Every barrel of oil produced from an offshore reservoir comes with water. In the early life of a field, the volume of produced water may be relatively small. As the field matures, water cut increases and on some FPSOs, the volume of produced water handled every day significantly exceeds the volume of oil produced.
For Project Directors, Procurement Managers and anyone new to FPSO operations, understanding what happens to that water from the moment it arrives at surface with the production fluid to the point where it is either discharged or reinjected is essential context for understanding why produced water monitoring equipment matters and what it is designed to do.
This article explains the full journey of produced water on an FPSO, from reservoir to final disposal, and why the oil-in-water monitoring step at the end of the treatment process is the critical compliance checkpoint. For a more detailed look at the treatment process itself, see What Is a Produced Water Treatment Package and How Does It Work?.
| Key thing to remember Produced water on an FPSO does not simply flow from the reservoir to the sea. It passes through multiple separation and treatment stages before it reaches the monitoring point and it can only be discharged if the oil-in-water monitor confirms that oil concentration is within the permitted limit. If it cannot be discharged within the limit, it must be reinjected or retained onboard. |
Where Does Produced Water Come From?
Produced water is water that has been trapped in the pores of the reservoir rock alongside oil and gas for millions of years. When hydrocarbons are produced from the reservoir, this formation water comes to the surface with them as an unavoidable by-product of production.
The quantity of produced water relative to oil expressed as water cut varies considerably between fields and changes over the life of the field. In a new field, water cut may be relatively low. In a mature field, water cut can exceed 90 per cent meaning that for every 10 barrels of fluid produced, nine are water and only one is oil.
On a large FPSO, this can mean treating and managing hundreds of thousands of barrels of produced water every day. The volume, composition and characteristics of the water vary with reservoir conditions, production chemistry and the mix of wells contributing to the production stream.
What Is the Full Journey of Produced Water on an FPSO?
Produced water follows a defined path from the reservoir to its final destination overboard discharge or reinjection. The table below summarises each stage of that journey.
| Step | Stage | What happens |
| 1 | Reservoir | Produced water is trapped in the rock formation alongside oil and gas, under pressure |
| 2 | Wellbore | Oil, gas and water are co-produced to surface through the production well |
| 3 | Production manifold | Fluids from multiple wells are combined before entering the topsides processing system |
| 4 | Three-phase separator | Oil, gas and water are separated by gravity into three distinct streams |
| 5 | Produced water treatment package | The water stream is treated through hydrocyclones, flotation and filtration to remove residual oil and solids |
| 6 | Oil-in-water monitoring point | Oil concentration in the treated water is measured continuously before any discharge decision |
| 7 | Discharge or reinjection | If within permit limits, water is discharged overboard; if not or if reinjection is preferred it is injected back into the reservoir |
| 8 | Compliance record | All discharge operations are recorded in the format required by the applicable regulatory authority |
Each stage in this journey is important but the oil-in-water monitoring point at stage 6 is the compliance gateway. Without a functioning, calibrated and correctly integrated monitor at this point, the operator cannot demonstrate that discharge is within the permitted limit, and discharge cannot legally proceed.
What Happens During Primary Separation?
When the production fluid a mixture of oil, gas and water arrives on the FPSO topsides from the production wells, it enters the primary separation system. A three-phase separator or free water knockout (FWKO) vessel uses gravity and residence time to separate the three phases:
- Oil rises to the top of the separator and is directed to the oil processing train
- Gas is released from the top of the separator and directed to the gas processing or compression system
- Water the produced water stream settles to the bottom and is directed to the produced water treatment package
At this stage, the bulk separation is complete but the produced water stream still contains significant quantities of dispersed oil droplets, dissolved hydrocarbons, fine solids, production chemicals and dissolved salts. It is not yet suitable for discharge and must pass through further treatment stages.
What Happens in the Produced Water Treatment Package?
The produced water treatment package receives the water from primary separation and progressively removes residual oil and solids through a series of treatment stages:
Hydrocyclone de-oiling
The water passes through a bank of hydrocyclones compact vessels that use centrifugal force to separate dispersed oil droplets from the water. Hydrocyclones are effective at removing oil droplets above approximately 10–15 microns but cannot remove very fine oil droplets, dissolved hydrocarbons or solids.
Gas flotation
The water then passes through a flotation unit Induced Gas Flotation (IGF) or Dissolved Gas Flotation (DGF) where fine gas bubbles are introduced to carry residual oil droplets to the surface for skimming. Flotation targets the finer oil droplets that hydrocyclones cannot remove.
Filtration (where required)
Where the required discharge quality is particularly tight, or where the water will be reinjected rather than discharged, a filtration stage walnut shell filter, sand filter or cartridge filter may be added to remove residual fine solids and oil.
Chemical dosing
Production chemicals demulsifiers, scale inhibitors, corrosion inhibitors, biocides are dosed at various points in the treatment system to support performance. The chemical environment of the produced water stream must be considered when selecting and calibrating the oil-in-water monitor, as some chemicals can affect measurement accuracy.
What Happens at the Oil-in-Water Monitoring Point?
After the treatment package, the produced water reaches the oil-in-water monitoring point the compliance gateway before any discharge decision is made. The SMART PFM 107 and other Rivertrace produced water analysers are installed at this point to continuously measure the oil concentration in the treated water stream.
The monitoring system works as follows:
- A sample of the treated produced water is taken from the discharge line and conditioned pressure reduced, temperature stabilised, solids removed before entering the analyser
- The analyser continuously measures oil concentration in parts per million and transmits the reading to the control system
- If oil concentration is within the permitted limit, the discharge valve remains open and water flows to the overboard discharge point
- If oil concentration exceeds the permitted limit, the analyser triggers an alarm and the discharge valve closes automatically, stopping discharge and diverting the water back to the treatment system
- All readings, alarm events and discharge operations are recorded in the compliance record
For a full explanation of how the monitoring system is designed, installed and integrated, see the Produced Water Discharge Monitoring for FPSOs explainer page.
What Are the Two Possible Destinations for Treated Produced Water?
Once the treated produced water has passed through the monitoring point, it has two possible destinations overboard discharge or produced water reinjection (PWRI). The table below summarises the key differences between the two options.
| Overboard discharge | Produced water reinjection (PWRI) | |
| Regulatory basis | Governed by field permit and national environmental law | Governed by reservoir management plan and formation injection permit |
| Quality standard | Oil concentration within permit limit (e.g. 15–40ppm depending on jurisdiction) | Typically tighter than discharge solids and bacteria must also be controlled |
| Monitoring required | Continuous oil-in-water monitoring at treatment package outlet | Oil, solids and bacterial monitoring to protect reservoir and injection well integrity |
| Key driver | Environmental compliance staying within permitted discharge limits | Reservoir pressure support, zero-discharge requirement, or permit restriction on discharge |
| Fate of the water | Discharged to sea after passing compliance check | Injected back into the reservoir or a disposal formation |
Overboard discharge
Where treated produced water meets the discharge limit set by the field permit or applicable regulation, it is discharged overboard through the sea chest or overboard discharge point. The discharge limit varies by jurisdiction some countries apply a 15ppm limit consistent with the MARPOL bilge water standard; others set limits of 25ppm, 30ppm or 40ppm. For a detailed overview of the regulatory frameworks that govern produced water discharge, see MARPOL and FPSO Compliance: Which Regulations Apply Above and Below the Waterline?.
Produced water reinjection
Where overboard discharge is not permitted because the oil concentration cannot be reduced to within the permitted limit, because the operating location has a zero-discharge requirement, or because reinjection is preferred for reservoir management reasons the treated produced water is injected back into the reservoir or into a dedicated disposal formation via water injection wells.
Produced water reinjection requires a higher water quality standard than overboard discharge solids and bacterial content must also be controlled to protect the injection well and the reservoir. The monitoring requirements for PWRI are therefore more extensive than for discharge monitoring alone.
| What happens if the treated water does not meet the discharge limit? If the oil-in-water monitor reading exceeds the permitted limit, discharge must stop. The produced water must either be recirculated through the treatment system until the oil concentration is reduced to within the limit, or reinjected if reinjection capacity is available. Continued discharge above the permitted limit constitutes a compliance breach with potentially serious consequences for the operator. |
What Compliance Records Must Be Kept?
The operator must maintain a compliance record that demonstrates to the satisfaction of the applicable regulatory authority that produced water discharge was within the permitted limit at all times. This record typically includes:
- Continuous or interval oil concentration readings from the oil-in-water monitor
- Timestamps for all readings, alarm events and discharge operations
- Records of any alarm activations and the corresponding operator response
- Calibration dates and reference information for the oil-in-water monitor
- Discharge volumes or flow rates where required by the field permit
Inspectors reviewing produced water discharge compliance will examine both the physical installation and the documentary record. For a detailed explanation of what inspectors check and the consequences of a failed inspection, see What Happens If FPSO Produced Water Discharge Fails an Inspection?.
Frequently Asked Questions
What is produced water on an FPSO?
Produced water is the water that is co-produced with oil and gas from the reservoir. It arrives at surface mixed with the production fluid and must be separated, treated and monitored before it can be discharged overboard or reinjected into the formation. On mature FPSOs, produced water can represent the majority of the total fluid volume handled by the topsides processing system.
Can produced water be discharged directly overboard from an FPSO?
No. Produced water cannot be discharged directly overboard. It must first be separated from the production fluid, treated through the produced water treatment package to remove residual oil and solids, and then pass through the oil-in-water monitoring point to confirm that oil concentration is within the permitted limit before any discharge decision is made.
What determines whether produced water is discharged or reinjected?
The decision depends on the discharge limit set by the field permit, the quality of the treated produced water, the operating location’s environmental requirements and the availability of reinjection capacity. Where overboard discharge is not permitted or the treated water does not meet the discharge limit, reinjection is the alternative. Some FPSO operations choose reinjection as the primary disposal route regardless of discharge permit conditions.
What happens if the oil-in-water monitor shows that produced water does not meet the discharge limit?
If oil concentration exceeds the permitted limit, the discharge valve closes automatically and discharge stops. The produced water must be recirculated through the treatment system until oil concentration is reduced to within the limit, or reinjected if reinjection capacity is available. Discharge above the permitted limit even briefly constitutes a compliance breach.
Why does the volume of produced water increase over the life of an FPSO?
As the hydrocarbon reservoir is depleted, reservoir pressure decreases and more formation water enters the production stream. Water cut the proportion of water in the total production fluid typically increases over time. In mature fields, water cut can exceed 90 per cent, meaning the topsides produced water treatment system must handle far larger volumes than in the early production phase.