Tag: buoy

Why drag loads are underwhelming at damping floating system motion

Posted on July 18, 2022
Ryan Nicoll

Snow is rare in my Canadian city. This might surprise you, given Canada’s reputation as a northern and wintry country, but I am lucky to live near the coast. The marine climate helps keep the weather surprisingly mild. Despite this, I do own a snow shovel. It’s a big one, too. It has a wide scoop, all the better to clear a large swath along the path on our driveway. I expected it to do well and help quickly clean up any snow that came along.

But that expectation threw me off on our last rare snowfall. The problem with snow in a marine climate is that it often comes down super wet and thick. Because wet snow is so dense, a massive scoopful in my large shovel was too heavy to lift. Piling up a giant heavy glob of dripping snow almost broke my shovel and my back at the same time! I had big expectations for my large shovel, but in wet, heavy snow, it was underwhelming.

An underwhelming response can throw anyone off. It can cause nasty surprises, especially if there are big expectations. In the world of hydrodynamics, there are often big expectations with drag loads. Any analyst should be wary of drag forces – after all, they are very sensitive to current speed and can become a dominating effect. But there are some circumstances when they fall flat and have little impact on ships and buoys. But first, it helps to understand why drag loads can be so powerful in the first place.

In wet, heavy snow, larger shovels are underwhelming

Why can drag loads be overwhelming?

Viscous drag forces appear when there is relative flow past a structure. Whether the structure is moving, the flow is moving, or a combination of both, you’ll see drag loads. The key is that there needs to be some relative motion involved. Generally, the greater this relative motion, the greater the magnitude of forces involved.

One portion of drag on a hull is a skin friction effect, and while important in some ways, it tends to be a much smaller effect in general. The other critical portion of drag is a pressure differential caused by eddies and wake structures that form in the lee of the relative flow behind the hull. Eddies and wakes are regions of low pressure in the flow field, and the resulting imbalance in pressure on the hull causes the drag force. But when are these effects overwhelming?

Drag forces from current can be overwhelming

The drag on the side of a ship hull or buoy in a current can be so significant as to dominate all other effects, overpowering a ship or destroying a mooring system. When a hull and flow have a large relative velocity, the pressure differential from these wake structures grows larger.

These forces increase rapidly with the square of relative flow speed. But it doesn’t mean they are always significant just because they are so sensitive. Viscous forces can also be weak in certain circumstances, and in particular, this can happen in oscillation.

Drag forces are underwhelming at damping oscillation

Oscillation, or any cyclical back-and-forth motion, is common for any hull in the marine environment. For example, a ship or a buoy will bob up and down vertically in heave. You see this in the tilt degrees of freedom of pitch and roll, too. So why does oscillation mean underwhelming drag forces?

Buoys or ship hulls, like the HMCS Halifax, have lots of drag. But drag from steady effects like currents or forward speed is different from viscous drag loads in oscillation.

A fully developed wake structure doesn’t appear instantly: it takes time for the low-pressure region to form behind the hull. Only once it is fully formed is the pressure differential greatest. This means there will be a time lag for the drag forces to come up to full strength.

But when a hull oscillates back and forth, it’s also constantly changing the flow structure back and forth. There’s almost no chance for a wake to form on either side of the system as the flow regularly reverses one way and the other. In the short term, fluid flows over the contours of the hull, causing some skin friction drag but without forming these larger low-pressure regions, preventing significant drag forces from forming.

The key is that in any kind of back-and-forth motion, the relative flow speed hovers around zero, as you would expect when you see a buoy or ship roll back and forth. The flow speeds can be high at a moment in time if the motion is violent, but the resulting drag loads are underwhelming because the flow structures haven’t had time to form and create large pressure differentials.

The Generic Frigate hull head on. The yellow portion is the wetted hull in this displacement. In a lateral ocean current, a steady wake forms in the lee of the hull and drag would be enormous. But drag during side-to-side sway oscillations, it is less likely these pressure regions can form and the effects of drag can be underwhelming.

But aren’t there other more significant hull damping forces?

Drag and viscous effects are not the only sources of damping in motion oscillations. Wave radiation can be substantial in certain conditions. Ship motion in pitch is usually controlled with damping by a totally different force: wave radiation. Smaller structures like buoys can some wave radiation effects, but it’s not as significant. Regardless, buoys often use a mooring that helps to control motion to some degree.

Sometimes drag is the only source of motion damping

This is a particular issue in specific buoy systems in pitch and roll and spar hulls in heave. It’s also a common problem in ship roll. These are only a few particular cases. Generally, it’s best if you are careful to understand both how your real system will work and how that reflects in your analysis of the floating hull dynamics.

Let’s look at an example

Here are two motion decay tests of the Generic Frigate computed by ShipMo3D. These are time-domain simulations in calm conditions with an initial deflection. The first is a decay test with the Generic Frigate starting at 10 degrees pitch. In this case, wave radiation dominates the damped system response, and after about 20 seconds, the motion is entirely damped.

Generic Frigate pitch decay test. Wave radiation effects are substantial and create a strong damping effect that stops pitch motion after about 20 seconds.

The second scenario is a decay test with the Generic Frigate starting at 10 degrees roll. The big difference from pitch motion is that there’s no significant wave radiation in roll because of the slender hull, and this means the damping effects are essentially completely from viscous drag loads. The decay response is far different than in pitch: after 100 seconds, it’s still going!

Generic Frigate roll decay test in calm seas. In roll, motion damping is almost completely from viscous drag effects. The system is still oscillating after 100 seconds and will likely keep going for a while!

In summary

Drag loads can be devastating when there are high relative flow speeds. Low-pressure wake structures cause a large differential pressure on the hull that we think of as the drag force. But when a floating hull oscillates, these wake structures don’t often have time to develop fully. Even if the oscillation velocity can be high, it’s only for a moment in time before the system reverses backward. It can mean drag loads become vanishingly small and ineffective at damping oscillatory motion. The result can be underwhelming – much like my disappointing giant snow shovel. By the way, my son’s toy snow shovel ended up working much better at clearing my driveway of heavy snow. Because it had a small scoop, it was far easier to quickly scoop and heave loads of snow out of the way, and it took far less time to get the job done!

Next step

In this article, we saw how viscous drag forces fizzle in oscillation motion – particularly in rotation.  In the next article, we focus on roll motion and see why it’s such an important topic in ship design. Read more on why the sensitivity of roll damping makes naval architects sweat here.

 

Why it’s hard to distinguish between the effects of buoy and line drag on mooring knockdown

Posted on June 14, 2022
Ryan Nicoll

The southern border between the Canadian provinces of British Columbia and Alberta is, in places, very irregular. It might even seem to be random at first glance. Yet, there is some logical reasoning behind what is there. This border was measured and established over one hundred years ago – far before GPS or other advanced survey techniques were around to help out. So how was the border established?

It has to do with hydrology. In other words, at the highest peak, the direction of rainfall flow establishes the border. Where rainfall flows to the west, it’s British Columbia, and when it flows to the east, it’s Alberta. It’s this clear reasoning that helps to distinguish things.

But we’re not always so lucky to have something to help distinguish things. At first glance, you might be looking at a seemingly random result without any clue how you landed there. When it comes to oceanographic mooring deflection, there are two primary sources of drag: the buoy and the mooring. However, distinguishing between what is more critical to mooring deflection can be a real challenge.

The southern portion of the border between BC and Alberta highlighted in red is highly irregular, but it’s not random. The direction of the flow of rainfall is used to define the border and distinguish between provinces. Picture credit: Google Maps

The vertical deflection of a mooring is crucial to understand

This vertical deflection, also called knockdown, is an increase in depth of all the mooring components from steady drag loads. Because it’s an increase in depth, you need to watch any parts approaching their depth rating. Flotation that exceeds depth rating may fail and result in the mooring collapsing to the seabed. But knockdown may have detrimental effects on instrumentation data quality, too. But regardless, you can’t get a good idea of mooring knockdown without considering the drag loads on the system.

Why is drag significant in calculating knockdown?

Without any currents and the resulting drag load, subsurface moorings are perfectly vertical in the water column. Knockdown results when a subsurface mooring leans over from the effects of steady loads like current drag. All components on the mooring will have some drag. But often, there are two primary sources of drag that tend to dominate mooring deflection and knockdown. The first is the drag on the primary buoy.

The primary buoy is a significant source of drag for a few reasons

The primary buoy is at or near the top of the mooring and often does the bulk of the work holding everything up in the water column. This means it tends to be relatively large compared to other components. On top of this, generally, but not always, currents tend to be larger higher up in the water column. This means the primary buoy is likely in higher flows than the rest of the mooring. Drag is proportional to structure size and flow speed, which explains why the primary buoy is a significant source of total drag on the mooring. But it’s not the only source to consider. The other source of drag to consider is the mooring line itself.

The NOAA CO-OPS DWEAB mooring uses a low-profile Mooring Systems Inc ellipsoid buoy. Picture credit: Laura Fiorentino

At first glance, a mooring line may seem minuscule compared to the primary buoy

Often the diameter of the mooring wire is only a tiny fraction of the size of the primary buoy. But the problem is that the drag area of the mooring is a function of both the diameter and the mooring length. When you consider the entire length of the mooring, the drag area can often be comparable to the top float. There is often at least some kind of current profile through the water column, and this means there can be a substantial amount of drag that accumulates over the whole system.

However, there are more details to the drag on the mooring than the drag area. The actual current profile is vital to get right. The inclination angle the mooring makes in the flow is a factor, too. The effect of inclination on drag is not something you can estimate as quickly by hand as by comparing drag areas. But suffice to say, it’s important enough that you shouldn’t ignore it, and it may even surprise you how important it can be.

How are these effects calculated?

Mooring deflection is typically calculated using well-established numerical techniques that factor in all mooring components, dimensions, and forces in a specific current profile. Purpose-built software like ProteusDS Oceanographic uses geometry, weight and buoyancy of the parts, and typical drag parameters to resolve the mooring deflection. The mooring deflection is a calculation of all these effects together.

So what’s more important: mooring or buoy drag?

It’s not easy to know, in general, what is more important. But we can use a validated example to show mooring and buoy drag contributions. NOAA CO-OPS uses the Deep Water Elliptical ADCP Buoy (DWEAB) design as a standardized design to measure the top portion of the current profile in various locations in the coastal United States. In a recent deployment in a high current region of the Gulf Stream, mooring knockdown field measurements show 48m in 2m/s flow. Mooring knockdown calculated by ProteusDS Oceanographic reached 52m in a 2m/s flow, reaching reasonably close to the field data results. The primary buoy is a large 58″ Mooring Systems Inc Elliptical float, while the mooring is only a 1/4″ in diameter. At first glance, it may seem like the primary buoy would dominate the mooring deflection because of its larger drag area. So how can we get an idea of how much buoy and mooring drag contribute to the knockdown?

To conceptually illustrate this, we looked at two additional load cases. Mooring deflection was calculated with drag on the primary buoy turned off and then again with the drag on the mooring line turned off.

The resulting numerically calculated knockdown with only primary buoy drag was 25m, while the knockdown with only mooring drag was 20m. Note that the total expected knockdown is not calculated by adding these deflections together. This particular example aims to give a rough idea about their contributions to the mooring knockdown for this system. In this specific example, the mooring drag is almost as significant as the buoy drag.

The 200m long DWEAB mooring deflection to scale in 325m water depth in 2m/s current

What if you don’t know the current profile ahead of a deployment?

A knockdown calculation is only as good as the information you have. Of course, before deployment, you may not have much information about the water flow in a region. In this case, it helps to think more about the boundaries of the problem rather than finding the most accurate or probably current profile. The most conservative approach is a uniform current profile. But that may only be appropriate in areas of intense flow. A linear shear, or power profile, may be more realistic. Still, it’s up to the designer to use their judgement and experience for what works best.

Summary

Mooring knockdown is crucial to understand ahead of deployment. This deflection can compromise data quality and damage equipment if depth ratings are exceeded. Knockdown happens in subsurface moorings as it leans over from steady current drag. The primary buoy is often a sizable source of drag in the system. But it’s a mistake to ignore the drag from the mooring line because it can make a surprising contribution to deflection. Unlike considering rainfall to determine a border between two Canadian provinces, there isn’t a way to easily distinguish the contribution of deflection from the primary buoy and mooring. Instead, the best approach is to make sure your analysis considers drag from all components at some point in the design process.

Next step

As we saw with the DWEAB mooring in the high flow Gulf Stream current, drag loads can be a dominating, even overwhelming, force. But there are times when drag doesn’t do much at all to floating systems, even if there is a lot of motion. Read the next article here about why viscous drag forces can be underwhelming at damping floating system motion.

Thanks

Thanks to Laura Fiorentino and Robert Heitsenrether from NOAA CO-OPS for providing data and pictures for the DWEAB mooring deployment used in the example.

PS

Download and explore the ProteusDS Oceanographic DWEAB 325m mooring file in the collection of designs available for download here.

When you need to fine tune surface buoy damping (and when you don’t)

Posted on May 9, 2022
Ryan Nicoll

Professional athletes make artistic synchronized swimming look effortless. Yet it is an incredibly demanding sport: it requires tremendous muscle power, endurance, and control to tread water and perform an elaborate dance simultaneously. On top of that, swimmers need to simultaneously keep in perfect sequence with a group in the water. So how do they keep synchronized while many of them are entirely underwater?

It’s essential to use special underwater speakers. The swimmers can each hear the music used for their performance loud and clear both above and below the water at the same time. Without this, there’s no way to keep the routine synchronized. Underwater speakers are crucial to fine tune the details.

Sometimes you have no choice but to fine tune the details to succeed in what you are doing. But sometimes a quick approximation might be all you need to get to the next step. This is certainly the case in surface buoy hydrodynamic damping: you can spend a lot of time and energy fine tuning hydrodynamic details, but there are workarounds. What we’re going to cover is:

  • Ignoring buoy damping altogether
  • Using a simple approximation when buoy motion may be important
  • Dialing in the details when buoy motion is crucial

First, we will cover the first point on ignoring damping altogether.

Special underwater speakers help artistic synchronized swimmers keep coordinated even when their ears are below the water surface

When can you ignore damping altogether?

Buoy damping comes into play when there is lots of relative motion between the buoy relative to the water surface. Suppose a surface buoy is light enough and has enough flotation to track the water surface, even in extreme storm conditions. In that case, buoy damping won’t have much of an effect on the system response. This kind of scenario often happens in deepwater oceanographic moorings.

In many deepwater oceanographic systems, the surface buoy may have a primary role in supporting the weight of the mooring itself. Full ocean depth moorings may have 5 kilometers of mooring line or more, bristling with dozens of instruments along most of the span. Surface buoys of a mooring like this are often very light, with a lot of extra flotation to support the weight of the mooring line and instruments. These buoys may be so large and light in the water they need a substantial mooring weight to stay stable and upright in the water!

In deepwater mooring, buoy heaving, or up and down motion, drives the dynamic mooring loads. Many surface buoys closely follow the water surface in moderate and extreme storm conditions. In this case, there isn’t much need to resolve the damping effects of the buoy itself when computing mooring loads. But not all surface buoys follow the wave surface perfectly, which can affect dynamic mooring loads. Also, buoy motion and acceleration may introduce constraints and limits on equipment and needs to be better understood. In these cases, ignoring buoy damping may not be good enough. This brings us to the second point about approximating buoy damping.

Some moored systems have instrumentation only in the buoy

The surface buoy may also be much larger and heavier to accommodate an extensive suite of devices to measure waves, wind, and current conditions. If the buoy is larger and heavier, it is less likely to merely follow the water surface in heave in most sea states. On top of this, if the system is in moderate or shallow depth water, the more detailed motion of the buoy – beyond only heave motion – may significantly influence the dynamic mooring loads. In these cases, it may be good to look into more detailed buoy motion, which means you need a rough approximation of buoy damping. But how can you make this rough approximation?

It doesn’t take much to make a rough approximation of buoy damping

It takes relevant experience or knowledge of the particular buoy you’re using. The main idea is you need to know roughly how the buoy responds in calm water to heave and tilt. This is sometimes referred to as the decay response. After a slight disturbance in heave or tilt, are there many motion oscillations before it calms down, or does the motion damp out and settle down right away?

You don’t need to be exact. Nevertheless, the expected decay response combined with the hull geometry, mass, and inertia defines the linear buoy damping. The buoy damping becomes an additional input into a dynamic analysis tool to resolve the mooring loads and buoy motion in various sea states.

While it’s a rough approximation, it’s still an excellent way to make progress on a mooring design problem. It’s an improvement on ignoring damping completely, too. However, more detail in buoy damping is warranted when accurate buoy motion is crucial. This brings us to the third point on dialing in the details on buoy damping.

In some cases, buoy motion has a critical impact on sensors or safety systems

Buoys using current or LIDAR wind profilers are particularly sensitive to surface buoy tilt motion. Visibility of Aid to Navigation buoys, and therefore safety at sea, is directly affected by the amount of tilt. If you’re uncertain about buoy damping, this directly translates into uncertainty about the buoy’s motion affecting data quality and equipment performance. So how do you dial in the detail of buoy damping?

The source of linear damping for floating systems is wave radiation

Wave radiation effects are often solved using potential flow software tools. These hydrodynamic software tools resolve how a moving hull shape creates radiating wave patterns at a range of motion frequencies. Dynamic analysis tools can then use the resulting forces to refine buoy motion.

An example of a potential flow software tool like this is ShipMo3D. The wave radiation forces are computed for a range of motion frequencies and all degrees of motion for a particular surface buoy hull shape. The dynamic analysis tool ProteusDS uses hydrodynamic data from ShipMo3D. Using these tools, mooring designers working with ProteusDS can incorporate more detailed buoy damping effects.

Do all buoys need to resolve wave radiation forces?

Not necessarily. Wave radiation effects are often only significant in larger and heavier buoy hulls that tend to displace a lot of water. These larger and heavier buoys may not simply follow the water surface in most conditions, and a more detailed look is warranted. Experience with a particular buoy form factor may provide the data and expertise to work with a rough approximation only without the need to get into the details of a tool like ShipMo3D. But the decision to use a tool like ShipMo3D should not be taken lightly: it takes time to collect necessary information, set up and compute the system hydrodynamics and validate it. A simple approximation may take a small fraction of the time and still get reasonable results.

Let’s look at an example

In this example, we are going to illustrate what happens to buoy motion when linear damping is ignored altogether. In a partnership between Nortek, AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS, a shallow water surface mooring was deployed to explore the influence of buoy tilt on ADCP measurements. The buoy was self-stable with heavy ballast plates to keep it upright, while a low tension mooring helped keep it on station. This is an ideal system to compare with simulated motion predictions from ProteusDS because the buoy is self-stable and the mooring should not have a dominating effect on tilt.

Surface buoy configuration with ballast plates and Nortek Signature 1000 ADCP

A nearby bottom-mounted AWAC was used to record and verify the sea state condition independent from any measurements on the buoy itself. The Nortek Signature 1000 ADCP mounted on the buoy recorded the tilt angle of the buoy in a range of sea state conditions.

A ProteusDS model of the mooring and buoy was constructed to compare with measured results. The buoy was modelled as a rigid body with a cylindrical hull with viscous drag coefficients and no additional linear damping. The resulting buoy tilt in a 3m significant wave height sea state was measured at 13 deg standard deviation with maximums around 50 degrees. The ProteusDS simulated buoy tilt response showed a 12 degree standard deviation with extremes around 60 degrees tilt.

Surface buoy moored profile in 19m water depth

It’s summary time

All surface buoys will have some damping that will affect their motion. Some of this damping can be from wave radiation effects. But the critical question is how much does this damping affect the function of the buoy and mooring? You may be able to ignore it entirely if the focus is on mooring loads. But larger buoys used for measurements that are sensitive to motion or navigation safety may need to take a closer look at evaluating the buoy motion. A simple approximation based on experience can help. Still, there are also advanced hydrodynamic tools like ShipMo3D that can shed light on the problem, too.

A synchronized swimming performance may look like magic in how the athletes keep their timing. Swimmers need to be precise to get through their routine. When it comes to mooring design, you may or may not need this kind of precision to get through a design process. At least you don’t have to hold your breath the whole time!

Thanks

Thanks to David Velasco from Nortek for providing data and insight into the shallow water buoy used for the example, and for AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS for publishing their work on the collaboration. Read more on their work published here.