Study on Severe Slugging in an S-Shaped Riser: Small-Scale Experiments Compared With Simulations
Severe slugging is a transient multiphase-flow phenomenon that can occur in pipeline-riser systems, particularly in offshore production of oil and gas. It is characterized by large pressure fluctuations at the base of the riser and is accompanied by fluctuations in fluid delivery from the top of the riser. This unstable phenomenon is undesirable because production and equipment are affected adversely by the large pressure and flow-rate fluctuations. In this study, air-/water-flow experiments have been carried out at the S-shaped-riser facility in the multiphase-flow laboratory of the Norwegian University of Science and Technology (NTNU) and have been compared with results from a flow simulator (OLGA®). The results obtained in the work show that stability maps, pressure amplitudes, and slug frequencies are in acceptable agreement with each other; however, some deviations are seen regarding the slug frequencies at low flow rates.
The petroleum industry has been forced to focus on development in deep seas because the demand for oil is increasing rapidly and few new fields have been discovered recently. This has led to a remarkable growth in subsea technology over the past few decades, and novel engineering solutions have been implemented to reduce costs, thereby making marginal fields economically and technically viable. One of these engineering solutions is the transportation of reservoir fluids from wells to processing units in the form of multiphase flows. A broad term, “flow assurance,” is used for the large range of challenges related to safe design and operation of such multiphase-transportation systems. Some typical flow-assurance concerns are (Bai and Bai 2010)
- System deliverability: pressure drop vs. production, pipeline sizing, pressure boosting, slugging, and emulsion
- Thermal behavior: temperature distribution, temperature change because of startup and shutdown, insulation options, and heating requirements
- Solids and chemical inhibitors: hydrates, waxes, asphaltenes, and scaling
Among these flow-assurance concerns, management of slugging in system deliverability has received much interest in recent years.
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01 June 2018