Reduce the risks of tank gauging by using work practice control
2 July 2017
Manual tank gauging is a common oilfield activity that’s resulted in multiple worker deaths in recent years. By using work practice control, operators can reduce the risks it poses significantly.
Every year, in survey after survey, oil and gas producers cite environmental health and safety as their number one priority. Many, however, continue to perform routine oilfield tasks that can be significantly improved by employing available technology.
Tank gauging is a prime example of this. Today, the vast majority of operators gauge their tanks manually. Among all of the archaic, business as usual-type activities that still take place in the oilfield, manual tank gauging is one of the most concerning because of the significant health and safety risks it poses to workers.
In 2016, the Occupational Health and Safety Administration (OSHA) and National Institute for Occupation Safety and Health (NIOSH) announced that from 2010 – 2014 nine fatalities occurred during manual gauging and sampling
of production tanks. Since the announcement, a number of reports and warnings have been issued informing operators of the dangers associated with completing this task; however, little has been done to educate companies on effective methods of prevention. One of the most reliable ways is by installing a radar level measurement device.
Dangers of Manual Tank Gauging
Manual tank gauging is a labour-intensive process that requires a worker to travel to a production tank (usually multiple tanks per day), climb a ladder to the top of it, open the thief hatch, and drop one end of a tape measure down to check liquid level.
When a thief hatch is opened, hydrocarbon gases and vapours that were previously in the equilibrium with production liquids (i.e., crude oil) are released into the atmosphere. In many cases, this can result in the creation of an acutely toxic and oxygen-deficient environment, which poses an immediate and life-threatening risk to the individual(s) gauging the tank.
The specific type and volume of vapor released when the hatch is opened is dependent on a number of factors, including product composition, temperature, wind speed, presence of a vapor recovery unit (or VRU), etc. Vapours com-monly encountered include low molecular weight hydrocarbons, such as methane, ethane, pro- pane, and butane. Depending on ambient conditions, heavier hydrocarbons, some of which are carcinogenic (i.e., benzene), may be present as well. On sites with “sour” wells, poisonous hydrogen sulfide gas (H2S) can also be a concern.
Without proper PPE, exposure to the gases and vapours released when the tank hatch is opened can affect the eyes, lungs, and central nervous system. If present in high enough concentrations, it can cause abnormal rhythms and cardiac arrest. Even very brief exposure (30 seconds or less) to a low-oxygen atmosphere can lead to the sudden onset of hypoxia and fatal cardiac arrhythmias, which can result in incapacitation within seconds, followed shortly by death if the individual is not removed from the environment .
The leading cause of death in the fatalities identified by OSHA and NIOSH was cardiac arrest brought on by exposure to hydrocarbon vapors and oxygen-deficient atmospheres.
Proper PPE Only Goes So Far
Oil and gas environments often allow workers to work independently and remotely. Climbing tanks requires awareness, knowledge and experience using fall protection equipment in some cases. In some cases, respirators require a proper fit and a cleanly shaven face for the seal to be effective, provided that the correct cartridges and (if needed) breathable quality supplied air is used. Perhaps the biggest challenge in this activity is complacency: “it will never happen to me. I have been doing this for years.” There are engineered control methods to minimize and eliminate several of these known and recognised risks.
When it comes to ensuring worker safety in manual tank gauging, proper PPE can only go so far. While air-purifying respirators with organic vapor cartridges are effective in pre- venting worker inhalation of heavier hydrocar- bon vapours, they are largely ineffective against light hydrocarbon gases, which can pass through the activated charcoal sorbent in respirator cartridges.
In addition, the respirators do not provide protection against low-oxygen atmospheres or concentrations of hydrocarbons exceeding the maximum use concentration. In at least one of the fatalities identified by OSHA and NIOSH, the worker was found unconscious wearing an air-purifying respirator. Supplied air respirators or self-contained breathing apparatus can protect workers from toxic exposures and oxygen-deficient atmospheres — however, their use in most instances is impractical.
Gas monitors are another piece of PPE that workers are often required to wear when manually gauging tanks — but like respirators, they are not a full-proof prevention method.
As mentioned previously, in certain instances, when the thief hatch is opened, the displacement of oxygen in the workers breathing zone is so rapid that it can result in almost immediate incapacitation. In such cases, the gas monitor alarm may sound but the worker will only have seconds to remove him or herself from the vicinity before going unconscious. In at least one of the fatalities identified by OSHA and NIOSH, the worker was wearing a 4-gas monitor, which reported an oxygen-deficient atmosphere and the presence of hydrocarbons exceeding 100% of the lower explosive limit at the approximate time of death.
