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World War II depth charge detonated in the Fehmarn Belt: “It still had its full explosive force”

So-called bubble curtains were used during the explosion to protect porpoises, seals, and other marine mammals. Illustration: Femern A/S

A mandrel, two sledgehammers, a fretsaw, a chisel, a power drill, a gramophone, and 16 explosive charges were enough for the danish movie Olsen Gang, when they, with the orchestra playing overture in the background, stole a rare Ming vase and a suitcase full of banknotes from the Royal Danish Theatre.

Although the list of materials looked somewhat different, it required equally meticulous planning for the largest and last of several World War II explosive remnants found near the Fehmarnbelt tunnel project to be neutralised at the start of the week.

An operation that had been planned and prepared several months in advance reached its climax when a 180-kg German depth charge containing 125 kg of explosive charge was detonated in a controlled explosion at 15:04 on Monday afternoon.

Explosives engineer and bomb expert Michael Seckington described the process to Ingeniøren. He works as a project manager at the Dutch construction company Boskalis, which was in charge of removing the bomb from the bottom of the Fehmarn Belt.

Anti-submarine bomb

The bomb is a German depth charge of the D2 type from World War II, and it weighs 180 kg, 125 kg of which is explosive charge.

It is designed so that it can be set to go off at specific depths based on water pressure. It was used in attacks against submarines and usually dropped from ships.

See how the explosion looked from the surface.

Michael Seckington’s theory is that the German ship that dropped the bomb overestimated the water depth, so the bomb is set to go off at a higher water pressure than is present where it was found, at a depth of 26 meters.

As the underwater image suggests, it is barrel-shaped. The barrel is 570 mm long and has a diameter of 450 mm.

The depth charge could be set to go off at a depth of 30 m, 60 m, 90 m, and 120 m. Illustration: Boskalis

“It’s a relatively common type of bomb, but it’s somewhat unusual to find it at a depth of 26 metres, as it’s designed to hit submarines, which normally operate in deeper waters,” he says.

That is why it is so dangerous.

“If it’s set to, for example, go off at a depth of 30 metres, it doesn’t take much for it to detonate.”

16 compressors power the central protection

The construction company Boskalis was tasked with carrying out a detonation that was as gentle to the marine environment as possible.

Normally, a task like this would go to the Royal Danish Navy’s mine and bomb unit, but the Dutch construction company, which is also responsible for excavating the tunnel trench for the Fehmarnbelt Fixed Link, got the job because they have more experience with bubble curtains. A mitigation tool against underwater noise.

Femern A/S has entered into an agreement with the German authorities that in the event of a bomb discovery, bubble curtains must be used to protect marine animals, which is not normal practice in Denmark.

The “curtains” are actually bubbles created with two perforated hoses in a circle shape, which are usually placed at a radius of 130 and 180 meters from the bomb.

The hoses are connected to 16 compressors on a special ship which, according to Michael Seckington, can “generate an enormous amount of air”.

The 16 compressors have a total capacity of 1,600 CFM (cubic feet per minute). This corresponds to approximately 45,000 litres of air per minute. Illustration: Boskalis

The location of the bomb is actually determined as precisely as possible several months in advance, but since the bomb can move a little in the meantime, a remote-controlled underwater robot was sent down to verify the position immediately before the hoses were submerged from the special ship.

The underwater robot can also calibrate the location further when the cables are on the seabed.

Finally, a test is carried out in which the compressor runs at full power, while drones confirm via video and images that the bubbles form two concentric circles on the water surface at a desired distance to the bomb.

The optimal number of bubble curtains and their distance to the bomb was determined based on the bomb type. Illustration: Femern A/S

Detonation day

In the hours leading up to the explosion, the main task is getting marine animals out of the perimeter of the explosion.

At 1 p.m.—two hours before the planned detonation—five ships called MMOs (marine mammal observers), designed to detect marine mammals, sail around a radius of five kilometres from the bomb.

At 1:30 p.m. underwater noise is emitted from so-called pingers to scare the animals away. The pingers are mounted on buoys at a good distance from the bomb. Together, they form a safety zone that one wants to keep porpoises, seals, and other marine mammals out of. The pingers are active for 40 minutes.

But since the noise could also scare animals and cause them to swim into the zone and closer to the explosion, four ships with ultrasound transmitters set off at 2:10 p.m. from the bomb’s position.

They sail at low speed (two knots) towards the north, south, east, and west respectively, while emitting sound waves and thereby scaring off the animals still in the zone.

They sail out to a radius of 3,500 meters from the bomb and past the perimeter of the security zone. This lasts 50 minutes—until the planned detonation.

At 2:45 p.m. the two bubble curtains at 130 and 180 meters are activated at full pressure. At the same time, drones in the air are monitoring whether the bubble curtains are working and documenting the imminent explosion.

The detonation device is installed in parallel with the safety zone procedure. It consists of a buoy, a radio receiver, two plastic detonation tubes with a flammable mixture inside and plastic explosives. At 2 p.m. the underwater robot takes the whole package down to the seabed.

Boskalis’s underwater robot is a Seaeye Panther-XT Plus model produced by Saab Seaeye. It is equipped with three cameras and two arms, and it handled various tasks in connection with the detonation.

The underwater robot measures: 2140 mm length, 1060 mm width,1217 mm height . Illustration: Boskalis

The explosive, which is connected to the buoy via the detonation tubes, is placed against the German depth charge. The underwater robot then releases the buoy, which makes its way to the surface. The radio receiver (which is encrypted, Seckington emphasises) is also mounted on the buoy.

When the explosive kit is ready, a warning signal is sent to other ships, warning them that they no longer just have to stay 500 meters away, but 1500 meters. Patrol ships reconnoitre and confirm that the warning is complied with.

At this point, it is 2:45 p.m. and the bubble curtains have just been activated.

“And then we just wait until we get an ‘all clear’ from the five MMOs, so we are absolutely sure that there are no marine mammals nearby,” Michael Seckington says.

It happens at 3:04 p.m., and a ship with a radio transmitter sends a signal to the receiver on the buoy, initiating the explosion. Boom.

The explosion does not look like much on the surface. But the bomb was certainly potent a good 80 years after it was submerged, Michael Seckington says.

“It was a complete explosion, and it still had its full explosive force.”

This was confirmed when Boskalis subsequently once again sent the underwater robot down into the depths and established that the whole depth charge had been neutralised.

In the area around the explosion, 10 hydrophones were set up in advance to provide data for an analysis of, among other things, the power of the bomb and the effect of the bubble curtains.