How Multi-Fuel PCTCs Change the Deck

The interesting change on a multi-fuel car carrier is not a thermal gradient. It is that ammonia gas detection alarms at 25 ppm in an enclosed space while ammonia does not burn until roughly 150,000 ppm. Those thresholds are set by toxicity, not fire — so a hull that already carries an enclosed deck of lithium-ion batteries acquires a second sensing duty answering to a completely different regime.

What the Aurora class actually is

Bigger than the fleet it joins, and "ready" rather than running. Höegh Autoliners' Höegh Aurora, delivered in 2024, is 83,687 GT and 9,100 CEU on 199.90 m by 37.8 m, with cargo on 14 decks including five liftable ones, a maximum deck height of 6.5 m and a 375-tonne ramp. Every deck is laid out for electric vehicles and strengthened for heavier project cargo, so the EV share on these hulls can be far higher than on the 2000s-built tonnage they supplement.

The fuel position needs stating precisely, because the shorthand misleads. These are LNG dual-fuel ships carrying DNV's ammonia-ready and methanol-ready notations — the first in the PCTC segment to hold them. "Ready" is a design-and-approval notation for a future conversion, not a fuel the ship burns today. The series runs Höegh Aurora in August 2024, Höegh Borealis in October 2024 and Höegh Australis on 5 January 2025, with two more expected every six months to the first half of 2027.

Ammonia detection is a toxicity system, not a fire system

This is the inversion, and the numbers make it plain. The IMO's Interim Guidelines for the Safety of Ships Using Ammonia as Fuel — MSC.1/Circ.1687, issued 26 February 2025 after approval at MSC 109 in December 2024 — set ammonia detection thresholds at 25 ppm in enclosed spaces, 110 ppm in the secondary enclosure, and 220 ppm to trigger alarms and shutdowns as appropriate.

Set those against combustion. Ammonia's lower flammable limit sits around 15% by volume, which is about 150,000 ppm. The enclosed-space alarm therefore fires at roughly one six-thousandth of a flammable atmosphere, and the shutdown threshold at around one seven-hundredth. That 25 ppm figure is not a fire number at all — it is the long-standing occupational exposure limit. A crew member is in danger long before anything can ignite, which is the reverse of every assumption built into a hydrocarbon fire-detection philosophy.

The code that governs this does not exist yet

There is no prescriptive chapter to comply with, which changes how these ships get approved. The IGF Code — the International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels — provides mandatory criteria for fuels with a flashpoint below 60°C, and its common principles cover tank protective location, double barriers on fuel supply lines, ventilation and gas detection, hazardous area classification and explosion mitigation. But it does not cover methanol, ethanol, LPG, ammonia or hydrogen.

So an ammonia- or methanol-fuelled ship is approved on a combination of non-mandatory interim guidelines, class rules written to align with IGF principles, and — where the arrangement has no prescriptive slot at all — SOLAS Chapter II-2 Regulation 17, alternative design and arrangements, which substitutes an engineering analysis submitted for Administration approval for a certificate held against a clause. That is the same route a detection arrangement takes when it does not map onto an EN 54 category, and it means two different systems on the same newbuild can both be riding on Reg 17 approvals rather than prescriptive compliance.

What actually changes on the vehicle deck

Less than the marketing suggests, and in a different place than expected. The vehicle-deck fire problem on an Aurora-class hull is the one the whole corpus already describes: an enclosed space full of lithium-ion batteries under the amended FSS Code's heat-detection requirement. What the multi-fuel architecture adds sits adjacent to it rather than inside it.

• Bunker, fuel-preparation and tank-connection spaces with their own detection duty, physically close to cargo spaces on a hull with little spare volume.
• A higher EV share by design — every deck laid out for electric vehicles rather than a subset — which raises the baseline the fire side has to work against.
• Five liftable decks, so the geometry a fixed detection layout was surveyed against changes with the stow.
• Two sensing regimes in adjacent compartments with thresholds three orders of magnitude apart, and different reasons for existing.

What it means for owners and underwriters

For owners, the question to put to a newbuild specification is which regime each detection system was written under. The vehicle-deck system answers to SOLAS II-2 and the FSS Code with heat detection now mandatory since 1 January 2026. The fuel-side system answers to interim guidelines and class rules with thresholds set on human toxicity. They are different duties with different acceptance criteria, and a yard package that presents them as one "detection scope" is worth reading twice.

For underwriters, an ammonia-ready notation on a vehicle carrier is not yet an operational ammonia risk — the ships are burning LNG. It is a signal about where the fleet is going and about which approvals the hull already holds. The useful question at renewal is not whether the vessel is multi-fuel but whether the conversion, when it happens, will re-open a Regulation 17 case that the original detection arrangement was approved under. A conventional PCTC and an ammonia-converted one carry identical cargo and a materially different hazard inventory.

Sources

• 1. IMO — MSC.1/Circ.1687, Interim Guidelines for the Safety of Ships Using Ammonia as Fuel, issued 26 February 2025; approved at MSC 109 (December 2024) following development by the Sub-Committee on Carriage of Cargoes and Containers at CCC 10 (September 2024). Detection thresholds agreed at 25 ppm in enclosed spaces, 110 ppm in the secondary enclosure and 220 ppm triggering alarms and shutdowns as appropriate. Non-mandatory, aligned with IGF Code and SOLAS principles.
• 2. IMO — International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels (IGF Code): mandatory criteria for fuels with a flashpoint below 60°C, covering fuel tank protective location, double barriers on fuel supply lines, ventilation and gas detection, hazardous area classification and explosion mitigation. The Code does not cover methanol, ethanol, LPG, ammonia or hydrogen; class rules for those fuels are written to align with its requirements.
• 3. Höegh Autoliners and Baird Maritime vessel review — Höegh Aurora (2024): 83,687 GT, 9,100 CEU, Norwegian flag, 199.90 m × 37.8 m, maximum deck height 6.5 m, ramp capacity 375 t, cargo on 14 decks including five liftable, all decks laid out for electric vehicles and strengthened for project cargo; DNV ammonia-ready and methanol-ready notations, the first in the PCTC segment; MAN main engine, Kongsberg bridge system. Deliveries: Höegh Aurora August 2024, Höegh Borealis October 2024, Höegh Australis 5 January 2025, with two vessels expected every six months to H1 2027 and an option for four more.