Riser Vortex-Induced Motion and Mitigation

Drill strings or production risers extend from an offshore oil platform to the seabed, and experience a unique fluid/structures interaction problem.  When subjected to ocean currents, these long cylindrical shapes shed periodic von Karman vortex streets.  Because the risers are almost neutrally buoyant and have very low structural damping, periodic forcing can induce transverse motion.  The motion varies with depth, and takes the form of waves along the riser’s length.  Motion amplitudes on the order of a riser diameter are not uncommon, and cause severe fatigue problems.  Designers need to know under what conditions the motion starts, how large and fast the motion will be, and whether any design feature can control the effect.   Riser motion is Reynolds number dependent, so RANS has an advantage over model tests where achieving full scale is difficult and expensive. 

AFT RANS codes have been used to predict riser motion for many years.  They are applied both for identifying conditions under which motion first starts, and for predicting the frequency and amplitude of response once it does.  They are used to predict the impact of mitigation features such as packaging of risers into arrays, the performance of helical strakes, and the behavior of fairings.   AFT has also developed turbulence models that include the effect of marine fouling, and validated the resulting motion predictions against experiment.  Click on a menu item at left to view examples of how AFT assists with riser design and analysis problems.