Suppression devices can have a profound impact on extending the fatigue life of a subsea structure which might otherwise fail due to VIV. Rarely considered though are their effects on downstream tubulars. For configurations such as riser arrays where tubulars are in close proximity to one another, suppression device selection can be very important. Many times, the suppression device selected for the upstream tubular can cause large motions of the downstream tubular regardless of the suppression device present on the downstream tubular.
Helical strakes are effective because they break up the correlation of vortices along a tubular and produce random alternating forces against its surface; however, the vortices rapidly reattach to one another producing large well-correlated vortices. These vortices can impose substantial pressure fluctuations on a downstream tubular and induce fairly large motions of downstream tubulars.
Fairings with a narrow-shaped tail “smooth” the flow of water around a riser. Small vortices shed from the surface of the riser will eventually roll up together and form a large single vortex but, for a given separation distance, the vortex will be substantially weaker than if it was shed from a helical strake. Thus, for a given separation distance the impact of vortices shed from a fairing is smaller than the impact of vortices shed from a helical strake.
Wider-tail fairings shed shear layers that are further apart than the shear layers shed from a narrow-tail fairing. Wider-tail fairings therefore can produce very large vortices a relatively short distance downstream of the fairings. To date, good test data for the dynamic effects of wide-tail fairing vortex shedding on downstream tubulars is lacking.
The pattern and spacing of risers in an array often dictate which type of suppression device will give the greatest overall performance benefits. VIV suppression requirements, the presence of marine growth, the need for low drag to prevent contact of adjacent tubulars, and the potential enhanced fatigue of downstream risers (even with suppression devices on them) are all considerations for suppression device selection.