Top 12 Points for Cleaning Cargo Tanks with the IG System

Discover the top 12 critical points for cleaning cargo tanks using the inert gas (IG) system. This essential guide covers safety, procedures, IMO compliance, and real-world tips for tanker operations.

Why Tank Cleaning with the IG System Matters in Maritime Operations

Cleaning cargo tanks on oil and chemical tankers is not just a procedural step—it’s a high-risk, high-importance operation that protects both vessel and crew. Inert Gas (IG) systems play a pivotal role in this task, particularly by reducing the oxygen content inside tanks, thereby preventing fire or explosion during tank cleaning and gas freeing.

The International Maritime Organization (IMO), through MARPOL Annex I and SOLAS Chapter II-2, mandates strict procedures for tank cleaning and atmosphere control, especially on vessels carrying flammable cargo. Failing to properly inert, clean, and ventilate tanks can lead to catastrophic outcomes—from tank explosions to cargo contamination and Port State Control (PSC) detentions.

This article presents the 12 most important considerations when cleaning cargo tanks with an IG system, blending industry best practices, regulatory requirements, and real-world tanker operations.

Understanding the Role of Inert Gas in Tank Cleaning

Before we delve into specific procedures, it’s important to understand why inert gas is so critical.

Inert gas is usually a byproduct of flue gas from the ship’s boiler uptake, scrubbed and cooled before entering tanks. The goal is to lower the oxygen concentration below 8%, minimizing the chance of combustion. IG is mainly used before and during tank washing, and before gas freeing.

According to the International Safety Guide for Oil Tankers and Terminals (ISGOTT), inerting must be done before introducing water or air into tanks that have carried volatile or flammable cargo.

Ensuring the IG System Is Fully Operational

A tank cleaning operation cannot begin unless the IG system is verified. This means:

  • Blower fans are functional
  • Deck water seal or dry-type seal is filled and operational
  • Oxygen analyzer is calibrated and giving reliable readings
  • Alarms and trips are tested

In recent PSC reports by Paris MoU and Tokyo MoU, malfunctioning IG systems have led to ship detentions.

Conducting Pre-Cleaning Risk Assessment and Toolbox Talk

Tank cleaning is inherently dangerous. Crews must conduct a detailed risk assessment addressing:

  • Explosion risk
  • Entry procedures
  • Pressure differences
  • Communication gaps

A toolbox meeting ensures all personnel understand their responsibilities and emergency procedures. According to BIMCO and ICS guidelines, this step must be documented and acknowledged by all watchkeepers involved.

Reducing Tank Oxygen Levels Before Cleaning

Oxygen content must be brought below 8% by volume before starting the wash. In many cases, this figure is brought to around 5% as a safety margin.

Why this matters: Injecting water or introducing hot air into an oxygen-rich, hydrocarbon-laden tank can create an explosive atmosphere. The flammability triangle (oxygen, fuel, ignition) must be disrupted, and IG does that effectively.

Maintaining Positive Pressure in Tanks

During cleaning, tanks must remain under positive pressure to prevent the ingress of atmospheric air, which contains about 21% oxygen.

Best Practice: The IG system should maintain a minimum overpressure of around 200 mmWG (millimeters of water gauge). Pressure must be monitored continuously during cleaning.

Sequential Washing with IG Purging

Once initial inerting is done, tanks are cleaned using hot or cold water (depending on cargo residue). After each wash cycle:

  • Purge the tank again with IG to reduce oxygen levels that may have increased due to residual air
  • Recirculate inert gas to ensure a stable atmosphere before resuming

This approach aligns with the ICS Tanker Safety Guide (Chemicals) and company ISM Code safety management systems.

Monitoring Oxygen and Hydrocarbon Content Continuously

Fixed gas detection systems and portable gas analyzers should be used throughout the operation.

Critical readings to monitor:

  • Oxygen level (% vol)
  • Hydrocarbon (HC) concentration (usually expressed as %LEL – lower explosive limit)

These figures must be logged and reported as part of the cargo operation documentation.

Water Washing Techniques Must Match Tank Type and Cargo Residue

Different cargos leave different residues:

  • Crude oil forms clingage and waxy deposits
  • Vegetable oils leave film-like layers
  • Chemical cargoes may require detergent wash or chemical-neutralizing agents

Important: IG must remain ON during hot water cleaning of cargoes like gasoline, benzene, or aviation fuel to prevent vapor ignition.

Many shipowners follow OCIMF guidelines to standardize the process for charterer audits.

Avoiding Static Electricity Buildup

The movement of water and vapors during cleaning generates static electricity. This becomes a serious hazard in the presence of flammable vapor.

IG helps neutralize this hazard by creating a moist, non-combustible atmosphere inside the tank. Additionally:

  • Bonding and grounding must be verified
  • Wash machines must be anti-static and pressure-checked

As highlighted in MAIB incident reports, static-induced explosions are among the leading causes of tank-cleaning accidents.

Post-Cleaning IG Purge Before Gas Freeing

After cleaning, before ventilation with fresh air (gas freeing), a final IG purge should be conducted. This ensures residual hydrocarbons are diluted in a non-flammable environment.

Order of operations:

  1. Wash
  2. Final IG purge
  3. Controlled gas freeing (blowers ON, vents open)
  4. Tank entry only after verified safe by portable gas readings

Failure to follow this sequence has resulted in serious casualties, according to IMO Marine Safety Committee (MSC) circulars.

Coordination with Terminal and Port State Authorities

Port authorities may require certificates confirming inerting and safe tank conditions before berthing or loading. This is common in terminals across the European Union, Singapore, and USA.

Vessel crew must:

  • Keep a Tank Atmosphere Log
  • Present IG performance records
  • Show last calibration data for analyzers

Some terminals (e.g., Rotterdam, Houston) have strict zero-tolerance rules for oxygen levels above 8%.

Documentation and Post-Operation Review

After completion, ensure that:

  • Logs are updated with time-stamped entries
  • Wash records, purge volumes, and gas readings are archived
  • Deviations or incidents are discussed in the post-operation review

These records are often checked by Class Surveyors, Port State Control inspectors, and charterer vetting teams.

Real-World Case Studies

Case 1: In 2019, a tanker in Fujairah experienced a near-miss explosion after beginning cold water cleaning in an oxygen-rich tank. Investigation showed the IG blower had failed, and the operation had continued without positive pressure.

Case 2: In 2023, a chemical tanker in Antwerp successfully cleaned and gas-freed a benzene-contaminated tank using sequential IG purging and hot water washing, following ISGOTT protocol. The vessel passed inspection with commendation.

Frequently Asked Questions

Is IG required for every tank cleaning?
No, but for tanks previously carrying flammable cargo, IG is essential before washing or gas freeing.

Can I clean tanks with air ventilation alone?
Not safely. Air introduces oxygen and may create an explosive atmosphere unless the tank is already gas-free.

What are the safety precautions before tank entry?
Triple-check gas readings (O2 < 21%, HC = 0% LEL, H2S if applicable), issue a permit to work, and maintain standby watch.

How often should IG analyzers be calibrated?
Before each operation or as per PMS (Planned Maintenance System). Calibration gas must be within expiry.

What happens if the IG system fails during cleaning?
Stop operations immediately, re-inert tanks when system is restored, and inform the Master and shore authorities.

Which IMO codes apply to tank cleaning with IG?

  • SOLAS Chapter II-2
  • MARPOL Annex I
  • ISGOTT (Industry standard)
  • IBC Code (for chemicals)

Conclusion: Cleaning with Confidence and Compliance

Cargo tank cleaning using the inert gas system is a balancing act of science, safety, and seamanship. It’s more than just pumping water into a dark tank. It’s about understanding flammability thresholds, maintaining atmospheric control, and coordinating systems in real time.

By mastering these 12 critical points—from oxygen reduction and pressure control to proper documentation and gas monitoring—seafarers ensure safer operations, regulatory compliance, and smoother port interactions. Tank cleaning is an art form backed by engineering and governed by international law.

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