Helical Strakes: Tried and True for Vortex-Induced Vibration Suppression
Vortex-Induced Vibration (VIV) can occur any time a long slender structure is exposed to a flowing fluid. Examples of structures subject to VIV include chimney stacks, cables, and car antennas that experience wind. VIV also commonly occurs in the ocean on pipelines, marine risers, tendons, jumpers, and umbilicals.
VIV is caused by vortex shedding. As a fluid flows past a structure, eddies known as vortices are formed on the structure's surface and quickly convected downstream. It can be easily observed by immersing a stick or other structure in a flowing stream and examining the swirling vortices that travel in the wake of the stick.
As the vortices form, they generate a low pressure region on the structure's surface. Once a vortex forms on one side of the structure, it grows until it interacts with fluid flowing around the opposite side of the structure which causes the vortex to separate and convect downstream. The process is then repeated on the opposite side of the structure, so that an alternating vortex shedding pattern is produced. In practice, patterns of vortex shedding other than the common alternating vortex pattern may be observed.
The alternating shedding of vortices causes alternating forces on the structures surface. The frequency of vortex shedding is often expressed by the Strouhal relationship, expressed as f = (V * S) / D, where V is the velocity of the fluid, D is the structure's diameter (or other characteristic width if the structure is not circular in cross section), and S is a proportionality constant known as the Strouhal number. While the Strouhal number is dependent upon a number of fluid flow parameters, a value of 0.2 is commonly used when estimating the frequency f. This frequency is known as the vortex shedding frequency and is identified with the vortex forcing frequency if the structure is stationary.
If the structure is sufficiently long and slender, or if the structure is supported by an elastic system, the vortex shedding frequency approach a natural frequency of the structure. For long slender tubulars, the natural frequency is usually associated with a bending mode, and thus the vortex shedding can cause bending vibrations of the structure. If the vibrations are sufficient in persistence and magnitude, then the structure may eventually fail due to fatigue.
In order to prevent fatigue failure, it is necessary to either re-design the structure or to place a device onto the structure to mitigate the vibration. Such devices are known as VIV suppression devices. The most common VIV suppression devices in use today are helical strakes and fairings.
While fairings consist of a structure having two sides that streamline the flow past the structure (such as an airplane wing), helical strakes consist of one or more fins that spiral along the structure's length. If the fins are effectively sized and designed, then the fins cause the vortices to break up into short lengths. This causes the vortices along the structure's length to be broken up into shorter and weaker segments. Further, the breaking up of the vortices reduces their ability to correlate along the span, resulting a series of vortices that are randomly phased in time. The net result is that, while the structure still experiences strong local forces, the randomness of the forcing frequency along the structure's length produces only a small net vibration of the structure.
Christopher Scruton and D. E. J. Walshe, working at the National Physics Laboratory in Great Britain, invented the helical strake and first published the results in 1957 (1). The helical strake was subsequently used to suppress VIV on a range of structures exposed to wind. For cylinders or tubulars in wind, an effective helical strake geometry consists of three fins (each spaced 120 degrees apart around the cylinder circumference) with each fin having a height of about 10 percent of the cylinder diameter (0.1D) and a pitch for each fin of approximately 5 times the cylinder diameter (5D). The fins are called "starts" and thus a helical strake with three starts is called a "triple-start helical strake." This geometry is still popularly used today to suppress VIV of structures in air.
While the 5D pitch, 0.1D height, triple-start helical strake system has worked well for many years in wind, this geometry did not gain popular use in water. In 1988 Don Allen and Dean Henning, working at Shell Oil Company, discovered that the optimal helical strake configuration for ocean structures was significantly different than that for structures in air [2]. Allen and Henning discovered that triple-start helical strakes with a height of 0.25D and a pitch of 12D-18D were substantially more effective in water than the shorter strakes with a tighter pitch that were popular for wind VIV suppression. This newer geometry has since received use on most all drilling and production oil and gas pipelines that employ helical strakes for VIV suppression. A second popular device for suppression of VIV for oil and gas tubulars, the short fairing, was also pioneered by Allen and Henning [2,3]. While longer fairings were moderately successful when used on offshore drilling risers in the late 1970s and early 1980s, Allen and Henning that discovered that short, and even ultra-short [4], fairings could be very effective at VIV suppression (fairings also have lower drag than helical strakes). Both short fairings and tall helical strakes should continue to be popular for many years.
References
1. C. Scruton and D.E.J. Walshe, A Means for Avoiding Wind-excited Oscillations of Structures with Circular Or Nearly Circular Cross-section, National Physics Laboratory (Great Britain), 1957.
2. Allen, D.W., An Experimental Evaluation of Vortex Suppression Devices, Technical Progress Report BRC 22-89, Shell Development Co., Bellaire Research Center, Houston, 1989 (now public).
3. Allen, D. W. and Henning, D. L., Small Fixed Teardrop Fairings for Vortex-Induced Vibration Suppression, U. S. Patent No. 5,410,979, May 2, 1995.
4. Allen, D. W., Henning, D. L., and Lee, L., Drilling Riser Fairing Tests at Prototype Reynolds Numbers, Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, 2007.
Copyright 2013. Content provided by VIV Solutions LLC.
VIV is caused by vortex shedding. As a fluid flows past a structure, eddies known as vortices are formed on the structure's surface and quickly convected downstream. It can be easily observed by immersing a stick or other structure in a flowing stream and examining the swirling vortices that travel in the wake of the stick.
As the vortices form, they generate a low pressure region on the structure's surface. Once a vortex forms on one side of the structure, it grows until it interacts with fluid flowing around the opposite side of the structure which causes the vortex to separate and convect downstream. The process is then repeated on the opposite side of the structure, so that an alternating vortex shedding pattern is produced. In practice, patterns of vortex shedding other than the common alternating vortex pattern may be observed.
The alternating shedding of vortices causes alternating forces on the structures surface. The frequency of vortex shedding is often expressed by the Strouhal relationship, expressed as f = (V * S) / D, where V is the velocity of the fluid, D is the structure's diameter (or other characteristic width if the structure is not circular in cross section), and S is a proportionality constant known as the Strouhal number. While the Strouhal number is dependent upon a number of fluid flow parameters, a value of 0.2 is commonly used when estimating the frequency f. This frequency is known as the vortex shedding frequency and is identified with the vortex forcing frequency if the structure is stationary.
If the structure is sufficiently long and slender, or if the structure is supported by an elastic system, the vortex shedding frequency approach a natural frequency of the structure. For long slender tubulars, the natural frequency is usually associated with a bending mode, and thus the vortex shedding can cause bending vibrations of the structure. If the vibrations are sufficient in persistence and magnitude, then the structure may eventually fail due to fatigue.
In order to prevent fatigue failure, it is necessary to either re-design the structure or to place a device onto the structure to mitigate the vibration. Such devices are known as VIV suppression devices. The most common VIV suppression devices in use today are helical strakes and fairings.
While fairings consist of a structure having two sides that streamline the flow past the structure (such as an airplane wing), helical strakes consist of one or more fins that spiral along the structure's length. If the fins are effectively sized and designed, then the fins cause the vortices to break up into short lengths. This causes the vortices along the structure's length to be broken up into shorter and weaker segments. Further, the breaking up of the vortices reduces their ability to correlate along the span, resulting a series of vortices that are randomly phased in time. The net result is that, while the structure still experiences strong local forces, the randomness of the forcing frequency along the structure's length produces only a small net vibration of the structure.
Christopher Scruton and D. E. J. Walshe, working at the National Physics Laboratory in Great Britain, invented the helical strake and first published the results in 1957 (1). The helical strake was subsequently used to suppress VIV on a range of structures exposed to wind. For cylinders or tubulars in wind, an effective helical strake geometry consists of three fins (each spaced 120 degrees apart around the cylinder circumference) with each fin having a height of about 10 percent of the cylinder diameter (0.1D) and a pitch for each fin of approximately 5 times the cylinder diameter (5D). The fins are called "starts" and thus a helical strake with three starts is called a "triple-start helical strake." This geometry is still popularly used today to suppress VIV of structures in air.
While the 5D pitch, 0.1D height, triple-start helical strake system has worked well for many years in wind, this geometry did not gain popular use in water. In 1988 Don Allen and Dean Henning, working at Shell Oil Company, discovered that the optimal helical strake configuration for ocean structures was significantly different than that for structures in air [2]. Allen and Henning discovered that triple-start helical strakes with a height of 0.25D and a pitch of 12D-18D were substantially more effective in water than the shorter strakes with a tighter pitch that were popular for wind VIV suppression. This newer geometry has since received use on most all drilling and production oil and gas pipelines that employ helical strakes for VIV suppression. A second popular device for suppression of VIV for oil and gas tubulars, the short fairing, was also pioneered by Allen and Henning [2,3]. While longer fairings were moderately successful when used on offshore drilling risers in the late 1970s and early 1980s, Allen and Henning that discovered that short, and even ultra-short [4], fairings could be very effective at VIV suppression (fairings also have lower drag than helical strakes). Both short fairings and tall helical strakes should continue to be popular for many years.
References
1. C. Scruton and D.E.J. Walshe, A Means for Avoiding Wind-excited Oscillations of Structures with Circular Or Nearly Circular Cross-section, National Physics Laboratory (Great Britain), 1957.
2. Allen, D.W., An Experimental Evaluation of Vortex Suppression Devices, Technical Progress Report BRC 22-89, Shell Development Co., Bellaire Research Center, Houston, 1989 (now public).
3. Allen, D. W. and Henning, D. L., Small Fixed Teardrop Fairings for Vortex-Induced Vibration Suppression, U. S. Patent No. 5,410,979, May 2, 1995.
4. Allen, D. W., Henning, D. L., and Lee, L., Drilling Riser Fairing Tests at Prototype Reynolds Numbers, Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, 2007.
Copyright 2013. Content provided by VIV Solutions LLC.