Month: February 2021

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 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


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.


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

Introducing DSA Ocean

Dynamic Systems Analysis Ltd. is an organization that cares about the ocean; it plays the central role in our business. It is a source of beauty and inspiration to our team. Whether we’re surfing, sailing, looking at it, or working on it, we are passionate about the ocean that lies beneath all of what we do.

As is somewhat self-evident from the name Dynamic Systems Analysis Ltd., when Ryan Nicoll and I started our company, we had the vision to create a business centered on dynamic simulation technology. We were interested in helping engineers to design better products. We were passionate about enabling others and growing. It wasn’t a completely ocean-exclusive idea at conception. But coming from the Subsea Robotics Lab at UVic, we never fully broke away from marine applications. The ocean is a challenging place to work, and there is no shortage of opportunity to apply digital technology to help solve problems.

Indeed, over the years, the use of ProteusDS, our marine dynamic analysis software product, has grown substantially. And we’ve become thoroughly committed to supporting the ocean industries we work in through organizations such as Marine Renewables Canada, ABCMI, BC Salmon Farmers, and CIMarE. Our staff are naval architects, ocean technologists, ocean engineers, civil engineers, and fluid dynamicists. Our customers and hence our business is ocean-based.

So today we’re eager to announce that from now on DSA will be doing business as DSA Ocean. Our new brand, DSA Ocean, pays tribute to where we’ve come from. The new logo preserves our cable-dynamics heritage – with that iconic ROV umbilical cross-section. It retains the familiar DSA as a reference to our legal name Dynamic Systems Analysis Ltd. But the new name and logo strongly emphasize our connection to the ocean and how we engage in the broader blue economy. We’re excited to take this new focus forward with our new and future clients!

Dean Steinke, CEO
DSA Ocean

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.

ProteusDS v2.56.1

Release Date: 2021-02-04 (v2.56.1)


  • Improved stability and performance of seabed contact model
  • Resolved issue with drag and added mass calculations in small volume fraction of submergence of cylinder and sphere hulls
  • Resolved issue with inaccurate accelerations reported by rigid body acceleration probes when motion RAO was active
  • General performance enhancements and usability improvements to ProteusDS Simulation Toolbox