Characterising NORM
hazards within subsea oil
and gas facilities.Daniel Emes – SA Radiation
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
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.
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.
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.
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”
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.
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).
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.
168mm OD steel pipe
NORM scale thickness = 5mm
=NORM material
= Detectable NORM gamma rays
240mm
140mm
Sodium Iodide 5cm Scintillator
Wall thickness = 11mm
Spreadsheet with assumptions
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
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.
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 0.1-15000Bq/g Ra226.
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.
Questions?