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doi:10.2204/iodp.proc.332.106.2011

Discussion

Dramatic reductions in VIV are clearly demonstrated by the simple expedient of attaching lengths of ropes (24 mm OD) to the upper part of the drill string where strong ocean currents (e.g., the Kuroshio Current) have the greatest impact on the drill string. During preliminary field tests using the Chikyu (Japan Agency for Marine-Earth Science and Technology Cruise CK10-01) and the newly redesigned sensor carrier, ropes were attached to the bottom section of the BHA to simulate electrical cables to be used in the final design (Kitada et al., 2011). Analysis found that an unexpected side effect of adding the simulated cables was a reduction in VIV, presumably through the disruption of vortex formation by breaking up the current flow around the drill pipe.

Expanding on these field tests, laboratory tests confirmed the vortex dampening by rope-attachment to the drill string (Kyo et al., 2011). During SmartPlug retrieval and LTBMS installation during Expedition 332, the ropes were also attached along the drill pipe just above the BHA (Fig. F3), which reduced the amplitude of VIV to <0.5 g (Figs. F4, F5). Attaching ropes along the drill pipe is effective for VIV suppression, especially when comparing with results from the dummy run test during IODP Expedition 319 (Fig. F6) (see also Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). This method has other significant advantages, being simple and easy to apply and low cost, with no maintenance requirements, especially when compared with more traditional vortex suppression steel devices by Blevins (1990).

One key factor in managing VIV is careful planning and control of the ship’s drift during deployment through the current. Acceleration data collected during SmartPlug recovery showed that managing drifting speed was crucial for reducing VIV compared to the actual speed of the current (Fig. F4). However, the difference in angle between drifting direction and the sea current direction remains an important factor for VIV suppression. This was illustrated by the sudden drifting direction changes while moving to the LCA during SmartPlug recovery operations; when the difference in angle increased, amplitude of VIV also increased (Fig. F4). The results of VIV monitoring showed that VIV was further suppressed with a drifting speed <1 kt and a drifting angle <45° during LTBMS installation at Site C0002 (Fig. F5).

In the shallow part of the Kuroshio Current (<300–500 m depth), the current speed can reach >3 kt. This can have considerable negative effects on LTBMS, or more generally observatory instrument installation and/or recovery procedures and the integrity of the component sensors themselves. To mitigate these effects, the LTBMS sensor assembly was first lowered to 1700 m DRF in the LCA (<1 kt) before drifting to Site C0002 began. These procedures increase the amount of operation time needed for deployment (~2 days in this installation). However, when considering the strong current (maximum near 6 kt) (Fig. F4), the time lost is a small price to pay for reducing VIV and maintaining the integrity of the CORK structure and instruments.

Another factor in VIV generation and suppression is resonance of the drill pipe. VIV reduction in this case focuses on reducing the tendency for the natural frequency of the drill pipe coinciding with the frequency of vortex shedding. Applying a heavy mass to the drill string, in this case by adding drill collars to the sensor assembly, can help to reduce this source of VIV. Data from the field test performed during the Chikyu Cruise CK10-11 shows VIV suppression, especially at high frequency (Kitada et al., 2011); however, more work remains to more carefully quantify VIV suppression using drill collars.

One additional source of concern—unrelated to VIV—are the sudden accelerations observed when the drill string and BHA impact the wellhead and the rotary table while running pipe. With maximum peak-to-peak amplitudes reaching 6 g, special care needs to be taken to prevent damaging the delicate sensors comprising the LTBMS assembly.

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