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Marine Inert Gas Systems

Marine inert gas systems (IGS) are critical safety equipment on oil tankers, chemical tankers, gas carriers, and certain other vessels carrying flammable cargoes, providing the inert atmosphere within cargo tanks that prevents fire and explosion. The fundamental principle is straightforward: by maintaining oxygen concentration in the tank atmosphere below the level needed to support combustion (typically below 8 percent oxygen by volume), an explosive atmosphere cannot form even if cargo vapours and ignition sources are simultaneously present. The implementation of this principle through reliable industrial equipment, integrated with cargo tank operations and ship safety systems, has substantially reduced the frequency of catastrophic tanker fires and explosions that plagued the industry through the mid-20th century. ShipCalculators.com hosts the relevant computational tools and a full catalogue of calculators.

Contents

Background

The history of tanker safety is punctuated by major casualty events that drove the development and progressive adoption of inert gas systems. The losses of Mactra (1969) and Marpessa (1969) (two large VLCCs that exploded during tank cleaning operations) were instrumental in identifying static electricity buildup in inert atmospheres as a fire ignition mechanism, leading to revised tank cleaning procedures and improved IGS design. The Amoco Cadiz (1978), Prestige (2002), and various other tanker casualties involving cargo tank fires have driven progressive tightening of IGS requirements and operational practices. The current regulatory framework under SOLAS Chapter II-2 Regulation 4 and the FSS Code, supplemented by the various tanker codes (ISGOTT, IMO MSC.1/Circ.1389), establishes mandatory inert gas requirements for the relevant ship types and provides detailed engineering and operational guidance.

Regulatory Framework

The international regulatory framework for marine inert gas systems combines SOLAS, IMO performance standards, ISGOTT industry guidance, and class society rules.

SOLAS Chapter II-2 Regulation 4 establishes the principal requirements for inert gas systems on oil tankers, chemical tankers, and gas carriers. Key requirements include:

  • Mandatory IGS on all oil tankers above 8000 deadweight tonnes (DWT)
  • IGS for chemical tankers and gas carriers based on cargo specifications
  • IGS performance requirements and approval criteria
  • Operational requirements for tank atmosphere management
  • Documentation and record-keeping requirements

IMO Resolution MSC.282(86) (Revised Guidelines for the Recognition of Equivalent Arrangements for Inert Gas Systems on Ships) provides the detailed performance standards and approval criteria for IGS equipment.

IMO Resolution A.567(14) (Guidelines for Inert Gas Systems) and the various IMO MSC circulars (particularly MSC.1/Circ.1389 for tanker operations) provide operational guidance for tanker IGS use.

The FSS Code (International Code for Fire Safety Systems), Chapter 15 covers Fixed Inert Gas Fire Extinguishing Systems and includes some inert gas applications.

ISGOTT (International Safety Guide for Oil Tankers and Terminals), now in 6th Edition, is the principal industry guidance for tanker operations. ISGOTT covers tank atmosphere management, safety procedures during cargo operations, equipment specifications, and operational best practices for inert gas systems.

Class society rules (DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, RINA, KR) implement SOLAS and IMO performance standards through detailed engineering requirements covering IGS design and certification, oxygen analyser certification, valve and piping specifications, and survey procedures.

OCIMF (Oil Companies International Marine Forum) provides industry guidance through publications like ISGOTT and various recommendations on tanker operations.

National regulations including USCG, EU regulations, and various flag state requirements may impose additional operational requirements for tankers in specific waters.

Principle and Theory

The fundamental principle of cargo tank inerting is reduction of oxygen concentration to below the level supporting combustion of cargo vapour mixtures.

Combustion requires three elements (the fire triangle): fuel, oxygen, and ignition source. Removing any one prevents combustion. In cargo tanks, fuel (cargo vapour) is unavoidable, ignition sources cannot be reliably eliminated (static electricity, electrical equipment, friction during operations), so oxygen reduction is the practical control method.

Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) define the concentration range within which a fuel-air mixture can ignite. Below LEL there is insufficient fuel to support combustion; above UEL there is insufficient oxygen.

Limiting Oxygen Concentration (LOC) is the oxygen concentration below which combustion cannot occur regardless of fuel concentration. For most cargo vapours, LOC is around 11-12 percent oxygen. For greater safety margin, IGS operations target oxygen below 8 percent.

The 5 percent rule for cargo tanks specifies that tank atmosphere oxygen must remain below 5 percent during cargo operations to provide safety margin against atmosphere fluctuations. The 5 percent target is below the 8 percent LOC providing approximately 60 percent safety factor.

Tank atmosphere management during cargo operations requires:

  • Pre-discharge pressurisation with inert gas (to prevent air ingress during discharge)
  • Continuous oxygen monitoring throughout cargo operations
  • Tank atmosphere maintenance during sea passages (preventing oxygen ingress through breathing)
  • Inerting before tank cleaning to prevent flammable atmosphere during cleaning
  • Gas freeing only after specific procedures and atmosphere verification

Static electricity buildup in inert atmospheres is a particular concern. Inert gas does not eliminate the possibility of static charge generation, only reduces the consequences (no ignition with low oxygen). Cargo handling procedures still address static electricity risks.

Inert Gas Sources

Several technologies provide the inert gas for cargo tank operations, with selection depending on ship type, cargo characteristics, and operational requirements.

Flue gas inert gas generators use exhaust gas from main boilers, treated to remove harmful constituents. Flue gas typically has:

  • Carbon dioxide: 12-14 percent (from combustion)
  • Nitrogen: 76-78 percent (from atmospheric air)
  • Oxygen: 2-5 percent (after combustion adjustment)
  • Sulfur oxides: variable depending on fuel
  • Particulates: requires removal

Flue gas treatment in the IGS:

  • Gas cleaner (scrubber) removes sulfur oxides and particulates with sea water spray
  • Oxygen analyzer ensures oxygen below specifications
  • Cooling reduces gas temperature
  • Pressurization for delivery to cargo tanks

Flue gas IGS is the dominant technology on oil tankers, with flue gas drawn from main boiler exhaust providing inert gas without dedicated generation equipment.

Independent inert gas generators (IIG, also called “dedicated generators”) burn fuel oil specifically for inert gas production. The combustion is controlled to produce gas with low oxygen content, with cooling and treatment as needed.

IIG advantages include independence from main propulsion (allowing IG generation at any time), better control over IG quality, simpler installation on ships without suitable boiler exhaust source.

IIG disadvantages include additional fuel consumption (the IG generation burns fuel specifically rather than recovering exhaust), more complex installation, and additional maintenance.

Nitrogen membrane systems use selective gas membranes to extract nitrogen from compressed air. The membrane allows oxygen to permeate through preferentially, leaving nitrogen-enriched air on one side. Membrane systems produce 95-99% nitrogen, providing very low oxygen content.

Nitrogen membrane advantages include very pure inert gas (low oxygen), no combustion (avoiding sulfur and particulate concerns), simpler installation than flue gas systems, suitable for chemical tankers and gas carriers handling sensitive cargoes.

Nitrogen membrane disadvantages include higher capital cost, energy consumption for compression (1-3 kWh per cubic metre of N2), and limitations on flow rate (membrane systems are typically smaller capacity than flue gas systems).

Pressure swing adsorption (PSA) nitrogen systems use molecular sieve adsorbents to separate nitrogen from compressed air. Air is alternately compressed through one of two adsorbent beds, with one bed adsorbing oxygen while the other regenerates by depressurization. PSA systems provide similar pure nitrogen as membrane systems but with different operational characteristics.

Inert Gas System Components

A complete inert gas system includes multiple components from gas source to tank discharge.

Inert gas source provides the gas at appropriate pressure and composition. Source equipment varies by technology (flue gas, IIG, membrane) but produces gas at moderate pressure (typically 0.5 to 3 bar gauge) ready for distribution.

Gas treatment removes contaminants and conditions the gas for cargo tank use. Treatment varies by source:

  • Flue gas: scrubber, demister, cooler
  • IIG: similar treatment plus combustion control
  • Nitrogen: less treatment needed (already pure)

Pressure-vacuum (PV) valves on cargo tanks regulate tank pressure. PV valves are bidirectional:

  • Pressure relief: opens when tank pressure exceeds set point (typically 0.14 bar above atmospheric)
  • Vacuum relief: opens when tank pressure drops below set point (typically 0.04 bar below atmospheric)

PV valve flame arrestors prevent flame propagation through the valve from outside the tank. Gauze flame arrestors meeting specific test standards (e.g., IMO Resolution MEPC.184(59)) are required.

PV breaker (PV breaker valve) provides ultimate over-pressure protection at higher pressure than the standard PV valves. It releases at typically 1.5 to 2 bar above the tank’s design pressure.

Inert gas main piping distributes gas throughout the cargo deck to individual tank connections. Pipe materials are typically carbon steel with corrosion-resistant coating.

Tank connections include cargo manifold connections (where IG joins cargo manifold piping for tank delivery) and direct tank connections (where IG enters specific tank vapor spaces).

Oxygen analysers monitor tank atmosphere and IG distribution. Oxygen analysers must meet specific accuracy requirements per FSS Code.

Permanent vapor sample lines from each cargo tank to fixed analyser locations provide reliable tank atmosphere monitoring.

Pressure gauges and recorders track tank pressure, IG main pressure, and other key parameters.

Alarm systems alert bridge personnel to critical conditions including high tank pressure, IG quality deviation, system failures.

Oxygen Analysers

Oxygen analysers are the primary instrument for monitoring tank atmosphere and IGS quality. Several technologies are used.

Paramagnetic oxygen analysers exploit the magnetic susceptibility of oxygen molecules. Oxygen in a magnetic field deflects a balanced rotor (the dumb-bell), with deflection proportional to oxygen concentration. Paramagnetic analysers are accurate, stable, and reliable, with typical service life of years.

Electrochemical analysers use galvanic cells with oxygen-permeable membranes. Oxygen reacts at the cell electrodes producing electrical current proportional to oxygen concentration. Electrochemical analysers are simpler than paramagnetic but require periodic membrane replacement and have temperature sensitivity.

Zirconia analysers operate at elevated temperature (around 700°C) using zirconium oxide solid electrolyte. Oxygen ions migrate through the heated zirconia in proportion to oxygen partial pressure differential. Zirconia analysers provide very fast response and high accuracy.

Continuous on-line analysers with automatic sample collection and analysis provide real-time tank atmosphere data. The systems include sample lines from each tank, sample conditioning (filtration, drying), the analyser, and result display/logging.

Portable oxygen analysers for spot measurements provide flexibility for tank checks at multiple locations. Portable analysers are typically electrochemical for portability and ease of use.

Alarm thresholds typically include:

  • High oxygen alarm at 5% (cargo operation criterion)
  • Higher alarm at 8% (LOC indication)
  • IGS quality alarm at 8% (output gas quality verification)

Calibration with reference gas at periodic intervals (typically monthly) verifies analyser accuracy. Modern installations include automatic calibration with built-in reference cells.

Cargo Tank Atmosphere Management

Operating an inert gas system requires careful management of cargo tank atmosphere throughout the cargo cycle.

Tank atmosphere status terminology per ISGOTT:

  • Inert: oxygen content below 8% (safe for cargo operations)
  • Marginally inert: oxygen 8-11.9% (borderline, verify and improve)
  • Pre-inert: 11.9-21% (atmosphere being changed)
  • Gas free: below 1% LEL hydrocarbons (safe for entry, after gas freeing procedures)

Pre-inerting before cargo loading prepares the tank by replacing air with inert gas. The tank is purged with IG, with displaced air vented overboard, until oxygen drops below 5%. This ensures safe atmosphere when cargo enters the tank.

Loading operations maintain tank atmosphere management:

  • Vapor return: cargo vapors returned to shore vapor recovery system
  • IG injection: IG topped up if vapor return is insufficient
  • Pressure control: maintaining slight positive pressure (preventing air ingress)

Loaded passage atmosphere management maintains inert atmosphere:

  • Daily oxygen content checks at multiple tank levels
  • Weekly comprehensive atmosphere assessment
  • Inert gas top-up if oxygen rises (typically every 2-7 days)

Discharge operations require:

  • Pre-discharge pressurization (raising tank pressure with IG before pump start)
  • Continuous IG injection during discharge (replacing cargo volume with inert gas)
  • Pressure maintenance throughout discharge
  • Post-discharge tank atmosphere verification

Tank cleaning operations under inert atmosphere:

  • Initial atmosphere verification (oxygen below 5%)
  • Continuous atmosphere monitoring during cleaning
  • Compatible cleaning chemicals (avoiding any creating oxygen or flammable byproducts)
  • Final atmosphere verification before completion

Gas freeing for tank entry requires extended controlled procedure:

  • Initial atmosphere verification of inert status
  • Controlled introduction of fresh air through ventilation
  • Continuous oxygen and hydrocarbon monitoring
  • Multiple sample points throughout tank
  • Personnel entry only after verified safe atmosphere

Tanker Operations

Tanker operations integrate inert gas with cargo handling, ballast operations, and tank cleaning. Several operational scenarios are typical.

Loaded cargo passage maintains inert atmosphere with periodic top-ups. Tank pressure typically remains within 0.5 to 1 bar below the relief setting, with daily monitoring of all tanks.

Empty cargo passage (returning ballast) maintains the inert atmosphere during the ballast voyage, preventing flammable atmosphere from forming in tanks that may still contain residual cargo vapors.

Tank cleaning and gas freeing follows specific procedures with verification at each stage:

  • Continuous IGS operation during cleaning
  • Atmospheric monitoring at multiple locations
  • Specific time intervals for atmosphere verification
  • Documentation of all readings and actions

Inerting between cargoes when changing from one cargo type to another requires complete tank atmosphere change. Procedures depend on cargo compatibility and may include cleaning, multiple inerting cycles, and additional atmosphere verification.

Ballast tank inerting is required on chemical tankers and gas carriers carrying certain cargoes. Ballast tanks adjacent to cargo tanks require atmosphere management to prevent any flammable atmosphere development.

Specific Tanker Applications

Different tanker types have characteristic IGS configurations matched to their cargo and operational profile.

Crude oil tankers (VLCC, ULCC, Suezmax, Aframax) typically use flue gas systems with main boiler exhaust as the IG source. Cargo tank capacity is large (50,000-300,000 cubic metres total), requiring substantial IGS capacity.

Product tankers (carrying refined petroleum products like gasoline, diesel, jet fuel) use flue gas IGS or independent IG generators. Different products may require different inert atmospheres depending on flammability characteristics.

Chemical tankers (carrying various chemical cargoes per the IBC Code) often use nitrogen membrane or PSA systems for the cleaner inert gas. Many chemical cargoes require nitrogen specifically due to compatibility issues with flue gas constituents.

Gas carriers (LPG, LNG, ethylene) use nitrogen IG for cargo tank atmosphere management. The cryogenic LNG tank atmosphere management is specifically a vapor return and N2 injection scheme.

Combination carriers (OBO ships) carry oil and bulk cargoes alternately, requiring particularly careful atmosphere management between voyages.

Maintenance and Inspection

IGS maintenance combines daily attention, periodic preventive maintenance, and major overhauls aligned with class survey requirements.

Daily attention includes monitoring of IG output quality (oxygen content), system pressure, scrubber operation (for flue gas systems), and inspection of all visible components.

Weekly maintenance includes oxygen analyzer calibration verification, scrubber media inspection, sample line cleanliness, and PV valve testing.

Monthly comprehensive maintenance includes detailed system inspection, alarm testing, sensor calibration, and review of operational logs.

Annual major maintenance includes scrubber cleaning, oxygen analyzer recalibration, valve overhauls, and system performance verification testing.

5-year major surveys involve comprehensive inspection during dry-docking. Major equipment overhauls including scrubber internal inspection and renewal, oxygen analyzer replacement (typical 5-year cycle), valve and PV valve overhauls, and re-certification testing.

PV valve testing per the FSS Code requires periodic verification of relief and vacuum settings. Hydrostatic testing or pressure decay testing verifies valve operation.

Flame arrestor inspection at periodic intervals verifies cleanliness and integrity. Damaged or contaminated arrestors are replaced.

Oxygen analyser annual inspection includes calibration verification, sample line cleanliness, and electronic system check.

Safety Considerations

Several safety considerations are particularly important in IGS operations.

Personnel safety from inert gas exposure is critical. Inert gas at concentrations above approximately 18% (i.e., oxygen below 18%) is dangerous to humans through asphyxiation. Tank entry without verified gas-free atmosphere has caused multiple fatalities.

Static electricity prevention through proper grounding, slow flow rates during loading, and use of conductive equipment prevents ignition during operations.

Tank breathing during temperature changes (cargo heating in tropical operations, cooling at night) creates internal pressure variations. PV valves manage these variations within design limits.

Cargo compatibility verification ensures the inert gas does not react with the cargo. Most petroleum products are compatible with both flue gas IG and nitrogen IG, but some chemicals require specific inert gas types.

Emergency response procedures address IGS failure including:

  • Cargo operations cessation (until atmosphere verified)
  • Emergency atmosphere verification
  • Tank entry restrictions
  • Contact with port authorities and operator

Training and certification of cargo officers (typically Cargo Operations Officer holding STCW certificate) covers IGS operation, atmosphere management, and emergency procedures.

Future Developments

Marine inert gas systems continue to evolve in response to environmental regulations, operational efficiency drivers, and technological advances.

Cleaner inert gas technologies including improved scrubber systems and alternatives reduce environmental impact. Modern flue gas IG systems have substantially lower SOx emissions than earlier installations.

Nitrogen membrane technology improvements provide higher purity inert gas with lower energy consumption, expanding applicability to more cargo types and ship sizes.

PSA technology improvements offer compact systems with high reliability and energy efficiency.

Smart monitoring and analytics integrate IGS data with overall ship operations, providing better visibility, predictive maintenance, and operational optimization.

Combined IG systems with multiple sources (flue gas plus nitrogen) provide flexibility for ships handling diverse cargo types.

Hybrid arrangements with battery-powered IG generation provide options for ships seeking emission reduction during port operations.

Conclusion

Marine inert gas systems are essential safety infrastructure on tankers and other ships carrying flammable cargoes. The combination of properly designed IG sources, comprehensive treatment and distribution, reliable monitoring instruments, and disciplined operational procedures produces the safe cargo tank atmospheres that prevent fire and explosion. Crew members responsible for these systems must understand the regulatory framework (SOLAS Chapter II-2 Regulation 4, FSS Code), engineering principles, operational practices, and maintenance requirements that together ensure safe ship operation. As the maritime industry decarbonises and adopts new fuels with different combustion characteristics, IG systems are evolving to handle the changing operational landscape, but the fundamental principle, maintaining inert atmospheres in cargo tanks to prevent fire, remains a constant of tanker safety engineering.

References

  • SOLAS Chapter II-2 Regulation 4 - Fire Probability
  • IMO International Code for Fire Safety Systems (FSS Code), Chapter 15
  • ISGOTT (International Safety Guide for Oil Tankers and Terminals) 6th Edition
  • IMO Resolution MSC.282(86) - Revised Guidelines for Inert Gas Systems
  • DNV Rules for Classification of Ships - Pt 4 Ch 6 Piping Systems