Background
The history of tank cleaning is closely tied to the evolution of cargo regulations and environmental protection. The original MARPOL Convention of 1973 introduced the first comprehensive international rules for cargo residue management on tankers, with subsequent amendments and the supporting MEPC resolutions establishing the detailed operational framework. The introduction of crude oil washing on VLCCs in the 1970s revolutionised tanker operations by reducing tank washing costs while improving cargo outturn. Modern tank cleaning combines decades of accumulated operational experience with advanced equipment including programmable washing machines, automated discharge management, and continuous environmental monitoring. Understanding tank cleaning requires knowledge of both the engineering equipment used and the layered regulatory framework under MARPOL, IMO performance standards, ISGOTT and IBC Code provisions, and various port state requirements.
Regulatory Framework
The international regulatory framework for tank cleaning combines MARPOL Annex I (oil) and Annex II (noxious liquid substances), supplemented by IMO performance standards, IBC Code, ISGOTT, and class society rules.
MARPOL Annex I (Oil Pollution) covers tank washing operations on oil tankers, with detailed provisions for:
- Cargo tank washings discharge requirements
- Slops tanks and their use
- Crude oil washing requirements and benefits
- Pumproom and machinery space drainage discharge
- Reception facility requirements
MARPOL Annex II (Noxious Liquid Substances) covers chemical tanker prewash and washing operations, with provisions for:
- Cargo categorisation (X, Y, Z categories based on environmental hazard)
- Mandatory prewash for certain cargoes
- Discharge requirements for prewash water
- Tank cleaning standards and verification
IMO Resolution A.446(XI) (Standards for Tankwashing Procedures) and supporting resolutions provide detailed guidance on tank cleaning methods, equipment, and operational practices.
IBC Code (International Bulk Chemical Code) covers chemical tanker construction, equipment, and cargo handling including tank cleaning provisions.
OIL/HNS Convention provisions and various supporting protocols establish liability frameworks for cargo residue spills.
ISGOTT (International Safety Guide for Oil Tankers and Terminals) provides industry guidance on tanker operations including tank cleaning, with detailed safety procedures.
Class society rules implement these regulations through certification of cleaning equipment, slop tank arrangements, and operational procedures.
National regulations may impose additional requirements for ships in specific waters. For example, the US Clean Water Act imposes strict tank cleaning effluent requirements in US waters.
Cargo Tank Cleaning Methods
Several methods are used to clean cargo tanks, with selection depending on previous cargo, planned next cargo, and operational requirements.
Sea water washing uses cold or warm sea water to flush tanks of cargo residues. Sea water is cheap, readily available, and effective for many cargoes. The water is typically discharged through an oily water separator or to a slop tank for shore disposal.
Hot water washing uses heated water for improved cleaning efficiency. Steam-heated water at 60-80°C is significantly more effective than cold water for cargoes with viscosity-sensitive residues.
Steam washing uses steam injection into the tank for cleaning. Steam directly heats and dissolves residues. Steam washing is energy intensive and creates substantial vapour load, but is highly effective for sticky cargoes.
Crude oil washing (COW) uses crude oil itself, recirculated from cargo tanks, as the cleaning medium. COW is unique to crude oil tankers and is covered in detail below.
Chemical tank cleaning uses specific cleaning chemicals selected for the previous cargo. Detergents, solvents, alkaline cleaners, and acid cleaners are used depending on the residue characteristics. Chemical cleaning is essential for chemical tankers preparing for sensitive cargoes.
Inert gas atmosphere cleaning maintains inert tank atmosphere during cleaning operations to prevent fire and explosion. Inert gas systems are mandatory for tankers and provide essential safety during cleaning.
Manual cleaning by crew entering tanks is the final stage on chemical tankers preparing for very sensitive cargoes (foodstuffs, pharmaceutical intermediates). Manual cleaning includes detail wiping, deposit removal, and final inspection. Manual cleaning requires gas-free tank atmosphere and detailed safety procedures.
Tank Cleaning Machines
Tank cleaning machines (washing machines) are the principal equipment for mechanised tank cleaning. Several types are used.
Single-rotation tank cleaning machines have nozzles that rotate during operation, with the rotation rate controlled by an internal mechanism (typically a turbine driven by the cleaning fluid). The single-rotation machine sweeps a single spherical pattern.
Multi-rotation tank cleaning machines combine vertical and horizontal rotation, allowing the spray pattern to cover the entire tank surface during a single cleaning cycle. Modern multi-rotation machines complete coverage in 1 to 8 hours depending on tank size and machine capacity.
Programmable tank cleaning machines (PTCM, also called “auto wash” machines) provide controlled rotation patterns with programmed sequences, ensuring complete tank coverage. Modern PTCMs typically achieve 90-95% surface coverage with proper programming.
Common manufacturers include:
- Scanjet (Denmark): leader in tanker cleaning machines
- Salwico
- Butterworth (USA)
- Toftejorg
- Various others
Machine selection depends on:
- Tank size (larger tanks need higher capacity machines)
- Tank geometry (tanks with internal structures need more flexible coverage)
- Cargo type (some cargoes require specific machine arrangements)
- Operational profile
Capacity is measured in cubic metres per hour. Typical machines deliver:
- Small tanks: 30-60 m³/hr
- Large crude tanks: 100-300 m³/hr
- Multiple machines per tank for very large tanks
Machine pressure is typically 8-15 bar. Higher pressure provides better cleaning of stubborn residues but requires more pumping capacity.
Hot water capability allows steam-heated washing with better cleaning efficiency.
Crude Oil Washing (COW)
Crude oil washing (COW) is a specific tank cleaning method unique to crude oil tankers, using cargo oil itself as the cleaning agent.
COW principle: hot crude oil from cargo tanks is recirculated through tank cleaning machines, where it impacts and dissolves cargo residues from tank walls. The dissolved residues are then pumped out with the cargo, effectively reducing tank residues to very low levels.
COW advantages over conventional water washing:
- Eliminates water-based slops that require disposal
- Reduces oil content in slops by 70-80%
- Reduces tank cleaning water demand
- Improves cargo outturn (more cargo recovered)
- Saves time during discharge operations
- Required by MARPOL Annex I for VLCCs and ULCCs
COW disadvantages:
- Limited to crude oil cargoes
- Requires specific tanker configuration
- Operational complexity
- Cargo flammability requires careful management
COW operation typically occurs:
- During discharge: during cargo discharge, with COW continuing as discharge progresses
- After discharge: continuing COW with reduced cargo to clean residues
COW machines are typically programmable washing machines installed in each cargo tank. Multiple machines per tank ensure complete coverage.
COW pump arrangement supplies the cleaning oil at appropriate pressure. Cargo pumps with COW capability are typical, with tank cleaning pressure of 8-12 bar.
COW programs are detailed in the ship’s COW Operations Manual, with tank-by-tank procedures, time schedules, and verification methods.
COW efficiency typically achieves:
- Pre-COW residues: 100-200 m³ per cargo tank
- Post-COW residues: 5-15 m³ per cargo tank
- Reduction: 90-95%
MARPOL Annex I requirement for COW on VLCCs and ULCCs derives from the OIL/HNS Convention provisions on cargo residue management.
Slop Tanks
Slop tanks store residue oil and water from cargo operations and tank cleaning, allowing controlled handling rather than overboard discharge.
Slop tank configurations on tankers:
- Two slop tanks (port and starboard): allows continuous use during operations
- Sometimes additional dedicated slop tanks for water-emulsion separation
Slop tank capacity is typically 5-10% of total cargo capacity, providing substantial buffer for cleaning operations.
Slop tank construction is typically integral steel tanks similar to cargo tanks, with appropriate coatings and internal arrangements.
Slop tank heating may be provided to assist water-oil separation.
Slop tank operation includes:
- Receiving cleaning water and recovered cargo
- Allowing gravity separation of water and oil
- Pumping clean water overboard (through OWS)
- Retaining oil in slop tank for shore disposal
Discharge to shore reception facilities is the appropriate method for slop tank emptying, with reception facilities at most major tanker terminals.
ORB documentation per MARPOL Annex I records all slop tank operations including transfers, discharges, and shore reception.
Chemical Tanker Tank Cleaning
Chemical tankers face particularly demanding tank cleaning requirements due to the wide variety of cargoes handled and strict purity requirements for next cargoes.
Pre-loading prewash per MARPOL Annex II is required for ships unloading certain Category X and Y noxious liquid substance cargoes. The prewash must use specific volumes of water, with the wash water requiring shore disposal.
Prewash water volumes per Annex II range from 0.075 to 1.0 cubic metres per cubic metre of tank capacity, depending on the cargo category and tank emptying performance.
Stripping requirements ensure cargo tanks are emptied below specified residue thresholds before next cargo loading. Pumping and stripping capability requirements are part of the tank cleaning equipment specifications.
Tank cleaning chemicals selected for the previous cargo:
- Caustic soda for acidic cargoes
- Sulphuric acid for alkaline cargoes
- Various detergents for organic cargoes
- Steam cleaning for residual sticky cargoes
- Hot water washing for many polymer/glycol cargoes
Cleaning verification through wall washing tests, tank atmosphere testing, and visual inspection ensures the tank is suitable for the next cargo.
Wall washing test uses a small volume of water flushed against tank walls, with the recovered water tested for residue presence. The test indicates how clean the tank surface is.
Tank atmosphere testing for hydrocarbons and other vapours ensures safe atmosphere for personnel entry.
Compatibility verification confirms the tank cleaning operations have prepared the tank appropriately for the next cargo, with documentation of all procedures and test results.
Tank Atmosphere Management
Tank atmosphere management during cleaning is critical to safety and operational success.
Inert atmosphere maintenance prevents flammable atmosphere development during tank cleaning. Inert gas system operation continues throughout cleaning, with atmosphere monitoring at multiple tank locations.
Atmosphere transition stages during cleaning:
- Inert: oxygen below 8%, before cleaning
- Pre-cleaning: oxygen verification before starting wash
- During cleaning: continuous atmosphere monitoring
- Post-cleaning: atmosphere verification before tank entry
Gas freeing for tank entry follows controlled procedure with multiple atmosphere checks. Personnel entry is only permitted after verified safe atmosphere.
Hydrocarbon vapour management during cleaning may include:
- Vapour return to shore facilities (where available)
- Vapour treatment systems aboard ship
- Atmospheric venting through pressure-vacuum valves (where permitted)
Static electricity prevention through proper grounding, controlled flow rates during washing, and use of conductive equipment prevents ignition during operations.
Hot work restrictions prevent any hot work (welding, cutting) during tank cleaning operations due to combustible atmosphere risks.
Tank Cleaning Operations
Tank cleaning operations follow detailed procedures matched to the tank type, cargo history, and planned next cargo.
Pre-cleaning preparation includes:
- Atmosphere verification (inert status)
- Ship’s position verification (within permitted areas)
- Tank cleaning machine preparation
- Cleaning fluid availability verification
- Slop tank capacity verification
Cleaning sequence typically follows tank-by-tank procedure:
- Initial clean water rinse
- Detergent or cleaning agent application
- Hot water rinse (where applicable)
- Final clean water rinse
- Verification testing
Atmosphere monitoring throughout cleaning ensures safe conditions:
- Continuous oxygen monitoring (cargo tanks)
- Hydrocarbon monitoring (if needed)
- Hydrogen sulphide monitoring (specific to crude oil)
- Personnel safety verification
Discharge management includes:
- OWS discharge for clean water (after separation)
- Slop tank for retained oil residues
- Shore reception for full slop tank contents
- Documentation of all flows
Post-cleaning verification confirms the tank is clean enough for next cargo:
- Wall washing test (chemical tankers)
- Visual inspection (where safe to enter)
- Atmosphere testing
- Documentation
Specific Applications
Different tanker types have characteristic tank cleaning operations.
VLCC and ULCC crude oil tankers:
- Use crude oil washing (COW) routinely
- Slop tank disposal in port through reception facilities
- Bunker, ballast, and crude oil tank cleaning at major dry-dockings
Suezmax and Aframax crude oil tankers:
- COW practices similar to VLCC
- More frequent voyage-to-voyage tank cleaning
- May use combination of COW and water washing
Product tankers (carrying refined petroleum products):
- Use water washing typically
- Different products may require different cleaning between voyages
- Cleaning agents matched to product residues
Chemical tankers (small, medium, large parcel):
- Substantial cleaning operations between cargoes
- Wide variety of cleaning chemicals and procedures
- Detailed operational manuals for cleaning specific cargoes
- Verification testing for each cargo change
Combination carriers (OBO ships):
- Particularly extensive cleaning between liquid and solid cargoes
- Full holding tank cleaning between voyages
- Major cleaning operations supported by shore reception facilities
LNG and LPG carriers:
- Tank cleaning using cold-out and warm-up procedures
- No traditional washing (cargo is gas at ambient temperature)
- Atmosphere management with nitrogen during cargo changes
Maintenance and Inspection
Tank cleaning system maintenance combines daily attention, periodic preventive maintenance, and major overhauls.
Daily attention includes:
- Cleaning machine inspection before use
- Pump and piping system verification
- Slop tank level monitoring
- Atmosphere monitoring system status
Weekly maintenance includes:
- Cleaning machine performance verification
- Pump strainer cleaning
- Slop tank water-oil separation verification
Monthly comprehensive maintenance includes:
- Cleaning machine internal inspection
- Pump performance testing
- Slop tank atmosphere testing
- Operational procedures review
Quarterly and annual maintenance includes:
- Cleaning machine major overhauls (typical 3-5 year cycles)
- Pump rebuilds (where indicated)
- Heater performance verification
- System testing under various conditions
5-year major surveys involve comprehensive inspection during dry-docking. Slop tank internal inspection, cleaning machine replacement (where significantly worn), pump rebuilds, and re-certification of all systems.
Cleaning machine performance verification through field testing periodically confirms proper operation. Coverage tests with photo verification or other methods ensure intended coverage is achieved.
Future Developments
Tank cleaning continues to evolve in response to environmental regulations, cargo diversity, and operational efficiency drivers.
Stricter discharge regulations continue to tighten allowable cargo residue discharge to sea. The trend is toward zero or minimum discharge with shore reception for nearly all tank wash water.
Smart cleaning systems with continuous monitoring of cleaning effectiveness, predictive analytics, and automated documentation provide better visibility and reduced manual oversight.
Reduced cleaning chemical use through better selection, optimised application, and recovery/reuse of cleaning chemicals.
Membrane separation technology for slop tank water/oil separation provides higher quality effluent than traditional gravity separation.
Crude oil washing efficiency improvements through better machine design, optimised procedures, and electronic control reduce cleaning time and improve outturn.
Closed-loop cleaning systems where tank cleaning fluid is recovered, treated, and reused on the same vessel reduce both environmental impact and operational cost.
Conclusion
Marine tank cleaning and crude oil washing operations are essential procedures that prepare cargo tanks for new cargoes while complying with environmental regulations and ensuring crew safety. The combination of properly designed cleaning machines, controlled cleaning procedures, slop tank management, atmosphere monitoring, and disciplined operational practice produces the safe efficient cleaning that tanker operations require. Crew members responsible for these operations must understand the regulatory framework (particularly MARPOL Annex I and II), engineering equipment, operational procedures, and verification requirements that together ensure compliance and safety. As the maritime industry decarbonises and environmental regulations continue tightening, tank cleaning operations are evolving toward better technology, lower environmental impact, and more comprehensive monitoring, but the fundamental tasks, cleaning cargo tanks safely between voyages, remain a constant focus of tanker operations.
Related Calculators
- Cargo COW Efficiency Calculator
- Cargo COW Tank Cycle Calculator
- Cargo Tank Cleaning Time Calculator
- Cargo Tank Cleaning Water Volume Calculator
- Tanker Crude COW Time Calculator
- Chemtank Prewash MEPC Calculator
- Chem Prewash Check Calculator
- Class LR COW Calculator
Additional calculators:
Additional formula references:
- Tanker Crude Oil Washing Cycle
- Tanker Tank Cleaning Crude Wash
- Tanker Crude Oil Washing Bottom Wash
- Tanker Crude Oil Washing Top Wash
Additional related wiki articles:
Related Wiki Articles
- Marine Inert Gas Systems
- Marine Cargo Pumps and Piping
- Marine Oily Water Separators and Bilge Water Treatment
- Oil Tanker
- Chemical Tanker
- IBC Code
References
- MARPOL Annex I - Regulations for the Prevention of Pollution by Oil
- MARPOL Annex II - Regulations for the Control of Pollution by Noxious Liquid Substances
- IMO Resolution A.446(XI) - Standards for Tankwashing Procedures
- ISGOTT (International Safety Guide for Oil Tankers and Terminals) 6th Edition
- IBC Code (International Bulk Chemical Code)