Marine safety service
Tank Cleaning Services
Cargo, ballast, fuel and slop tank cleaning, gas freeing and residue removal by specialist crews, with atmosphere testing and waste disposal records for the port.
Overview
Ship tank cleaning and gas freeing services
Ship tank cleaning services remove cargo residues, sludge, sediment and scale from cargo tanks, fuel and lube oil tanks, ballast tanks, slop tanks and fresh water tanks. Cleaning is needed before a change of cargo grade, before class or vetting inspections, before hot work or repairs, when converting fuel types, and when sediment in ballast or fuel tanks starts to affect operations. It almost always involves gas freeing and entry into enclosed spaces, which makes planning and supervision as important as the cleaning itself.
vslSafety arranges specialist tank cleaning crews, gas-freeing equipment, waste collection and, where required, independent certification at the vessel’s port or anchorage, working around cargo operations and port restrictions.
Regulatory requirements and standards
Enclosed space entry is the main safety risk. SOLAS regulation XI-1/7, in force since 1 July 2016, requires ships to carry portable atmosphere testing instruments capable of measuring at least oxygen, flammable gases or vapours, hydrogen sulphide and carbon monoxide. In June 2025 IMO adopted revised Recommendations for entering enclosed spaces aboard ships by resolution MSC.581(110), superseding resolution A.1050(27); the revision introduces a ship-specific enclosed space register and places more emphasis on risk assessment and atmosphere testing, including for carbon dioxide. For oil tankers, ISGOTT (sixth edition) sets out industry practice for tank cleaning, gas freeing and hot work.
Disposal of residues is governed by MARPOL. Annex I covers oily residues and slops, recorded in the Oil Record Book, and regulates crude oil washing on crude carriers. Annex II covers noxious liquid substances from chemical tankers: regulation 13 requires a tank from which a Category X substance has been unloaded to be prewashed before leaving the port of unloading, with residues discharged to a reception facility, and operations are recorded in the Cargo Record Book in accordance with the ship’s Procedures and Arrangements Manual. Annex V covers dry cargo residues and hold wash water. Port authorities often require a licensed waste contractor and issue their own receipts.
Typical triggers and records
| Item | Requirement | Timing | Reference |
|---|---|---|---|
| Enclosed space entry | Risk assessment, permit, atmosphere testing, rescue plan | Every entry | SOLAS XI-1/7; MSC.581(110) |
| Category X cargo tanks | Prewash and discharge to reception facility | Before leaving port of unloading | MARPOL Annex II reg. 13 |
| Oily slops and sludge | Discharge to reception facility, Oil Record Book entries | As generated | MARPOL Annex I |
| Hot work in tanks | Tank cleaned and gas-freed, certificate issued | Before hot work | ISGOTT; company SMS; port rules |
| Ballast and structural tanks | Clean enough for close-up survey and gauging | Before class surveys | Class rules |
What the service includes
- Pre-job risk assessment, method statement and enclosed space entry plan with the ship’s officers
- Gas freeing and ventilation with portable fans, and continuous atmosphere monitoring
- Manual and machine washing, mucking out of sludge and sediment, and hand cleaning of structure
- Cleaning of fuel oil, lube oil and sludge tanks, including preparation for fuel changeover
- Ballast tank sediment removal and preparation for close-up survey and thickness gauging
- Cargo tank cleaning for grade change, with wall-wash sampling on chemical tankers where required
- Fresh and potable water tank cleaning and disinfection
- Collection and disposal of slops, sludge and residues by licensed contractors, with receipts
- Gas-free certificate from a competent person or marine chemist where the port requires it
Tank types and ship types covered
Crews routinely work on product and chemical tankers, crude carriers, bulk carriers, container ships, gas carriers in refit and offshore vessels. Tanks include cargo tanks, slop tanks, double bottom and wing ballast tanks, fore and aft peaks, heavy fuel and distillate tanks, settling and service tanks, sludge and bilge holding tanks, and potable water tanks. Where a ship does not carry enough calibrated gas detectors or breathing apparatus for a large job, these can be supplied through equipment rental services, and instrument checks can be arranged with gas detection equipment servicing.
Common problems and PSC findings
- Atmosphere testing instruments missing, out of calibration or unable to measure the required gases
- Enclosed space entry without a permit, adequate testing or rescue arrangements, a recurring cause of fatalities
- Oil Record Book or Cargo Record Book entries inconsistent with tank soundings and receipts
- Heavy sediment in ballast tanks hiding corrosion and delaying class survey
- Residues left in cargo tanks causing contamination claims on the next cargo
- Missing reception facility receipts for slops and sludge discharged
Certificates and documents you receive
Typical deliverables include a cleaning report listing tanks cleaned, methods and quantities removed, gas-free or safe-for-entry certificates, atmosphere test records, waste delivery receipts from the reception facility, wall-wash analysis results where applicable, and photographs. These support Oil Record Book or Cargo Record Book entries, the next charterer’s inspection and any class survey that follows.
How to prepare the vessel
Strip tanks as far as possible using ship’s pumps and record remaining quantities. Confirm with the agent whether tank cleaning, gas freeing and waste collection are allowed at the berth or anchorage, and book reception facilities early. Prepare enclosed space entry permits, atmosphere testing instruments, ventilation, lighting and rescue equipment. Isolate valves, heating coils and pumps connected to the tanks. For work before class surveys, coordinate timing with the thickness gauging firm and the surveyor so the tanks are presented only once.
Information needed for a quotation
- Vessel name, IMO number, flag, class and ship type
- Port, berth or anchorage, ETA, ETD and time available
- Tanks to be cleaned, with capacities, dimensions and access arrangements
- Last cargo or contents, residue type and estimated quantity remaining
- Required standard: gas-free for entry, hot work, grade change, survey or potable water
- Waste disposal needs and whether reception facilities are already booked
- Safety data sheets for last cargoes and any charterer cleaning requirements
vslSafety coordinates a suitable tank cleaning contractor at the vessel’s port and confirms waste disposal and permit arrangements in the quotation.
Frequently asked questions
What does gas-free mean for tank entry?
A gas-free tank has been ventilated so that oxygen is at normal atmospheric levels and flammable and toxic gases are below the limits set in the company’s procedures. It must be confirmed by testing with calibrated instruments at several levels in the tank and repeated during the work. A certificate is often issued by a competent person or marine chemist, particularly before hot work.
Who can issue a gas-free certificate?
On board, a responsible officer trained in atmosphere testing issues the entry permit under the safety management system. For hot work in shipyards or ports, the authority or terminal often requires a certificate from an independent competent person or marine chemist. Requirements differ by country, so the agent should confirm what the port expects before arrival.
How often should ballast tanks be cleaned?
There is no fixed international interval. Sediment should be managed in line with the ship’s Ballast Water Management Plan, and tanks must be clean enough for class inspection and thickness measurement at intermediate and renewal surveys. Ships trading in silty waters may need more frequent removal to prevent loss of capacity and hidden corrosion.
Can tank cleaning be done at anchorage?
Often yes, but permission depends on the port, cargo type, weather and whether residues can be discharged to a barge or reception facility. Some ports restrict gas freeing to specific areas or times. Anchorage work can save berth time but requires suitable launch and waste barge logistics, so early planning with the agent is important.
What is a Category X prewash?
Under MARPOL Annex II regulation 13, a tank from which a Category X noxious liquid substance has been unloaded must be prewashed before the ship leaves the port of unloading, with the residues discharged to a reception facility. A surveyor authorised by the port state verifies the operation and endorses the Cargo Record Book. The procedure follows the ship’s Procedures and Arrangements Manual.
