Estimation of flow-induced vibration parameters of marine risers

Abstract

Flow-induced vibration of marine risers may be characterized by modal frequencies, mode shapes, and participation factors. These parameters cannot always be estimated accurately using methods that require a priori knowledge of the mode shape matrix. Therefore a combined system and subspace identification method was developed to estimate the risers’ modal parameters in the presence of unknown input, i.e., fluid forces. Data were analyzed, from an experiment involving a model riser under uniform and shear flow. ARMA models were used to describe the data dependence in state space. Subspace identification was used to reduce computational time. Modal parameters including structural damping were estimated from the state matrix of the system. Contrary to previous studies, it was found that mode shapes are complex, i.e., the system’s damping is nonproportional. The estimated damping ratios were in the range 0.03–0.07. The deviation of the results from those obtained using conventional methods, which assume that mode shapes are real and may be described approximately by sine functions, was significant when the structure was subjected to shear flow. Thus such a priori assumptions are inaccurate in these cases. In contrast, system identification methods which use only the available data are appropriate.