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How vortex shedding can indirectly sink your mooring

Heavy steel train wheels are built to take a lot of abuse, but they struggle to handle floppy, wet leaves. In many places in autumn, leaves fall from trees and build up on the railway tracks. At the start, this isn’t much of a problem, though. The heavy train wheels will pulverize the leaves, leaving a layer of mush. But this is when the problems begin. Over time, this mush breaks down into a black, slippery sludge. You’d think the heavy weight of the train and steel wheels would cut through the sludge, but it doesn’t work like that.

The slippery gunk causes enough of a difference in friction that the railway wheels can slip. This causes two new problems. One, when the wheels slip, there’s a localized force on one part of the wheel, which starts to make a flat spot. And two, if the wheels slip too much, an automated safety system can trigger a full stop. It’s slamming on the brakes that causes the most wear and makes even more of a flat spot on the wheels. This kind of accelerated wear is a serious failure. It causes a lot of delays when it keeps happening. But even worse, too much wear means the train needs a costly intervention to be pulled out of commission to replace wheels. Something as flimsy as wet leaves have huge indirect impacts.

Indirect impacts are often a cause of design failures. In some cases, there can be small effects that build up into major headaches. In the world of mooring design, vortex-induced vibration (VIV) can cause a lot of problems through indirect impacts. VIV, or line strumming, can cause issues with accelerated wear. But there’s other effects that can be even worse, requiring an intervention if the mooring goes out of commission. In this article, we’re going to explore what VIV is and what kinds of impacts it can have on an oceanographic mooring.

What is vortex-induced vibration?

Vortex-induced vibration is a complex phenomenon that can appear on structures in a steady flow. The first stage of this effect is an oscillating vortex pattern. A vortex, or swirl of water, forms and sheds off alternating sides of the structure in time. This vortex shedding effect is highly predictable and has been observed for hundreds of years. It shows up at many different physical scales and fluids – in wind blowing past islands, buildings, or smoke stacks, and in currents flowing past pilings, hulls – and mooring lines.

Very generally, the rate of vortex shedding tends to increase with the flow speed past the structure. The vibration comes into play based on the characteristics of the structure. For example, vortex shedding on an island in the ocean isn’t going to cause vibration! On the other hand, something like an electrical power line in wind or an oceanographic mooring in current might start to move back and forth at the same rate as the changing pressure field from the vortex pattern. The resulting motion, particularly in mooring lines, is called vortex-induced vibration, or VIV.

VIV can cause all sorts of headaches on structures like moorings.

Cyclical loading of any kind is hard on a system. The more vibration in the line, the more abrasion between components. This accelerates wear and directly degrades the strength rating of components. If there’s enough wear, it can trigger a failure when the system is under significant load in a heavy current or wave condition at a future time. Yet accelerated wear is only one problem. Other indirect impacts from VIV can trigger failures in the system.

VIV can also amplify the drag loading on an oceanographic mooring

To understand why this happens, it helps to understand some of the characteristics of the vibration. Namely, it is self-limiting. It tends to reach a maximum vibration amplitude of about one diameter. A vibration larger than this disrupts the vortex shedding pattern, and the motion automatically decreases. So, in the worst-case scenario, a vigorously vibrating line may interact with the steady flow through a motion envelope of about three line diameters.

This greater interaction with the flow is linked to the amplification of drag. The drag amplification from vibration is seen on lines, pipes, and moorings. It is seen in experimental lab tests and the field with amplification as high as a factor of three on the normal drag on the system without any VIV. But what does this amplified drag mean for oceanographic moorings?

VIV drag amplification can push components much deeper into the water

The amplified drag will cause larger mooring deflections, especially for subsurface mooring systems. Many components like sensors and flotation have depth ratings, which, when exceeded, can cause catastrophic failures. Data loss is possible. In addition, compromised flotation can mean the mooring may collapse to the seafloor, significantly complicating recovery.

When does VIV happen?

Though many of the characteristics of vortex shedding and VIV are well-known, predicting a line’s vibration response and hydrodynamic characteristics is challenging. However, VIV only sometimes happens. So, what are the conditions in which this vibration is possible?

The rate of vortex shedding needs to line up with the rate the mooring line wants to vibrate

The rate of vortex shedding is related to the line diameter and also the bulk flow speed. Generally, vortex shedding frequency goes up with flow speed. The frequency also goes up as the line diameter gets smaller. On the other hand, lines vibrate largely in proportion to their tension – much like a guitar string! The rate at which the line wants to vibrate is also called the natural frequency. If the vortex shedding frequency gets close to the line vibration frequency based on the local line tension, there’s a good chance there will be VIV. But you still need the flow speeds in the proper range to get VIV.

VIV is less likely in extremely high flow speeds

Moorings in very high flow speeds seem to reach conditions that are too fast, or too turbulent, for a regular vortex shedding pattern to appear. Also, higher flow speeds often mean more significant tilt of mooring lines, and more deflection. The angle the line makes in the flow can naturally disrupt vortex shedding as the effect works best when the line is vertical in the water column. But high speeds aren’t the only natural limiting factor for VIV.

Vortex shedding is too weak at lower flow speeds to cause any significant vibration

Remember that vortex shedding rate is proportional to flow speed. So, at much lower flow speeds, even if the frequencies line up and there is VIV, the resulting motion is too slow to cause any significant interaction with the bulk flow past the mooring. This means it is less likely to amplify drag in these conditions. While these are both examples of natural limits of drag amplification, you also have design options to mitigate the effect.

You can mitigate VIV by disrupting the vortex shedding effect from the start

Commercial products like hairy fairings or helical jackets are designed to disrupt the vortex shedding pattern on the mooring line. This breaks up the vortex shedding pattern and prevents it from triggering a vibration in the line. Other commercial products like clip-on fairings will reduce or eliminate vortex shedding by streamlining the mooring line interaction with bulk flow.

Another way to mitigate VIV is by adjusting the mooring line properties

This is harder to do than using the add-ons mentioned above. But generally, lower line tension will reduce the vibration intensity in the system and reduce the chances of significant impacts like drag amplification from VIV. But be careful with this because line tension resists the nominal mooring deflection from drag in the first place.

What about vortex shedding on other components?

The shape complexity of many components, such as instrument frames, chains, and connectors, means that vortex shedding is not coherent and won’t cause any regular vibration. However, floats may cause significant problems. The rate of vortex shedding on floats is proportional to the float diameter and may trigger a different vibration effect on the mooring. However, the more streamlined the float, the weaker the vortex shedding effect. Much like the effect of a clip-on fairing on a mooring, using a streamlined float may help you avoid headaches from this effect.

It’s example time

We covered a few aspects of how VIV can affect moorings, and now it’s time to look at a few examples. These examples are part of a much more in-depth R&D program we’ve been working on that’s produced new functionality in ProteusDS Design Checks.

Example 1 – mid-atlantic subsurface mooring

The first example is mooring M480C deployed by the Rosenstiel School of Marine and Atmospheric Science (RSMAS). This subsurface mooring was deployed in 5000m water depth. Sensors along the span of the mooring measured the current profile and local pressure. The field data from the recovered mooring showed substantial mooring deflections. Yet, mooring deflection is expected, and generally more with increasing current speeds. But what was notable was that in some particular current profiles, the vertical knockdown mooring deflection was hundreds of meters greater than that predicted by standard mooring analysis. One explanation for this difference is drag amplification on the mooring from VIV.

This ProteusDS Design Check plot shows the risk of drag amplification on the mooring line from VIV. This scenario corresponds to a current profile measured at the same point in time as one of the maximum mooring vertical deflections. When a new ProteusDS mooring analysis is completed with the amplified drag coefficients, the difference between the field measurements of mooring deflection decreases dramatically to less than 80 meters. This mooring was recovered safely and intact, but that isn’t always true. Now, let’s look at another example.

Example 2 – South Atlantic subsurface mooring

The second example is an Ocean Observatories Initiative (OOI) subsurface mooring. This mooring was designed and deployed with Woods Hole Oceanographic Institution (WHOI), and was part of an array measuring mesoscale eddies in the Argentine Basin. The mooring was recovered, although the top float was significantly damaged. DSA, WHOI, and OOI staff teamed up to investigate possible reasons for the failure. Field measurements of the upper water column current profile showed several extreme events, including severe mesoscale eddy events before the failure of the float. The ProteusDS Design Check plot using the available current data shows a significant risk of drag amplification on the mooring during these extreme current events. Re-evaluating the mooring with a worst-case possible mooring drag amplification shows the top float reaching a depth value far in excess of its depth rating. The drag amplification from VIV in combination with the extreme current events gives a plausible explanation for the damage to the float.

It’s summary time

We covered a few concepts on vortex-induced vibration impacts on oceanographic moorings, and it’s time to review. It’s common for blunt structures to generate a regular vortex shedding flow pattern in steady flow conditions. These swirls of water, or vortices, cause an oscillating pressure field around the structure, and it can cause a vibration in the structure under the right conditions. Oceanographic moorings may vibrate when the vortex shedding frequency is close to the line’s natural frequency, primarily governed by local tension. This can accelerate wear on components and cause premature failure through a reduction in component strength and fatigue life. It may also amplify the drag on the mooring and cause significantly larger-than-expected deflections and failure of components that exceed depth ratings. It’s not all bad news, though, as mitigation is possible through design and use of add-on components that disrupt the vortex shedding pattern.

Careful planning can help mitigate indirect impacts from VIV. As it turns out, careful planning is also the solution to the problem of leaf sludge on train tracks. In some countries, a special train equipped with high powered lasers makes a daily pass around the tracks, blasting leaf sludge off them. And it gets good results, too, with a significant reduction in wear and replacement of train wheels!

Next step

See data from the National Science Foundation-funded OOI portal from the many moorings deployed around the ocean here.

Thanks

Thanks to Don Peters and John Kemp from Woods Hole Oceanographic Institution, and John Lund and Andrew Reed from the Ocean Observatories Initiative for sharing data and collaborating on the investigation of the subsurface mooring. Thanks also to Bill Johns at the Rosenstiel School of Marine, Atmospheric, and Earth Sciences for collaborating and sharing field data from the M480C mooring deployment.