Back to MARPOL
Safety

MARPOL Annex I — Prevention of Pollution by Oil

IOPP, the 15 ppm OWS, discharge criteria, the Oil Record Book Parts 1 & 2, SOPEP and the ten special areas.

10 min read
Intermediate
Safety
Key Principles at a Glance 5 points
  • Annex I (Regulations for the Prevention of Pollution by Oil) applies to all ships, has 7 chapters and 39 regulations, and requires the International Oil Pollution Prevention (IOPP) Certificate, valid for 5 years.
  • Key machinery-space regulations include: 12 sludge tanks, 13 standard discharge connection, 14 oil filtering equipment, 15 control of operational discharge, 16 segregation of oil and water ballast, and 17 Oil Record Book Part 1.
  • Ships of 400 GT and above but less than 10,000 GT must be fitted with oil filtering equipment with an alarm and automatic stop of discharge when the oil content exceeds 15 ppm.
  • Overboard discharge of oil or oily mixtures is prohibited unless the ship is en route, processing through oil filtering equipment, the effluent does not exceed 15 ppm without dilution, it does not originate from cargo pump-room bilges and is not mixed with oil cargo residues; in the Antarctic all discharge is prohibited.
  • A SOPEP is required on oil tankers ≥150 GT and other ships ≥400 GT; SOPEP defines the reporting and response plan, and Annex I has 10 special areas where discharge is prohibited so residues must go to reception facilities.

1. Application, IOPP Certificate & Key Regulations

MARPOL Annex I: Regulations for the Prevention of Pollution by Oil applies universally to all ships, establishing rigorous technical standards for machinery spaces and tanker cargo areas:

All Ships Annex I Universal Scope
7 Ch / 39 Regs Regulatory Structure
5 Years IOPP Certificate Validity
≥ 400 GT Machinery Threshold (Tanker ≥ 150 GT)

Chapter 3: Machinery Space Regulations

Reg 12: Oil residue (sludge) tanks • Reg 13: Standard discharge connection • Reg 14: Oil filtering equipment (15 ppm OWS) • Reg 15: Control of operational bilge discharge • Reg 16: Segregation of oil/ballast and forepeak oil carriage ban • Reg 17: Oil Record Book Part 1.

Chapter 4: Tanker Cargo Area Regulations

Reg 18: Segregated Ballast Tanks (SBT) protective location • Reg 29: Cargo slop tank arrangements • Reg 31: Oil Discharge Monitoring & Control System (ODMCS) • Reg 34: Tanker cargo discharge criteria • Reg 35: Crude Oil Washing (COW) • Reg 36: Oil Record Book Part 2.

Chapter 5: Oil Pollution Incident Response

Reg 37: Shipboard Oil Pollution Emergency Plan (SOPEP) mandatory on every oil tanker ≥ 150 GT and every other ship ≥ 400 GT, approved by the Administration with reporting and coordination protocols.

Statutory IOPP Survey Cadence

Initial: Before entering service • Annual: Within ±3 months of anniversary • Intermediate: Within ±3 months of 2nd or 3rd anniversary (extended checks on OWS, ODMCS, COW) • Renewal: Every 5 years max.

MARPOL ANNEX I: SHIP INTERNAL ARCHITECTURE & STATUTORY COMPONENT ZONES MACHINERY SPACE OWS 15ppm Sludge Tk Bilge Holding Tank SLOP TK (Reg 29) CARGO TANKS (COW - Reg 35) SBT (Reg 18) FOREPEAK NO OIL CARRIAGE (Reg 16 Ban) Reg 13 Flange ODMCS SOPEP CORE ANNEX I REGULATORY MAPPING (ORAL EXAM QUICK MATRIX): Machinery Space (Ch 3) • Reg 12: Sludge Tanks • Reg 13: Standard Flange • Reg 14: 15 ppm OWS • Reg 17: ORB Part 1 Cargo Area (Ch 4) • Reg 18: SBT Protection • Reg 29: Slop Tanks • Reg 31: ODMCS System • Reg 36: ORB Part 2 Response & Survey • Reg 37: SOPEP Plan • Reg 16: Forepeak Oil Ban • IOPP Cert: 5-Yr Validity • Annual ±3m / Intermed KEY STATUTORY THRESHOLDS 1. Tonnage Thresholds • Oil Tankers: ≥ 150 GT • All Other Ships: ≥ 400 GT 2. Machinery Space Discharge • En route + 15 ppm OWS filter • No dilution • No cargo pump room 3. Tanker Cargo Discharge • > 50 nm from nearest land • Rate ≤ 30 L/nm • Total ≤ 1/30,000 ORB PRESERVATION RULE: Part 1 & Part 2 must be kept on board readily available for Port State Control PRESERVED FOR MINIMUM 3 YEARS
Figure 1: MARPOL Annex I shipboard internal architecture and regulatory layout. The vessel is divided into machinery spaces (Chapter 3: Reg 12 sludge tanks, Reg 13 standard connection, Reg 14 15 ppm OWS, Reg 16 forepeak ban, Reg 17 ORB Part 1) and tanker cargo blocks (Chapter 4: Reg 18 SBT, Reg 29 slop tanks, Reg 31 ODMCS, Reg 35 COW, Reg 36 ORB Part 2), backed by Chapter 5 SOPEP oil spill emergency response.

2. Oil Filtering Equipment & Standard Discharge Connection

Two essential technical installations under Regulations 13 and 14 ensure oily residues are either cleaned to legal limits or transferred to shore reception facilities:

15 ppm Maximum Discharge Alarm Trip
≥ 400 GT Mandatory OWS Fitting Threshold
215 mm OD Standard Flange Outer Diameter
600 kPa Standard Flange Working Pressure

1. Oil Filtering Equipment (Regulation 14)

Any vessel of 400 GT and above must be equipped with approved oily water separating / filtering equipment. The unit must feature an audible/visual bilge alarm and an automatic fail-safe stop valve that instantly halts sea discharge if the effluent exceeds 15 parts per million (ppm).

2. Fail-Safe 3-Way Diverter Valve

Upon detection of >15 ppm oil content, loss of electrical power, or sample water failure, the spring-loaded pneumatic diverter valve automatically trips closed to overboard and directs the oily stream back to the bilge holding tank.

3. Standard Discharge Connection (Regulation 13)

To enable universal coupling with port reception facilities worldwide, the sludge and bilge manifold discharge pipelines must be fitted with an international standard flange of uniform dimensions, suitable for up to 125 mm inner pipe diameter at 600 kPa (6 bar).

4. Oil-Proof Gasket & Slotted Bolt Holes

The flange has six open slots extending to the outer periphery to facilitate rapid alignment with shore hoses, clamped with six 20 mm bolts and sealed with an oil-proof aramid/nitrile gasket.

Standard Discharge Flange Statutory Dimensions (SOLAS / MARPOL Reg 13):

Feature Statutory Dimension Surveyor Verification Note
Outside Diameter 215 mm Total flange circular outer boundary
Inner Diameter Equal to pipe outside diameter Accepts pipes up to 125 mm bore maximum
Bolt Circle Diameter (PCD) 183 mm Pitch circle for 6 bolt slots
Flange Slots 6 slots of 22 mm diameter Slotted through to flange outer edge for quick fit
Flange Thickness 20 mm Machined flat face steel plate
Bolts & Nuts 6 bolts, each 20 mm diameter Steel fasteners of suitable length with flat washers
Maximum Working Pressure 600 kPa (6 bar / 6 kg/cm²) Fitted with oil-proof gasket
REGULATION 13: STANDARD DISCHARGE CONNECTION — MECHANICAL CAD DRAWING FRONT FACE ELEVATION & BOLT SLOTS PCD = 183 mm INNER BORE (Up to 125 mm ID) OUTSIDE DIAMETER = 215 mm • 6 SLOTS × 22 mm FLANGE CROSS-SECTION & BOLTING DETAIL Ship Pipe 20 mm Gasket Shore Hose M20 Bolt M20 Bolt SPECS: • 6 Bolts M20 • 6 Slots 22mm • Thk: 20 mm • PCD: 183 mm • OD: 215 mm • Press: 6 bar • Flat face • Oil gasket
Figure 2: MARPOL Regulation 13 Standard Discharge Connection mechanical engineering drawing. The universal flange features an outside diameter of 215 mm, bolt circle diameter (PCD) of 183 mm, flange thickness of 20 mm, and six 22 mm radial slots extending to the periphery, fastened by six 20 mm steel bolts with an oil-proof gasket rated for 600 kPa (6 bar).

3. Control of Discharge of Oil: Criteria

Surveyors rigorously test the legal criteria governing oil discharges, distinguishing sharply between machinery-space bilges and tanker cargo spaces:

En Route Mandatory for All Sea Discharges
≤ 15 ppm Machinery Effluent Limit (No Dilution)
> 50 nm • 30 L/nm Tanker Cargo Limits (Outside Special Areas)
PROHIBITED Special Areas (Cargo) • Antarctic (All)

1. Machinery-Space Bilges (All Ships ≥ 400 GT — Regulation 15):

1. En Route

The ship must be proceeding en route under its own propulsion (never stationary, at anchor, or drifting in port).

2. Approved 15 ppm OWS

The oily mixture must be continuously processed through type-approved oil filtering equipment conforming to IMO Resolution MEPC.107(49).

3. No Dilution ≤ 15 ppm

The oil content of the effluent without artificial dilution must not exceed 15 parts per million, monitored by an active bilge alarm.

4. Origin & Cargo Residue Exclusion

The oily mixture must not originate from cargo pump-room bilges on oil tankers, and must not be mixed with oil cargo residues.

2. Tanker Cargo Operations (Regulation 34):

Outside Special Areas

• Vessel proceeding en route • Distance > 50 nautical miles from nearest land • Instantaneous discharge rate ≤ 30 litres per nautical mile • Total quantity discharged ≤ 1/30,000 of total cargo for new tankers (1/15,000 for existing tankers) • Operational ODMCS and approved slop tank arrangement.

Inside Special Areas & Antarctic

Inside any designated Special Area: Any discharge of tanker cargo oil residue is STRICTLY PROHIBITED — all slops and washings must be retained on board for discharge to shore reception facilities. In the Antarctic area: ALL discharges whatsoever are prohibited.

TWO-STAGE 15 PPM OIL FILTERING EQUIPMENT (OWS) & FAIL-SAFE OVERBOARD P&ID BILGE HOLDING TANK Feed Pump (Screw Type) STAGE 1: GRAVITY Coalescing Plates Separated Oil Steam Heating Coil Auto Oil Valve to Sludge Tk ← STAGE 2: FILTER Fine Cartridge Adsorption Elements 15 PPM ALARM Optical Sensor MEPC.107(49) Fresh Water Flush 3-Way Valve NRV OVERBOARD (≤ 15 ppm) ← FAIL-SAFE RECIRCULATION TO BILGE HOLDING TANK (IF > 15 PPM OR POWER LOSS) ← FAIL-SAFE LOGIC: Bilge alarm optical sensor trips 3-way diverter valve to recycle within 20 seconds of detecting >15 ppm.
Figure 3: Two-stage 15 ppm oil filtering equipment (OWS) piping and instrumentation diagram (P&ID). Oily bilge water is pumped via a low-shear screw pump into the Stage 1 gravity coalescer, followed by Stage 2 fine adsorption cartridges. The sample is continuously interrogated by an optical 15 ppm bilge alarm. If oil content exceeds 15 ppm or power fails, the 3-way pneumatic diverter valve instantly trips shut to sea and recirculates effluent back to the bilge holding tank.

4. Oil Record Book, SOPEP & Special Areas

MARPOL Annex I demands meticulous operational accountability through the Oil Record Book, emergency preparedness via SOPEP, and zero-discharge compliance in designated Special Areas:

3 Years ORB Minimum Preservation
Part 1 (Machinery) Tankers ≥ 150 GT • Other ≥ 400 GT
Part 2 (Cargo) Oil Tankers ≥ 150 GT Only
10 Special Areas Zero Cargo Discharge Regimes

Oil Record Book Part 1 — Mandatory Letter Codes (Surveyor Recital):

Code A & B: Fuel Tanks

Code A: Ballasting or cleaning of oil fuel tanks • Code B: Discharge of dirty ballast or cleaning water from oil fuel tanks.

Code C: Sludge & Residues

Collection & disposal of oil residues (sludge): Manual/automatic sludge transfers, incinerator burning, evaporation, or shore disposal — recorded weekly.

Code D & E: Bilge Operations

Code D: Non-automatic discharge overboard or disposal of bilge water • Code E: Automatic discharge overboard via 15 ppm OWS (stop/start times and vessel positions).

Code F, G & H: Bunkering & Events

Code F: Condition/failure of oil filtering equipment • Code G: Accidental or exceptional discharges • Code H: Bunkering of fuel or bulk lube oil.

Master Signature Rule

Each operational entry must be signed and dated by the officer in charge upon completion. Each completed page must be signed by the Master. The book must be retained on board for 3 years after the last entry.

SOPEP (Regulation 37) & Oil Spill Response Equipment:

Four Core Sections of SOPEP

1. Procedure for reporting oil pollution incidents to coastal states • 2. List of coastal authorities, port contacts, and company PIC • 3. Immediate shipboard damage-control actions to mitigate discharge • 4. Procedures for coordinating with national/local authorities.

Mandatory SOPEP Equipment Inventory

Dedicated SOPEP locker containing: Sorbent pads and rolls, sorbent booms, sawdust/chemical absorbents, rubber scupper plugs, wilden air-driven non-spark diaphragm pump, anti-static shovels and scoops, buckets, heavy-duty disposal drums (200 L), and chemical-resistant PPE.

The 10 Special Areas Under MARPOL Annex I:

European & Mediterranean

1. Mediterranean Sea • 2. Baltic Sea • 3. Black Sea • 4. North West European Waters (including North Sea, English Channel, Irish Sea, Celtic Sea).

Middle East, Africa & Polar

5. Red Sea • 6. Gulfs Area • 7. Gulf of Aden • 8. Oman Area of the Arabian Sea • 9. Southern South African Waters • 10. Antarctic Area (all discharges prohibited).

REGULATION 31: OIL DISCHARGE MONITORING & CONTROL SYSTEM (ODMCS) P&ID CARGO SLOP TANK Interface Det. Stripping Pump DEGASSER Conditioner OPTICAL CELL ppm Sensor ODMCS COMPUTER UNIT • Flowmeter (m³/h) • Speed Log (Knots) • Oil Content (ppm) CALCULATES: Litres / NM Overboard Valve OVERBOARD (≤ 30 L/NM) ← FAIL-SAFE SLOP TANK RECIRCULATION (SHUTS SEA VALVE IF > 30 L/NM OR TOTAL > 1/30,000) ← STATUTORY CRITERIA: Outside special area • >50 NM from land • ≤30 L/NM instantaneous rate • Total ≤ 1/30,000 of cargo.
Figure 4: Oil Discharge Monitoring and Control System (ODMCS) functional P&ID for oil tanker cargo operations. The computing unit continuously integrates effluent oil concentration (ppm), overboard flow rate (m³/h), and GPS speed log (knots). If the instantaneous discharge rate exceeds 30 litres per nautical mile or cumulative discharge exceeds 1/30,000 of cargo, the system immediately shuts the motorized overboard valve and diverts the stream back to the slop tank.