Characterising NORM hazards within subsea oil and gas facilities. Daniel Emes SA Radiation

Transcription

1 Characterising NORM hazards within subsea oil and gas facilities. Daniel Emes SA Radiation

2 What is in Oil and Gas NORM? Naturally Occurring Radioactive Material (NORM) can be characterized into many forms. It is found almost everywhere, in soil, air, water, and even our bodies. In small concentrations it is not a problem, but when NORM becomes concentrated, that s when it can become a hazard. Oil and gas operations deal with predominantly two forms of NORM Hard radium scales; and Sludges

3 How does NORM form? Hard radium scales are formed when radium dissolved in the formation water or injection water plates out due to temperature and pressure variations during extraction to the surface. Sludges are formed during processing once the product is pumped from underground.

4 Radium scale detection Because radium scales are often formed deep underwater, they are difficult to detect, and difficult to characterize. The scale however does emit gamma radiation, which is penetrating enough to be detected from the outside of the pipe. The dose-rate detected outside of the pipe allows activity approximations for the scale within the pipe to be calculated.

5 Case Study: Subsea heat exchanger A previous radiological survey of a subsea heat exchanger found elevated levels of gamma readings using a scintillation detector (measured in gross counts cps). Using previous data, the challenge was to estimate activity within the heat exchanger.

6 What information is needed to approximate activity? What radionuclides are likely to be present and their gamma characteristics (such as gamma yield and energy) The age of the material The thickness of the pipe The size of the detector and an approximation of the area it can see

7 What radionuclides are present? Radium 226 and 228 plate out on the insides of pipes, and their daughter products grow in with their own half lives. In nature, the uranium 238 and thorium 232 series are in very similar concentrations. Therefore the mobilized radium in formation and injection water in terms of activity is approximately 45% radium 226 (from the U238 series) and 55% radium 228 (from the Th232 series). In time, the relative abundance changes due to the difference in half lives.

8 What radionuclides are present? The oil platform had not been used for production in 5 years, so the shorter lived radionuclides have had the opportunity to grow into the scale. After 5 years, the approximate abundance of Ra226 and its progeny is around 75% of the total activity, and Ra228 and its progeny the remaining 25%. (in 5 years the Ra228 activity has halved twice, compared to relatively no change to the Ra226).

9 What can the detector see? An estimation of the area within the pipe the detector can see is required, in order to estimate the activity concentration of the scale. 240mm =NORM material 140mm = Detectable NORM gamma rays 168mm OD steel pipe Sodium Iodide 5cm Scintillator Wall thickness = 11mm NORM scale thickness = 5mm

10 Spreadsheet with assumptions

11 Assumptions used in activity calculation The NORM scale stopped being deposited 5 years prior to measurements The natural abundance of U and Th was approximately 45/55 within the well (UNSCEAR 2000) The system was closed (to prevent escape of Radon) Pb-210 and progeny were in equilibrium (conservative) The NORM scale was 5mm thick Detector was discriminatory below 160KeV (mentioned in initial report to reduce background dose) Measurements were taken with the detector in contact with the pipe surface

12 Activity calculation Using a model that takes each of variables mentioned into account, the gross counts detected by the crystal can be used to approximate the activity of the scale in the pipe. Using the model, the maximum total activity of scale was estimated to be 7100Bq/g within the heat exchanger. From the total activity of 7100Bq/g, approximately 1400Bq/g comes from combined Ra228 and Ra226.

13 Comparison to other examples and literature One heat exchanger that was removed from the ocean floor had combined radium activities of approximately 4000Bq/g (Sonter, 2009). ARPANSA s RPS15 cites up to 4300Bq/g for combined Ra226 and Ra228, and the range given in IAEA report no 34 is from Bq/g Ra226.

14 Further Action Approximating the activity concentrations while the equipment is still subsea allows oil and gas decommissioning projects to determine what risk the activity concentrations may have on any employees handling or dismantling any equipment. If the decision is made to leave the equipment as is on the ocean floor, the activity concentrations could also be used for ERICA assessments for environmental impacts.

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