High-pressure CO2 Pipelines Research Articles

Assessment of Failure Frequency Methodology Applied to Dense Phase Carbon Dioxide Pipelines

This paper determines the capability of current failure frequency methodology and models for assessing dense phase CO2 pipelines. An overview of these models is given in order to explain their applicability to CO2 pipelines, structure and to highlight the similarities and differences between each model. A pipeline must comply with the relevant design code in order to ensure conformity with pipeline safety regulations, for example in the UK it is PD-8010 (BSI, 2015). In this particular code the maximum allowable operating stress in the pipeline and the minimum allowable distance between the pipeline and any occupied buildings near to the pipeline route must be defined by the product being transported. The code was originally written to be applied to products which pose a thermal hazard and therefore there is currently no hazard category included in the code for dense phase CO2 and therefore cannot be applied to the design of dense phase CO2 pipelines. Where no design code for dense phase CO2 pipelines exists, to ensure the safe design, construction and operation, a quantitative risk assessment (QRA) approach is required (Barnett, 2014; Cooper, 2014). The purpose of a pipeline QRA is to determine the risks posed by the failure of the pipeline to people located nearby. The procedure involves the identification of hazard scenarios and considers both the probability of failure and the consequences of failure in order to calculate values for the individual and societal risk due to the pipeline. In general a QRA procedure covers the following steps: i) Identify hazards ii) Identify failure causes iii) Calculate the frequency of failure for each cause iv) Evaluate the consequences of failure v) Calculate the individual and societal risk a specific locations along the pipeline vi) Assess the tolerability of the calculated risks by comparison with recognised criteria In terms of failure causes, pipeline failure can occur due to numerous different mechanisms including, third party external interference, corrosion, material or construction defects, natural events and operational error; all of which must be considered as part of the assessment (Goodfellow, 2006). For oil and gas pipelines, the frequency of pipeline failure due to third party external interference has traditionally been calculated using models based upon probabilistic, structural reliability methods. Structural reliability methods are applied by combining the following: • Limit state functions, the mathematical models which define the conditions for failure; • Probability distributions based around selected random variables; • A mathematical technique to calculate the probability of failure (e.g. Numerical Integration, Monte Carlo, First Order Reliability Methods). For pipelines the limit state functions are based on semi-empirical fracture mechanics failure models; and the probability distributions are based on pipeline damage and derived from historical operational data. The failure probability is converted into a failure frequency to take into account the regularity of third party external interference damage. The various models currently in use within the oil and natural gas pipeline industry differ in their subtleties; however all are based upon a methodology originally developed in the 1980s. To calculate pipeline failure frequency for dense phase CO2 pipelines it would be desirable to extend the use of the current pipeline failure frequency methodology. The methodology has been employed for over 25 years, and as a result is tried, tested and well understood. The transportation of dense phase CO2 by pipeline however requires operational pressures in excess of the CO2 triple point; potentially up to 200bara. This high design pressure requirement necessitates the use of thick wall linepipe in pipeline construction, potentially with wall thickness dimensions outside of the limits of current operational experience. Consequently, the reliance of the failure frequency methodology on empirical data and semi-empirical relations needs to be examined. The requirement to develop a robust Quantitative Risk Assessment (QRA) methodology for high pressure CO2 pipelines has been recognised as critical to the implementation of CCS. Consequently, failure frequency models are required that are appropriate for high pressure CO2 pipelines. This paper addresses key components from the failure frequency for QRA methodology development.

Experimental study on dispersion behavior during the leakage of high pressure CO2 pipelines

Pipeline transportation is considered as the most economical and efficient method for transporting large amounts of CO2. The potential for pipeline leakage, possibly resulting in catastrophic accidents, makes it vitally crucial to predict and assess the consequences of accidental CO2 pipeline releases. To investigate the dispersion behavior of high pressure CO2 leaked from a pipeline, a new laboratory scale experimental setup with a total length of 14.85 m and an internal diameter of 15 mm was constructed. Upward releases of supercritical, liquid and gaseous CO2 containing 2%, 4%, 6% mol nitrogen were carried out through an orifice of 1 mm. Evolutions of temperature and movement of dry ice particles in the far-field were studied. An improved dispersion model was proposed to analyze the abnormal low-temperature area found in pure CO2 leakage. Furthermore, the effects of N2 concentration, initial inner pressure and temperature on the temperature characteristics in the far-field were studied. The results show that the addition of N2 has different effects on the temperature characteristics for CO2 leakage with different initial phase states. The experimental results can be used to determine safety distances and verify outflow and dispersion models.

An experimental study on the flow characteristics during the leakage of high pressure CO2 pipelines

High pressure pipeline transportation is dominant mode for economically transporting large amounts of CO2. However, potential leakage is one of the main risks associated with pressurized CO2 pipeline transportation. Thus, to investigate the leakage behavior of high pressure CO2 and to de-risk such transportations, a new laboratory scale experimental setup (total length of 14.85 m and the inner diameter of 15 mm) was constructed. Leakage experiments for pure CO2 and CO2 mixtures containing various N2 concentrations were carried out for different initial phase states (supercritical, liquid and gaseous respectively). The pressure and temperature characteristics and phase transitions of high pressure CO2 were then studied following pipeline leakage through small diameter nozzles. The results show that a minimum temperature occurs during the leakage process of various initial phase states; while the temperature characteristics of supercritical and gaseous CO2 leakage were rather different from that of liquid CO2 leakage. Impacts of impurity (N2), initial inner pressure and nozzle sizes on the leakage behavior were investigated, and two minimum temperature lines, based on N2 concentrations and initial inner pressures were obtained, respectively. These findings will ascertain that the pipeline is operated above the ductile-brittle transition temperature.

Numerical study of the effect of heat transfer on solid phase formation during decompression of CO2 in pipelines

CO2 solid phase formation accompanying rapid decompression of high-pressure CO2 pipelines may lead to blockage of the flow and safety valves, presenting significant hazard for safe operation of the high-pressure CO2 storage and transportation facilities. In this study, a homogeneous equilibrium flow model, accounting for conjugate heat transfer between the flow and the pipe wall, is applied to study the CO2 solid formation in a 50 mm internal diameter and 37 m long pipe for various initial thermodynamic states of CO2 fluid and wide range of discharge orifice diameters. The results show that the rate of CO2 solid formation in the pipe is limited by heat transfer at the pipe wall. The predicted amounts of solid CO2 are discussed in the context of venting of CO2 pipelines.

Study of the consequences of CO2 released from high-pressure pipelines

The development of the Carbon Capture and Storage (CCS) technique requires an understanding of the hazards posed by the operation of high-pressure CO2 pipelines. To allow the appropriate safety precautions to be taken, a comprehensive understanding of the consequences of unplanned CO2 releases is essential before the deployment of CO2 pipelines. In this paper, we present models for the predictions of discharge rate, atmospheric expansion and dispersion due to accidental CO2 releases from high-pressure pipelines. The GERG-2008 Equation of State (EOS) was used in the discharge and expansion models. This enabled more precise ‘source strength’ predictions. The performance of the discharge and dispersion models was validated against experimental data. Full-bore ruptures of pipelines carrying CO2 mixtures were simulated using the proposed discharge model. The propagation of the decompression wave in the pipeline and its influence on the release rate are discussed. The effects of major impurities in the CO2 mixture on the discharge rate were also investigated. Considering typical CO2 mixtures in the CCS applications, consequence distances for CO2 pipelines of various sizes at different stagnation pressures were obtained using the dispersion model. In addition, the impact of H2S in a CO2 mixture was studied and the threshold value of the fraction of H2S at the source for which the hazardous effects of H2S become significant was obtained.

Modelling the non-equilibrium two-phase flow during depressurisation of CO2 pipelines

The development, testing and validation of a two-fluid transient flow model for simulating outflow following the failure of high pressure CO2 pipelines is presented. Thermal and mechanical non-equilibrium effects during depressurisation are accounted for by utilising simple constitutive relations describing inter-phase mass, heat and momentum transfer in terms of relaxation to equilibrium. Pipe wall/fluid heat exchange on the other hand is modelled by coupling the fluid model with a finite difference transient heat conduction model. The two-fluid transient flow model's performance is tested by comparison of the predicted transient pressure and temperature profiles along the pipeline against those based on the simplified homogeneous equilibrium model (HEM) as well as real data captured during the full bore rupture of a 260m long, 233mm internal diameter pipeline containing CO2 at 36bara and 273°C. The two-fluid model is found to produce a reasonably good degree of agreement with the experimental data throughout the depressurisation process. The HEM based flow model on the other hand performs well only near the rupture plane and during the early stages of the depressurisation process.

A risk assessment methodology for high pressure CO2 pipelines using integral consequence modelling

This paper presents a risk assessment methodology for high pressure CO2 pipelines developed at the Health and Safety Laboratory (HSL) as part of the EU FP7 CO2Pipehaz project. Until recently, risk assessment of dense phase and supercritical CO2 pipelines has been problematic because of the lack of suitable source term and integral consequence models that handle the complex behaviour of CO2 appropriately. The risk assessment presented uses Phast, a commercially available source term and dispersion model that has been recently updated to handle the effects of solid CO2. A test case pipeline was input to Phast and dispersion footprints to different levels of harm (dangerous toxic load and probit values) were obtained for a set of pipeline specific scenarios. HSL's risk assessment tool QuickRisk was then used to calculate the individual and societal risk surrounding the pipeline. Knowledge gaps that were encountered such as: harm criteria, failure rates and release scenarios were identified and are discussed.

Risk assessment methodology for high-pressure CO2 pipelines incorporating topography

This paper presents a risk assessment methodology for high-pressure CO2 pipelines developed at the Health and Safety Laboratory as part of the EU FP7 project CO2Pipehaz.Traditionally, consequence modelling of dense gas releases from pipelines at major hazard impact levels is performed using integral models with limited or no consideration being given to weather bias or topographical features of the surrounding terrain. Whilst dispersion modelling of CO2 releases from pipelines using three-dimensional CFD models may provide higher levels of confidence in the predicted behaviour of the cloud, the use of such models is resource-intensive and usually impracticable. An alternative is to use more computationally efficient shallow layer or Lagrangian dispersion models that are able to account for the effects of topography whilst generating results within a reasonably short time frame.In the present work, the proposed risk assessment methodology for CO2 pipelines is demonstrated using a shallow-layer dispersion model to generate contours from a sequence of release points along the pipeline. The simulations use realistic terrain taken from UK topographical data. Individual and societal risk levels in the vicinity of the pipeline are calculated using the Health and Safety Laboratory's risk assessment tool QuickRisk.Currently, the source term for a CO2 release is not well understood because of its complex thermodynamic properties and its tendency to form solid particles under specific pressure and temperature conditions. This is a key knowledge gap and any subsequent dispersion modelling, particularly when including topography, may be affected by the accuracy of the source term.

Modelling of accidental releases from a high pressure CO2 pipelines

In the near future large quantities of CO2 will be transported over a large distance from Carbon dioxide Capture plants to onshore and off-shore underground Storage (CCS) sites. The risk assessments for the existing CO2 pipelines show distances to harmful threshold concentrations from 1 to 7.2 km. Such large differences in safety distances are not acceptable. For the design, construction and operation of new high pressure CO2 pipelines through populated areas it will be necessary to have a validated risk assessment model. This paper describes the applied models for the outflow and dispersion and the causes of the uncertainties in the outflow and dispersion of CO2 after an accidental release from a high pressure pipeline. © 2011 Published by Elsevier Ltd.