Tag: CSIRO

How to avoid wuzzles when recovering oceanographic moorings

Posted on October 15, 2021
Ryan Nicoll

It was Christmas time, but I didn’t expect to get a haircut as a present. On this particular holiday, one of my young son’s presents was a small automatic flying drone. It was a seemingly harmless small toy that merely hovers and slowly drifts around indoors automatically, without the need for a pilot. But sometimes, it would change direction quickly and catch us off guard. It bumped into my head within less than 5 minutes, and the propellers snarled into my hair. It took a lot of working on getting it unwound and even a few snips to get it free. It was a tangled mess!

Nobody likes to deal with a tangled mess, so if you can avoid one entirely, so much the better. When it comes to oceanographic mooring recovery, there is a considerable risk for the entanglement of the line. When the line forms one of these giant tangles (also known as wuzzles!), it can take a lot of time to sort out and even damage equipment. But with the right precautions, it’s possible to avoid a wuzzle altogether.

Getting a small flying drone tangled in your hair is a serious tangled mess. It pays to avoid problems like this!

So what exactly is a wuzzle?

Strange as it might seem, wuzzle is an actual term to describe a tangled mess in a line. Why does this happen? If a mooring line loses tension and has a chance to bunch up in an area, it may drift into a convoluted mess. When there is low line tension, the residual twist can easily throw the system into a tangle.

During mooring recovery, there’s ample risk for a wuzzle

This is because there is both low line tension and the opportunity for the mooring line to bunch up. There’s a strong chance of this if you’re working with a mooring recovered using an acoustic release.

A wuzzle in an oceanographic mooring is also a serious tangled mess. Picture credit: Danny McLaughlan, courtesy of CSIRO/IMOS

An acoustic release facilitates mooring recovery

An acoustic release is fired when a unique coded signal is transmitted. The result is the device opens a mechanical connection on the mooring line. Acoustic releases are usually placed near the anchor along with a significant amount of flotation.

It’s this significant flotation that brings the tail end of the mooring up to the water surface. The problem is when the tail end catches up to the top float or surface buoy. In the worst-case scenario, the entire mooring can be bunched up in one location in a low tension state. It’s a wuzzle in waiting, and unless you take action, it’s going to be the biggest headache of your life to untangle.

How can you avoid a wuzzle on oceanographic mooring recovery?

You can reduce the risk of a wuzzle with careful observation and operations management. The recovery vessel needs to have a rough idea of the ocean currents at the time, so they expect where the mooring and ship will drift through the process. Typically, the recovery crew can get a location fix of the acoustic release, so they know a safe distance to wait.

However, the recovery vessel may attach a towline for a surface buoy and start pulling gently on the buoy laterally before even firing the release. For subsurface moorings, the recovery crew is on the lookout for the top buoy to reach the surface first. Once they see it, they’ll steam over in the recovery vessel and attach a tow line. Either way, the key is for the recovery vessel to pull the surface buoy or top float away from where the tail end of the mooring is expected to come up. It keeps the mooring from pooling up and reduces the chances of a tangled mess.

How do you know how much time you have before the acoustic release reaches the surface?

There are a few ways to anticipate this. The easiest way is to keep acoustically pinging the release as it rises through the water column. A fix on the release tells you how fast it is rising in real-time. But it’s also possible to complete a recovery analysis in a software tool like ProteusDS Oceanographic. Depending on how much flotation and the line properties you use in your mooring, you can get an idea of how much time you might expect to see it reach the surface.

Snapshot from mid-simulation of mooring recovery. The acoustic release has already been fired and the flotation pack is bringing that end of the mooring to the surface at A. The primary buoy is already at the surface at B.

Summary

When it comes to entanglement, the best approach is prevention. When my son wants to bring out his automatic drone, I now keep a safe distance. I am also considering wearing a hat, and possibly just going into another room altogether. When it comes to oceanographic mooring recovery, prevention is key as well through ship operations.

Next step

Read more on tools for oceanographic mooring design with ProteusDS Oceanographic here.

Thanks to WHOI and CSIRO

Thanks to engineer Don Peters at WHOI for the discussion and feedback on the article topic and also to CSIRO/IMOS for the use of the image.

What to keep an eye on during oceanographic mooring deployment

Posted on August 26, 2021
Ryan Nicoll

The longest continuously running experiment started in 1930 and is still going right now. The Pitch Drop Experiment looks like an hourglass filled with a thick, black substance called pitch. Pitch is a resin that is so thick it looks and acts like a solid. It is so much like a solid that it can even shatter into small bits. But despite all this, it will indeed flow over time as a liquid. In fact, it flows so painfully slow that it takes about ten years to form and separate a drop! In the Pitch Drop Experiment, because things move so slowly, there isn’t a need to constantly keep a close eye on things.

On the other hand, you need to keep a close eye on things when there are rapid changes in a system. Rapid changes mean it is easy to miss important information that may affect a design. When it comes to deploying oceanographic moorings, it may seem painfully slow for a mooring to settle in deep water. But don’t be fooled by the time it takes to settle because several rapid changes happen to the mooring and its components that can affect the design. This article will cover an anchor-last deployment’s characteristics that you may want to look at closely.

What we’re going to cover is:

1) peak load during deployment

2) lateral deflection

3) acoustic release effects

First, we’re going to cover the peak load during deployment.

Professor John Mainstone was the custodian of the Pitch Drop Experiment for 52 years. Picture credit: University of Queensland

You might expect a mooring’s maximum loads to appear during an extreme storm

But that’s not always necessarily the case. There can be significant loads during deployment, too. Many moorings are deployed anchor-last, with the top float and mooring strung out along the water surface and the anchor released in the final step for deployment.

Because the anchors tend to be so heavy and with nothing to support it the moment they are dropped from the ship, there can be a sizeable local load on the mooring. This instant of time can be when mooring tension may reach a local maximum. At the very least, the mooring anchor assembly, including acoustic release, may see a load close to the anchor’s full weight at that moment.

The anchor-end of the mooring then accelerates down into the water

As the mooring is pulled into the water, drag relieves the abrupt initial accelerations and resulting loads from the anchor’s free fall. Drag forces on the mooring mean the anchor will descend steadily to the seabed. As mooring flotation is submerged, the descent rate may further slow.

Once the anchor lands on the seabed, the soil will then support the anchor’s weight and further reduce this significant load on the lower portion of the mooring. Either way, the peak deployment load may be between dropping the anchor from the ship and the anchor landing on the seabed. But checking loads during the anchor’s initial descent is not the only thing to take a closer look at during a mooring deployment. This brings us to the next point on the lateral deflection of the mooring.

This snapshot from ProteusDS Oceanographic shows the SOFS5 mooring during anchor-last deployment. After the anchor (A) is dropped, the mooring is pulled down toward the seabed. The buoy (B) stays at the surface for some time and travels horizontally during the transition

Anchors can be so heavy that it seems unlikely they would drift sideways in free fall

There is a lot of concentrated weight in the anchor, and it is hard to imagine it budging much while in free fall. But don’t underestimate the relentless pull from the mooring on the anchor. While the anchor descends and more of the line submerges, there is a reaction load from the mooring. This builds with time. The greater the descent time, the more this acts on the anchor and steers it laterally.

By the time the anchor rests on the seabed, the anchor’s lateral fallback could be ten percent of the water depth for a surface mooring (but less for a subsurface mooring). This is assuming no ocean currents are present. But if there are currents, it can add an interesting wrinkle to the problem. Any prevailing current profile can introduce additional drag loads on the mooring and cause even more lateral movement. You have to be aware of these lateral deflections because it will tell you how far off your intended placement you may be. It all depends on the specifics of the mooring, of course.

However, once the anchor hits the seabed, there’s ideally enough holding capacity to resist lateral loads on the mooring system from any prevailing currents. You may think deployment dynamics are over once the anchor lands, but there’s one more critical stage to check. This brings us to the last point on the effects of the acoustic release.

Everything that moves has momentum

The entire mooring has built up momentum as it descends through the water. After the anchor impacts the seabed, it will take time for the mooring to slow down. The time it takes to slow down and the distance the mooring covers in this time are critical to the acoustic release. It is not a lot of distance, but it’s not zero.

The acoustic release must not impact the anchor assembly or the seabed

If the acoustic release hits the seabed, there’s a chance it can get damaged. If it’s damaged, the release may not work, making mooring recovery extremely difficult or impossible. But clearing the seabed is only half the battle.

When the mooring has slowed down and stopped, and ideally, the acoustic release is still clear of the bottom, the flotation will pull up the lower portion of the mooring back up. This snapback is also not a very large distance, but it’s not the distance that’s important.

The snapback load can cause a problem, too

There’s typically only a short line between the acoustic release and the anchor. If this line is a stiff material like chain, there isn’t a lot of compliance. This means the snapback forces can be very high. If the snapback load is high, it means there’s a chance it can damage components, including the acoustic release – again risking the ability to release and recover the mooring.

What’s vital here is introducing a line with a bit of elastic compliance in the material. It doesn’t need to be a bungee rope – but something a bit softer than chain will reduce the snapback loads significantly and protect the acoustic release. The SOFS5 mooring uses 20 meters of Nystron rope between the acoustic release and the anchor to absorb the snap back load that results from 40 glass floats just about the acoustic release.

Detail on the anchor assembly of the SOFS5 mooring that includes Nystron rope to absorb deployment snapback load

But how can you quantify these loads?

Experience, rules of thumb, and design practices can help a lot to refine mooring designs. This kind of experience is valuable and always plays a vital role in mooring design. But complementing this kind of experience are dynamic analysis software tools like ProteusDS Oceanographic. It’s often worth ensuring there are no surprises or failures in components due to these loads as the system goes through the deployment process.

How to set up anchor-last deployment analysis in ProteusDS Oceanographic

It’s possible to set up an anchor-last deployment scenario in ProteusDS Oceanographic. This can help provide insight into some of the dynamic effects you might see during deployment. Check out the video tutorial below that shows how to set up the scenario.

It’s summary time

There are crucial operational and design aspects introduced by mooring installation and anchor descent to the seabed. You may see considerable tensions during initial anchor release and from snapback after the anchor lands on the seabed. Operationally, the lateral drift of the mooring is critical to understand where the anchor will actually land. You should check the acoustic release clearance from the seabed, too, and be aware of snapback loads because you’re in for big headaches on mooring recovery if it gets damaged.

A full ocean depth mooring might take a while to descend to the seabed fully. But at least it will take a lot less than ten years like it does for a drop to fall in the Pitch Drop experiment!

Thanks

Thanks to Brian Hogue at the WHOI FIXIT Lab for providing feedback and discussion on the article.

PS

The Pitch Drop Experiment has a live webcam feed. Keep a close eye on the experiment in progress here.

 

 

 

How the anchor-last oceanographic mooring deployment process works

Posted on August 6, 2021
Ryan Nicoll

The first time I started running for exercise, it was a disaster. It was a disaster because there are pitfalls that aren’t obvious to people who are new to running. My problem was I just jumped right in without a detailed plan. I had a general idea that I needed to build up distance slowly. But that wasn’t specific enough, and I ended up with painful shin splints that stopped me in my tracks.

It was over a year before I started training again. Before starting the second time around, I learned a lot more about preventing injuries like this and, more specifically, how to increase mileage slowly. Before starting the second time around, I had a much more detailed process in place.

A detailed process lays out the steps you need to take to move forward. Whether you realize it or not, it helps you avoid problems that may or may not be obvious. When it comes to deploying oceanographic moorings, there is no shortage of challenges, especially when working with longer systems. The longer the mooring, the greater the risk of entanglement and damage to the components. We’re going to talk about the key steps in a process involved in a common way to deploy long oceanographic moorings: anchor-last.

You need a process when running, especially when starting for the first time, to avoid injuries like shin splints

What is anchor-last deployment?

Anchor-last deployment is a process in which the assembled mooring is laid out on the water surface, starting with the top float or surface buoy. The anchor assembly is then connected and dropped as the final step in the deployment of the mooring.

Anchor-last is by far the most common way to deploy moorings for a few key reasons. Following this process makes it possible to keep the mooring aligned and prevent any loops or snags from forming that can cause severe damage before the mooring is even in place. It also provides some easy control over the deployment location – that is to say, where the anchor lands on the seabed. Finally, the cost of a mooring deployment is affected by the size of the vessel you need to deploy the mooring. The nice thing about anchor-last deployments is that vessel requirements are typically the minimum possible.

How are moorings actually deployed anchor-last?

We’ve talked about anchor-last deployment in broad strokes, but now it’s time to dig into the details a little for clarification. Before anything is even put in the water, the ship needs a proper starting position.

The ship needs to advance slowly toward the deployment location. There needs to be enough distance such that at the ship’s forward speed, there is enough time to comfortably assemble the mooring and string it out on the water behind the vessel. Depending on the water depth and length of the mooring involved, the ship may start several kilometers from the deployment location!

Next, we get to the first stages of putting equipment in the water

The top float or surface buoy is prepared and connected to a short section of the upper mooring. What’s vital here is that the lower portion of the mooring is also tied off on the ship’s deck. Mooring wire may be spooled on a winch or laid out in segments on the deck. Segments may be tied off on the deck with the integrated sling or pear links in the mooring.

A crane or A-frame is then used to hoist the buoy off the deck and into the water. The buoy may be one of the heaviest components in the mooring, and the crane or A-frame needed to hoist the buoy may be one of the essential requirements for vessel size.

Now the assembly of the mooring begins

As the ship continues forward at a slow pace, the buoy is towed behind the vessel. Because the mooring is tied off on the deck with a sling link, additional modular segments of the mooring can be easily laid out on the deck, along with assembled instruments and connected when ready.

The ship’s deck length limits how long these mooring segments can be, which may be an essential factor in the mooring design. Nevertheless, as each section of the mooring is attached, it is then deployed into the water. Gradually, the mooring is assembled and strung out and towed behind the ship.

The final stages involve the anchor assembly. For deepwater moorings, it’s common to use a glass float cluster and acoustic release along with the anchor. Once these are in place, the anchor assembly is prepared. Either a crane drops the anchor or a skid plate slides the anchor assembly into the water. Once the anchor assembly is in the water, the final stage of deployment begins.

1) Buoy is deployed 2) mooring is assembled and spooled out behind the ship 3) Anchor assembly prepared 4) anchor released. Image courtesy of J. Doucette © WHOI

What happens after the anchor is dropped?

Even though oceanographic moorings can be completely different, there are still a few typical dynamic stages the mooring goes through once the anchor is dropped.

After the anchor is dropped, it begins its descent to the seabed. A subsurface mooring will often reach a constant descent rate – its terminal velocity – once the mooring is totally submerged. But a surface mooring will always have a varying descent rate because of the effect of drag on the mooring.

Eventually, the anchor will land on the seabed. There is some settling, including a bit of overshoot, as the entire mooring slows down and the flotation pulls it taut to its expected static tension.

This snapshot from a ProteusDS Oceanographic analysis shows a surface mooring in the middle of an anchor-last deployment. The anchor (A) moves vertically downward to the seabed, while the buoy (B) stays at the surface and travels horizontally during this transition.

Why isn’t anchor-last deployment used everywhere?

There aren’t many different ways to deploy moorings. But in some cases, anchor last deployments just can’t be used. For moorings deployed in ice-covered seas, an anchor-first process has to be used. In an anchor-first process, the ship crane supports the entire anchor and mooring weight as the system is deployed through a small hole cut in the ice. As you can imagine, there’s no room to lay out the mooring on the water and steam an icebreaker to the deployment location in these circumstances!

In summary

The anchor-last process is used widespread for longer moorings. Typically, a ship will slowly advance toward the final deployment location. This allows the mooring to be assembled and strung out behind the deployment vessel to prevent any snags. The last step is dropping the anchor near its installation point.

Many new runners may jump right into the sport and end up with injuries without training that follows a tried and true process. Likewise, anchor-last deployment is a tried and true process for oceanographic moorings. There are always many details and specifics that arise with different mooring designs and lengths, but many fundamental steps follow this anchor-last deployment process.

Next step

We covered what the anchor-last process looks like. But what about specific details to look closely at during the deployment process? Read more to learn about specific effects to look out for during deployment here.

Thanks to WHOI

Thanks to Rick Trask at WHOI for sharing technical pointers and details on the anchor-last deployment process.

Why you see modular oceanographic mooring design everywhere

Posted on April 16, 2021
Ryan Nicoll

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.

When to check instrument depth in oceanographic mooring design

Posted on February 24, 2021
Ryan Nicoll

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 132m. This compares reasonably well with the maximum measured knockdown of 160m. 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 135m of vertical knockdown at the top ADCP float 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

Read more on validation cases, including the EAC 4200m mooring, in the oceanographic validation document published here. Note the EAC 4200m mooring is also a sample mooring 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.