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

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.