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Vortex-induced vibration measurements

In order to establish long-term borehole observatories in the NanTroSEIZE region, one of the primary challenges is to install high-precision sensitive sensors into the borehole without damaging them. In particular, strong vibration can damage sensors during lowering of the drill pipe through the water column and into the borehole, especially in areas of strong ocean currents such as the Nankai Trough where the Kuroshio Current prevails (see example in Fig. F4 in the “Site C0010” chapter [Expedition 332 Scientists, 2011b]).

Given that strong vibrations during a dummy run test of the first-generation instrument carrier conducted during Expedition 319 (Saffer et al., 2009) caused severe damage to both sensors and the carrier itself, alternate installation methods have been explored to reduce VIV. A new H-beam type instrument carrier (i.e., 3½ or 5 inch tubing size) was developed for the sensor tree assembly, and drill collars to balance the weight were discussed. In addition, it was suggested that VIV reduction ropes be attached on the drill pipe. These countermeasures were tested and approved for operations during the CK10-01 shakedown cruise on the Chikyu in March 2010.

Acceleration measurements were carried out to monitor VIV on the drill pipe during SmartPlug retrieval at Site C0010 and long-term borehole monitoring system installation at Site C0002. These measurements had two main objectives: (1) to evaluate environmental conditions, such as VIV, shock, and acceleration during operations, and (2) to improve operational procedures for installing high-precision sensitive sensors into the borehole by avoiding VIV-induced damage by the strong Kuroshio Current.

A self-contained accelerometer (94 mm diameter and 760 mm length) equipped with a triaxial acceleration sensor, internal lithium batteries, and a data logger with 2 GB SD memory card was attached to the 5½ inch drill pipe through the attachment tool before running into the hole (Fig. F32). The device measures acceleration in three directions (x, y, and z) at a sampling rate of 250 Hz, where the z-axis corresponds to the downward direction of the drill pipe. The orientation of the three axes is shown in Figure F32.

An attachment tool was manufactured to secure the accelerometer to the drill pipe. A copper sheet was set between the attachment blocks and drill pipe to prevent sliding or rotation during deployment (Fig. F32). Self-locking nuts were used to tighten the bolts to prevent them from loosening from vibration. Figure F33 shows a photograph of the accelerometer instrument on the drill string.