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Environmental complianceLesson 1 of 6

MARPOL Annex I in the engine room

You will be able to say, for any drop of oily water in the engine room, whether it may go overboard and what has to be recorded.

14 minutes · Open to everyone · Rules checked 20 October 2026

Start here

In August 2013 a newly hired engineer on a cruise ship reported that a bypass pipe, a so-called “magic pipe”, had been used to pump oily waste into the sea off England. One discharge that month was about 4,227 gallons, 23 miles off the coast. He left the ship at Southampton. In 2016 the operator agreed to plead guilty to seven felonies and pay a US$40 million penalty, then the largest ever for deliberate vessel pollution. The court papers said the ship had been discharging through bypass equipment since 2005, a year after it entered service.8

None of this needed new technology to prevent. The core engine-room rule of MARPOL Annex I has applied since 1993 and was carried into the revised Annex I that entered into force on 1 January 2007: oily water goes overboard only if the ship is under way, the water has passed through approved equipment, and what leaves the ship holds no more than 15 parts per million (ppm) of oil. Everything else stays on board.132 This lesson sets out that rule, the equipment and tanks it relies on, and who checks it.

Try it

Overboard or keep it on board?

In plain words

Engines leak. Every ship collects a mix of water, fuel, lubricating oil and dirt in its engine-room bilges, and has to get rid of it somehow. MARPOL Annex I, the international rule on oil pollution from ships, allows only one route into the sea: through an approved oily water separator (the rule calls it “oil filtering equipment”), while the ship is moving, with no more than 15 ppm of oil in what comes out. As a rule of thumb, 15 ppm is about one teaspoon of oil in a bathtub of water.

The heavier waste, called sludge, comes mainly from cleaning fuel and lubricating oil in the purifiers. Sludge never goes into the sea. It is kept in its own tank and either pumped ashore, burned in an incinerator or boiler that is approved for it, or dealt with in another way listed on the ship’s certificate.

Three things make the system work: the equipment (separator, alarm, automatic stop, tanks and pipes), the record (the Oil Record Book, where every transfer and discharge is written down), and the surveys (the flag state or class society checks the equipment and issues the International Oil Pollution Prevention, or IOPP, Certificate). Port state control officers can check all three in any port.

In detail

Which ships

Annex I’s survey and Oil Record Book rules apply to every oil tanker of 150 GT and above and every other ship of 400 GT and above.34 The oil filtering equipment and sludge tank rules of regulations 12 and 14 apply from 400 GT for every ship, tankers included.15 Smaller ships are not exempt from the discharge ban: below 400 GT the ship either keeps all oily water on board or discharges it through equipment of an approved design at 15 ppm or less while en route (regulation 15.6).2

The equipment (regulation 14)

Ship size Equipment required
400 GT to under 10,000 GT Oil filtering equipment of an approved design that gives 15 ppm or less (regulation 14.6)
10,000 GT and above The same, plus an alarm when 15 ppm cannot be maintained and an automatic stop of the discharge above 15 ppm (regulation 14.7)
400 to 10,000 GT that discharge ballast water from fuel oil tanks Must meet regulation 14.7 as well (regulation 14.1 and 16.2)
Stationary ships (hotel ships, storage vessels) No filtering equipment needed, but a holding tank and all oily bilge water to shore (regulation 14.3)

The Administration may waive the equipment for ships trading only inside special areas, and for certain high-speed craft on short scheduled services, if they keep all oily bilge water on board for reception facilities and the IOPP Certificate is endorsed (regulation 14.5).1 In practice the approval standard for equipment installed since 1 January 2005 is resolution MEPC.107(49), which defines the “pollution prevention equipment” as three items: the 15 ppm bilge separator, the 15 ppm bilge alarm and the automatic stopping device.13 Lesson 2 takes that equipment apart.

The discharge rule (regulation 15)

Outside special areas, a ship of 400 GT and above may discharge oil or oily mixtures only when all of these are true (regulation 15.2):2

  1. the ship is proceeding en route;
  2. the mixture has passed through oil filtering equipment meeting regulation 14;
  3. the oil content of the effluent without dilution does not exceed 15 ppm;
  4. on an oil tanker, it does not come from cargo pump-room bilges; and
  5. on an oil tanker, it is not mixed with oil cargo residues.

Inside a special area the same five conditions apply, but the equipment must meet regulation 14.7, with alarm and automatic stop (regulation 15.3). The Annex I special areas include the Mediterranean, Baltic, Black Sea, Red Sea, Gulfs area, Gulf of Aden, Antarctic, North West European waters and the Oman area of the Arabian Sea (regulation 1.11), and Southern South African waters were added with effect from 1 August 2008.110 The stricter special-area discharge rules apply in an area only from the date IMO sets once its coastal states confirm reception facilities; no such date has yet been set for the Oman area.16 In the Antarctic area (south of 60 degrees S) no discharge of any kind is allowed (regulation 15.4). Two more sentences close the loopholes: no discharge may contain chemicals added to get round the conditions (15.8), and whatever cannot be discharged lawfully stays on board for reception facilities (15.9).2

Regulation 4 is the only way around regulation 15: a discharge to secure the ship’s safety or save life, or one resulting from damage where all reasonable precautions were taken and neither the owner nor the master acted with intent to cause damage, or recklessly and with knowledge that damage would probably result.12

Sludge and the tanks (regulations 12 and 13)

Every ship of 400 GT and above must have a sludge tank of adequate capacity for oil residues that cannot be dealt with otherwise, and its piping must have no direct connection overboard other than the standard discharge connection (regulation 12).5 For ships delivered on or after 1 January 2014, resolution MEPC.187(59) also bans discharge connections from the sludge tank to the bilge system, the oily bilge water holding tank, the tank top or the oily water separator. The only exception is a drain with a manually operated self-closing valve, and arrangements to check the settled water visually, leading to the bilge holding tank or a bilge well.1016 Both the bilge and sludge lines must end in the standard discharge connection of regulation 13, so that any shore hose fits.5

Surveys and the IOPP Certificate (regulation 6)

The IOPP Certificate is issued after an initial survey and renewed at intervals of not more than five years. An intermediate survey falls within three months either side of the second or third anniversary and specifically covers the oily-water separating equipment and oil filtering systems; annual surveys fall within three months either side of each anniversary.4 The Supplement to the IOPP Certificate lists the ship’s sludge and bilge tanks and the approved means of disposal. That list is what the Oil Record Book entries must match (lesson 3).

Records (regulation 17)

Every operation in the engine room that moves oil or oily water (bunkering, sludge collection and disposal, bilge transfers and discharges, ballasting of fuel tanks) is entered in the Oil Record Book Part I, and any failure of the oil filtering equipment must be recorded (regulation 17.5).3 Since 1 October 2020 the book may be an electronic record book approved by the Administration.7

What is coming

MEPC 84 (April to May 2026) approved a new regulation 12B for integrated bilge water treatment systems (IBTS), with matching changes to the IOPP Certificate (a new section 3.4) and a new Oil Record Book entry under code D for forced evaporation of oily bilge water. Adoption is planned for MEPC 85; the entry-into-force date will be fixed then.14

What the rules say

  • MARPOL Annex I regulation 14.6 requires the oil filtering equipment of ships of 400 GT and above to ensure that any oily mixture discharged into the sea has an oil content “not exceeding 15 parts per million”.1
  • Regulation 14.7 adds, for ships of 10,000 GT and above, an alarm when 15 ppm cannot be maintained and arrangements that automatically stop any discharge above 15 ppm.1
  • Regulation 15.2 prohibits discharges from ships of 400 GT and above unless the ship is proceeding en route, the mixture is processed through regulation 14 equipment, and the effluent “without dilution” does not exceed 15 ppm.2
  • Regulation 15.4 prohibits any discharge of oil or oily mixtures from any ship in the Antarctic area.2
  • Regulation 15.8 forbids discharges containing chemicals or other substances “introduced for the purpose of circumventing the conditions of discharge”.2
  • Resolution MEPC.381(80) made the special-area discharge rules of regulation 15.3 take effect in the Red Sea and Gulf of Aden special areas on 1 January 2025.6
  • Regulation 12 requires a sludge tank of adequate capacity, with no direct connection overboard other than the standard discharge connection.5
  • Regulation 6.1 requires an IOPP renewal survey at intervals not exceeding five years and an intermediate survey that confirms the oily-water separating equipment is “in good working order”.4
  • Regulation 17.1, as amended by resolution MEPC.314(74) from 1 October 2020, allows the Oil Record Book to be an electronic record book approved by the Administration.7

Flag and class notes

Flags restate Annex I in their own notices, and those notices are where flag-specific choices sit. Liberia’s Marine Notice POL-001 (Rev. 11/24), for example, requires the Oil Record Books to be the ones issued by the Administration or an electronic record book it has approved, keeps a public list of approved electronic record books, and lists the effective dates of recent amendments such as the Red Sea and Gulf of Aden special areas.10 Check the equivalent notice of your own flag, and the IOPP Supplement issued by your recognised organisation, which fixes the tank names and disposal routes your records must use.

In the United States, a knowing violation of MARPOL or the US Act to Prevent Pollution from Ships (APPS) is a class D felony, and the court may pay up to half of the fine to the person whose information led to the conviction.15 That reward is why so many US cases start with a crew member’s photographs (lesson 3).

Key numbers

What Value
Oil Record Book Part I and IOPP surveys required from 400 GT (oil tankers 150 GT)34
Oil filtering equipment and sludge tank required from 400 GT15
Alarm and automatic stop required from 10,000 GT (and in special areas)12
Maximum oil content of effluent, without dilution 15 ppm2
Discharges allowed in the Antarctic area None2
Red Sea and Gulf of Aden special-area rules in effect 1 January 20256
Standard discharge connection 215 mm outside diameter, 183 mm bolt circle, 6 bolts of 20 mm, 600 kPa5
IOPP Certificate renewal Not more than 5 years4
Annual and intermediate survey window 3 months either side of the anniversary date4
Oil Record Book kept on board 3 years after the last entry3

What inspectors find

  • In the Caribbean MoU’s campaign on MARPOL Annex I (September to November 2013, 182 ships), 7 ships had a 15 ppm alarm that was not correctly adjusted or operable and 5 had a three-way valve or stopping device that did not work. No ship was detained under the campaign, but the report notes that one ship with “many detainable deficiencies” could only be detained under other national environmental legislation.11
  • The same campaign found 18 ships whose incinerator suitable for burning oil residues was not marked in the IOPP Certificate (and 33 such auxiliary boilers), a mismatch between the certificate and the disposal routes actually on board.11
  • In the US case against the bulk carrier ASL Singapore (2025), a routine Coast Guard inspection found that the crew had been pumping oily bilge water overboard directly from the bilge holding tank with a portable pump and flexible hose since at least June 2023, while the Oil Record Book showed nothing of it.9
  • On the cruise ship in the Princess case (2016), engineers ran clean sea water through the overboard equipment while discharging through the bypass, to create “a false digital record” of a legal discharge.8

Questions people ask

Does the 15 ppm limit apply to what goes into the separator or to what comes out?

To what comes out. Regulation 15 limits the oil content of the effluent “without dilution”, measured after the separator. What goes in can be much dirtier, which is why the separator, the alarm and the stopping device must work together.

We are under 400 GT. Do we need a separator?

Not necessarily, but you still may not discharge oily water freely. Regulation 15.6 lets ships under 400 GT either keep oil and oily mixtures on board for reception facilities or discharge through approved equipment at 15 ppm or less while en route. Regulation 14.4 asks the flag to make sure such ships are equipped for this as far as practicable.

Can bilge water be moved into the sludge tank?

Yes, and MEPC.1/Circ.736/Rev.2 shows how to record it, but from then on the whole content of a sludge tank counts as sludge and must go ashore, to an incinerator or to another approved means. The only thing that may return to the bilge side is settled water drained off through the permitted self-closing drain to the bilge holding tank, which is recorded as a code C.12.2 entry. Lesson 3 covers the entries.

Check yourself

6 questions. Pass with 60% to mark the lesson as done.

  1. 1 A 30,000 GT bulk carrier is en route in the Mediterranean. Which arrangement allows it to discharge processed bilge water?
  2. 2 The separator outlet reads 22 ppm. The second engineer suggests opening a sea-water line into the overboard pipe to bring it down. What is correct?
  3. 3 Where must purifier sludge go?
  4. 4 Which ship needs an Oil Record Book Part I?
  5. 5 A ship floods after a collision and the master orders oily bilge water pumped over the side to keep her afloat. What applies?
  6. 6 At which survey does the class surveyor specifically confirm that the oily-water separating equipment is in good working order, apart from the renewal?

Sources

  1. 1 IMO Resolution MEPC.117(52), Amendments to the Annex of the Protocol of 1978 relating to MARPOL (revised Annex I), adopted 15 October 2004, in force 1 January 2007, regulations 1.11 and 14. Later amendments are in the IMO consolidated edition of MARPOL (paywalled). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  2. 2 MARPOL Annex I (revised by resolution MEPC.117(52)), regulation 15, Control of discharge of oil, paragraphs 1 to 9. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  3. 3 MARPOL Annex I (revised by resolution MEPC.117(52)), regulation 17, Oil Record Book Part I, paragraphs 1 to 7. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  4. 4 MARPOL Annex I (revised by resolution MEPC.117(52)), regulation 6, Surveys, paragraph 1. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  5. 5 MARPOL Annex I (revised by resolution MEPC.117(52)), regulation 12, Tanks for oil residues (sludge), and regulation 13, Standard discharge connection. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  6. 6 IMO Resolution MEPC.381(80), Establishment of the date on which regulations 15.3, 15.5 and 34.3 to 34.5 of MARPOL Annex I, in respect of the Red Sea and the Gulf of Aden special areas, shall take effect (adopted 7 July 2023), operative paragraph 1. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.381(80).pdf
  7. 7 IMO Resolution MEPC.314(74), Amendments to MARPOL Annexes I, II and V (Electronic Record Books), adopted 17 May 2019, in force 1 October 2020, annex paragraphs 1 to 3. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.314(74).pdf
  8. 8 US Department of Justice, Office of Public Affairs, Princess Cruise Lines to Pay Largest-Ever Criminal Penalty for Deliberate Vessel Pollution, 1 December 2016. https://www.justice.gov/archives/opa/pr/princess-cruise-lines-pay-largest-ever-criminal-penalty-deliberate-vessel-pollution
  9. 9 US Attorney’s Office, Eastern District of Louisiana, Companies That Own and Operate Bulk Carrier Guilty, Sentenced For Environmental Crimes (M/V ASL Singapore), 24 February 2025. https://www.justice.gov/usao-edla/pr/companies-own-and-operate-bulk-carrier-guilty-sentenced-environmental-crimes
  10. 10 Liberia Maritime Authority, Marine Notice POL-001 Rev. 11/24, Prevention of Pollution from Ships, paragraphs 1.1.5, 1.1.7, 1.1.15 and 2 (.7). https://www.liscr.com/sites/default/files/online_library/POL-001.pdf
  11. 11 Caribbean MOU, Report on the CIC on MARPOL Annex I carried out by the Caribbean MOU, 1 September to 30 November 2013, sections 2 and 3 (table 2). https://caribbeanmou.org/sites/default/files/MARPOL%20Annex%20I%20CIC%20Outcome%20Report.pdf
  12. 12 MARPOL Annex I (revised by resolution MEPC.117(52)), regulation 4, Exceptions. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.117(52).pdf
  13. 13 IMO Resolution MEPC.107(49), Revised guidelines and specifications for pollution prevention equipment for machinery space bilges of ships (adopted 18 July 2003), preamble (1992 discharge amendments in force 6 July 1993), operative paragraph 2(a) and annex section 3.1. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.107(49).pdf
  14. 14 ABS, Regulatory News: IMO MEPC 84 Brief, 1 May 2026, section Amendments to MARPOL Annex I and associated guidelines related to IBTS. https://ww2.eagle.org/content/dam/eagle/regulatory-news/2026/ABS-MEPC-84-Brief.pdf
  15. 15 United States Code, title 33, section 1908(a) (Act to Prevent Pollution from Ships, criminal penalties). https://www.law.cornell.edu/uscode/text/33/1908
  16. 16 IMO Resolution MEPC.187(59), Amendments to MARPOL Annex I (adopted 17 July 2009, in force 1 January 2011), annex 1, new regulation 12.2.2. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.187(59).pdf

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