Long liquid slugs reaching several hundreds pipe diameter may appear when transporting gas and liquid in horizontal and near horizontal pipes. The long slugs cause system vibration and separation difficulties that may lead to operational problems. Although mainly short hydrodynamic slugs have been observed in offshore production fields, the appearance of the long slugs becomes more likely as the fields become older, giving reduced production rates and a reduced operational pressure.

In this paper we present a method for reducing the negative effects of the long slugs in horizontal pipes. The method is based on increasing the slug frequency, at constant flow conditions, by using an upwardly inclined inlet section. The flow in the inclined section results in a faster formation of slugs upstream in the pipeline. As a result, the slug frequency increases and slugs become fully developed earlier in the pipe preventing further growth into the long slug region.

For validation of the method, experiments have been carried out in a 137 m long air-water horizontal pipe flow with an internal diameter of 0.052 m, using different inlet sections. The results show that, at relatively large flow rates, there is an inverse relationship between the slug length and frequency. However, the slug length becomes less sensitive to the change in slug frequency at low flow rates.

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