Testing an oil-in-water monitor offshore is not a single event; it is an ongoing programme of verification that begins before the equipment leaves the manufacturer and continues throughout the operating life of the FPSO. Each type of test serves a different purpose, is carried out at a different stage of the project and answers a different compliance question.
For commissioning managers, instrumentation engineers and operations teams, understanding what each test involves, what it confirms and when it must be carried out is essential both for achieving first-oil readiness and for maintaining a defensible compliance record throughout service.
This article explains the main testing methods for oil-in-water monitors in FPSO produced water applications, from Factory Acceptance Test through to routine operational verification. For guidance on specifying the right monitor before testing begins, see How to Specify an Oil-in-Water Monitor for FPSO Produced Water Discharge.
| Key thing to remember
Testing an oil-in-water monitor confirms that the equipment is performing correctly at a specific point in time. It does not guarantee future performance. Regular testing and calibration verification throughout the operating life of the FPSO is required to maintain a defensible compliance record and is a standard expectation of regulatory inspectors. |
What Are the Main Types of Testing for an Oil-in-Water Monitor?
Testing for an oil-in-water monitor falls into several distinct categories, each carried out at a different stage and for a different purpose. The table below summarises the main test types relevant to FPSO produced water monitoring applications.
| Test type | When | Purpose | Where |
| Factory Acceptance Test (FAT) | Pre-shipment | Verify analyser meets specification before it leaves the manufacturer | Manufacturer’s facility or approved test house |
| Site Acceptance Test (SAT) | Commissioning | Verify installed system performs correctly in the FPSO environment | Installed location on the FPSO topsides |
| Grab sample correlation | Operations | Verify ongoing monitor accuracy against independent laboratory analysis | Sample point at the treatment package outlet |
| Calibration verification | Operations | Confirm calibration remains valid for current crude oil and process conditions | Onsite using calibration reference sample or laboratory correlation |
| Alarm and shut-off functional test | Commissioning / periodic | Confirm alarm output and discharge shut-off respond correctly to a simulated high-oil signal | Control room and discharge valve location |
| Sample conditioning check | Commissioning / periodic | Confirm pressure, temperature, flow and filtration at the sample point are within the analyser operating range | Sample conditioning skid at the monitoring point |
Each test type answers a different question. The FAT confirms that the equipment meets its specification. The SAT confirms that the installed system works correctly in the FPSO environment. Grab sample correlation confirms that the monitor continues to read accurately in service. Calibration verification confirms that the calibration remains valid for the current process conditions.
What Is a Factory Acceptance Test and What Does It Cover?
A Factory Acceptance Test (FAT) is a formal performance verification carried out at the manufacturer’s facility or at an approved test house before the oil-in-water monitor is shipped to the FPSO. It is the primary opportunity to confirm that the equipment meets the specification before installation, when deficiencies are still straightforward to address.
For produced water monitoring applications, the FAT should be witnessed by the EPC instrumentation engineer or a nominated representative and documented in a formal FAT report that forms part of the commissioning handover package.
The table below sets out the key checks that should be covered in a produced water monitor FAT.
| FAT check | What is verified |
| Measurement accuracy | Monitor reads accurately across the required measurement range using a reference sample of the specified crude oil type |
| Response time | Monitor responds to a step change in oil concentration within the time specified in the applicable standard |
| Automatic shut-off output | Discrete output activates correctly when oil concentration exceeds the alarm set point |
| Analogue output signal | 4–20mA analogue output transmits correct oil concentration values across the full measurement range |
| Alarm function | Alarm activates at the correct set point and resets correctly when oil concentration returns below the limit |
| Data logging | Monitor records readings, alarm events and timestamps correctly in the required format |
| Hazardous-area compliance | ATEX or IECEx certification is verified against the zone classification for the installation location |
| Documentation package | Type approval certificate, calibration certificate, test records and O&M manual are complete and available |
One of the most important and most frequently overlooked aspects of the FAT is confirming that the reference sample used for the accuracy test is representative of the actual crude oil type that will be present in the FPSO produced water stream. A FAT carried out with a generic reference oil provides limited assurance of accuracy in service if the FPSO handles a significantly different crude.
| FAT with the correct crude oil reference
Where the FPSO produced water will contain a specific crude oil type, the FAT accuracy test should be conducted using a reference sample of that crude or the closest available equivalent. A monitor calibrated and tested against a different crude may give inaccurate readings in service, even if it passed the FAT with excellent results. |
What Is a Site Acceptance Test and How Is It Different From a FAT?
A Site Acceptance Test (SAT) is carried out after the oil-in-water monitor has been installed on the FPSO, connected to the sample conditioning system, integrated with the DCS or ICSS, and connected to the discharge control valve. It confirms that the complete installed system not just the analyser in isolation performs correctly in the FPSO environment.
The SAT is a critical milestone before the FPSO enters service. It must be completed and documented before produced water discharge operations begin.
Key elements of the SAT for a produced water monitoring system include:
Sample conditioning verification
Confirm that the pressure, temperature, flow rate and filtration at the sample point are within the analyser’s specified operating range. This requires measurement at the sample conditioning system not just a desk check of the engineering drawings.
Live reading verification
Confirm that the analyser is producing a stable, credible reading from the actual produced water stream or from a reference sample injected at the sample point and that the reading is transmitted correctly to the DCS or ICSS display.
Alarm and shut-off functional test
Simulate a high-oil condition either by injecting a calibrated reference sample above the alarm set point, or by using an electronic test signal and confirm that the alarm activates in the control room and the discharge shut-off valve closes within the required response time. This is the single most important functional test of the entire monitoring system and must be documented in the SAT report.
Data logging verification
Confirm that readings, alarm events and discharge operation timestamps are being captured correctly in the data logging system or Oil Record Book interface.
Calibration confirmation
Confirm that the analyser calibration, as established during the FAT, remains valid in the installed configuration or recalibrate if conditions have changed.
| The alarm and shut-off test cannot be skipped
It is not sufficient to confirm that the analyser is wired to the control system and that the discharge valve is connected. The complete functional loop from a high-oil reading at the analyser through the alarm signal to the physical closure of the discharge valve must be tested and the result documented. A monitoring system where this test has not been completed cannot be relied upon to prevent a discharge compliance breach. |
How Is Grab Sample Correlation Used to Verify Monitor Accuracy in Service?
Once the FPSO is in service, grab sample correlation is the primary method for verifying that the oil-in-water monitor continues to read accurately over time. It involves taking a manual sample of the produced water from the same point as the monitor sample, analysing it in a laboratory, and comparing the laboratory result with the monitor reading at the time the sample was taken.
The process works as follows:
- A grab sample is taken from the sample point at the treatment package outlet as close as possible to the monitor sample tap and at the same time as the monitor reading is recorded
- The sample is preserved correctly typically by acidification to prevent biological degradation before analysis
- The sample is analysed by an accredited laboratory using a recognised method typically GC-FID (gas chromatography with flame ionisation detection) or infrared spectroscopy
- The laboratory result is compared with the monitor reading recorded at the time the grab sample was taken
- A consistent discrepancy between the monitor reading and the laboratory result indicates calibration drift, sample conditioning problems or a change in crude oil composition that has affected calibration validity
The frequency of grab sample correlation depends on the permit conditions and the operator’s calibration management plan. Monthly correlation is common practice; some permits specify minimum correlation frequency. Any systematic discrepancy should trigger a calibration check and, if necessary, recalibration.
How Is an Oil-in-Water Monitor Calibrated Offshore?
Calibration of an oil-in-water monitor confirms that the relationship between the monitor’s measurement signal and the actual oil concentration in the sample is correct for the specific crude oil type and process conditions on the FPSO. For a full explanation of the calibration requirements that apply to oil content monitors, see Calibration of Oil Content Monitors.
Offshore calibration of a produced water monitor typically involves one or more of the following approaches:
Onsite calibration using a reference sample
A prepared reference sample of known oil concentration made up using the FPSO’s actual crude oil is introduced to the analyser at the sample point and the monitor reading is compared with the known concentration. If necessary, the calibration is adjusted to bring the reading into agreement with the reference. The OCD Xtra can be adjusted onsite against laboratory analysis providing a practical offshore calibration capability without requiring specialist attendance.
Correlation-based calibration adjustment
Where a sufficient number of grab sample correlation results are available, and where they show a consistent offset between the monitor reading and the laboratory result, the calibration can be adjusted to correct the offset. This approach requires a statistically meaningful dataset of correlation results and should be documented with reference to the supporting laboratory data.
Equipment replacement
Where the monitor has reached the end of its calibration interval and cannot be recalibrated offshore, replacement with a newly calibrated unit is an accepted alternative to onsite calibration. The replaced unit is returned to the manufacturer or approved service centre for recalibration or refurbishment.
What Should Be Included in the Testing and Calibration Record?
Every test and calibration event must be documented to provide a defensible compliance record that can be produced during a regulatory inspection or audit. The testing and calibration record should include:
- FAT report signed and dated, including test results, reference sample details and equipment serial numbers
- SAT report signed and dated, including sample conditioning verification results, live reading check, alarm and shut-off functional test result, and calibration confirmation
- Grab sample correlation log date, time, sample point, monitor reading at time of sampling, laboratory result and any corrective action taken
- Calibration records date, method, reference sample details, results and any adjustment made
- Calibration certificate issued by the manufacturer or approved service centre, with expiry date
- Alarm event log all alarm activations, timestamps, readings and operator responses
For a detailed explanation of what regulators and inspectors look for when reviewing the produced water discharge compliance record, see the Produced Water Discharge Monitoring for FPSOs explainer page.
What Are the Common Testing Mistakes to Avoid?
Many of the issues that cause problems during regulatory inspection can be traced back to avoidable testing and documentation failures. For a detailed breakdown of what inspectors find and what the consequences are, see What Causes High Oil Readings in Produced Water Monitors? which covers the diagnostic approach to monitor performance problems in service.
The most common testing mistakes are:
- FAT conducted with the wrong crude oil reference results are not representative of actual FPSO service conditions
- Alarm and shut-off functional test not completed during SAT the complete loop has never been confirmed to work
- Grab sample correlation not carried out regularly no independent verification of monitor accuracy in service
- Calibration certificate expired before the next IOPP survey or regulatory inspection non-compliance even if the monitor is functioning correctly
- SAT report incomplete or not signed documentation gap that cannot be filled retrospectively
- Sample conditioning not verified during SAT monitor installed but operating outside its specified conditions
Frequently Asked Questions
What is the difference between a FAT and a SAT for an oil-in-water monitor?
A Factory Acceptance Test (FAT) is carried out at the manufacturer’s facility before shipment and confirms that the analyser meets its specification in a controlled environment. A Site Acceptance Test (SAT) is carried out after installation on the FPSO and confirms that the complete installed system analyser, sample conditioning, control integration and discharge shut-off performs correctly in the actual operating environment.
How often should an oil-in-water monitor be tested against grab samples offshore?
The minimum frequency depends on the permit conditions and the operator’s calibration management plan. Monthly grab sample correlation is common practice. Any consistent discrepancy between the monitor reading and the laboratory result should trigger a calibration check regardless of how recently the last correlation was carried out.
Can an oil-in-water monitor be calibrated offshore without specialist attendance?
Yes, in many cases. Some produced water analysers including the OCD Xtra can be adjusted onsite against a laboratory reference sample without requiring the manufacturer’s service engineer to attend offshore. The calibration adjustment must be documented with reference to the supporting laboratory data.
What happens if the alarm and shut-off functional test is not completed during commissioning?
If the alarm and shut-off functional test is not completed during the SAT, the operator cannot confirm that the monitoring system will automatically stop discharge if oil concentration exceeds the permitted limit. This represents a significant compliance risk the system may be installed but not able to provide automatic discharge protection.
What testing documentation should be retained onboard an FPSO?
The following should be retained onboard: FAT report, SAT report, grab sample correlation log, calibration records, current calibration certificate, alarm event log, and O&M manual. These documents form the compliance record for the produced water monitoring system and must be available for inspection by regulatory authorities.