VALIDATION AND APPLICATION OF COMPUTED RADIOGRAPHY (CR)

TANGENTIAL TECHNIQUE FOR WALL THICKNESS MEASUREMENT OF 10 INCH

CARBON STEEL PIPE

Noorhazleena Azaman(a), Khairul Anuar Mohd Salleh(a), Amry Amin Abas(a), Arshad Yassin(a),
Sukhri Ahmad(b)
(a)DigitalIndustrial Radiography (DIR) Laboratory, Leading Edge NDT Technology (LENDT) Group, Industrial
Technology Division, Malaysian Nuclear Agency, Bangi, 43000, Kajang, Selangor Darul Ehsan, MALAYSIA.
(b)Material and Structural Integrity (MSI) Group, Industrial Technology Division, Malaysian Nuclear Agency, Bangi, 43000,

Kajang, Selangor Darul Ehsan, MALAYSIA.

hazleena@nm.gov.my

Abstract

Oil and gas industry requires Non Destructive Testing (NDT) to ensure each components, in-service
and critical, are fit-for-purpose. Pipes that are used to transfer oil or gas are amongst the critical
component that needs to be well maintained and inspected. Typical pipe discontinuities that may lead
to unintended incidents are erosion, corrosion, dent, welding defects, etc. Wall thickness assessment,
with Radiography Testing (RT) is normally used to inspect such discontinuities and can be performed
with two approaches; (a) center line beam tangential technique (b) offset from the centre pipe tangential
technique. The latter is a method of choice for this work because of the pipe dimension and limited
radiation safe distance at site. Two successful validation approaches (simulation and experimental) were
performed to determine the probability of successfulness before the actual RT work with tangential
technique is carried out. The pipe was a 10 inch diameter in-service wrapped carbon steel. A 9 Ci Ir-
192 and white Imaging Plate (IP) were used as a gamma radiation source and to record the radiographic
image. Result of this work suggest that RT with tangential technique for 10 inch wrapped in-service
carbon steel pipe can be successfully performed.

Keyword: Nondestructive Testing (NDT), radiography testing (RT), computed radiography (CR), tangential radiography
technique

Abstrak

Industri minyak dan gas memerlukan teknik-teknik Ujian Tanpa Musnah (NDT) untuk memastikan
setiap komponen, dalam perkhidmatan dan yang dianggap kritikal, adalah sesuai dan selamat untuk
digunakan. Paip yang digunakan untuk memindahkan minyak atau gas adalah antara komponen kritikal
yang perlu dijaga dengan baik dan diperiksa. Ketidakselanjaran yang wujud pada paip tersebut boleh
membawa kepada kejadian yang tidak diingini. Antara ketidakselanjaran yang dimaksudkan ialah
hakisan, kakisan, lekuk, kecacatan kimpalan, dan lain-lain. Pengukuran ketebalan dinding paip dengan
menggunakan Teknik Radiografi (RT), (a) samada teknik tangen garis tengah paip atau (b) teknik
tangen menjauhi dari garis tengah paip, biasanya digunakan untuk menilai ketidakselanjaran yang
dinyatakan. Teknik tangen merupakan kaedah pilihan untuk kerja-kerja ini kerana dimensi paip dan
jarak perlindungan radiasi selamat yang terhad di tapak. Dua pendekatan pengesahan (simulasi dan
eksperimen) telah dijalankan untuk menentukan kebarangkalian kejayaan sebelum kerja RT dengan
teknik tangen dijalankan. Paip yang diuji diperbuat dari keluli berkarbon berdiameter 10 inci dan
berbalut. Sumber sinaran gamma, Ir-192 dengan kekuatan 9 Ci dan plat pengimejan putih (IP) telah
digunakan didalam kerja ini. Hasil kerja ini mencadangkan bahawa RT dengan teknik tangen untuk 10
inci yang dibalut dalam perkhidmatan paip keluli karbon boleh berjaya dilakukan.

Kata kunci: Ujian tanpa musnah (NDT), ujian radiografi (RT), radiografi berkomputer (CR), teknik radiografi tangen.
INTRODUCTION

The introduction of powerful computers and reliable imaging technologies has significant impact on
the currently used NDT methods. New radiologic imaging technique in Digital Radiography; Computed
Radiography (CR) with phosphor imaging plates have increased the capacity and accuracy for
visualization and measurement of defects.

Phosphor Imaging Plate (IP) is media for film-free radiography. This technology is commonly called
Computed Radiography or CR. This technology has been introduced and applied in the fields of NDT.
CR could have the way to new areas of applications since higher sensitivities allow shorter exposure
times and the results can be directly evaluated digitally.

Imaging plates are exposed analogously to films. The image information is readout from the plates with
a laser scanner and the plates are erased at the same time or in a follow up erasure unit. This makes any
chemical developing process as for films obsolete, the results are available on the computer for
evaluation immediately after the readout. The plates can be reused up to thousand times without any
significant loss in quality if no mechanical damages appear.

A typical application for Computed Radiography (CR) is the quantification of corrosion effects in pipe
walls (shadow technique or projection radiography). The inspection for corrosion in pipes is one of the
most important NDT precautions measures in the chemical industry. The pipes are thermally insulated
and the insulation material is held together with an aluminium collar. The radiographic inspection is
accomplished without removing the insulation. This is an advantage over the ultrasonic method that
needs a direct contact with the pipe.

The classification of tangential radiographic technique are divided into two classes. There are basic
technique TA and improved technique TB. The basic technique, TA are intended for tangential
radiography of generalized wall loss, such as that due to erosion or large scale corrosion. The improved
technique, TB should be used for the more demanding tangential radiography of localized corrosion
pitting flaws, which require higher sensitivity for detection and sizing.

TANGENTIAL RADIOGRAPHY TECHNIQUE

Two techniques recommended for making tangential radiograph. There are radiation source located on
the pipe centre line as figure 1 and radiation source located offset from the pipe centre line as figure 2.
The suitable technique used in this validation, application and inspection is the technique TA where
there are intended for tangential radiography of generalized wall loss and the radiation source is located
offset from the pipe centre line.
d

SFD
SDD

PDD

Figure 1: Test arrangement and distances for tangential radiography with the source located on the
pipe centre line
 d

SFD
SDD

PDD

Figure 2: Test arrangement and distances for tangential radiography with the source offset from the
pipe centre line

For tangential radiography, the choice of radiation source should be determined by the maximum
penetrated thickness of the pipe, Wmax which occurs for the path forming a tangent to the pipe inner
diameter, as shown in figure 3.
d

Wmax.

Figure 3: Maximum penetrated thickness, Wmax, for the tangential radiography technique

Below (equation 1) is the mathematical equation to calculate the penetrated thickness, Wmax. Where t
is the nominal thickness of the pipe and De is the outside diameter of the pipe.

Wmax = 2 t ( De – t ) (1)

Table 1 gives recommended limits on the maximum penetrated thickness for different radiation sources.
 Table 1 – Maximum penetrated thickness range for different radiation sources for steel

Radiation Source Limits on maximum penetrated thickness

Wmax
mm
Basic Improved
(for generalized wall loss) (for pitting flaws)
X-ray (100 kV) ≤ 10 ≤7
X-ray (200 kV) ≤ 30 ≤ 20
X-ray (300 kV) ≤ 40 ≤ 30
X-ray (400 kV) ≤ 50 ≤ 35
Se 75 ≤ 55 ≤ 40
Ir 192 ≤ 80 ≤ 60
Co 60 ≤ 120 ≤ 85

For digital radiographs, somewhat higher values for the limits on maximum penetrated thickness than
those given in Table 1 may be used.

To identify and determine the appropriate radiation source for a particular pipe, the maximum
penetrated thickness, Wmax should be determine using mathematical formula (1) and compared with the
values given in table 1.

In this case, where radiographs are produced using gamma rays, the total travel–time to position and
rewind the source shall not exceed 10% of the total exposure time.

For offset tangential radiography, the true wall thickness tact at the tangential pipe position can be
calculated from the measured wall thickness tmeas using the approximate formula as below:

tact = SPD x tmeas (2)

SDD

TANGENTIAL RADIOGRAPHY USING SIMULATION

Before carrying out the actual radiographic exposure, a simulation using the software aRTist has been
done. One object is created in this simulation where the pipe schematic diagram and the details for the
dimension of the pipe is as figure 4.

Figure 5 is a preparation for the radiography simulation set-up. It consists of a radiation source Ir-192
(source strength activity 9 Ci), carbon steel object with nominal thickness 9mm and inside there is a
groove at the centre of the part measured 45% wall loss (4.95mm), the detector is white Imaging Plate
(IP) and also used the duplex wire EN 462-5 as a determinant for the calibration of the measurement.
In this simulation, the tangential radiography technique is selected to identify and determine the
thickness of the pipe wall.
 Dimensions of carbon steel pipe:
9mm T
Nominal thickness : 9mm
Outer diameter (OD) : 254mm
ID Inner diameter (ID) : 246mm
OD Thickness of groove (T) : 4.95mm

Figure 4: Schematic diagram of the pipe

Figure 5: Simulation using aRTist software

Image from the figure 6 below shows the result from the simulation. At the area where the 45% groove
is created, the thickness calculated is 4.95mm. It is mean the value for measured and calculated is equal.
Here, the simulation of Computed Radiography (CR) tangential technique for wall thickness
measurement of 10 inch carbon steel pipe was identified and determined.

Figure 6: Result radiography image from simulation

TANGENTIAL RADIOGAPHY INSPECTION AT FSO PUTERI DULANG

FSO Puteri Dulang is a facilities floating storage and transfer vessel of crude oil. It is located
approximately 175Km from the east coast of Peninsular Malaysia at Dulang field. Radiography work
is carried out here in order to identify and determine the thickness of the carbon steel pipe wall of 10
inch outer diameter with wrapping as shown in figure 7. The most suitable technique used for this test
is tangential radiograph technique.

Side A

Side B

Side A

Figure 7: Location of radiographed 10 inch pipe with wrapping

In this inspection, CR and white IP were used as an equipment and tools in order to get the image as
figure 8 below. For the figure 9, it is the set-up for the real radiography exposure.

Figure 8: Computed Radiography (CR)

Figure 9: Experimental tangential radiography set-up in real inspection

RESULT AND DISCUSSION

Figure 10 and 11 shows that the image from the result of inspection through a real exposure
experimental using computed radiography and the imaging plate as a detector. The reading are collected
from the three points at location side A and B. From table 2 was tabulated the result for wall thickness
of 10 inch carbon steel pipe.

Raw Image Location side A Point 1 Location side A

Point 2 Location side A Point 3 Location side A

Figure 10: Image at location side A of the pipe

Raw Image Location side B Point 1 Location side B

Point 2 Location side B Point 3 Location side B

Figure 11: Image at location side B of the pipe
 Table 2: Result of thickness at location side A and B
Location side Source to Measured Pipe Centre to Corrected
Detector thickness, tmeas Detector thickness, tact
Distance, SDD (mm) Distance, PDD (mm)
(mm) (mm)

A - Point 1 400 17.9 136.5 6.11

A - Point 2 400 8.84 136.5 3.02
A - Point 3 400 18.5 136.5 6.31
B - Point 1 430 19.1 136.5 6.06
B - Point 2 430 21.2 136.5 6.73
B - Point 3 430 20.5 136.5 6.51

The high priority or the critical interest area for this inspection is at the side A and the inspection at the
side B is for the confirmation average of the thickness remaining for that pipe. From the results, it shows
the remaining wall thickness of outer diameter 10 inch carbon steel pipe is around 6mm. While result
on the area at side A point 2 shows the remaining thickness is 3mm. On the thinnest area of 3mm it is
because of the high critical area of the corrosion to happen. From the results, it gives the reliable data
for the wall thickness measurement.

CONCLUSION

The tangential radiography is useful tool that allow to inspect the wall thickness with the wrapped pipe.
The image of radiography for simulation and real exposure (experiment) were analysed. The result
image from simulation shows that the calculated and measured pipe with groove 4.95mm thickness
results are equal. The result calculated is according to the standard of EN 16407-1 for tangential
radiography. The real radiography exposure (experiment) at the field were analysed and the true wall
thickness tact was calculated from the measured wall thickness tmeas based on the equation for the offset
tangential radiography. According to the standard, for digital radiographs, somewhat higher values for
the limits on maximum penetrated thickness than those given in Table 1 may be used.

ACKNOWLEGEMENT

The author wish to express their sincere thanks to Ria Solution Sdn Bhd located Paka, Terengganu for
giving us the opportunity and confidence to carry out the inspection at FSO Puteri Dulang and also
thanks to everyone who has given extensive support for this work.

REFERENCE

1. EN 16407-1: 2014, DIN EN 16407-1:2014, Non-destructive testing Radiographic inspection of

corrosion and deposits in pipes by X- and gamma rays, Tangential radiographic inspection.
2. Procedure for Computed Radiography inspection of 10”nps pipe using tangential profiling
technique (written in according to EN 16407-1:2014).
3. Radiographic evaluation of corrosion and deposits in pipelines: Results of an IAEA co-ordinated
research programme, Uwe Zscherpel, Uwe Ewert, Silvia Infanzon, Aendur, Nasser Rastkhan, P.
R. Vaidya, Isaac Einav, Sinasi Ekinci.