In the offshore subsea environment, weather disruptions and changes in the oceanic environment are common considerations for operators. Reducing the effects of such factors is key to keeping costs low and production high.
When a current flows past a cylindrical object such as a riser, tendon, jumper or horizontal pipeline span, it creates a phenomenon called vortex-induced vibration (VIV). The friction of the cylinder surface causes boundary layers to form on each side of the cylinder. The retardation of the flow due to the friction ultimately causes the boundary layers to separate from the tubular.
Tubulars experiencing VIV can eventually fail due to fatigue. To prevent substantial fatigue damage, it is often prudent to install VIV suppression devices over at least part of the tubular span (to reduce the vibration amplitude and/or frequency). VIV can usually be minimized with the careful selection and design of VIV suppression devices such as helical strakes and fairings.
Helical strakes are a popular choice for suppressing VIV of subsea tubulars. Protruding fins disrupt the correlation of vortex shedding along a tubularís span, resulting in lower and randomly-phased lift and drag forces. Strakes are often a feasible candidate for risers, tendons, jumpers, and horizontal pipeline spans.
Helical strakes consist of one or more fins that are spiraled around a tubular.† Traditional helical strakes designed for applications in wind had three starts (three fins each 120 degrees apart around the tubular), pitch per start of approximately 5 times the tubular diameter, and a fin height of 10 percent of the diameter.† Early tests of helical strakes for deepwater tubular applications found that the traditional wind parameters were inadequate and that the strakes performed best with a fin height closer to 25 percent and a pitch per start in the range of 12 to 20 times the tubular diameter.† The configuration utilizing three starts was generally maintained.
Helical strakes are effective when they break up the correlation of vortices along a tubular and produce random alternating forces along the tubular length.† An exception to the optimal geometry for deepwater tubulars has been made for short cylinders, such as spar production platforms, where considerations such as drag and the need for shorter correlation lengths (due to the limited length of the structure) have driven helical strake designs closer to the geometries of tradition helical strakes for wind applications.
Because helical strakes are relatively bluff to the incoming flow and often produce early separation of the incoming flow, they are associated with higher drag.† Depending upon various parameters such as the Reynolds number, the surface roughness, or the presence of marine growth, etc., the drag coefficient for helical strakes typically varies from about 1.3 to 2.0 for most deepwater tubulars (due to the short aspect ratio and short fin height, the drag coefficients can be a little lower for spar platforms).† However, helical strakes can be very effective at reducing VIV for a wide range of applications and remain a popular choice.†
Fairings provide unrivaled protection against VIV forces by streamlining the flow of currents around a tubular, effectively dispersing the vortices that cause oscillating forces on its surface. Fairings are designed to rotate freely around the tubular and self-orient with the tail pointing downstream.
One key advantage of fairings is that they significantly reduce drag. Fairings are especially beneficial in high-current regions, or near the top of the water column where surface currents dominate.
Tail geometry can be customized to achieve maximum performance. Additionally, fairings are somewhat less susceptible to marine growth performance degradation than other types of suppression devices.
Thrust collars are installed between fairing bodies and serve as a bearing surface for fairing rotation.† They help the fairings maintain their axial position along the riser string. Specialized collars are used on tubulars with insulation to accommodate diameter changes caused by hydrostatic shrinkage.
Not all fairings are equal.† Fairing performance can be dependent upon the fairing chord (the distance from the fairing nose to the fairing tail), the fairing thickness, the Reynolds number, surface roughness, the shape of the fairing side walls, the shape of the fairing tail, the annulus between the fairing and the tubular, etc.
Because fairings streamline the flow similar to an airplane wing, effective fairings produce substantially lower drag than helical strakes.† Fairings are also less sensitive to soft marine growth on their surface than helical strakes, and fairings usually produce lower motions on downstream tubulars than helical strakes.† While fairing drag coefficient depends on the fairing geometry, the Reynolds number, and a few other variables, it is possible to design fairings with drag coefficients less than about 1.0 for a production riser and in the range of 0.6 or less for a drilling riser.
While fairings, like helical strakes, can experience substantially reduced effectiveness due to the presence of hard, barnacle type, marine growth, tests have shown that soft marine growth can be well tolerated by fairings with sometimes a negligible reduction in the fairing performance.† While it is possible to apply anti-fouling coatings, it is most important that the bearing surface between a fairing and adjacent fairings or thrust collars is kept relatively free of marine growth so that the fairing can properly weathervane.†
Fairings can be very effective at reducing VIV and produce additional damping for the tubular to which they are applied.† This damping can further reduce the motion of the underlying tubular.
The pattern and spacing of risers in an array often dictate which type of suppression device (helical strake or fairing) 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 operators must keep in mind for making the right suppression device selection.