Modal analysis of measurements from a large-scale VIV model test of a riser in linearly sheared flow |
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| Institution: | 1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China;2. Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, China;3. Marintek, Trondheim, Norway;4. Mechanical Engineering School of Engineering, University of Kansas, 1450 Jayhawk Blvd, Lawrence, KS 66045, USA;1. National University of Singapore, Department of Civil and Environmental Engineering, 1 Engineering Drive 2, Singapore 117576, Singapore;2. School of Marine Science and Technology, Newcastle University, Armstrong Building, Newcastle upon Tyne NE1 7RU, UK;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 Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China;2. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China;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 |
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| Abstract: | Large-scale model testing of a tensioned steel riser in well-defined sheared current was performed at Hanøytangen outside Bergen, Norway in 1997. The length of the model was 90 m and the diameter was 3 cm. The aim of the present work is to look into this information and try to improve the understanding of vortex-induced vibrations (VIV) for cases with very high order of responding modes, and in particular to study if and under which circumstances the riser motions would be single-mode or multi-mode. The measurement system consisted of 29 biaxial gauges for bending moment. The signals are processed to yield curvature and displacement and further to identify modes of vibration. A modal approach is used successfully employing a combination of signal filtering and least-squares fitting of precalculated mode-shapes. As a part of the modal analysis, it is demonstrated that the equally spaced instrumentation limited the maximum mode number to be extracted to be equal to the number of instrumentation locations. This imposed a constraint on the analysis of in-line (IL) vibration, which occurs at higher frequencies and involves higher modes than cross-flow (CF). The analysis has shown that in general the riser response was irregular (i.e. broad-banded) and that the degree of irregularity increases with the flow speed. In some tests distinct spectral peaks could be seen, corresponding to a dominating mode. No occurrences of single-mode (lock-in) were seen. The IL response is more broad-banded than the CF response and contains higher frequencies. The average value of the displacement r.m.s over the length of the riser is computed to indicate the magnitude of VIV motion during one test. In the CF direction the average displacement is typically 1/4 of the diameter, almost independent of the flow speed. For the IL direction the values are in the range 0.05–0.08 of the diameter. The peak frequency taken from the spectra of the CF displacement at riser midpoint show approximately to be equal to the Strouhal frequency. The peak frequency in IL direction was typically twice the Strouhal frequency. |
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