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Why indirect relationships drive wave buoy mooring design
A marching column of leafcutter ants is a mesmerizing sight. The tiny ants carry a massive piece of leaf cut in angular forms like a puzzle piece. The leaf wobbles awkwardly as it’s taken along a convoluted path back to a nest. Indeed, there is typically a whole column of ants following a way back to their nest with these leaves, with soldier ants patrolling nearby and viscously attacking anything else that may disrupt the foraging lines. It’s an incredible amount of organization and effort they go through. Yet the ants don’t eat the leaves. So why do they go to so much effort? It’s actually food for the fungus that they carefully cultivate in gardens in their nest. The fungus is, in turn, a critical food supply for their young, without which they won’t survive. The leafcutter ant puts in a lot of effort for an indirect relationship.
Similarly, there are many indirect relationships in wave buoy mooring design. In many oceanographic mooring designs, the primary focus is on mooring loads and checking the required component strengths. But in wave measurement buoy moorings, the loads are often very low to minimize the impact on data quality. The result is that many indirect relationships drive the mooring design rather than component strength. In this article, we’re going to cover several of these indirect relationships and why they’re essential:
- Buoy submergence
- Mooring watch circle
- Self-entanglement
First, we’re going to cover buoy submergence.
What is buoy submergence?
Moorings always have some give and take to keep a buoy on station. In this case, it needs to provide the space for a wave buoy to do its job to get an accurate measurement of the sea state. But there are limits: a mooring must also ensure the buoy stays on station. In certain conditions, the buoy can be overwhelmed by larger mooring loads and submerge. Different combinations of current, current and waves, and even high sustained winds can cause buoy submergence. So understanding how the mooring and buoy performance and when it submerges can help in anticipating problems with data quality.
A wave measurement buoy can’t measure waves when it’s underwater
It may still get a sense of what motions are like, but there’s a significant amount of uncertainty on data quality. Buoys that submerge in extreme conditions aren’t capturing the full height of the waves, so they may be reporting smaller sea state conditions than it is actually in. Buoys that submerge in high currents may not report anything for long periods of time at all. If the buoy is real-time, communications will be interrupted, making headaches for interpreting the data and post-processing. Yet submergence isn’t the only thing that affects data quality: the deflection of the mooring itself can have an impact. This brings us to the second point on indirect relationships on the mooring watch circle.
A mooring watch circle is like a footprint of possible motion
It’s an area that a moored buoy may be in during deployment. For a single leg mooring like those most commonly found in oceanographic systems, over time, they tend to sweep out a circle as the changing winds and currents cause them to drift around. The watch circle is the boundary of this motion footprint. So what’s so important about the watch circle?
The watch circle itself is an indicator of uncertainty
The larger it is, the less certain you can be about the location of the associated metocean measurements. This may not matter as much in a lone oceanographic buoy thousands of kilometers from shore. But in a coastal environment where the wave intensity may be strongly affected by bathymetry and shielding from islands, you want to keep an eye on the watch circle.
There’s an element of safety, too. Collisions with boats are a significant problem with wave buoys. If there’s marine traffic or channel nearby, a larger watch circle might affect the chances of a collision. The watch circle can be reduced by decreasing the length of mooring line in the water. But be careful because shortening the line length can increase mooring loads and interfere with wave measurement. Mooring loads and watch circle aren’t the only things affected by the amount of line in the water. This brings us to the last point on indirect relationships on wave buoy mooring design: self-entanglement.
Self-entanglement is when a mooring winds up around itself
It might catch on a specific component, like a mid-water float, or acoustic release, and wind up around these components more and more over time. This is possible because of the low-tension nature of these moorings and the changing direction of metocean conditions that cause them to drift around in their watch circle.
Self-entanglement is a problem because it affects mooring performance
As the line is wound up, there’s less free line in the water column and less compliance in the mooring. As the free line length reduces, the reaction loads on the buoy increase, affecting wave measurements and increasing uncertainty. It can also lead to more buoy submergence. Tangled rope can increase wear and degrade components, causing failures. Even worse, it causes operational headaches, preventing proper mooring recovery if the acoustic release is entangled. It can be costly to deal with, requiring contingencies like diver intervention to address the problem.
So how do you deal with self-entanglement?
Watch circle and submergence is something you can check in a dynamic analysis tool like ProteusDS. But it’s not easy to predict when self-entanglement happens. Nevertheless, considering the limits of mooring design, a taut mooring can not self-entangle. Though taut moorings are not always practical for wave measurement moorings, the idea is that any mooring with line length greater than water depth has a chance of self-entangling. The larger the scope, or other words, the more line that’s in the water, the greater the chance there is of an issue with self-entanglement. You can’t reduce mooring scope too much though because mooring loads will start to increase in more severe conditions, affecting buoy measurements.
Summary
We covered a few examples of indirect relationships when designing wave buoy moorings, and its time for a review. Generally, in mooring design, there’s a focus on mooring loads and ensuring that components meet strength requirements. But wave buoy moorings tend to have low loads to avoid measurement interference. Yet several critical indirect relationships come up through the design process that can affect performance.
Buoy submergence is possible in extreme conditions: different combinations of wind, currents, and waves may cause the buoy to submerge, which can have a significant impact on data quality. Another factor is the watch circle: this indicates the maximum limits of motion, like a footprint, of the system. The larger it is, the less confident you are on the correlation between measurements and station. But it can have implications for safety and collision if there’s marine traffic around, too. Finally, self-entanglement is a problem, particularly if there is a lot of line in the water. It’s not easy to predict when it can happen, but it’s worth keeping an eye on because of the significant implication on operational costs and data quality. You can directly change component sizes to adjust for strength when you know what the mooring loads are. But these other factors are essential but related in an indirect way. Much like how leafcutter ants indirectly rely on their fungal gardens for survival!
Next step
The Datawell Waverider and Sofar Spotter buoy are both examples of wave measurement buoys. Check out the sample mooring layouts in ProteusDS Oceanographic Designer to use as a starting point for these kinds of indirect design factors here.
