Seabed warfare has come into sharp focus in the last twelve months, with several major interruptions to the security of seabed infrastructure, the most notorious being the as yet unattributed intruder attack to the Nord Stream pipeline. 

Such deep-sea operations are hard to carry out, not only because of a myriad of factors derived from the great distances being traversed and the intense underwater pressure, but also because they are difficult to complete covertly and without notice, says Ioseba Tena, commercial director at global maritime defence company Forcys. “If you’re running an operation in the middle of the Atlantic, at a significant depth, it’s a lot harder. There’s a footprint and a presence you’re going to generate in order to carry out a mission. It’s very hard to do that covertly.”

An alternative vector of attack comes through the littoral environment, the home of most seabed infrastructure, rich with refineries and power stations, cables and pipelines. Even infrastructure that spans the ocean will make its way through the shallows at some point, and it is here that they are most vulnerable to expeditionary forces seeking to interfere with a state’s energy or communications networks. 

Intruder detection on the seafloor

Representing a new advancement in seabed warfare protection, Forcys has developed a subsea intruder detection system, typically deployed in the littoral environment, designed to alert the operator to threats that would otherwise go unobserved, that has demonstrated success against both uncrewed underwater vessels (UUV) and divers trained to avoid detection. 

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The underwater environment has a lot of background noise that makes targets hard to detect and challenging to follow after detection. For conventional sonar detection systems, tracking a UUV presents a special challenge because of their small profile and the speed they travel at. Spotting a diver looking to target marine infrastructure is also a fraught with obstacles: even under normal circumstances they are very difficult to detect because of their small size, but this difficulty is heightened by the shrewd tactics special forces employ on such sorties. 

Tena offers as an example, the capacity for divers to elude sonar detection by hugging close to a harbour wall and staying still. “Traditionally, when you’re playing around with these systems, divers that will try and play games. So, they’ll play those games where they stop next to a wall, and you can’t see them again until they start swimming towards you again.” 

Forcys has developed technology that couples sonar detection methods with sensors that listen for the natural sound signature of its target. “One of the things that we noticed,” says Tena, “is that if you start to evaluate that passive signature at that frequency of a contact, you can start to tell apart those contacts from the background.” 

‘At this bearing, there is someone breathing.”

For UUVs this means listening for the mechanical components acting within the vessel; for divers the intruder detection system listens for the sound of the rebreather apparatus. “What we’re able to do now is track them, and then use the passive component to keep that bearing while they stop.”

It is a misnomer to call these emissions ‘natural sound’ as the frequencies being detected are inaudible to the human ear, but the signal is distinct from normal sonar detection because the system is not relying on use of sonar to locate threats. The intruder detection system is truly utilising sound created by the adversary. However, this leap forward in protection is due to a combination of both the audio detection and a sonar component. “In effect,” says Tena, “what it’s doing is to say: at this bearing, there is somebody breathing.”

The intruder detection system that Forcys has developed is a 35kg unit emitting a 70 kilohertz sonar and a passive listening sensor at 72 kilohertz.  “It’s a really delicate microphone, to be fair,” adds Tena. When stationed on the seafloor they can sit at rest for two weeks before being raised for cleaning and maintenance, when they are then ready for redeployment, at the same site, or elsewhere.  

Although the system has been used as a defence against intrusion, it has applications for expeditionary sorties, mounted upside-down on a tripod from a battery-operated topside vessel. “We’ve had clients tell us that we’ve said they save their lives, but that’s as much as we can share on that.”