The Health and Safety Executive (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.
Source: HSE.