High concentration of hydrogen sulfide (H2S) in cargo and slop tanks

Health and Safety Executive - Safety notice

Department name: Energy Division – maritime integrity, offshore

Bulletin number: ED2-2023

Issue date: 04/23

Target audience: Operators of floating production storage and offloading (FPSO) and floating storage and offloading (FSO) vessels and floating storage units (FSU) in the UK continental shelf (UKCS).


HSE has identified high concentrations of hydrogen sulfide (H2S) in ullage spaces of cargo and slop tanks onboard some FPSOs and FSUs operating in UKCS. The high concentrations (up to 16,000ppm) is above the upper measuring limits of the standard portable gas monitoring equipment generally used onboard tankers and FPSOs, and may be undetected.

Outline of the problem

Sulphate reducing bacteria present in cargo and slop tanks, especially those containing oil with high water cut, can generate hydrogen sulfide.

Hydrogen sulfide concentrations over 1,000ppm can cause immediate collapse with loss of breathing, even after inhalation of a single breath. This corrosive gas can also pit and corrode steel, which can affect the integrity of cargo containment systems and hull structures.

Standard equipment may not detect high concentrations of H2S

Investigations using specialist measuring equipment has revealed high concentrations of hydrogen sulfide even when process trains do not identify such high concentrations. Personnel using standard equipment onboard FPSOs and FSUs to monitor tank atmosphere may be unaware of these high concentrations.

Risk of exposure

There is a risk of exposure to hydrogen sulfide during operational activities involving breaking of containment, unintentional releases due to poor integrity of pipework attached to cargo tanks or during regular venting operations to maintain cargo tank pressure within operating limits. Toxic gas from such low-pressure release (often less than 800mm of WG), may not disperse in low wind conditions, can accumulate in isolated pockets onboard offshore installations.
Cargo tank ullage space gas monitoring requires the gas to be diverted to measuring equipment and vented to the atmosphere following measurement. Venting inert gas containing high levels of hydrogen sulfide of unknown concentration without appropriate risk control measures, such as preventing presence of persons on the main deck during venting or specifying minimum wind speed and/or direction, could expose crew to hydrogen sulfide.

Action required

Monitor ullage spaces of cargo and slop tanks

Duty holders operating FPSOs and FSUs must regularly monitor ullage spaces of cargo and slop tanks for the presence of toxic gas, such as hydrogen sulfide, using appropriate equipment, even if the process trains may not indicate high concentrations.

Assess and manage risk

If hydrogen sulfide is detected, duty holders must ensure that the installation has adopted and implemented a suitable hydrogen sulfide management process in accordance with HSE offshore information sheet no. 6/2009, 'managing hydrogen sulphide detection offshore'.

Review and revise management and control of major accident hazards to ensure all hazards with the potential to cause a major accident including exposure to high concentrations of hydrogen sulfide have been identified, evaluated and suitable risk control measures have been identified and implemented.
Revisit task risk assessments for routine ullage space gas monitoring, sampling and other tasks where individuals could be exposed to hydrogen sulfide. Provide suitable respiratory protective equipment (RPE), emergency escape breathing devices (EEBD), personal gas monitoring equipment and pressure containment equipment. Ensure inspection regimes are fit for purpose to identify any anomalies that can lead to a low pressure release of hydrogen sulfide.

Review integrity management and inspection arrangements for hull in way of cargo/slop tanks to identify any accelerated degradation mechanisms that can affect the structural or water-tight integrity of the hull.

Provide information and training

Provide necessary information, instruction and training to anyone onboard about the risks and any additional control measures implemented.

Make all marine crew aware of this safety notice so that they can identify any creeping changes in hydrogen sulfide concentration measured during routine monitoring.

Fixed gas monitoring

Consider fixed gas monitoring arrangements for cargo and slop tanks to avoid people being exposed to toxic gas while monitoring with portable gas monitoring equipment.

Other reasonably practicable measures

Consider implementing other preventive strategies such as increased frequency of dewatering cargo/slop tanks, biociding, routine maintenance of cargo tank stripping systems to ensure availability or any other reasonably practicable measures.


Relevant legal documents

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