A fatal accident took place at an onshore processing facility for slop water from the offshore petroleum industry.
Drilling fluids, or mud, are typically oil-water emulsions consisting of base oil (continuous phase), water (dispersed phase), and emulsifying agents. Used drilling mud, or slop, is mud enriched with water and rock cuttings from drilling --- typically 60-80% water, 10-20% emulated base oil, and 10-20% rock cuttings. The used drilling fluids are collected in slop tanks on oil platforms and later shipped to onshore facilities for further processing.
On the day of the accident, two operators were trying to remove the lid from a manhole on top of a 1600-cubic meter storage tank. However, they were not able to unscrew the rusted bolts holding the lid in place, and decided to use an angle grinder equipped with a cutting disc to cut the bolts. Shortly after the cutting began, an explosion occurred inside the tank. The explosion caused sections of the weld between the tank wall and the tank roof to rupture. The most serious damage to the tank roof was in the vicinity of the manhole. One of the operators was thrown over the railing and was killed. No other tanks in the tank farm were damaged. T
he accident investigation focused on finding the direct causes of the accident, and in particular on identifying the fuel component in the explosive mixture. A sample of base oil collected from the liquid surface inside the tank was tested by an external laboratory, and the closed cup flash point according to ASTM D6450 and ISO-2719 was determined to be 92° C. Hence, there was no reason to suspect that oil vapors played a significant role in this accident.
The liquid surface contained small bubbles trapped in a thin oil film, and gas chromatography (GC) revealed that the primary constituent in these bubbles was hydrogen gas. Microbiological analysis by direct counting with an epifluorescence microscope and quantification of DNA sequences by quantitative polymerase chain reaction (Q-PCR) showed extremely high biological activity: more than 100 million bacteria per milliliter. Most of the organisms identified in the solution belonged to groups of fermenting bacteria: Clostridia (Acetobacterium woodii), Bacilli (Alkaliebacterium, Desemzia), Fusobacteria (Propionigenium/Ilyobacter), and Spirochaeta. These are all capable of producing hydrogen under anaerobic conditions as part of the energy metabolism. The solution contained only about 0.1% metanogene bacteria: Methanosaeta (Metanofollis limninatans), Methanogenium and Methanoplanus. This is consistent with the negligible content of methane observed in the GC analysis. Hence, the fuel was hydrogen gas produced by bacterial fermentation of hydrocarbons inside the tank. Unfortunately, the growth substrate that sustained the high bacteriological activity could not be unambiguously identified from analysis with gas chromatography-mass spectroscopy (GC-MS). The ignition source was most likely mechanical sparks produced by the angle grinder, which could have entered the tank through an open 6-inch inspection hatch on the roof of the tank. This is consistent with the very low minimum ignition energy of hydrogen-air mixtures.
- Hand Tools
- Angle Grinder
- Hydrogen Storage Equipment
- Atmospheric Storage Tank
Many accidents reported from paper mills have much in common with this incident. Microorganisms in the process water with pulp produce hydrogen gas that mixes with air to form an explosive atmosphere. The ignition source is typically sparks produced by hot work, but ignition by electrostatic discharges in a cloud of mist in a storage tower has also been reported (see Explosion Caused by Microbial Hydrogen Formation).
Provided the hazard posed by bacterial hydrogen production has been recognized, there are several preventive and/or mitigating measures that can be considered for reducing the risk:
It is advantageous to avoid anaerobic and stagnant conditions in the solution by providing sufficient circulation and/or aeration (bubbling air through the solution), but this requires monitoring and could result in foaming or generation of biogas.
The formation of an explosive atmosphere may be avoided by providing sufficient natural or forced ventilation to keep the hydrogen concentration below the lower flammability limit (LFL), or by using an open-top floating roof tank to remove the space where an explosive mixture could form.
Forced ventilation can be used, but this solution requires monitoring, and care must be taken to avoid introducing a potential ignition source.
The free space above the liquid level can be blanketed with an inert gas to prevent the formation of an explosive mixture, but this is a relatively expensive solution that requires monitoring, and the inert atmosphere will promote anaerobic conditions that favor bacterial hydrogen production.
Strict enforcement of hot work procedures, including gas measurements and the use of non-sparking tools, may prevent certain ignition sources, and especially those that are directly associated with personnel risk.
Technical solutions such as grounding to prevent build-up of electrostatic charges and installation of flame arresters on all openings in the tank to prevent ignition by lightning, etc. can also reduce the risk, but the possibility of having an explosive atmosphere inside the tank should still be regarded as a severe hazard.
Risk management strategies should include a continuous focus on preventive maintenance, mandatory safety training for all workers, regular reviews of risk assessments, and learning from previous accidents in related industries.
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- = Explosion
- = Fire
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