A CFD-Based Model to Locate Flow-Restriction Induced Hydrate Deposition in Pipelines

Abstract

Abstract Hydrate is a major risk in all high pressure natural gas transport lines including the connecting lines and manifold systems in all offshore production facilities. Marine transportation of compressed natural gas is one example where prediction of hydrate formation in loading and unloading lines and manifold systems is a requirement for the safe transport of gas to and from ocean going ships. Production facility components such as chokes, velocity-controlled subsurface safety valves, and conventional valves and fittings can all act as restrictions to the flowing fluids, resulting in changes in flow conditions, which could lead to the formation of hydrate in the pipeline. The principal objectives of our research can be summarized as:to identify the location where hydrate blockage would most likely develop,to study the effect of parameters such as orifice geometry, gas composition, real gas behaviour, and surrounding conditions on the agglomeration spot, andto validate the numerical results with available experimental data. Numerical simulation using computational fluid dynamics (CFD) techniques is underway to model the mechanism of the deposition based on the most recent theories of the deposition phenomenon. The model uses CFD algorithm for assisting to configure the flow field using real gas models and predicting the actual fluid properties. The nucleation theory and driving force models and their relationships to hydrate formation are also used to predict the incipient hydrate particle size and growth rate. In this paper, the theory of the deposition mechanism is briefly discussed. The application of the theory in turbulent regime for different hydrate particle sizes is then presented. Finally the approach used for deposition process is discussed. The study concludes that two phenomenon control the deposition mechanism, namely: the Brownian diffusion mechanism by which the movement of small particles (<1µm) can be explained and the inertia mechanism which controls the dynamics of the relatively large particles. The collection efficiency, the indicator of the deposited particles, decreases as the size of the particles increases in the diffusion region whereas in the impaction region, the collection efficiency increases with particle size. Introduction Particle deposition is a process that plays a key role in many fields ranging from atmospheric applications to material sciences. In the oil and gas field, the accumulation of hydrate is one of the most challenging aspects in flow assurance studies. It could partially plug and eventually completely block the natural gas pipeline, causing serious risk to the safety of operating personnel and equipment as well.