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doi:10.2204/iodp.proc.332.106.2011

Introduction

The Nankai Trough, where the Philippine Sea plate subducts beneath the Eurasian plate off southwest Japan, is one of the most active earthquake zones in the world (Fig. F1). As part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a series of long-term borehole monitoring systems (LTBMS) that combine elements of circulation obviation retrofit kits (CORKs) (e.g., Ocean Drilling Program [ODP] Leg 196 [Mikada, Moore, Taira, Becker, Moore, and Klaus, 2005]) and ’neath seafloor equipment for recording Earth’s internal deformation (NEREID) (ODP Leg 186 [Suyehiro, Sacks, Acton, and Oda, 2003]) to investigate fault mechanics and seismogenesis along subduction megathrusts are being installed into three holes along the NanTroSEIZE transect offshore the Kii Peninsula.

The Kuroshio Current, one of the world’s fastest ocean currents, can flow at >5 kt in the Nankai Trough area. When the drill pipe is placed in a strong current, an asymmetrical flow pattern caused by the unsteady separation of flow over bluff bodies (e.g., the vertical drill pipe in the water column), creates alternating vortices (Karman vortex) on the downstream side of the drill pipe. These vortices then create periodic lateral forces on the drill string, causing it to vibrate, which is known as vortex-induced vibration (VIV).

During Integrated Ocean Drilling Program (IODP) Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010), a field test simulating the future planned deployment of the LTBMS was run into the hole to investigate two main objectives: (1) to evaluate environmental conditions and their effects on the LTBMS sensors for their development (Kimura et al., 2011), such as shock, acceleration, and vibration during installation; and (2) to confirm sensor installation operational procedures, such as onboard assembly of the sensor tree, ship maneuvers to reenter the sensor tree, and entry into the hole. During these tests, VIV effects on the sensor carrier and associated sensors were extremely severe, damaging or destroying the sensors sensitive internal components and cracking the sensor carrier itself (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010).

A second phase of field tests was conducted during the D/V Chikyu’s March 2010 CK10-01 cruise to examine VIV suppression/countermeasures (Kitada et al., 2011). Four accelerometers were attached to the drill pipes and instrument carrier to collect VIV data at separate points along the test string. VIV reduction ropes (24 mm outside diameter [OD]), using the drill collars as a heavy mass weight, and the newly redesigned instrument carrier were tested to establish the VIV reduction method for the LTBMS installation. The possibility of using 5 inch drill pipes and 3½ inch tubing for the bottom-hole assembly (BHA) inside the borehole was also explored.

VIV can be controlled by various passive methods (Blevins, 1990; Kwon et al., 2002) (e.g., streamlined fairings are one kind of effective device in general use for VIV suppression [Miyazaki et al., 2008]). Here we show acceleration data on drill pipe VIV collected during IODP Expedition 332 (Kopf et al., 2010). First, we evaluate the characteristics and causes of the vibration on drill pipe in order to suppress VIV. Then, we establish the installation method of the LTBMS in strong current areas such as the Nankai Trough. Finally, the operation procedure is verified and further modification points are discussed.

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