The upsurge of deepwater E&P activities during the last 20 years has caused operators to think smarter and prioritize complex matters to minimize financial, health, environmental and safety risks while simultaneously improving project economics and insuring asset integrity.
One important issue that impacts the outcome of all of these goals is vortex-induced vibration (VIV). While VIV was a well-known occurrence before this time, its importance has grown due to an increased probability of encountering large ocean currents and the difficulty associated with predicting and suppressing VIV in deepwater.
When a current flows past a cylindrical object such as a riser, tendon, jumper or horizontal pipeline span, it creates VIV. The friction against the cylinder’s 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 and form vortices. The vortices shed from the cylinder in an alternating pattern, thereby imposing alternating (oscillating) forces on the cylinder. For a long deepwater tubular, the frequency of this vortex shedding will be sufficiently close to one or more of the natural frequencies associated with bending, thereby causing VIV.
Tubulars experiencing VIV can eventually fail due to fatigue. To prevent substantial fatigue damage, it is usually beneficial to install VIV suppression devices over at least part of the tubular span to reduce the vibration amplitude and/or frequency. VIV suppression devices such as helical strakes and fairings can often minimize VIV if care is taken to select, design and install these devices during the project execution phase.
Strakes or fairings?
One popular solution for suppressing VIV of subsea tubulars is helical strakes. Protruding fins disrupt the correlation of vortex shedding along a tubular’s span, resulting in lower and randomly phased lift and drag forces. Strakes are a common candidate to solve VIV issues with regard to risers, tendons, jumpers and horizontal pipeline spans.
Strakes consist of one or more fins that are wound helically around a tubular. Traditionally, strakes were designed for wind applications and consisted of three starts (three fins, each 120 degrees apart around the tubular), a pitch per start of about five times the tubular diameter and a fin height of 10% of the diameter. Upon early testing of strakes for deepwater tubular applications, results found traditional wind parameters were inadequate and that the strakes performed best with a fin height closer to 25% and a pitch per start in the range of 12 to 20 times the tubular diameter. The configuration using three starts for deepwater operations was generally maintained.
Because helical strakes directly encounter the oncoming flow and often produce early separation of it, they are associated with higher drag. Depending upon various parameters such as the Reynolds number (a dimensionless quantity that relates the inertial forces to the diffusion forces of the flow), the surface roughness or the presence of marine growth, the drag coefficient for strakes typically varies from about 1.3 to 2.0 for most deepwater tubulars.
In comparison, 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 and causing them to shed farther downstream. 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 also can be customized to achieve maximum performance. Fairings also are less susceptible to marine growth performance degradation than other types of suppression devices such as strakes.
Assisting the performance of fairings are thrust collars, which are installed between fairing bodies to serve as a bearing surface for fairing rotation. These collars also help the fairings maintain their axial position along the riser string, and specialized collars can be used on tubulars with insulation to accommodate diameter changes caused by hydrostatic shrinkage.
However, not all fairings are equal. As with strakes, fairing performance is dependent upon a number of factors including the fairing chord (the distance from the fairing nose to the fairing tail), the fairing thickness, Reynolds number, surface roughness, the shape of the fairing side walls, fairing tail thickness and shape, the annulus between the fairing and the tubular, etc. It is prudent to obtain proper consultation on fairing design to insure that performance of the fairing system is optimized.
Because fairings streamline the flow similar to an airplane wing, effective fairings produce substantially lower drag than helical strakes. Fairings are also a little less sensitive to soft marine growth on their surface than strakes and usually produce lower motions on downstream tubulars than other VIV suppression devices. While fairing drag coefficients depend on various factors, 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.
Fairings, like helical strakes, can experience substantially reduced effectiveness due to the presence of hard, barnacle-type marine growth. However, tests have shown that soft marine growth can be well tolerated by fairings with a sometimes negligible reduction in the fairing’s VIV suppression performance. While it is possible to apply antifouling 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 with the flow.
Suppressing for success
Sailing in the seas off the Kii Peninsula and engaged in research, the R/V Chikyu, a Japanese scientific drilling ship built for the Integrated Ocean Drilling Program, faced a major problem requiring resolution before riser drilling could succeed: how to handle the effects of VIV resulting from the Kuroshio Current, which can flow at 3 knots (1.5 m/sec or 5 ft/sec) or more, as noted in the March 2010 issue of Chikyu Hakken–Earth Discovery. When 1.2-m (4-ft) diameter riser pipes were lowered from the Chikyu and deployed to the seafloor drilling site, the riser would shake due to VIV. The solution was to use VIV Solutions Tail Fairings, a modular type of suppression device consisting of a pointed tail and two straps, to help channel the flow of water around the riser and reduce vortex shedding.
Lowered about 305 m (1,000 ft) below the surface, the riser pipe, equipped with 132 fairings, showed almost no vibration despite currents reaching anticipated speeds well in excess of 3 knots. Results were confirmed by sensors placed on the fairings as well as by data collected from an analysis system that recorded the movements of the riser pipe.
This application is just one of many examples showcasing the power of VIV suppression devices to mitigate devastating downtime during drilling operations and prevent fatigue damage of critical operating equipment. Both strakes and fairings can effectively reduce VIV; however, various considerations such as the presence of marine growth, the need for low drag to prevent contact of adjacent tubulars and the potential of vortex shedding affecting downstream risers are all important to evaluate when designing and procuring VIV suppression devices.