If you work in marine environmental compliance, you have probably encountered both terms. But produced water and bilge water are not the same thing and using the wrong monitoring equipment for either one creates serious compliance risk, particularly on FPSO projects.
This article explains what produced water is, how it differs from bilge water, and why the distinction matters when specifying oil-in-water monitoring equipment for offshore and marine applications.
| Key thing to remember Produced water comes from oil and gas production. Bilge water comes from a vessel’s machinery space. They have different compositions, different regulatory frameworks and require different monitoring technology. A standard 15ppm bilge monitor is not suitable for produced water discharge on an FPSO. |
What Is Produced Water?
Produced water is water that is trapped underground alongside oil and gas reserves and brought to the surface as a by-product of hydrocarbon production.
When oil and gas are extracted from a reservoir, water naturally comes with them. This water has been in contact with the hydrocarbon reservoir for millions of years and contains:
- Formation water and dissolved salts
- Dispersed and dissolved hydrocarbons
- Production chemicals added during the separation process
- Suspended solids and fine particles
- Gas in solution or as fine bubbles
- Naturally occurring radioactive materials (NORM) in some fields
On a Floating Production Storage and Offloading (FPSO) vessel, produced water is separated from the hydrocarbon stream in the topsides processing system and must be treated before discharge or reinjection.
The volume of produced water can be substantial. In mature fields, the ratio of produced water to oil can exceed 10:1, making it one of the largest waste streams in offshore oil and gas operations.
What Is Bilge Water?
Bilge water is the water that accumulates in the lowest part of a ship’s hull, the bilge. It is a normal feature of any vessel in operation and collects from:
- Leaks from machinery, pipes and seals
- Condensation from engines and equipment
- Drainage from deck and machinery spaces
- Water used in cleaning machinery spaces
Bilge water typically contains oil, fuel residues, lubricants, detergents and general machinery contamination. It must be processed through an oily water separator (OWS) before any discharge overboard, and the discharge is strictly regulated under MARPOL Annex I.
The international standard for bilge water discharge is a maximum oil content of 15 parts per million (ppm) which is why the monitoring equipment used is called a 15ppm bilge alarm.
What Are the Key Differences Between Produced Water and Bilge Water?
The table below summarises the main differences between the two water streams:
| Bilge Water | Produced Water | |
| Source | Machinery space leaks, condensation, drainage | Oil and gas reservoir separated from hydrocarbons during production |
| Composition | Oil, fuel, lubricants, detergents, water | Formation water, crude hydrocarbons, chemicals, salts, gas, solids |
| Oil concentration | Typically up to 1,000ppm before treatment | Highly variable can range from <10ppm to several hundred ppm |
| Regulatory standard | MARPOL Annex I 15ppm discharge limit via OWS | Field permits, national regulations, operator standards no single global limit |
| Monitoring equipment | 15ppm bilge alarm (MEPC.107(49)) | Specialist oil-in-water analyser or ODME e.g. SMART PFM 107, OCD Xtra |
| Treatment process | Oily water separator (OWS) | Produced water treatment package (hydrocyclone, flotation, filtration) |
| Measurement challenges | Relatively stable composition | Gas bubbles, solids, emulsions, chemical dosing, crude variability |
Why Does the Distinction Matter for Oil-in-Water Monitoring?
The difference between produced water and bilge water is not just technical; it has direct implications for which monitoring equipment is required, which regulations apply and what happens if the wrong equipment is specified.
Different regulations apply
Bilge water discharge from a vessel’s machinery space is governed by MARPOL Annex I and IMO MEPC.107(49), which sets the 15ppm discharge limit and defines the type of bilge alarm required.
Produced water discharge from an FPSO topsides system is typically governed by field permits, national environmental regulations and operator-specific standards. There is no single global limit equivalent to the 15ppm bilge rule. Discharge limits vary by location, field permit conditions and applicable legislation.
Different measurement challenges exist
A 15ppm bilge alarm is designed and calibrated for the relatively stable composition of bilge water. Produced water presents a much more complex measurement challenge:
- Oil droplet size and distribution vary with reservoir conditions and production chemistry
- Gas bubbles can interfere with optical measurement systems
- Suspended solids can cause false readings
- Chemical dosing during water treatment affects the sample characteristics
- Temperature and pressure variations affect sensor performance
A monitor that appears to give accurate readings in a stable bilge water environment may perform unreliably in a variable produced water stream creating compliance risk that only becomes visible during commissioning or an audit.
Different equipment is required
For bilge water discharge monitoring, the required equipment is a type-approved 15ppm bilge alarm meeting MEPC.107(49). Rivertrace’s SMART BILGE is designed for this application.
For produced water discharge monitoring on an FPSO, the appropriate equipment depends on the discharge stream and the applicable regulatory basis. Options include:
- SMART PFM 107 an MEPC.107(49)-approved oil-in-water monitor using microscopy and optical recognition, suitable for produced water, oily water separator discharge and drill rig applications
- OCD Xtra a produced water analyser measuring oil concentration across 0–200ppm, calibratable on up to six oil types for site-specific accuracy
- SMART ODME relevant where an FPSO is converted from a tanker or where tanker slop, tank wash or ballast discharge monitoring is required under MEPC.108(49)
| Important Specifying a standard 15ppm bilge monitor for an FPSO produced water discharge application is a common and costly mistake. The equipment may not be suitable for the process conditions, may not satisfy the applicable permit requirements, and may create compliance exposure during regulatory audit or inspection. |
What Does This Mean for EPC and FPSO Project Teams?
For EPC, EPCI and FPSO contractor teams, the produced water versus bilge water distinction is a specification decision that must be made early in the project before procurement, before detailed engineering and well before commissioning.
Getting it wrong creates downstream risk across multiple project disciplines:
- Incorrect technology selection requiring late-stage specification changes
- Commissioning delays if analyser performance cannot be verified against permit requirements
- Documentation gaps that prevent a supplier from progressing through bid evaluation
- Regulatory exposure during inspections or environmental audits
Rivertrace works with EPC teams at the specification stage to define the right oil-in-water monitoring approach taking into account the discharge stream, applicable regulations, process conditions and long-term serviceability requirements.
Frequently Asked Questions
Can a 15ppm bilge monitor be used for produced water discharge on an FPSO?
No. A 15ppm bilge alarm is designed and type-approved for oily water separator discharge from a vessel’s machinery space. It is not suitable for FPSO produced water discharge, which requires specialist oil-in-water analysers capable of handling variable composition, gas, solids and chemical interference.
What regulations apply to produced water discharge from an FPSO?
Produced water discharge from FPSO topsides systems is typically governed by field permits, national environmental legislation and operator standards rather than a single global MARPOL standard. The applicable requirements depend on the operating location, the discharge stream and the field permit conditions.
What is the difference between MARPOL bilge water rules and produced water regulations?
MARPOL Annex I sets a universal 15ppm discharge limit for bilge water from vessel machinery spaces, enforced via type-approved bilge alarms. Produced water regulations vary by country, field and permit; there is no single equivalent limit, and monitoring equipment must be specified to match the applicable requirements.
Which Rivertrace product is right for FPSO produced water monitoring?
The right product depends on the discharge stream and regulatory basis. SMART PFM 107 is suitable for produced water discharge monitoring under MEPC.107(49). OCD Xtra is designed for produced water applications requiring calibration against multiple oil types. SMART ODME is relevant for tanker-converted FPSOs with slop or ballast discharge requirements under MEPC.108(49). Rivertrace can advise on the correct selection at the specification stage.
When should Rivertrace be involved in FPSO produced water monitor specification?
As early as possible ideally at the concept or FEED stage, before procurement and detailed engineering. Early advisory input helps avoid incorrect technology selection, reduces documentation risk and supports smoother commissioning and first-oil readiness.