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?