Fiber-Optic Linear Heat Detection on RoRo

Linear heat detection stopped being an alternative option on 1 January 2026. The amended FSS Code now requires heat detection in vehicle, special category and ro-ro spaces alongside the smoke detection already fitted, and a line-type detector satisfies that with a single continuous run rather than a lattice of point devices. That is a regulatory driver, not a technology preference.

What changed on 1 January 2026

Two resolutions adopted at MSC 108 moved heat detection from optional to required on vehicle decks. Resolution MSC.550(108) amended SOLAS Regulation II-2/20 and MSC.555(108) amended the FSS Code, both entering into force on 1 January 2026. FSS Code paragraph 4.1.6 mandates the installation of heat detectors additional to any existing smoke detectors in special category spaces and in open and closed ro-ro and vehicle spaces — or, alternatively, replacement of the existing smoke detectors with combined smoke and heat detectors.

The application split is what determines whether this is a design question or a retrofit question. Ships with keels laid on or after 1 January 2026 take the amended requirements immediately. Existing vessels have until the first survey on or after 1 January 2028. For detector spacing, the instrument to work from is MSC.1/Circ.1695, the unified interpretation of the FSS Code's spacing requirements.

Why a line-type detector fits the mandate

Because the requirement is for heat detection across a space, and a line-type detector measures along its whole length rather than at discrete points. Distributed fibre-optic sensing — DTS for temperature, DAS for acoustic and strain — turns one cable into a continuous measurement, so there is no untreated gap between devices, which is the failure mode a point-detector layout has to be designed around.

• Continuous coverage along the cable run rather than sampled coverage at intervals.
• Unaffected by the optical problems that degrade camera-based sensing — smoke obscuration, lens contamination, condensation.
• One interrogator unit reads kilometres of fibre, so installed cost scales with cable length rather than device count.
• Intrinsically safe along the sensing length: the fibre carries light, not electrical power.

What FIRESAFE II actually demonstrated

One full-scale test in one environment, with a clear result and a clear limit. EMSA's FIRESAFE II study — carried out by Bureau Veritas, RISE Research Institutes of Sweden and Stena Rederi, with final reports in December 2018 — selected two detection technologies for full-scale fire testing aboard a commercial ro-pax under work package 4: fibre-optic linear heat detection for open ro-ro spaces, and a thermal imaging camera for the weather deck. Both were found functional and suitable for their respective environments.

The caveat published alongside that result matters more than the result. WP4 identified the main challenge for open ro-ro spaces and weather decks as the potentially significant ventilation those spaces carry, which can both delay and delocalise detection. Airflow does not only push the signal below a threshold; it moves the apparent source, so a system can report late and in the wrong place. A continuous cable is less exposed to the first of those problems than a sparse point layout, and no more protected from the second.

Where it is the wrong primary

On a densely packed enclosed deck, where the question is which vehicle rather than which zone. Distributed fibre resolves position in metres, not centimetres, so localisation to an individual pack is approximate. Temperature resolution degrades with distance along the run, so a small per-vehicle delta can sit inside the noise. And on an enclosed deck stowed to the lashing plan, a cable cannot be routed close to every battery pack — the geometry that makes continuous coverage cheap on an open deck works against it below.

That is a statement about spatial resolution, not about quality. It also runs directly into the other half of the 2026 package: the same amendments require video monitoring and recording in vehicle, special category and ro-ro spaces, and the direction of regulatory travel is toward knowing which object is involved rather than which zone alarmed. A technology whose native output is a position along a cable answers part of that question and not all of it.

What this means for specification

Match the technology to the space rather than to the vessel. On weather decks and open ro-ro spaces — where ventilation is highest, point detectors are least reliable, and FIRESAFE II's evidence actually sits — a continuous line-type detector is the better-supported primary. On enclosed decks, the resolution argument runs the other way, and the honest position is that a line-type run satisfies paragraph 4.1.6 without on its own delivering object-level localisation.

For anyone writing a specification against the 2026 amendments, three things are worth separating: what the Code requires (heat detection present in these spaces), what is demonstrated (fibre LHD tested and found suitable in open ro-ro spaces, on one vessel, in 2018), and what is inferred (how any given technology performs on an enclosed deck full of electric vehicles, which FIRESAFE II did not test and which post-dates its fieldwork). Vendor claims tend to blur the three.

Sources

• 1. Lloyd's Register — Class News 07/2026, SOLAS amendments on fire safety requirements for ro-ro passenger ships: Resolution MSC.550(108) amending SOLAS Regulation II-2/20 and Resolution MSC.555(108) amending the FSS Code, both adopted at MSC 108 and in force 1 January 2026; FSS Code paragraph 4.1.6 requiring heat detectors additional to existing smoke detectors in special category spaces and open and closed ro-ro and vehicle spaces, or replacement with combined smoke and heat detectors; paragraphs 4.4.1 and 4.4.2 requiring an effective video monitoring and recording system in those spaces; paragraph 6.2.3 requiring a fixed water-based monitor system on weather decks used for vehicles; application immediate for keels laid on or after 1 January 2026 and not later than the first survey on or after 1 January 2028 for existing ships; MSC.1/Circ.1695 cited as the unified interpretation for detector spacing.
• 2. EMSA — FIRESAFE II (initiated 2017; Bureau Veritas, RISE Research Institutes of Sweden and Stena Rederi; final reports December 2018), work package 4 on fire detection technologies for open ro-ro and weather decks: fibre-optic linear heat detection (open ro-ro spaces) and a thermal imaging camera (weather deck) selected for full-scale fire testing aboard a commercial ro-pax, both found functional and suitable for the relevant environments; ventilation identified as the main challenge because it can delay and delocalise detection.