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doi:10.2204/iodp.proc.319.104.2010

Observatory

Sensor dummy run test

Instrument preparation

All equipment except the strainmeter was loaded onto the Chikyu by supply boat on 3 August 2009; the strainmeter was loaded on 10 August because it needed repairs from shipping damage incurred between 10 and 13 July during transportation to Shingu, Japan. Prior to the dummy run test, sensor running tests for the strainmeter and accelerometer-tiltmeter were conducted to confirm that sensors were working well with no damage during shipment to the Chikyu (Figs. F33, F34). The instrument carrier was passed through a 9⅝ inch casing joint before the sensor dummy run test to ensure that there would be sufficient clearance to reenter the hole (Fig. F35). Two seismometers, an accelerometer-tiltmeter, and eight miniature temperature loggers (MTLs) were attached to the instrument carrier on 17 August and a ninth MTL was attached to a pup joint at the bottom of the assembly (Fig. F36). Dummy run test sensor tree configurations are shown in "Observatory" in the "Methods" chapter. Additional brackets were attached to the carrier at the top and bottom of each sensor to prevent loss or damage during drifting through the high-current area and reentry (Fig. F36).

First dummy run test

The sensor dummy run test began on the morning of 18 August. The dummy run assembly was made up and lowered from the rig floor at 1502 h on 18 August. The current speed was ~0.7 kt. The sensor tree was lowered to 1000 m DRF at 1730 h on 18 August when the Chikyu started drifting to Site C0010 at 1 kt. The sensor tree was located 4 nmi from Site C0010 at 1700 m DRF at 0915 h on 19 August. The Kuroshio Current was too strong (4.8 kt) to deploy the ROV around this area. Therefore, the Chikyu needed to move to a low-current area to deploy the ROV from 1345 to 1930 h on 19 August. The ROV was deployed to visually examine the sensor tree while in the low-current area and documented loss of the strainmeter and one seismometer (CMG3T) from the instrument carrier. Therefore, we decided to retrieve the sensor tree before attempting reentry in Hole C0010A (Fig. F37).

After the sensor tree was retrieved, the seismometer and accelerometer-tiltmeter were removed from the instrument carrier and the condition of the accelerometer-tiltmeter sensor was checked. Data recording had stopped by the time the check was conducted on board the Chikyu. Recorded data in the memory were checked and found to cover the time period from 0645 h on 18 August to 0911 h on 19 August. A capacitor on the accelerometer sensor was found to be damaged (connections to the circuit board snapped), probably as a result of strong vibration. After repairing the snapped capacitor, the accelerometer-tiltmeter worked well.

Second dummy run test

The second dummy run reentry test employed only the accelerometer-tiltmeter, to evaluate shock acceleration and vibration during reentry (Fig. F38). Because we lost the strainmeter, we replaced it with a dummy strainmeter for the reentry test. We also needed to strengthen the instrument carrier to prevent losing sensors. Therefore, additional protectors were welded to the instrument carrier to hold the sensors, and we patched the cracked sections of the instrument carrier with welded plates (Fig. F39). The dummy run test assembly was made up and lowered to 1689 m DRF at 1507 h on 20 August. The current speed was ~0.7 kt, and drifting to Site C0010 at a speed of 0.7 kt began at 1815 h on 20 August, arriving at Hole C0010A at 0923 h on 21 August.

The reentry test was conducted in strong currents (4.3 kt) under good sea-surface conditions from 0923 to 1034 h on 21 August. The bottom of the sensor tree was stabbed into the wellhead three times to 5–7 m DSF (Fig. F40). During the third reentry, the bottom thread of the 3½ inch VAM top tubing (the bottommost parts of the sensor tree) hit the reentry cone two times. During observation of the sensor tree via the ROV camera, there was no obvious repeated slamming or extreme vibration during the reentry procedure. After reentry testing, the sensor tree was pulled out of the hole.

After the dummy run sensor tree was recovered on deck, the accelerometer-tiltmeter status was checked. Data recording had already stopped when checked on board the Chikyu. The recording period in the second dummy run test only covered from 0746 to 2226 h on 20 August. No acceleration and tilt data were recorded during hole reentry. One fuse on the electronic circuit board was discovered to be loose, which may have been caused by strong VIV from the Kuroshio Current during drifting.

Acceleration and tilt data

In the first dummy run test, acceleration and tilt data were collected from 0645 h on 18 August to 0911 h on 19 August. Examples of the time series data are shown in Figure F41. Power spectral density (PSD) images of acceleration for each axis were generated from the collected acceleration data (Fig. F42). In the first dummy run, three events (A, B, and C) can be identified from the PSD image (Fig. F42A). Event A is characterized by the broad high PSD at a frequency of 0.1–10 Hz at ~0542 h on 19 August. The location of Event A (33°18.2067′N, 136°34.2046′E) is indicated in Figure F43. Event B shows a clear offset in resonance frequency occurring at 0652 h on 19 August, implying a change in the mass of the sensor tree occurred at this time. Before this event, the resonance frequencies were 0.3–0.5 and 0.9–1.6 Hz for x-axis acceleration, 0.7–1.3 and 1.4–1.6 Hz for y-axis acceleration, and 2.2–2.8 and 3.7 Hz for z-axis acceleration. After this event, the resonance frequencies changed to 1.9 and 6.0 Hz for x-axis acceleration; 1.4–2.2 Hz for y-axis acceleration; and 4.0, 5.0, and 8.3 Hz for z-axis acceleration. This change likely identifies the event when the strainmeter and the assembly below (~900 kg in total) were dropped. The location (33°17.3197′N, 136°35.3139′E) where this occurred is indicated on the map (Fig. F43). The peak to peak amplitudes for acceleration data are 19 m/s² for x-axis, 21 m/s² for y-axis, and 25 m/s² for z-axis before Event B (Fig. F41A). After the event, the amplitudes are 26 m/s² for x-axis, 19 m/s² for y-axis, and 34 m/s² for z-axis (Fig. F41B). Event C is characterized by high PSD at a frequency of 0.1–10 Hz at ~0905 h on 19 August. The map location of Event C (33°15.5078′N, 136°37.5752′E) is also indicated in Figure F43. In the second dummy run test, acceleration and tilt data were collected from 0746 to 2226 h on 20 August. The data indicate a problem with the accelerometer-tiltmeter data after 1625 h on 20 August (Fig. F42B), with no acceleration and tilt data collected during the reentry.

Tilt data from the first and second dummy run tests are characterized by a lower frequency than the acceleration data (e.g., Fig. F41). This may not fully represent the exact tilt of the instrument carrier during testing because of the limited dynamic range of the bubble-type tilt sensor under strong vibration.

Temperature data

One MTL was set inside the pup joints at the bottom part of the sensor tree and eight were attached to the instrument carrier in the first dummy run test (see "Observatory" and Table T21 in the "Methods" chapter). Temperature data were recovered from six sensors (see Table T21 in the "Methods" chapter). No temperature data from inside the borehole are available, as the first dummy run test was aborted before reentry. Figure F44 shows water temperature results from the dummy run. High-frequency temperature variations between 2.32° and 2.44°C are observed at 1700 m DRF (Fig. F44B). This variation does not directly correspond to the ship's motion.

Further modifications for future installments

Modifications of sensors and equipment are needed to overcome strong VIV for future installations under high sea current conditions. Vibration tests are also needed to evaluate sensor performance before the planned installation of long-term observatories.

Temporary monitoring system

Instrument preparation

A small instrument package designed to monitor pore pressure and temperature attached to the bottom (i.e., downward-looking) end of a mechanically set retrievable packer at Site C0010 represents an assembly hereafter referred to as the smart plug. Two instruments (8A and 82) (see "Observatory" in the "Methods" chapter) were shipped for Expedition 319 and were recording data during shipment from 12 April 2009 for test purposes so that during the expedition these data could be downloaded and checked for quality and overall performance of the pressure transducers and loggers. Additional status checks were successfully performed on board the Chikyu between 5 and 11 August before the pore pressure data logger memory was cleared and the instrument was "deployed" (i.e., was set to start recording) at 1459:20 h on 11 August. In parallel, the self-contained MTL was not reprogrammed and continued logging from its initial programming on 12 April 2009. Data logging includes pore pressure as well as three separate temperature readings (one in each pressure transducer for compensation and a separate platinum chip thermistor set for 60 s intervals, plus the self-contained MTL for Unit 8A set for 30 min logging intervals; see "Observatory" in the "Methods" chapter).

After programming the data loggers, the bottom end cap of each smart plug was greased and carefully closed, and the lower pore pressure tubing designed to monitor formation pressure was mounted and secured (Fig. F45A). Instrument 8A was chosen for deployment at Site C0010, and Instrument 82 was kept for backup. Given the experience with the observatory dummy run, in which strong ocean currents imparted dynamic force onto the drill string and the attached instruments (see above), we secured all 16 bolts holding the upper ("packer coupler") and lower ("bullnose") (Fig. F45A) end caps of smart plug Instrument 8A using a threadlock compound (Fig. F45B). The instrument was then coated with white paint to facilitate recognition during ROV-aided reentry of the borehole.

Installation

The smart plug instruments built for Expedition 319 were designed for deployment immediately beneath a Baker-Hughes A3 Lok-Set retrievable casing packer seal, as illustrated in Figure F46. In this configuration, the smart plug terminated the drill string and was first threaded onto the bridge plug. To ensure that the instrument did not detach during running through the Kuroshio Current, it was tack-welded at the crossover connection (Fig. F47). The smart plug assembly (Fig. F48) was then lowered into the water in a low-current area 10 nmi from Site C0010, where current speed was only 1.5 kt. Once the drill string depth was 1000 m DRF, the Chikyu slowly approached Site C0010 by drifting with the current, the ROV was deployed for reentry, and the drill pipe was lowered until the instrument package was near the seafloor. Reentry of the hole with the smart plug was successfully carried out at 0404 h on 23 August, and deployment was completed by setting the packer (with the instrument package below) at 365 m DSF at 0800 h on 23 August. The drill string was then detached from the retrievable packer by counter-clockwise rotation, and the drill string was pulled out of the hole at 0913 h (see "Operations").

The instrument and screen placement were designed for the smart plug to provide a time series of fluid pressure and temperature in an isolated interval of formation including the splay fault (downward-looking pressure tube protruding from bullnose; Fig. F45A) and also monitor hydrostatic pressure as a reference (upward-looking pressure tube) (see also Fig. F46). Retrieval of the smart bridge plug is anticipated for 2010 or 2011, when a more sophisticated long-term monitoring system will be deployed at Site C0010.

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