Flow-Induced Vibration Analysis Using a Low-Cost Inertial Measurement Unit

Abstract Flow-induced movements have been investigated for more than 20 years, especially in offshore platforms, where Vortex-Induced Movements (VIM) impact platform productivity, mooring lines, and risers fatigue life. However, monitoring these systems with standard methods (i.e. robots) is very expensive. In this work, a low-cost alternative is developed: we carried out experiments in the towing tank at The University of Tokyo (UTokyo), measuring the movements of a spar-type platform model with a low-cost Inertial Measurement Unit (IMU). The IMU application is beyond the computation of orientation: by measuring movements, displacement amplitudes and frequencies, we analyzed Flow-Induced Vibrations (FIV) in the spar model. The model was free to move in six degrees of freedom (6DOF), while its acceleration, rotation rate and magnetic field were recorded using the commercial IMU GY-87. A robust signal treatment methodology is developed based on the Kalman filter, Fast Fourier Transform (FFT) filter, Intrinsic Mode Function (IMF) decomposition, and coordinate manipulation using quaternion rotations, to remove drift and noise problems, and extract the model displacement in the three axes. The IMU application is evaluated by comparing its results with a high-accuracy optical tracking system, showing the potential of low-cost sensors for FIV analysis. Maximum amplitudes were found for cross-flow displacement at the reduced velocity VR = 10.13, with a significant amplitude of 0.7D for the IMU and 1D for the optical system.