Losing an eye might seem like a critical point of failure, but not for a starfish. This is because starfish have multiple eyes on the tip of each arm. While their eyes aren’t very complex and consist only of a few photosensitive cells, they’re still simple and effective enough to navigate around their underwater habitat. However, they aren’t the fastest animal underwater. It’s common to lose a limb or two to predators from time to time. But even when things go wrong like this, it isn’t a big headache for them because they can keep going just fine with all the other arms and eye spots they have as backup.
Backup is one way to increase reliability. If something can take over when things go wrong, you still have a way forward. When it comes to recovering a mooring, the acoustic release is a critical point of failure. If the acoustic release doesn’t work, it can be a big headache to get your equipment and data back. In this article, we’re going to talk about when to use a simple and effective way to use a backup acoustic release.
Starfish have light-sensitive cells they use for vision at the end of each arm. Talk about backup!
What is a backup acoustic release?
An acoustic release consists of a mechanical release that opens when it receives a specific coded acoustic signal. In theory, only one is needed to recover a mooring. A backup acoustic release is when two devices are used in tandem in such a way that either unit can fully release the mooring. So what can this configuration look like more specifically?
A redundant configuration like this is simple and effective to set up. Two acoustic releases are attached to a single frame. Both release mechanisms are connected to a short line of chain that runs through a ring. The ring is then connected to the remainder of the mooring and anchor assembly. This way, when one of the acoustic releases is triggered, the chain will slip through the ring, allowing the rest of the mooring to float to the surface for recovery.
Tandem acoustic release configuration. Only one unit needs to trigger to release the mooring. Picture credit: Mooring Systems Inc.
Why is a backup acoustic release important?
When configured correctly, either acoustic release will disconnect the mooring from the anchor for recovery. This adds redundancy, so you can trigger the second one during recovery if there are any problems. Because of this redundancy, it significantly reduces the risk of losing the mooring. An acoustic release is unusual because it must function perfectly for a successful mooring recovery, so it needs to be reliable. So what affects acoustic release reliability?
Some reliability factors are in your control
A mooring design that keeps the acoustic release clear of the seabed during deployment and operation will help. Completing pre-deployment checks and standard battery life testing is crucial for a successful operation and recovery. But not all factors can be controlled, and one of these is battery reliability during a long-term deployment.
It isn’t easy to predict acoustic release battery life down to the minute
The longer the deployment, the more uncertainty there is. Many factors affect battery life: storage temperature, storage time, and operational subsea temperature, to name a few. How the energy is used affects battery life, too, and each time the acoustic release runs routines to listen for their specific release code, a little bit of battery energy is used up. Of course, some amount of energy is needed in reserve to trigger the mechanical release.
Yet modern acoustic release designs do an excellent job of managing their battery energy to maximize deployment life. Regardless of how sophisticated the energy management is, there’s going to be some uncertainty in energy levels, especially if you are pushing the limit of the deployment life of the acoustic release. Nevertheless, mooring designers can reduce the uncertainty and risk of totally depleting batteries on long deployments by using two separate acoustic releases in tandem. While this is a good technical reason for using a backup acoustic release, another good reason is the context of the project itself.
Consider the consequences if a single acoustic release doesn’t fire
Whether or not to use a backup release may depend on your backup plan to recover the mooring. A small surface mooring in shallow water could be retrieved by a diver or simply by pulling on the mooring line. But in deeper waters or for subsurface moorings, these are not practical options. Using a grappling line to recover the mooring is one possibility. But it can take a lot of time to drag the grappling hook back and forth in the hopes that you snag the mooring and pull it in. You also risk damaging instrumentation and equipment from the grapple!
Another factor to consider in the context of the project is the cost of the mooring equipment and ship time. Ship time alone can be tens of thousands of dollars per day, and the expenses of replacing lost mooring equipment and extra ship time to troubleshoot a recovery can be substantial. In this context, doubling up on an acoustic release makes sense!
It’s summary time
An acoustic release is a critical component for mooring recovery. It’s possible to use two acoustic release units together in a redundant configuration to increase the chances of a successful recovery. Though acoustic release units have excellent energy management, battery life can be uncertain, especially when pushing the limits of deployment life. A backup release helps increase the chances of recovery in a long term deployment. But the context of the project should be considered, too. The more complex the mooring and the higher cost involved in deployment and recovery make it worth having a redundant release assembly. Redundancy can pay big dividends in the long run – just like a starfish with eyes on each of its arms!
Next step
Check out a simple way to configure a redundant acoustic release assembly in ProteusDS Oceanographic Designer in this video tutorial from our YouTube channel:
Thanks to Teledyne Benthos
Thanks to Paul Devine and Summer Farrell at Teledyne Benthos for the information on acoustic releases.