One of the flow assurance challenges in subsea production systems is the occurrence of erosion damage due to the existence of sand particles and high production Gas Oil Ratio (GOR) as such erosion mostly occurs in highly gas dominated operating conditions in the annular flow regime. The erosion rate for an elbow with a constant flow velocity and with all other factors equal is higher in gas systems than liquid systems as more particles will impact on the inner wall of the outer curvature of the elbow. The maximum wear location and the penetration rate for multiphase flows are often an intermediary of gas and liquid systems occurring at 55 degrees from the inlet of the elbow, however this depends heavily on the multiphase flow regime. A challenge facing industry is availability of erosion prediction models; the majority of available models are based on singlephase liquid or gas as the carrying medium. This can result in large discrepancies in erosion rates and potentially increased wall thickness, fabrication and subsequent intervention costs.To predict the flow regime in greater clarity requires the use of computational power and / or instrumentation that can accurately characterize the flow within the pipes. Since experimental work is costly and unlikely to be representative of a large integrated production system, Computational Fluid Dynamics (CFD) is used to perform erosion assessments and can also aide in corrosion prediction and inhibitor selection. Only erosion assessments by CFD methods are discussed in detail within this paper. CFD has been extensively applied for erosion analyses; it is commonly used for identifying potential failure locations, improving understanding of failure mechanisms and only qualitatively used for erosion rates.CFD erosion modelling capability in this paper has been enhanced by simulating flow regime characteristics, in particular the liquid film for annular flow. This benefits the simulation to obtain greater accuracy for sand particle impact angles, area, speed and thus the erosion rate is significantly enhanced. In addition, the local volume fraction of sand has been considered in order to accurately evaluate the impact force. The research to date shows that a promising agreement is obtained between predicted erosion rate and the empirical predictions (Salama, Salama & Venkatesh and DNV RP-501 methods).Further comparisons to empirical model predictions are carried out to address the importance of flow regime on the results as current empirical models lack this consideration. The influence of the flow orientation (upwards and downwards flow), has also been investigated in this work due to current lack of publically available data. The paper presented hereafter illustrates that considerable difference in flow orientation is revealed and the prediction can be improved by considering the flow characteristics. An example is provided highlighting the use of liquid film and droplet velocity to replace the mixture velocity implemented in empirical models for annular flow. All of the fi...
The Pseudo Dry Gas (PDG) technology / concept has been demonstrated for transporting wet gas in a long subsea tieback pipeline (200 km) in deep water depths (1.8 km) under wet gas conditions (water saturated gas) [Ref.1] along with a state of the art technology review of existing solutions. When a multiple of these in-line / piggable liquid removal units are used, they help to reduce the well back pressure by reducing the liquid content to an extent where ‘dry gas’ pressure losses are seen. Therefore, this mitigation of the gravitational pressure drop allows the use of larger pipelines to minimise the frictional pressure drop. This in turn increases recovery of reserves and allows tie back distances to be enhanced. The objective of this paper is to investigate a Pseudo Dry Gas System (PDGS) for an ultra-long deep-water gas condensate development, building upon the research and development already conducted with Strathclyde University. This work was undertaken using non-standard flow assurance methodologies and simulations recycling data and results with the advanced Computational Fluid Dynamics simulations of the liquid removal units behaviour, over various operational boundary conditions. Engagement with subsea equipment suppliers based on the flow assurance results has been undertaken. This paper describes how gas condensates within a subsea tieback system behave very differently to condensed water from a wet gas system and therefore a pseudo dry gas system needs to be configured differently for gas condensate developments. These differences include how and where the liquid drops out of the gas phase, where and if the free liquid is reabsorbed back into the gas stream and how the bubble point of condensate is equal to or very close to liquid removal units operating pressure; this greatly impacts the liquid handling system compared to a wet gas (water) design. Therefore, to ensure controlled liquid only transportation, careful examination of the liquid removal units performance, the liquid pump selection criteria and optimisation of the system needs to be undertaken. This results in a trade-off between maximum reserve recovery and system complexity. The paper demonstrates that the liquid condensate system will remain as a single liquid phase pipeline, where the number of pumps can be reduced and the pump power requirements are very low and within the existing technically qualified subsea pumps.
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