Frequency Domain Model for Prediction of Stochastic Vortex Induced Vibrations for Deep Water Risers

Abstract

This paper describes a new model for prediction of fatigue damage from VIV in risers. The method will overcome some of the shortcomings of previous methods. A fully 3D model is proposed, “cross-flow” and “in-line” response are predicted, response at higher order harmonic components will be added, and the stochastic nature of the response is accounted for by introducing a time varying envelope function combined with “time sharing” between dominating response frequencies. A model that reflects this behaviour is considered to be more realistic and is more likely to predict lower fatigue damage than the traditional discrete-frequency models. The model will predict a response that will appear as a combination of standing and travelling waves depending on boundary conditions, damping and load distribution. Fatigue damage will therefore become more evenly distributed along the riser, and less concentrated at anti-nodes for (dominating modes) than seen from traditional discrete frequency models. The proposed model needs empirical coefficients for simultaneous IL and CF response. In principle this requires a data base of added mass, excitation and damping coefficients for varying flow conditions and response frequencies, combinations of response amplitudes in both directions, varying phase between the two response components and even the presence of higher order motion components. Such data do not exist. We have therefore proposed to use the limited information we have on this matter at present. Future improvement of the model might therefore be possible if more data becomes available. The new model will be implemented in the VIVANA program and the enhancement of the code is in progress. The paper will present the background of the model, the basic assumption of the new model and a comparison between preliminary results obtained from a preliminary code and model test results. The cases include both 2D uniform current conditions and 3D (non-uniform) current conditions.

Empirical sensitivity of two-dimensional nonlinear wake–cylinder oscillators in cross-flow/in-line vortex-induced vibrations

“…Based on a frequency domain approach, Lie et al (2008) accounted for the stochastic VIV response of flexible risers by introducing a time varying envelope function combined with a time sharing between the dominating frequencies. This approach allows the combination of response standing and travelling waves and the improved fatigue estimation.…”

Stochastic modelling of cross-flow vortex-induced vibrations

“…The experimental time series show varying degree of frequency bandwidth for every towing velocity, although the tendency is towards larger frequency bandwidth for increasing velocity (see figure 12). The second consideration used to determine the stochastic parameters relates to experimental findings reported by Lie et al (2008). In their study, the limit for random response was found to be at about 10th mode and above in uniform and sheared flow.…”

“…The observed randomness is in good agreement with what found in the literature. Recall that Lie et al (2008) classified the limit for stochastic response at about 10th mode and above in uniform and sheared flow.…”

“…This stochastic prediction tool was able to provide a rough estimate of fatigue damage found directly from the measurements. Lie et al (2008) proposed a method for analysis of combined cross-flow and in-line response, using a time sharing approach combined with a stochastic model of displacement amplitudes. The method was based on frequency domain analysis and could potentially include the effect of higher harmonics in addition to the fundamental frequency.…”