What is VIV?
Vortex-induced vibration (VIV) is produced by the flow of ocean currents past long slender tubulars such as production risers, steel catenary risers, drilling risers, tendons, pipelines, jumpers, and cables. 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.
The separated boundary layers, commonly called “shear layers,” roll up into vortices due to differences in pressure near the tubular’s surface. Vortices grow until they entrain the shear layer from the opposite side of the tubular at which time they shed from the tubular’s surface. The process then repeats itself on the opposite side of the tubular’s surface. Thus, vortices shed from alternating sides of the tubular and create a phenomenon known as vortex shedding.
Vortex shedding produces oscillating forces on the tubular surface primarily due to the low pressure in the growing vortex while it is attached to the tubular surface. The resulting oscillating forces can cause the tubular to experience an oscillatory motion. This motion is generally known as VIV, though VIV is usually most pronounced when the vortex shedding excites a structural natural frequency of the tubular. When VIV is initiated the vortices will grow in strength as they correlate along the tubular span which further increases the tubular vibration. Fortunately, this process is self-limiting but can produce substantial vibration that can cause early failure of the tubular by fatigue.
What are the differences between helical strakes and fairings?
Helical strakes consist of one or more fins that are spiraled around a tubular. They break up the correlation of vortices along the tubular and produce small, random, alternating forces along the tubular’s length. The net effect is a very small motion of the tubular hence VIV is effectively suppressed. Strakes are normally fixed in place on a tubular and do not rotate.
Fairings are devices that surround a tubular and streamline the flow so that only small vortices are shed at the fairing tail area. They are usually free to rotate with changes in the oncoming current direction. Effective fairings both suppress VIV and produce substantially lower drag than do helical strakes. They also introduce hydrodynamic damping to the system which can further reduce dynamic motions both from VIV and from other sources such as waves or platform motions.
How much coverage do I need?
Suppression efficiency improves as the coverage density increases; however, it is nearly impossible to completely cover a tubular with strakes or fairings. Allowances must be made for bolster gaps, lifting points, anode bracelets, and other appurtenances.
At VIV Solutions, we can help provide recommendations for optimizing suppression performance. We analyze historical site conditions and current speed profiles for your project to best determine the type and quantity of suppression devices needed.
For a detailed discussion of coverage length and density considerations, please visit the Technical Notes page of our site.
Do you offer VIV analysis services?
Yes, VIV Solutions’ staff often perform VIV analyses for our customers. Results are typically used to predict long-term fatigue over the life of the offshore system. We can even assist with suppression layout optimization, saving valuable time and money on large-scale projects. Visit the Analysis page to learn more.
Can a strake or fairing design be customized?
VIV Solutions has a longstanding history of solving unique VIV challenges, which is why clients often turn to us for help. We can customize our suppression products to address a variety of technical objectives. This differentiates us from competitors who typically only manufacture off-the-shelf products. Some of our previous successes include:
- Helical strakes for heated jumpers with high external temperatures (145 deg C)
- Low drag fairings (Cd ~0.3-0.35) with a long chord that are stable (do not flutter)
- Strakes or fairings with improved cathodic protection and heat transfer properties
- Thrust collars containing integral hydrostatic shrinkage springs for s-lay installation
- Fairings systems with centralizers to eliminate friction with underlying pipe coatings or tubulars
- Captive collars for drilling risers with external control lines running parallel to the buoyancy
We are eager to tackle your next suppression challenge.
How do I mitigate marine growth?
Anti-fouling coatings can be applied to the surface of suppression devices to deter marine organisms from attaching. These coatings are specifically designed for use subsea and are frequently used to reduce friction along the hulls of maritime vessels. VIV Solutions has developed its own unique coating protocols to ensure the coating forms a strong mechanical bond to the underlying substrate and does not chip or flake off. We offer silicone- and copper-based products, as well as other lesser known blends.
Longevity of anti-fouling coatings varies from manufacturer to manufacturer. Marine organisms can differ greatly from one region of the world to another, making it difficult to know with certainty whether a particular coating will be effective at deterring growth. For this reason, we recommend weighing the cost of anti-fouling coatings against the perceived benefits of delaying the initial cleaning operation.
A valuable strategy for removing marine growth is to periodically inspect and clean the suppression with a scrubber tool or water blaster. These activities are typically performed by a remotely operated vehicle (ROV). VIV Solutions can provide consulting to help determine when a tubular’s VIV suppression devices need to be cleaned.
To read more about how marine growth affects suppression performance, please download and read VIV Solutions’ technical notes available on our website.
What is a hybrid system?
Hybrid systems consist of both helical strakes and fairings together on a single tubular. Most commonly, fairings are installed at the top of the water column where surface currents dominate. They offer lower drag and are less susceptible to performance degradation from marine growth. Strakes are installed below the fairings, sometimes all the way to the touchdown region on the seafloor. Strakes provide the same amount of VIV suppression reduction but at a lower price point, hence the reason they may be preferred for extended coverage lengths.
To read more about hybrid suppression systems, please download and read VIV Solutions’ technical note available on our website.
Can suppression devices be retrofitted?
Both helical strakes and fairings can be installed onto a subsea tubular after it is already in the water. A retrofit scenario is common for site locations with previously undetected currents or pipeline spans with shifting seabed profiles.
A subsea ROV performs the installation operations. Specialty hardware is added the suppression devices which allows the ROV to handle them underwater and fasten them onto the tubular.
Since the cost of retrofitting can be quite high due to the resources required for offshore installation, we encourage clients to consider purchasing additional pre-installed suppression. For example, it may be more cost effective to initially cover a longer length of pipeline with helical strakes so that shifting freespan locations are always suppressed as opposed to going back at a later date with an ROV to retrofit changing span locations.
VIV Solutions’ team members were the first to design and develop a retrofit method for suppression devices in the early 1990’s. Our strakes and fairings have been ROV-installed on many deepwater top-tension risers, production risers, and tendons in the Gulf of Mexico. If you have questions about retrofit suppression technologies, please reach out to us so that we can best meet your project needs.
How do fairings decrease rig downtime?
Fairings allow deepwater rigs to continue drilling in high current speeds when other operators may be shut down. They streamline the flow of water around the riser which reduces drag on the entire string and significantly improves both top and bottom angles. Fairings also suppress motions caused by oscillating vorticity fields. This extends the life of the riser over time while also minimizing wellhead fatigue.
Drilling without fairings can be precarious, especially when unexpected conditions arise (for example, the Loop Current Eddy). VIV analysis results may even mandate the use of fairings in regions where currents are extremely high, such as the Cape of South Africa and near the mouth of the Amazon River.
We recommend utilizing fairings whenever possible.
Are alternative buoyancy methods effective?
It is becoming increasingly popular to use specialty buoyancy modules with unique surface profiles when drilling deepwater wells; however, our experience indicates that they actually do very little to reduce drag and VIV on the riser string. Grooves, ridges, or bumps along a module’s exterior surface are designed to disrupt the flow of oncoming ocean currents and introduce early separation of the boundary layers. In many cases, the increased correlation length will produce more VIV than a bare riser.
Laboratory test results for alternate buoyancy products are frequently misleading and measure much lower drag than what would be experienced in the field. Sometimes these tests are conducted at low Reynolds numbers which do not mirror high Reynolds number field conditions offshore. For high Reynolds number testing, it is common to test very short buoyancy sections on rigidly-mounted pipes. A short rigid setup artificially limits the amount of VIV the system can achieve because vortices are not able to correlate along the cylinder’s span since only a short amount of span is even present in the test.
Drilling riser fairings offer consistently lower drag than alternate buoyancy products and have been the preferred suppression method for more than twenty (20) years. Fatigue life can be improved by a factor of 1,000-10,000, which is far superior to any other designs on the market.
To read more about our investigation of alternate buoyancy methods, please download and read VIV Solutions’ technical note available on our website.
How long does it take to install fairings offshore?
Previous generations of drilling riser fairings were cumbersome to handle and very slow to install. Many were manufactured from heavy fiberglass and fitted with expensive fastening hardware. The first generation of tail fairings utilized pockets which were difficult for inserting the ends of the straps into. While these tail fairings were improved relative to earlier full fairing designs, they were often still lower than they could have been.
VIV Solutions’ saw this as an opportunity for improvement and subsequently developed the Generation II Tail Fairing. Our new fairing utilizes lightweight materials, quick-turn fasteners, and significantly faster installation speeds. Each piece can be easily lifted to the riser by a single person.
Upon completion of the Generation II Tail Fairing development, we observed that a significant portion of installation time was comprised of installing the thrust collars. Previously, tail fairings required one fairing per collar to keep the tails from rotating above the strap of an adjacent fairing. Our new patented FlexLock™ technology allows for multiple fairings to be stacked between thrust collars, which eliminates well over half of the collars of a system. FlexLock technology produces a further decrease in installation time.
A well-trained crew with good moonpool access can install a joint of FlexLock Tail Fairings in about 45 minutes or less.
Working with VIV Solutions
Can suppression be purchased from existing inventory?
It depends upon the type of products you are seeking. Please inquire for additional details.
What sort of lead times are required?
Production rates vary from project to project. We can often reduce the time to delivery by using existing manufacturing molds or expediting shipping to the final destination.
Are quality and safety programs compliant with industry standards?
Yes, we maintain a quality system that is compliant with ISO 9001:2015 and strive for 100% customer satisfaction on all projects. Our safety policy aligns with OSHA guidelines. The company has had zero lost time incidents since its inception in 2010. We work closely with our vendors to ensure these same standards are applied during manufacturing and delivery. Every VIV Solutions employee completes annual training requirements related to quality, safety, environmental and social responsibility, anti-bribery and anti-corruption, and other job-specific duties.
Do you have a corporate profile in common vendor management systems?
VIV Solutions takes pride in its internal processes and procedures. Our customers appreciate the transparency with which we do business. As such, we maintain active up-to-date profiles in the following supplier management systems. We invite you to connect with us online through these third-party platforms and learn more about our organization. If your company manages its own Approved Vendor List, we will gladly complete any necessary qualification paperwork.
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