Valve failure on CalMac ferry saw uncontrolled escape of flammable gas

ByCalum Watson

BBC Scotland

A faulty valve on a CalMac ferry led to a sudden and uncontrolled release of highly flammable methane gas last summer, BBC Scotland News has learned.

The operator's investigation found that 700kg of the gas - enough to fill a four-bedroomed house - was vented from the side of MV Glen Sannox over the space of 12 minutes while it was carrying 82 passengers from Arran to the mainland.

So much methane was released that some of it was sucked into a ventilation intake, setting off gas alarms in the ship's generator room.

While the risk of explosion is believed to have been low, CalMac described it as a "critical failure" in the gas fuel system and said lessons had been learned.

The propulsion systems for Glen Sannox were designed, supplied and commissioned by Finnish firm Wartsila, a world leader in liquefied natural gas (LNG) engines, who worked as subcontractors to the Ferguson Marine shipyard.

Details of the incident have not previously been made public and have only emerged after a freedom of information request from BBC Scotland News.

Since it happened, CalMac has not run the ship's main engines on LNG, and the methane tank was completely emptied last November.

On 3 June 2025, the 15:10 sailing of Glen Sannox, the first UK-built ship capable of directly powering its engines with LNG, left Troon in South Ayrshire, bound for Brodick on Arran.

During the 80-minute crossing, while running on LNG, gas detector alarms sounded inside the ship, and crew in the gas tank room reported hearing a strange noise.

The port main engine also "tripped" out - an automatic shutdown when the safety systems detect an irregularity such as unusual gas pressure.

The ferry continued its journey to Brodick running on diesel.

An internal CalMac investigation later found the crew should have avoided using the gas systems until they had identified the cause of the alarms and unusual noise.

Instead, testing was carried out while berthed in Brodick, which found the engines seemed to work satisfactorily on gas, albeit with some abnormal gas flow readings.

The ferry set off at 17:20, bound for Troon, but during this journey a large and uncontrolled release of gas occurred from a small vent at the side of the ship.

Glen Sannox can run on LNG while out at sea, but the ship's masters switch the engines to diesel as it approaches harbour because they don't want to run the risk of the engines tripping out as they approach the dock.

Whenever the ship switches to diesel, any residual methane gas has to be flushed out of the engine pipework and vented into the atmosphere.

What happened during this voyage was that a valve used in this flushing process failed and defaulted to the open position, meaning that high pressure LNG fuel destined for the engines escaped through the vent pipe.

The crew initially tried to restart the engine on gas before they realised that methane was escaping from the side of the ship.

Engineers had to physically shut the valve because it was not responding to electrical control.

The unusually high quantity of vented methane meant some of it was sucked back into ventilation intakes located further back from the gas outlet, triggering gas detector alarms in the ship's generator room and other spaces.

The failed valve was sent for examination by Wartsila which found that a metal casing was damaged, probably worn away by another component that had worked loose.

Glen Sannox has a known vibration problem while moving backwards, a fault which has been linked to the propellers which were also designed by Wartsila and are due to be replaced.

CalMac's investigation concluded the valve began to fail on the journey to Arran, then failed completely when the engines were powered up on the sailing to Troon.

The methane used on the ships is stored under pressure and at minus 162C as a liquid, but in its gaseous state it is similar to the gas used in domestic boilers and cookers, with potential to form an explosive mixture with air.

But methane is lighter than air, and most of the released gas would have quickly dispersed once vented outside the ship.

Andy Alderson, a marine safety expert with 35 years experience of working with LNG ships, said the gas sucked back inside Glen Sannox was unlikely to have reached the concentration levels that could have triggered an explosion.

And while some safety systems were absent, he said the gas detectors apparently functioned as they should do, triggering an automatic shut down of the fuel supply.

The biggest issue, he said, was to understand why opportunities to prevent the incident happening were missed.

"I think the mistake, and it's been acknowledged, was the decision to restart the engine without having discovered what the problem was in the first place, and therefore restarting the engine with the valve open," he told BBC Scotland News.

Alderson, who runs AMA Maritime Safety, said he would characterise it as a "significant incident" that deserves proper scrutiny so that lessons can be learned.

There is evidence to suggest that engine maker Wartsila was previously aware of reliability issues with the valve, but it is unclear who was given this information.

The firm published three versions of a service notice - in June 2020, March 2021 and June 2024, a few months before Glen Sannox was delivered - recommending that the valve either be upgraded using a service kit or replaced with a new model.

Wartsila also recommended tightening bolts and locking them in place with superglue to improve reliability, particularly involving air leakages, failed silencers and solenoid coils becoming loose.

All these issues were found in the valve that subsequently failed on Glen Sannox.

But BBC Scotland News understands Ferguson Marine did not have access to this information, which was issued to "owners and operators" via an online portal.

Operator CalMac said it also did not have access to this service portal until after the ship was delivered.

CMAL, the ship's owner, said it would have expected engineering documentation to have been sent directly to the shipyard prior to delivery.

Another issue identified by the investigation was that no fail-safe system - involving valve position indicators and gas pressure sensors - had been installed to alert the crew that the valve was failing.

Wartsila had shipped the main engines to Ferguson's in early 2017 without such monitoring systems - although it would later fit them as standard on new engines and had fitted them to the smaller auxiliary engines on the ship.

CalMac's report suggests that standards body Lloyds Register may have signed off the system based on the assumption that such safety features were fitted on all engines.

These sensors have now been retrofitted to Glen Sannox and will also be fitted on its sister ship MV Glen Rosa which is due for delivery later this year.

The valve that failed has been replaced with the new model and CalMac has provided more training to its staff. The ship is not currently operating on gas.

In a statement CalMac said: "Lessons were learned following the incident in June 2025, and remedial actions taken.

"The vessel is absolutely safe to operate on LNG, and the decision not to re-gas her following annual overhaul at the beginning of the year was purely due to our desire to get her back in service as quickly as possible."

The Marine Accident Investigation Branch has reviewed CalMac's report and indicated that it is satisfied with the firm's remedial action.

Wartsila said client confidentiality prevented it answering questions on why fail-safe monitoring equipment was not fitted on Glen Sannox, or who had access to its service notices.

In a statement it said: "We are dedicated to sustainability and maintaining the highest safety standards, quality, and reliability for our customers.

"We work closely with our customers throughout the lifecycle of our solutions and provide technical guidance, updates, and support where relevant."

The incident will likely reignite debate on whether LNG was the right choice for Glen Sannox and its sister ship Glen Rosa, whose construction has been marred by delays and cost rises.

CMAL, which procures and owns the ships used by CalMac, first started talking to Wartsila about the possibility of powering a Scottish ferry with LNG as long ago as 2012.

Gas burns more cleanly than diesel, and it offered a means of complying with new regulations due to come into force on sooty pollutants, known as NOx and SOx.

Manufacturers also claimed their engines emitted less CO2 - although the green credentials have been challenged because they also release unburnt methane which is itself a very powerful greenhouse gas.

CalMac was content to go along with the choice of LNG as a fuel, which at the time was cheaper than diesel, when it helped it draw up the specifications for the new ships in 2014, a project then known as "Super Eco 1000 Ro Pax Ferry".

However, the technology proved a major challenge for the Ferguson shipyard after it was awarded the contract the following year.

The LNG equipment added weight to the ship, making it harder to meet the demanding cargo carrying requirements without adopting a bulky hull shape, which in turn had an impact on fuel efficiency and speed.

The large gas tank took up a lot of space, and the engine room ended up very congested. Ferguson workers have dubbed it the "snake pit" because of the amount of pipework that had to be fitted inside.

Glen Sannox was also the first LNG ship of its kind ever constructed in the UK, which introduced uncertainty over what was required for regulator approval.

A protracted dispute over claims for extra costs eventually led to the firm falling back into administration and being nationalised.

Ferguson managers have at various times over the years asked CMAL to reconsider the LNG specification.

The ferries agency has, however, insisted the shipyard deliver the ships with the gas systems fully commissioned in accordance with the original contract.