Some rock formations contain naturally present radioactive isotopes or Naturally Occurring Radioactive Material (NORM). Examples of NORM isotopes include uranium and thorium. When oil & gas is extracted from reservoirs where the rocks contain NORM, the radioactive isotopes will be carried with the oil & gas stream. Most radioactive decay products contained in extracted oil & gas will be eliminated in upstream processing before the product is loaded onto a Ship.
Radon gas is an exception, as it can remain entrained in LNG and Propane. Radon can decay whilst being shipped, which results in deposits of radioactive decay products as dust / scale on internal surfaces of vessel cargo tanks and associated liquid phase cargo handling systems .
The amount of deposit and levels of radioactivity depend on a number of factors including the length of time the vessel has been in service, or time since last decontamination, and the origin of the cargos carried.
NORM found in LNG and propane liquid handling systems on Ships are mainly Alpha & Beta radiation emitters. Alpha & Beta radiation is primarily an internal radiation hazard. It is a risk to health if the radiation is inhaled, ingested or enters the body via cuts in the skin.
There are no acute (short term) health effects associated with the levels of radiation form NORM. Any radiation dose an individual is exposed to will increase the risk of chronic (long term) effects such as cancer.
The greater the dose (concentration of radiation; frequency & duration of exposure), the greater the risk of health effects will be.
NORM can deposit in LNG and propane cargo tanks and associated liquid phase cargo handling systems, including vapourisers; compressor knock out drums; cargo pumps; cargo line valves and cargo lines.
Monitoring for NORM must be undertaken on internal surfaces of LNG & propane cargo tanks and liquid handling systems using specialist contamination monitors. The monitoring must be carried out by a trained Radiation Protection Supervisor (RPS).
If NORM is detected the surfaces must be cleaned by a trained cleaning team before general access to the cargo tanks, or work on liquid handling systems can proceed.
- Limit / control access to personnel monitoring / cleaning
Designate a decontamination area at tank access point with
- Plastic sheeting; wet wipes/water for washing; waste bags and labels.
- Contamination checks and decontamination area for personnel monitoring / cleaning
Designate a clean area at tank access point with :
- Clean Respiratory Protective Equipment (RPE) & Personal Protective Equipment (PPE)
Wear appropriate RPE & PPE :
- Disposable coveralls with wrists and ankles tapped
- Long nitrile / rubber gloves
- Disposable shoe covers
- Full face mask with P3 / P100 particulate filters. . (This may be relaxed to half face mask after internal inspection – The RPS will advise.)
- Avoid generating dust whilst monitoring and cleaning
- Cleaning in LNG and propane cargo tanks cannot use water
- High Efficiency Particulate (HEPA) vacuum cleaner; sticky rollers; damp cloths