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Tanker Safety & SIRE 2.0 · FSS Code Ch. 15 & MARPOL Annex I

Inert Gas System (IGS) & Crude Oil Washing (COW) Guide: FSS Code Ch. 15, SOLAS II-2 & MARPOL Annex I

An inoperative Inert Gas System or uncalibrated oxygen analyser will immediately halt cargo discharge at an oil terminal and trigger a Port State Control Code 30 detention or a high-risk SIRE 2.0 Chapter 8 finding. This guide covers FSS Code Chapter 15 (IMO Res. MSC.457(101)), SOLAS II-2/4.5.5 & 16.3.3, MARPOL Annex I Regulations 33 & 35, IMO Resolution A.446(XI) and ISGOTT 6th Edition operational rules.

Updated · Written by SeaReady in our own words. Not affiliated with OCIMF, RightShip, CDI, any MoU or any administration — always check the current official documents.

1. SOLAS II-2, FSS Code Ch. 15 (MSC.457(101)) & MARPOL Annex I Reg. 33/35

Hydrocarbon gas/air mixtures inside oil and chemical tanker cargo tanks are flammable between approximately 1% and 10% hydrocarbon vapour by volume when oxygen exceeds 11% by volume. By displacing tank air with flue gas or nitrogen so that oxygen remains below 8% by volume across the entire cargo tank (with inert gas supplied at ≤ 5% oxygen by volume), an Inert Gas System (IGS) keeps the tank atmosphere safely outside the flammability envelope during loading, loaded passage, discharge, Crude Oil Washing (COW) and tank cleaning.

  • SOLAS Reg. II-2/4.5.5 (Mandatory IGS applicability): Fixed Inert Gas Systems complying with the FSS Code are mandatory on oil tankers and chemical tankers of ≥ 20,000 DWT constructed before 1 January 2016, oil tankers and chemical tankers of ≥ 8,000 DWT constructed on or after 1 January 2016, and all tankers operating a Crude Oil Washing (COW) system regardless of deadweight (Reg. II-2/4.5.5.1.3).
  • FSS Code Chapter 15 (IMO Res. MSC.457(101)): Entered into force on 1 January 2024 (promulgated in the FSS Code January 2024 Supplement QQQB155E), completely replacing Chapter 15 to unify requirements for combustion flue-gas IGS, inert gas generators, and nitrogen generator (N2 membrane / PSA) systems.
  • MARPOL Annex I Regulations 33 & 35 + IMO Res. A.446(XI): Every crude oil tanker of ≥ 20,000 DWT must be fitted with a cargo tank cleaning system using Crude Oil Washing (Reg. 33.1), carry an Administration-approved Operations and Equipment Manual (COW Manual, Reg. 35.1) and a VOC Management Plan (MARPOL Annex VI Reg. 15.6 & MEPC.185(59)), and wash a sufficient number of tanks on ballast voyages (≥ 25% of tanks on rotation per Res. A.446(XI) § 6.1) to control sludge build-up.

2. Inert Gas System (IGS) Oxygen, Pressure, Scrubber & Non-Return Cutoffs

Under FSS Code Chapter 15 (IMO Res. MSC.457(101)) and SOLAS Regulation II-2/16.3.3, every officer of the cargo watch and marine engineer must know the exact statutory design and operational numbers:

ParameterFSS Code Ch. 15 / SOLAS ClauseMandatory Limit & Design Requirement
Volumetric Delivery CapacityFSS Code Ch. 15 § 2.2.1.1At least 125% of the maximum rate of discharge capacity of the ship expressed as a volume, ensuring positive tank pressure even during maximum all-pumps discharge.
Supply Oxygen Content (IG Main)FSS Code Ch. 15 § 2.2.1.2 & § 2.2.4.5.1.1Not more than 5% O2 by volume in the inert gas supply main at any required flow rate; high-oxygen audible/visual alarm at 5% O2 by volume.
Cargo Tank Oxygen ContentSOLAS II-2/16.3.3 & Res. A.446(XI) § 6.6Cargo tanks kept below 8% O2 by volume and at positive pressure at all times (verified at 1 m below deck and mid-ullage height before COW).
Two Non-Return DevicesFSS Code Ch. 15 § 2.2.3.1At least two non-return devices in the inert gas main (one being a deck water seal or equivalent double-block-and-bleed arrangement + one mechanical non-return valve forward of the deck seal) plus a positive isolating valve.
250 mm External Supply Flange ConnectionFSS Code Ch. 15 § 2.2.3.2.6A 250 mm nominal pipe size bolted flange, isolated from the inert gas main by a valve and located downstream of the non-return valve, for connecting an external/shore inert gas supply.
100 mm WG Low-Pressure AlarmFSS Code Ch. 15 § 2.2.4.5.1.3Audible and visual alarm on the bridge and in the Cargo Control Room (CCR) whenever pressure in the inert gas main forward of the non-return devices falls below 100 mm water gauge (~0.98 kPa).
50 mm WG Very-Low-Pressure Trip / AlarmFSS Code Ch. 15 § 2.2.4.5.1.3 & § 2.2.4.5.3Automatic shutdown of the cargo pumps or an additional audible/visual alarm when inert gas main pressure falls to 50 mm water gauge (~0.49 kPa).
High-Pressure Alarm & P/V BreakerFSS Code Ch. 15 § 2.2.2.3 & § 2.2.4.5.1.4High-pressure alarm before the liquid-filled Pressure/Vacuum (P/V) breaker blows; P/V breaker filled with antifreeze/water mixture to the exact gauge mark while the IG main is depressurised (0 mm WG).
High Gas Temperature TripFSS Code Ch. 15 § 2.2.4.5.1.2 & § 2.2.4.5.2.3Audible/visual alarm and automatic shutdown of the inert gas blowers and regulating valve if inert gas temperature at the blower discharge exceeds 65°C (or > 5°C above seawater cooling inlet temperature at scrubber outlet).

3. Mandatory IGS Alarms & Automatic Shutdown Setpoints (FSS Code Ch. 15)

Before every discharge port and during weekly/monthly PMS checks, the Chief Officer, 2nd Engineer and ETO must test and record the automatic trips and alarms required by FSS Code Chapter 15 § 2.2.4.5:

  • Automatic shutdown of inert gas blowers and gas regulating valve (§ 2.2.4.5.2): Triggered automatically upon (1) low scrubber cooling water pressure or flow rate (§ 2.2.4.5.1.6), (2) high water level inside the flue-gas scrubber tower (§ 2.2.4.5.1.7), (3) high inert gas temperature (above 65°C at blower outlet, § 2.2.4.5.1.2), and (4) failure of the inert gas blowers (§ 2.2.4.5.2.2).
  • Automatic closure of the gas regulating valve & external supply flange (§ 2.2.4.5.3 & § 2.2.3.2.6): The gas regulating valve closes automatically upon failure of the inert gas blowers or any blower shutdown condition while keeping the deck water seal water supply running continuously; downstream of the non-return valve, a 250 mm nominal pipe size bolted flange isolated by a valve is fitted for external inert gas supply.
  • Continuous Oxygen & Pressure Recorder (§ 2.2.4.2): Located in the Cargo Control Room (CCR), continuously recording both oxygen content (% O2) and inert gas main pressure (mm WG / kPa) throughout all cargo, ballast and tank-cleaning operations, calibrated with certified zero (100% N2) and span (20.9% clean dry air) before every operation and cross-checked with the ISGOTT 6th Edition Ship/Shore Safety Checklist.

4. Crude Oil Washing (COW): Res. A.446(XI), MARPOL Reg. 35 & VOC Plan

  • Pre-Arrival Pressure Test (Res. A.446(XI) § 6.3): Before arrival at the discharge port, the entire deck COW piping system must be pressure-tested to working pressure and inspected for leaks. Any leak found must be rectified before COW begins, and the pressure test must be recorded in the Oil Record Book Part II (Code G).
  • Pre-Washing Tank Atmosphere Verification (Res. A.446(XI) § 6.6): Before washing any tank, the oxygen level must be measured with a portable oxygen analyser at a point 1 metre below the deck and at the middle level of the ullage space and confirmed not to exceed 8% O2 by volume, while the IGS delivers inert gas at ≤ 5% O2 and positive pressure (above 250 mm WG).
  • Two-Stage (Top-Wash + Bottom-Wash) Method & 125% Stripping Capacity (Res. A.446(XI) §§ 4.4 & 6.5): Top washing is performed as the cargo level falls down the bulkheads during discharge; bottom washing is performed only after the tank has been discharged and stripped dry (≤ 1 m heel with positive eductor discharge to the slop tank). The stripping system capacity must be at least 1.25 times (125%) the total throughput of all COW machines operated simultaneously, verified dry after washing at at least 4 hand-dipping locations per tank.

5. SIRE 2.0 Chapter 8 & PSC Detention Traps on IGS and COW

Deficiency / Observation AreaWhat the Inspector ChecksHow to Prevent the Finding
Fixed O2 Analyser vs Portable Meter DiscrepancyInspector compares the CCR fixed IGS oxygen recorder reading against a calibrated portable oxygen meter at the deck test cock.Calibrate the fixed O2 analyser with certified zero (100% N2) and span (20.9% air or 2–5% O2 span gas) before every discharge; log both readings in the IGS log.
Deck Water Seal Sight-Glass & Vent DrainInspector inspects the deck water seal sight-glass for fouling, checks the continuous water supply pump pressure, and verifies the open-funnel drain behind the non-return valve.Clean the deck water seal sight-glass regularly, test the low-water-level/flow alarm before discharge, and keep the vent/drain pipe between the non-return valve and deck seal clear.
Cargo Tank P/V Valves & Flame ScreensInspector checks individual tank high-velocity pressure/vacuum (P/V) relief valves (≥ 30 m/s exit velocity), flame arrestor meshes, and manual lifting check-levers.Inspect flame screens for corrosion, paint blockage or tears; verify P/V valve vacuum/pressure test certificates (SOLAS II-2/11.6 & MSC/Circ.677 / MSC.1/Circ.1324).
COW Machine Rotation Indicators & Hydrant BlanksInspector checks whether programmable deck-mounted COW machines have working rotation/angle indicators and whether unused COW hydrant connections are blanked.Verify rotation indicators during COW, keep all unused COW line connections bolted with full-face blanks, and isolate any external cargo heater with physical spectacle blanks (Res. A.446(XI) § 4.2.5).
Closed Ullaging / UTI Tape EarthingInspector watches the pumpman or officer taking post-COW dry-dip checks through vapour locks (Res. A.446(XI) § 4.4.4).Ensure portable Hermetic/MMC UTI tapes are bonded to the hull before opening the vapour lock valve, and verify all 4 dip points per tank are clear of sludge.

Questions seafarers ask

What are the mandatory oxygen limits for Inert Gas System (IGS) supply and cargo tank atmosphere under SOLAS II-2 and FSS Code Chapter 15?

Under FSS Code Chapter 15 § 2.2.1.2 (as replaced by IMO Resolution MSC.457(101), effective 1 January 2024) and SOLAS Regulation II-2/16.3.3, the Inert Gas System must deliver inert gas with an oxygen content of not more than 5% by volume into the inert gas main at any required rate of flow (at least 125% of the maximum discharge capacity of the ship expressed as a volume). A high-oxygen audible and visual alarm must activate when the oxygen content of the inert gas reaches 5% by volume (§ 2.2.4.5.1.1). Inside cargo tanks, the atmosphere must be maintained at positive pressure with an oxygen content not exceeding 8% by volume in any part of any cargo tank (especially before commencing Crude Oil Washing under MARPOL Annex I Regulation 35 and IMO Resolution A.446(XI) § 6.6).

At what pressure does FSS Code Chapter 15 require the inert gas system and cargo pumps to alarm or trip, and what is the 250 mm flange requirement?

Under FSS Code Chapter 15 (IMO Resolution MSC.457(101)), two low-pressure thresholds in the inert gas main forward of the non-return devices are critical: (1) below 100 mm water gauge (§ 2.2.4.5.1.3), an audible and visual low-pressure alarm must activate on the bridge and in the cargo control room / machinery space; and (2) at 50 mm water gauge (§ 2.2.4.5.1.3 & § 2.2.4.5.3), an automatic shutdown of the cargo pumps or an additional very-low-pressure alarm must activate so cargo tanks cannot be drawn into vacuum. Additionally, under § 2.2.3.2.6, the inert gas main must be fitted with a 250 mm nominal pipe size bolted flange, isolated from the inert gas main by a valve and located downstream of the non-return valve, for connecting an external inert gas supply.

Which oil tankers are required to be fitted with an Inert Gas System and a Crude Oil Washing (COW) system?

Under SOLAS Regulation II-2/4.5.5.1, oil tankers of 20,000 tonnes deadweight and upwards constructed before 1 January 2016, and oil tankers of 8,000 tonnes deadweight and upwards constructed on or after 1 January 2016, must be fitted with a fixed Inert Gas System complying with FSS Code Chapter 15. In addition, any tanker operating a Crude Oil Washing (COW) system must be fitted with an Inert Gas System regardless of deadweight (SOLAS Reg. II-2/4.5.5.1.3). Under MARPOL Annex I Regulation 33.1, every crude oil tanker of 20,000 tonnes deadweight and upwards must be fitted with an approved COW system complying with Regulation 33 and IMO Resolution A.446(XI) (as amended by Resolutions A.497(XII) and A.897(21)).

What pre-arrival and pre-washing checks are mandatory before starting Crude Oil Washing (COW)?

Under MARPOL Annex I Regulation 35 and the Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems (IMO Resolution A.446(XI) §§ 6.1–6.7), the Person in Charge (holding a valid STCW Reg. V/1-1 Advanced Oil Tanker Cargo Operations endorsement and experience under § 5.2) must verify before COW commences that: (1) the COW system has been pressure-tested to working pressure before arrival and any leaks rectified; (2) the oxygen content of the inert gas being delivered does not exceed 5% by volume; (3) the atmosphere in every tank to be washed is at positive pressure and measured at ≤ 8% O2 by volume using portable oxygen analysers at a point 1 metre below the deck and at the middle level of the ullage space; (4) the tank bottom is dry / stripped before bottom washing; and (5) heater isolation blanks are in place if a tank crude-oil heater is fitted outside the cargo tanks.

How must deck water seals and double-block-and-bleed non-return arrangements be maintained on inerted tankers?

Under FSS Code Chapter 15 § 2.2.3.1 (IMO Resolution MSC.457(101)), at least two non-return devices must be fitted in the inert gas supply main to prevent the return of hydrocarbon vapour and liquid to machinery spaces or gas-safe spaces, one of which must be a deck water seal (wet, semi-dry or dry type) or an equivalent double-block-and-bleed valve arrangement. A water supply must be maintained to the deck water seal at all times, particularly when the inert gas plant is shut down, with low water-pressure/flow alarms (§ 2.2.4.5.1.5) and heating arrangements to prevent freezing in cold weather.

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