Radiation protection in the offshore petroleum industry

Radium and barium are normally dissolved in the formation water of oil reservoirs. After sea water, which contains sulfates, is injected to the reservoir to maintain production pressure radium can precipitate with barium as barium(radium)sulfate. This salt plates out on internal surfaces of pipes, valves and vessels and highly radioactive scale is formed. The specific activity of 226Ra and 228Ra in scale formed on some internal surfaces of an offshore oil production facility was measured as 250,000 Bq/kg and 300,000 Bq/kg respectively. About ten times lower specific activity of radium was measured in sludge. Naturally occurring radioactive materials in petroleum facilities are referred to as NORMs. To manage occupational radiation exposures contaminated pipes and vessels were visibly marked, all personnel on the facility attend regular training and a radiation protection monitoring program was implemented. Even though the external dose rate on external surfaces of some pipes and vessels contaminated with scale already reached 60 μSv/h it was possible to manage the maximum personal annual effective dose equivalent below 1 mSv. During shutdowns external dose rates inside contaminated vessels may reach more than 150 Sv/h and the potential alpha energy concentration of radon and thoron daughters in non ventilated vessels may reach several WLs. Therefore, all entries of vessels contaminated by scale and protection of employees must be planned and breathing protection provided in order to keep personal doses at low levels. The paper summarized practical experience in assessment of personal doses and management of personal exposures which involve job planning and exposure scenario modelling. (author)