Issue
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.
