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Vortex-induced vibration measurements

Two accelerometers were affixed to the drill string when the LTBMS sensor assembly was lowered in the LCA (<1 kt; see Fig. F5 in the “Site C0010” chapter [Expedition 332 Scientists, 2011c]). Accelerometer 1 was attached at 1002 m DRF (just above the CORK head running tool) at 2030 h on 5 December 2010 and Accelerometer 2 was attached at 1720 m DRF at 0732 h on 6 December. The Chikyu’s drifting speed was ~0.5 kt to further reduce VIV, as suggested from VIV measurements collected during the SmartPlug retrieval at IODP Site C0010 (see Fig. F13 in the “Site C0010” chapter [Expedition 332 Scientists, 2011c]). The angle between the Chikyu’s drifting direction and sea current was ~45°, and the ship began drifting toward Site C0002 at 0830 h on 6 December, arriving at 0045 h on 8 December.

An abundance of acceleration data was collected during LTBMS installation at Site C0002. An example of the full data set collected is shown in Figure F20. The same features of acceleration shocks caused by the drill pipe contacting the rotary table during lowering and pulling up as seen in the results from SmartPlug operations are observed in the acceleration data. Acceleration during reentry was much lower (0.5 g peak to peak amplitude) than during SmartPlug retrieval, mostly in vertical and horizontal directions where significant force was applied to latch on. The shocks during ROV operations on the CORK head ROV platform and cementing after lowering the sensor assembly are new observations. They appear to increase in strength because the drill string is “fixed” in the borehole while the ROV applies significant push force to connect to the ODI connectors. The maximum peak to peak amplitude of the acceleration was ~6 g. These results show that significant acceleration shocks can occur after successful placement in the wellhead and that further care should be taken to avoid damaging the delicate sensors to help insure the success of the LTBMS installation.

The results during drifting from the LCA to Site C0002 show the amplitude of VIV was successfully reduced to <0.5 g, especially the amplitude of the vertical component of the acceleration during drifting (<0.2 g). Similar to operations at Site C0010 (see Fig. F13 in the “Site C0010” chapter [Expedition 332 Scientists, 2011c]), two types of vibrations are also observed in the Site C0002 acceleration data set: one induced by the current and a second caused by the drill string hitting the carts in the moonpool (i.e., spikes extending beyond the background drifting VIVs in Fig. F20 in the yellow shaded region).