A New Approach for Identification of Forces on Slender Beams Subjected to Vortex Induced Vibrations

Vortex induced vibration (VIV) has been subjected to extensive research during the last 20 years. A large number of laboratory and ocean tests with long slender beams or cables have been reported. Key results from such experiments have been response frequencies and amplitudes, but also information on mode composition and traveling waves. Due to the difficulty of direct force measurement, accelerometer and bending strain measurement are used in such experiments. Formally, it should be possible to identify the forces that have created the measured response, but so far few results from such attempts have been reported. An inverse force estimation method is adopted to provide an accurate way of reconstructing the unknown hydrodynamic forces from measured dynamic response data. The method is based on state space formulation of a finite element beam model. It incorporates the Kalman filtering and recursive least squares algorithm to remove the noise from measurement and obtain force estimation in discrete time domain. The inverse force estimation method is verified with numerical simulations. The input force of a tensioned beam structure is estimated from response. The result indicates its capability to accurately estimate the input forces from stochastic response. The method is applied to the data from Rotating Rig Test to identify hydrodynamic forces along the riser. The lift force and added mass coefficients are calculated and compared with existing data.