An Optimisation Framework to Reduce the Risk of Hydrate Blockages in Flowlines

The formation and subsequent blockage of natural gas hydrates in subsea oil and gas flowlines remains one of the most prevalent flow assurance concerns for a deepwater operator today. The risk for a hydrate blockage to occur is particularly exasperated during any restart operation that follows an extended shut-down period when the fluids contained in the flowline cool to the surrounding sea water temperature and come into contact with any free water that has accumulated at the flowline low-spots. Through the pioneering experimental work of Leporcher et al. [2002] and Volk et al. [2007], it is apparent that a hydrodynamic strategy could potentially be adopted as a means of reducing the risk of hydrate blockage during restart operations, such that the current techniques used to avoid hydrate blockages, including chemical inhibition, thermal control and de-watering exercises, are not required, allowing for production to be brought back online immediately. This dissertation explores the possibility that Computational Fluid Dynamics (CFD) may be used in conjunction with common meta-heuristic optimisation techniques as a tool to assess the risk of hydrate blockage during restart operations in gas-dominated flowlines featuring accumulated water zones, without chemical inhibition and thermal control. For this purpose, a high-fidelity CFD model of a 3-D half-pipe is first constructed using OpenFOAM and SALOME in order to investigate the capability of the twophase interFoam solver to capture turbulent mixtures of air and water and oil and gas at different pipe inclinations. A total of 120 simulations are performed and observations made for stratified, slug, dispersed bubble, annular and froth flow. The values of liquid holdup and pressure drop obtained from the CFD simulations are compared to results obtained from the mechanistic model developed by Petalas and Aziz [2000] at three separate inclinations of +10◦, 0◦ and -10◦. The results show that the flow pattern behaviour and numerical values of liquid holdup and pressure drop compare reasonably well to the mechanistic model across all flow regimes considered, giving confidence that the model can be used for optimisation studies, albeit at a computational cost that is too high for practical use. In order to overcome the excessive computational cost incurred from the highfidelity CFD model, a modified CFD modelling strategy is proposed using scalablewall functions for the purpose of minimising the computational expense whilst still providing an acceptable representation of the likely flow patterns involved during a restart operation; stratified and slug flow. This approach is verified using three experimental studies focussing on stratified and slug flow in long pipes and a gassweep in a pipe featuring a single low-spot with an accumulated water region. These studies are used to investigate the influence of the longitudinal mesh resolution on the simulation accuracy, where the mesh resolution recommended for use in the framework has been successfully reduced by over 97% when compared to the original model. Using the modified CFD modelling strategy, a CFD model is constructed to emulate a restart operation of a single low-spot bend using the same geometry presented in the experimental study of Leporcher et al. [2002]. Through this study, a novel strategy for the implementation of a CFD-based optimisation approach to reduce the risk of hydrate blockages in flowlines is proposed. Here, the most appropriate metaheuristic optimisation techniques available in the DAKOTA toolkit are assessed and a modification is made to the interFoam solver in order to capture the interfacial area between the gas and water phases at each time step, allowing for an estimate of the total amount of hydrate growth to be made using a combination of the kinetic model by Turner et al. [2005] and mass transfer-limited model by Skovborg and Rasmussen [1994]. The results obtained from this study have demonstrated that the surrogatebased optimisation strategy facilitated using DAKOTA has been particularly effective as a means to efficiently project the hydrate growth results obtained from a defined number of sampling points throughout the design space, providing an avenue for the implementation of the optimisation approach in the form of a comprehensive framework. The optimisation approach used for the single low-spot bend is then further refined through the use of an applied example featuring the Ormen Lange gas field located offshore of Norway. The methodology used in this example is assessed for the purpose of usability and then adapted to form the basis of a comprehensive optimisation framework, applicable to different types of flowline geometries and fluid properties. The final form of the framework is presented as a black box interface, facilitating communication between OpenFOAM, SALOME and DAKOTA through the use of a text-based user interface to allow for streamlined user input and ensure that the framework may serve as a useful investigative tool for a flowline operator.