The high concentrations (up to 16,000ppm) of H2S was located in ullage spaces of cargo and slop tanks onboard some floating production storage and offloading (FPSO) and floating regasification units (FSUs) operating in UKCS. The high concentrations are above the upper measuring limits of the standard portable gas monitoring equipment generally used onboard tankers and FPSOs, and may be undetected.
Sulphate reducing bacteria present in cargo and slop tanks, especially those containing oil with high water cut, can generate H2S.
H2S 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.
Investigations using specialist measuring equipment has revealed high concentrations of H2S 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.
There is a risk of exposure to H2S 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 H2S 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 H2S.
Duty holders operating FPSOs and FSUs must regularly monitor ullage spaces of cargo and slop tanks for the presence of toxic gas, such as H2S, using appropriate equipment, even if the process trains may not indicate high concentrations.
If H2S is detected, duty holders must ensure that the installation has adopted and implemented a suitable H2S management process in accordance with HSE offshore information sheet no. 6/2009, 'managing hydrogen sulphide detection offshore'.
Other actions include:
Reviewing and revising 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 H2S have been identified, evaluated and suitable risk control measures have been identified and implemented.
Revisiting task risk assessments for routine ullage space gas monitoring, sampling and other tasks where individuals could be exposed to H2S. Providing suitable respiratory protective equipment, emergency escape breathing devices, personal gas monitoring equipment and pressure containment equipment. Ensuring inspection regimes are fit for purpose to identify any anomalies that can lead to a low pressure release of H2S.
Reviewing integrity management and inspection arrangements for hull in way of cargo/slop tanks to identifying any accelerated degradation mechanisms that can affect the structural or water-tight integrity of the hull.