It was Christmas time, but I didn’t expect to get a haircut as a present. On this particular holiday, one of my young son’s presents was a small automatic flying drone. It was a seemingly harmless small toy that merely hovers and slowly drifts around indoors automatically, without the need for a pilot. But sometimes, it would change direction quickly and catch us off guard. It bumped into my head within less than 5 minutes, and the propellers snarled into my hair. It took a lot of working on getting it unwound and even a few snips to get it free. It was a tangled mess!
Nobody likes to deal with a tangled mess, so if you can avoid one entirely, so much the better. When it comes to oceanographic mooring recovery, there is a considerable risk for the entanglement of the line. When the line forms one of these giant tangles (also known as wuzzles!), it can take a lot of time to sort out and even damage equipment. But with the right precautions, it’s possible to avoid a wuzzle altogether.
Getting a small flying drone tangled in your hair is a serious tangled mess. It pays to avoid problems like this!
So what exactly is a wuzzle?
Strange as it might seem, wuzzle is an actual term to describe a tangled mess in a line. Why does this happen? If a mooring line loses tension and has a chance to bunch up in an area, it may drift into a convoluted mess. When there is low line tension, the residual twist can easily throw the system into a tangle.
During mooring recovery, there’s ample risk for a wuzzle
This is because there is both low line tension and the opportunity for the mooring line to bunch up. There’s a strong chance of this if you’re working with a mooring recovered using an acoustic release.
A wuzzle in an oceanographic mooring is also a serious tangled mess. Picture credit: Danny McLaughlan, courtesy of CSIRO/IMOS
An acoustic release facilitates mooring recovery
An acoustic release is fired when a unique coded signal is transmitted. The result is the device opens a mechanical connection on the mooring line. Acoustic releases are usually placed near the anchor along with a significant amount of flotation.
It’s this significant flotation that brings the tail end of the mooring up to the water surface. The problem is when the tail end catches up to the top float or surface buoy. In the worst-case scenario, the entire mooring can be bunched up in one location in a low tension state. It’s a wuzzle in waiting, and unless you take action, it’s going to be the biggest headache of your life to untangle.
How can you avoid a wuzzle on oceanographic mooring recovery?
You can reduce the risk of a wuzzle with careful observation and operations management. The recovery vessel needs to have a rough idea of the ocean currents at the time, so they expect where the mooring and ship will drift through the process. Typically, the recovery crew can get a location fix of the acoustic release, so they know a safe distance to wait.
However, the recovery vessel may attach a towline for a surface buoy and start pulling gently on the buoy laterally before even firing the release. For subsurface moorings, the recovery crew is on the lookout for the top buoy to reach the surface first. Once they see it, they’ll steam over in the recovery vessel and attach a tow line. Either way, the key is for the recovery vessel to pull the surface buoy or top float away from where the tail end of the mooring is expected to come up. It keeps the mooring from pooling up and reduces the chances of a tangled mess.
How do you know how much time you have before the acoustic release reaches the surface?
There are a few ways to anticipate this. The easiest way is to keep acoustically pinging the release as it rises through the water column. A fix on the release tells you how fast it is rising in real-time. But it’s also possible to complete a recovery analysis in a software tool like ProteusDS Oceanographic. Depending on how much flotation and the line properties you use in your mooring, you can get an idea of how much time you might expect to see it reach the surface.
Snapshot from mid-simulation of mooring recovery. The acoustic release has already been fired and the flotation pack is bringing that end of the mooring to the surface at A. The primary buoy is already at the surface at B.
Summary
When it comes to entanglement, the best approach is prevention. When my son wants to bring out his automatic drone, I now keep a safe distance. I am also considering wearing a hat, and possibly just going into another room altogether. When it comes to oceanographic mooring recovery, prevention is key as well through ship operations.
Next step
Read more on tools for oceanographic mooring design with ProteusDS Oceanographic here.
Thanks to WHOI and CSIRO
Thanks to engineer Don Peters at WHOI for the discussion and feedback on the article topic and also to CSIRO/IMOS for the use of the image.