November 2020, Vol. 247, No. 11
Features
New Model Design Against Running Ductile in Dense-Phase CO2 Pipelines
By Bente Helen Leinum, Senior Principal Engineer, and Erling Østby, Senior Principal Specialist, DNV GL-Oil & Gas
Carbon capture and storage (CCS) is vital to decarbonize a few important sectors, including the most energy-intensive industries: power, heat and transport. It will also enable future operations to become more competitive and sustainable in a net-zero economy.
Removing carbon from natural gas –to reach hard-to-abate sectors before or after combustion – is anticipated to be a catalyst for deep decarbonization after 2035, according to DNV GL’s latest Energy Transition Outlook (ETO).
Currently, about 40 mtpa of anthropogenic carbon dioxide (CO2) is captured and stored in geological formations each year. While this is almost a threefold increase on projections in 2019, it will not scale early enough to meet the targets set by the Paris Agreement.
Pipeline Transport
The need to transport CO2 is expected to increase significantly in the years to come as part of the widespread realization that CCS is a viable means to reduce CO2 emissions into the atmosphere. Safely and reliably transporting from where it is captured to a storage site is, therefore, of utmost importance.
There are two main transport options, ship or pipeline. Generally, transport via ship is the cheapest solution for small volumes and distances, while pipelines are the least expensive for larger volumes and longer distances. The successful large-scale implementation will require a basis that allows for combined safe and cost-effective pipeline design solutions.
There are current challenges in this respect, predominantly the need to mitigate against potential running ductile fractures. A running ductile fracture can run along the pipeline in both directions, potentially splitting hundreds of meters of the pipeline, resulting in large leaks if not arrested. In response to this, the CO2SafeArrest project was set up between DNV GL and the Australian Energy Pipelines Cooperative Research Centre (EPCRC).
The goal of the collaboration was to better understand what governs the running ductile fracture and bring such knowledge into codes and an updated recommended practice: DNVGL-RP-F104 “Design and operation of carbon dioxide pipelines.”
Technical Background
CO2 is transported in ordinary steel pipelines of the same kind used for natural gas. Normally, CO2 is always compressed to a high pressure before pipeline transportation. This reduces the volume that needs to be transported and ensures the CO2 is in an efficiently transferrable state. A consequence of this is that the CO2 inside the pipeline will be in a dense phase.
In the event of an uncontrolled release of CO2 (e.g., caused by damage to a pipeline), the escaping fluid will quickly expand to CO2 gas while the pressure in the region of the crack tip remains high; in other words, a pressure plateau develops (Figure 1).
This behavior is different from the decompression behavior in natural gas, where a continuous drop in pressure is observed (Figure 2). Thus, it is crucial to understand the implications of this difference to control the risk of running ductile fracture in pipelines.
The Battelle two-curve method (BTCM) is still the dominating method to assess running ductile fracture in pipelines. Due to the different decompression behaviors for CO2, it has been proposed that a simplified method using the so-called “arrest pressure equation” may be applied to assess whether arrest will take place or not.
The arrest pressure is a function of the pipe geometry, material strength and toughness. Arrest is predicted if this pressure is higher than the predicted plateau pressure in the decompression curve.
One challenge with this approach is that there is an increasing body of evidence from large-scale test observations showing crack propagation for conditions predicted to results in crack arrest using the arrest pressure equation in the Battelle model (Figure 2). Therefore, the safe design of CO2 pipelines requires a revised approach to assess the risk of running ductile fracture and specify cost-effective measures to avoid this.
Arrest Testing
The CO2SafeArrest project carried out two large-scale CO2 crack arrest tests. Testing was performed at the DNV GL Spadeadam Test and Research facility in Cumbria, U.K.
Manufactured by Europipe, 24-by-65-inch (610-by-1,651-mm) pipes with wall thicknesses of 0.53 to 0.59 inches (13.5 to 15 mm) were applied in the tests. Test layouts with telescopic variation in properties like strength or toughness, i.e., properties increasing with crack propagation, were investigated to explore their influence on fracture propagation and arrest behavior. The first test was carried out with the full test length, consisting of eight pipes, buried under ca. 3 feet (1 meter) of clay.
For the second test, half of the test was left unburied with the other half partially buried in a similar way as in the first test. The running fracture was triggered by setting off an explosive charge cutting a slit ca. 3 feet into each of the two pipes in the middle of the test layout.
Both tests were carried out successfully. In the first test the crack propagated through the two first pipes on each side and arrested in the third pipe on each side. In the second test, as shown in Figures 3 and 4, the fracture propagated in the three first pipes on each side and arrested in the fourth pipe. The crack velocity and the pressure decompression were captured in both tests. Some key observations follow:
The crack did propagate with almost constant pressure at the crack tip until arrest took place.
The decompression of the CO2 did not follow a fully flat plateau as predicted for a theoretical model, but rather a “sloping” plateau (also observed in previous tests).
The crack propagated faster in the unburied compared to the buried pipes; however, eventual arrest did take place in pipes with similar properties.
Empirical Model
Observations from the CO2SafeArrest large-scale tests have been used together with observations available in the public domain from previous large-scale test programs (CO2 Pipetrans, Cooltrans and SarCO2B) to propose a new empirical model for the assessment of running ductile fracture in CO2 pipelines.
The model is based on the observations shown in Figure 5, pointing to a slant demarcation line between observed arrests (crosses) and propagations (circles), which evidently were seen from the experimental data. This demarcation has been used to propose a new limit for crack arrest in CO2 pipelines.
The new limit is interpreted using the existing parameters in the original Battelle model, keeping a link to the existing approach. However, the limits for crack arrest have been adjusted to reflect observations from large-scale tests and remove the existing non-conservatism in the Battelle approach.
No arrest observations exist for materials with lower toughness. Thus, the empirical model applies a cut-off below which it is not defined.
Efficient design rules require optimizing the combination between the need for safety and allowing for cost-effective solutions and execution of the design process. The empirical model for crack arrest described above is included in a proposed update of the DNVGL-RP-F104 “Design and operation of CO2 pipelines” document soon ready for external hearing.
For pipe designs falling into the right-hand corner, safety against running fracture, is documented if the general requirements are fulfilled. For cases falling in the left corner, special considerations, for example, typical execution of large-scale testing will need to be made to document safety against arrest.
Cases falling in the upper part of the figure are considered difficult to achieve because crack arrest is based mainly on the steel wall arrest capability. The main improvements are considered to be:
A design equation in line with actual observations of conditions for crack arrest in CO2 pipelines
Opening for a possibility in which crack arrest capabilities in CO2 pipelines may be documented without large-scale tests
Further perspectives
The proposed empirical model is believed to provide a more efficient way of qualifying pipelines with basically high Charpy V-notch (CVN) toughness (>250 J), achievable in most newly built pipelines. There is, however, also a potential for re-use of existing pipelines for CO2 transport. In that respect, pipes with poorer, or more uncertain, properties may need to be evaluated.
There is currently a lack of experimental observations of crack arrest conditions for lower toughness pipes. A better understanding of the governing conditions for arrest in such pipes should be developed, to eventually expand the regions for simplified assessment of crack arrest.
Acknowledgments:
The following are acknowledged for funding support for this work:
The Commonwealth Government of Australia, Department of Industry, Innovation and Science for funding supplied under the Carbon Capture and Storage Research Development and Demonstration fund
Gassnova SF for funding supplied under the Norwegian CLIMIT funding scheme.
Further, the CO2SafeArrest JIP partner, the Australian Energy Pipelines Cooperative Research Centre (EPCRC), is acknowledged for good cooperation with the team lead by Valery Linton.
References
- Michal, E. Østby, B. J. Davis, S. Røneid and C. Lu, “An empirical fracture control model for dense phase CO2 carrying pipelines” in Proceedings of the 2020 19th International Pipeline Conference IPC2020, Calgary, Canada, 2020.
- Linton, B. H. Leinum, R. Newton and O. Fyrileiv, “CO2SAFE-ARREST: A full-scale burst test research program for carbon dioxide pipelines – Part 1: Project overview and outcomes of Test 1,” in 2018 18th International Pipeline Conference, Calgary, Canada, 2018.
- Michal, B. J. Davis, E. Østby, C. Lu and S. Røneid, “CO2Safe-Arrest: A full-scale burst test research program for carbon dioxide pipelines – Part 2: Is the BTCM out of touch with dense-phase CO2?,” in Proceedings of the 2018 18th International Pipeline Conference IPC2018, Calgary, Canada, 2018.
- Di Biagio, A. Lucci and E. Mecozzi, “Fracture propagation prevention on CO2 pipelines: Full scale experimental testing and verification approach,” in 21st Joint Technical Meeting, Colorado Springs, Colorado, USA, 2017.
- Cosham, D. G. Jones, K. Armstrong, D. Allason and J. Barnett, “Analysis of a dense phase carbon dioxide full-scale fracture propagation test in 24 inch diameter pipe,” in International Pipeline Conference (IPC), Calgary, Alberta, Canada, 2016.
Authors: Bente Helen Leinum, who holds a master’s degree in science from the Norwegian Institute of Technology, specializing in physical metallurgy, is senior principle engineer for DNV GL, Pipelines in Operation and has held various positions in DNV GL including business development leader Subsea, business development leader Pipeline Operation and head of section for Pipeline Operations.
Erling Østby holds a doctorate in mechanical engineering from the Norwegian University of Science and Technology and is senior principal specialist DNV GL, Structural Integrity. He mainly works with structural integrity issues toward code development and as advisory services.
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