Tag: ProteusDS

Why you see modular oceanographic mooring design everywhere

Something strange happens to you when you start thinking about buying a new car or bicycle. These are relatively big purchases for most people, so you spend a bit of time thinking about a few choices. When you spend time thinking about different options, the subtle design details and shape of the vehicle become more familiar. This is when something weird starts to happen. The lines and shape of a specific car or bicycle you are considering will stand out when you out around town – you start seeing them everywhere.

This effect is actually quite common and can happen with anything – not just a new bicycle or car. In oceanographic mooring design, you will find a lot of subtle design details. For example, many oceanographic moorings use are modular in nature. You may not pick up on these kinds of design details at first glance. Once you realize they are there, you might start seeing them everywhere. In this article, we’re going to cover a few of these modular design details:

1) working on a limited deployment ship deck space

2) adjusting sensors to reach target instrument depth

3) adapting moorings to different water depths

First, we’re going to talk about how moorings are assembled for deployment on the deck of a ship.

My trusty Toyota Yaris: I still see them everywhere!

It’s easy to get lost in the details of the mooring design

After all, there are often dozens of components to keep track of. But ultimately, there’s going to be a time when you need to assemble the whole kit and get it ready to deploy in the ocean. There’s no way around this part of the operation.

You have to assemble the mooring on the deck of a ship. It’s rare for longer moorings to build the entire mooring line in one go. They usually need to be laid out in finite lengths, with all the components and clamp-ons added before connecting to the next segment. In this way, a notable limiting factor is the amount of deck space you have available to you.

The mooring segments laid out to prep for assembly are often some factor of the ship length itself. Quite literally, the ship’s deck size can be a factor in the mooring design. If the mooring segments are too long, you won’t be able to lay them out properly on the ship deck to assemble them sequentially. This is why particularly long moorings can look like they are made of modular segments of rope with connectors instead of one long string. But these modular segments also help in another critical facet of oceanographic mooring design. This brings us to the next point in how the mooring design is adjusted to reach target instrument depths.

The ultimate goal of oceanographic moorings is to measure something in the ocean

This might be temperature, salinity, or pressure, but it can be many other things, too. But these measurements can’t just be made anywhere the sensor ends up in the water column. There is a plan for where these measurements need to be placed: this is the target instrument depth.

But there might be dozens of sensors over the entire mooring line. As you go through the design cycle, adding components here, adjusting flotation there, the instruments’ position in the water column will shift around. The final design needs to show how each sensor is reaching its target instrument depth. So, do you go through and adjust every single sensor’s clamp-on position on their segment? Not necessarily.

It’s usually faster and easier to adjust the lengths of the modular segments above or below

In this way, groups of instruments on a mooring segment move together up or down in the water column. Adjusting the segment length is a much easier way to quickly and easily reach the required target instrument depths. But this also comes into play when changing the mooring for new locations and water depths. This brings us to the third point on adapting a mooring design for different water depths.

In any design problem, it helps to know about other designs that have worked in the past

It’s the same for oceanographic moorings, too. It’s possible an identical mooring with the same target instrument depth needs to be deployed at another location – with a major change being water depth.

Adopting the design to a different water depth is easy to deal with by adjusting the mooring segments’ lengths – either removing and shortening some or adding even more segments as needed. This modular nature of mooring designs helps old designs provide a lot of helpful insight for new designs.

What if I don’t have a mooring design to start from?

Everyone has to start somewhere. It can be hard to find details on existing moorings. Mooring diagrams you aren’t intimately familiar with might also have missing design details you need to be wary of. But ultimately, the oceanographic community is very open and collaborative and asking others for help can be a way to get started. On top of this, we are building a library of sample moorings, largely based on actually deployed moorings where possible, that work with Proteus Oceanographic.

Let’s look at a specific example

CSIRO maintains an array of subsurface moorings that measure flow conditions in the East Australia Current (EAC). This array of moorings covers several hundreds of kilometers along the continental shelf. The array of moorings covers a water depth ranging from 200m to full ocean depth at 5000m. Each mooring measures salinity, temperature, and also flow speed over the water column.

Wire segments and fibre rope segments are used to fine tune instrument position and accomodate different water depths

The moorings in the array have a similar design in spite of the significantly different water depths. You can see a picture of the subsurface mooring used in 4200m of water depth below. The length of segments of wire in between the ADCP buoys and clusters of flotation were fine-tuned to reach target instrument depths. The longer fibre rope segments at the bottom of the mooring were adjusted to accommodate the specific water depth at this location. In this way, a similar modular mooring design was used across all locations in the EAC array.

Summary

It’s no accident that oceanographic moorings are naturally modular. It arises from constraints from available ship length. But it also makes adjusting the design to reach target instrument depths, or adapting old designs to new water depths, much easier.

This aspect of modular mooring design is fairly common, especially in long moorings. Now you might start seeing it elsewhere in your design work – just like when you are thinking of a new car or bicycle!

Next Step

We’ve added the EAC 4200m mooring mentioned in the example above as another sample mooring to the collection you can check out with ProteusDS Oceanographic. Read more and download the free version of ProteusDS Oceanographic here.

Thanks to CSIRO and IMOS

Thanks to mooring engineer Pete Jansen from CSIRO Marine National Facility / IMOS for sharing technical pointers, sharing data, and helping to assess the EAC mooring system.

How to correct subsurface mooring knockdown

Manatees are sophisticated, high-performance swimmers. You might look at one of these cuddly blubberous hulks and think otherwise, but it’s true. By high performance, I don’t mean they are incredibly fast. Instead, it’s how they keep their ungainly bulk in control. Because they are so large, it would be a big challenge to rely on muscle power alone to navigate around. This is where Manatees have a trick up their sleeve.

They rely on the careful use of digestive gasses to keep the right balance in the water. It’s not all about flotation, either. Critically important for them is they use this gas to control their tilt, too. They have special pockets that hold these digestive gases through their intestines that enable this control.

But it requires precision: too little or too much gas and they are out of alignment in the water. If they have too much gas, Manatees end up swimming awkwardly around with their tails way up over their heads. So what do they do if they have too much gas? Well, it turns out they just need to fart. With judicious use of farts, their swimming tilt can be corrected.

Similarly, you will be faced with making corrections when you work on a design problem. Often, a design will get out of balance, and you need to make some adjustments. When it comes to subsurface mooring design, any amount of steady current can lead to significant mooring knockdown. In this article, we’re going to talk about correcting subsurface mooring deflection by adjusting:

  1. flotation
  2. line size
  3. sensor location

All these parameters can affect knockdown. But they each affect knockdown differently, and it pays to consider them separately. First, we’re going to talk about adjusting flotation.

As a general rule, consider not swimming behind Manatees

You might think adding flotation is a brute force approach

It certainly makes sense: the larger the flotation used, the greater the mooring’s static tension. The larger the static tension, the more it will resist deflection from an ocean current.

But there are limits to the amount of flotation you can add. After all, working with larger and larger floats starts to get awkward and heavy to handle on a ship during mooring deployment. There is a cost factor too: larger floats are heavier and will be more expensive to manufacture and transport. So while it is a simple approach to use more flotation, there are certain practical limits. You may find that you need as much flotation as you can handle to reduce knockdown in your mooring deployment. So it’s good to be aware of what your practical limits for flotation are.

You might wonder about the drag force on the float itself, too. While this can be an important factor, there is a lot of nuances to float drag. Before considering the float’s drag, it is far more essential to consider the drag on the mooring line itself. This brings us to the second point: adjusting line size.

Mooring knockdown is proportional to the total amount of drag

And drag force is always proportional to the amount of structure area in the flow. Simply put, for mooring lines, this area is a factor of line length and diameter. You can’t do much about line length if you need to get sensors in a specific position in the water column. So that leaves line diameter to play with.

A few millimeters in line diameter can make a massive difference to knockdown. While a few millimeters in line diameter may seem insignificant, the effect adds up over the entire span of the mooring line. The longer the mooring line, the more significant impact it has.

So what does it mean to adjust the line diameter? In a subsurface mooring, the line tension is often driven by how much flotation you’ve added to the mooring. If you are using a smaller line diameter, the same rope material will have a smaller minimum breaking load. Of course, you’ve got to make sure the line’s minimum breaking load is sufficient given the tensions you expect.

If you want to try a smaller diameter, you may need a different line material

There are lots of sophisticated fibre ropes that have incredible minimum breaking loads at smaller diameters. Higher performance ropes will come with a higher cost. But the cost of the line should be considered in the context of the entire project. Again, only a few millimeters in line diameter can significantly change knockdown, so it is worth investigating in the design stage to see what happens.

Knockdown is proportional to the drag on the mooring. But it also makes a big difference if certain portions of the mooring line are in areas of higher current than the rest. It’s rare to have a completely uniform current profile, especially in full ocean depth conditions. This brings us to the last point on sensor location in the water column.

Sometimes you can work around the problem

Every mooring is driven by requirements to make measurements in the water column. In some cases, there is some flexibility on where sensors can be located to make these measurements. If you know something about the current profile for where the mooring will be deployed, you can use this to your advantage about where to place sensors.

For instance, there are many different kinds of ADCP, each with their own capability to measure ocean currents over various distances. If you’re not constrained to a specific ADCP, you can choose an alternate instrument and make adjustments to where these sensors are located in the water column to measure current profiles.

Knockdown is proportional to the amount of drag, and drag is driven by current speed. If you can make measurements of currents from a distance out of the main flow, so much the better. Because there are so many options for ADCP, the trick is evaluating which is the right one that will work well enough for your mooring. However, there aren’t ways of measuring things like salinity or temperature from a distance, in which case there isn’t a way to adjust sensor location.

So how do I evaluate all these options on flotation, line diameter, and sensor location?

Calculating the drag on the mooring line from each design is a process. Drag is not an obvious calculation you can do in your head or on paper for most typical moorings. You can use a software tool like ProteusDS Oceanographic to lay out flotation size, line diameter, and sensor types in different current profiles and calculate the resulting knockdown. It’s a straightforward process to adjust each parameter and see the effect on mooring knockdown.

It’s time for an example

We set up three subsurface mooring configurations in ProteusDS Oceanographic to highlight the effect of making current measurements out of the main flow speed. We assumed a water depth of 3000m. A current profile representative of extreme open ocean conditions was used with a current profile of 1m/s at the surface, tapering to 0.5m/s at 1000m, and further to zero at the seabed.

The objective is to measure the top 1000m of the water column. We used three typical subsurface mooring configurations to compare the effect on mooring knockdown and tilt by measuring flow speed away from the main ocean current. The three mooring configurations were:

  • Configuration A: uses one Teledyne 45kHz Pinnacle unit – a newer ADCP with double the measurement range from older models – in a single float, located at 1000m nominal depth
  • Configuration B: uses two Teledyne 75kHz Workhorse units – an older ADCP technology with a moderate measurement range, located at 500m nominal depth
  • Configuration C: uses two floats, each with a single Teledyne 75kHz Workhorse unit, located at 500m and 1000m nominal depth

The goal was to use ProteusDS Oceanographic to determine the float sizes such that the knockdown was less than 50m, and allowable tilt around 5 degrees. For simplicity, only the floats, 1/4″ mooring wire, and wet weight of the ADCP units were accounted for in the mooring layout. We selected float sizes based on the spherical ADCP models listed on the DeepWater Buoyancy website.

Three typical subsurface mooring configurations are used to compare the effect on mooring knockdown and tilt by measuring flow speed away from the main ocean current

The results for each configuration were:

  • Configuration A: 56″ sphere 1500m depth rating: knockdown 22m and 1deg tilt
  • Configuration B: 62″ sphere 1500m depth rating: knockdown 87m and tilt 3 degrees (we would have needed a bigger float or a second float to bring the knockdown below 87m!)
  • Configuration C: Two 56″ spheres 1500m depth rating: knockdown 32m, tilt 5.6deg

The results highlight how an ADCP with a longer measurement range, like the Teledyne Pinnacle, allows a mooring design with float and mooring out of larger currents. This reduces the total amount of drag on the mooring and the resulting knockdown. The other configurations needed more flotation or much larger floats to achieve the same knockdown requirements.

Summary time

We covered a few aspects of correcting knockdown in a subsurface mooring design, and now it’s time to review. The first thing to work with is the amount of flotation. Increasing this should directly help reduce knockdown, but there are practical limits in cost and logistics, too. The next thing to review is line diameter. Even a few millimeters reduction of line diameter can make a huge difference, especially on longer moorings. It may mean using a more expensive line material, but it may be a small additional cost compared to the entire project. Finally, consider sensor placement and what you actually need to measure. If you can measure high-speed currents without the mooring sitting in the middle of it, you could avoid significant drag loading on the mooring and the resulting knockdown. But it depends on what sensor options you have available, and in some cases, there might not be a workaround.

Manatees can get into trouble when they are out of alignment in the water. But they can always make some corrections to keep things running smoothly. Similarly, when there’s an ocean current around, your mooring might get into trouble and get out of alignment in the water. But now you have a roadmap for making corrections to the design to help reduce knockdown and keep things running smoothly, too.

Next Step

Try out the free version of ProteusDS Oceanographic. The Official Parts Library includes a host of DeepWater Buoyancy floats to make trials of different equipment easy to do. Read more and download the software here.

Thanks

A huge thank you to Paul Devine at Teledyne Marine and David Capotosto and Dan Cote from Deepwater Buoyancy for collaborating on the example configurations. Also a big thanks to Chris Beck at DSA Ocean for running the analysis through ProteusDS Oceanographic!

When to check instrument depth in oceanographic mooring design

In the late 19th century, the Hawaiian sugar industry was sweating, but it wasn’t from the heat. It was because of rats. The rat population was eating up way too much of their sugar crop. And the problem was getting worse. So what could be done to help keep things under control? One idea was to bring in Mongoose to help out. There was a lot to like about Mongoose: they’re relatively small animals, they’re carnivores, and they’re great hunters. But the best part? They like to go after rodents.

The plan looked good on paper. So Mongoose was then imported to Hawaii. But there wasn’t any impact on the rat population. Everyone came to realize the big problem: a rat is nocturnal, a Mongoose isn’t. The plan was a total failure – and chiefly from this crucial detail.

Many plans can look good on paper. But without carefully checking the details, things can go sideways very quickly. In oceanographic mooring design, a key detail to check is instrument depth. There are a few steps to do this through the mooring design process to make sure they are not far off target depth. What we’re going to cover in this article is calculating instrument depth from:

  1. static mooring stretch from weight and buoyancy
  2. static mooring deflection in mean current
  3. dynamic mooring deflection in current and waves

These form steps of a plan to check how the instrument depth might have strayed from a target location as you go through the mooring design. It’s also an opportunity to make adjustments to the mooring to get sensors where you want them in the water column. First, we will look at static mooring stretch from weight and buoyancy.

Mongoose: wily and capable hunters, but since they aren’t nocturnal, aren’t effective against rats

It may seem a bit anticlimactic

The ocean has intense forces from wind, waves, and current, but all we’re looking at, to begin with, is the mooring sitting by itself in calm water. But intense forces can come from the mooring itself, too. Flotation can come in all sizes and pull on the mooring with tons of static load. All materials will stretch some amount. But in certain circumstances with software materials or very long moorings, this stretch can be significant, indeed.

It’s the effect of this stretch on instrument depth you need to get a handle on first

Maybe the error from target instrument depth isn’t much, but you should still check it as a first step. Besides, many locations in the ocean spend a lot of time in low current condition, so this is an excellent time to make sure you’ve dialed in your instrument depths to where you want them to be on the mooring.

At this point, adjustments you might need to make in the mooring design might look like shifting the instruments’ location clamped on the mooring line, or more likely, adjusting mooring component lengths.

While many moorings may spend a lot of time in a calm environment, it doesn’t take much ocean current to cause problems. This brings us to the next step, calculating static mooring deflection to a mean current.

You might think you need massive current speeds to cause any mooring deflection

But that’s not necessarily the case. The longer the mooring, the more it acts like a giant lever, where small currents through the water column can cause significant mooring deflection. The more the mooring deflects, the more there can be an error from target depth. So it’s this deflection in a mean current we need to tease out and properly understand. It’s often the knockdown in subsurface moorings that really throws off instrument depth.

Evaluating this deflection and the resulting instrument depth error gives designers another stage to adjust their mooring. Fortunately, a calculation of static mooring deflection is a rapid calculation. The mooring response to ocean current can often be the most significant influence on instrument depth. But it’s not necessarily the last thing to check.

If there are parts of the mooring at or even near the surface, it’s a good idea to check the effect of ocean waves. This brings us to the final section, calculating the dynamic mooring deflection in mean current and ocean waves.

Evaluating the effects of ocean waves is the most complex stage

It also takes the longest to evaluate. There may be a variety of wave states at any location. It’s not always obvious which waves will have the most significant influence on the mooring. This means there’s often no way around systematically checking a range of different wave heights and periods to see what happens to the mooring, how it deflects, and what this does to the instruments. It may be that ocean waves do not significantly impact the instrument location, but it’s still useful to rule it out all the same.

Nevertheless, this process reveals a new mooring profile. On top of this mean profile is a dynamic variation caused by ocean waves. It’s this dynamic variation you want to get an understanding of. Position in the water isn’t the only thing affected by ocean waves. The mooring motion caused by ocean waves can introduce errors in the measurement of ocean current velocity. You have to be comfortable with the amount the sensors are moving about the target depth.

But do I always need to check instrument depth in current and waves?

Not necessarily. If you’re redeploying a mooring and the site conditions are expected to be the same as a previous deployment, you should expect similar performance. But if the mooring configuration has changed, it’s a good idea to check what will happen to the instrument depths.

You might be tempted to skip a detailed check for very simple moorings that are either short or only have one or two sensors. But if you skip this step, you have to be comfortable with the additional headaches it can make for you in post-processing the data you get from your mooring. Regardless, instrument depth becomes more complicated with longer and more complex moorings, so extra care is needed.

Let’s look at an example of a real mooring’s deflection

CSIRO and IMOS maintain an array of subsurface moorings that monitor the East Australia Current. This massive and complex ocean current has a significant impact on the environment. This impact can only be understood if the current is measured and understood. The array consists of a series of subsurface moorings in a region stretching along the continental shelf to full ocean depth 200km away from Brisbane, Australia.

 

The East Australia Current is a massive and complex flow along the coast of Australia. CSIRO/IMOS maintains a subsurface mooring array to measure the complexity and magnitude of this flow in the yellow box indicated off the coast of Brisbane. These currents can cause a significant deflection of the moorings.

 

One of the subsurface moorings in the array is located in 4200m water depth. We compared the deflection of a ProteusDS Oceanographic model of the mooring with measurements from the real deployment in the 95th percentile ocean current measurement. The maximum knockdown calculated was 151m. This compares well with the measured maximum knockdown of 160m depth. The corresponding maximum tilt of the primary floats was 15 degrees.

The knockdown has to be carefully accounted for to make sure the ADCP devices near the top of the mooring can still measure the ocean current all the way to the water surface even when the mooring is deflected.

A) schematic of the 4200m depth EAC subsurface mooring, showing an array of temperature, pressure, and current sensors (ADCP) B) Deflection of the 4000m+ mooring in 95th percentile current profile with 150m of vertical knockdown at the top of the mooring

Summary

We covered a few aspects of checking target instrument depth error, and it’s time to review. Once you’ve laid out your mooring, the first thing to check is the effect of static stretch on the mooring from weight and buoyancy in calm conditions. The next step is to consider environmental effects – first, steady current and then after that both current and waves together – particularly for surface moorings or subsurface moorings near the ocean surface. Each step along the way increases analysis complexity and offers you a chance to adjust the mooring as you go along.

Importing Mongoose to stop a rat problem for the 19th century Hawaiian Sugar Industry seemed like a good idea at the time. But they missed a critical detail and their plan failed. Using a systematic approach to check and adjust an oceanographic mooring is key to checking the crucial details like instrument depth.

Next Step

We’ve added the EAC 4200m mooring mentioned in the example above as another sample mooring to the collection you can check out with ProteusDS Oceanographic. Read more and download the free version of ProteusDS Oceanographic here.

Thanks to CSIRO and IMOS

Thanks to mooring engineer Pete Jansen from CSIRO Marine National Facility / IMOS for sharing technical pointers, sharing data, and helping to assess the EAC mooring system.

PS

Check out a numerical visualization of the East Australia Current on Windy.com here.

How mooring deflection disrupts oceanographic data quality

Sloths may be slow, but they’re not stupid. They rely heavily on stealth and blending in with their surroundings. Now, frankly, relieving themselves up high in a tree would make a lot of noise and attract unwanted attention. So how do they handle the call of nature? It is risky, but they climb right down to the base of their tree and completely unload. And unload they do, as they can hold it for about a week at a time. Slow-moving as they are, it takes them a long time to climb down and back up again. Answering the call of nature is a big disruption.

For sloths, this kind of disruption is unavoidable. But not all disruptions are necessary. In fact, many of them can be avoided entirely. In oceanographic systems, instrument data quality is disrupted by mooring deflection. All moorings will deflect from the environmental effects of wind, current, and waves. In this article, we’re going to cover three elements of mooring deflection and how they disrupt data quality:

  • knockdown
  • excursion
  • tilt

First, we’re going to talk about knockdown.

Slow-moving Sloths take a long time to climb up and down trees

Knockdown is a vertical effect

Sometimes also referred to as blowdown or subduction, this vertical effect is a change in water depth that happens as the mooring deflects. When you’re working with subsurface moorings, it’s a significant effect to keep an eye on. The more the mooring deflects, the more all the sensors along the span of the mooring shift to a different water depth. It’s this shift that can give you a big headache in truly understanding what’s going on in the water column.

After all, there can be remarkably abrupt changes in water properties like temperature, salinity, velocity, and so on, especially with depth. It can be a real challenge to look over data collected from a mooring and establish just what exact water depth the sensors were actually at when they made the measurements. In addition, some sensors will have a depth rating, and they may be destroyed if the knockdown takes them too deep. But knockdown isn’t the only parameter to keep track of when you’re designing an oceanographic mooring. This brings us to the next point: excursion.

Excursion is a horizontal effect

This effect is a horizontal change in position that happens as the mooring deflects. Knowing excursion is quite essential to both surface and subsurface moorings. The big challenge is with excursion because you don’t know exactly where the sensors are in that horizontal envelope when they make their measurements. As the mooring deflects in the water column, it can be challenging to sort out exactly where the sensors are.

Without a lot of careful consideration, the data you collect from your mooring can only correlate to the entire footprint the mooring sweeps around. Ocean properties can change quite abruptly in the water both horizontally as well as vertically, so excursion is another crucial factor to keep an eye on. However, neither knockdown or excursion necessarily directly interfere with individual instrument measurements. This brings us to the third point: mooring tilt.

All moorings deflect from the effects of wind, currents, and waves. In a current with speed U, a subsurface mooring will have horizontal excursion A, knockdown B, and tilt C. These are relative measurements from the mooring profile in calm conditions.

Tilt is an angular deflection of the mooring

Since most moorings are often vertical in the water column when there’s no current, tilt is usually indicated as an angular deflection from vertical. Unlike knockdown and excursion, tilt causes an entirely different issue when collecting data.

It doesn’t affect all sensor types. But some sensors stop properly working when they tilt beyond a particular range. For example, an ADCP is affected by tilt. Most ADCP units simply can’t tolerate a tilt greater than about 15 degrees. Also, since an ADCP makes measurements at a distance from their location in the water, tilt affects accuracy. If disturbances from ocean waves cause dynamic tilt, it means the ADCP measures a large swath of ocean volume, and it reduces how precise the measurements are.

But how do you know what these effects will be before deploying the mooring?

This is a key question that we try to answer through the process of mooring design. These are often key parameters computed by a mooring design tool. Mooring design tools like ProteusDS Oceanographic calculate these parameters to show how the mooring performs. Nevertheless, the first step is understanding the terminology and concepts. Once you start working on your mooring design, no matter what tools or processes you use, you’ll know what to be looking for.

It’s summary time

We introduced three concepts of mooring deflection:

  • The vertical effect of knockdown
  • The horizontal effect of excursion
  • The angular effect of tilt

These are three fundamental effects you need to keep an eye on for the entire mooring when going through the design process.

Like every living thing, including sloths, the call of nature can be a disruption you can not avoid. But in mooring design, using the right tools, we can prevent disruptions caused by mooring deflection with careful and appropriate design.

Next step

With these concepts of mooring deflection in mind, you can start to account for them when designing oceanographic moorings. Read more on convenient times to adjust instrument placement in the design process here.

Why drag force is more than just resistance

It’s tough to beat Nature. You don’t have to look in a top secret lab to find one of the most miraculously slippery fluids on the planet. You can just look at your knee! Lining many of your joints is an egg-white coloured substance called Synovial Fluid. Among its many purposes is to keep your joints moving. But not just for a few days – for your entire life. And through all this time, it manages to control and minimize joint resistance.

Certainly, when resistance gets out of control, everything can come to a grinding halt. When it comes to hydrodynamics, when you think of resistance, drag forces may be the first thing that comes to mind. But there’s more to drag than just resistance, and this is what we’re going to cover in this article.

What is the drag force?

Forces appear when there are changes in momentum. Momentum means mass in motion, so for the specific case of fluids, this could mean water or air flowing in a current. The drag force arises when there is a change in momentum in fluids. More specifically, the drag force transfers momentum between a structure and a fluid it is immersed in. This might sound like an abstract idea, but you already have some experience with it every day.

You feel this effect when your hand is in water

A big part of this is the drag force. But if you are in a pool or bathtub, the water isn’t moving around – it’s your hand – and so the drag force in this case will feel like resistance to you. In this case, it’s your hand that has momentum: it is the mass in motion. The drag force is then transferring momentum from your hand into the water.

You can feel drag when moving your hand around in water

What does momentum look like in water?

Well, if it’s your bathtub, there will be swirling and churning water – turbulence, and probably some waves. All this moving water is mass in motion, too. But the drag force does work in reverse, too – it can transfer momentum from water into structures.

What happens when there are ocean waves or currents in the water?

Ocean waves and water currents are also examples of mass in motion, too. If a structure, like a mooring buoy or ship gets in the way of moving water, there’s going to be some drag forces. These forces will transfer momentum from the waves and currents into the structure. So in this way, drag force is really about the exchange and transfer of momentum.

Sometimes this means it creates resistance and slows down a structure. But it can create motion in floating structures, too. The key is that drag is proportional to the relative velocity between a structure and the fluid.

The drag force is one of the essential forces in hydrodynamics

It acts like a resisting effect on many structures. If you are trying to understand what will happen to an ocean robot driving around in the water, it will have a big impact on energy consumption as well as how it can maneuver.

But the drag force is also a key element in mooring designs. In oceanographic mooring designs, the aggregate drag on all the components and the mooring line causes the system to deflect in an ocean current. In reality, the water current loses some momentum from this drag force from the mooring – the wake from the float and mooring components reduces the ocean current flow speed by some small amount.

In an ocean current with speed U, the drag force from all mooring components causes deflection.

Drag is also a key element in the excitation forces when ocean waves are around. You need to know these excitation forces to properly design a system to perform the way you want in the ocean environment. But knowing drag forces is one thing. Knowing how big they are is really the million-dollar question.

How do we know what the actual drag force is?

The drag force is measured from an experiment. These experiments might be a physical test in a lab, or in more modern times, they might be virtual tests using fluid dynamics software. These experiments resolve what the drag forces are in certain specific conditions. There have been hundreds of thousands of tests completed in laboratories for many decades measuring the drag force on a vast array of shapes in different flow speeds and fluid mediums. But how do we take all this information on drag force and then use it in a specific application?

The key is the drag coefficient

Similar shapes produce similar and predictable drag forces. The concept of a drag coefficient works well and covers a wide range of fluid types and conditions. Ultimately, if you’re looking at something like a sphere, you can use a drag coefficient associated with a sphere and predict reasonably well what the drag forces will be for a good range of wind or water current speeds. These drag coefficients are often available in look up tables to help the design process.

But what about the drag on mooring lines?

A mooring line is indeed a much more complicated structure than a sphere. Depending on its orientation to the flow, the local drag can change drastically. The drag forces on mooring lines require a calculation that considers the local flow speed and tilt of the line of the whole system. Suffice to say, it’s not a back-of-the-envelope type calculation you can do like that of the drag on a sphere! But this is what we have programs like ProteusDS Oceanographic that help address these complexities.

It’s summary time

The drag force is an effect that arises when momentum transfers between a fluid and a structure. Yes, a structure will feel resistance when it’s moving in a fluid, but the reverse is true, too: a moving fluid, like ocean currents and waves, can also cause a structure to move around, too – and the drag force plays a big role in that. The drag force is a very important effect that affects a range of systems from vehicle dynamics to oceanographic mooring design.

Even super low friction fluids like those in your knee joints have some drag effects, too. While you don’t need to do much over your life to keep your knee joints going, you do need to be mindful of drag when designing structures in the ocean.

Next step

Figuring out a drag coefficient for a particular structure isn’t always obvious. While ProtesuDS Oceanographic includes drag coefficients for a variety of shapes, you can learn more about different ways to resolve drag coefficients here.