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How to make the most out of flotation to minimize mooring deflections

For their body size, whales have smaller lungs than humans. Put another way, you would have a larger lung volume if you were as big as a whale. Yet Whales can hold their breath for hours under the water, but humans are typically in the realm of a few minutes. So how can this be possible?

Whales have advantages that aren’t obvious. One significant advantage is how they can get so much out of the lungs they have. Humans only use about 15% of their lung volume for every normal breath. In an extreme situation, if you really push it, you might get 80%, but this is a limit that most people can’t reach. In comparison, every single breath a whale makes at the water surface uses a whopping 90% – almost their entire lung volume! This is because whale lungs are much more elastic than ours, and it is easy for them to rapidly and completely reach such large air exchanges every single time. Whales do well to make the most out of what they have.

Likewise, in oceanographic mooring design, you want to make the most out of what you have, and sizing flotation is no different. Yet it can be overwhelming because there’s a wide range of sizes of floats available to you, from small plastic or glass floats to large syntactic foam shapes. However, larger floats have advantages that aren’t obvious. Minimizing deflection from drag means you need to take a closer look to understand why, and make sure every bit of flotation you have is put to use.

What do you mean by minimizing mooring deflection from drag?

The forces from current drag push oceanographic moorings over, causing deflection. Too much deflection can cause problems like significant issues with data quality to damaged equipment or lost moorings when components exceed their depth ratings. So a practical effort to minimize mooring deflection is always a good idea in the design phase. You can do a few things to adjust a mooring to reduce deflection, and one of the most obvious ones is to increase the amount of uplift force on the mooring. This means adding more buoyancy.

But you can’t just mindlessly add more and more flotation because there’s a catch: the floats add drag to the mooring, too. You can end up in a destructive design spiral by adding more flotation to reduce the increasing drag because you are adding more flotation! But this is where there is a hidden advantage to larger floats: drag efficiency.

What is float drag efficiency?

Drag efficiency is the relationship between the amount of float uplift to drag. Here is the kicker with spherical floats: the net uplift grows with the diameter cubed, while the drag only grows by the diameter squared. So what does this mean for different float sizes?

For the most enormous diameter floats, you have an excellent amount of uplift relative to the drag it adds to the mooring. Conversely, much smaller diameter floats have a fair bit of drag for only a moderate amount of uplift added. This creates an essential guideline for mooring design: consolidate flotation when you can.

Consolidating flotation means using a larger float in place of many small ones.

Rather than using a cluster of small trawl floats, can you use a single syntactic foam float? Bringing it together in a single entity significantly reduces the drag for the same amount of uplift. Considering the drag efficiency in this way can dramatically help improve mooring performance, especially when there are a lot of little floats in use.

Let’s look at an example

We set up an arbitrary mooring to make a comparison of drag efficiency of small and large floats. The scenario is 1000m water depth with 1.0m/s surface current tapering to zero at the seabed. The primary mooring line is neutrally buoyant 11mm stiff fiber rope. One mooring uses a collection of small plastic trawl floats, and the other uses a single large syntactic foam float.

The first configuration uses 156 twelve-inch plastic trawl floats. The second configuration uses a single 49 inch DeepWater Buoyancy Hydro-float. The net uplift of both float systems is approximately 560kg.

The static calculation in ProteusDS reveals a knockdown of 13m and an excursion of 130m for the mooring with the single larger Hydro-float buoy. On the other hand, the mooring with trawl floats has a knockdown of 23m and an excursion of 180m. The deflection of the mooring with trawl floats is almost double even though they have the same net uplift!

Don’t obsess about drag efficiency

It’s a valuable design guideline, but there are other ways to reduced mooring deflection in high flow currents. In a location with powerful flow speeds, you may need a massive float to resist the loads from currents. Rather than building a gigantic (and very expensive) custom syntactic foam sphere, there may be a way to avoid the most intense flow regions by using different instrumentation. An alternative low-drag float form factor may help, too, like an ellipsoid or torpedo-shaped buoy.

It’s time for a summary

One way to reduce mooring deflection from drag is by adding more flotation. But floats also introduce drag, too. For spherical floats, the net uplift grows much faster than the amount of drag. This means consolidating them to use fewer but larger diameters instead of many smaller ones helps you get the most out of your flotation. While it’s a useful idea, it’s also not the only requirement in design, so don’t obsess about it. Take a deep breath (in your inefficient non-whale lungs) and see if you can use the idea in your next mooring design!

Next step

Drag efficiency is one thing to consider when looking at the flotation needed for the mooring as a whole. But there’s more than one kind of shape that flotation comes in, so how do you decide what’s best? It might be a mistake to focus too much on different float drag coefficients. Read more on drag considerations of different oceanographic floats in the next article here:
proteusds.com/flotation-fixation