Factory Acceptance Testing of an LNG high-pressure pump unit can produce a perfectly acceptable test report and still leave some of the most important operational questions unanswered.
Before attending a recent FAT, I prepared my own list of questions.
Guaranteed capacity and discharge pressure were obviously important. But they were not the areas that interested me most.
I wanted to understand what happens between the guaranteed operating points.
How does the unit behave when gas demand changes rapidly? What happens when operation moves from three cold ends to two, or from two to one? What capacity remains available? How quickly can the required pressure be established? How does the control philosophy interact with the actual FGSS arrangement on the vessel? What happens when a component or signal disappears unexpectedly?
And once the machinery has been operating continuously for several hours: what happens to vibration, supports, insulation, icing, instrumentation and maintainability?
Those questions changed the way I approached the FAT.
Proving that an LNG HP Pump system can achieve its guaranteed capacity is not the same as proving that it will be a good system to operate at sea.
1. Before the FAT: define what you actually want to learn
A FAT protocol naturally contains acceptance criteria. My preparation started somewhere else.
I wanted the selected operating points to correspond, as closely as practicable, to the future operating profile of the vessel and its main engine.
That meant looking beyond maximum capacity and considering minimum stable operation, intermediate loads, high-load operation and transitions between different cold-end configurations.
Steady-state readings are necessary. But transitions often tell us more about how the machinery will actually behave.
A stable operating point tells me whether the equipment can perform. A transition tells me whether the system can operate.
For an LNG HP fuel supply system, this distinction is particularly important because the pump unit will ultimately become only one component of a much larger dynamic system.
2. FAT operating points must mean something on board
Factory testing may be performed with nitrogen rather than LNG. That difference should not turn the FAT results into numbers that lose their meaning once the equipment leaves the factory.
Where N₂ is used, the measured operating points should be converted using the agreed engineering basis and relevant fluid properties so that they can subsequently be related to real LNG operation and recognisable main-engine operating points on the vessel.
The objective is not to suggest that an N₂ factory test is identical to operation with LNG. It is to establish a meaningful engineering reference.
These points should ultimately be relatable to the expected M/E gas demand. That creates something extremely valuable: a baseline.
When the unit is commissioned with LNG, I have a reference. When the next unit of the same newbuilding series is tested, I have a comparison. As further units are completed, I begin to understand what normal behaviour for that particular design actually looks like.
3. One FAT is a test. A series of FATs becomes data.
For a multi-vessel newbuilding programme, I would not treat each LNG HP Pump FAT as an isolated acceptance event.
The first unit establishes a baseline. The second provides the first meaningful comparison. By the third and subsequent units, the owner begins to establish a technical fingerprint of the series.
- cooldown behaviour
- pressure build-up
- stable operating points and maximum capacity
- cold-end transitions
- temperatures and pressure losses
- vibration and control response
- observed deviations
The results should not be identical. Manufacturing tolerances, instrumentation and individual equipment characteristics will naturally create differences. But units of the same design should begin to show a recognisable pattern.
This is why an unexpected result deserves attention even when an individual acceptance limit has not necessarily been exceeded.
“Is it still within tolerance?” is not always the first question. Sometimes the more useful question is: “Why is this unit behaving differently from the others?”
An outlier is not automatically a defect. But it is information.
4. Understand the hydraulic performance — not only total capacity
Total LNG HP Pump capacity is only one part of the hydraulic picture. The FAT should also establish the pressure loss across the skid at relevant operating points.
This provides an important reference for understanding how much pressure is consumed within the unit itself and how much pressure margin remains for the downstream installation.
A satisfactory pressure at the pump discharge does not automatically mean sufficient pressure is available at the engine. This becomes increasingly important when the LNG HP Pump is located a significant distance from the GVT and main engine.
5. FAT should establish the first LNG HP Pump performance fingerprint
The FAT should not only confirm that the LNG HP Pump reaches the required capacity. It should also establish the first performance fingerprint of each pump and cold end.
At selected operating points, pump frequency or speed should be correlated with volumetric flow and, where relevant, converted mass flow, together with suction and discharge pressure, differential pressure, temperature, power demand and vibration.
The purpose is not simply to demonstrate a guaranteed point. These measurements create a reference for the individual LNG HP Pump.
When the machinery is later commissioned with real LNG, the same relationship should be checked again during gas trials. The FAT data can then be converted from the N₂ test condition to the applicable LNG density basis and compared with the actual onboard operating points.
This creates a useful chain of reference:
Where the automation architecture permits, the relevant parameters should remain visible to the operator through the vessel mimic or monitoring system. Operating hours and starts should also be recorded for each cold end individually, particularly where the LNG HP Pump is designed for 3–2–1 cold-end operation.
Over time, the operator can then compare frequency or speed, flow, differential pressure, power, vibration and operating hours against the established reference. A gradual deviation may indicate hydraulic degradation, wear, fouling or a change in system resistance. A sudden deviation deserves immediate attention.
For cryogenic LNG pumps, the same trend information can also help identify abnormal suction behaviour. Unstable flow, pressure fluctuations, increasing vibration or an unexpected change in the pump characteristic may be associated with reduced suction margin, local flashing or cavitation. Vapour or gas formation within the cold end can likewise disturb stable pumping conditions.
This does not mean that one parameter alone proves cavitation. The value lies in recognising a change from the known operating fingerprint and correlating several measurements before the condition develops into equipment damage.
The control philosophy should therefore define what the operator sees when the LNG HP Pump moves outside its expected operating envelope: information, warning, alarm, load limitation and, where necessary, controlled unloading or cut-out of the affected cold end.
The FAT should create the first performance fingerprint of the LNG HP Pump — a reference that follows the equipment from factory test to gas trial and into service.
7. NPSH must become a vessel operating limit
Another parameter that should be clearly established is NPSH — Net Positive Suction Head.
In practical terms, NPSH describes the suction-pressure head available relative to the liquid's vapour-pressure condition. For a cryogenic LNG pump, inadequate suction margin can promote local flashing/cavitation, unstable pump behaviour, increased vibration and potential damage.
The minimum NPSH required by the pump for the stated operating condition, as defined by the maker's performance basis.
The NPSH actually provided by the vessel installation under the prevailing tank, fuel and operating conditions.
For the owner, NPSH should not remain simply a number on a pump datasheet. It needs to be translated into the actual suction conditions at the LNG HP Pump.
The essential question is whether sufficient suction-pressure margin above the LNG saturation pressure is maintained under all relevant operating conditions. If the available NPSH approaches the pump NPSHr, local vaporisation may occur at the pump inlet, leading to cavitation, unstable performance and potentially a cold-end trip.
Loss of this margin does not necessarily originate at the HP pump itself. Increasing pressure drop across an upstream simplex filter, filter contamination, an incorrect or malfunctioning differential-pressure indication, or other restrictions in the upstream piping can progressively reduce the pressure available at the HP LNG Pump inlet.
This is also why NPSH is an important design consideration when selecting the LNG LP Pump. Its duty should not be assessed on capacity alone. The LP pump must provide sufficient flow and sufficient discharge pressure so that, after the pressure losses through filters, valves and piping, the required suction-pressure margin remains available at the HP LNG Pump.
This should be verified across the specified operating envelope, including the maximum main-engine gas demand — for example 100% or 110% M/E load where such conditions form part of the design basis. Although vessels may rarely operate continuously at these loads, they remain important design and verification points for the LNG fuel supply system.
Capacity tells us whether the LP LNG Pump can supply enough LNG. NPSH tells us whether the HP LNG Pump can receive it without entering an unstable hydraulic condition.
8. Do not only prove that a cold end runs — prove that it can be safely removed
The FAT should also demonstrate the protection and isolation philosophy of individual cold ends.
A particularly useful test is the controlled operation of the relevant safety or isolation valves to cut out a selected cold end while the system is operating.
I want to observe the complete response: does the affected cold end unload and stop in the intended sequence? Do the relevant valves reach their correct positions? How does HP discharge pressure react? How do the remaining cold ends respond? Is capacity redistributed correctly? Does another cold end start if required? Can stable HP supply be maintained?
And importantly: what does the operator see?
Testing the valve alone demonstrates valve movement. Testing the complete sequence demonstrates whether the LNG HP Pump system behaves as a coordinated system.
9. Three cold ends. Two cold ends. One cold end.
Once individual cold-end protection and cut-out behaviour are understood, the next question is what the complete unit can actually deliver with 3 → 2 → 1 cold end.
For each configuration I want to establish the maximum available capacity and stable operating range and relate these to the corresponding M/E gas demand.
Then come the transition questions. When does an additional cold end cut in? When does it cut out? How is load shared? What happens to discharge pressure during the transition? Can one cold end become unavailable without causing an unnecessary interruption of gas operation? Are there restricted or barred operating ranges? Could the control system repeatedly cut units in and out close to a transition point?
Three installed cold ends tell me how much equipment has been installed. The 3–2–1 test tells me how much operational capability remains.
10. Test demand reduction — not only maximum demand
Maximum flow is normally an obvious FAT point. A rapid reduction in demand can be equally informative.
The main engine does not guarantee the fuel system that its demand will always change gently. An engine event, load reduction, individual unit cut-out or complete gas-demand rejection can change the required flow rapidly.
I therefore want to understand what happens to HP pressure; how quickly the control system responds; which cold end unloads or stops; how excessive pressure is avoided; whether the control system remains stable; how the unit recovers; and what the operator sees while all of this is happening.
This is where control philosophy becomes just as important as pump capacity.
11. The LNG HP Pump does not operate alone
At the factory we test a package. On the vessel we operate a system.
The LNG HP Pump system will eventually operate together with the vessel FGSS, high-pressure piping, filters and valves, GVT and main engine. The physical distance between the HP pump unit and the GVT therefore matters.
So do pipe diameter, pipe length and internal volume, valves and fittings, filters, pressure losses, maximum pump discharge pressure, pressure-control philosophy, and safety-valve settings and margins.
The entire pressure budget needs to make sense.
Where is the controlling pressure measured? What is the maximum pressure the unit can generate? How does the control system respond to changing M/E demand? Is engine-demand feed-forward used in addition to pressure feedback? How does the physical distance between the LNG HP Pump system and the engine affect response? What pressure loss is expected between the HP unit and the GVT at maximum demand? Are the selected piping dimensions consistent with that calculation? What margin remains between normal operating pressure, maximum control pressure and the safety-valve setting?
We may have proved the LNG HP Pump system. But have we proved that the unit is correctly matched to the system designed around it?
12. Maximum capacity is not enough — watch the machinery
Process values on the FAT screen tell only part of the story. I also watch the equipment.
At different operating points and cold-end configurations: does vibration change? Does anything move unexpectedly? How do supports behave? Are small-bore connections stable? Are valve and safety-valve arrangements mechanically secure? Does operation at maximum capacity reveal behaviour that is not visible at partial load?
Cryogenic machinery is exposed to thermal contraction and repeated cold-to-warm cycles in addition to pressure and vibration. Flange connections, bolting, supports, valves and connected instrumentation therefore deserve physical observation during FAT.
A unit may satisfy its required flow and still show mechanical behaviour worth investigating.
13. Cold operation also tests the insulation concept
Some of the most valuable observations may never appear on the acceptance sheet.
Once the machinery has been operating continuously for several hours, the actual icing pattern becomes visible. That can immediately challenge assumptions made from drawings.
Where does the ice stop? Does it progress toward flanges? What happens around stainless-steel supports and PSV connections? Are long valve stems affected? Can position sensors continue to operate reliably? Can manual levers remain accessible? Should additional insulation be provided? Would local heat tracing be justified?
This is not merely a cosmetic insulation issue. If ice reaches instrumentation, position indication or manual operating points, it becomes an operability and maintainability issue.
14. Maintainability becomes real when you stand beside the equipment
A drawing may show maintenance clearance. Standing next to the actual equipment asks a different question: how will we actually remove this component?
The size and mass of a cold end become considerably more meaningful when seen physically. Eventually it needs to be disconnected, supported, lifted, moved clear of the HP pump arrangement and installed again.
Is there sufficient vertical and horizontal clearance? Where will lifting equipment be attached? Can two chain blocks provide controlled removal and installation? Is a dedicated trolley or lifting arrangement required? Can the work be completed without unnecessary dismantling of adjacent equipment?
Maintainability is part of performance.
15. Emergency Stop should be tested as a sequence
An Emergency Stop test should demonstrate more than the fact that the machinery stops when a button is pressed.
The sequence matters. The final condition matters. And what the operator sees matters.
If equipment stops but the operator cannot immediately understand what happened, part of the system response is missing. The same principle applies to protective functions and interlocks throughout the LNG HP Pump system.
16. Introduce the unexpected
A predefined FAT protocol naturally verifies known alarms, trips and shutdown conditions.
I also value selected, safely coordinated simulated failures or disconnections of dedicated components and signals. The objective is not to create random chaos. It is to determine whether the system recognises an abnormal condition and responds coherently.
Operator information
Does the MOP clearly identify what has failed or disappeared?
Control-system decision
Does the automation initiate the expected alarm, limitation, standby changeover, controlled stop or emergency response?
Equipment response
Does the physical LNG HP Pump system actually reach the intended degraded or safe operating condition?
Testing a predefined alarm proves that the alarm was programmed. A controlled unexpected condition tells us more about whether the system understands failure.
17. FAT findings should follow the equipment to the vessel
I do not consider FAT complete when the protocol is signed.
Some observations naturally become design-review and commissioning questions. Piping dimensions may need to be checked again against the final pressure-loss calculation. The pressure-control philosophy may need to be considered together with the actual distance to the GVT and engine. NPSH assumptions need to be translated into real tank and LNG operating conditions. Insulation details may require refinement after observing icing. Lifting arrangements may need improvement after seeing the physical dimensions of the cold ends. FAT operating points should follow the equipment into LNG commissioning and sea trials.
That is not evidence that FAT failed. Quite the opposite.
A good FAT should create questions.
Seeing the actual machinery operate should refine some of the assumptions previously made from drawings. Otherwise, I would ask whether we were looking closely enough.
Capacity is a test result.
Operability is a system property.
I went to the FAT to verify an LNG HP Pump system. I came away with a broader conclusion: the owner's task is to connect the two.
That means looking beyond guaranteed flow and pressure into operating envelope, NPSH, skid pressure losses, 3–2–1 cold-end operation, transient response, control philosophy, remaining capacity after failure, vibration and mechanical behaviour, insulation and icing, maintainability, safety-valve and isolation behaviour, Emergency Stop response, operator information, and the relationship between the LNG HP Pump, FGSS, GVT and main engine.
For a vessel series:
The question at the end of FAT should therefore not only be “Did the unit pass?”
What have I learned today that will help the crew operate this system — and help the owner recognise when it is no longer behaving normally?
Independent owner-side technical judgement for vessel investment, newbuilding and complex machinery decisions.
This article reflects general engineering observations and professional experience. Project-, vessel- and manufacturer-specific information has been intentionally excluded.