The combination of declining reservoir pressure and increasing water production in aging fields along with certain common pipeline/riser geometries can create or exacerbate riser slugging. The severity of riser slugging typically becomes worse with increasing riser height and may be especially problematic for some aging deep-water production systems. Severe riser slugging is characterized by large fluctuations of liquid (oil and water) and gas production arriving at the topsides first stage separator. The severe slugging cycle is divided in four steps as described in Figure 1. The classic pipe geometry for severe slugging is a slightly downward section upstream of a riser. In step 1, gas and liquid velocities are low enough to allow stratified flow in the downward sloping pipe section followed by liquid bridging and accumulation at the bottom of the riser. The hydrostatic pressure of the accumulated liquid initially increases equal to or faster than the build-up of gas pressure upstream of the liquid plug (Step 2). When the gas pressure eventually exceeds the hydrostatic head of the liquid slug, the gas will begin to push the liquid plug out of the riser and start to penetrate the riser (Step 3). The pressure in the gas reduces as the liquid is removed from the riser and the gas expands increasing the velocities in the riser. After most of the liquid and gas exits the riser, the velocity of the gas is no longer high enough to sweep the liquid upwards. Liquid film not swept from the riser starts falling back down the riser (Step 4) and the accumulation of liquid starts again.

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