A high oil reading in a produced water monitor on an FPSO is one of the most operationally disruptive events in offshore produced water management. It can trigger a discharge alarm, halt overboard discharge and if the reading is not investigated and resolved quickly stop production.
But not every high reading means there is actually too much oil in the produced water. Some high readings are real, they reflect genuine treatment underperformance or process upsets that must be addressed before discharge can resume. Others are false, they are caused by measurement errors, sample conditioning problems or monitor limitations rather than by an actual increase in oil content.
Understanding the difference between a real high reading and a false high reading is critical for FPSO operations teams, instrumentation engineers and commissioning personnel. This article explains the most common causes of high oil readings in produced water monitors and how to identify which cause applies in each situation. For guidance on selecting the right monitor for the application in the first place, see How to Specify an Oil-in-Water Monitor for FPSO Produced Water Discharge.
| Key thing to remember
A high reading in a produced water monitor may indicate a real increase in oil content — or it may indicate a measurement problem. Before any operational response is made, it is important to determine which type of high reading has occurred. A false high reading that is treated as real can cause unnecessary production shutdown. A real high reading that is dismissed as false can result in a discharge compliance breach. |
What Are the Main Causes of High Oil Readings in Produced Water Monitors?
High oil readings in produced water monitors fall into three broad categories — process-related causes, installation and sample conditioning causes, and maintenance-related causes. The table below summarises the ten most common causes across all three categories.
| Cause | Category | What it means |
| Gas bubbles in the sample | Process | Gas in solution released under pressure reduction causes false high optical readings |
| Suspended solids and fines | Process | Solid particles scatter light in the same way as oil droplets in optical monitors |
| Production chemical interference | Process | Some chemicals alter oil droplet size, surface properties or optical characteristics |
| Change in crude oil type | Process | Monitor calibrated on one crude may give inaccurate readings when crude composition changes |
| Sample conditioning failure | Installation | Incorrect pressure, temperature or filtration at the sample point affects measurement |
| Fouled or contaminated sensor | Maintenance | Oil or scale build-up on the measurement cell or sensor surface causes elevated readings |
| Calibration drift or expiry | Maintenance | Monitor reading has drifted from the calibrated baseline due to age or environmental factors |
| Upstream treatment underperformance | Process | Hydrocyclone or flotation unit not removing oil effectively — reading reflects real high oil content |
| Sample line contamination | Installation | Oil residue in the sample line from a previous event carried forward into current readings |
| Incorrect monitor type specified | Specification | Monitor not designed for the specific water stream characteristics of the FPSO process |
Each of these causes produces a different pattern of high readings which is why understanding the pattern is the first step in identifying the root cause.
Which Process Conditions Cause False High Readings?
Gas bubbles in the sample stream
Gas interference is one of the most common causes of false high readings in produced water monitors — particularly on FPSOs where produced water passes through pressure reduction stages before reaching the monitoring point.
When dissolved gas comes out of solution as pressure drops — a phenomenon known as flashing — it creates fine gas bubbles in the water stream. In optical oil-in-water monitors that use light scattering to detect oil droplets, gas bubbles scatter light in the same way as oil droplets, producing elevated readings that do not reflect the actual oil content of the water.
The SMART PFM 107 uses optical microscopy to actively differentiate between oil particles, gas bubbles and solids in the 1–500 micron range — directly addressing gas interference as a source of false high readings in produced water monitoring applications.
Suspended solids and fine particles
Fine reservoir particles, scale and other solids in the produced water stream can cause false high readings in optical monitors. Like gas bubbles, solid particles scatter light in the measurement cell and can be misinterpreted as oil droplets by monitors that cannot distinguish between different types of particles.
Solid particle interference is most common where upstream filtration is absent or underperforming, or where the produced water stream carries a higher-than-expected solids loading from the reservoir. Sample conditioning — specifically filtration ahead of the monitor — is the primary defence against solids-related false readings.
Production chemical interference
Production chemicals demulsifiers, scale inhibitors, corrosion inhibitors, biocides — are injected at various points in the produced water treatment system. Some of these chemicals can affect the physical properties of oil droplets in the water stream, altering their size, surface tension or optical characteristics in ways that affect monitor accuracy.
Chemical interference is most likely to appear as a step change in readings following a change in chemical type, dosage or injection point rather than as a gradual drift. It can also affect the validity of the monitor’s calibration if the chemical environment has changed significantly since the last calibration was performed.
Change in crude oil type or composition
An oil-in-water monitor is calibrated against a specific crude oil reference — or, in the case of the OCD Xtra, against up to six crude oil types. If the crude oil type or composition changes — due to reservoir conditions, production blending or a change in the producing well — the existing calibration may no longer accurately reflect the optical properties of the oil in the produced water stream, resulting in readings that are higher or lower than the actual oil content.
This is a particularly important consideration on FPSOs that produce from multiple reservoir zones or that receive commingled production from different fields. Recalibration against the new crude oil type is required whenever a significant change in crude composition occurs.
Which Installation and Sample Conditioning Issues Cause High Readings?
Sample conditioning failure
The produced water sample must reach the oil-in-water monitor at the correct pressure, temperature and flow rate — and free of large solids — for the monitor to give accurate readings. If any of these conditions are outside the monitor’s operating range, measurement accuracy is compromised.
Common sample conditioning failures include pressure too high or too low at the sample cell, temperature outside the instrument’s operating range, insufficient filtration allowing solids into the measurement cell, and sample flow rate outside the specified range. For a full explanation of sample conditioning requirements, see the Produced Water Treatment Package article.
Sample line contamination
Oil residue left in the sample line or measurement cell from a previous high-oil event, a maintenance operation or a commissioning flush can carry forward into subsequent readings, causing elevated measurements that do not reflect the current condition of the produced water stream.
Sample line contamination is most likely to appear immediately after a real high-oil event, after maintenance on the sample conditioning system, or during initial commissioning before the sample system has been fully flushed. Systematic flushing of the sample line before taking compliance readings is good practice.
Which Maintenance Issues Cause High Readings?
Fouled or contaminated sensor
Over time, oil, scale or biofilm can accumulate on the measurement cell or sensor surface of the oil-in-water monitor. This contamination reduces the amount of light passing through the cell and can cause the monitor to register elevated oil readings even when the water being measured is within the permitted limit.
Sensor fouling typically produces a gradual upward drift in readings over time rather than a sudden step change. Regular cleaning of the measurement cell in accordance with the manufacturer’s maintenance schedule — is the primary preventive measure.
Calibration drift or expiry
All oil-in-water monitors are subject to calibration drift over time the relationship between the electrical signal produced by the sensor and the actual oil concentration in the sample changes gradually due to component ageing, temperature cycling and environmental factors.
A monitor whose calibration has drifted will give readings that are systematically higher or lower than the actual oil content of the sample. Calibration drift is best identified by correlating monitor readings against regular grab samples analysed by a laboratory a discrepancy between the monitor reading and the lab result indicates drift.
| Do not dismiss a high reading without investigation
It is tempting to attribute an unexpected high reading to a measurement problem rather than a real increase in oil content particularly if the upstream treatment system appears to be operating normally. However, treatment performance can change without any visible indication at the process level. Every high reading should be investigated systematically before a conclusion is reached. |
When Is a High Reading a Real Compliance Problem?
A high reading is a real compliance problem not a measurement error when the oil concentration in the produced water stream is genuinely above the permitted discharge limit. This can occur when:
- The upstream treatment system hydrocyclone, flotation unit or filtration is underperforming due to equipment degradation, upset conditions or process changes
- Water cut has increased significantly, increasing the volume and loading of produced water through the treatment system
- A process upset separator upset, chemical injection failure, high-flow event has caused a step change in the oil content of the produced water entering the treatment system
- The treatment package has not been correctly sized for current production conditions
Where a high reading is confirmed as real, discharge must stop until the treatment system performance is restored and the monitor confirms that oil concentration is within the permitted limit. Continuing to discharge above the permit limit even unintentionally constitutes a compliance breach with potentially serious consequences.
How Do You Diagnose the Cause of a High Reading?
The diagnostic approach to a high oil reading depends on identifying the pattern of the reading and correlating it with process and equipment conditions. The table below sets out the most common observation patterns and where to look first for each.
| What you observe | Where to look first |
| Reading rises sharply on pressure change or pump start | Gas release from solution — check sample conditioning pressure reduction |
| Reading elevated across all conditions, not just peak flow | Sensor fouling or calibration drift — check and clean measurement cell, verify calibration |
| Reading elevated only at high flow rates | Turbulence in the sample line causing gas entrainment — check sample conditioning flow |
| Reading normal on grab sample but high on monitor | Sample conditioning issue — grab sample taken after settling; monitor sees undisturbed stream |
| Reading rises after chemical injection point change | Chemical interference — check compatibility of new chemical with monitor calibration |
| Reading high following crude oil type change | Calibration no longer valid for new crude — recalibrate against new crude reference |
| Reading high despite normal upstream treatment performance | Solids loading in sample — check filtration in sample conditioning system |
| Reading consistently elevated with no process changes | Upstream treatment underperformance — check hydrocyclone and flotation unit performance |
Correlating the monitor reading against a grab sample taken from the same point is always a useful first step — if the grab sample analysis confirms a high oil content, the reading is likely real. If the grab sample shows oil within the permitted limit while the monitor reading is high, the cause is likely a measurement error.
How Does Correct Monitor Selection Prevent False High Readings?
Many of the false high reading causes described in this article are avoidable through correct monitor selection at the specification stage. The two most common sources of false high readings — gas bubble interference and solids interference — can both be addressed by selecting a monitor that uses measurement technology designed to discriminate between oil, gas and solids. For a full guide to the specification process, see the Produced Water Discharge Monitoring for FPSOs explainer page.
Key selection criteria that directly affect false reading risk include:
- Measurement technology — optical microscopy monitors that differentiate oil, gas and solids reduce gas and solids interference compared to simple light-scattering monitors
- Calibration capability — monitors that can be calibrated on multiple crude oil types reduce the risk of calibration-related inaccuracy when crude composition changes
- Sample conditioning design — monitors with integrated or well-defined sample conditioning requirements are less susceptible to sample-related measurement errors
- Maintenance accessibility — monitors with easily accessible measurement cells are more likely to be cleaned regularly, reducing the risk of fouling-related drift
Frequently Asked Questions
What is the most common cause of false high readings in a produced water monitor?
Gas bubble interference is one of the most frequently encountered causes of false high readings in optical produced water monitors on FPSOs. When dissolved gas comes out of solution as pressure drops through the sample conditioning system, it creates fine bubbles that scatter light in the same way as oil droplets, producing readings higher than the actual oil content of the water.
How do you tell the difference between a real high reading and a false high reading?
The most reliable method is to take a grab sample from the same point as the monitor sample and have it analysed by a laboratory. If the lab result confirms high oil content, the reading is likely real. If the lab result shows oil within the permitted limit while the monitor reads high, the cause is likely a measurement error — gas, solids, sensor fouling or calibration drift.
Can production chemicals cause false high readings in a produced water monitor?
Yes. Some production chemicals — particularly demulsifiers and certain scale inhibitors — can alter the optical properties of oil droplets in the produced water stream, affecting measurement accuracy. A step change in readings following a change in chemical type, dosage or injection point is a common indicator of chemical interference.
How often should a produced water monitor be calibrated to avoid drift-related high readings?
Calibration requirements depend on the monitor type and the applicable permit conditions. As a general principle, monitor readings should be correlated against laboratory grab sample analysis regularly typically monthly or more frequently where readings are variable. Any systematic discrepancy between the monitor reading and the lab result should trigger a calibration check.
What should be done if a high oil reading cannot be resolved quickly?
If the cause of a high reading cannot be identified and resolved quickly, discharge must be suspended until the reading returns to within the permitted limit and the cause has been investigated. Continuing to discharge while a high reading is unresolved creates compliance risk regardless of whether the reading is real or a measurement error.