A scatter diagram approach to the selection of design currents for prediction of marine riser vortex-induced vibration

Abstract

This paper describes a scatter diagram approach for the classification of large numbers of current profiles for use in the prediction of riser fatigue damage due to vortex-induced vibration. Scatter diagrams have long been used to characterize the probability of various combinations of wave height and period, which are then used to assess wave forces. To predict VIV fatigue damage the designer needs to know which current profiles have the combined property of long regions of relatively constant velocity and relatively high speed. A sorting algorithm is proposed which searches every current profile for long regions of relatively constant flow speed. The probability of each length and speed combination is assessed and the data is used to populate the bins of the scatter diagram. The designer need only select relatively few representative profiles for detailed VIV analysis from those bins that would account for the most damage. The method is tested by making comparison to a brute force approach in which each of many thousands of profiles is evaluated for fatigue damage by running it in the SHEAR7 VIV response prediction program.