ABSTRACT

An accurate calculation of a pipeline's hydraulic capacity is crucial for an optimal utilization of the gas transport network. The calculated capacity is the capacity that is made available for sale to the owners and shippers of the gas. This capacity is found by means of a capacity test, where the wall roughness is used to tune the model to match the flow conditions from the steady state test period. Based on this roughness the friction factor is extrapolated along the appropriate Colebrook-White friction factor curve to find the hydraulic capacity. This method is based on the assumption that the Colebrook-White curves are the best tool for capacity estimation available. The Colebrook-White formula is a combination of the law of the smooth and the rough turbulent flow found by Nikuradse. These two formulas have been widely employed for almost one century. However, the transition region where the roughness elements start to protrude significantly into the viscous sublayer bears significant uncertainty. Most of the pipelines operated by Gassco are in the early phase of this transition. Research conducted by many parties suggests different descriptions of the fluid behaviour and hence the friction factor in this region, which subsequently leads to different capacity calculations. A more accurate description of the pressure loss would enable Gassco, and other pipeline operators, to perform more accurate calculations of the pipeline capacity, and hence in most cases increase the transport capacity. Furthermore, the online models will be more accurate, allowing an improved capacity exploitation. This was the reason for starting the work that will be presented here. This paper documents research indicating that the use of the Colebrook-White friction factor correlation in commercially available pipeline simulation tools may be improved. A database of historical steady state operational data from real pipelines have been collected and analyzed. The friction factor is used to tune the pipeline model to match the measured pressure drop and flow rates for a range of operational conditions. It has been found that the friction factor, when applied to the simulation models, decreases faster than predicted by the Colebrook-White correlation. Reynolds numbers in the range has have been investigated. Transition from smooth to rough turbulent flow is expected for such operating conditions in systems having the pipeline feautures treated in this paper. 6104010⋅− It is shown how these results influence the capacity calculations. A low flow rate in the capacity test will lead to a conservative calculation of the capacity as the friction factor decreases faster than predicted by Colebrook-White. The potential for increased calculated capacity is shown to be in the range 0.5-2.0 %. This amounts to a potential annual increase in the gas export from the Norwegian Continental Shelf by 100-400 million $.

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