Short fairings are quickly becoming the most popular choice for suppression of vortex-induced vibration (VIV) on marine risers. VIV is a complex phenomenon that can cause, and has caused, premature fatigue failure of marine risers and pipelines. The suppression of VIV can be essential to safe production thru a riser, and most all risers being installed in deepwater today have some kind of VIV suppression on them.
This paper presents the performance characteristics of short fairings for riser and tendon type applications. These characteristics include:
the suppression capability of fairings on smooth risers;
the effects of marine growth on fairing performance;
the effects of upstream tubulars (risers or tendons) on fairing suppression capability;
the effect of Reynolds number on fairing effectiveness; and
the drag coefficients associated with fairings.
Vortex-induced vibration of deepwater tubulars requires good analysis methodology that, in turn, often requires the use of VIV suppression equipment to insure a safe design. While analysis methodology can be critical to insure tubular safety, the selection of proper suppression equipment is equally critical and can also be a significant cost component of the tubular (riser, tendon, etc.) system.
The selection of VIV suppression equipment for marine applications has often depended upon research and experimentation conducted in air (wind tunnels). Zdravkovich1 developed an excellent review of this research. However, the author has learned that the "conventional wisdom" developed from applications in air often does not apply to marine applications. In addition, by performing a large number of experiments in water, the author (see Allen2) was able to discern that short fairings (Figure 1) could provide excellent VIV suppression characteristics with drag coefficients that were still lower than those corresponding to helical strakes (herein the use of the term "short" related to fairings implies fairings with chord to thickness ratios of less than about 1.7).
The performance characteristics presented herein are derived from experiments, most of which have been conducted at prototype Reynolds numbers for production risers, and at least into the critical Reynolds number range. The sections below briefly describe the experiments (which are more thoroughly described in previous publications) and then present the findings of those experiments for short fairings (with some comparisons to conventional helical strakes).
In 1998, the author performed tests with a long pipe in the critical Reynolds number range. Allen and Henning3 previously published the bare pipe results from these experiments. These tests were performed in the Rotating Arm Facility at the Naval Surface Warfare Center in Carderock, Maryland. Figure 2 shows the test setup on the rotating arm for these tests. The arm has a span of 129 ft, a width of 20 ft, and a weight of 44,000 lbs. The arm pivots on a pedestal at the center of the basin. A maximum arm speed of 30 knots can be achieved in 1/2 of a revolution at a radius of 120 ft and speeds up to 50 knots can be achieved at this radius in two revolutions.