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Vortex-induced vibration measurements

The Chikyu was maneuvered into the LCA to begin lowering the running tool to recover the SmartPlug and set the GeniusPlug instruments (see Fig. F4). Two accelerometer instruments were attached to the drill string, and ropes were additionally fastened to reduce vibration in the upper ~500 m of water column. Accelerometer 1 was attached at 461 m DRF (just above the BHA) at 1520 h on 3 November 2010 and Accelerometer 2 was attached at 1598 m DRF at 0530 h on 4 November. Drifting to Site C0010 began at 2100 h on 3 November at the speed of ~1 kt. For details, refer to “Operations.”

Acceleration data were collected to monitor VIV on the drill pipe once SmartPlug retrieval at Site C0010 was started. Hence, a record when drifting in and reentering without the bridge plug/SmartPlug, as well as a data set with the assembly attached to the end of the string, were collected. The timeline of acceleration data collected during these operations is shown in Figure F13.

Acceleration shocks caused by the drill pipe contacting the rotary table during lowering and pulling up the drill string were measured by the attached accelerometer. The peak to peak amplitudes of the shocks reached ~1.5 g. When the accelerometer was closer to the rotary table at the beginning of the lowering drill pipes, the peak to peak amplitude became 6 g with higher horizontal acceleration than vertical (Fig. F13). The difference between signals recorded during lowering and pulling up drill pipes may be explained by the horizontal acceleration during pulling up drill pipes having been lower (peak to peak amplitude = 0.5 g) than during lowering. Two other shocks were measured during the reentry and SmartPlug retrieval. The shock amplitude during reentry exceeded 2.5 g. These acceleration measurements suggests that shocks endured while lowering near the rotary table and during reentry have to be considered to further reduce sensor damage.

During drifting (yellow shading in Fig. F13), two types of vibrations are observed in the acceleration data on the drill pipe. One is VIV, which corresponds to the drifting speed and the drifting angle between drifting direction and sea current. By attaching rope along the drill string, the amplitude of VIV was successfully reduced to <1 g. The results show the vertical components of the accelerometer are much lower than the horizontal components. The second type of vibration observed is shocks caused by the drill pipe slamming into the rotary table during lowering and pulling up, which is actually similar to those observed during lowering pipe (see above). Although this signal is less regular in nature, it may reach larger magnitudes depending on the strength of the impact.

From VIV monitoring over a period of several days during Site C0010 operations, the following three conclusions can be drawn for long-term borehole monitoring system installation at Site C0002:

  1. The BHA should be lowered in the LCA, with the relative current speed being as low as possible.

  2. Drifting speed should be kept as low as possible (should be well below 1 kt).

  3. Drifting angle between drifting direction and sea current should be kept as small as possible (and definitely <45°).