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.
This scatter diagram method could reduce the cost of risers by reducing the over-conservatism that is introduced by using an envelope design current profile. It also reduces analysis time by helping the designer choose a small number of profiles to examine, rather than having to consider the tens of thousands of profiles contained in a typical data set.
Before a riser can be designed and deployed in a new location, information on the local currents must be obtained. Because this data often consists of tens of thousands of measured current profiles, it is difficult to know which ones are important for riser design. To overcome this difficulty, current industry practice is to prescribe a conservative design profile, based on the data set. One type of design profile is the envelope of maximum values. This profile is found by plotting the maximum velocity found in the data set at each measured depth, and incorporating these points into a single "envelope profile". Therefore, at any point along the riser, the greatest velocity to be found at that depth in the entire data set will be used. Sometimes a "slab" of constant velocity is drawn in the region of the peak of the envelope profile (the area of highest velocities). An example of this method is shown in Figure 1. In this case, the envelope and slab were produced using 1000 profiles gathered over the course of one current ring event. These conservative design profiles don't actually occur in nature, and usually result in much higher damage rates due to vortex-induced vibration (VIV) than any measured profile. So any riser designed to this specification will be built far more conservatively than the actual currents in the region require.
