France’s Frankensub: How a damaged nuclear sub was repaired with leftover parts

PARIS — Naval Group has delivered the Perle nuclear-powered attack submarine back to the French Navy after an extraordinary — and, the company claims, first-ever — repair that involved replacing its front half, damaged by fire, with the front of a decommissioned sister sub of the same Rubis-class.

The navy said the vessel had been inspected by all necessary authorities who had confirmed its capacity to return to normal operations completely safely.

Naval Group gave no indication of the cost of the repair but according to former armed forces ministry spokesman Hervé Grandjean speaking in October 2021, repair costs were about €110 million ($120 million) with the defense ministry coughing up €60 million, and the rest being covered by Naval Group’s insurance.

The 73.6-meter-long, 2,500-ton Rubis-class submarines are the most compact nuclear-powered attack submarines in the world. First produced in the late 1970s, the model is slated to be replaced in the future by six Barracuda-class submarines. (The first Barracuda, the Suffren, was launched July 12, 2019 and was commissioned on June 3, 2022.)

The 26-year old Perle caught fire on June 12, 2020 whilst undergoing major renovations by Naval Group in dry dock number 3, in the Missiessy zone of the naval base shipyard in Toulon on France’s Mediterranean coast. Soldering work had taken place a few days beforehand.

During the 14-hour blaze the high temperatures of the fire altered the qualities of the steel at the fore of the submarine, making the ship unusable going forward.

But the aft of the vessel, the parts that house the nuclear power plant and propulsion, were intact. And when the blaze broke out the Perle was almost empty: the nuclear material, the weapons, the fuel, the electronic cabinets, pumps, ventilators and other equipment had all been removed.

Hence, the decision was made to follow an unusual repair plan: rather than try to repair the Perle’s fore section, the damaged part was cut off. Replacing it was the fore of a decommissioned sister sub, the Saphir, which was commissioned in 1984 and exited service in 2019.

The “frankenstining” of the two submarines took place in the naval port of Cherbourg, on France’s north-west coast. It was transferred there on a “roll dock” barge. Once that was done the boat returned to Toulon, also by barge, on Nov. 1, 2021 so that the major upgrades it had been undergoing when the fire broke out could continue, including the integration of the F21 torpedo.

Vicent Martinot-Lagarde, director of the Naval Group Services directorate, told Breaking Defense in an exclusive phone interview that when the fore of the Saphir was joined to the aft of the Perle, an additional hull section was added so the Perle is now 75 meters long, more than a meter longer than it was originally.

He explained the repair was undertaken in Cherbourg “because that is where we dismantle decommissioned vessels so the Saphir was there. We also had a long dry dock available so that we could cut both submarines and we also have there the ‘walkers’ [objects like large caterpillar feet which, when placed under the vessel, allow it to move]. Our specialized cutters and welders are also in Cherbourg so it was logical to bring the Perle there for this operation.”

When the decision was taken to repair the Perle, Florence Parly, Minister for the Armed Forces at the time, had said “this is a complex operation that few countries are able to consider doing, but we are undertaking it with confidence. Because despite the circumstances, each stop of this repair corresponds to proven knowhow: these are technologies that Naval Group and the state services are familiar with and master.”

For retired Rear-Admiral Jean-François Morel “there are no scruples to be had in diving aboard the Perle. After all, these days we build submarines in slices and it’s this know-how that enabled this repair to be undertaken,” he told Breaking Defense.

Perle exited the dry dock on Nov. 10, 2022 to begin quayside testing and its first dive took place on May 22, 2023.

Martinot-Lagarde remarks that the world-first was “an industrial challenge but we managed within a reasonable budget to give this submarine another 10 years of active service.”

If all goes to schedule, the Perle will be replaced by the sixth Barracuda-class submarine due to be delivered in 2029.