Day: 5 December 2022

Why you need breathing space between glass floats on oceanographic moorings

When you’re in a rush, finding a place to park your car can be a relief. But relief can turn to dread when you eyeball the spot and start to think it might be a squeeze. Trying to park a car in a place that doesn’t quite have enough space is stressful. When it’s tight, it’s more likely to damage things, leaving a gash in a wall or another car.

You know you need to get the angle right, but without much space, you might be frustrated going backward and forward in little bits to adjust. And when you’re next to a pillar or wall, somehow, you have to squeeze yourself in and out awkwardly through a narrow gap in your door. A little extra breathing space makes all the difference.

Similarly, breathing space makes all the difference when it comes to groups of glass floats on oceanographic moorings. There’s a good reason you need more than a narrow gap between them: it’s more likely to damage things when it’s tight. But the damage we’re talking about is more severe than bumps or scrapes.

Parking a car in a tight spot is stressful!

Why is breathing space between glass floats important?

Each glass float creates an uplift force because they’re hollow. They’re often made with particularly thick walls that make them incredibly strong and useful for all ocean depth ranges. But the problem is that although they’re incredibly strong, they’re not invincible. Glass is also very brittle – when it breaks, it tends to shatter catastrophically all at once. So when a glass float fails at some water depth, under immense hydrostatic pressure, there is an instantaneous and catastrophic implosion – an implosion so violent it creates a shockwave.

Glass floats in protective hard hats in a bundle on a mooring line. Picture credit: James Potemra U. Hawaii SOEST

You want a buffer from the implosion shockwave

If other glass floats are too close by, the shockwave can be enough to trigger even more failures – an event referred to as sympathetic implosion. But what exactly do we mean by breathing space?

It means spacing things out

It’s common to see clusters of glass floats on moorings. Because glass floats are small, they’re often used in bundles to provide enough flotation to support a mooring in the water column. Typically, these bundles are made up of a few pairs of glass floats spaced out on a short length of chain – perhaps a total of four on a 4m chain. The connecting hardware required to string the lengths of chain together introduces even more spacing, too. This spacing helps spread out the impact from the shock wave when any single float might fail. Spacing is helpful, but there’s another way to guard against the effects of implosion.

Implosion of a single float means an instant loss in uplift.

Too much loss in net uplift can mean the mooring may collapse to the seabed. Fortunately, redundancy is a direct way to counteract this. With some redundancy, the mooring can remain upright and recovered even if some floats implode.

A destroyed hard hat wtih imploded glass float. Picture credit: Dan Kot WHOI

But how many more floats should you add? It depends on the level of risk involved and how important the data and equipment on the mooring are. Understanding design measures are practical, but it’s not all you can do. What about reducing the chances of implosion from happening?

But just how common is implosion before rated depth?

Modern glass float manufacturing techniques have steadily improved quality and reliability over the years. Glass floats are always pressure tested to prove some capacity of depth rating when manufactured. Through years of designing moorings, it may be rare to see any implosions before hitting the depth rating.

Yet there are some cases when failures can be more common, though the underlying causes are not always obvious. Nevertheless, glass floats are usually reused over many deployments, and the risk of implosion increases as they age. Fortunately, this is where there is more control over the risk of implosion.

It’s not a snowball! Remains of an imploded glass float after implosion. Picture credit: James Potemra U. Hawaii SOEST

What can you do to reduce the chances of implosion?

Inspection of each of the glass floats used before and after deployment is helpful. Cumulative damage can show up in the form of spalling that looks like small chips off the surface of the float. Glass dust will start to appear inside the float from the load cycling as it is deployed at working depth and recovered. Too much of this dust, or glass chips above a specific size, means it’s time to retire the float. In addition to inspection, glass floats have their own particular handling needs.

Because glass is brittle, it doesn’t have the same capacity to absorb bumps and scrapes while handling them. Bumping a glass float on a bulkhead might seem like a minor accident, but it can cause a microscopic flaw in the glass. But under the incredible hydrostatic pressures at working depths, these flaws can be enough to trigger an implosion. Yet this is easily addressed by specific training in properly handling glass floats and is readily available from manufacturers like Nautilus.

Are there any alternatives to glass floats?

Syntactic foam is one alternative. These solid floats aren’t brittle and don’t have the same kind of handling requirements as glass floats need. They can also be engineered to withstand the immense pressures at most water depths. But there’s a downside. The larger the depth rating, the denser the foam must be for increased strength. This makes the float heavier and less effective at creating lift for the same volume as glass floats at more significant depth ratings.

Let’s look at examples of spacing

The CSIRO EAC mooring and SOFS moorings both have large groups of glass floats to support the mooring, particularly near the acoustic release assembly. Both use multiple groups of glass floats on short sections of chain, with varying levels of spacing. There are examples of a range of four to six floats spaced evenly on 4m chain section.

It’s time to summarize

Glass floats are commonly used, especially in applications reaching the deepest places in the ocean. They are cost-effective and extremely strong. But they are also brittle and can spectacularly fail when they implode at depth. An implosion shockwave can lead to a cascading failure through sympathetic implosion when used in clusters on a mooring. But you can minimize the problem by inspecting them before and after use, handling them properly, and using redundancy. Just don’t forget some space between them, too. A little breathing room makes everything easier – just you wish you had some when you’re stuck in a tight parking spot!

Next step

Check out some of the sample mooring layout files in ProteusDS Oceanographic Designer to see more detailed examples of glass float spacing. The CSIRO EAC and SOFS moorings have many glass floats near the acoustic release assembly. Download ProteusDS Oceanographic Designer and the sample files here.

Thanks to Nautilus GmbH

Thanks to Steffen Pausch at Nautilus GmbH for the helpful discussion and information on their Vitrovex glass flotation systems.