MARPOL Annex I Reg 29 & 30: slop tanks, piping

An oil tanker generates oily waste on every voyage even when nothing goes wrong. Tank washing produces a slurry of seawater and clingage. Ballast carried in a cargo tank picks up oil from the tank bottom and structure. Stripping the cargo pumps and the cargo lines after discharge leaves a few cubic meters of oil-rich liquid with nowhere obvious to go. MARPOL Annex I solves this with a closed set of regulations that work as one mechanism. Regulation 29 provides the tank that holds and settles the waste. Regulation 30 provides the piping and the discharge points that move it. Regulation 32 supplies the instrument that finds the oil/water boundary inside the slop tank, Regulation 31 supplies the monitor that controls what crosses the ship’s side, and Regulation 34 sets the numerical limits that any discharge must meet. Read alone, each regulation looks like a hardware specification. Read together, they are the legal and physical basis of the load-on-top method that has kept operational oil out of the sea since the 1970s.

This article treats Regulations 29 and 30 at the level a tanker officer, a port-state-control inspector, or a SIRE vetting inspector needs. It states the slop-tank capacity tiers and the conditions that trigger each one, walks the slop-tank design & the load-on-top cycle, sets out the pumping and piping rules including the small-diameter line and the part-flow arrangement, and names every interlocking regulation so the whole discharge chain is visible. It sits inside the wider MARPOL Annex I regime, itself one annex of the MARPOL Convention.

Regulation 29: the slop-tank requirement

Regulation 29 opens with a threshold and a default. Every oil tanker of 150 gross tonnage and above must be provided with slop-tank arrangements in accordance with the requirements of paragraphs 2.1 to 2.3 of the regulation, subject to the small-tanker relief in Regulation 3.4. For tankers delivered on or before 31 December 1979, any cargo tank may serve as a slop tank; newer tankers are expected to have dedicated arrangements. The 150 GT floor matters because it sets the dividing line between a tanker that must hold & process its residues internally and a small vessel that can rely on shore reception. It is the cargo-area counterpart to the 400 GT machinery-space threshold in Regulation 15.

The function of the slop tank is to receive, settle, and hold. Paragraph 2.1 requires adequate means to clean the cargo tanks and to transfer the dirty ballast residue and the tank washings from the cargo tanks into a slop tank approved by the Administration. Paragraph 2.2 requires the arrangement to be such that any effluent finally discharged into the sea will comply with Regulation 34, the cargo-area discharge limits. The slop tank is not a holding tank in the casual sense. It is a settling vessel sized and shaped to separate oil from water so that the water can leave legally and the oil can stay.

Aggregate slop-tank capacity: 3% and its reductions

The capacity rule is the part of Regulation 29 most often misquoted, so it pays to state it exactly. Under paragraph 2.3, the total capacity of the slop tank or combination of slop tanks shall not be less than 3% of the oil carrying capacity of the ship. In symbols, the baseline requirement is

where $V_{\text{slop}}$ is the aggregate slop-tank volume and $V_{\text{oil}}$ is the ship’s oil carrying capacity. The 3% baseline assumes a tanker with no special residue-reducing equipment, so the slop tank must absorb the full burden of tank washings and dirty ballast. The regulation then grants three reductions, each tied to a documented reason the residue volume is smaller.

The first reduction, paragraph 2.3.1, drops the floor to 2% where the tank-washing arrangements are such that, once the slop tank or tanks are charged with washing water, this water is sufficient for tank washing and, where applicable, for providing the driving fluid for eductors, without the introduction of additional water into the system. This is the recirculation case. If a ship washes with its own retained slop water rather than fresh seawater, it adds no new water to the system, so the slop tank does not have to hold the extra volume that fresh wash water would generate.

The second reduction, paragraph 2.3.2, also drops the floor to 2% where segregated ballast tanks or dedicated clean ballast tanks are provided in accordance with Regulation 18, or where a crude oil washing system is fitted in accordance with Regulation 33. Both of those systems attack the residue at source. Segregated ballast means a tanker never has to ballast a cargo tank, so it never generates oily ballast in the first place. Crude oil washing dissolves the clingage off the tank structure during discharge using the cargo itself, so far less oil remains to be washed out with water afterward. Where the ship also meets the no-added-water washing condition of paragraph 2.3.1, this reduction goes further, to 1.5%.

The third reduction, paragraph 2.3.3, drops the floor to 1% for combination carriers where oil cargo is only carried in tanks with smooth walls. A smooth-walled tank, free of the internal stiffening of a typical cargo tank, holds far less clingage, so the residue volume after discharge is small. Again the no-added-water washing condition stacks: it reduces the floor to 0.8%. The full tier structure is best read as a table.

Two points stop officers tripping on this. First, the percentages are a floor, not a target: a ship may carry more, and many do, because slop capacity is operationally convenient. Second, the reduction is conditional on the equipment actually being fitted and used, and a port-state inspector can ask for the evidence. A tanker certificated at 2% on the strength of crude oil washing that is not, on the day, operable has lost the basis for its reduced capacity.

Two slop tanks for the largest crude carriers

Paragraph 3 of Regulation 29 adds a count requirement on top of the volume requirement. Oil tankers of 70,000 tonnes deadweight and above delivered after 31 December 1979 must be provided with at least two slop tanks. The reason is operational redundancy in the settling process. A very large crude carrier produces a large volume of slop, and a single tank cannot both receive fresh washings and hold a quiet, settled column at the same time. Two tanks let the crew decant a settled tank while still receiving into the other, so the load-on-top cycle never stalls for want of a quiet column. This is one of several Annex I thresholds that sort tankers by size class, in this case carving out the VLCC and ULCC band.

Slop-tank design: settling, decanting, and the interface

Paragraph 2.4 governs the internal design. Slop tanks must be designed, particularly in respect of the position of inlets, outlets, baffles, or weirs where fitted, so as to avoid excessive turbulence and entrainment of oil or emulsion with the water. The physics behind this sentence is the whole point of the tank. Oil and water separate by density: oil rises, water sinks, & a sharp interface forms in between if the column is left undisturbed. Turbulence destroys that interface, emulsifying oil into the water so that no decant can produce clean effluent. So the inlets are arranged to fill the tank without a violent jet, and weirs or baffles let the crew draw water from low in the tank without pulling down the oil layer above. A well-designed slop tank is a settling tank first and a storage tank second.

Finding the interface is not done by eye. The slop tank carries an oil/water interface detector approved under Regulation 32. That instrument, lowered into the tank or fixed at sounding points, reads the boundary between the settled oil layer on top and the cleaned water below. The crew uses it to confirm there is enough clear water under the oil to decant, and, during the decant, to stop before the falling interface reaches the suction. If the oil layer is, for example, 1.5 m thick in a tank of 12 m sounding, the crew know roughly 10 m of water column is available and can decant most of it while watching the interface descend. The interface detector is the link between the tank’s settling function and the discharge that the monitoring system controls. The brief on the slop tank also addresses static electricity: the regulation framework requires arrangements that prevent the excessive accumulation of static electricity, because decanting and washing move charged hydrocarbon liquid through the tank and a spark in an unpurged ullage space is a fire and explosion hazard. Slop tanks on modern tankers sit inside the inert gas envelope for the same reason.

Load-on-top built on the slop tank

The slop tank is the hardware; load-on-top is the method that runs on it. Load-on-top is the sequence by which a tanker keeps its oil residues and discharges only cleaned water, and it is the operational reason the slop tank exists. The cycle runs in four moves.

First, collect. After cargo discharge, the crew washes the cargo tanks and strips the dirty ballast and tank washings into the slop tank, consolidating the voyage’s oily waste in one settling column. Second, settle. The slop tank is left undisturbed, often for many hours, so the oil rises and a sharp interface forms above the cleaned water. The longer and quieter the settle, the cleaner the water below. Third, decant. Once the interface detector confirms a clean water column, the crew decants that water overboard through the oil discharge monitoring and control system, watching the oil content and stopping the discharge the moment the monitor signals the limit or the interface approaches the suction. Fourth, retain and reload. The oil layer left in the slop tank stays on board, and the next cargo is loaded on top of that residue, so the oil is never landed as waste but is delivered with the next parcel.

The method’s elegance is that it turns a waste-disposal problem into a cargo-accounting problem. The oil that would otherwise be pumped to the sea or to a shore reception facility instead becomes the bottom few centimeters of the next cargo. The whole cycle is only legal because the decant step passes through the monitoring and limit framework of Regulations 31 and 34, and it is only practical because the slop tank of Regulation 29 gives the oil a place to separate and wait.

Where load-on-top and the slop tank came from

The slop tank is a fix for a problem the early tanker trade created and ignored for decades. A tanker that ballasted its cargo tanks and water-washed them at sea produced thousands of tons of oily slops per voyage, and before any international limit existed, that slurry went over the side. The first attempt to control it was the 1954 International Convention for the Prevention of Pollution of the Sea by Oil, OILPOL, which prohibited oil discharges within zones extending 50 miles from most coasts and further off sensitive areas. OILPOL set a zone rule, not a quantity-per-cargo rule, and outside the zones the discharge was effectively unrestricted.

Industry, not regulators, developed load-on-top during the 1960s as a way to recover the oil rather than lose it. Operators found that pumping the washings and dirty ballast into a single tank, letting them separate on the long sea passage, decanting the clean water, and loading the next cargo on top of the retained oil recovered cargo that had been thrown away, several hundred tons on a large tanker. The 1962 and 1969 amendments to OILPOL tightened the regime around this practice; the 1969 amendments moved from a pure zone rule toward limits on the rate, concentration, & total quantity discharged, the conceptual ancestor of the 30 liters per nautical mile and 1/15,000 figures now in Regulation 34.

The 1967 grounding of the Torrey Canyon, which released on the order of 119,000 tonnes of crude off the southwest of England, exposed how little international law then governed tanker operation. MARPOL 73/78 was the response: the 1973 Convention was reshaped by the 1978 Protocol from the International Conference on Tanker Safety and Pollution Prevention, and Annex I entered into force in 1983. The 1978 Protocol made crude oil washing mandatory for new crude tankers of 20,000 tonnes deadweight and above under Regulation 33, and pushed segregated ballast onto new tankers under Regulation 18. The slop-tank capacity tiers of Regulation 29 are the residue of that history: the 3% baseline reflects the water-washing tanker, and each reduction reflects a technology, COW, segregated ballast, smooth-walled combination tanks, that the post-Torrey-Canyon reforms brought in to shrink the slop in the first place.

Regulation 30: pumping, piping, and discharge arrangements

Regulation 29 holds and separates; Regulation 30 moves and discharges. It governs how cargo-area liquid reaches the ship’s side, where it may cross that side, and the controls on that crossing. The recurring theme is that every overboard route is engineered to be observable and, in the default case, above the waterline where a film of escaping oil is visible rather than hidden under the hull.

The manifold on the open deck, both sides

Paragraph 1 places the discharge manifold for connection to reception facilities on the open deck on both sides of the ship. Both sides matters operationally: a tanker discharges to a shore reception facility on whichever side the terminal lies, and the manifold has to reach either. The open-deck placement matters for inspection: a manifold and its valves on deck can be seen, sealed, & checked, where a buried connection cannot. This is the same manifold a SIRE tanker inspection and a port-state-control officer head for when verifying that overboard valves are shut and sealed.

Discharge above the waterline, and the named exceptions

Paragraph 2 sets the default for the ballast and oil-contaminated water lines. In every oil tanker of 150 GT and above, pipelines to sea for the discharge of ballast water or oil-contaminated water from cargo tank areas must be led to the open deck or to the ship’s side above the waterline in the deepest ballast condition. The deepest ballast condition is the worst case for waterline height, so a line that is above water then is above water always in normal operation. The principle is visibility: oil leaving above the waterline produces a sheen an observer can see, where a below-waterline discharge can dump oil unseen under the hull.

Paragraph 6 then carves out the exceptions where below-waterline discharge is allowed, each tied to a check that the water is clean. Paragraph 6.1 permits segregated ballast and clean ballast to be discharged below the waterline in port or at sea by gravity, provided the surface of the ballast water has been examined to ensure that no contamination with oil has taken place. Paragraph 6.4 permits dirty or oil-contaminated ballast to be discharged below the waterline by gravity after the oil and water have separated in the slop tank, but only after the oil/water interface has been checked with the Regulation 32 interface detector to confirm the interface height keeps oil away from the discharge. The protective-location logic of Regulation 18 feeds in here through paragraph 6.3, which references the protected ballast arrangements of older tankers. The exceptions do not weaken the rule; they extend it to cases where a different check, surface examination or interface confirmation, provides the same assurance that no oil leaves the ship.

The means to stop the discharge

Paragraph 3 requires the discharge to be stoppable from the right place. For oil tankers delivered after 31 December 1979, the means for stopping the discharge of effluent into the sea must be located in a position from which the manifold in use and the discharge of the effluent to the sea by the lines referred to may be visually observed. The intent is that whoever controls the discharge can see what they are controlling. The regulation accepts an alternative: the stopping position need not have line of sight if a positive communication system, such as a telephone or radio system, links the observation point to the control point. In practice the control point is the cargo control room, and the operator decanting from the slop tank watches the overboard discharge, the oil/water interface reading, and the oil discharge monitoring and control system readout together, so the monitor’s alarm and the interface trend both reach the hand on the stop control.

Oil retention, draining, and the small-diameter MARPOL line

Paragraph 4 attacks the oil left in the pipework. For oil tankers delivered after 1 June 1982, the piping must be designed so that oil retention in the lines is minimized, and means must be provided to drain all cargo pumps and all oil lines on completion of cargo discharge, where necessary by connection to a stripping device. The draining must be capable of being discharged both ashore and to a cargo or slop tank. The reason is simple: the few cubic meters of oil standing in the cargo lines after a discharge is oil that should reach the shore tank, not be flushed to the slop tank or, worse, lost overboard at the next ballast.

To land that last residue, paragraph 4 requires a special small-diameter line connected to the manifold valves outboard of the manifold. This is the MARPOL line. Its small bore lets the stripping pump push the final, low-flow residue ashore at a velocity that keeps the oil moving, where the large main cargo line would let it stall and drain back. Unified Interpretation 57 fixes the bore: for tankers delivered after 1 June 1982, the cross-sectional area of the small-diameter line must not exceed 10% of the cross-sectional area of the main cargo discharge line; for earlier tankers the limit is 25%. Connecting the MARPOL line outboard of the manifold valve is deliberate. It lets the crew strip ashore through the small line with the main manifold valve shut, so cargo cannot run back into the ship’s lines during the operation. Officers using the line keep that manifold valve closed for exactly that reason.

The part-flow arrangement and the monitoring tie-in

The part-flow arrangement is the sampling system that feeds the oil content meter. The oil discharge monitoring and control system of Regulation 31 has to measure the oil content of the water leaving the ship’s side, but it cannot put a meter in the full overboard stream of a large pump. So a part-flow system draws a small, representative portion of the discharge off the overboard line, passes it through the meter, and returns it, giving the monitor a continuous reading of oil content in parts per million while the bulk flow goes to sea. The reading drives the calculation the monitor performs: instantaneous rate of discharge of oil in liters per nautical mile, computed from the oil content, the flow rate, & the ship’s speed. When that rate, or the running total, reaches the Regulation 34 limit, the monitor commands the discharge to stop. The part-flow arrangement is therefore the physical bridge between the piping of Regulation 30 and the control logic of Regulation 31: it is how the law’s numbers get applied to a real overboard stream.

Sea chests and isolation

Paragraph 7 closes a leakage path on newer tankers. For oil tankers of 150 GT and above delivered on or after 1 January 2010, the sea chest serving the cargo-area discharge must have both a sea-chest valve and an inboard isolation valve, plus a means of positive isolation to prevent, under all circumstances, the section of line between the sea-chest valve and the inboard valve from being filled with cargo. The provision answers a real failure mode: a single valve passing slightly can let cargo seep into the sea chest and then to the sea, undetected, between discharges. Two valves and a positive isolation, often a spectacle blank or a removable spool, make that seepage path closeable and inspectable.

How Regulations 29, 30, 31, 32, and 34 interlock

The cargo-side discharge regime is a chain, and a single broken link makes a legal discharge impossible. It helps to walk the chain in the order the liquid moves.

Regulation 29 is the start: it gives the ship a slop tank that can receive the washings and dirty ballast and settle them into an oil layer over a water column. Regulation 32 supplies the instrument that reads the boundary between those two, so the crew know how much clean water is available and when to stop. Regulation 30 supplies the piping that takes the settled water to the ship’s side, the discharge points that keep that route above the waterline and observable, and the means to stop the flow from a position that sees the overboard outlet and the monitor. Regulation 31 supplies the oil discharge monitoring and control system, built to the revised specification in Resolution MEPC.108(49), that measures the oil content of the effluent through the part-flow arrangement and automatically stops the discharge when a limit is reached. Regulation 34 supplies those limits: for a discharge from the cargo area outside a special area, the tanker must be en route, more than 50 nautical miles from the nearest land, discharging at an instantaneous rate not exceeding 30 liters per nautical mile, with the total quantity discharged not exceeding 1/15,000 of the previous cargo for a tanker delivered on or before 31 December 1979 or 1/30,000 for a tanker delivered after that date, and with the monitoring system and the slop-tank arrangement of Regulations 29 and 31 in operation.

Two design programs lighten the load on this chain. Segregated ballast tanks under Regulation 18 stop the tanker generating oily ballast at all, because ballast never enters a cargo tank, which is why their presence earns the 2% slop-capacity reduction. Crude oil washing under Regulation 33 strips the clingage off the tank structure with the cargo itself during discharge, so the residue left to wash out with water afterward is far smaller, which is the second route to the 2% reduction. A modern crude tanker with both fitted runs a small, clean slop budget compared with a 1970s tanker that ballasted its cargo tanks and water-washed everything. The slop tank is still there, but it works less hard.

The oily residues generated by the machinery spaces, the sludge and bilge side, run on a parallel track through the oil residue (sludge) tanks of Regulation 12 and the oil filtering equipment of Regulation 14, governed on discharge by Regulation 15. A tanker therefore runs two separate residue systems: the cargo-area chain of Regulations 29, 30, 31, 32, and 34, and the machinery-space chain of Regulations 12, 14, and 15. Keeping them straight matters during an inspection, because the limits, the equipment, & the record entries differ between the two.

The monitoring system that closes the loop

Regulation 30 builds the piping; Regulation 31 builds the brain that decides what may pass through it. The oil discharge monitoring and control system, the ODMCS, is the automatic governor on every cargo-area discharge, and the part-flow arrangement of Regulation 30 is its sensing tap. Its specification was reset by Resolution MEPC.108(49), adopted 18 July 2003, which revised the earlier guidelines and specifications and is the standard a modern tanker’s monitor is type-approved against. The system continuously records and controls: it logs the oil content of the effluent in parts per million, the discharge rate in liters per nautical mile, the total quantity discharged, the time, the ship’s speed, & the ship’s position, and it commands an automatic stop when a Regulation 34 limit is reached.

The two figures the monitor enforces are worth stating in the form it computes them. The instantaneous rate limit is 30 liters of oil per nautical mile, so for a ship making speed $v$ in knots the permitted oil throughput per hour is $30v$ liters. The total-quantity limit caps the whole discharge against the previous cargo: not more than $1/15{,}000$ of that cargo for a tanker delivered on or before 31 December 1979, or $1/30{,}000$ for a tanker delivered after that date. The monitor integrates the instantaneous discharge against both limits in real time. When either is hit, the overboard valve closes or the discharge pump trips, depending on the system’s control philosophy, and the event is recorded. This is why the means-to-stop position of Regulation 30 puts the operator where they can see the monitor readout, the interface reading, & the overboard discharge together: the automation backstops the operator, but the operator is meant to be watching the same data the automation acts on.

MEPC.108(49) also contemplates a manually operated alternative for decanting when the monitor fails at sea, but it frames that as an abnormal condition that needs deliberate operator judgment, not a routine mode. A tanker that decants manually because its monitor is defective is running on the crew’s gauging and interface readings alone, with no automatic backstop, and the records of such a voyage attract close attention at the next inspection. The integrity of the whole cargo-side discharge regime rests on the monitor working and being seen to work, which is why the part-flow sampling, the recording function, and the automatic stop are specified together rather than left to the operator’s discretion.

Operational practice and port-state control

The regulations describe equipment; the daily reality is procedure, gauging, and records. The points below are where Regulations 29 and 30 meet the deck.

Slop-tank gauging is the routine that keeps load-on-top honest. The crew sounds the slop tank for total liquid and reads the oil/water interface with the Regulation 32 detector, recording both before and after each decant. The two readings together give the oil layer thickness and the clean water depth available, and the trend across a settle shows whether separation is complete. A common error is decanting too early, before the interface has fully formed, which sends emulsified oil to the monitor and triggers an automatic stop, or worse, a discharge over the limit if the monitor is in a degraded mode.

A short worked decant shows how the regulations bite in practice. Take an Aframax with an oil carrying capacity near 90,000 cubic meters. Fitted with crude oil washing and segregated ballast, it certifies its slop tanks at the 2% floor, so $V_{\text{slop}} \ge 0.02 \times 90{,}000 = 1{,}800$ cubic meters of aggregate slop capacity. Suppose after a discharge the consolidated slop column stands at 1,400 cubic meters and, after a 24-hour settle, the interface detector puts the oil layer at the top 180 cubic meters with about 1,220 cubic meters of clean water below. The crew can decant most of that water column. With the ship en route at 13 knots, the Regulation 34 instantaneous limit of 30 liters of oil per nautical mile means the monitor allows up to $30 \times 13 = 390$ liters of oil per hour to pass while the bulk water goes to sea, and it cuts the discharge automatically if the oil content climbs as the falling interface nears the suction. The crew stops the decant with a margin of clean water still under the oil, leaving the 180 cubic meters of recovered oil to ride out as the bottom of the next cargo. The numbers are illustrative, but the structure, capacity floor, settle, interface-bounded decant, rate limit, & retained oil, is exactly how a real operation runs.

Interface detection failure is treated seriously precisely because the whole decant depends on it. If the interface detector is unreliable, the crew cannot prove where the oil sits, and a conservative operator stops decanting rather than risk an over-limit discharge. The same logic applies to the monitor: Regulation 31 contemplates a manually operated alternative for decanting in the event of a monitor failure, but that is an abnormal condition requiring deliberate care, not a routine fallback, and an inspector will look hard at the records of any voyage run that way.

Sealing the manifold and sea valves is the most visible port-state-control check on the cargo-area discharge arrangement. In port, the overboard discharge valves and the manifold valves not in use are shut and seal-wired, and the seal numbers are logged. A broken or missing seal on an overboard valve is a finding because it is the physical evidence that the ship cannot discharge to the sea while alongside. The two-valve sea-chest isolation of paragraph 7 on newer tankers is checked the same way. PSC officers under the Paris MOU and Tokyo MOU routinely inspect these points, and the IOPP certificate and its Form B supplement record the slop and discharge arrangements the ship is certified to have.

Recording every operation in the Oil Record Book is the legal trail behind the hardware. Cargo and ballast operations on an oil tanker are recorded in the Oil Record Book Part II, the cargo and ballast counterpart to the Part I machinery-space book. Every loading, internal transfer, discharge of cargo, ballasting and deballasting of cargo tanks, cleaning of cargo tanks, discharge of dirty ballast, discharge of water from slop tanks, and disposal of residues is entered with quantities, tank identities, times, and positions, and the Oil Record Book is one of the first documents a port-state inspector examines. A decant entry that does not match the monitor printout, or a slop quantity that cannot be accounted for across a voyage, is the thread an inspector pulls. The discipline of accurate, contemporaneous entries is what turns the slop tank and the MARPOL line from hardware into provable compliance.

Limitations

This article states the structure of Regulations 29 and 30 as they apply to a typical oil tanker, but several boundaries deserve a practitioner’s caution.

The regulations distinguish tankers by delivery date, and the date bands carry real differences. The two-slop-tank rule, the small-diameter-line bore limit, the means-to-stop provision, the oil-retention requirement, and the sea-chest isolation each attach to a specific delivery-date threshold (31 December 1979, 1 June 1982, or 1 January 2010). An older tanker may legitimately lack a feature a newer one must have, and a compliance check has to start from the ship’s keel-laying and delivery dates, not from the consolidated text read flat. This article gives the bands but does not trace every grandfathering provision for pre-MARPOL tonnage.

The percentage tiers in Regulation 29 are conditional, and certification is only as good as the equipment’s working state. A 2% or 1.5% slop capacity rests on crude oil washing or segregated ballast actually being fitted and operable; if the equipment is defective on the day, the reduced capacity loses its basis, and the consequence is a regulatory and operational problem the article describes but does not resolve for any specific ship. The figures here are the regulatory floors, not advice on sizing a particular tanker, which is a class and Administration matter.

The Antarctic and special-area regime layers further restrictions on top of Regulations 29 and 30 that this article only touches. Any discharge of oil or oily mixture from the cargo area is prohibited inside a special area, so the load-on-top decant described here is a non-special-area operation, and a tanker operating in or transiting a special area must retain everything on board. The interaction of the slop-tank capacity with full retention in the Antarctic, where the slop tanks must be sized to hold all residues until a port outside the area, is governed by additional provisions not detailed here.

Finally, this is reference prose, not a substitute for the certificated procedures. The authoritative text is the consolidated MARPOL Annex I and its Unified Interpretations as adopted by the IMO, read with the ship’s IOPP certificate, its approved procedures & arrangements, and the flag Administration’s instructions. Where this article and a ship’s approved documentation differ, the approved documentation and the regulation text govern.

See also

• MARPOL Annex I: the full oil-pollution-prevention annex these regulations sit within
• MARPOL Annex I Reg 18: segregated ballast tanks: the design that earns the slop-capacity reduction
• MARPOL Annex I Reg 33: crude oil washing: the wash method that cuts the residue burden
• MARPOL Annex I Reg 15: discharge control: the machinery-space discharge counterpart
• MARPOL Annex I Reg 12: oil residue (sludge) tanks: the parallel machinery-space residue system
• MARPOL Annex I Reg 14: oil filtering equipment: the 15 ppm separator side
• MARPOL Annex I special areas: where cargo-area discharge is prohibited
• Oil Record Book: where slop and discharge operations are logged
• International Oil Pollution Prevention certificate: the certificate recording the arrangements
• Port state control: the inspection regime that verifies them