Severe Slugging

What is Severe Slugging?

At low gas and liquid flow rates, unsteady state flow may occur in such two-phase pipeline-riser systems. The cyclic unsteady state flow characterised by large-amplitude, relatively long-period pressure and flow rate fluctuations is know as severe slugging.

At relatively low flow rates, liquid accumulates at the bottom of the riser, blocking the gas, until sufficient upstream pressure has been built up to surge the liquid slug out of the riser followed by gas surge. After fluid and gas surge, part of the liquid in the riser falls back to the riser base to create a new blockage and the cycle repeats. This transient cyclic phenomenon causes period of no liquid and gas production at the riser top followed by very high liquid and gas surges, and is called severe slugging.

Figure 1 shows the different stages of a cycle of severe slugging

Fig. 1 : Stages of Severe Slugging

Types of Severe slugging

Severe Slugging of type 1

To highlight the differences between all types of severe slugging, we can better describe a cycle of SS1 in five stages: (1) Blockage of the riser base ; (2) slug growth; (3) liquid production; (4) fast liquid production; (5) gas blowdown. In Fig. 2 these five stages are illustrated.

Fig. 2: Stages for severe slugging of type 1 (a) a graphical illustration (b) marked on a cycle of an experimental riser $\Delta\:P$ trace (USL​=0.2 m/s & USG0​ =1 m/s)

Severe Slugging of type 2

The transitional severe slugging of type 2 is qualitatively similar to SS1, but the slug length is shorter than the height of the riser and it often has intermittent unstable oscillations. In Fig. 3 four stages of SS2 are illustrated.

Fig. 3: Stages for severe slugging of type 2 (a) a graphical illustration (b) marked on a cycle of an experimental riser $\Delta\; P$ trace (USL= 0.10 m/s & USG​=2.00 m/s)

Severe Slugging of type 3

We describe a cycle of SS3 in four stages: (1) transient slugs; (2) aerated slug growth; (3) fast aerated liquid production; (4) gas blowdown. In Fig. 4 these four stages are illustrated.

Fig. 4: Stages for severe slugging of type 3 (a) a graphical illustration (b) marked on a cycle of an experimental riser $\Delta P$ trace (USL= 0.39 m/s & USG​=2.33 m/s)

Severe Slugging Risks

Severe slugging creates potential problems in the platform facilities downstream of the riser top which have been designed to operate under steady state conditions, e.g. separators, pumps and compressors. During the liquid and gas surges, the peak flow rates might cause overpressurization of the separator, which consequently might lead to the complete shutdown of a production facility. Moreover, an increased back pressure at the wellhead may lead to the end of the production and abandonment of the well. These repeating impacts provoke a faster mechanical fatigue and can eventually lead to a rupture.

Therefore, the accurate prediction of severe slugging characteristics is essential for the proper design and operation of two-phase flow in these systems