A laser as a watchman

Ports, offshore wind farms and data cables form the backbone of supply security and are increasingly under threat. The Institute for the Protection of Maritime Infrastructures of the German Aerospace Center (DLR) in Bremerhaven is conducting research on how best to monitor and protect them.

On a lake near Bremerhaven, scientists are testing distributed acoustic sensing (DAS) as a means of detecting surface and underwater drones.

Credits: DLR

Targeted collisions in wind farms, destroyed pipelines and severed data cables are scenarios that no one wants to think about, but it is important to prepare for them. “Security threats like these are usually the result of human action – for example, with ships and other vehicles operating in the vicinity of these facilities,” says Frank Sill Torres, Director of the DLR Institute for the Protection of Maritime Infrastructures in Bremerhaven. Cargo ships measuring 100 metres or more in length, in particular, are potentially dangerous, whilst smaller boats can damage data cables – for example, with trawl nets.

“Wind farms, in particular, need to be protected – especially the central converter platforms,” says Sill Torres. Another threat is ships of the so-called shadow fleet that often deactivate or manipulate the position signal of their automatic identification system (AIS) to conduct their illegal activities without detection. “The challenge is identifying these ships specifically – the ones that want to remain invisible or the rust bucket tankers that masquerade as harmless fishing boats.”

According to Sill Torres, an electrical engineer with a PhD, that is why it is so important to know what is going in the direct vicinity of maritime facilities “Situational awareness, the ongoing tracking of developments, is of the utmost importance.” The DLR can draw on many years of experience in the aerospace industry and use satellites and airborne sensor systems for monitoring.

The greatest challenge in the maritime industry is the large expanses of water. He adds: “You can’t install cameras everywhere, and existing satellites can’t monitor everywhere at the same time.” As they circle the earth, there are inevitably temporal and spatial gaps in surveillance.

Too expensive, too short, too many gaps

Theoretically, monitoring would be possible with a large number of satellites, but that would be very costly and just about impossible to finance. “There need to be more affordable security solutions,” says Sill Torres. And drones are still hampered by their limited flight times. “Even high-performance models with up to twelve hours of flight time would have to return twice a day for recharging – it would take multiple drones in the air at the same time to ensure full coverage.”

The institute’s ongoing project “Analysis systems for the early detection of potential threats to critical undersea infrastructure” (AFKUI), which has received funding of 3.3 million euros from the Ministry for Economic Affairs and Energy, has therefore opted for a different approach. It adds an underwater component to the operational picture for authorities, demonstrating the additional benefit of a technology that does not provide images, but listens. “A promising technology that we’re currently researching is distributed acoustic sensing,” explains Enno Peters, Head of the Sensors Working Group at the institute. DAS, as it is abbreviated, simply reports that there is something here. A pulsed laser emits light pulses approximately every millisecond – ultimately 1,000 hertz – in a glass fibre that is up to 100 kilometres long – for example, along a power or export cable to an offshore wind farm in the North or Baltic Sea. The key trick here lies in analysing the phase of the light: “If a vibration or an acoustic signal hits the fibre, there’s a minimal shift in its structure. And that can be measured through the phase of the light.”

The light is continuously backscattered, and the transit time can be measured to pinpoint the exact section of the fibre that the signal is coming from. This, in turn, makes it possible to determine if something is happening and where – without a camera or visual contact, but simply by listening in the cable. Further information – like the speed and size of an object – can be obtained in this way.

Two men are standing at the open rear of a vehicle, looking at a monitor displaying measurement data; one is wearing an orange high-visibility jacket and is pointing at the screen.

Though it looks like a typical server rack, the interrogator unit transforms a 100-kilometre submarine cable into a sensor. The enormous volume of raw data is analysed directly on site, with only a warning sent afterwards.

Every ship has its own sound

“Due in large part to their propellers and engines, ships generate distinctive vibrations, which travel through the water and along the sea floor to the cable,” explains Peters. For larger ships, the frequencies typically lie between 10 and 60 hertz. “Because every ship generates a slightly different vibration profile, we’re optimistic that we’ll be able to classify them in the future – whether it’s a large or small ship, container ship or smuggling vessel. It’s an exciting field of research that’s still evolving.” The quality of the signal is also dependent on the depth of the cable in the sediment and water. The hardware was not developed by the institute, but is a proven technology already used along railway lines for train location and monitoring, for example.

The system has two areas of application in the project. For perimeter protection, a cable is installed around an infrastructure like an LNG terminal, a wind farm or a port entrance. It responds when something like a diver, an autonomous underwater vehicle or a motorboat approaches at night. No cameras would be able to capture that.
The second area of application is maritime surveillance, which uses the existing submarine cable between the coast and offshore wind farm, with the DAS device connected on land. The cable acts as a hydrophone array, registering everything that passes above. As the system can identify both approved and suspicious ships, other sources of data must be connected. If the cable reports a ship without AIS signal, radar satellite can be used to determine if something is actually there. Authorities can then respond accordingly – for example, with a drone or a visual satellite image.

A live demo at the end of the project in 2029 will prove that it works in practice: a ship with deactivated AIS will pass over the cable and an autonomous underwater vehicle will pass underneath it – the system should be able to identify both and automatically sound the alarm. (cb)

“Security threats are usually the result of human action.”

Frank Sill Torres, Director of the DLR Institute for the Protection of Maritime Infrastructures in Bremerhaven.

“Ships generate distinctive vibrations, which travel through the water and along the sea floor to the cable.”

Enno Peters, Head of the Sensors Working Group at the institute

Facts

DLR Institute for the Protection of Maritime Infrastructures

Established: 2017, as one of seven new DLR institutes
Location: Bremerhaven
Aim: Better protect maritime infrastructures through applied research in collaboration with all those involved, which translates to identifying dangers early on, neutralising them in a targeted fashion and responding flexibly to new threats.
Employees: 75

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