The use of manual tank gauging is still common in the oil and gas industry, but it is a method which can compromise the safety of the workers making the measurements. In February 2016, the U.S. National Institute for Occupational Safety and Health (NIOSH) and U.S. Occupational Safety and Health Administration (OSHA) jointly issued a hazard alert 1 for workers involved in manual tank gauging and fluid sampling at oil and gas extraction sites. This alert identified the health and safety risks posed by exposure to concentrated hydrocarbon gases and vapors, and exposure oxygen-deficient atmospheres and the potential for fires and explosions when hatches are opened on production and flow back tanks, or other tanks containing process fluids.
The alert states how opening tank hatches, often known as “thief hatches,†can cause high concentrations of hydrocarbon gases and vapors to be released. For workers performing this task, these exposures can have immediate damaging health effects and can even prove fatal. NIOSH and OSHA identified nine worker deaths from 2010-2014 that occurred while workers manually gauged or extracted samples from production tanks. Exposure to hydrocarbon gases and vapors and/or oxygen-deficient atmospheres was believed to be either a primary or contributory factor in those deaths.
What happens when a thief hatch is opened?
When a thief hatch is closed, hydrocarbon gas and vapors in the tank are often at a pressure higher than atmospheric. When the hatch is opened, a large volume of gas, mostly propane and butane, rush out. These gases can displace oxygen in the immediate work area, possibly asphyxiating workers in the vicinity. As the hatch remains open, heavier hydrocarbon molecules (pentane, hexane, heptanes) also begin to leave the tank and enter the workspace. The rate of release is high, and these gases and vapors may reach toxic or flammable concentrations. As the hatch remains open, the gases and vapors in the tank approach equilibrium with the environment, significantly slowing the rate of emission.
When workers suffer acute exposure to hydrocarbon gases and vapors, their eyes, lungs and the central nervous system can be affected. The NIOSH/OSHA alert states such exposure can sensitize the heart to stress hormones, such as catecholamine’s, causing abnormal rhythms and ventricular fibrillation, which can lead to sudden death. Even an exposure of fewer than 30 seconds can result in fatal cardiac arrhythmias, the alert adds.
Improving safety
Operators can take various measures to prevent workers from putting their health and safety at risk through exposure to hydrocarbon gases, vapors and flammable atmospheres, including:
Engineering controls such as fitting tanks with sampling taps and thief hatch pressure indicators
Improved training and work practices, and
Use of protective equipment such as flame-retardant clothing, respiratory protection, and impermeable gloves.
However, number one on the NIOSH/OSHA list of recommendations for improving safety is the implementation of alternative tank gauging and sampling procedures, which enable workers to monitor tank fluid levels and take samples without having to open the thief hatch.
Automatic tank gauging systems
Automatic tank gauging (ATG) systems perform a range of measurements within a tank (Figure 1), and can also issue an alert to an operator when action is necessary, for example in the event of the fuel level is too high or low. Critically, these fully automatic systems eliminate the need for workers to climb tanks and open thief hatches and therefore provide significant safety benefits.
