Improved techniques, mixing units can minimize crew exposure to caustic soda on drilling rigs
By Shahram Vatanparast, Contributing Editor
Hazardous substances cause 74,000 work-related deaths in EU countries each year. This indicates that more people die from dangerous substances than from workplace incidents. Caustic soda is one of the hazardous substances. A significant number of employees are away from work due to caustic soda skin burns, which can take weeks or months to heal.
Caustic soda is used in water-base mud as a source of hydroxyl ions to control pH. Caustic soda, caustic, alkali and lye are all common names for sodium hydroxide (NaOH). It is a strong base that is extremely soluble in water and dissolves into sodium (Na) and hydroxyl (OH) ions in solution. Due to its corrosive nature and high reactivity, caustic soda is known as a hazardous chemical for people who work on drilling rigs.
Numerous incidents within the drilling industry involving the mixing of caustic soda have resulted in personal injury, ranging from minor burns to blindness and, in some cases, death. All too often, the incident can be traced back to poor training and supervision practices, inadequate work and safety procedures, or inadequate equipment design. According to IADC’s 2013 statistics, approximately 4% of fatal incidents in the worldwide drilling industry that year were caused by employees’ contact with chemicals.
Accidental exposure to caustic soda may occur under the following conditions:
• During transportation and handling of caustic soda buckets or sacks;
• During the mixing and diluting operation;
• During maintenance and cleaning of the caustic barrel or unit;
• During disposal of caustic buckets or sacks after usage; or
• During decontamination following spills and equipment failures.
Reactivity and corrosive nature
High reactivity is one of the characteristics of caustic soda. For instance, if dry caustic is added to water with agitation, the caustic will dissolve rapidly, and the resulting heat will be dissipated throughout the solution volume. However, “if a large quantity of caustic is added rapidly to water without sufficient agitation, or if a large solid mass is added, the caustic mass will form a large hydrate, dissolve slowly and result in local overheating, boiling and spreading of the solution. A similar condition will occur if water is added to dry caustic,” according to an industry technical paper. Heat generation will take place on concentrations of 40% or greater.
The corrosive nature of caustic soda is well known. For instance, strong alkali substances, such as caustic soda, cause injury to tissue by liquefaction necrosis, which is saponification of fats and solubilization of proteins, allowing deep penetration into the tissue. While the corrosive destruction of the tissue caused by exposure to alkalines takes longer, they penetrate more deeply into the tissues than is the case with exposure to strong acids.
Traditional techniques in mixing caustic soda
A majority of the drilling industry, both offshore and onshore, still use traditional techniques for mixing caustic soda. This includes using 55-gal barrels with connected paddles to facilitate mixing operation. Some drilling companies have upgraded their units by adding an electrical motor for agitation purposes, instead of using manual handles.
In addition, dumping dry caustic into a suction pit or mixing hopper also have been observed at drilling sites. Both are considered hazardous practices. From operational aspects, direct addition of dry caustic to the active drilling fluid system has proved less effective for pH control, and dumping dry caustic into the mixing hopper can blow dry caustic into the worker’s face and flocculate in the drilling fluid.
Drawbacks of traditional techniques
Considering the hazardous nature of caustic soda, traditional mixing techniques involve various risks to the rig crew and are not considered safe practices. For instance, due to splashing or “boil over” effects, exposure is possible when the caustic barrel lid is opened for the addition of caustic and water. In addition, improper rinsing after each use, the flat bottom of the caustic barrel and incorrect location of the discharge valve on the caustic barrel are factors that can cause a solid mass of caustic to be gathered at the base of the caustic barrel. Past incidents involving the caustic barrel indicate that the buildup of solid caustic due to the incorrect location of the discharge valve and the flat bottom have caused injury to maintenance crews.
Further, ergonomic features have not been considered in the design of traditional caustic mixing barrels, where the operator must hold a 50-lb sack while he is adding caustic to the barrel. This can lead the operator to add the caustic too rapidly due to fatigue.
Advanced techniques in mixing caustic soda
Recent improvements in the design of caustic soda mixing units have begun to address hazards that could result from “boil over” and the potential for personnel to come into contact with the caustic soda solution during the mixing process. Other issues being addressed include the elimination of dust and mist exposure and the addition of ergonomic features.
Various companies have devised new caustic mixing units capable of minimizing workers’ exposure to this dangerous chemical. Some have devised caustic soda mixing units that allow caustic soda bags to be cut manually with a closed chamber, which eliminates personnel’s dust exposure. The powder is emptied into a 400-liter tank, where mixing of the caustic is performed by circulation by means of a diaphragm pump. Readily mixed caustic soda can be pumped to the desired tank.
Another mixing unit works by generating a turbulent swirl, providing rapid volume turnover and mixing for chemical dissolution and uniform blending. In this device, the contents of the caustic soda bags are emptied into the tank. It has features such as a safety cover for more security and overflow ball valve as a precaution against leakage.
Do’s and don’ts while mixing caustic soda
• Do review the job safety analysis (JSA)/material safety data sheet before mixing caustic soda.
• Do follow safe operational procedures for mixing caustic soda.
• Do add one sack of caustic to the mixing barrel at a time.
• Do add caustic to the mixing barrel slowly and with constant stirring.
• Do handle caustic waste in a proper manner.
• Do follow emergency procedure in case of any accidental contact with caustic soda or spillage.
• Do not add water to the caustic as it will cause a violent reaction.
• Do not add more caustic to the mixing barrel than the amount required.
• Do not start to mix the caustic without having performed the proper JSA.
• Do not start to mix the caustic without the required PPE, including face shield, rubber apron, goggles, rubber boots, respirator and rubber gloves.
• Do not come into contact with caustic in any chemical form, even in low concentrations.
• Do not dump caustic through the mixing hopper or mud pit grating.
• Do not use very cold or hot water due to the high reactivity of caustic. Water temperature should between 26-37°C.
Conclusion
Traditional methods of mixing caustic in the drilling industry increase the risks of injury and ill health effects due to a high level of employee exposure to the caustic. By utilizing improved engineering controls, such as newly designed caustic soda mixing units, companies can reduce employees’ exposure and decrease the number of incidents that result from the use of inappropriate equipment. Providing a safe system of work, training, supervision, required PPE and administrative controls are among other measures that can help to minimize employees’ exposure to caustic soda. DC
Shahram Vatanparast is a chartered safety and health practitioner with more than 13 years of experience at major oil and gas companies in the Middle East and South Asia, including PTTEP, Sinopec International and Total E&P.
References:
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- European Agency for Safety and Health at Work, 2008. Expert forecast on emerging chemical risks related to occupational safety and health. Luxembourg: European Communities publication office
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