Robert M. Andrews

A Single Specimen CTOA Test Method for Evaluating the Crack Tip Opening Angle in Gas Pipeline Steels

Failure information from recent full-scale burst experiments on modern TMCP gas pipeline steels having a yield strength level of 690MPa and higher has shown that the CTOA fracture criterion can be effectively used to predict the arrest/propagation behaviour of the pipe against possible axial ductile fractures. The use of CTOA as an alternative or an addition to the Charpy V-notch and DWTT fracture energy in pipelines is currently under review. A significant difficulty currently limiting the more extensive use of CTOA in pipeline assessment is its practical evaluation either in the real structure or in a laboratory scale test. Different combinations of experimental and finite element analyses have been proposed for the measurement of the CTOA of a material. Although most of these models are able to predict the CTOA effectively, their implementation requires extensive calibration processes using the test load-deflection data. The authors have recently developed a novel test technique for direct measurement of the steady state CTOA using a modified double cantilever beam geometry. The technique uses optical imaging to register the uniform deformation of a fine square grid scored on the sides of the specimen. The slope of the deformed gridlines near the crack tip is measured during crack growth from captured images. Its value is a representative of the material CTOA. This paper presents recent results from the implementation of the technique to determine the steady state CTOA (steady state in this work refers to regions of ductile crack growth where CTOA values are constant and independent of crack length) of API X80 and X100 grade gas pipeline steels. In each case the approach was able to produce large amounts of highly consistent CTOA data from both sides of the test sample even from a single specimen. This extensive data set allowed an evaluation of the variance of the stable CTOA as the crack grew through the microstructure. The test method generated a steady CTOA value of 11.1° for X80 and 8.5° for X100 steels tested, respectively.Copyright

The Fracture Arrest Behaviour of 914 mm Diameter X100 Grade Steel Pipelines

The drive to reduce the installed cost of high-capacity long-distance pipelines has focused attention on increasing the strength of the pipe material, in order to reduce the tonnage of material purchased, transportation and welding costs. In parallel with developments in plate rolling and pipe fabrication, the properties and performance of prototype pipe materials and construction welds have already been extensively evaluated. While these studies have provided considerable confidence in the performance of X100 pipe, a major remaining issue in the introduction of such steels has been an understanding of the resistance to propagating fractures. The scarcity of relevant fracture propagation data and concerns about the measurement and specification of toughness in high strength steel pipes have led to doubts that the existing methods for control of ductile fracture can be extrapolated to X100 strength levels. In order to provide experimental data on which to base fracture control approaches, a Joint Industry Project has been undertaken using conditions representative of potential applications. Results are presented from two full-scale fracture propagation tests on 914mm pre-production grade X100 pipes pressurised using natural gas. The full-scale results are compared with small-scale test specimen data and also with results from other full-scale tests on high strength steel pipes. This provides a valuable insight into the fracture response of these materials. Information has also been obtained concerning the predictive capability of current gas decompression models. These results provide a contribution to the development of fracture control plans in pipelines using X100 steel linepipe.Copyright

Experimental Study of Thickness and Fatigue Precracking Influence on the CTOA Toughness Values of High Grade Gas Pipeline Steel

Recent experimental and computational studies have revealed that the CTOA fracture criterion has the potential to assess steady state ductile rupture resistance during large amounts of crack growth. One of the major difficulties currently limiting the more extensive use of CTOA is its practical estimation either on a real structure or in a laboratory scale test. The authors have recently developed a novel test technique for direct measurement of CTOA. It is a single CTOA test (SCT) method using a modified double cantilever beam (DCB) specimen. Photographic and video techniques are used to register the progression of the crack tip in real time on the faces of the DCB. The CTOA profile is estimated from the uniform variations of the slope of a reference fine mesh scored on the surface of the specimen. Its value is representative of the material CTOA. So far the method has been successfully tested on gas pipeline steels of grade API X80 and X100 and 6000 series aluminium alloy. In all tests, it generated large amounts of highly consistent CTOA data, even from a single test specimen. This paper describes recent results from the CTOA testing of X100 steel specimens with different ligament thicknesses in the range of 8 to 12 mm, equivalent to 42 to 63% of the original pipe wall thickness. In total 18 sets of CTOA test data were obtained from opposite sides of fractured specimens. Analysis of these showed that the CTOA values were thickness independent in the examined range. An average CTOA value of 8.5° was found for X100 steel in the steady state crack propagation phase. The development of slant fracture associated with the steady CTOA region was completed after an average crack growth of 1.3–2.7 times the specimen ligament thickness. The fatigue precracking resulted in a 10–23% drop in the initiation load depending on the gauge thickness of the specimen and the initial crack length.Copyright

Measurement and modelling of the crack tip opening angle in a pipeline steel

Recent developments in the control of propagating ductile fractures in gas pipelines have proposed using the Crack Tip Opening Angle (CTOA) as a measure of fracture resistance. This is attractive as it can be related directly to the geometry of the fracturing pipe and also can be implemented easily in finite element models of the propagating fracture process. Current methods of determining CTOA in linepipe have been based on the standard DWTT specimen. This geometry often does not allow a fully slant fracture to develop, and is loaded in bending rather than tension. A novel specimen design has been developed to measure CTOA under quasi-static conditions and applied to a X80 (Grade 555) pipeline steel. The experimental work involved development of the design to ensure crack path stability. CTOA was obtained directly by measurement from video images. The CTOA values dropped from an initially high value to a steady state value of about 8 degrees when fully slant crack growth was achieved. This required crack growth over a distance of about 5 to 12 times the test section thickness. The crack growth was modeled numerically using the Gurson ductile void growth material model. The finite element modeling was able to qualitatively reproduce the crack path instability observed in practice, and the fall of CTOA from the initial high value to a steady state condition. Although further work is required to improve the modeling, the work carried out to date has demonstrated that there is the potential to apply damage mechanics methods to predict the laboratory specimen response and then to model the structural response.Copyright

Analysis and Testing of a 13Cr Pipeline to Demonstrate “Leak Before Break”

As part of a safety case for a subsea 13Cr pipeline, the operator wished to demonstrate that if a circumferential through wall crack developed, the crack would remain stable as a leak rather than growing to a full bore rupture. An initial fracture mechanics analysis had suggested that the margins on crack length were too small to make such a “leak before break” argument. This paper reports an integrated programme of small scale testing, numerical modelling and full scale testing which showed that a leak before break case could be made.13Cr martensitic steel generally shows excellent toughness at the service temperature, as does the super duplex weld metal that was used for the girth welds. However, as the pipeline had been installed by reeling, there was some concern that the toughness may have been reduced. Hence a programme of fracture toughness testing was designed to generate tearing resistance curves for both as-received and pre-strained parent material and weld metal. Deep and shallow through thickness notched specimen geometries were tested to explore the effect of constraint on the toughness. Finite element analysis was used to predict the stress intensity for a range of crack lengths, including the effects of misalignment. Non-linear analyses were used to estimate the limit load for the cracked pipe. The test results were used as input to tearing analyses to Level 3 of BS 7910. These showed that the tolerable length of a through wall crack exceeded the length of anticipated defects by a factor of at least two.To confirm the fracture mechanics predictions, two full scale tests were carried out. These used pressure cycling to grow a through wall crack by fatigue. These cracks were stable under an internal pressure equal to the pipeline design pressure. The cracked specimens were then axially loaded to failure. Extensive tearing occurred before final failure at loads above those predicted by the fracture analysis, confirming the conservatism of the predictions.Copyright

The Significance of Low Toughness Areas in the Seam Weld of Linepipe

There are concerns that there may be areas of low toughness in the seam welds of submerged are welded linepipe. These areas are typically associated with the Coarse Grained Heat Affected Zone and manifest themselves through low values obtained in Charpy impact and crack tip opening displacement (CTOD) tests. Although it is possible to locate areas of low toughness in linepipe seam welds, it is not clear if these are structurally significant. If it can be shown that low toughness areas in the seam weld HAZ do not affect the fitness for service of the pipe as a structure, these could be accepted for use. Under funding from PRCI, a study has been carried out to investigate this problem quantitatively. Experience in the offshore structural field, where the similar problem of local brittle zones in weld HAZs has received considerable attention, was reviewed. A constraint based fracture mechanics analysis was developed using the T-stress approach. Cracked body finite element analyses were used to obtain the T-stress for a range of surface breaking and buried defects in typical linepipe geometries. The results from these models were used to develop a constraint modified Failure Assessment Diagram for a fracture analysis. Fracture analyses showed that the structural constraint is low and failure will occur by plastic collapse for practical seam weld defect sizes. This shows that even when the seam weld toughness is very low, the dominant failure mode for the structure will be plastic collapse. Hence the low toughness values obtained in fracture mechanics tests are not structurally significant for practical defect sizes likely to occur in linepipe.Copyright

Design of Pipeline Damage for the BP X100 Operational Trial

As part of an ongoing pipeline technology program for BP Alaska, a 1 km, 48-inch diameter, X100 demonstration pipeline was constructed and operated for a period of two years. Artificial defects were introduced into one of the two test sections. These defects were intended to demonstrate that current assessment methods could be used to predict the behaviour of pipeline defects in a very high strength steel under realistic conditions including accelerated pressure cycling and a range of cathodic protection levels. The defects included in the trial were volumetric corrosion, mechanical damage, arc strikes and girth weld defects. The volumetric corrosion defects included both isolated defects and pairs of interacting defects. All the defects and details such as the girth welds were assessed for fatigue failure in addition to failure at the Maximum Operating Pressure. This paper describes the design of the defects for the trial. The defects were designed to be close to failure, so as to provide a realistic test of the predictive methods. Current methods were used including the Pipeline Defect Assessment Manual (PDAM) and ongoing work sponsored by PRCI.Copyright

Estimation of Slant Tearing Energy for High-Grade Pipeline Steel From Instrumented Charpy Test Data and its Transferability to Large Structures

For several decades, the Charpy upper shelf energy has been used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. However, significant discrepancies have been observed between the results of full-scale burst experiments on modern pipeline steels and those predicted by Charpy-based fracture models. This indicates that fracture models calibrated in the past on lower-grade pipeline steels (Charpy toughness below about 100J) cannot be extrapolated beyond their calibration range to assess the fracture behaviour of higher-strength high-toughness steels. One reason for this is the high level of energy often required for crack initiation in these steels. Accordingly, in the short term different correction factors ranging from 1.4 to 2 have been proposed to refine these fracture prediction models. The use of alternative failure parameters like CTOA is currently under review. In this paper a novel experimental technique is given to apportion the upper shelf Charpy fracture energy into its different components, i.e. crack initiation energy and flat and slant tearing energy. The experimental data from instrumented Charpy tests on standard impact specimens made from an X100 grade pipeline steel is used to estimate crack initiation and propagation energy. The areas associated with flat tearing in the centre and slant shearing at the edges of the fracture surface of Charpy test specimens are estimated optically using a fine measurement grid with 0.5 mm spacing. The energy required for generating the flat and slant fracture areas is calculated by the use of associated multipliers, i.e. the specific flat and slant fracture energy (in terms of J/mm2 ). These are measured separately using flat and slant crack growth data from fracture tests on standard C(T) and modified DCB like specimens. The results showed that the Charpy energy from a test is dominated by non-crack propagation energies. Around 36% of the measured impact energy appeared to be associated with flat and slant tearing processes. As the latter is the important failure micro-mechanism in pipeline steel only that part of the overall Charpy shelf energy which is associated with slant shearing might be used to evaluate the crack growth resistance of modern steels. This suggests the possible use of correction factors for high toughness pipeline steels of the order of 1.7 to transfer the slant fracture energy measured on small-scale specimens to the real structures for predicting their crack arrest/propagation behaviour. The correction factor proposed here from the laboratory test programme agrees with those obtained from costly full-thickness burst experiments on similar class of pipeline steel.Copyright

Assessment of Corrosion Defects in Old, Low Toughness Pipelines

Corrosion metal loss in one of the major damage mechanisms to transmission pipelines worldwide. The remaining strength of corroded pipe subjected to internal pressure loading has been extensively researched and guidelines for assessing corrosion are well defined. Methods including ASME B31G, RSTRENG and LPC have been developed, validated and matured to the extent that they are now incorporated in standards and regulatory requirements. However, these methods are based on the assumption that the pipe fails via a ductile mechanism, i.e., the line pipe material has sufficient toughness to prevent a toughness dependent failure. This limits the application of the existing methods to materials that have sufficient toughness. It is possible that some older pipelines operate with the material in the ductile / brittle transition region of the Charpy transition curve, or even on the lower shelf. It is also possible that under fault conditions, a pipeline normally operating on the upper shelf could be temporarily in the transition region. In these circumstances, existing assessment criteria may be non-conservative. At present there are no rigorous criteria available for assessing corrosion defects in low toughness pipe. This paper presents an approach for removing the uncertainty in the use of existing methods for assessing corrosion defects in older, low toughness pipelines based on the Beremin approach to brittle cleavage fracture. Comparison of the Beremin results with existing assessment methods allows an ‘effective transition temperature’ to be defined as the temperature at which the existing method is no longer conservative. The results suggest that, for the corrosion defects investigated, the effective transition temperature is sufficiently low that existing assessment methods will remain conservative.Copyright

A Time Dependent Model for Assessing the Significance of Mechanical Damage

A small but significant number of pipeline failures occur each year and many in populated regions arise as a result of mechanical damage from external interference. These failures generally occur at the time of damage, but a small number are delayed, with the final failure reportedly taking place following periods of up to 40 years from the original damage. This type of failure is of concern to pipeline operators as they occur without warning. The mechanisms governing time-delayed failures are poorly understood due to the extremely limited experimental data available. A greater understanding of the failure mechanism would help operators to improve their strategies for managing external interference damage. In the event of combined dent and gouge mechanical damage, the model described in this paper provides an approach for the assessment of the time dependent failure modes of the defective pipeline. The model, developed using Matlab code, is capable of investigating a complex load history consisting of constant internal pressure hold periods and cyclic fluctuations and can predict time dependent phenomenon such as pressure reversals sometimes experienced following hydrostatic pressure testing. The approach is based on the BS 7910 material specific Failure Assessment Diagram (FAD) in conjunction with R5 (British Energy) creep procedures. The result is a time dependent FAD which allows the combined influence of time dependent material properties (toughness, strength and cold creep) and loading to be investigated.Copyright