Bilge Oil & Oily Water Inspections: Marine Compliance for Separation & Record Accuracy
The oily water separator is one of the most inspected — and most frequently deficient — systems on any vessel. PSC inspectors know it, surveyors know it, and port state authorities worldwide continue to identify defective or non-compliant OWS installations during routine inspections. The consequences of getting it wrong range from MARPOL violations and PSC detention to criminal prosecution for illegal discharges, with fines reaching millions of dollars and crew members facing imprisonment. The good news: the compliance requirements are clear, the inspection process is predictable, and with systematic preparation, there is no reason for a bilge-related deficiency to appear on your vessel's record. This guide covers what inspectors actually check, how the 15 ppm alarm system must perform, and how to keep your Oil Record Book beyond reproach. Operators looking to digitize OWS maintenance and ORB tracking can sign up for Marine Inspection's compliance platform to manage everything in one auditable system.
Bilge Oil & OWS Compliance: Key Reference Numbers
15 ppm
Maximum Oil Content
In effluent discharged overboard (MARPOL Annex I)
≤5 sec
Alarm Response Time
From oil content change to display reading
≤20 sec
Auto-Stop Time
From exceeding 15 ppm to stopping discharge
18 months
Data Retention
Minimum alarm recording storage requirement
The Legal Framework: When Can You Discharge?
MARPOL Annex I Regulation 15 sets strict conditions for legal discharge of bilge water. Understanding these rules is the starting point for compliance — violating any one condition makes the discharge illegal regardless of oil content.
Legal Discharge Conditions (Ships ≥400 GT)
Ship must be proceeding en route — Not at anchor, not alongside, not stationary. The vessel must be making way through the water.
Processed through approved oil filtering equipment — The OWS must be type-approved per MEPC.107(49) for equipment installed on or after 1 January 2005.
Oil content must not exceed 15 ppm — Monitored continuously by the 15 ppm bilge alarm (Oil Content Monitor). Automatic stopping device must halt discharge if exceeded.
Not from cargo pump-room bilges (tankers) — And the mixture cannot be mixed with oil cargo residues.
Zero discharge in the Antarctic area — Any discharge of oil or oily mixtures is completely prohibited regardless of oil content or treatment.
What PSC Inspectors Actually Check
PSC officers follow a systematic process when inspecting OWS installations. AMSA (Australia) has published detailed guidance that reflects the approach used by inspectors worldwide. Here is exactly what they examine — and what triggers a detention.
Detainable
1. 15 ppm Alarm Response Test
The PSCO simulates a discharge exceeding 15 ppm. The alarm must activate within 5 seconds and the automatic stopping device (3-way valve) must halt overboard discharge within 20 seconds. If either fails, the system is non-compliant and the vessel is likely to be detained. The PSCO also confirms the alarm activates when sample flow is stopped — if it doesn't alarm within 5 seconds of flow loss, and the auto-stop doesn't activate within 20 seconds, this is a failure.
Detainable
2. Sample Line Integrity
The sample line to the Oil Content Monitor (OCM) must deliver a truly representative sample with adequate pressure and flow. All valves on the sample line must be in normal operating position during inspection. Closed or partially closed sample valves — or evidence that valves are routinely closed during operation — indicate the system is being bypassed. The sampling point installation must comply with MEPC.107(49) requirements.
Detainable
3. Fail-Safe Arrangements
The system must include fail-safe arrangements that prevent overboard discharge if the alarm malfunctions. If the OCM fails, loses power, or the sample flow is interrupted, the system must default to stopping discharge — not allowing it. PSCOs test this by interrupting sample flow and confirming the auto-stop activates. Clean water flushing for calibration must also trigger the alarm and stopping device.
Deficiency
4. Calibration & Service Records
The 15 ppm bilge alarm must be calibrated by the manufacturer or authorized agent at intervals not exceeding 5 years (or per manufacturer's instructions, whichever is shorter). The calibration certificate must be retained onboard. Accuracy must be within ±5 ppm. Missing or expired calibration certificates are a deficiency; an inaccurate alarm is detainable.
Deficiency
5. Data Recording Device
The alarm must record date, time, and operating status of the separator. Data must be stored for at least 18 months and be displayable or printable. PSCOs may request to view historical operating data — gaps, anomalies, or evidence of tampering will raise serious concerns and may constitute clear grounds for expanded inspection.
Deficiency
6. Physical Condition of Equipment
PSCOs inspect the separator, filters/coalescers, piping, valves, and overboard discharge arrangements for condition, leaks, and evidence of tampering or bypass. Oil traces in pipes connected to the OWS but downstream of the separator suggest illegal bypass arrangements. All valves must operate satisfactorily with no signs of unauthorized modifications.
Oil Record Book: The Documentation That Protects You
The Oil Record Book (ORB) Part I is the legal record of all machinery space operations involving oil. It is one of the first documents PSCOs review — and inconsistencies or gaps are treated as serious findings.
Oil Record Book: What Must Be Recorded
A
Ballasting/cleaning of fuel oil tanks — Tank identification, whether cleaned, if connected to OWS discharge.
B
Discharge of dirty ballast or cleaning water — Method, quantity discharged, through OWS or to shore.
C
Collection/disposal of oil residues (sludge) — Quantities, transfer between tanks, disposal to shore.
D
Non-automatic discharge overboard of bilge water — Through OWS with OCM, quantity, oil content, position.
E
Automatic discharge overboard of bilge water — Through OWS, time period, position at start/finish.
H
Bunkering operations — Date, port, type and quantity of fuel received, tank identification.
I
Additional operational procedures and remarks — OWS maintenance, seal logs, overboard valve locking/numbering.
✓
Signed by Officer-in-Charge — Each completed operation signed. Each completed page signed by the Master.
3yr
Retained onboard 3 years — Must be available for PSC inspection at any time. Readily accessible, not stored away.
Record-Keeping Best Practice
PSCOs cross-reference ORB entries against engine room logbook entries, sludge tank soundings, bunker delivery notes, and OWS alarm data recordings. Inconsistencies between these records are treated as evidence of falsification — which can trigger criminal investigation, not just a deficiency. The best protection is entries that are timely (made at the time of operation, not retrospectively), accurate, complete, and consistent across all related documents.
The overboard discharge valve should be locked with a numbered seal, and the seal number recorded in the ORB under code I. Log every seal break and replacement with the new seal number. Maintain a seal inventory.
Check bilge water level and composition (avoid feeding heavily oiled or emulsified water that overwhelms the separator). Verify all valves in correct position. Confirm 15 ppm alarm is displaying and functional. Check sample line flow is unobstructed.
Operational readiness check
M
Monthly
Test 15 ppm alarm by simulating high oil content — confirm alarm activates and 3-way valve stops overboard discharge. Inspect and clean filter/coalescer elements. Check all pipe connections and valve operation. Verify data recording device is storing data.
Functional test + physical inspection
6M
Every 6 Months
Internal inspection of separator — clean all separation chambers, check coalescers for blockage or degradation, inspect heater elements (if fitted), and verify all internal seals and gaskets. Replace filter elements per manufacturer schedule.
Internal overhaul per maker's instructions
5Y
Every 5 Years (Max)
OCM calibration by manufacturer or authorized agent — calibration certificate must be retained onboard. This is required at IOPP Certificate renewal survey at minimum. Alternatively, replace the measuring cell with a factory-calibrated unit and retain the new calibration certificate.
MEPC.107(49) requirement — certificate mandatory
Track Every OWS Operation, Test & ORB Entry
Marine Inspection connects your OWS maintenance schedule, alarm test records, calibration certificates, and ORB documentation into one auditable platform — so compliance is continuous, not reactive.
These are the findings that appear repeatedly in PSC reports worldwide — each one is preventable with proper procedures and maintenance.
Detention Risk
Alarm Bypassed or Non-Functional
The 15 ppm alarm is found disconnected, sample valves closed, or the auto-stop mechanism disabled. Some operators block the OCM sensor to force a zero reading — PSCOs know this trick and will request an actual test using bilge tank water. An alarm that doesn't respond to real oily water is a detention-level finding.
Detention Risk
Illegal Bypass Piping
Any piping arrangement that allows bilge water to be discharged overboard without passing through the OWS and OCM is an illegal bypass — regardless of whether it was installed for "emergency" purposes. PSCOs inspect all piping around the OWS for unauthorized connections, oil traces in clean-side pipes, and cross-connections that shouldn't exist.
Serious Deficiency
ORB Irregularities
Entries that don't match sludge tank levels, bilge well soundings, or engine log records. Retrospective entries (clearly written in the same pen/handwriting after the fact rather than at the time of operation). Missing entries for bunkering or sludge disposal. Unsigned pages. Any of these raise suspicion of falsification — which can escalate beyond a deficiency to criminal investigation.
Serious Deficiency
Expired Calibration Certificate
The OCM calibration certificate has expired or is missing. Without a valid certificate, there is no evidence the alarm is reading accurately — meaning every discharge since expiry may have been non-compliant. PSCOs will require immediate recalibration or cell replacement before the vessel can legally discharge bilge water overboard.
Zero MARPOL Deficiencies Starts with Digital Tracking
Marine Inspection tracks OWS maintenance, alarm test records, calibration expiry dates, seal logs, and ORB consistency — giving you the audit trail that proves compliance at every PSC inspection.
What is the maximum oil content allowed in overboard discharge?
15 parts per million (ppm) as set by MARPOL Annex I Regulation 15. The 15 ppm bilge alarm continuously monitors effluent from the oily water separator and must automatically stop overboard discharge if this limit is exceeded. The alarm must respond within 5 seconds and the automatic stopping device must activate within 20 seconds. Zero discharge is permitted in the Antarctic area regardless of oil content.
How often must the 15 ppm bilge alarm be calibrated?
At intervals not exceeding 5 years after commissioning, or within the term specified in the manufacturer's instructions — whichever is shorter. Calibration must be performed by the manufacturer or an authorized agent. Alternatively, the measuring cell can be replaced with a factory-calibrated unit. The calibration certificate must be retained onboard for inspection. Accuracy must remain within ±5 ppm at all times.
How long must the Oil Record Book be kept onboard?
The Oil Record Book Part I must be retained onboard for 3 years from the date of the last entry and must be readily available for inspection by PSC officers at any time. All operations involving oil in the machinery spaces — including OWS discharge, sludge disposal, bunkering, and bilge water transfers — must be recorded with date, time, details, and signatures.
What happens if the OWS is found non-functional during PSC?
A non-functional OWS — including a bypassed alarm, failed auto-stop device, or inoperative separator — is a detainable deficiency. The vessel cannot legally discharge bilge water overboard until the system is repaired and verified as compliant. In cases where the PSCO finds evidence of deliberate tampering or bypass, the matter may be referred to the flag state and potentially to criminal prosecution authorities.
How does the PSCO test the 15 ppm alarm during inspection?
The PSCO requires the OWS to be configured to recirculate (bilge tank to bilge tank) while supplying an effluent sample to the OCM. They then simulate a condition exceeding 15 ppm and verify the alarm activates within 5 seconds and the auto-stop halts discharge within 20 seconds. They also test the fail-safe by stopping sample flow — the system must alarm and stop discharge. All sample line valves must be in normal operating position during testing.
