Pennsylvania State University, United States

This study reports on vortex-induced vibration (VIV) suppression methods for long-term borehole monitoring system (LTBMS) installation in areas of strong ocean currents such as the Kuroshio Current. One of the primary challenges in installing the LTBMS was to deploy high-precision, sensitive sensors into the borehole without damaging them during their trip through the water column. Two field tests were performed using accelerometers attached to the instrument carrier and/or drill pipes to collect data on the characteristics and effects of drill pipe VIV. These tests demonstrated that vortex suppression can be achieved by using drill collars and attaching ropes to the drill pipe. Therefore, these methods were applied to the drill pipe (above the sensor assembly) for the actual recovery and installation of the observatory systems during the Integrated Ocean Drilling Program (IODP) Expedition 332. From the observations and data collected over a period of several days of VIV monitoring during the LTBMS installation and SmartPlug retrieval, the following three points can be made regarding VIV suppression: (1) the bottom-hole assembly should be lowered in the low-current area, with the relative current speed being as low as possible, (2) the drifting speed should be kept well below 1 kt, and (3) the drifting angle between drifting direction and sea current should be kept as small as possible (definitely <45 degrees ). The results of VIV monitoring during Expedition 332 attest that the amplitude was further reduced to <0.5 g, eventually contributing to the success of the first LTBMS installation at IODP Site C0002.