikyu could drift back toward Hole C0002F with 72 joints of 20 inch casing trailing underneath. However, during the afternoon of 1 August, probably due to a sudden intensifying of the current, the string sheared off just above the casing running tool and most of the casing was lost on the seafloor. The decision was made to return to Shingu, Japan, and load up new casing and try again.

The port call lasted less than a day, and by 3 August the Chikyu returned to sea to stand by while decisions were made on shore about how to proceed. It was decided to procure a sturdier but untested casing running tool, which was brought out by the Heisei-maru on 7 August. While decisions were still being made on shore, we surveyed current conditions with help from the Heisei-maru. Currents sometimes exceeded 4 kt, with an average around 3 kt.

Preparations for the second casing operation were completed by 12 August. We reamed down another 4 m (to 872.5 mbsf) and continued wiper trips until 14 August then returned upstream to a low current area to prepare for setting casing. On 15 August, the new casing was lowered with four ropes attached to the drill pipe to reduce VIV. Drifting downstream began in the early morning of 16 August. By midday on 17 August, the casing was stabbed into Hole C0002F under current speeds of >4 kt. On 18 August, the casing was successfully set and cemented, and the drill pipe could be tripped back up. The casing shoe is at 860.3 mbsf. By nighttime on 19 August, the ROV dove with the corrosion cap and set it before midnight. On 20 August, the ROV was retrieved back on the Chikyu and we headed toward Shimizu, Japan. In the evening of 20 August, the Chikyu arrived in Shimizu, Japan, and Expedition 326 came to an end at 0000 h.