Slop Tank: What It Is and How It Works on Oil Tankers

A slop tank is a cargo-area tank on an oil tanker that collects tank washings, drainings and other oily mixtures. Oil and water separate in it by gravity. This guide explains how slop tanks work, where they sit, and what MARPOL Annex I requires, for ship’s officers, surveyors and maritime students.

What Is a Slop Tank?

MARPOL Annex I Regulation 1 defines it as “a tank specifically designated for the collection of tank drainings, tank washings and other oily mixtures”. It holds them until the water is decanted and the oil disposed of.

The mixture itself is called slops. Without a slop tank, every tank cleaning and every dirty ballast discharge would put oil straight into the sea. The slop tank is the reason a tanker can clean its cargo tanks at sea and still comply with MARPOL Annex I.

What Goes Into a Slop Tank?

Regulation 29 requires a tanker to be able to transfer dirty ballast residue and tank washings from the cargo tanks into an approved slop tank. In practice the slop tank receives:

• Tank washings from water washing of cargo tanks
• Residues stripped from cargo tanks and lines after discharge or crude oil washing
• Oily water from dirty ballast carried in cargo tanks
• Drainings from cargo pumps, lines and the cargo system

What Is the Difference Between a Slop Tank and a Sludge Tank?

A slop tank holds cargo-area oily mixtures on a tanker and is recorded in Oil Record Book Part II. A sludge tank sits in the engine room of every ship and holds oil residues from fuel and lube oil treatment. Its entries go in Oil Record Book Part I.

Sludge comes mainly from the fuel oil purifiers and is either landed ashore or burned in a waste oil incinerator. Slops are handled in the cargo system instead: the water is decanted and the oil goes ashore or stays on board under load on top.

Where Is the Slop Tank Located on a Ship?

On most oil tankers the slop tanks are the aftermost cargo tanks, one port and one starboard. They sit next to the pump room or engine room bulkhead, inside the double hull.

The aft position suits the trim most tankers keep during discharge and stripping, so drainings run aft to the tank. Regulation 29 requires at least two slop tanks on tankers of 70,000 tonnes deadweight and above delivered after 31 December 1979. Some older tankers may designate an ordinary cargo tank as the slop tank.

The slop tanks appear on the ship’s capacity plan and in the Supplement to the IOPP Certificate. Check both when you join a ship: the designated slop tanks are the ones the Oil Record Book entries must refer to.

What Does MARPOL Require for Slop Tanks?

MARPOL Annex I Regulation 29 requires slop tank arrangements on every oil tanker of 150 gross tonnage and above. Minimum capacity is 3% of oil carrying capacity, with lower figures for specific designs.

The capacity rules in Regulation 29 paragraph 2.3 are:

Ship arrangement	Minimum slop tank capacity	Reduced capacity allowed
Standard oil tanker	3% of oil carrying capacity	2% if the slops, once filled, supply all tank washing and eductor water without adding more water
Tanker with segregated ballast tanks, dedicated clean ballast tanks or crude oil washing	2%	1.5% under the same water condition
Combination carrier carrying oil only in smooth-walled tanks	1%	0.8% under the same water condition

Regulation 29.2.4 adds a design rule. Inlets, outlets, baffles or weirs must be arranged to avoid excessive turbulence and entrainment of oil or emulsion with the water.

Two further regulations equip the slop tank for discharge:

• Regulation 31 requires an oil discharge monitoring and control system (ODME) for any discharge of oily water from the cargo area.
• Regulation 32 requires approved oil/water interface detectors to measure the boundary between oil and water in the slop tanks.
• IMO resolution MEPC.5(XIII), adopted in 1980, sets the interface detector specification: accuracy within 25 mm of a sharply defined interface.

What Are the US Slop Tank Requirements?

US-flag and US-trading tank vessels follow 33 CFR 157.15, which mirrors MARPOL closely. A new vessel below 70,000 DWT needs at least one slop tank, and one of 70,000 DWT or more needs at least two.

Capacity is 3% of oil carrying capacity, or 2% for vessels with qualifying segregated ballast tanks or no eductor arrangement using extra water. Slop tanks must have separate inlet and outlet connections to limit turbulence and emulsions, and may carry bulk oil when not used as slop tanks.

How Does a Slop Tank Work?

A slop tank works by gravity separation: oil floats to the top and water settles to the bottom. The interface detector finds the boundary, and the water is decanted through the ODME while the oil stays on board.

Most tankers run two slop tanks in series, with washings going first into the primary or “dirty” slop tank. Water from its bottom passes through a decanting line or weir to the secondary or “clean” slop tank. That second settling stage gives cleaner water at the overboard discharge.

Separation is not instant. Crude oils can form stable water-in-oil emulsions that slow it down, so settling time is planned into the tank cleaning programme. Slop tanks are commonly fitted with heating coils to speed separation.

What Is Load on Top?

Load on top (LOT) means keeping the separated oil in the slop tank and loading the next crude cargo on top of it. It avoids discharging the oil into the sea and was the basis of the original MARPOL slop tank rules.

1. Tank washings and dirty ballast residues are transferred to the slop tank.
2. The mixture settles during the ballast voyage.
3. The interface is measured and the water underneath is decanted through the ODME.
4. The remaining oil is either discharged to a shore reception facility or kept, and the next cargo is loaded on top.
5. Every transfer, decant and quantity retained is entered in Oil Record Book Part II.

When Can Slop Tank Water Be Discharged at Sea?

MARPOL Annex I Regulation 34 allows a tanker to discharge oily water from the cargo area only when all six conditions are met:

• The tanker is not within a special area.
• It is more than 50 nautical miles from the nearest land.
• It is proceeding en route.
• The instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile.
• Total oil discharged does not exceed 1/30,000 of the cargo the residue came from (1/15,000 for tankers delivered on or before 31 December 1979).
• The ODME and slop tank arrangement required by Regulations 29 and 31 are in operation.

Inside a special area, any discharge of oil or oily mixture from the cargo area is prohibited. Only clean or segregated ballast may go overboard there.

How Are Slops Disposed of Ashore?

Slops that cannot be decanted or retained go to a port reception facility, either ashore or to a slop barge alongside. MARPOL requires governments to provide adequate reception facilities in loading terminals, repair ports and other ports that receive oily residues.

The ship records the delivery in Oil Record Book Part II and keeps the reception facility receipt. A gap between the slop tank ullage and the quantity on the receipt is one of the first things an inspector checks.

Do Chemical Tankers Have Slop Tanks?

Yes, but their washings follow different rules. Residues of noxious liquid substances fall under MARPOL Annex II, which requires a prewash discharged to a reception facility for some cargo categories. The types of chemical tankers page explains how those ships are arranged.

How Is a Slop Tank Built?

A slop tank is built like the cargo tanks around it, with the same steel, coatings and inert gas connection. Its internal arrangement differs: inlets, outlets and weirs are placed to limit turbulence.

• Coating: oil tanker slop tanks are normally coated steel; chemical tankers may use stainless steel slop tanks.
• Inlets and outlets: separate connections, placed to avoid stirring up the settled layers.
• Weir or decanting line: moves bottom water from the primary to the secondary slop tank.
• Interface detector access: a gauging point for the portable or fixed oil/water interface detector.
• Level gauging and high-level alarm: the same overfill protection as the cargo tanks.
• Venting and inerting: connection to the inert gas system and to the cargo tank P/V valves.

Slop tanks see more water, heating and agitation than the cargo tanks around them. That makes their coatings and heating coils a frequent corrosion point at survey.

What Are the Safety Hazards of Slop Tanks?

Slop tanks hold hydrocarbon-water mixtures that release flammable gas, so they carry the same fire and explosion risk as cargo tanks. Tank washing adds static electricity risk, and slops from sour crude can release toxic hydrogen sulphide (H₂S).

• Flammable atmosphere: the slop tank stays inerted with the rest of the cargo tanks, with oxygen content kept within the inert gas system limits.
• Static electricity: washing machines and water spray can generate static charges inside tanks being cleaned or filled with washings.
• Toxic gas: H₂S and benzene vapour can be present, so gas readings are taken before any work near open slop tank fittings.
• Tank entry: entering a slop tank follows the enclosed space entry procedure, with gas-freeing, ventilation and a permit.

Air emissions are handled at the ship and terminal level, not by the slop tank itself. Crude oil tankers must carry a VOC Management Plan under MARPOL Annex VI Regulation 15 to control vapour release during loading and voyage.

What Do Inspectors Check on Slop Tanks?

Inspectors check that slop tank records match reality. PSC compares Oil Record Book Part II with tank ullages and ODME records. Surveyors test the ODME and interface detector; vetting inspectors review decanting procedures.

Port State Control. PSC officers check the IOPP Certificate and its Supplement, which records slop tank capacity. They read Oil Record Book Part II for the slop-related codes and compare the entries with tank ullages, ODME printouts and reception facility receipts.

• Code G: cleaning of cargo tanks, including washings sent to the slop tank
• Code H: discharge of dirty ballast, including oily water transferred to the slop tank
• Code I: discharge of slop tanks into the sea
• Code J: collection, transfer and disposal of residues and oily mixtures not otherwise dealt with
• Code M: condition of the oil discharge monitoring and control system

Insufficient remaining slop tank capacity for the intended voyage is listed among the MARPOL grounds for ship detention.

Class and IOPP surveys. At annual, intermediate and renewal surveys, the surveyor confirms the slop tank arrangement matches the IOPP Supplement and tests the ODME and interface detector. Tank internals, coatings and heating coils are examined when the tanks are gas-free.

Vetting. A vetting inspection looks at how the crew runs the system. Inspectors review the load on top and decanting procedures in the ISM Code safety management system and check that officers can operate the ODME.

Findings to prepare for include:

• Oil Record Book Part II quantities that don’t match slop tank ullages
• Missing or inconsistent ODME printouts for a recorded discharge
• ODME failures not recorded under Code M
• An interface detector that is faulty, missing or never checked
• Slop tanks too full for the planned tank cleaning or ballast voyage

Related Topics

• Oil Tanker Types and Classification
• Cargo Handling Systems and Equipment on Oil Tankers
• Bitumen Tanker: How Ships Carry Hot Asphalt Cargo
• VLCC: Very Large Crude Carriers

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