Under pressure: when the parts don’t match the maintenance plan

The failure of a diesel generator aboard Kommandor Susan highlights the risks of treating spare parts, overhaul intervals and maintenance planning as separate decisions – and why knowing exactly what is inside an engine matters

When one of the diesel generators on the survey vessel Kommandor Susan failed during sea trials in the Firth of Forth, Scotland, in January 2025, the consequences were severe. The vessel suffered an engine-room fire and complete power blackout. Although the crew were able to extinguish the fire, the vessel lost propulsion and began to drift. Limited propulsion was eventually restored and, with the help of two tugs, Kommandor Susan returned to Leith Harbour.

The subsequent investigation by the UK Marine Accident Investigation Branch (MAIB) revealed extensive damage inside one of the diesel generators. Pistons in two cylinders had fractured, releasing their connecting rods, which subsequently punched through the sides of the crankcase. The investigation ultimately traced the failure back to wear of the connecting rod bearings.

The incident was not simply an unforeseeable mechanical breakdown. The investigation traced the failure back to decisions made years earlier, during major overhauls of the vessel’s Caterpillar diesel generator engines.

Caterpillar had previously approved an extension of the engines’ major-overhaul interval from 10,000 to 24,000 running hours, on the basis that genuine Caterpillar components would be used. During the 2019 overhaul, however, non-OEM pistons, connecting rods, and bearings were installed. The engine failed 13,466 hours after the 2019 overhaul – well short of the planned 24,000 hours.

What makes a part suitable?

As the name suggests, original equipment manufacturer (OEM) parts are components produced by, or supplied through, the original manufacturer of the machinery.

Non-OEM parts come from other manufacturers. Some are produced by established companies and supplied with detailed technical documentation, while others may have less certain specifications or provenance. They can be cheaper than genuine OEM parts, although there are ways to reduce costs within the OEM system too, including remanufactured and reconditioned components.

Non-OEM parts are not inherently poor quality. Operators need to consider whether the component is suitable for the machinery, how and to what specification it was manufactured, what documentation supports it, and whether its origin and history can be traced. Depending on the component, certification or approval may also be required.

“In practice, the safest spare parts decisions are those that remain predictable, traceable and explainable long after installation,” says Henrik Wilhelms, Director, Agreement Sales at Wärtsilä Marine – a marine technology company that, among other things, manufactures and services ship engines.

“Genuine OEM parts are designed and continuously developed to work as intended inside a specific vessel system and real operating conditions…throughout their operational life. They are supported by traceable documentation, expertise, and lifecycle services,” says Wilhelms.

In Kommandor Susan’s case, the non-OEM parts were manufactured by Costex Tractor Parts (CTP), which supplies replacement parts for Caterpillar machinery. CTP was not listed as an authorised dealer on Caterpillar’s official dealer locator, and the MAIB found no evidence that the CTP-manufactured parts had been class-approved by a marine classification society.

Parts and overhaul intervals

The suitability of a replacement part depends on many factors including how long it is expected to remain in service and under what operating conditions. This becomes particularly important when an engine is operating on an extended overhaul interval.

Time between overhauls (TBO) refers to the recommended operating interval between major overhauls of an engine or particular components. “Overhaul intervals have traditionally been based on a combination of engineering design parameters, operating experience and running hours,” explains Wilhelms.

For engines operating under similar loads to Kommandor Susan and using genuine Caterpillar components, extending the TBO from 10,000 to 24,000 hours was not uncommon. Such extensions are based on evidence about the vessel’s operation, including fuel use, oil analysis and historic power loading, as well as assumptions about the components being used. In this case, one of those assumptions was that genuine Caterpillar parts would be fitted.

The bearings installed during the 2019 overhaul were constructed differently from the genuine Caterpillar components. The MAIB found weaker bonding between the layers of the non-OEM bearings, reducing their durability and making them unsuitable for the extended service interval being applied.

Neither Hays Ships Limited, which owned Kommandor Susan at the time of the 2019 overhaul, nor Gardline, which bought the vessel in 2022, knew that the engine had been fitted with non-OEM parts. Had that substitution been known, perhaps a different decision on the extended TBO would have been reached.

Looking beyond purchase price

One attraction of non-OEM components is their potential to reduce the upfront cost of spare parts. The Kommandor Susan incident, however, shows why the choice between OEM and non-OEM components cannot be reduced to price alone, or separated from questions of expected service life and maintenance requirements.

“Ultimately, the greatest value comes from taking a lifecycle approach,” says Wilhelms. “Decisions should be based not only on upfront purchase price, but on their impact on reliability, compliance, availability and long-term asset performance.”

Reducing maintenance costs is not simply a matter of stretching overhaul intervals as far as possible. “The most sustainable savings typically come from improving maintenance and spare parts planning, resource allocation, asset management and proper fluid management, rather than simply reducing maintenance activity,” says Wilhelms.

Condition-based and predictive maintenance are one way of doing that. Rather than relying only on running hours or calendar dates, operators can draw on actual engine data, maintenance history, fluid analysis, and inspections to build a clearer picture of equipment condition. Engine-performance monitoring, predictive analytics, and AI-enabled diagnostics could also help identify developing problems before they affect reliability or vessel operations.

“By analysing patterns, trends and weak signals in operational data, predictive maintenance can highlight potential problems that may not be immediately visible – even to the most experienced humans – through conventional monitoring or routine operational review,” says Wilhelms.

But condition-based and predictive maintenance still depends on knowing what is actually inside the engine. It cannot by itself correct for an unknown change in component specification or expected service life.

As for Kommandor Susan, following the accident current owner Guideline Shipping rebuilt the affected engine and overhauled the remaining units using OEM parts.

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Image: Close-up pipes, tubes and pressure gauge of marine diesel engine. Credit: Shutterstock