Knowledge Centre
NORM Scale
Source of emissions
Produced water, having been in contact with various rock strata at elevated pressure and temperature, contains many soluble components including Barium and the radioactive intermediates of the Uranium and Thorium decay series. As the water is produced the temperature and pressure decreases creating conditions in which the Barium and radionuclides can co-precipitate inside separators, valves and pipework, forming an insoluble NORM (naturally-occurring radioactive material) scale. Some of the soluble radionuclides and particles of NORM scale will pass through the system and be discharged with the produced water (see section on produced water). Similarly, some particulate scale and soluble radionuclides will be entrained with the exported oil by pipeline and will be discharged from the onshore terminal.
NORM scale may significantly reduce the efficiency of the production equipment on an installation and must be periodically removed. Large fixed vessels, such as separators, can be opened and the scale removed using high pressure water jets. The removed scale is macerated to a particle size of 1mm or less and is then discharged to sea. Smaller items of plant, such as valves and pipework are transported ashore to be cleaned at an authorised facility prior to being refurbished and re-used.
Although certain parts of the produced water process system may be cleaned periodically, there is the potential for a quantity of NORM scale to remain within the system which must be dealt with at the time of decommissioning of the installation. Whether the NORM scale is cleaned and discharged offshore or at an onshore location will be installation specific and determined during development of the decommissioning plan.
Potential environmental impacts
The most significant radioactive element in NORM scale and produced water is Radium and in particular the isotope Ra226 which is an alpha emitter with a half life of 1620 years. The behaviour of Radium in the marine environment and its effects on marine organism have been well studied. It should be noted that the same radioactive salts found in NORM scale are found in all seawater. The sea has been naturally radioactive for millions of years and marine organisms have evolved with this background activity.
Discharge of radium would be of concern if it were to bio-accumulate in marine organisms and then pass up the food chain to humans. Generally there is an inverse relationship between taxonomic position and sensitivity to radiation – marine invertebrates are extremely tolerant to radiation toxicity whilst humans are the most sensitive. Marine animals can potentially bio-accumulate radium from solution in the ambient seawater, from ingested seabed sediments or from their food.
However, the NORM scale discharged from offshore installations is insoluble in seawater and when produced water rich in barium and radium is discharged to sulphate rich seawater, the radium precipitates rapidly as a complex of barium, radium and sulphate which is also insoluble. As a result, radium has a very low concentration in solution in seawater and has a low bio-availability to marine organisms. It is also known that dissolved cations in seawater, particularly calcium and magnesium, inhibit the bio-accumulation of radium.
Similarly, any radium associated with precipitated barite in seabed sediments would not be bio-available to benthic organisms. Because of its low concentration in seawater and sediments, radium is rarely accumulated to high concentrations in the tissues of marine plants and animals. Where bio-accumulation does occur in marine animals the radium concentrations are higher in calcified skeletal structures of molluscs, crustaceans and fish than in their muscle and organ tissue. For example, more than 40 percent of the radium accumulated by fish is in the bone, with only 6 percent in the edible flesh.
Radium concentrations decrease with increasing trophic level, due to the poor assimilation efficiency and studies in the North Sea and the Gulf of Mexico have shown that the risk to the human population through the consumption of fish and shellfish is not significant.
The radioactivity discharged from offshore oil and gas operations is not considered to have a significant environmental impact.
Key control and mitigation measures
The keeping of radioactive materials and the accumulation and disposal of radioactive waste is regulated in the UK under the Radioactive Substances Act 1993 (RSA) and permitted under authorisation from the Scottish Environment Protection Agency (in Scotland), the Environment Agency (in England and Wales).
For naturally occurring radionuclides a substance is radioactive if it contains radionuclides with elemental activities above levels given in Schedule 1 of the RSA. Norm waste is currently exempted from the disposal requirements of RSA under the Radioactive Substances (Phosphatic Substances, Rare Earth etc.) Exemption Order 1962. This effectively exempts a significant amount of solid oilfiled NORM with elemental activities of radium and polonium below 14.8Bq/g.
It should however be noted that the present exemption regime falling under RSA93 is being reviewed and a revised exemption regime been proposed by the Department of Energy and Climate Change.
Reports of the types and quality of wastes disposed of both offshore and onshore are required and reports are to be submitted to SEPA or EA as appropriate on an annual basis.
References
- NEFF (2002) Bioaccumulation in Marine Organisms
- NRPB (2004) Assessment of the doses arising from the discharge of produced water
Updated: November 2009
