Severe slugging is reported from some field operations, when an increase in the production rate leads to a transition from steady stratified flow to slug flow in the pipeline. The slugs can be longer than anticipated for normal hydrodynamic slugging and the flow transients can then be a limitation for the production capacity of the system.

Long initial slugs at the transition to slug flow has been demonstrated in air-water laboratory experiments earlier, where a delay in the stratified-slug transition causes excess liquid buildup in the stratified layer, which blows out as a long plug at the time of slug initiation.

New experiments have been made in order to investigate this flow phenomenon at high pressures. The slug initiation at low pressure has also been studied with a slug tracking code, allowing numerical experimentation to be carried out.

The flow experiments were conducted at the SINTEF Multiphase Flow Laboratory (69 mm inner diameter, 100 m long, inclination −0.1°, gas densities 1, 19, and 46 kg/m3). Liquid height was measured with gamma densitometers at four axial locations. Two different inlet flow conditions were applied to the test section (slug flow or stratified flow) in order to promote early or delayed flow regime transitions. Transient experiments were made, in which the gas flow rate was kept constant, and the liquid flow rate carefully increased across the flow regime transition.

In the low pressure experiments, the first observed slugs at the point of transition could become very long, but the subsequent slugging after the initial slugs was less severe. Long initial slugs were not observed in the high pressure experiments. The phenomenon can be reproduced by a slug tracking model, provided an appropriate slug initiation model is applied.

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