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When profilers are specially adapted to ride ocean waves

Originally published on dsaocean.com

When you think of Owls, “ninja” might not be the first thing that crosses your mind. But there’s a good argument for it: like a ninja, owls are incredibly stealthy. These birds are the only animal that can fly absolutely silently. Their silent flight ability stems in part from their special serrated and flexible feathers. These feathers significantly reduce airflow noise over their wings. They also have a remarkable downy plume that helps absorb any amount of noise that they do make while in the air. They’re incredibly successful at stealth because of these special adaptations.

Special adaptations are often the key to success in any niche. When it comes to oceanographic moorings, profilers have a few adaptations of their own. Some profilers are adapted to take advantage of, and even thrive on, the power of ocean waves. In this article, we’re going to talk more on wave powered profilers.

Wave powered profilers may sound complicated at first glance

After all, it’s no easy task to harness the power of ocean waves. Now, wave powered profilers certainly do use the power of ocean waves to drive themselves up and down mooring lines to make measurements in the water. But they are a far cry from complex machines and are instead quite simple and elegant in their design.

It’s this simplicity that is their real advantage

Profilers need energy to crawl up and down mooring lines while their on-board instruments make measurements in the water column. A powered profiler will need a lot of extra hardware, like batteries and electric motors, to creep along the line. But all this additional hardware can also introduce problems. A powered profiler system is also heavier, and more space is taken up by batteries.

A wave powered profiler doesn’t need any of these extra components: it uses the extra energy in ocean waves to drive the profiler along the mooring line. So it’s lighter, and also less complicated. After all, the fewer moving parts and components there are, the fewer chances of something breaking or otherwise going wrong. But their simplicity also leads to another advantage.

Wave powered profilers can last a long time

A mooring deployment with a powered profiler may be limited by the time it takes for the profiler’s batteries to run out. But a wave powered profiler can keep making profiles as long as there are ocean waves around. In the open ocean, there is quite often some wave action almost all the time. But how do the waves power the profiler?

You won’t find a tiny generator somehow capturing wave energy in these profilers

A wave powered profiler will only work when it’s matched with a properly designed mooring as an integrated system. The mooring certainly needs a surface buoy and a small amount of sustained tension in the line to keep it upright in the water column.

All wave powered profilers work using a similar principle

They grip tightly onto the mooring line in one direction but slide easily in the other direction. Wave powered profilers make this happen by using pairs of rotating cams that clamp onto the mooring line. While the mooring moves up and down in the water column from wave action, the cams allow the profiler hull to ratchet along in a stick-slip fashion.

But there’s another trick too: the wave powered profiler needs to be just a little bit off neutrally buoyant in the water: it needs to be able to just barely float up to the surface if left free to slide on the mooring.

Now, the stick-slip action happens lightning quick

It has to, as the system makes use of an ocean wave crest that passes by the surface buoy. These small wave crests may only be 1-2 seconds in time. As an ocean wave crest comes along and pushes upward on the surface buoy, the entire mooring line moves upward with it. As the mooring line accelerates and moves upward, the cams on the wave powered profiler slip on the mooring line. While the mooring line moves up, the wave powered profiler actually doesn’t move upward with it.

In essence, it’s the mooring line that’s moved a little bit upward, pushed up by the ocean wave, while at the same instant, the wave powered profiler stays put in the water column. This is the “slip” – there’s a small amount of relative motion that creates a ratcheting effect.

At the moment the ocean wave crest is passing, the surface buoy starts to move down again

Now, the mooring is moving back down in the water. As the mooring line moves down through the profiler, the profiler cams clamp back on in a fraction of a second. This is the “stick” – the wave powered profiler is now securely clamped on and moves down the water with the mooring.

This stick-slip ratcheting motion can happen extremely quickly. As a result, even small ocean waves will cause the wave powered profiler to move down, perhaps only a few centimetres at a time, but inexorably downward into the water.

So the waves can help it get down quite a ways into the water

But what then? At a certain point on the mooring line, there is an end stop clamped in position. The wave powered profiler bumps into this end stop, and the cams are flipped fully open. Since the wave powered profiler has just a little bit of positive flotation, it slides up the mooring line at a constant speed, making measurements all the way. It hits another end stop at the top, and the cams are flipped back on in contact with the mooring line, ready to start ratcheting back down the mooring line again. It will keep going through this cycle over and over as long as the ocean waves keep coming.

It’s time for an example

Del Mar Oceanographic develops the WireWalker Profiler. This device is generally used with a slight net positive flotation. This amazing stop motion video produced by Dr. Melissa Omand and Dr. Collen Durkin and shows how the WireWalker ratchets along the mooring.

Wave powered profilers will work well in more exposed areas

The larger the ocean waves, the faster it can ratchet down the mooring line. But speed isn’t the only advantage. Wave powered profilers can be deployed on tiny vessels with a small team. It doesn’t necessarily need a large ship and crew to deploy. Remote locations, far from major ports, can work just fine, too.

In spite of their elegant design, they can still run into trouble

If there’s too much water current, the mooring might tilt too much, or the drag force might stop the wave powered profiler from ratcheting down the mooring line. Of course, it needs ocean waves to work, too.

But doesn’t it need large waves to work?

You might think the system will work better and run more profiles in large ocean waves. However, the mooring surface buoy and profiler are typically small enough to respond even to very short period wind waves. But there is a lower limit – ultimately, the waves need to be big enough so the surface buoy and mooring will move.

We covered the operating principles of wave powered profilers and now it’s time to review

A relatively simple mechanical design makes it possible for profilers to use the power of ocean waves to climb up and down mooring lines. Ultimately, these designs rely on a mooring that moves in ocean waves, and a cam-driven stick-slip action to ratchet along the line.

This simple mechanical design makes it both reliable and also a good option for longer term deployments. But you still need ocean waves and a surface buoy to make them work.

Owls are successful stealthy hunters in large part due to their unique feathers

It’s these feathers that form part of their special adaptations. Likewise, the special adaptations in mechanical design allow profilers to make use of ocean wave power. This opens up a lot more opportunities to make use of profilers for oceanographic data collection.

Next step

Read more on Del Mar Oceanographic’s WireWalker wave powered profiler here.

Thanks to Del Mar Oceanographic

Thanks to Chris Kontoes and Drew Lucas from Del Mar Oceanographic for the discussion and information on profilers. Thanks also to Dr. Melissa Omand and Dr. Collen Durkin for making the very creative stop motion video, too!