(84a) A Comprehensive Pipeline Source Term Model for Pressurized/Flashing Liquids in Phast/Safeti (PIPEBREAK-II)

PIPEBREAK is an integral model in Phast/Safeti that simulates the evolution of pressure and temperature at the outlet and upstream ends of a pipeline as well as the attendant mass flow rate out of a breach. PIPEBREAK supports the modelling of breaches at any location along the pipeline length, pumped input as well as valve closure effects. However, PIPEBREAK assumes instantaneous depressurization to saturated conditions along the entire pipeline length at the onset of a breach. For long (>100m) pipelines, where conveyed fluids are stored at pressures significantly above their bubble point, this assumption may potentially lead to:

• Underprediction of fluid inventory contributing to the overall release process.
• Underestimation of initial source terms for release scenarios involving small breaches (e.g., with aperture ratios <20%).

A model (PiRRaM), has been developed for the UK Health and Safety Executive (HSE), that closely resembles PIPEBREAK and attempts to address the key limitation in PIPEBREAK described above. However, PiRRaM was developed to satisfy HSE’s requirements and focusses on the initial 30s of a release. As such, PiRRaM does not support many features currently in PIPEBREAK, such as: pumped input, valve closure and heat transfer effects.

This paper presents an updated model, PIPEBREAK-II, the outcome of modelling work to address the limitations described in PIPEBREAK and PiRRaM. PIPEBREAK-II has been developed to simulate the full range of hole sizes, from full bore ruptures to small breaches while accounting for pumped input, valve closure effects, heat transfer between the fluid in the pipeline and its surroundings as well as providing support for an interactive flow model. PIPEBREAK-II tracks both rarefaction and feedback waves, frictional effects as well as the presence of pre-existing (pumped/non-stationary) flow during the expanding liquid depressurization process. Furthermore, this paper describes work to improve the liquid density (pipeline inventory) predictions of the traditional cubic equations of state methods employed within PIPEBREAK and the results of an extensive validation exercise to support the selected approach.

The results of validation of PIPEBREAK-II against a wide range of field/experimental data as well as verification against predictions from other long pipeline models based on available data in published literature are also presented. The practical implications of observations from selected case studies, illustrating the impact of pumped input, valve closure effects as well as the potential impact of ignoring interactive flow on simulated results are also discussed.