Ballast tank in which H2S was detected; Source: BSEE

‘Dangerous’ concentrations of hydrogen sulfide found at offshore oil & gas facilities in U.S.

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.

Ballast tank in which H2S was detected; Source: BSEE

According to the U.S. regulator, offshore oil and natural gas operators have unexpectedly encountered “dangerous” concentrations of H2S in both process and non-process areas over the previous two years on facilities producing from fields determined to be H2S absent. As a result, the offshore energy facilities did not have H2S contingency plans in place.

Even though H2S 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 H2S production may occur unexpectedly in areas such as bilges, storage tanks, ballast tanks, chemical totes, and out-of-service process equipment.

Four recent H2S incidents

The U.S. Bureau of Safety and Environmental Enforcement outlines that personnel recently reported an H2S odour downwind of the floatation cell during normal production operations. At the time, a reading of 124 parts-per-million (ppm) H2S 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 H2S. 

The second incident occurred during routine samplings when operators encountered a reading of 40 ppm H2S 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 H2S was bacterial activity and not the wells. The regulator underlines that the operator took immediate action to remediate the H2S 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 H2S 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 H2S reading of 0 ppm, as the 4-gas monitor began alarming with H2S 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 H2S 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 H2S 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 H2S 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 H2S capabilities to survey for the presence of H2S. The presence of H2S 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 H2S production in vessels, tanks, or other areas that may routinely hold stagnant liquids with low oxygen levels.

Source: BSEE

Moreover, oil and gas operators and contractors should make sure the Job Safety Analysis includes a discussion of unexpected encounters of H2S and any mitigation necessary when conducting work in areas that have a higher threat of bacterial-generated H2S production such as ballast tanks.

Additionally, the BSEE recommends reviewing Safety Alert 366 titled: ‘Dangerous Levels of H2S Detected at Offshore Facilities’ and following H2S 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.