After looking into the circumstances surrounding four recent detections of hydrogen sulfide (H2S) during offshore operations at facilities on the Outer Continental Shelf (OCS), the U.S. Bureau of Safety and Environmental Enforcement (BSEE) has outlined a set of recommendations for oil and gas operators and contractors to assist in bringing down the number of such incidents in the future.
According to the U.S. regulator, offshore oil and natural gas operators have unexpectedly encountered “dangerous” concentrations of H 2 S in both process and non-process areas over the previous two years on facilities producing from fields determined to be H 2 S absent. As a result, the offshore energy facilities did not have H 2 S contingency plans in place.
Even though H 2 S is mostly associated with production from sour petroleum reservoirs, it can also form in low-oxygen environments from microbial activity during the decomposition of organic material, or by microbial reduction of sulfates. Due to this, BSEE explains that H 2 S production may occur unexpectedly in areas such as bilges, storage tanks, ballast tanks, chemical totes, and out-of-service process equipment.
Four recent H 2 S incidents
The U.S. Bureau of Safety and Environmental Enforcement outlines that personnel recently reported an H 2 S odour downwind of the floatation cell during normal production operations. At the time, a reading of 124 parts-per-million (ppm) H 2 S was observed at the vacuum breaker using a portable multi-gas detector. Readings taken 5 feet away were observed at 10 ppm, and at 10 feet the level dropped to 0 ppm. In a bid to tackle the issue, the team isolated the block valve, barricaded the area, and introduced biocide treatments to kill the bacteria and eliminate the presence of H 2 S. The second incident occurred during routine samplings when operators encountered a reading of 40 ppm H 2 S in the process stream via Draeger Tube measurements. The following day a reading of 250 ppm was found in the cargo tanks. Investigations led to the conclusion that the source of the H 2 S was bacterial activity and not the wells. The regulator underlines that the operator took immediate action to remediate the H 2 S in both the liquid and gas phase. The third incident took place during a transfer of a defoamer between day tanks with workers recognising a sour/rotten egg odor. Using a 4-gas monitor, a reading of 136 ppm H 2 S was taken near the tank’s opening. While the tank was quickly secured, the exposed personnel were sent in for evaluation as a precautionary measure. The fourth incident happened when a worker entered a ballast tank for cleaning, after observing an initial H 2 S reading of 0 ppm, as the 4-gas monitor began alarming with H 2 S readings as high as 128 ppm, after agitating the sludge layer. BSEE emphasises that the worker immediately exited the tank and work was suspended until an entry plan could be developed to include H 2 S mitigating procedures.
What should oil & gas operators do?
Bearing in mind the nature and frequency of these incidents, the BSEE recommends that operators and their contractors make sure personnel understand that H 2 S may be encountered as a by-product of non-hazardous materials even in non-sour service facilities and conduct periodic inspections of stagnant holding tanks to ensure no H 2 S presence while also conducting crew drills focused on unexpected H2S encounters, even on non-sour facilities.
Furthermore, oil and gas operators and contractors are urged to advise their workers to stay upwind when opening chemical tanks and utilising a multi-gas detector with H 2 S capabilities to survey for the presence of H 2 S. The presence of H 2 S should also be assessed at the bottom of the tank when personnel is planning to enter it and agitate any sludge that may be present.
In addition, operators and contractors should ensure that operating procedures include routine sampling where oil compositions present an increased potential for souring in the production system. Robust risk assessments should be carried out when conducting facility-level hazard analysis to adequately assess the threat of H 2 S production in vessels, tanks, or other areas that may routinely hold stagnant liquids with low oxygen levels.
Moreover, oil and gas operators and contractors should make sure the Job Safety Analysis includes a discussion of unexpected encounters of H 2 S and any mitigation necessary when conducting work in areas that have a higher threat of bacterial-generated H 2 S production such as ballast tanks.
Additionally, the BSEE recommends reviewing Safety Alert 366 titled: ‘Dangerous Levels of H 2 S Detected at Offshore Facilities ’ and following H 2 S guidelines per 30 CFR 250.490 and NTL 2009-G31.
Recently, the U.S. regulator also examined the events surrounding four recent high-potential, near-miss tagline entanglements during offshore operations and provided recommendations to lower the number of similar incidents going forward.