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When to check instrument depth in oceanographic mooring design

Originally published on dsaocean.com

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.

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 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.

One of the subsurface moorings in the array is located in 4200m water depth. We compared the deflection of a ProteusDS 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.

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 is also one of the publicly available designs you can explore in Oceanographic Designer. Download the sample mooring files from the Documentation area 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.