Fire onboard the car-ferry Ytterøyningen: Preliminary investigation results.

Updated 03.12.2020

Following the incident on the car-ferry MF Ytterøyningen on October 10th, 2019, a thorough investigation of both the course of events and the cause of the incident has been ongoing.

The investigation and findings so far show that the fire was most likely due to a coolant leak from a gasket in the Corvus liquid-cooled energy storage system and that it was a one-off event,” says Geir Bjørkeli CEO of Corvus Energy.

“For Corvus it is important to release the preliminary conclusions on the cause of the fire on board Ytterøyningen, thus avoiding speculation and uncertainty regarding the safety of lithium-ion battery-based energy storage systems. Safety has been, is and will always be Corvus Energy’s first priority.”

The preliminary findings presented below are supported by the parties involved in the investigation, including Norwegian Maritime Authority and DNV-GL.

The investigation has been led by the Kvinnherad police in Hordaland, Norway. Fire engineering expertise from the Norwegian National Criminal Investigation Service, DNV GL, Norwegian Maritime Authorities, Corvus Energy, insurance companies, and other stakeholders have also been involved.

The car ferry, built in 2006 and converted to battery-hybrid in 2019, was in regular operation, running on its diesel engines when the fire started. There were twelve passengers and three crew members aboard the vessel at the time. The vessel was able to moor safely, and all passengers and vehicles disembarked as scheduled. No personal injuries were suffered in connection with the incident and no environmental damage has been reported.

Based on the investigations aboard the vessel, supported by external experiments and analysis, the following preliminary conclusion has been made:

The most probable cause of the fire was a leakage in the battery system’s liquid cooling circuit. Findings indicate that a twisted gasket, intended to seal the cooling plate outside of a battery module, is the most probable cause of the leakage.  

It is too early to conclude whether the twisted gasket was a result of the recent service work on the cooling system or if it was caused by other reasons.  

The leakage created arcing between electrical components, at pack voltages of 1000Vdc, igniting a fire. The fire was fueled by ethylene glycol components from the coolant and caused external heating of battery modules. 

Due to the ongoing service work, no part of the battery system was connected to the shipside systems at the time of the incident. Consequently, no alarms from the battery system were sent through the ship’s alarm system.

Findings have shown that the patented and certified Corvus Passive Single Cell Thermal Runaway Isolation safety system worked as designed and intended, most likely limiting the damage from the fire.

Both the vessel’s Novec 1230 inert gas system and the vessel saltwater fire sprinkler system were deployed during the event. The saltwater sprinkler system was installed as an additional safety barrier. Indications are that the activation of the saltwater sprinkler system contributed to escalating the incident.

The further investigation will focus on how the extent and severity of the following events were able to develop towards an explosion 12 hours later in the switchboard room adjacent to the battery room.

Norwegian Maritime Authority has issued a press release supporting the preliminary findings and conclusions.

Update 03.12.2020:

Note from Corvus:

End of November 2020 the investigation is finalized. The conclusion supports the preliminary findings from Corvus.

West police district has sent out the following pressrelelease (translated from norwegian)

West police district has completed the investigation of the fire on the hybrid ferry MF Ytterøyningen. The case is closed.

The fire occurred in the ferry’s battery system which had not yet been put into operation. After the fire broke out, the ferry went to the quay and was allowed to drop off passengers and cars.

The fire was eventually extinguished, but there was still an explosion several hours after the fire itself was extinguished.

The results of the investigation strongly suggest that the cause of the fire is creeping currents caused by a leak in coolant from a cooling module belonging to a battery module. The study strongly suggests that the fire started in a battery rack where connected to the battery module. The explosion that occurred later happened due to build up of gases in the battery room. The source was the batteries themselves.

The battery technology on ferries is relatively new and the investigation has provided several learning points. For example, the Norwegian Maritime Directorate has made recommendations that alarms for bridges should be connected to battery packs even when the battery is not in use. In this case, the fire was reported by the ferry’s ordinary fire alarm system.

There is also a focus on which extinguishing agents are best suited and how they are used. It looks at the needed changes to increase the safety of larger battery installations on board ships. This applies to both electrical safety and safety systems. Efforts are being made to increase the competence of seafarers when it comes to incidents with new energy carriers.

The emergency services on land have varying expertise with regard to fire in larger battery systems. Bergen Firebrigade and others have taken the initiative to increase the competence related to batteryfire.

End pressrelease from West Police District

For questions related to Corvus, please contact

Mr Geir Bjørkeli

CEO, Corvus Energy

+47 952 73 398


Sonja Vernøy Hansen

Marketing & Communications Manager, Corvus Energy

+47 993 09 309

Corvus Q&A

Where did the leak occur?

We have very clear indications that the source of the leak was a twisted gasket on the cooling plate outside of one of the battery modules.

Did the leak cause a short-circuit in the battery?

No – the leak caused arcing which over time ignited a fire. Based on the preliminary results it is a reason to believe that saltwater has caused short-circuits.

Did the complete battery system catch fire?

No – from the investigation this far we know there has been a fire in one rack. We have no indications so far that other racks have similar indications of a fire.

Would this have been avoided if the battery systems had been connected to the ship’s systems?

We don’t know, but early detection is key. We think that the most important learning from this is that even if batteries are not in use – alarms need to be forwarded to the ship’s system.

Did the batteries explode?

No – the batteries did not explode. The fire started in the battery room, but it was a small fire and it was extinguished the date of the incident according to the fire brigade. The explosion was in a different room 12 hours later. What has caused the explosion must still be concluded as a result of further investigation, but it is a theory that the use of saltwater sprinklers could have contributed to the escalation of the event.

Why did the incident end with an explosion the day after in an adjacent room?

This is yet to be investigated by the police. We have to wait for the investigation to be finalized.

We often hear about the concern for a Thermal Runaway event in connection with lithium-ion batteries. What is a Thermal Runaway?

Thermal runaway occurs when a cell has reached the temperature at which the temperature will continue to increase on its own and it becomes self-sustaining as it creates oxygen which feeds the fire (literally). A Thermal Runaway can be caused by:

  • A manufacturing defect in a cell
  • Overcharging
  • Lack of cooling or ventilation
  • External fire/ heating of a battery module

That is why we use air-cooled or liquid-cooled systems and carefully monitor and isolate the battery cells to prevent any thermal runaway from spreading.

How does Corvus Orca Single-cell Passive Thermal Runaway Isolation system work?

Corvus Orca passive single-cell isolation reduces the effects of a thermal event to a minimum, leading the gases out through a closed pipe system and preventing the heat from cascading to neighboring cells and modules. The system has been extensively tested and exceeds the strict requirements of Norwegian Maritime Authorities and multiple class societies.

Is the Corvus battery system safe?

Yes – we consider this a one-off event and investigations have shown that the Thermal Runaway safety system worked as it should.

Can this happen on an air-cooled battery system?

No. This particular event cannot happen on an air-cooled battery system.

As these are preliminary results – how sure are we that we know what happened?

We need to be open to changes as a result of the ongoing investigation, but we are so confident about the current findings that we have chosen to make them public.

How long will it take to finalize the investigation?

The investigation is still ongoing and it is unknown how long it will take for the police to finalize.

Is the ferry back in operation?

The ferry is not in operation yet.

Is it considered safe to travel with all-electric or battery-hybrid ferries?

Yes. There is no reason why it should be considered dangerous to travel with all-electric or hybrid ferries.