On the Prediction of Fatigue Damage From VIV
Empirical methods for calculation of response from vortex shedding are based on a set of coefficients that determines response frequencies, excitation force and damping, but also how competing frequencies will appear in time and along a structure. It is easy to formulate a mathematical model for Vortex Induced Vibrations (VIV), but the key challenges are to find the necessary hydrodynamic coefficients and a model for how the active frequencies appear in time and space. Since the original version of VIVANA was released more than ten years ago, new information has been become available through new test techniques, as well as from new methods for analysis of old tests. In this study, recent re-examination of the results of the NDP 38 m tests is presented. The observed response behavior is compared to predicted VIV response and fatigue. The consequences of assuming that response frequencies will be active concurrently or consecutively are investigated and predicted response and fatigue are compared to results based on the measurements. Another method that has provided valuable information has been inverse analysis, by which forces on a flexible beam are estimated from measured strains and/or accelerations. When forces are known, the underlying coefficients can be calculated from the standard equations. Inverse analysis has been carried out from a set of experiments, and a new set of coefficients has been estimated for use in coefficient based programs such as VIVANA. Initial comparisons between measured and predicted response show that the new coefficients give an improved agreement with regard to amplitudes and frequency composition.