Any catastrophic rupture scenarios of a steel pipe should be taken into considerations in the design and during the maintenance stage as the loss-of-containment may be accompanied by either property damage or fatal accidents. Ductile fracture of wrinkled (buckled) steel pipes on the tensile side of the cross-section is studied in this research as the most plausible case of ultimate failure for pressurized buried pipelines being subjected to monotonically increasing curvature. The results from two full-scale bending tests on X80 line pipe specimens that are pressurized up to 60% of specified minimum yield strength (SMYS) are considered as an input for the current study. The specimens possess the same dimensions and are made of X80 steel grade with different yield strength to tensile strength ratios (Y/T) of 90% and 83%. The specimen with higher Y/T ratio ruptured on the tensile side of the cross-section while experiencing post-buckling deformations. However, the specimen with lower Y/T ratio was unloaded after the formation of the local buckling.
Finite element analysis (FEA) of the full-scale tests were conducted and verified using the experimental data. The power law is calibrated to model the post-necking plasticity of steel using material test data, and, cumulative fracture criterion in conjunction with general fracture strain locus for the pipelines’ high-strength steel is implemented to predict the ductile fracture initiation in the pipe's wall. It is shown that the FE model accurately reproduces the load-displacement response and final rupture of the specimen with the higher Y/T ratio. For the other specimen, numerical simulation shows no rupture until the inner surface of the buckle comes into contact with itself which reveals that the lower Y/T ratio reduces the chance of rupture. Further numerical studies postulate that both Y/T ratio and internal pressure have a coupled effect on the rupture of wrinkled pipes and play a key role in triggering that kind of failure. That is, higher values of Y/T ratio and internal pressure increases the probability of the rupture of wrinkled pipes.
Single Edge Notched Tension (SENT) fracture toughness test specimens are being used for a wide range of applications due to the higher fracture toughness that is measured by SENT specimens compared to an equivalent specimen under bending (SENB). The testing of SENT specimens is now standardized in BS 8571:2014 (BSI, 2014) and there is potential to use SENTs for high and low temperature tests, in a wider range of material thickness, and for different applications.
This paper describes the validation of a method for carrying out SENT tests at very low temperatures, using threaded ends to allow testing inside a temperature controlled test chamber, while preventing the specimen from yielding at locations away from the intended notch tip. Two designs of threaded-end specimens were assessed for their feasibility; directly machining a round threaded portion onto a square section specimen is the simpler approach, while the alternative is attaching to the square section specimen (in this case by friction welding), a wider diameter round bar for threading.
Numerical models of the notch location and of the threads under load were compared for their respective strain capacity. A successful SENT specimen design has greater strain capacity in the threaded end than around the notch. The results of the numerical models were compared to experimental tests on both designs with different notch depths.
Machined threads were shown to be limited to applications with low weld strength over-match, deeper notch depths and single point fracture toughness testing (including multi-specimen R-curve testing). Friction welding the threaded portion allowed threaded-end SENT specimens to be successfully used for unloading compliance tests R-curve tests, shallower notched specimens and where there is weld strength over-matching.
Single Edge Notched Tension (SENT) fracture toughness test specimens (Fig.1) are now being used for a wider range of applications. The test specimen became established for use with fitness-for-service assessments of flaws in pipeline girth welds under high strain conditions during installation, as described in DNV-RP-F108:2008 (DNV, 2008), and were later further established for all subsea pipelines in DNV-OS-F101:2013 (DNV, 2013). The lower constraint of the SENT specimen, compared to the historically more common Single Edge Notched Bend (SENB) specimen, results in higher values of fracture toughness being measured in SENTs at ambient temperature.
There is an ongoing debate in the industry about the requirement for Fatigue Crack Growth calculations (FCG) as part of a holistic fracture mechanics based ECA in preference to traditional S-N damage, which is not yet wholly sorted out. At present the methods are complementary and engineers will usually find that both S-N fatigue and FCG as part of the ECA are available for use, especially for HP/HT pipelines or pipelines that are installed using the reeling method. S-N fatigue calculations are relatively straight-forward and always used in design stage whilst fatigue crack growth is more involved computationally but more realistic and mainly used for fabrication and installation stages.
It is proposed that FCG should be carried out instead of S-N fatigue life estimates, where Fatigue Crack Growth calculations are practicable and necessary and all the appropriate input has been collected. This paper includes a brief history of S-N fatigue and its characteristics and the background to fracture-mechanics based fatigue crack growth calculations before demonstrating with practical, numerical examples that, for subsea pipeline and riser girth welds, the Fatigue Crack Growth method as part of ECA is preferable and provides a more realistic picture compared to an S-N fatigue damage assessment whilst bounding the design on the safe side by the use of conservative Paris Law parameters.
It is recognized by practitioners that S-N calculations merely give an estimate of fatigue life whereas fracture mechanics provides a method for calculating and quantifying the time history of a flaw’s growth under the influence of cyclic stress. The ECA approach used by installation contractors includes requirements for testing and accumulation of analysis input data such as material toughness values, J-R curves and the like, which fulfils all the requirements for enabling reliable fracture mechanics evaluations.
ECA enables the provision not only of post-weld allowable defect curves but also installation holding periods for the pipeline and riser systems that would be required during project execution and the influence of effective fatigue crack growth calculations on decisions that need to be made in the field will be explained in the paper.
The approach of using FCG has been successfully used for many years on live projects, otherwise offshore operations could have been hampered and/or significant costs incurred.
Modern subsea pipelines are now routinely exposed to high strains. When above yield they receive appropriate attention since Engineering Critical Assessment (ECA) derived flaw acceptance criteria would normally be considered mandatory. For stress based designs, (applied axial strain up to 0. 4%), standard flaw acceptance criteria, that originated with empirically based workmanship levels, will be applied unless an optional ECA has been performed. Some of the best current guidance for ECA on pipeline girth welds can be found in DNV-OS-F01 [
In this paper the tolerability of the DNV-guided workmanship level flaw sizes has been assessed using ECA. The methodology used in this study is in line with the guidance as per DNV-OS-F101 (2013) [
New initiatives have recently been launched, (e.g. TWI) to standardize ECA for pipeline girth welds and based on the findings here it is recommended that these studies also check workmanship flaw acceptance and if necessary restrict further the domain in which they can be applied, e.g. adjust the maximum applied stress to well below yield. It would be useful if a standard ECA approach could consistently allow less stringent criteria over the domain that ‘workmanship’ acceptance criteria can be applied when overmatching weld strength and good toughness data are ensured by testing and enhanced welding control.
Stress corrosion cracking (SCC) and Hydrogen Induced Cracking (HIC) continue to be significant challenges to the pipeline industry. The initial challenge is identifying “where” along a pipeline the damage may exist through inline inspection (ILI) or direct assessment (DA) methods. However, additional challenges must be overcome to realistically assess the significance of the damage to safe pipeline operation. These include characterizing the true extent of the SCC or HIC, estimating future crack behavior and predicting failure pressures.
This paper presents the methods used to destructively determine the actual crack depths in the PRCI(1) project as well as the statistical comparison of those values to the NDE measurements. We also briefly introduce advanced fracture mechanics analysis techniques that can be used to assess the safety significance of crack-like pipeline anomalies.
One of the challenges with any integrity management program is accurate characterization of anomalies detected during integrity assessments. Evaluation of stress corrosion cracking anomalies presents a special challenge because of the branched nature of the cracking and the lack of significant planar or volumetric metal loss accompanying this failure mechanism.
NDE service providers and technology manufacturers publish detection specifications for their equipment or inspection methodology. These specifications consider the statistical distribution of results based on their testing as well as the inherent measurement error associated with the detection components, commonly reported at varying degrees of confidence. Once the technology is used in the field to characterize actual anomalies, the level of control over these parameters along with human performance factors greatly influence the effectiveness of NDE performance. Flaw dimensions (i.e., depth and width), flaw orientation relative to inspection probe, interaction effects between closely positioned flaws, surface irregularities (both inner and outer), as well as other factors all contribute as sources of potential measurement inaccuracies. However, for meaningful engineering assessments about a pipe’s continued fitness for service, we must understand the confidence level and variability of an NDE or ILI solution to predict the depth and length of these flaws.
In Oil & Gas industry Engineering Criticality Assessments (ECAs) are routinely used to provide defect acceptance criteria for pipelines welds joints. Methodologies for industrial application are given in BS7910 (2013), API579-1 (2007), R6, DNV-OS-F101 (2013). Failure Assessment Diagram (FAD) is commonly used for ECA of flawed components. However for ductile material, the use of stress-based approach leads to very conservative assessment. In such case the strain based design could be more appropriated. The purpose of this paper is to introduce a method in order to establish the threshold value of plastic failure criterion for ECA Strain-Based design application.
This paper discusses the role played by FEA (Finite Element Analysis) in the development of basic understanding and procedures for the use in association with strain-based fracture assessments of pipelines. The basic fracture mechanics parameters and their assumptions are briefly presented and special challenges and possible limitations with respect to applications in large plastic strain scenarios are identified. The motivation for use of FEA in solving some of these challenges is discussed from different perspectives. A series of examples, although not claimed to be exhaustive, of the use of FEA in relation to strain-based facture assessment of pipelines is included to give a representative picture of the state of the art. The relation to general codes is also discussed, and the current lack of clear guidance on how to carry out such analyses is highlighted. The paper concludes with some perspectives regarding further development of the field, and some possible general steps are proposed in this respect.
Structural components subjected to high cyclic loadings may lead to plastic deformation accumulation and result in fracture failure. The low-cycle failure (LCF) is the coupling result of accumulative plastic damage and low-cycle fatigue damage. The crack tip opening displacement (CTOD) is one of the important parameters for studying the low-cycle fatigue of plate sustaining large scale yielding. The CTOD value can reflect the ability of material resistance to crack initiation and propagation, and it is an important parameter to evaluate material toughness as well as the main controlling parameter in analyzing the crack propagation due to low-cycle fatigue damage. An analytical model is presented in this paper to determine the CTOD for central-through cracked plates subjected to cyclic axial in-plane loadings. Combined with Dugdale model, the plastic strain accumulation at the crack tips are used as controlling parameter to estimate cyclic CTOD for low-cycle failure analysis of the central-through cracked plate. Also in the present work, the finite element analysis is conducted to investigate the influence of the accumulative plastic strain at crack tips, mean stress, and crack geometry. The second order polynomial for the normalized CTOD defined as a function of the accumulative plastic strain, the ratio of mean stress to the yield stress, and the crack length is fitted by the least square method. The new accumulative plastic strain based on CTOD estimation presented in this paper provides a new way for low-cycle fatigue analysis, considering accumulative plastic damage for central-through cracked plates under high cyclic loadings.
Chen, Hongyuan (Xian Jiaotong University) | Ji, Lingkang (China National Petroleum Corporation) | Chi, Qiang (Key Lab of Oil Tubular Mechanical and Environmental Behaviour of CNPC) | Wang, Peng (China National Petroleum Corporation) | Wang, Yalong (Key Lab of Oil Tubular Mechanical and Environmental Behaviour of CNPC) | Qi, Lihua (China National Petroleum Corporation) | Ren, Jicheng (Key Lab of Oil Tubular Mechanical and Environmental Behaviour of CNPC)
The specimens of girth weld joint of Φ813mm×14.7mm pipeline cracked at "near-seam zone" in tensile test. It was considered unacceptable for strain-based design pipelines in some current standards. The girth weld joint for X70 pipelines with "near-seam zone" crack were researched by CWP test and single-edge notch test (SENT). The high strain capacity is demonstrated by resistance curve tangency approach and CWP test results. The results imply the considerable tensile fracture resistance and consequently strain capacity when the strength overmatch.
Fairchild, D.P. (ExxonMobil Upstream Research Company) | Tang, H. (ExxonMobil Upstream Research Company) | Shafrov, S.Y. (ExxonMobil Upstream Research Company) | Cheng, W. (ExxonMobil Upstream Research Company) | Crapps, J.M. (ExxonMobil Upstream Research Company)
There are generally two reasons for conducting full-scale tests (FSTs) for the measurement of pipe or weld strain capacity, (1) to generate data useful in verifying the accuracy of a strain capacity prediction model, or (2) to test materials being considered for use. The former case involves exploring variables important to the scope of the model, while the latter involves project specific materials and girth weld procedures often combined with upper bound cases of weld misalignment. Because the challenge of strain-based design is relatively new, FSTs should be used for both reasons cited above.
This paper provides observations, lessons learned, and recommendations regarding full-scale pipe strain capacity tests. This information has been developed through the conduct, witness, or review of 159 FSTs. One of the most important aspects of full-scale testing is the preparation of welded pipe test specimens. It is imperative that the specimens be fabricated with materials of known properties and that all possible measures be taken to limit variations from the intended specimen design. It has been observed that unexpected results are often due to irregularities in pipe material strength, weld strength, weld toughness, or the presence of unintended weld defects in a specimen designed to contain just man-made defects. Post-test fractography and metallurgical examination are very useful in explaining the performance of a FST; therefore, aspects of failure analysis are discussed.