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Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

OSHA Hazard Information Bulletins
Confined Space Entry on Drilling Rigs

May 30, 1990




  • Director
  • Office of Field Programs


  • Director
  • Directorate of Technical Support


  • Safety Hazard Information Bulletin on Confined Space Entry on Drilling Rigs

The Directorate of Technical Support issues Hazard Bulletins (HIB) as needed to provide relevant information regarding unrecognized or misunderstood safety and health hazards, inadequacies of materials, devices, techniques, and engineering controls. HIB's are initiated based on information provided by the field staff, studies, reports and concerns expressed by safety and health professionals, employers, and the public. Information is complied based on a comprehensive evaluation of available facts, literature, and in coordination with appropriate parties. HIB's do not establish an OSHA policy.

The Marine Chemist Qualification Board of the National Fire Protection Association (NFPA) is currently investigating two near mishaps involving confined space entry on drilling rigs. Although this case may be outside of OSHA jurisdiction because the Coast Guard is the principal Federal agency on matters of safety and health on the Outer Continental Shelf, some of the case particulars are of interest for general safety procedures in confined spaces.

The Coast Guard requires that confined spaces which have contained or are suspected of containing flammable, combustible or toxic materials be tested by a Marine Chemist. The Marine Chemist is then required to perform an evaluation of the confined space environment using the guidelines of NFPA standard 306, Standard for the Control of Gas Hazards on Vessels. If the environment meets NFPA 306 requirements, "Safe for Workers" and/or "Safe for Hot Work" certificates are issued. (See attached NFPA 306 excerpts.) These requirements specify that the oxygen content of the air be between 19.5 and 22% and that the concentration of flammable materials be below 10% of the lower explosive limit.

In the first near mishap, the Marine Chemist and two Coast Guard inspectors were preparing to enter the final 35 feet of a 60 foot column of a mobile offshore drilling unit. Access to this confined space was made by unbolting a hatch door. The Chemist dropped a 25 foot testing hose attached to his MSA Miniguard monitor into the void. The meter indicated 20.2% oxygen. Then one inspector's Biosystems 310 Oxygen Monitor sounded an alarm. The Chemist pumped his unit's test bulb several times and found the reading to be 8% oxygen and dropping. The delay in receiving a proper oxygen reading could have been created by a venturi effect of the column pulling air up from the untested 10 feet at the column bottom.

In the second near mishap, the Marine Chemist had cleared entrance into a bow spud can (drilling unit "foot" or base of its lattice legs) for hydrogen sulfide exposure by noting a detector tube reading of less than 1 part per million (ppm) An inspector entered the space wearing a Neotox hydrogen sulfide meter. The meter's alarm sounded after 30-35 seconds with a reading of 15 ppm. Subsequent testing by the Chemist yielded a reading of 20 ppm.

Coast Guard personnel question the Chemist's use of a 25 foot line in a 35 foot cavity and the lack of a back-up monitor. There is nothing wrong with the use of an MSA Miniguard monitor with a 25 foot hose. It is important that the volume of the confined space closest to the access hatch be thoroughly checked before samples are taken at a lower level. Moreover, it is important that the sampling be done in an as large a horizontal area as possible at the upper regions of the space closest to the hatch. Then, after entry, the lower depths could be examined.

The Chemist's use of a detector tube for hydrogen sulfide monitoring is highly questionable. OSHA uses Industrial Scientific and Draeger pump-powered monitors for such applications. The units are intrinsically safe and have external alarms. Since the units are powered, sample hoses of varying lengths can be employed. Moreover, three range monitors are available that can simultaneously monitor oxygen, hydrogen sulfide and combustibles at the lower explosive limit.

Please distribute this bulletin to all area offices, state plan states and consultation project offices.

Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.
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