Terrain slugging

Terrain slugging is caused by the accumulation of significant amounts of liquid in low points along the line. Once the liquid bridges to the top of the pipe, the gas trapped upstream of the liquid slug starts to be compressed, until it reaches a pressure sufficient to overcome the hydrostatic head of the liquid and a chaotic blowout expansion will then occur.

As the slug then moves through an uphill section of the line, liquid is shed from its rear and runs back down the slope to the low point, while at the same time stratified liquid is scooped up in front of the slug and added to its front. If insufficient liquid is available to be scooped up in front of the slug to replace that lost at the rear, then the slug will collapse before it reaches the next high point in the line. In systems with a steady liquid inflow, the amount of liquid in the line eventually accumulates to the point where terrain induced slugs successfully emerge from the system. Due to gravity effects, terrain slugging is worse in downward-sloping lines.

Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards


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