On 25 January 2025, during sea trials in the Firth of Forth, a diesel generator on the site investigation vessel Kommandor Susan failed catastrophically. A fire followed in the engine room and the vessel lost power. The crew extinguished the fire and mustered safely. The UK Marine Accident Investigation Branch (MAIB) published its report (10/2026) on 6 May 2026 (read the report).
The failure sequence
The engine was a large high-speed diesel generator set. The MAIB describes a cascade:
- connecting rod bearing number 5 failed;
- the connecting rod cap fractured;
- pistons 5 and 6 fractured and the connecting rods detached;
- the loose connecting rods breached the crankcase, and fire followed.
The root cause: non-genuine parts
After the accident, inspection by an authorised agent of the engine manufacturer found that major rotating components fitted during a January 2019 overhaul were not original equipment manufacturer (OEM) parts. The MAIB reports a key difference in the bearings: the OEM bearings used an aluminium-based layer under the white-metal bearing surface, while the replacements used a copper-based layer. The replacements showed weak bonding between layers, and other connecting rod bearings showed excessive wear.
Separately, an extended overhaul interval of 24,000 hours had been approved. That extension was only valid for genuine components. The engine failed about 13,466 running hours after the overhaul. The MAIB also found weaknesses in the owner's oversight of contractors and critical maintenance.
Why this matters
Plain bearings are layered materials. Their performance depends on the chemistry of each layer, the bond between layers and the structure of the bearing surface. None of these can be checked by fit or appearance alone. The same is true of fasteners, valve trim, seals, springs and many other replacement components.
Practical checks for critical spares
- Specify materials, not just dimensions. Purchase specifications for critical parts should state material and, where relevant, construction and bonding requirements.
- Verify on receipt. Positive material identification (PMI), hardness testing and, for critical items, metallographic sectioning of a sample can confirm what you have bought.
- Keep traceability. Record the source, certificates and batch of parts fitted, so that intervals and risk assessments rest on facts.
- Review intervals when parts change. An interval justified for OEM parts should be reassessed if anything else is fitted.
- Examine failed parts properly. Preserve failed components and have them examined. The MAIB's finding came from looking at what was actually installed.
How MTIS can help
MTIS provides failure investigation and materials characterisation and testing, including verification of replacement components against specification. Contact MTIS if you need a failed part examined or a batch of spares checked.
Source: UK Marine Accident Investigation Branch, report 10/2026, catastrophic engine failure and subsequent fire on board Kommandor Susan, published 6 May 2026. Summarised in our own words.