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How deepwater moorings disrupt wave-powered profiling (and what you can do about it)
Moths thrive at night but are threatened by many hunters in the darkness. Chief among these predators are bats, who are incredibly adapted to snatch prey in the pitch black night using sound waves and echolocation. But moths are not mere hapless and plucky flying snacks for predators: millions of years of adaptation have produced sophisticated capabilities for protection. Some moths have features on their wings which make their own ultrasonic clicks. But they don’t use these clicks as a warning or distraction. It’s completely for interference. The bats are left empty handed when they become temporarily blind as their echolocation system is disrupted.
Some disruptions are good. But not all disruptions are helpful. Some can be a pain in the neck and need special attention. In the world of oceanography, longer mooring lines can disrupt wave powered profiling rates. If you don’t understand how these systems work in deeper waters, you may be left empty handed and with blind spots in your data when you don’t get the profiling rate you expected. In this article, we’re going to cover what causes interference with wave powered profilers with longer moorings in greater water depths.
What we’re going to cover is:
- line weight
- resistance from current profile
- downweight limits
Let’s start with line weight.
Wave-powered profilers rely on inertia to work correctly – but not in a way you’d expect
There are two phases to how wave-powered profilers work: profiling and ratcheting. The whole concept works well when the profiler itself is a smidge buoyant but has a lot of inertia. The small amount of positive (or negative) buoyancy means the profiler can make a constant speed measurement of all parameters as it slides along the profiling section of the mooring. But it must ratchet back to its original position to make another measurement profile.
The basis for this ratcheting behaviour to reset the profiler is wave action on the surface buoy. But the mooring has to be pretty light so that the surface buoy can lift the whole profiling section in a moment when a wave crest pops by. In a span of a second or two, the mooring zips up a small distance through the inside of the profiler, which stays more or less in one place in the water column.
So the deeper the profiling section, the longer – and heavier! – the profiling wire is going to be. This heaviness means the inertia goes up, and as the wire has more inertia, the buoy won’t be able to lift the entire line as much as the waves pass by. So, the line movement and, consequently, the profiling rate starts to degrade.
But what about using a larger float with more buoyancy capacity?
You could definitely use a bigger buoy with more capacity to lift a heavier line. But as the buoys get larger, the entire system becomes less responsive to heave motion from smaller waves. Depending on your specific site, it may also attenuate mooring movement and the profiling rate.
The increased inertia of the system isn’t the only issue that comes up with longer lines. A longer line has more complex considerations when it comes to drag, which brings us to the second point on resistance from the current profile.
The longer the line, the more drag area there is
The current speed at your specific site may not be much, but it’s the aggregate effect of drag over the entire system, including the profiling line, that you need to consider. The problem with too much drag on the system is that it deflects the system, and too much deflection starts to cause a tilt in the profiling line.
Wave powered profilers work best when the mooring is vertical
It doesn’t have to be perfectly vertical, but that’s when you get the fastest profiling rate. So, as the drag and mooring tilt increases, the profiling rate decreases.
There’s also a secondary effect of drag directly on the profiler itself. Whenever there’s any line tilt, some amount of the prevailing current also acts on the profiler itself. In the most extreme circumstances, this might lead to the profiler staying stuck at the top of its range on the mooring. But that’s the limiting case and certainly not going to be what happens with a well-designed mooring in moderate to mild currents. That said, there are things you can do to address the design in the face of prevailing currents. But there are limitations to what you can do, too, and digging into this brings us to the third point on downweight limitations.
The downweight is a crucial component that keeps the profiling section vertical
This weight from this clump mass pulls down on the profiling section, helping keep the line under control. The mooring system may float around the watch circle area from drift loads from wind and wave action on the buoy. The downweight is essential because the longer the line, the more weight is needed to keep it vertical while the system drifts around. But the tension in the line from the downweight also directly resists the effects of drag from any prevailing current profile. The larger the downweight, the more it resists drag and keeps the line under control. But this is where the problem lies.
There are limits to how big of a downweight you can use
For example, there are practical limits on how much weight you can handle when putting the mooring together or deploying it. Larger downweights also mean you may need to upgrade other components in the system with increased strength ratings. The increased line tension might even be enough for you to consider a larger buoy at the top of the system, too, which in smaller waves can reduce overall mooring motion and the profiling rate.
Getting the heaviest downweight you can possibly handle is also not always a good idea from a dynamics perspective. In extreme storms, when buoy motion is the highest, the downweight might have enough inertia to start moving out of sync with the buoy and mooring – leading to loss of tension and shock loading. But that’s pretty extreme!
Nevertheless, the main point is that the best size of downweight is not obvious and requires careful consideration in the design process if you don’t have extensive experience with a specific system.
It’s time to look at an example
The Ocean Time-Series and Multiscale Ocean Dynamics groups at Scripps Institution of Oceanography have designed and deployed moorings in depths greater than 4000 m for over 20 years. In partnership with Del Mar Oceanographic, several wave-powered profiling moorings were developed and deployed to collect high-resolution data to calibrate NASA’s Surface Water and Ocean Topography (SWOT) satellite. These SWOT calibration moorings used Del Mar Oceanographic Wirewalkers to capture profiles of the upper 500m mooring section. This profiler provided real-time data on the relationship between sea surface height and distribution of temperature, salinity, and density, which was crucial for calibrating the SWOT satellite.
Jeff Sevadjian completed an extensive mooring design and analysis using ProteusDS to evaluate the mooring tilt and effects of the downweight size in different current profiles and wave conditions. ProteusDS provided insight into the mooring deflection and motion, particularly around the region of the downweight and profiling section, which helped build confidence in the project’s success.
The array of SWOT calibration moorings was successfully deployed and recovered in 2023, resulting in thousands of profiles after several months of deployment in the Pacific Ocean.
It’s summary time
We covered a few topics on how to address wave-powered profiler mooring design in deep water, and it’s time to summarize. Wave-powered profilers rely on inertia to work. The mooring has to be light enough for the buoy to lift it and slide it through the profiler a small amount as waves pass by.
The longer the profiling section of the mooring is, the more inertia and weight it has, and the less mooring motion you get from the same size buoy and waves.
The other factor is the tilt of the line: the more line tilt there is, the less efficient the profiling action is. The longer the mooring is, the more it is susceptible to significant tilt from prevailing currents or from the simple weight and inertia of the line when it drifts around from wind and wave action on the buoy. A larger downweight can balance this, but the size of the downweight is also a practical limitation in terms of handling and the impact on the dynamics of the mooring in more extreme conditions.
It’s possible for wave-powered profilers like the Wirewalker to operate through hundreds of meters of water depth. If you don’t keep an eye on factors like this that might disrupt or even halt your profiling rate, you may be surprised to find yourself empty-handed – much like a hungry bat zeroing on a moth he just can’t catch!
Next Step
The publicly available sample mooring files include a template Wirewalker mooring configured for 200m water depth. Use this as a starting point to learn what might be possible in different water depths with ProteusDS. Download the sample mooring design files from the Documentation page or direct link here.
Thanks
To Jeff Sevadjian from Scripps Institute of Oceanography for the figures and data used in the example and feedback and discussion on the article.
PS
Read more on the calibration of the SWOT satellite and Scripps work in collecting data from multiple moorings for the project:
