Andrew Cosham

The Pipeline Defect Assessment Manual

Oil and gas transmission pi pelines h ave a g ood s afety record. This is due to a combination of good design, materials and operatin g practices . How ever, lik e an y en gineering structure, pipelines do occas ionally f ail. T he m ajor cau ses of pipeline failures around the world are external interference and corrosion; th erefore, as sessment m ethods are needed to determine the severity of such defects when they are detected in pipelines. Defects occu rring du ring th e f abrication of a pipelin e are usually assessed ag ainst reco gnised an d p roven q uality co ntrol (workmanship) limits. These workmanship limits are somewhat arbitrary, but they h ave been prov en ov er tim e. How ever, a pipeline w ill in variably co ntain lar ger defects at some stage during its life, and th ese w ill req uire a ‘ fitness-for-purpose’ assessment to d etermine w hether o r n ot to repair the pipeline. Consequently, the past 40 y ears has seen a large number of full scale tests of def ects in pipelin es, an d th e dev elopment of a number of m ethods f or as sessing the significance of defects. Some of th ese m ethods h ave been in corporated in to in dustry guidance, o thers are to b e f ound in th e published literature. However, there is no definitive guidance that draws together all of th e as sessment tech niques, or assesses each method against the p ublished test d ata, o r reco mmends b est p ractice in th eir application. To ad dress this industry need, a J oint Industry P roject has been sp onsored b y fifteen international oil and gas companies 1

The Decompression Behaviour of Carbon Dioxide in the Dense Phase

Pipelines can be expected to play a significant role in the transportation infrastructure required for the successful implementation of carbon capture and storage (CCS). National Grid is undertaking a research and development programme to support the development of a safety justification for the transportation of carbon dioxide (CO2) by pipeline in the United Kingdom.The ‘typical’ CO2 pipeline is designed to operate at high pressure in the ‘dense’ phase. Shock tube tests were conducted in the early 1980s to investigate the decompression behaviour of pure CO2, but, until recently, there have been no tests with CO2-rich mixtures.National Grid have undertaken a programme of shock tube tests on CO2 and CO2-rich mixtures in order to understand the decompression behaviour in the gaseous phase and the liquid (or dense) phase. An understanding of the decompression behaviour is required in order to predict the toughness required to arrest a running ductile fracture.The test programme consisted of three (3) commissioning tests, three (3) test with natural gas, fourteen (14) tests with CO2 and CO2-rich mixtures in the gaseous phase, and fourteen (14) tests with CO2 and CO2-rich mixtures in the liquid (or dense) phase. The shock tube tests in the liquid (dense) phase are the subject under consideration here.Firstly, the design of the shock tube test rig is summarised. Then the test programme is described. Finally, the results of the dense phase tests are presented, and the observed decompression behaviour is compared with that predicted using a simple (isentropic) decompression model. Reference is also made to the more complicated (non-isentropic) decompression models. The differences between decompression through the gaseous and liquid phases are highlighted.It is shown that there is reasonable agreement between the observed and predicted decompression curves.The decompression behaviour of CO2 and CO2-rich mixtures in the liquid (dense) phase is very different to that of lean or rich natural gas, or CO2 in the gaseous phase. The plateau in the decompression curve is long. The following trends (which are the opposite of those observed in the gaseous phase) can be identified in experiment and theory:• Increasing the initial temperature will increase the arrest toughness.• Decreasing the initial pressure will increase the arrest toughness.• The addition of other components such as hydrogen, oxygen, nitrogen or methane will increase the arrest toughness.Copyright

Fracture Control in Carbon Dioxide Pipelines: The Effect of Impurities

The fourth report from the Intergovernmental Panel on Climate Change states that “Warming of the climate system is unequivocal...” It further states that there is a “very high confidence that the global average net effect of human activities since 1750 has been one of warming.” One of the proposed technologies that may play a role in the transition to a low-carbon economy is carbon dioxide capture and storage (CCS). The widespread adoption of CCS will require the transportation of the CO2 from where it is captured to where it is to be stored. Pipelines can be expected to play a significant role in the required transportation infrastructure. The transportation of CO2 by long-distance transmission pipeline is established technology; there are examples of CO2 pipelines in USA, Europe and Africa. The required infrastructure for CCS may involve new pipelines and/or the change-of-use of existing pipelines from their current service to CO2 service. Fracture control is concerned with designing a pipeline with a high tolerance to defects introduced during manufacturing, construction and service; and preventing, or minimising the length of, long running fractures. The decompression characteristics of CO2 mean that CO2 pipelines may be more susceptible to long running fractures than hydrocarbon gas pipelines. Long running fractures in CO2 pipelines may be preventable by specifying a line pipe steel toughness that ensures that the ‘arrest pressure’ is greater than the ‘saturation pressure’ or by using mechanical crack arrestors. The preferred choice is control through steel toughness because it assures shorter fracture lengths. The ‘saturation pressure’ depends upon the operating temperature and pressure, and the composition of the fluid. ‘Captured’ CO2 may contain different types or proportions of impurities to ‘reservoir’ CO2 . Impurities, such as hydrogen or methane, have a significant effect on the decompression characteristics of CO2 , increasing the ‘saturation pressure’. The implication is that the presence of impurities means that a higher toughness is required for fracture arrest compared to that for pure CO2 . The effect of impurities on the decompression characteristics of CO2 are investigated through the use of the BWRS equation of state. The results are compared with experimental data in the published literature. The implications for the development of a CCS transportation infrastructure are discussed.Copyright

GASDECOM: Carbon Dioxide and Other Components

Carbon dioxide (CO2 ) pipelines are more susceptible to long running fractures than hydrocarbon gas pipelines because of the decompression characteristics of CO2 . The key to understanding this issue is the phase diagram and the liquid-vapour phase boundary. GASDECOM — based on the BWRS equation of state — is a program widely used for calculating the decompression behaviour of mixtures of hydrocarbons. The calculated decompression wave velocity curve is then used in models such as the Battelle Two Curve Model to determine the toughness required to arrest a propagating ductile fracture. GASDECOM is capable of modelling mixtures of hydrocarbons (methane through to hexane), nitrogen and carbon dioxide. It therefore can (and has) been used to investigate the effect of methane and nitrogen on the decompression characteristics of CO2 . Pipelines can be expected to play a significant role in the transportation infrastructure required for the successful implementation of carbon capture and storage (CCS). The composition of the carbon dioxide rich stream to be transported in a pipeline depends on the capture technology, e.g. post-combustion, pre-combustion and oxy-fuel. Post-combustion tends to result in an almost pure stream. The other capture technologies produce a less pure stream, containing potentially significant proportions of other components such as hydrogen, nitrogen, oxygen, argon and methane. One of the factors that will constrain the design and operation of a carbon dioxide pipeline is the effect of these other components on the decompression characteristics, and hence the arrest toughness (amongst other issues). Components such as hydrogen, oxygen and argon cannot currently be considered using GASDECOM. Through a study of the underlying algorithms implemented in GASDECOM, it is shown how GASDECOM can be modified to include these additional components relevant to carbon capture and storage. The effect of impurities such as hydrogen on the decompression characteristics is then illustrated, and related back to their effect on the phase diagram and the liquid-vapour phase boundary. The sensitivity of the results to the use of equations of state other than BWRS is also illustrated. Simplifications that follow from the decompression behaviour of carbon dioxide are also highlighted. Finally, the small and large scale experimental studies that are required to validate predictions of the decompression behaviour and the arrest toughness are discussed.Copyright

Ruptures in Gas Pipelines, Liquid Pipelines and Dense Phase Carbon Dioxide Pipelines

Ruptures in gas and liquid pipelines are different. A rupture in a gas pipeline is typically long and wide. A rupture in a liquid pipeline is typically short and narrow, i.e. a slit or ‘fish-mouth’ opening.The decompression of liquid (or dense) phase carbon dioxide (CO2) immediately after a rupture is characterised by a rapid decompression through the liquid phase, and then a long plateau. At the same initial conditions (pressure and temperature), the initial speed of sound in dense phase CO2 is greater than that of natural gas and less than half that of water. Consequently, the initial decompression is more rapid than that of natural gas, but less rapid than that of water.A question then arises … Does a rupture in a liquid (or dense) phase CO2 pipeline behave like a rupture in a liquid pipeline or a gas pipeline? It may exhibit behaviour somewhere in-between the two. A ‘short’ defect that would rupture at the initial pressure might result in a short, narrow rupture (as in a liquid pipeline). A ‘long’ defect that would rupture at the (lower) saturation pressure might result in a long, wide rupture (as in a gas pipeline). This is important, because a rupture must be long and wide if it is to have the potential to transform into a running fracture.Three full-scale fracture propagation tests (albeit shorter tests than a typical full-scale test) published in the 1980s demonstrate that it is possible to initiate a running ductile fracture in a CO2 pipeline. However, these tests were on relatively small diameter, thin-wall line pipe with a (relatively) low toughness. The results are not applicable to large diameter, thick-wall line pipe with a high toughness.Therefore, in advance of its full-scale fracture propagation test using a dense phase CO2-rich mixture and 914×25.4 mm, Grade L450 line pipe, National Grid has conducted three ‘West Jefferson Tests’. The tests were designed to investigate if it was indeed possible to create a long, wide rupture in modern, high toughness line pipe steels using a dense phase CO2-rich mixture. Two tests were conducted with 100 mol.% CO2, and one with a CO2-rich binary mixture.Two of the ‘West Jefferson Tests’ resulted in short ruptures, similar to ruptures in liquid pipelines. One test resulted in a long, wide rupture, similar to a rupture in a gas pipeline. The three tests and the results are described. The reasons for the different behaviour observed in each test are explained. It is concluded that a long, wide rupture can be created in large diameter, thick-wall line pipe with a high toughness if the saturation pressure is high enough and the initial defect is long.Copyright

Analysis of Two Dense Phase Carbon Dioxide Full-Scale Fracture Propagation Tests

Two full-scale fracture propagation tests have been conducted using dense phase carbon dioxide (CO2)-rich mixtures at the Spadeadam Test Site, United Kingdom (UK). The tests were conducted on behalf of National Grid Carbon, UK, as part of the COOLTRANS research programme.The semi-empirical Two Curve Model, developed by the Battelle Memorial Institute in the 1970s, is widely used to set the (pipe body) toughness requirements for pipelines transporting lean and rich natural gas. However, it has not been validated for applications involving dense phase CO2 or CO2-rich mixtures. One significant difference between the decompression behaviour of dense phase CO2 and a lean or rich gas is the very long plateau in the decompression curve.The objective of the two tests was to determine the level of ‘impurities’ that could be transported by National Grid Carbon in a 914.0 mm outside diameter, 25.4 mm wall thickness, Grade L450 pipeline, with arrest at an upper shelf Charpy V-notch impact energy (toughness) of 250 J. The level of impurities that can be transported is dependent on the saturation pressure of the mixture. Therefore, the first test was conducted at a predicted saturation pressure of 80.5 barg and the second test was conducted at a predicted saturation pressure of 73.4 barg.A running ductile fracture was successfully initiated in the initiation pipe and arrested in the test section in both of the full-scale tests.The main experimental data, including the layout of the test sections, and the decompression and timing wire data, are summarised and discussed.The results of the two full-scale fracture propagation tests demonstrate that the Two Curve Model is not (currently) applicable to liquid or dense phase CO2 or CO2-rich mixtures.Copyright

Reduction Factors for Estimating the Probability of Failure of Mechanical Damage Due to External Interference

Quantified risk assessments (QRAs) are widely used in the UK to assess the significance of the risk posed by major accident hazard pipelines on the population and infrastructure in the vicinity of the pipeline. A QRA requires the calculation of the frequency of failures and the consequences of failures. One of the main causes of failures in onshore pipelines is mechanical damage due to external interference, such as a dent, a gouge, or a dent and gouge. In the published literature, two methods have been used to calculate the probability of failure due to external interference: • historical failure data and • limit state functions combined with historical data (i.e. structural reliability-based methods). Structural reliability-based methods are mathematically complicated, compared to using historical failure data, but have several advantages, e.g. extrapolation beyond the limited historical data, and the identification of trends that may not be apparent in the historical data. In view of this complexity, proposed supplements to the UK pipeline design codes IGE/TD/1 (natural gas) and PD 8010 (all substances) — on the application of QRAs to proposed developments in the vicinity of major accident hazard pipelines — include simple ‘reduction factors’ for use in ‘screening’ risk assessments. These ‘reduction factors’ are based on a comprehensive parametric study using a structural reliability-based model to calculate the probability of failure due to mechanical damage, defined as: gouges, and dents and gouges. The two ‘reduction factors’ are expressed in terms of the design factor and wall thickness of the pipeline. It is shown that, through appropriate normalisation, the effects of diameter, grade and toughness are secondary. Reasonably accurate, but conservative, estimates of the probability of failure can be obtained using these ‘reduction factors’. The proposed methodology is considerably simpler than a structural reliability-based analysis. The development and verification of these ‘reduction factors’ is described in this paper.Copyright

Crack-Like Defects in Pipelines: The Relevance of Pipeline-Specific Methods and Standards

Oil and gas transmission pipelines have a good safety record; however, like any engineering structure, pipelines do occasionally fail. The main causes of pipeline failures in North America and Europe are defects, such as damage due to external interference, corrosion defects, or material defects. Consequently, the pipeline industry developed its own ‘pipeline-specific’ methods for assessing the significance of these defects. These pipeline-specific methods have their origins in fracture mechanics, but the complexity of the underlying failure mechanisms was addressed through empiricism. Over the intervening years, more accurate ‘pipeline-specific’ methods have been developed that better model the underlying failure mechanisms. The toughness of line pipe steels is typically characterised in terms of the Charpy V-notch impact energy. This is a qualitative measure of toughness. The pipeline-specific methods use empirical correlations between Charpy V-notch impact energy and fracture toughness.In parallel to the development of the pipeline-specific methods, fracture mechanics has been generalised in standards such as BSI 7910: 2005 and API 579-1/ASME FFS-1, using the ‘failure assessment diagram’ (FAD). The pipeline-specific methods and the methods in these general (‘generic’) standards have common roots.The pipeline industry has used its pipeline-specific methods for 50 years, as there was little need to use the generic methods. This was because most of the defects detected in pipelines were corrosion or damage (e.g. dents), where pipeline-specific methods were well-researched and validated.The increasing sophistication of in-line inspection methods (‘intelligent pigs’) in the pipeline industry mean that cracks in pipelines are now easily detected. Additionally, recent failures caused by crack-like defects (e.g. the San Bruno failure in the USA in 2010), mean that the industry needs guidance on how to assess these crack-like defects. There is little or no guidance on pipeline-specific methods for crack-like defects. Therefore, the industry has adopted the generic methods, without any evaluation of their relevance or accuracy.The similarities and differences between the generic and pipeline-specific methods for crack-like defects in pipelines are illustrated in this paper through comparison with published full-scale test data for pipes containing notches and cracks. The significance of correlations between Charpy V-notch impact energy and fracture toughness is highlighted.In conclusion, a commentary is given on how and when to take advantage of either approach. The critical gaps in the existing methods are also identified.Copyright

Fracture Control in Pipelines Under High Plastic Strains: A Critique of DNV-RP-F108

Offshore pipelines experience strains greater than yield during pipelay and in service. Installation by reeling introduces high levels of plastic strain, typically on the order of 2 percent for a 12 in. flowline. Controlled lateral buckling in offshore pipelines, due to high operating pressures and/or temperatures, may also give rise to high strains and large cyclic loads. Similarly, frost heave or ground movement in onshore pipelines can cause high strains. To date, most of the cases involving high strains are to be found in offshore pipelines, in terms of both design and the assessment of accidental states. However, some of the experiences in the offshore industry have relevance to onshore pipelines. Fracture control in this context is designing pipelines to address the implications of these high static and cyclic strains during installation/construction and operation. Pipeline design codes such as DNV-OS-F101 and DNV-RP-F108 give guidance. Two issues to consider are: the degradation of the material properties, and the failure of the girth welds. High strains may cause failure or the growth — by stable ductile tearing — of preexisting flaws in the weld. Subsequent fatigue loading may cause pre-existing flaws to grow to failure. Engineering critical assessments (ECAs) are conducted during pipeline design to determine tolerable sizes for weld flaws. Standards such as BS 7910 and API 579 are primarily stress-based and it is not straightforward to apply them to strain-based situations. DNV-RP-F108 addresses this gap by providing additional guidance derived from UK and Norwegian research programmes. Assessing flaws subject to high strains is at the ‘cutting-edge’ of applied fracture mechanics. ECAs often have a reputation of being ‘over-conservative’. ECAs of pipelines subject to high strains may indicate that only very small flaws would be acceptable, whereas practical experience has shown that the girth welds are highly tolerant to the presence of flaws. It is therefore instructive to consider under what situations might ECAs be too conservative, and when they may be non-conservative. The available guidance for ensuring fracture control in pipelines under high plastic strains is discussed in this paper, and the wider issues are addressed.Copyright

ECAs, FE and Bi-Axial Loading: A Critique of DNV-OS-F101, Appendix A

Controlled lateral buckling in offshore pipelines typically gives rise to the combination of internal over-pressure and high longitudinal strains (possibly exceeding 0.4 percent).Engineering critical assessments (ECAs) are commonly conducted during design to determine tolerable sizes for girth weld flaws. ECAs are primarily conducted in accordance with BS 7910, often supplemented by guidance given in DNV-OS-F101 and DNV-FP-F108. DNV-OS-F101 requires that finite element (FE) analysis is conducted when, in the presence of internal over-pressure, the nominal longitudinal strain exceeds 0.4 percent. It recommends a crack driving force assessment, rather than one based on the failure assessment diagram. FE analysis is complicated, time consuming and costly. ECAs are, necessarily, conducted towards the end of the design process, at which point the design loads have been defined, the welding procedures qualified and the material properties quantified. In this context, ECAs and FE are not an ideal combination for the pipeline operator, the designer or the installation contractor.A pipeline subject to internal over-pressure is in a state of bi-axial loading. The combination of internal over-pressure and longitudinal strain appears to become more complicated as the longitudinal strain increases, because of the effect of bi-axial loading on the stress-strain response.An analysis of a relatively simple case, a fully-circumferential, external crack in a cylinder subject to internal over-pressure and longitudinal strain, is presented in order to illustrate the issues with the assessment. Finite element analysis, with and without internal over-pressure, are used to determine the plastic limit load, the crack driving force, and the Option 3 failure assessment curve. The results of the assessment are then compared with an assessment using the Option 2 curve. It is shown that an assessment based Option 2, which does not require FE analysis, can potentially give comparable results to the more detailed assessments, when more accurate stress intensity factor and reference stress (plastic limit load) solutions are used.Finally, the results of the illustrative analysis are used to present an outline of suggested revisions to the guidance in DNV-OS-F101, to reduce the need for FE analysis.Copyright