Pipelines for CO2 Transport: Verification and Control of Running Shear Fracture Failure Mode

Abstract

Abstract Scope of the paper is to describe a methodology for the verification and control of the running shear fracture in offshore pipelines to be dedicated to CO2 transport. The objective is to avoid through thickness failures and running fracture which can be deleterious for the pipeline, the environment, and the people safety. The running shear fracture failure mode is described, and a stepwise verification method is proposed. The application of the methodology includes the development of a simulation model (by means of patented software worldwide recognized for applications of hydrocarbon transport), with the aim to provide a basis for the definition of a reference temperature for the material testing (mainly Charpy impact test and Drop Weight Tear Test). A wide sensitivity has been performed considering different typical injection networks transporting high CO2-rich mixtures flow rates, injected both into shallow or deep-water reservoirs, at different initial operating conditions along the pipelines and different mixture compositions. The model development includes also a thermodynamic characterization suited for CO2 mixtures according to the most recent findings. The methodology and the predictive tool are considered suitable to describe the CO2 liquid isentropic decompression and conservatively estimate the minimum temperature reached along the pipeline during a running shear fracture event. The results show that, during the crack propagation, the fluid temperature in the pipeline sections next to the rupture running tip are maintained higher than critically very low values for material embrittlement, i.e. higher than ductile to brittle material transition temperature. Different decompression behaviors have been found, based on the initial operating conditions and depending on if injection is performed into shallow or deep-water reservoirs. For all the studied cases, issues relevant to the brittle propagation of the fracture is expected only locally at the start of the leakage but not seen by the running fracture eventually started. In order to compensate the limited experience of operations of CO2 pipelines and the lack of large full scale tests, a simulation model has been developed, combined with the available theory of the running shear fracture mechanics, with the challenge to predict the temperature and pressure trend along the pipeline during a running shear fracture event and with the main objective to rationalize the design of CO2 transportation.