WORK PACKAGE 6: FRACTURE PROPAGATION

LEADER: UCL PROF. H. MAHGEREFTEH

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Crack propagation is a problem in pipelines conveying gas or liquids with high vapour pressures. Fractures can propagate in either the fully brittle or fully ductile modes for long distances, and in theory, could propagate almost indefinitely. If the propagation of a long-running ductile or brittle fracture cannot be prevented through the specification of material toughness then measures such as the fitting of crack arrestors along the pipeline have to be adopted. Many of the operating pipelines in the US are fitted with crack arrestors. This work package is therefore aimed at understanding the effects of release of CO₂ in terms of fracture propagation and specifying the material properties to ensure that the fractures are arrested. This work package is divided into two sub-work packages; one which concentrates on fracture propagation in the brittle mode due to low temperature effects, and one which concentrates on fracture arrest of ductile fractures.

WP 6.1: Low Temperature-Induced Fracture Propagation (UCL: Prof H. Mahgerefteh, Cranfield University: Prof. F. Brennan)

Crack propagation is a problem in pipelines conveying gas or liquids with high vapour pressures. Fractures can propagate in either the fully brittle or fully ductile modes for long distances, and in theory, could propagate almost indefinitely. If the propagation of a long-running ductile or brittle fracture cannot be prevented through the specification of material toughness then measures such as the fitting of crack arrestors along the pipeline have to be adopted.

This research addresses the fundamentally important issue regarding the appropriate material selection for CO2 pipelines resisting low temperature induced running fractures leading to massive and rapid escape of CO2 into the environment. In the case of a stable through wall defect formed, for example, as a result of corrosion or third party damage in a CO2 pipeline, the rapid quasi-adiabatic depressurisation results in significant cooling of the escaping fluid to temperatures as low as -70 0C. If the pipe wall temperature reaches its ductile/brittle transition temperature (typically – 40 to – 70 oC), it is imperative to be able to predict, a priori, whether the corresponding transient thermal and pressure stresses at the crack tip could lead to its transformation into a propagating fracture [1]. Similar considerations also apply in the case of emergency blowdown of CO2 pipelines. Here a further consideration is the possibly of pipeline instability due to the freezing of the surrounding medium [2].

The recent jointly commissioned report [3] by Research Council of Norway, Gassco and Shell Technology Norway considers CO2 pipelines as being more susceptible to fast propagating fractures than hydrocarbon pipelines.  The same report classifies ‘further research in this area as important and in the blowdown context’ most critical short term (urgent) need’. 

Amongst other things [4], CO2 pipeline transportation presents a unique set of challenges due to the relatively high Joule Thompson coefficient of CO2 and its exclusive phase equilibrium behaviour resulting in prolonged sustained line pressure during the two phase liquid/vapour transition. In this region, despite inventory being lost from the pipeline, the line pressure will remain constant thus maintaining the pressure stresses responsible for propagating fracture.

Despite its obvious importance, no detailed study accounting for the role of such fluid/structure interactions considering the pertinent thermodynamic and heat transfer effects on fracture initiation and propagation in CO2 pipelines has been undertaken.  In addition, fracture mechanics analysis of this subject area carried out to date has been confined to hydrocarbon pipelines and entirely empirically based [2]. Addressing such issues require urgent attention given the temptation to employ the existing hydrocarbon pipeline infrastructure for transporting CO2 in view of the obvious cost advantages. There is evidence that pressure testing of pipelines prior to commissioning can induce some warm pre-stressing which to some extent reduces the susceptibility to low temperature induced failure. Once again though no systematic fundamental studies identifying the governing mechanism and quantifying the effectiveness of warm pre-stressing on pipelines has been undertaken. As such, there is no certainty on how warm pre-stressing will reduce the risk of low temperature induced fracture initiation and propagation in CO2 pipelines.

References:

1. Mahgerefteh H, Atti O. Modeling Low-Temperature-Induced Failure of of Pressurized Pipelines. AIChE Journal. 2006; 52(3), 1248-1256.
2. Oosterkamp A , Ramsen J ,  State-of-the-Art Overview of CO2 Pipeline Transport with Relevance to Offshore Pipelines, Polytech, report no.  POL-O-2007-138-A,  Jan 2008.
3. Mahgerefteh H, Denton G, Rykov Y. CO2 Pipeline Rupture. HAZARDS XX Process Safety and Environmental Protection, IChemE Sympsium. 2008; 154: 869- 882.
4. Duffy A R. Proceedings of the 4th Symposium on Line Pipe Research. 1969H10-H11.

WP 6.2: Prediction of Fracture Arrest (Newcastle University: Dr J. Race, Prof. M. Downie)

In the literature on CO₂ pipelines, many authors have also indicated that ductile fracture propagation may be an issue. The problem of fracture propagation was recognised in the gas industry over forty years ago and extensive world-wide research has led to the establishment of a number of models, which describe fracture propagation behaviour for gas pipeline systems. These models have been very successful in defining toughness requirements for pipe material, which ensure fracture arrest, however, they have not been verified for fracture propagation in CO₂ pipelines. In this task the suitability of these models will be reviewed and modelling undertaken to propose alternative approaches for the prediction of fracture arrest in CO₂ pipelines.