A recent investigation by CalMac, following a freedom of information request from BBC Scotland News, has revealed a significant and uncontrolled release of highly flammable methane gas from the MV Glen Sannox last summer. The incident, which occurred on 3 June 2025, involved approximately 700kg of liquefied natural gas (LNG) escaping from a faulty valve while the ferry was carrying 82 passengers on a journey from Arran to the mainland.
The CalMac report describes the event as a “critical failure” within the vessel’s gas fuel system, leading to gas alarms sounding in the ship’s generator room as vented methane was drawn back into ventilation intakes. Despite the operator’s belief that the risk of explosion was low, the incident underscores serious concerns regarding the ferry’s operational safety and the integrity of its propulsion systems.
MV Glen Sannox, the first UK-built ship capable of running directly on LNG, utilises propulsion systems designed, supplied, and commissioned by Finnish firm Wartsila, a prominent manufacturer of marine engines.
Incident Unfolds Amidst Operational Testing
The sequence of events on 3 June 2025 began during the 15:10 sailing from Troon to Brodick. While operating on LNG, gas detector alarms activated, and crew members in the gas tank room reported unusual noises. The port main engine subsequently “tripped out,” an automatic shutdown triggered by safety systems detecting an irregularity. The ferry completed its journey on diesel.
Despite these warnings, an internal CalMac investigation found that crew conducted engine testing on gas while berthed in Brodick, noting abnormal gas flow readings but deeming the engines satisfactory. It was during the return journey at 17:20, after the ferry switched to diesel as it approached Troon harbour (a standard procedure to prevent engine trips near docks), that the critical failure occurred. A valve intended to flush residual methane from the engine pipework failed, defaulting to an open position. This allowed high-pressure LNG fuel to escape uncontrollably through a vent pipe on the side of the ship for 12 minutes.
Engineers ultimately had to physically close the valve after it failed to respond to electrical controls, while the substantial methane release triggered further alarms in other ship compartments.
Safety and Systemic Oversight Questions
Methane, the primary component of LNG, is a colourless, odourless, and highly flammable gas that forms explosive mixtures with air in concentrations between 5% and 15%. However, marine safety expert Andy Alderson, with 35 years of experience in LNG ships, suggested the gas drawn back into Glen Sannox was unlikely to have reached explosive concentrations, primarily due to methane being lighter than air and quickly dispersing once vented externally. Alderson characterised it as a “significant incident” requiring thorough scrutiny to learn lessons.
A key concern raised by Alderson was the decision to restart the engine without identifying the initial problem. He told BBC Scotland News: “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.”
The investigation also revealed a broader issue: Wartsila, the engine manufacturer, had issued three service notices (in June 2020, March 2021, and June 2024) recommending an upgrade or replacement for this specific type of valve, citing reliability problems like damaged casings, loose components, and air leakages. These were precisely the issues found in the failed valve on MV Glen Sannox.
However, Ferguson Marine, the shipyard, and CalMac both stated they did not have access to Wartsila’s online service portal where these notices were published until after the ship’s delivery. CMAL, the ship’s owner, asserted it expected engineering documentation to be sent directly to the shipyard.
Furthermore, critical fail-safe systems, including valve position indicators and gas pressure sensors, were not initially installed on the main engines supplied by Wartsila in early 2017, although they were fitted as standard on auxiliary engines and later became standard on new Wartsila engines.
CalMac’s report suggests that Lloyds Register may have approved the system presuming these safety features were present. These sensors have since been retrofitted to Glen Sannox and will be installed on its sister ship, MV Glen Rosa.
Ongoing Debates
Since the incident, CalMac has not operated MV Glen Sannox’s main engines on LNG, and the methane tank was emptied in November. The failed valve has been replaced with the newer model, and staff have received additional training.
In a statement, CalMac said: “Lessons were learned following the incident in June 2025, and remedial actions taken.” They further clarified: “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.”
Wartsila declined to comment on client confidentiality regarding the lack of fail-safe monitoring equipment or access to service notices, stating: “We are dedicated to sustainability and maintaining the highest safety standards, quality, and reliability for our customers.” The Marine Accident Investigation Branch has reviewed CalMac’s report and expressed satisfaction with the corrective actions.
In Case You Missed It:
The incident is likely to intensify the ongoing debate surrounding the choice of LNG as fuel for MV Glen Sannox and MV Glen Rosa. While LNG offers reductions in harmful pollutants like NOx and SOx, meeting regulatory compliance, its “green” credentials have been challenged due to methane slip—the release of unburnt methane, a potent greenhouse gas.
The LNG specification has also contributed to significant construction challenges, delays, and cost escalations at the Ferguson Marine shipyard, which was eventually nationalised, with total costs for the two ferries quadrupling from £97m to £460m.
