By ShipCalculators.com · Published 14 June 2026

MARPOL Annex I Reg 34: cargo-area oil discharge

Regulation 34 of MARPOL Annex I, Chapter 4, controls the operational discharge of oil from the cargo area of an oil tanker. Outside a special area, a discharge is lawful only when six conditions hold at once: the tanker is not within a special area, is more than 50 nautical miles from the nearest land, is proceeding en route, holds the instantaneous rate of discharge of oil content at or below $30\ \text{L/nm}$ , keeps the total quantity discharged at or below $\tfrac{1}{15{,}000}$ of the cargo for a tanker delivered on or before 31 December 1979 (or $\tfrac{1}{30{,}000}$ for one delivered after that date), and has an oil discharge monitoring and control system (Regulation 31) and a slop tank arrangement (Regulation 29) in operation. It is the cargo-side counterpart to Regulation 15, which governs machinery-space bilge water.

Contents

Regulation 34 sits in Chapter 4 of MARPOL Annex I, the chapter that holds the construction, equipment, & operational rules specific to the cargo area of oil tankers. It answers a narrow question with a long list of conditions: when, if ever, may a loaded or part-loaded oil tanker put oily residue from its cargo tanks over the side. The short answer for most of the ocean is that it may not. The longer answer, the one the regulation actually writes, is a set of six cumulative conditions that have to be true at the same moment, plus an outright ban inside the areas the convention protects most.

The regulation is paired in practice with Regulation 15, which controls discharge of oil from the machinery space of any ship. The two read like siblings and are routinely confused on board, in port-state inspections, and in casualty files. They are not the same rule, they do not share the same numbers, and applying one in place of the other is a documented route to a detention or a prosecution. The distinction runs through this whole article.

What Regulation 34 controls and what it does not

Regulation 34 is part of Chapter 4 of MARPOL Annex I, the part of the MARPOL Convention that deals with prevention of pollution by oil. Chapter 4 applies to the cargo area of oil tankers: the cargo tanks, the slop tanks, the cargo and stripping pumps, & the piping that moves crude or product around the forward two-thirds of the hull. Regulation 34 is the operational discharge gate for that whole zone.

The substance the regulation controls is oil or oily mixtures originating in the cargo area. In service this is tank washings, the bottom residue that strips out of a tank after discharge, the water-and-oil emulsion that collects in a slop tank, and ballast that has been carried in cargo tanks on older ships not fitted with dedicated ballast tanks. It does not control the oily bilge water that drains to the engine-room bilge well from machinery leaks, fuel-oil purifier sludge water, or the boiler and economizer washings. That stream is the province of Regulation 15, and the wiki entry for Regulation 15 sets out its separate criteria.

The unit of measure is the oil tanker of 150 gross tonnage and above. Below that threshold the cargo-area discharge regime does not bite in the same way, and very small craft are generally required to retain on board. The 150 GT cutoff for tankers, against the 400 GT cutoff that governs the machinery-space rule for ships generally, is one more reason the two regulations are not interchangeable.

Where Regulation 34 sits in the revised Annex I

The numbering is recent. The revised MARPOL Annex I, adopted by resolution MEPC.117(52) on 15 October 2004 and in force from 1 January 2007, renumbered the whole annex and gathered the cargo-area rules into Chapter 4. The cargo-area operational discharge rule that practitioners had known for years as Regulation 9 of the old annex became Regulation 34 of the revised text. Older training material, casualty files, and some flag-state circulars still cite the pre-2007 numbers, so a reference to “Annex I Regulation 9 discharge” in a document from before 2007 is the same rule now found at Regulation 34. The substance, the 50 nautical miles, the 30 liters per nautical mile, the vintage ratio, and the special-area ban, carried across the renumbering essentially intact.

The 2007 reorganization did not invent the regime. The conditions trace to the original 1973 convention and the 1978 Protocol, the instrument that gave the world MARPOL 73/78 and that tightened the cargo-residue rules for the post-1979 fleet. The 1978 Protocol is why there are two divisors at all: it halved the permissible total discharge for new tankers and brought in the segregated-ballast and crude-oil-washing requirements that reshaped how tankers handle residue. Regulation 34 is the operational survivor of that history, restated in the 2004 renumbering.

A second placement point matters for combination carriers and for ships that carry oil in spaces other than dedicated cargo tanks. The consolidated text applies Regulations 16, 29, 30, 31, 32, 34, and 36 to the construction and operation of any space arranged to carry oil cargo on a tanker, with a relief for small aggregate capacities: where the total such capacity is under 1,000 cubic meters, the special-area provision of Regulation 34.6 can apply in place of the full outside-area discharge regime. This is the convention reaching the edge cases, the oil and bulk ore carriers and product tankers with marginal oil-carrying spaces, so the discharge gate cannot be sidestepped by carrying oil somewhere the cargo-area rules nominally do not name.

The six cumulative conditions outside special areas

Outside a special area, Regulation 34 permits a discharge of oil or oily mixtures from the cargo area only when every one of the following is satisfied. Miss one and the discharge is illegal, no matter how clean the rest of the picture looks.

The first condition is that the tanker is not within a special area. The special areas under Annex I are listed in the special areas regulation and include the Mediterranean, the Baltic, the Black Sea, the Red Sea, the Gulfs area, the Gulf of Aden, the Antarctic, & the North West European waters among others. Inside any of them, the cargo-side discharge gate is shut. That prohibition is Regulation 34.6 and it has its own section below.

The second condition is distance: the tanker is more than 50 nautical miles from the nearest land. This is a hard number. It is not the machinery-space formula of “as far as practicable” from the nearest land, and it is not the 12 nautical mile figure that appears in some older or related discharge regimes. Fifty miles, fixed, measured to the nearest land as the convention defines that term.

The third condition is that the tanker is proceeding en route. The vessel has to be underway and making a passage, not stopped, not drifting, not maneuvering on station to dump. The phrase is the convention’s way of forcing the discharge to be spread along a track rather than concentrated in one patch of sea, which is the whole point of capping the rate per mile rather than per hour.

The fourth condition is the instantaneous rate. The instantaneous rate of discharge of oil content must not exceed 30 liters per nautical mile. Regulation 1.9 defines the instantaneous rate of discharge of oil content as the rate of discharge of oil per hour at any instant divided by the speed of the ship in knots at the same instant. Putting the cap per mile rather than per hour ties the allowable oil flow to the ground the ship is covering, so a slow tanker is allowed less oil per hour than a fast one over the same stretch of water.

The fifth condition is the total-quantity ceiling, and this is where the vintage split lives. The total quantity of oil discharged into the sea must not exceed a fraction of the total quantity of the particular cargo of which the residue formed a part. For a tanker delivered on or before 31 December 1979, that fraction is $\tfrac{1}{15{,}000}$ . For a tanker delivered after 31 December 1979, it is $\tfrac{1}{30{,}000}$ . The next section pins down exactly which date and which definition map to each figure.

The sixth condition is equipment in operation: the tanker has an oil discharge monitoring and control system, as required by Regulation 31, and a slop tank arrangement, as required by Regulation 29, both running. Not merely fitted, not merely surveyed, but in operation during the discharge. This is the condition that converts the other five from a paper standard into something a machine measures and enforces in real time.

The 30 liters per nautical mile rate, stated as math

The instantaneous rate the oil discharge monitoring and control system computes and limits is, from Regulation 1.9:

R_{\text{inst}} = \frac{Q_{\text{oil}}}{V}

where $R_{\text{inst}}$ is the instantaneous rate of discharge of oil content in liters per nautical mile, $Q_{\text{oil}}$ is the rate of discharge of oil per hour in liters per hour at that instant, and $V$ is the speed of the ship over the ground in knots (nautical miles per hour) at the same instant. The regulatory limit is:

R_{\text{inst}} \le 30\ \text{L/nm}

The hour cancels: liters per hour divided by miles per hour leaves liters per mile. The monitoring system samples oil content in the effluent, multiplies by the discharge flow rate to get liters of oil per hour, divides by speed from the ship’s log, and compares the result against the 30 L/nm ceiling continuously.

The total-quantity ratio, stated as math

The cumulative cap over the whole discharge operation is:

Q_{\text{total}} \le \frac{1}{N}\, Q_{\text{cargo}}

where $Q_{\text{total}}$ is the total quantity of oil discharged into the sea during the operation, $Q_{\text{cargo}}$ is the total quantity of the particular cargo of which the residue formed a part, and $N$ is 15,000 for a tanker delivered on or before 31 December 1979 and 30,000 for a tanker delivered after that date. The instantaneous rate and the total quantity are independent gates: the discharge can pass the 30 L/nm test every second and still breach the cumulative ratio, at which point it must stop.

The vintage split: 1/15,000 versus 1/30,000

The two divisors are not interchangeable, and getting the wrong one onto a compliance calculation is a common error. The split keys off the delivery date of the tanker against the definitions in Regulation 1.28.

A tanker delivered on or before 31 December 1979 is an existing oil tanker in the sense of Regulation 1.28.1. For that vessel the total-quantity ceiling is the looser $\tfrac{1}{15{,}000}$ of the particular cargo. These are the ships built to the original 1973 convention standard, before the 1978 Protocol tightened the cargo-residue regime.

A tanker delivered after 31 December 1979 is a new oil tanker in the sense of Regulation 1.28.2. For that vessel the ceiling is the tighter $\tfrac{1}{30{,}000}$ , half the permissible total. The 1978 Protocol, which reworked Annex I and brought in segregated ballast and crude oil washing requirements for the new generation, halved the allowable cumulative discharge for ships ordered or built after the cutoff.

The delivery-date language matters because “delivered” is a defined event in Regulation 1.28, tied in turn to keel-laying and construction stages for ships where a simple delivery date is ambiguous. In day-to-day terms, almost every tanker trading today is a new oil tanker and runs on the $\tfrac{1}{30{,}000}$ figure; the $\tfrac{1}{15{,}000}$ tier survives mostly as a historical band for the handful of pre-1980 hulls still in any service and as a teaching point in the rule’s structure. Note one trap worth flagging: some secondary summaries online state the threshold as “31 December 2015,” which is wrong. The cutoff in Regulation 34 read with Regulation 1.28 is 31 December 1979, not 2015.

“Nearest land” and how the 50 nm rule differs from Regulation 15

The 50 nautical mile threshold is measured “from the nearest land,” and that phrase carries a precise convention meaning. Under Annex I, the nearest land means the baseline from which the territorial sea of the territory in question is established in accordance with international law, the same baseline used under the UNCLOS framework. It is not the visible coastline and not the low-water mark of the nearest beach: it is the legal baseline, which for an indented or island-fringed coast can sit well seaward of the mainland.

There is one notable exception built into the definition. Off the north-eastern coast of Australia, the convention draws “nearest land” as a line of stated latitude and longitude points running outside the Great Barrier Reef, so the protected reef waters count as land for discharge-distance purposes. The Australian Maritime Safety Authority publishes the coordinate line, and in places it lies as much as 150 nautical miles off the Queensland coast. A tanker that thinks it is 60 miles offshore there can in fact be inside the “nearest land” line and discharging illegally. The Great Barrier Reef is also a particularly sensitive sea area, so routeing and discharge restrictions stack on top of the baseline rule.

Now the contrast with the machinery-space regime, which is the point most likely to be muddled on board. Regulation 15 governs oily bilge water from machinery spaces. Its core operational test, for ships of 400 GT and above outside special areas, is a concentration standard: the oil content of the effluent must not exceed 15 parts per million, the ship must be proceeding en route, and the discharge must pass through oil filtering equipment meeting Regulation 14 with the 15 ppm alarm and automatic stopping device. That regime has no 50 nautical mile distance and no total-quantity ratio. The protection comes from cleaning the water to 15 ppm before it goes over the side, not from putting distance between the ship and the coast.

So the two rules protect the sea by different mechanisms. The cargo side, Regulation 34, allows a much dirtier effluent (30 liters of oil for every mile steamed is far above 15 ppm) but forces it 50 miles out, spreads it along a track, caps the cumulative total against cargo volume, and bans it entirely in special areas. The machinery side, Regulation 15, allows discharge much closer in but only after the oil content has been driven down to 15 ppm through filtering equipment. Reading a 15 ppm meter and concluding a cargo-tank slop discharge is legal, or reading the 30 L/nm cargo standard and applying it to bilge water, are both compliance failures with the regulator’s full attention.

Side by side: Regulation 34 against Regulation 15

The two operational discharge rules compared on their governing terms:

Term	Regulation 34 (cargo area)	Regulation 15 (machinery space)
Source of the oil	Cargo tanks, slop tanks, cargo piping	Engine-room bilge, fuel and lube oil leakage
Applies to	Oil tankers of 150 GT and above	Ships of 400 GT and above
Distance from nearest land	More than 50 nautical miles	None (no distance test in the core 15 ppm criterion)
En route	Required	Required
Concentration standard	None as such (rate-based)	15 ppm through Regulation 14 filtering equipment
Instantaneous rate	At or below 30 L/nm	Not applicable
Total-quantity ceiling	1/15,000 or 1/30,000 of the cargo	None
Monitoring	ODME, Regulation 31	15 ppm bilge alarm and stopping device, Regulation 14
Inside special areas	Prohibited (Regulation 34.6)	Stricter regime, generally 15 ppm with approved equipment
Logged in	Oil Record Book Part II, Regulation 36	Oil Record Book Part I

The table makes the core point visible: the cargo rule controls a relatively oily effluent by distance, rate, and cumulative quantity, while the machinery rule controls a relatively clean effluent by concentration. Reading across the wrong row is the recurring error the two regulations invite.

The special-area prohibition: Regulation 34.6

Inside a special area the calculus is simple: Regulation 34.6 prohibits any discharge of oil or oily mixtures from the cargo area of an oil tanker into the sea. There is no 50 mile carve-out inside a special area, no rate that makes it lawful, and no cumulative ratio to compute. The cargo-side gate is closed.

The Annex I special areas, set out in the special areas entry, are sea areas the convention recognizes as needing the strictest discharge control for oceanographic, ecological, or traffic reasons. Within them the only lawful destination for cargo-area slops is a reception facility ashore or retention on board until the tanker is clear of the area and back under the outside-area conditions of the previous sections.

The exceptions to the special-area prohibition are narrow and concern ballast that is genuinely free of significant oil. The prohibition does not bite on the discharge of clean ballast or segregated ballast. Segregated ballast is seawater carried in tanks completely separated from the cargo and fuel oil systems, so it never touches oil; the regulation for segregated ballast tanks is Regulation 18. Clean ballast is ballast from a cargo tank that has been cleaned to the point that effluent run through the monitoring system would not produce visible traces on a calm sea, or shows below 15 ppm on the meter. Neither of those is “oil from the cargo area” in the polluting sense, so neither is caught by the 34.6 ban.

A second narrow allowance covers a ship on a voyage only part of which lies in a special area. Nothing in Regulation 34 prohibits that ship from discharging, in the portion of the passage outside the special area, in accordance with the ordinary outside-area conditions. The ban follows the ship’s position, not the voyage as a whole: cross the boundary out of the special area, meet the six conditions, and the gate reopens.

Worked example: a tanker discharging cargo-tank washings en route

Take a product tanker delivered in 2008, so a new oil tanker on the $\tfrac{1}{30{,}000}$ tier. It has just discharged a cargo and is on a ballast passage in open ocean, 70 nautical miles from the nearest land, well clear of any special area, proceeding en route at 13 knots. The chief officer wants to discharge slop-tank contents through the oil discharge monitoring and control system. Two questions decide whether it is lawful: the instantaneous rate at every moment, and the cumulative total against the cargo quantity.

Start with the rate. Suppose the discharge pump is moving effluent at a measured flow such that the monitoring system reads an oil content giving 280 liters of oil per hour at one instant. The instantaneous rate of discharge of oil content is, from Regulation 1.9:

R_{\text{inst}} = \frac{Q_{\text{oil}}}{V} = \frac{280\ \text{L/h}}{13\ \text{kn}} = 21.5\ \text{L/nm}

That is below the 30 L/nm ceiling, so at that instant the discharge passes the rate test. Now push it: if the oil content climbs so the system reads 420 liters of oil per hour at the same 13 knots, the instantaneous rate is

R_{\text{inst}} = \frac{420\ \text{L/h}}{13\ \text{kn}} = 32.3\ \text{L/nm}

which exceeds 30 L/nm. The monitoring and control system stops the discharge automatically. The same 420 L/h would have passed if the ship were faster: at 15 knots it computes to 28.0 L/nm, under the limit, which is the per-mile logic doing its job. The rate gate is dynamic and speed-dependent.

Now the cumulative ceiling. Say the residue being discharged formed part of a particular cargo of 45,000 tonnes of gasoline. The total quantity of oil the tanker may discharge into the sea from that residue is

Q_{\text{total}} \le \frac{1}{30{,}000} \times 45{,}000\ \text{t} = 1.5\ \text{t}

so 1.5 tonnes of oil, roughly 1,800 to 2,000 liters depending on density, is the cumulative cap for the whole operation. The instantaneous rate can sit comfortably under 30 L/nm the entire time, yet once the running total of oil put over the side reaches 1.5 tonnes the discharge must stop, because the second gate has closed. Had this been an existing tanker delivered in 1977 on the $\tfrac{1}{15{,}000}$ tier, the ceiling would have been double, $\tfrac{1}{15{,}000} \times 45{,}000 = 3.0$ tonnes.

Pass or fail, then, is a two-test verdict. The discharge is lawful only while the position holds beyond 50 nm and outside any special area, the ship stays en route, the monitoring and control system reads at or under 30 L/nm at every instant, and the cumulative total stays under the vintage ratio. Trip any one and the operation is over.

The interlock with the cargo-area equipment regime

Regulation 34 does not stand alone. It is the operational rule that a whole chain of Chapter 4 equipment and procedure regulations exists to enforce, and the conditions of Regulation 34 reference that chain directly.

The oil discharge monitoring and control system is required by Regulation 31. In service it is the oil discharge monitoring equipment, the ODME. The system continuously measures the oil content of the effluent, takes the discharge flow rate and the ship’s speed, computes the instantaneous rate in liters per nautical mile and the running total in liters, records both, and operates an interlock that starts, records, and stops the discharge against the Regulation 34 limits. It must start recording when any discharge to the sea begins, keep a continuous record of oil content, instantaneous rate, total quantity, time, date, and ship position, and stop the discharge automatically when the instantaneous rate would exceed 30 L/nm or when the total would exceed the vintage ratio. The recorded printout is retained for at least three years and is a primary document a surveyor will ask for. A tanker whose ODME is inoperative cannot lawfully discharge cargo-area slops to sea at all, because the sixth Regulation 34 condition fails.

The slop tank arrangement is required by Regulation 29. A slop tank is a tank specifically designed for the collection of tank drainings, tank washings, and other oily mixtures. The slop tank is where the load-on-top method works: oily washings and dirty ballast are pumped into the slop tank, left to settle so the oil floats and clean water settles to the bottom, the clean water bottom is decanted off through the ODME under Regulation 34 conditions, and the remaining oil is kept on board to be loaded on top of the next cargo or pumped ashore. Load-on-top is the operational reason a tanker can clean its tanks without either polluting or filling up with un-disposable slops, and it depends on the slop tank arrangement Regulation 29 mandates. The related oil residue tanks of Regulation 12 serve the machinery-space sludge stream, a separate retention system, not the cargo slops.

Crude oil washing reduces the residue at source. Under Regulation 33, crude carriers of stated size must be fitted with a crude oil washing system, and Regulation 35 governs how it is operated. Instead of washing cargo tanks with seawater and creating an oily-water slop that then needs the Regulation 34 discharge gate, the tanks are washed with the cargo’s own crude during discharge, so the clingage goes ashore with the cargo and far less residue is left to deal with. The crude oil washing entry sets out the equipment side; the inert gas requirement that makes COW safe ties in through the cargo-tank atmosphere rules. COW does not replace Regulation 34, but a properly crude-oil-washed tanker generates a fraction of the slops a water-washed one does, so the cumulative ratio is rarely the binding constraint.

Segregated ballast removes a whole category of discharge from the Regulation 34 problem. Under Regulation 18, new tankers above stated sizes must carry ballast in segregated ballast tanks that never hold cargo or fuel. Ballast carried in dedicated tanks that never touch oil can be discharged anywhere, including inside special areas, because it is not oily. Before segregated ballast was mandatory, tankers ballasted in cargo tanks and the resulting dirty ballast was a major Regulation 34 discharge stream; the segregated ballast requirement engineered most of that stream out of existence for the modern fleet.

The discharge sequence in practice and load-on-top

Regulation 34 reads as a list of limits, but on a tanker it is a sequence of operations the officer runs in order. The sequence is worth setting out because it is where the abstract conditions become the buttons and valves an officer actually works.

After the cargo is out, the tanks that need cleaning are washed, either with crude during discharge under crude oil washing or with seawater on a clean-up or change-of-grade voyage. The washings, an oil-and-water emulsion, are pumped to the slop tank rather than overboard. The slop tank is then left to settle: the lighter oil rises and the heavier water sinks, and the longer the settling time the cleaner the separation. On a typical passage the slop is allowed to settle for a day or more before any decanting begins, which is one reason the en route condition exists, the ship is making the passage that buys the settling time.

When the master decides to decant, the officer confirms the position is outside any special area and beyond 50 nautical miles from the nearest land, confirms the ship is proceeding en route, lines up the slop-tank suction to the ODME, and starts the discharge of the settled water bottom. The ODME meters the oil content of the water leaving the tank. As the suction approaches the oil-water interface the oil content climbs, the computed instantaneous rate rises toward 30 L/nm, and the system stops the discharge before the interface is reached. What stays in the slop tank is the recovered oil, the “top.”

That recovered oil is then loaded on top of the next cargo, or pumped to a reception facility ashore, rather than discharged. This is the load-on-top method, and it is the reason a tanker can clean its tanks at sea without either polluting beyond the Regulation 34 limits or accumulating slops it has nowhere to put. Load-on-top depends on the slop tank arrangement of Regulation 29, the settling time the en route condition provides, and the ODME of Regulation 31 to find the interface and cut the discharge. A tanker that has crude-oil-washed its tanks arrives at this sequence with far less residue to settle and decant in the first place, which is the operational payoff of crude oil washing.

Two practical traps recur in this sequence. The first is the position fix at the start and end of the decant: the discharge can begin lawfully and then drift inside the 50 mile line or across a special-area boundary as the ship steams, and the regulation is breached from the moment the boundary is crossed even if the meter still reads under 30 L/nm. The officer has to watch the position, not only the meter. The second is the temptation to chase the last of the water bottom past the point where the ODME wants to stop, by overriding or spoofing the system; that is the cargo-side equivalent of defeating the oily water separator, and it is the act that turns a clean compliance record into a prosecution.

The Oil Record Book Part II and the paper trail

Every cargo-area oil operation that Regulation 34 touches has to be logged. The Oil Record Book Part II, governed by Regulation 36 and part of the wider Oil Record Book regime in Regulation 17, is the cargo and ballast operations record for oil tankers. Part I of the Oil Record Book covers machinery-space operations on all ships; Part II covers cargo and ballast operations on tankers. The two-part split mirrors the two-regulation split: Part I shadows Regulation 15, Part II shadows Regulation 34.

The Part II entries are itemized by operation: loading, internal transfer, unloading, ballasting of cargo tanks, cleaning of cargo tanks including crude oil washing, discharge of dirty ballast, discharge of water from slop tanks into the sea, disposal of residues, and discharge overboard of bilge water accumulated in the cargo area. For a Regulation 34 discharge to sea, the entry records the date, the ship’s position at start and end, the quantity discharged, the identity of the slop tank, the ODME record reference, & the signature of the officer in charge. Each completed operation is signed by the officer in charge and each page by the master.

The Oil Record Book is the document against which a Regulation 34 discharge is reconstructed after the fact. A port-state inspector cannot watch the discharge happen 70 miles offshore; the inspector reads the book, cross-checks it against the ODME printout, the slop-tank ullages, the cargo figures, and the engine and bridge logs, and looks for the discrepancy that says the recorded story and the physical evidence do not match. The book is where a false account of a discharge is committed to paper, which is why falsifying it is the offense that most enforcement actions actually charge.

Enforcement: the cross-check, port-state control, and the magic pipe

Enforcement of Regulation 34 turns on documentary cross-check more than on catching a discharge in the act. The instantaneous-rate and total-quantity limits are abstractions out at sea; what an inspector has is the Oil Record Book Part II, the ODME printout, and the physical state of the slop tanks and cargo tanks. The case is built on whether those agree.

Port-state control is the front line. A port-state control officer boarding a tanker checks the International Oil Pollution Prevention Certificate issued under Annex I, then tests the operational reality behind it: the ODME calibration and recording, the slop-tank arrangement, the Oil Record Book Part II entries, and whether the recorded discharges are consistent with the position fixes, the speeds, and the quantities. A slop-tank ullage that cannot be reconciled with the logged discharges, an ODME with a suspiciously clean or broken record, or a Part II entry that places a discharge inside a special area or under 50 miles is the thread an inspector pulls.

The cargo-side analogue to the machinery-space “magic pipe” is the unmonitored cargo or stripping line that routes slops overboard without passing through the ODME, or the deliberate manipulation of the ODME so it records a compliant discharge while a non-compliant one happens. On the machinery side the magic pipe is the bypass hose around the oily water separator; on the cargo side it is any arrangement that defeats the Regulation 31 monitoring and control system. The legal exposure is identical: the offense the regulator charges is usually not the discharge itself, which happened beyond any coastal jurisdiction, but the false Oil Record Book entry made or maintained while the ship was in port.

Worth walking through how the discrepancy surfaces, because it is the same shape in most cases. The Oil Record Book Part II records a slop-tank discharge to sea on a stated date at a stated position, of a stated quantity. The inspector takes the ship’s position fixes from the GPS log or the voyage data recorder for that date and checks whether the recorded discharge position was in fact beyond 50 miles and outside any special area. The inspector takes the ODME printout for that date and checks whether it shows a discharge at all, whether the instantaneous-rate trace stayed under 30 L/nm, and whether the recorded total matches the book. The inspector sounds the slop tanks and checks whether the present ullage is consistent with the discharges and retentions the book claims. When the four sources, book, position log, ODME record, and physical ullage, line up, the discharge was lawful and recorded honestly. When they do not, the gap is the case: a discharge the ODME never saw, a slop tank emptier than the logged operations explain, a position the GPS places inside the line. The physical evidence does not lie about volume the way a logbook can, so the reconstruction is built outward from the ullages and the position track.

That is the reach of the United States Act to Prevent Pollution from Ships, 33 U.S.C. 1901 and following, which gives MARPOL Annex I domestic effect. APPS enforcement, run by the Coast Guard and the Department of Justice, has for two decades prosecuted foreign-flagged ships not for discharges on the high seas but for presenting a false Oil Record Book to the Coast Guard in a U.S. port, an offense that occurs inside U.S. jurisdiction. Cases under 33 U.S.C. 1908 and the false-statement and obstruction statutes have produced multi-hundred-thousand-dollar fines, probation with court-supervised environmental compliance plans, & seafarer convictions, with a whistleblower-reward provision that pays crew who report the bypass and the falsified book. The lesson the case law teaches is consistent: the discharge happens at sea, but the conviction happens in port, on the strength of the document Regulation 36 requires the ship to keep honestly.

Limitations

This article states the operational discharge regime of Regulation 34 as it stands in the consolidated Annex I, but it is not the certificate, the survey, or the legal text. Several boundaries are worth stating plainly.

The figures here are the convention minima. A flag state, a port state, a coastal state, or a regional instrument can impose stricter limits, and many do. Special-area boundaries and entry-into-effect dates change as the IMO designates new areas; the list in force on any given voyage must be checked against the current Annex I, not against a static summary.

The vintage split between $\tfrac{1}{15{,}000}$ and $\tfrac{1}{30{,}000}$ turns on the delivery date as defined in Regulation 1.28, which itself depends on keel-laying and construction-stage criteria for borderline cases. The right divisor for a specific hull is a documentary question answered from the ship’s particulars and the IOPP certificate, not from a launch-year guess.

The “nearest land” definition is the territorial-sea baseline, with the Great Barrier Reef line as a defined exception, and other coastal states may publish their own baselines that a navigator must consult. A position that is 50 miles from the visible coast may be inside the legal nearest-land line, and the worked example assumes an open-ocean position where the baseline and the coastline roughly coincide.

The worked example uses round numbers to show the two-gate logic and is not a substitute for the ship’s own ODME, which performs the real continuous computation against the actual oil content, flow, and speed. Densities, cargo grades, and meter response all affect the conversion between liters and tonnes that the example glosses.

Finally, this entry covers operational discharge. The accidental-outflow, double-hull, and damage-stability regime that governs spills from collision or grounding lives in separate Annex I regulations and is outside the scope of Regulation 34. The cargo-side and machinery-side operational rules sit alongside the structural prevention rules, and a tanker’s full Annex I compliance picture is the combination of all of them, read through the IOPP certificate and verified at survey.

Frequently asked questions

How far from land can a tanker discharge cargo residue under Regulation 34?

More than 50 nautical miles from the nearest land, and only outside a special area. Inside a special area the discharge of oil from the cargo area is prohibited under Regulation 34.6. The 50 nm figure is fixed: it is not a 'practicable' distance like the machinery-space rule but a hard threshold the oil discharge monitoring and control system enforces.

What is the 30 liters per nautical mile limit in Regulation 34?

It is the cap on the instantaneous rate of discharge of oil content. Regulation 1.9 defines that rate as the oil discharged per hour divided by the ship's speed in knots at the same instant. The oil discharge monitoring and control system computes it continuously and stops the discharge if it would exceed 30 liters for every mile the tanker steams.

Is the total quantity limit 1/15,000 or 1/30,000 of the cargo?

Both, depending on vintage. A tanker delivered on or before 31 December 1979 (an 'existing oil tanker' under Regulation 1.28.1) may discharge a total of no more than 1/15,000 of the particular cargo. A tanker delivered after 31 December 1979 (a 'new oil tanker' under Regulation 1.28.2) is held to the tighter 1/30,000.

How does Regulation 34 differ from Regulation 15?

Regulation 34 governs oily residue from the cargo area of an oil tanker. Regulation 15 governs oily bilge water from the machinery space of any ship. The cargo-side rule carries a 50 nm distance, a 30 liters per nautical mile rate, and a total-quantity ratio; the machinery-side rule is a flat 15 ppm concentration standard with no distance or quantity ceiling.

What equipment must be running for a legal Regulation 34 discharge?

The tanker must have in operation an oil discharge monitoring and control system (Regulation 31) and a slop tank arrangement (Regulation 29). Without both running, no overboard discharge of cargo residue is lawful regardless of position or rate.