IJETAE_1212_51

7/30/2019 IJETAE_1212_51

1/5

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December 2012)

280

Investigation of Weld Integrity of X70 Grade Line Pipe by FullScale Burst TestA. A. Shaikh1, J. C. Purohit2

1Associate Professor, Mech. Engg. Dept. S.V. National Institute of Technology, Surat-7 (India)2Head operation of HPML, Essar Steel India Ltd, Hazira, Surat (India)

AbstractIn the oil and gas industry, high pressure long

distance pipe lines are widely used for efficient and effective

movement of natural gas, oil and other petroleum products.

The gases are transported at extremely high operating

pressures that can range from 200 to 1600 PSI depending on

the size and length of the line pipe. For operating safety,

Hydrostatic test is used for confirming mechanical integrity of

pipe lines and is applied to check for leakages before putting

the pipe lines in use. The test aims to evaluate the point of

failure of the material and assess the structural integrity of a

welded pipe.

In present work, an experiment is carried out to confirm

weld integrity of two X70 grade carbon steel pipes under the

manufacturing process of hot rolling with left and right hand

spiral plants. The tests are conducted at pressure in excess of

the UTS of the steel/ pipes and aimed to sustain the pressure

as per API convention. Both the pipes attained the expected

results; however the results of theoretical predicted pressure

minutely diverge as the thickness variation is not considered.

Keywords- Burst, HSAW, Tolerance, UTS, Weld Integrity.

I. INTRODUCTIONDuring the actual useful life of a Pipeline [1], it is

exposed to internal and external corrosion. Internal

corrosion is on account of hydrocarbon fluids which are

susceptible to CO2 corrosion and external corrosion is on

account of harsh environment of adjacent soil in case of

offshore and hostile marine environment in case of off-

shore pipe lines. These corrosion leads to wall thinning

process over periods of time. This leads to weakening of

mechanical strength when the pipelines are subjected to

high pressure conditions for longer periods of use.

Over period of time, macro-economic considerations [2]

have influenced the development of higher strength steelgrades for pipeline use with lower wall thickness. As a

result steel pipes of grades with strength levels X80, X100

and lately X120 are being proposed by pipeline designers.

Accordingly attempts are made to improve the grade of the

steel in order to have better mechanical properties. As

manufacturing of higher strength pipes of smaller wall

thickness becomes feasible, pipes would become lighter in

weight reducing the cost significantly.

To ensure better quality material, its testing becomes

mandatory. A large number of mechanical testing are

conducted to analyze the properties of the pipe and to

predict its bearing capacity.

Researcher [3] shows that pipelines with higher strength

levels are subjected to a variety of mechanical tests. For

pipelines designed for high operating pressures, theassessment of fracture behavior has been studied by means

of laboratory and full scale tests. Researchers have

attempted to define safety criteria against a possible

fracture by determining various fracture properties by

means of studying the initiation and propagation of fracture

upon failure. Also the influence of higher impact energy of

pipe to resist such failures has contributed such pipes to

sustain such hostile operating conditions.

Burst test [4] forms a major part of the entire testing

schedule in such pipes. During operating condition, the

primary load on any pipeline is the internal pressure.

Design pressures that result in hoop stresses in the pipe as

high as 80% of the specified minimum yield strength areallowed by many pipeline codes. So to ensure sufficient

pressure capacity, a series of burst tests are conducted. The

objective of the burst tests is to ascertain the fracture

behaviour of the pipes, integrity of the weld and thus

understand the ductile fracture propagation control for the

pipes so as to assure complete safety in their application.

During the formation of a pipe [5], the steel loses its

strength during the forming process due to mechanical

work hardening, Bauschingers effect and development of

corresponding residual stresses. The tendency of steel

softening becomes more prominent in steels of higher

grade, typically X70 and above. The consumable selection

during the welding process must therefore be ensured to

match this effect, and becomes very important aspect for

weld integrity.

The pre-service hydro test [6]or a full scale burst test with

water has been widely used to combat the consequence of

failure experienced as a result of pipeline testing using air

or product. In more recent years the purpose of the test has

moved from being a leak tightness test to one which

benefits the pipeline as a structural system.

7/30/2019 IJETAE_1212_51

2/5



281

Apart from assessing the integrity of weld, studies have

shown that a full scale burst test with water or a pre-servicehydro test offers several advantages: Such as any unusual

mid-wall lamination in the steel. In many cases,

manufacturing defects such as leaks concentrated at the

weld seam leading to surface morphology of pipes and

metallurgical properties of steel as revealed in the

Hydrogen induced cracking tests can be ascertained. Any

localized weld heat and the impact on property

misalignment at the weld leading to subsequent

susceptibility (acute) to hydrogen embrittlement can be

foreseen. Field pressure testing has been done usually at a

pressure that induced a hoop stress of less than the uni-

axial specified minimum yield stress (SMYS). In some

cases a high level pressure test are conducted tha t resultedin a hoop stress equivalent or greater than the uniaxial

SMYS.

During the actual operation, a line pipe experiences

cyclic pressure condition [7] which tend to fatigue the

material construction. Under various operating conditions,

pipe line experience different hoop stresses. Accordingly

designers recommend fixation of the maximum allowable

hoop stress compensating a fatigue cycle testing under

various design codes and recommended practices by the

American Petroleum institute.

Use of micro-alloying elements [8] such as Niobium has

led to improved weldability, particularly in field welding,

higher operating pressures and a decrease in wall thickness.

Pipelines made with lower wall thickness exhibit ease of

handling, lower cost of construction, in addition to high

impact toughness and the capability to arrest cracks

assuring high safety standards of operating pipelines which

have variable pressure conditions.

Burst resistance [9, 10, 11] of the pipe is related to the

Ultimate tensile strength of the pipe material, which

comprises of the Steel, Weld-seam and the Heat affected

zone. Since each of these zones of a pipe possess different

strength, the full-scale burst test reveals a composite

integral response of the pipe to a high pressure condition.

The control of ductile fracture initiation and propagation

has been a concern for regulatory agencies, it has been

studied that pipes with low toughness and lower ( % sheararea) tend to crack easily in-spite of their higher UTS. A

correlation therefore with impact values of weldment or

base steel and also with residual stresses developed during

the pipe forming process helps to ascertain crack

propagation mechanism.

Since limited experience on the characteristic of fracture

resistance was available for the given pipe manufacturingprocesses, in the experiment which is presented here, two

pipes manufactured from two different machines were

selected and subjected to full scale burst test.

The Pipes selected were as of API 5L X70 (480 MPa

yield strength) grade and were subjected to full scale field

burst test. Both the pipes were made from two opposite

direction left and right hand spiral plants in size

914mm*14.3mm [from HSAW-1 (Helical submerged arc

welded) and HSAW-2 machines]. Upon bursting it was

found that while the pipes burst well above the calculated

burst pressure; one of the pipe burst at a pressure very close

to the design or calculated burst pressure. Detailed study

was conducted to explore possible explanations andvariations.

II. EXPERIMENTATION AND TEST LAYOUTTwo pipes selected were manufactured using the SAW

process in two steps. The first step comprised of forming

followed with simultaneous GMAW welding and the

second stage comprised of off-line submerged arc welding.

To check the mechanical integrity of the pipes, the weld

seam on the pipes were subjected to X-ray and ultrasonic

testing for 100% of length. In addition the pipes were

subjected to Hydrostatic test for 15 seconds and also cyclic

hydro test for 24-hours before burst test.

Prior to burst testing, the two open ends of the pipeswere closed with metallic dish ends of thickness higher

than the wall thickness of the pipes. The dish end weld was

tested using NDT methods for its integrity and thus making

the complete sealing of the pipes. Pipe plugs were fitted on

the dish ends for filling the pressurizing media. The

pressurizing medium chosen for the test was water. For

safety, prior to filing water, the pipes were lowered into a

deep pit so that any unwarranted incident upon release of

high pressure water during pipe bursting can be proactively

avoided.

A high pressure, high volume flow pump was connected

to the sealed pipes and water was allowed to fill in. High

pressure water pump capable of delivering 28 liters/min

with 965 bar maximum capacity propelled by 55 kW motor

was used for the purpose.

III. EXPECTED BURST PRESSUREHSAW (Helical submerged arc welded) pipes as per API

5L X 70 PSL-2 with the dimensions mentioned below are

considered.

7/30/2019 IJETAE_1212_51

3/5



282

Thickness, t=14.3mm; Outer Radius, r0 = 457mm; Inner

Radius, ri = (457-14.3) mm = 442.7mmUTS, = 603MPa;

Burst Pressure, P= (t/ ri) = 194 bar

Considering API convention tolerance of 10%, the burst

pressure prediction from calculated formulae was

calculated to provide the experiment with a bursting

pressure range.

HSAW-1 and 2 pipes

Considering the wall thickness tolerances of pipe used, a

range of values of maximum and minimum bursting

pressure for the given value of Ultimate Tensile Strength,

603 MPa can be calculated as;

For r/t= maximumTolerance for thickness of pipe=0.1 * thickness=1.43mm

Nominal thickness, t = (14.3-1.43) mm=12.9mm

So maximum radius of pipe body under tolerance limit=

(457-12.9) mm=444.1 mm

Bursting pressure, P=175.16 bar

For r/t= minimumTolerance for thickness of pipe=0.1 * thickness=1.43mm

Nominal thickness, t= (14.3+1.43) mm=15.73mm

So maximum radius of pipe body under tolerance limit=

(457-15.73) mm=441.27mm

Bursting pressure, P= 214.95bar

Hence for a given tolerance of +10% on the wall

thickness of pipe, the expected burst pressure for HSAW-1

and HSAW-2 pipes can range between 175-215 bar for a

given average UTS of 603 MPa

The expected burst pressure in bar can be calculated

based on formulae available in literature and as per API

convention are listed below;

Barlows Prediction[4,9]:188.69

Zeus Prediction[4,9]:191.64

Papka Steven Prediction[4,9]:191.68

API : 194 calculated within Range (174-215)

IV. RESULTS AND DISCUSSIONThe actual burst pressure observed for both pipes with

same dimension, same material with welded on different

spiral plant are depicted in Table I.

TABLE I

Sr Pipes Dimensions Actual Burst

Pressure (bar)

1 HSAW 1 914x14.3 mm 194

2 HSAW 2 914x14.3 mm 180

Subsequent to the filling of the water, and continuously

increasing water pressure, the first pipe made from the

HSAW-1 unit burst at 194 bar pressure. Upon examination

of the failure,as shown in Figure 1, it was observed that the

crack was apparently a ductile fracture initiated from the

body of the pipe and traversed in two opposite longitudinal

directions parallel the pipe axis and perpendicular to theradial hoop stresses as the dimension detail is highlighted

in Figure 2.

The crack however did not propagate beyond the weld

seam, which meant that not only was the weld seam strong

enough, but it also acted as a crack arrestor. The second

pipe made from the HSAW-2 unit also revealed a similar

failure characteristic and crack propagation; however it

burst at 180 bar pressure which was lower than expected.

During the pressuring cycle of HSAW-2 pipe, it was

observed that the pressure did not increase for a significant

period of time, and the pipe burst after a long 130-minute

wait as shown in Figure 3 and the dimension detail is

shown in Figure 4. Before bursting, the pipe bulged fromthe centre like a balloon until it finally gave way.

Fig 1. Photographic view of HSAW 1 burst pipe

Fig 2. Dimensional detail of HSAW 1 burst pipe

7/30/2019 IJETAE_1212_51

4/5



283

Fig 3. Photographic view of HSAW 2 burst pipe

Fig 4. Dimensional detail of HSAW 2 burst pipe

The criteria for judging the initiation point of the crack is

from that portion of the pipe which has maximum bulge.

On observing the pipes, it is found that pipes of HSAW-1

and HSAW-2 both had the crack initiated at the base metalwhich indicates that the weld joint of each pipe is defect

free and stronger than the base metal. Fractured surfaces of

all the pipes show ductile failure as the surfaces appear

rough and show patches of cup and cone structure.

V. CONCLUSIONSIt is found that the thickness is the most sensitive

parameter for the calculation of burst pressure calculation.

In practice that localized wall thinning can cause bursting

at lower pressures. Therefore in pipeline applications

tolerance considerations play an important role in design.

Practical demonstration of a full scale burst test

represent results very close to those warranted by thetheory, which reflect consistency in the pipe manufacturing

process.

In case of the pipe made from HSAW, one of the

probable reasons for pressure not increasing beyond a

certain value was due to the plastic deformation, which

suggests localized lower yield strength. Due to increase in

the volume of pipe on expansion could accommodate

greater amount of the fluid.

This aspect can be controlled by streamlining the rolling

mill temperature controls[12,13]

, and by appropriatechemistry design.

There might be little divergence in the calculation of

minimum burst pressure which may be on the account of

the exact value of UTS or the diameter (external instead of

internal yields wrong results). This is because the actual

radial stresses act upon the ID of the pipe.

The another reason for divergence may be the presence

of air pocket inside the pipe prior the experiment.

Therefore on increase of pressure the air present inside may

be shrinking gradually, resulting in the pressure being

constant.

On taking tolerances into consideration, the minimum

expected pressure comes out be 175.16 bars where the pipeactually burst at 174 bars. The pressure has reached up to

179 bars before the failure. This means that the pipe has

attained its UTS value and then plastically deformed

leading to drop in pressure before failure.

REFERENCES

[1] Nasir Shafiq, Mokhtar Che Ismail, Chanyalew Taye, Saravanan Kand M F Nuruddin; Burst test Finite element analysis and structural

integrity of Pipeline system, Petromin Pipeliner Jul-Sept-2010-pp38-43

[2] M.K. Graf, H.G. Hillenbrand, C.J, Heckmann, K.A. Niederhoff,High-Strength large diameter pipe for long distance high pressuregas pipelines ISOPE-2003, May26-30, 2003, Honolulu, Hawaii,

Conference proceeding Paper No. 2003-SMYP-03 pp 1-9

[3] G. Demofonti, G. Mannucchi, C.M. Spinelly, L. Barsanti, H.G.Hillenbrand, Large diameter X100 gas line pipes Fracture

propagation evaluation by full-scale burst test, Eupropipepublication, Yr 2000, pp 1-12

[4] A. Liessam, M. K. Graef, G. Knauf, U. Marewski, Influence ofthermal treatment on mechanical properties of UOE linepipe, 4thInternational conference on Pipeline technology, May-9-12, 2004,

Ostend, BelgiumTavel, pp1-18

[5] I.Yu. Pyshmintsev, D.A. Pumpyanskyi, L.G. Marchenko, V.I.Stolyarov, Strength and Bauschinger Effect in TMCP Line Pipe

Steels, Publication of Russian Research Institute for the Tube and

Pipe Industries, Russia.2004, pp 1-5

[6] Mike Kirkwood, Andrew Cosham , Can the Pre-service Hydrotestbe Eliminated, Pipes & Pipelines International Vol. 45, No. 4, July-August 2000, pp 1-19

[7]

Leo Corcoran CEng MBA FIEI, Report on Corrib Gas PipelineDesign October 2005, pp 1-6

[8] Dr. Ronald Rittmann and Dr. Klaus Freier, Niobium ContainingSteels For Spiral And Electric Resistance Welded Line Pipe

Production, Development publication by Salzgitter Flachstahl

GmbH 38239 Salzgitter, Germany, pp 1-20

[9] S. D. Papka, J. H. Stevens, M. L. Macia, D. P. Fairchild, C. W.Petersen , Full-Size Testing and Analysis of X120 Linepipe,

Proceedings of The Thirteenth International Offshore and PolarEngineering Conference Honolulu, Hawaii, USA, May 25 30,

2003, ISBN 1 880653 60 5 (Set); ISSN 1098 6189 (Set), pp

50-59

7/30/2019 IJETAE_1212_51

5/5



284

[10] John F. Kiefner and Willard A. Maxey, Periodic HydrostaticTesting Or In-Line Inspection To Prevent Failures From Pressure-Cycle-Induced Fatigue, Report for American petroleum institute,

2000, pp 1-9

[11] Duan Qingquan, Zhang Hong,Yan Feng,Deng Changyi, HydrostaticBurst Test 0F X80 Grade Steel Pipe, Journal of Loss Prevention in

the Process Industries, Elsevier, 22 (2009) 897900

[12] Guedri , B. Merzoug , A. Zeghloul, Improving MechanicalProperties Of Api X60/X70 Welded Pipeline Steel Sciences &Technologie BN29, (Jun 2009), pp 51-58. 51,

[13] Choong-Myeong Kim, Jong-Bong Lee, Jang-Yong Yoo, A Studyon the Metallurgical and Mechanical Characteristics of the Weld

Joint of X80 Steel, Proceedings of the fifteenth International

offshore and polar engineering conference, Seoul, South Korea, June

19-24, 2005, ISBN, 1-880653-64-8 (Set), ISSN 1098-6189 (set),

pp158-162

Documents

IJETAE_1212_51