Investigation of Weld Integrity of X70 Grade Line Pipe by Full Scale Burst Test

Transcription

1 Investigation of Weld Integrity of X70 Grade Line Pipe by Full Scale Burst Test A. A. Shaikh 1, J. C. Purohit 2 1 Associate Professor, Mech. Engg. Dept. S.V. National Institute of Technology, Surat-7 (India) 2 Head operation of HPML, Essar Steel India Ltd, Hazira, Surat (India) Abstract In 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. INTRODUCTION During 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 CO 2 corrosion and external corrosion is on account of harsh environment of adjacent soil in case of offshore and hostile marine environment in case of offshore 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 steel grades 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, the assessment 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 are allowed 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, Bauschinger s 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. 280

2 Apart from assessing the integrity of weld, studies have shown that a full scale burst test with water or a pre-service hydro 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 uniaxial specified minimum yield stress (SMYS). In some cases a high level pressure test are conducted that resulted in 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 ( % shear area) 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 manufacturing processes, 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 and variations. II. EXPERIMENTATION AND TEST LAYOUT Two 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 pipes were 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 PRESSURE HSAW (Helical submerged arc welded) pipes as per API 5L X 70 PSL-2 with the dimensions mentioned below are considered. 281

3 Thickness, t=14.3mm; Outer Radius, r 0 = 457mm; Inner Radius, r i = ( ) mm = 442.7mm UTS, σ = 603MPa; Burst Pressure, P= σ (t/ r i ) = 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= maximum Tolerance for thickness of pipe=0.1 * thickness=1.43mm Nominal thickness, t = ( ) mm=12.9mm So maximum radius of pipe body under tolerance limit= ( ) mm=444.1 mm Bursting pressure, P= bar For r/t= minimum Tolerance for thickness of pipe=0.1 * thickness=1.43mm Nominal thickness, t = ( ) mm=15.73mm So maximum radius of pipe body under tolerance limit= ( ) mm=441.27mm Bursting pressure, P= bar 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 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; Barlow s Prediction [4,9] : Zeus Prediction [4,9] : Papka Steven Prediction [4,9] : API : 194 calculated within Range ( ) IV. RESULTS AND DISCUSSION The 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 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 the radial 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 from the 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 282

4 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 metal which 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. CONCLUSIONS It 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 the theory, 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. 283 This aspect can be controlled by streamlining the rolling mill temperature controls [12,13], and by appropriate chemistry 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 bars where the pipe actually 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 K and M F Nuruddin; Burst test Finite element analysis and structural integrity of Pipeline system, Petromin Pipeliner Jul-Sept pp38-43 [2] M.K. Graf, H.G. Hillenbrand, C.J, Heckmann, K.A. Niederhoff, High-Strength large diameter pipe for long distance high pressure gas pipelines ISOPE-2003, May26-30, 2003, Honolulu, Hawaii, Conference proceeding Paper No 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, Eupropipe publication, Yr 2000, pp 1-12 [4] A. Liessam, M. K. Graef, G. Knauf, U. Marewski, Influence of thermal treatment on mechanical properties of UOE linepipe, 4th International 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 Hydrotest be Eliminated, Pipes & Pipelines International Vol. 45, No. 4, July- August 2000, pp 1-19 [7] Leo Corcoran CEng MBA FIEI, Report on Corrib Gas Pipeline Design October 2005, pp 1-6 [8] Dr. Ronald Rittmann and Dr. Klaus Freier, Niobium Containing Steels For Spiral And Electric Resistance Welded Line Pipe Production, Development publication by Salzgitter Flachstahl GmbH 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 Polar Engineering Conference Honolulu, Hawaii, USA, May 25 30, 2003, ISBN (Set); ISSN (Set), pp 50-59

5 [10] John F. Kiefner and Willard A. Maxey, Periodic Hydrostatic Testing 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, Hydrostatic Burst Test 0F X80 Grade Steel Pipe, Journal of Loss Prevention in the Process Industries, Elsevier, 22 (2009) [12] Guedri, B. Merzoug, A. Zeghloul, Improving Mechanical Properties Of Api X60/X70 Welded Pipeline Steel Sciences & Technologie B N 29, (Jun 2009), pp , [13] Choong-Myeong Kim, Jong-Bong Lee, Jang-Yong Yoo, A Study on 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, (Set), ISSN (set), pp