CO2 pipelines material and safety considerations

This paper presents an overview of some of the most important factors and areas of uncertainty affecting integrity and accurate hazard assessment of CO2 pipelines employed as part of the Carbon Capture and Sequestration (CCS) chain. These include corrosion, hydrate formation, hydrogen embrittlement and propensity to fast running ductile and brittle factures. Special consideration is given to the impact of impurities within the CO2 feed from the various capture technologies on these possible hazards. Knowledge gaps in the modelling of outflow and subsequent dispersion of CO2 following the accidental rupture of pressurised CO2 pipelines, central to their safety assessment, are also presented

Modelling brittle fracture propagation in gas and dense-phase CO2 transportation pipelines

The development and application of a fluid–structure interaction model for simulating the transition of a through-wall defect in pressurised dense (150 bar, 283.15 K) and gas phase (34 bar, 283.15 K) CO2 pipelines into a running brittle fracture is presented. Given the economic incentives, the fracture model is employed to test the suitability of the existing stock of natural gas pipelines with the relatively high ductile to brittle transition temperatures of 0 and −10 °C for transporting CO2 in the terms of their resistance to brittle fracture propagation. The hypothetical but nevertheless realistic scenarios simulated involve both buried and above ground 10 km long, 0.6 m i.d. pipelines. Based on the assumption of no blowout of the surrounding soil upon the formation of the initial leak, the results show that the transition of the leak into a running brittle fracture in buried CO2 pipelines is far more likely as compared to above ground pipelines. In addition, gas phase pipelines are more prone to undergoing a propagating brittle fracture as compared to dense phase pipelines despite the lower operating pressures of the former. Furthermore, counter-intuitively, isolation of the feed flow following the discovery of a leak is shown to facilitate brittle fracture failure. The initial defect geometry on the other hand is shown to have a profound impact on the pipeline's resistance to propagating brittle fractures

Modelling Dry Ice Formation Following Rapid Decompression of CO₂ Pipelines

A fundamentally important issue regarding the safety assessment of CO2 pipelines is the possibility of solid or ‘dry ice’ discharge during an accidental release. This is particularly relevant given the near-adaibatic decompression process and the unusually high Joule Thomson coefficient of expansion of CO2. Solids discharge will affect many aspects of the ensuing hazard spanning the erosion of surrounding equipment, modification of the toxic dose duration, atmospheric dispersion and possibly, the pipeline’s propensity to fracture propagation. This paper describes the development of a Cubic Equation of State capable handling solid CO2 as a third phase. Pipeline rupture outflow data are reported based on the coupling of this new equation of state into a rigorous transient outflow model in order to investigate the impact of the pipeline design and operating conditions as well as the presence of the typical impurities on solid CO2 discharge

Hybrid fluid–structure interaction modelling of dynamic brittle fracture in steel pipelines transporting CO2 streams

Pressurised steel pipelines are considered for long-distance transportation of dense-phase CO2 captured from fossil fuel power plants for its subsequent sequestration in a Carbon Capture and Storage (CCS) chain. The present study develops a hybrid fluid–structure methodology to model the dynamic brittle fracture of buried pressurised CO2 pipeline. The proposed model couples the fluid dynamics and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in the rupturing pipeline a compressible one-dimensional Computational Fluid Dynamics (CFD) model is applied, where the fluid properties are evaluated using rigorous thermodynamic model. In terms of the fracture model, an eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model the dynamic brittle fracture behaviour of pipeline steel. Using the proposed model, a study is performed to evaluate the rate of brittle fracture propagation in a real-scale 48 in. diameter API X70 steel pipeline. The model was verified by comparing the obtained numerical results against available semi-empirical approaches from the literature. The simulated results are found to be in good correlation with the simulations using a simple semi-empirical model accounting for the fracture toughness, indicating the capability of the proposed approach to predict running brittle fracture in a CO2 pipeline

A model of the near-field expansion of CO2 jet released from a ruptured pipeline

The transportation of pressurised CO2 using pipelines is a crucial element of the Carbon Capture and Storage chain; for their safe design the ability to accurately predict the consequences of a failure, the jet release and ensuing dispersion is essential. Such phenomena are commonly modelled in stages: jet expansion followed by atmospheric dispersion. For jet expansion modelling, both analytical and Computational Fluid Dynamic (CFD) models are available to predict the fully expanded flow conditions which are subsequently used as inputs in dispersion modelling. Although analytical models are computationally efficient, due to the lack of experimental data, their predictions have yet been verified. In this work, a conservation law based multiphase analytical model is constructed with a rigorous equation of state. The predicted flow variables at full expansion are then compared to those from the Shear Stress Transport k-ω CFD model. The quantitative comparisons between two models provide necessary verification of the application of analytical models in accidental release modelling

Entropy – stagnation enthalpy interpolation tables for calculation of the critical flow properties of compressible fluids

High-pressure pipelines provide the most cost-effective and established method for long-distance transportation of large quantities of compressible fluids, such as natural gas, hydrogen and carbon dioxide. Given significant safety hazards associated with these pipelines, their design and operation requires using mathematical modelling tools quantifying consequences of accidental pipeline failure. Central to this is the accurate and robust prediction of the critical discharge flow from the pipeline, accounting for the real fluid thermodynamic behaviour, including the phase transition induced by the decompression process. In this work, a method of inverse interpolation tables is developed to calculate physical properties of compressible fluid, for use in a computational model of transient outflow from an accidently ruptured pipeline. In particular, the density – energy interpolation tables are applied to calculate the fluid pressure, temperature and phase composition as required for solving the mass, momentum and energy conservation equations describing the decompression flow inside pipeline, while the entropy – stagnation enthalpy interpolation tables are introduced to obtain the critical (choked) flow properties at the rupture section of the pipe. To construct the latter, the choked flow properties are calculated by solving simultaneously the total enthalpy conservation equation along with the constant entropy condition. The interpolation is performed using Akima splines fitted to the thermodynamic properties data predicted using highly-accurate Perturbed Chain-SAFT (PC-SAFT) equation of state. The interpolation tables are constructed for ethylene and carbon dioxide, covering pressures from 0.1 to 10 MPa and temperatures ranging from the triple point to 350 K. The study provides recommendations for the optimal resolution of the interpolation tables to achieve a balance between the accuracy and computational efficiency of the calculated physical properties. Practical implementation of the interpolation method in a pipeline decompression flow model is discussed. Acknowledgement. This research has received funding from the European Union’s Horizon 2020 Research & Innovation Programme under the Grant Agreement No 884418, and Qatar National Research Fund (a member of the Qatar Foundation) NPRP award 8-1339-2-569

Compression power requirements for Oxy-fuel CO₂ streams in CCS

CO2 compression systems are commonly designed assuming negligible amount of impurities in CO2 fluid, it is of practical interest to evaluate the impact of impurities in oxy-fuel streams on the compression power requirements. Compared to more traditional postcombustion and pre-combustion capture methods, oxy-fuel technology produces a CO2 stream with relatively high concentration of impurities that may require partial or a high level of removal and whose presence can be expected to increase the costs of CO2 compression. Four types of compression technologies employed include four-stage compressor with 4 intercoolers, single-stage supersonic shockwave compressor, three-stage compressor combined with subcritical liquefaction and pumping and three-stage compressor combined with supercritical liquefaction and pumping. The study depicts that decrement of the impurities content from 15 to 0.7%v/v in the CO2 streams reduced the total compression power in the compression system. The study also concludes that three-stage compressor combined with subcritical liquefaction and pumping can potentially offer higher efficiency than four-stage compressor with 4 intercoolers for almost pure CO2 streams. In the case of raw oxy-fuel mixture, that carries relatively large amount of impurities, subcritical liquefaction proved to be less feasible, while supercritical liquefaction efficiency is only marginally lower than that in the four-stage compressor with 4 intercoolers

Investigations on power requirements for industrial compression strategies for Carbon Capture and Sequestration

The main purpose of this study is to identify the optimum multistage compression strategies for minimising the compression and intercooler power requirements for pure CO2 stream. An analytical model based on thermodynamics principles is developed and applied to determine the power requirements for various compression strategies for pure CO2 stream. The compression options examined include conventional multistage integrally geared centrifugal compressors (option A), supersonic shockwave compressors (option B) and multistage compression combined with subcritical (option C) and supercritical liquefaction (option D) and pumping. In the case of determining the power demand for inter-stage cooling and liquefaction, a thermodynamic model based on Carnot refrigeration cycle is applied. From the previous study by [1], the power demand for inter-stage cooling duty was assumed to have been neglected. However, based on the present study, the inter-stage cooling duty is predicted to be significantly higher and contributes approximately 30% of the total power requirement for compression options A, C and D, while reaches 58% when applied to option B. It is also found that compression option C can offer higher efficiency than other compression strategies, while supercritical liquefaction efficiency is only marginally higher than that in the compression option A

A fully coupled fluid-structure interaction simulation of three-dimensional dynamic ductile fracture in a steel pipeline

Long running fractures in high-pressure pipelines transporting hazardous fluid are catastrophic events resulting in pipeline damage and posing safety and environmental risks. Therefore, the ductile fracture propagation control is an essential element of the pipeline design. In this study, a coupled fluid-structure interaction modelling is used to simulate the dynamic ductile fractures in steel pipelines. The proposed model couples a fluid dynamics model describing the pipeline decompression and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in a rupturing pipeline, a compressible one-dimensional computational fluid dynamics model is applied, where the fluid properties are evaluated using a rigorous thermodynamic model. The ductile failure of the steel pipeline is described as an extension of the modified Bai-Wierzbicki model implemented in a finite element code. The proposed methodology has successfully been applied to simulate a full-scale pipeline burst test performed by British Gas Company, which involved rupture of a buried X70 steel pipeline, initially filled with rich natural gas at 11.6 MPa and −5 °C

Simulation of two-phase flow through ducts with discontinuous cross-section

The development of an AUSM+-up based scheme for simulating transient single or two-phase flows through ducts with discontinuous or abrupt changes in area is presented. The non-conservative terms in the governing equations arising from the variation in the duct area are discretised to maintain exactly states at rest. An additional scaling of the pressure based dissipation is added to ensure numerical stability across the area change. The extensive application of the scheme to ideal gas and two-phase CO2 based on the Homogeneous Equilibrium Model (HEM) for both shock tube and other transient flow problems indicate the scheme’s capability to resolve such problems accurately and robustly