VIV response of a flexible cylinder with varied coverage by buoyancy elements and helical strakes |
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| Institution: | 1. Zachry Department of Civil Engineering, Ocean Engineering Program, Texas A&M University, College Station, TX 77843-3136, USA;2. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200030, China;1. Key Lab of Structures Dynamic Behavior and Control (Harbin Institute of Technology), Ministry of Education, Harbin, Heilongjiang 150090, China;2. School of Civil Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, China;3. Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA;1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China;2. Statoil, Trondheim, Norway;3. Marintek, Trondheim, Norway;4. Dept. of Marine Technology, Centre for Ships and Ocean Structures, NTNU, Trondheim, Norway;1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, Southwest Petroleum University, Chengdu 610500, PR China;2. Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan;1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China;2. Collaborative Innovation Centre for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China;3. Marintek, Trondheim, Norway;4. CNOOC Research Institute, Beijing 100027, China;5. Massachusetts Institute of Technology, Cambridge, MA 02139, USA;1. Department of Naval Architecture and Ocean Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan;2. Department of Marine Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway |
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| Abstract: | Many significant engineering challenges have emerged as the petroleum industry has moved their field development and production activities into increasingly deeper water depths. The design of deepwater marine risers presents the combined challenges to minimize top tensioning requirements, mitigate any flow-induced vibrations, and if possible to increase the expected fatigue life of these slender structural members. As part of the design process to achieve these goals external buoyancy modules and strakes have been employed. To gain insight into the complex multi-mode response behavior a recent experimental study was performed and the analysis of selected data sets is presented. In the experiments a horizontal cylinder with a length to diameter ratio of 263 was fitted with a variety of strake and buoyancy element configurations. The models were towed at uniform speeds ranging from 0.4 to 2.0 m/s and fiber optic strain gages were used to measure both in-line and cross-flow strain response. The resulting time series information was processed utilizing the method of time domain decomposition formulated for strain data input and the introduction of modal assurance criterion to resolve the modal strain information that included frequency, mode shape, and critical damping ratio information. The pre-tensioned cylinder without appendages was used as a base case and the results were basically consistent with expectations. In the case of 0.8 m/s low-tension test, multiple closely spaced non-overlapping peaks were observed in both in-line and cross-flow directions and were identified as being of the same mode with mode shapes distorted away from purely sinusoidal behavior. The test data for the 100% coverage by helical stakes demonstrated the effectiveness of that suppression device over the range of current velocities investigated. The most interesting case was that of a staggered combination of helical strakes and buoyancy element whose total for each type of coverage was equal. This effective asymmetric VIV suppression approach is presented and discussed in detail. |
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| Keywords: | VIV Fiber optic sensors Helical strakes Buoyancy elements Mixed coverage Time domain decomposition |
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