Yu, Jianhao; (2021) Optimal Emergency Shutdown Valve Configuration for Pressurised Pipelines. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Pressurised pipelines are the primary mode of choice for transporting large quantities of hazardous fluids across the globe. The failure of such pipelines can lead to the release of significant amounts of flammable or toxic inventories, which may in turn present significant risks to life, environment and property. In order to mitigate such risks, various types of emergency shutdown valves (ESDVs), including Check Valves (CVs), Automatic Shut-off Valves (ASVs) and Remote Control Valves (RCVs), are installed along such pipelines as the front-line emergency mitigation tool. Accounting for the critically important ensuing in-pipe transient fluid flow, this thesis presents the development and application of a multi-objective optimisation study for selecting the ESDV type, number and spacing as well as its combinations and operational settings for striking a balance between the minimum valve capital cost against the efficacy in minimising and ultimately isolating outflow following pipeline failures. Two types of pipeline failures, including Full Bore Rupture (FBR) and puncture are considered. Ethylene and natural gas (typical flammable and explosive hydrocarbons) as well as Carbon Dioxide (CO2) are chosen as the transported fluids. CO2 is selected given its hazardous nature (an asphyxiant at > 7% v/v) and the extensive use of pressurised pipelines being proposed as the main method for transporting large quantities of captured CO2 for permanent geological storage as part of the Carbon Capture and Storage chain. The pipeline decompression model employed is based on the Homogeneous Equilibrium Mixture assumption, where the constituent fluid phases are assumed to be at both thermodynamic and mechanical equilibrium. The Peng-Robinson Equation of State along with relevant hydrodynamic and thermodynamic relations are employed to determine the required fluid thermophysical properties and phase equilibrium data. The impact of the valve closure on the in-pipe fluid flow dynamics is accounted for through the implementation of appropriate boundary conditions. The resulting system of conservation equations is solved numerically using the Method of Characteristics. The first part of the study focuses on the investigation of ESDV dynamic response and characteristics on the fluid behaviour following the accidental failure of a hypothetical nevertheless realistic ethylene pipeline. This is divided into two parts: in the first, the results based on the simulation of the pressure surges upon CV closure are presented for a wide practical range of ethylene pipeline operating pressures and temperatures. It is found that for high operating pressures (over 90 bar), pressure surges upon CV closure can lead to a large thrust or bending forces acting on the pipeline segments, potentially damaging the pipeline in the event of exceeding the maximum safe design operating pressure. In the second part, the impact of ASV and RCV activation pressure is assessed by determining the amount of escaped inventory prior to complete isolation in the event of pipeline FBR and different generic sizes of puncture failures. The results show that for FBR and relatively large puncture failures (> 50 % pipe i.d.), decreasing valve activation pressure results in an insignificant reduction in the total inventory loss prior to valve closure. In the case of small puncture diameter failures (less than 5 % pipe i.d.), ASV may not be self-activated given the relatively small pressure drop across the valve throughout the decompression process. The above is followed by the analysis of the efficacy of ESDVs using different inline valve combinations and spacings by performing two sets of investigations. The first includes the simulations of fluid flow behaviour upon valve closure using a hypothetical CO2 pipeline puncture decompression scenario. Different valve spacings and combinations of inline RCVs and CVs are investigated, and the results are presented and discussed in terms of the release pressure and temperature, the discharge mass flowrate and, more importantly, the total mass released, as a function of time during decompression. Next, based on the FBR failure of a high-pressure ethylene pipeline, the efficacy of CVs, RCVs and ASVs and their combinations for the emergency isolation is investigated by comparing the amount of escaped inventory prior to complete valve closure. Building on the above work, the study culminates in optimising inline ESDV configuration by developing a multi-objective optimisation method combined with Principal Component Analysis and applying it to a real, 1016 mm i.d., 150.2 km long natural gas transmission pipeline in China operating at 80 bar and 307.24 K. Starting with defining a set of 6 characteristic variables for ESDV settings, PCA is first employed in order to reduce the number of variables to 3, whilst retaining good agreement for the problem solution and emphasis on computational simplification. Next, a description of the optimisation problem is performed, detailing the objective functions used, including valve capital cost, and the various parameter realistic values assumed. The results of the multi-objective optimisation are presented using scatter plots providing a geometrical visualisation of the Pareto Front and Set. Such results serve as a highly informative tool in assisting pipeline operators in selecting the optimal inline ESDV configurations for pressurised pipelines.

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