NORM (Naturally Occurring Radioactive Material) are present in rock formations and are related to the radioactive decay series of U-238 and Th-232, with Ra-226 and Ra- 228 being the most important daughter radionuclides. From the point of view of radiological protection, these radionuclides, if inhaled or ingested, are deposited in the bones, causing injuries and even cancer over time. During the production and exploration of oil and gas, these radionuclides migrate from the reservoir rock, flow through the production lines until they reach operational areas, and accumulate in pipes, tanks, valves, heat exchangers, etc., in the form of sludge (accumulation of heavy hydrocarbons and sediments) and scale (SrSO₄/BaSO₄ and CaCO₃ precipitates, causing a reduction in oil uptake). Once the presence of NORM has been confirmed by radiometric survey on this equipment, cleaning and decontamination of sludge and scale are carried out using five techniques: cleaning and manual aspiration (removes sand and sludge from surface equipment without involving any machinery), removal mechanical (drilling or enlarging to remove scale from tubular), chemical descaling (use of chemicals to prevent and remove scale), abrasive methods (removes scale from surfaces of accessible components) and fusion (separates NORM-contaminated metals from the rest of contaminated components). The choice of technique can be based on some reasons, such as the environmental conditions of the location, the equipment to be decontaminated, and the types of NORM material, with the ultra-pressure hydrojetting technique (abrasive method) being the most used. During cleaning and decontamination, radionuclides may disperse into the air, enabling inhalation and/or ingestion of particulates, leading to internal contamination by radionuclides from the U-238 and Th-232 series. Data from UNSCEAR and Hamilton show an assessment of external and internal exposures (inhalation and ingestion) of the operator and assistant. Personal air monitors, personal dose monitors, and radiation survey meters were used to perform the assessment during the cleaning and decontamination of pipelines with NORM. Cleaning work was carried out on 20 tubes/day for 250 days/year, resulting in annual effective doses of 3.3 mSv for the operator and 4.9 mSv for the assistant. The internal dose contributed to 12% and 17% of the annual effective dose, thus demonstrating the risk of internal exposure of workers during cleaning and decontamination. The results show the importance of carrying out a wide dose assessment methodology that covers external and internal dosimetry. The objective of this work is to present this dose assessment methodology for workers who carry out the cleaning and decontamination of oil and gas NORM equipment, consisting of the following steps: assessment of cleaning and decontamination site; background determination; monitoring workers with personal dose monitor Hp(10); monitoring workers with individual air sampling; monitoring workplace with cascade air sampling during the procedure, and evaluation of the worker’s internal contamination through urine using the in vitro bioanalysis technique. This assessment methodology is being applied during the cleaning and decontamination of equipment, both onshore and offshore, in the main petrochemical plants in Brazil.