Why Marine Equipment Passes FAT but Fails During Vessel Commissioning

A successful factory acceptance test proves that equipment operated under the conditions created for the test. It does not automatically prove that the equipment will perform correctly after installation as part of a vessel's complete engineering system. During commissioning, previously separate mechanical, electrical, automation and piping interfaces operate together for the first time. That is where apparently compliant equipment can reveal integration problems that were never present at the factory.

What a FAT actually proves

A Factory Acceptance Test confirms that a specific piece of equipment meets its own specification, under conditions the manufacturer has deliberately created to demonstrate that. It is a genuinely useful test — but it is a test of the equipment in isolation, not of the vessel it will eventually be installed on.

FAT conditions are typically bounded in ways that will not exist on board. Electrical supply is usually clean and stable, often from a dedicated test bay rather than a vessel's own generation and distribution system. Cooling water, compressed air and other services are frequently supplied from temporary or laboratory-grade sources, at conditions chosen to make the equipment perform well rather than conditions matching the vessel's actual machinery space. Signals from surrounding systems — permissives, interlocks, remote commands — are commonly simulated rather than generated by the real equipment they will eventually come from. Ambient temperature, vibration and electrical noise are usually far more benign than a real engine room.

None of this makes a FAT pointless. It is the correct point to catch a fault in the equipment itself, while it is still on the manufacturer's premises and easiest to rectify. What it does not do is prove that the equipment will behave the same way once it is receiving its cooling water from the vessel's own system, its electrical supply from the vessel's own generators, and its control signals from the vessel's own automation — rather than from a test rig built to make it succeed.

The interface gap between factory and vessel

Equipment performance is never purely a property of the equipment. It is a property of the equipment plus everything it is connected to. The following are common sources of a genuine gap between FAT conditions and vessel conditions:

• Electrical voltage, frequency, phase balance and general supply quality
• Cooling-water temperature, pressure and flow actually available at the equipment
• Suction conditions and the net positive suction head genuinely available, as opposed to assumed
• Discharge pressure and back-pressure imposed by the rest of the system
• Pipe sizing, internal cleanliness, flushing standard and contamination carried over from construction
• Alignment, foundation stiffness, resilient mounting and pipe strain on the equipment's own connections
• Ventilation and the ambient temperature of the machinery space under real operating load
• Instrument location, calibration and sensor scaling as actually installed, not as specified on paper
• Control power quality and earthing arrangements
• Network communications and protocol configuration between systems from different suppliers
• Alarm, shutdown, permissive and interlock logic, as it actually behaves once every system is live simultaneously
• Interaction with other equipment and with the vessel's various operating modes

Any one of these can be the difference between equipment that performed correctly at the factory and the same equipment struggling once it is actually part of a vessel.

Common commissioning failures after a successful FAT

The following are general engineering examples of how this gap shows up in practice — not case studies, and not a claim that any particular one occurred on a specific project.

• A pump that ran correctly at the factory begins cavitating on board, because the available suction head is lower than assumed, or because air is being drawn in through a suction-side fitting that only leaks under vacuum.
• A generator or motor exhibits instability once connected to the vessel's own electrical system, due to control settings, load-sharing behaviour or an interaction with other equipment on the same bus that a test-bay supply never exposed.
• Equipment overheats in service because the installed cooling circuit — as actually piped, flushed and balanced — cannot deliver the flow or temperature the equipment was designed around.
• A sensor reports a plausible-looking value that is simply wrong, because of an incorrect range, scaling factor or wiring configuration introduced during installation rather than at the factory.
• An automation sequence that worked perfectly with simulated permissives during FAT fails once every permissive is coming from a real, physical system, because the sequence was never actually proven end to end as a complete vessel process.
• Hydraulic noise or pulsation appears only after the equipment is connected to the vessel's actual pipework, pipe supports and structure — none of which existed in the same form on the factory test rig.
• Alarms, remote stops and emergency shutdowns each function correctly when tested individually, but the cause-and-effect logic between them has never been proven as an integrated sequence until commissioning.

In every case, the equipment itself may be entirely within specification. The fault lies in the interface between the equipment and the system it has been connected to — which is precisely the thing a FAT is not designed to test.

Commissioning should test the system, not merely the component

A structured commissioning approach treats the vessel as the thing being proven, with the equipment as one part of it:

1. Confirm the approved design basis and the acceptance criteria the system is actually being tested against.
2. Verify the installation against drawings, specifications and the manufacturer's own installation requirements.
3. Check the utilities and operating conditions genuinely available at the equipment boundary — not the conditions assumed at design stage.
4. Confirm instrumentation is correctly installed, ranged and calibrated.
5. Test alarms, trips, permissives and shutdowns individually before testing them as an integrated sequence.
6. Test normal, degraded and failure operating modes, not only the straightforward case.
7. Record objective measurements rather than relying on a simple running or not-running observation.
8. Investigate any abnormal result systematically, rather than accepting the first plausible explanation.
9. Retest after corrective work, rather than assuming a fix has worked.
10. Document results and close out defects against evidence, so the record shows what was actually proven.

Measurements that matter

Meaningful commissioning relies on actual figures, not a subjective impression of whether something seems to be working. The following are representative of the kind of measurement that reveals a genuine problem before it becomes an operational one.

FAT, HAT and SAT are complementary

A Factory Acceptance Test, a Harbour Acceptance Test and a Site or Sea Acceptance Test each prove something different, and none of them substitutes for the others.

The FAT proves the equipment against its own specification, under controlled factory conditions. The HAT proves installation and basic functionality once the equipment is on board and connected to the vessel's actual services, still alongside the quay. The SAT — whether a true sea trial or a site acceptance test — proves the complete system under real or realistic operating conditions, including the interactions between systems that only become fully apparent once the vessel is actually operating as intended.

Treating a passed FAT as sufficient evidence that the vessel-level system will work is the single most common way that integration problems are discovered later than they should be — often at handover, when they are most expensive and most disruptive to resolve.

Independent commissioning support

When equipment does not perform as expected during commissioning, the useful question is rarely "is this equipment faulty?" in isolation. It is usually: does the problem originate in the supplied equipment itself, in how it was installed, in the utilities actually available to it, in its control interfaces, or in the wider vessel system it now forms part of? Answering that correctly is what actually resolves the issue, rather than simply reallocating blame.

Independent oversight during this phase gives owners, managers and shipyards a technical view that isn't tied to any one supplier's equipment or any one contractor's installation — which matters, because the answer to that question often sits at the boundary between two parties' scope rather than cleanly inside either one. This is not a position taken against shipyards or manufacturers; in practice it means working alongside the project team, including the yard and the original equipment suppliers, while maintaining independent technical judgement about what the evidence actually shows.

Torven Marine provides this kind of independent commissioning & technical support, and — where a problem has already emerged and needs to be run to ground — independent troubleshooting & investigations. For owners and managers who want a technical presence through a build or refit programme more broadly, this also sits within our wider independent technical consultancy service, including for shipyards and vessels operating in Vietnam and Southeast Asia.

The same underlying discipline applies whether the fault appears during commissioning or years into a vessel's service: identify what the evidence actually shows, rather than what the first plausible explanation suggests. Our related article on root-cause analysis in marine engineering covers that process in more detail.

If equipment on your project has passed its factory testing but is behaving differently now that it's part of the vessel, get in touch and tell us what you're seeing.