Passive Slug Mitigation

The discussions below focus on the passive slug mitigation methods with emphasis on the one applying a flow conditioner to modify the flow regime in the pipeline so as to mitigate severe slugging i.e. flow conditioning. Three typical passive methods including slug catcher, self-gas lifting and flow conditioning are discussed.

Slug Catcher

The most commonly used method to suppress the effects of liquid slugs on the downstream facilities is slug catcher. It is a vessel located downstream of the riser outlet with sufficient volume to buffer the liquid slugs acting as a first stage gas/liquid separator. A slug catcher actually is designed to temporarily store the liquid slugs which will be treated afterwards.

A vessel type slug catcher is essentially a conventional vessel, which is simple in design and maintenance. A ‘finger type’ slug catcher consists of several long pieces of pipes (fingers) of large diameters, which form the buffer volume through the common manifolds. The advantage of the ‘finger type’ slug catcher is that it is much simpler to design the pipe segments for high pressure than a large vessel. A disadvantage is that its footprint can become excessively large.

Generally a slug catcher is not able to deal with all slug sizes due to its limited buffer volume. It is usually sized for hydrodynamic slugs, thus the problems induced by the long liquid slugs under severe slugging can only be mitigated rather than eliminated. In order to eliminate the impacts of severe slugging a much larger slug catcher has to be designed but it may not be achievable due to the limited space on the offshore surface structures. Furthermore, slugs might be larger than expected and consequently the pressure and flow fluctuations could still lead to unfavorable impacts on the downstream processing facilities. Therefore, the issues with slug catchers for slug mitigation are how to predict the slug sizes and how to size the required buffer volume.

Self-gas Lifting

The external-gas lifting needs compressors to compress the large amount of external gas and separate pipelines to transport the compressed gas to the designed injection places (Jansen et al., 1996). In contrast to the external-gas lifting method, the self-gas lifting does not need compressors, extra pipelines and external gas. The gas needed for lifting comes from the gas flow in the pipeline upstream of the riser base.

Sarica and Tengesdal (2000) proposed two types of self-gas lifting methods as illustrated in Figure 2-10. The principle of this technique is to connect the riser to the downwardly inclined segment of the pipeline with a small diameter conduit. The conduit can transfer the gas from the downwardly inclined segment to the riser at points above the riser base. The transfer process can reduce both of the hydrostatic head in the riser and the pressure in the pipeline; consequently the severe slugging can be mitigated or eliminated. It needs to be noted that there are some practical difficulties when applying these methods to the fields. The liquid dropping out from the transferred gas flow may accumulate in the dip of the bypass or inserted pipe; the insertion of the smaller diameter pipe causes inherent intrusion to the flow path, resulting in problems to pigging operations.

(a) External bypass (b) Smaller diameter pipe insertion

Figure 2-10 Self-

Flow Conditioning

A flow conditioner for severe slugging mitigation refers to a pipe device installed in the pipeline upstream of the riser base.

the necessary conditions for severe sluggi

pipeline upstream of the riser base is stratified flow. alter the stratified flow to a

Almeida and Gonçalves (1999

convergent nozzle section followed by a divergent diffuser section as shown in 2-11. This device was proposed to be

was claimed that the venturi mixing effect and converting They performed experimental study rig and verified their claims ( of the contraction section may pose

(a) External bypass (b) Smaller diameter pipe insertion

-gas lifting methods (Sarica and Tengesdal

A flow conditioner for severe slugging mitigation refers to a pipe device installed in the pipeline upstream of the riser base. As proposed by Schmidt et al. (1980,

conditions for severe slugging to occur is that the flow regime in the pipeline upstream of the riser base is stratified flow. The flow conditioner

a non stratified flow in the pipeline, i.e. flow conditioning Almeida and Gonçalves (1999 a) proposed a venturi-shaped device

convergent nozzle section followed by a divergent diffuser section as shown in

This device was proposed to be located near to the riser base in the pipeline. It he venturi-shaped device could introduce a pressure drop

ing the stratified flow to a non-stratified flow temporarily. performed experimental study of the venturi-shaped device on a small

rig and verified their claims (Almeida and Gonçalves, 1999 b). However, may pose problems to the pigging operations for (a) External bypass (b) Smaller diameter pipe insertion

Tengesdal, 2000)

A flow conditioner for severe slugging mitigation refers to a pipe device installed in the 1980, 1985) one of ng to occur is that the flow regime in the flow conditioner is used to

, i.e. flow conditioning. device comprising a convergent nozzle section followed by a divergent diffuser section as shown in Figure in the pipeline. It introduce a pressure drop causing a flow temporarily. a small-scale test However, the existence pigging operations for the pipeline.

Figure 2-11 Venturi

Another type of flow conditioner system was patented by Makogan

be positioned immediately upstream of the riser and comprise upwardly inclined pipe section upstream of a downwardly inclined horizontal pipe section as illustrated in

inclination angle of the upwardly inclined to 90° and the length of the upwardly inclined The lengths of the horizontal section

inclined section in Figure 2 Makogan and Brook (2007)

(a) Upward/downward pipe section

Figure 2-12 Pipe device proposed by

It was claimed that this device could eliminate severe slugging by establishing short liquid slugs in the pipeline. The volume of each liquid slug

enturi-shaped device (Almeida and Gonçalves,

type of flow conditioner for mitigating severe slugging in a pipeline was patented by Makogan and Brook (2007). The pipe device they proposed can positioned immediately upstream of the riser and comprises at least one unit

section upstream of a downwardly inclined pipe

section as illustrated in Figure 2-12 (a) and (b), respectively inclination angle of the upwardly inclined pipe section to the horizontal

and the length of the upwardly inclined pipe section ranges from horizontal sections in Figure 2-12 (a) and (b) and

in Figure 2-12 (a) were recommended to be less than 6.1 m Brook (2007).

ard pipe section (b) Upward/horizontal pipe sections

Pipe device proposed by Makogan and Brook

It was claimed that this device could eliminate severe slugging by establishing short liquid slugs in the pipeline. The volume of each liquid slug could be sufficiently small

, 1999 b)

ging in a pipeline/riser they proposed can least one unit of an pipe section or a respectively. The horizontal ranges from 5° ranges from 0.3 m to 9.1 m. the downwardly recommended to be less than 6.1 m by

(b) Upward/horizontal pipe sections

(2007)

It was claimed that this device could eliminate severe slugging by establishing short sufficiently small

to be transported by the gas pressure building up behind it. Consequently severe slugging could be changed into plug flow or intermittent flow.

et al. (2011) reported an experimental and simulation study on the proposed pipe device.

It was shown that their device however, there was no detailed

explanations to the working principle of

Adedigba (2007) examined the flow characteristics of

two-phase flows in helical pipes. The work performed by Adedigba (2007) focused on helical pipes with the internal diameter

a helical pipe was made by

diameter of 50 mm round a straight steel pipe shown in Figure 2-13.

Figure 2-13 Helical pipe

The helical pipe positioned

Adedigba (2007). It was found that

stratified flow or slug flow prevailed in a straight pipe while the bubbly flow in the tested helical pipe instead. T

potential for severe slugging mitigation because the stratified flow regime could be converted into bubbly flow if installed in the pipeline upstream of the riser base. the performance of the helical pi

transported by the gas pressure building up behind it. Consequently severe slugging could be changed into plug flow or intermittent flow. Most recently

. (2011) reported an experimental and simulation study on the proposed pipe device. It was shown that their device could reduce or eliminate the severe slugging in

detailed presentation of the flow behaviour in their device and working principle of it.

Adedigba (2007) examined the flow characteristics of the single phase and gas in helical pipes. The work performed by Adedigba (2007) focused on

internal diameter of 50 mm and low amplitude. In the experiment a helical pipe was made by ‘wrapping’ a reinforced flexible pipe with

ound a straight steel pipe with a smaller diameter

Helical pipe of low amplitude investigated by Adedigba (2007)

elical pipe positioned horizontally was tested with air/water two

. It was found that, at certain superficial air and water velocities stratified flow or slug flow prevailed in a straight pipe while the bubbly flow

in the tested helical pipe instead. Those findings showed that the helical pipe had a potential for severe slugging mitigation because the stratified flow regime could be converted into bubbly flow if installed in the pipeline upstream of the riser base. the performance of the helical pipe on severe slugging mitigation

transported by the gas pressure building up behind it. Consequently severe Most recently, Makogan . (2011) reported an experimental and simulation study on the proposed pipe device.

reduce or eliminate the severe slugging in risers; the flow behaviour in their device and

single phase and gas/liquid in helical pipes. The work performed by Adedigba (2007) focused on 50 mm and low amplitude. In the experiment with an internal a smaller diameter of 19 mm as

by Adedigba (2007)

water two-phase flow by at certain superficial air and water velocities, the stratified flow or slug flow prevailed in a straight pipe while the bubbly flow occurred hose findings showed that the helical pipe had a potential for severe slugging mitigation because the stratified flow regime could be converted into bubbly flow if installed in the pipeline upstream of the riser base. Then pe on severe slugging mitigation was justified

experimentally. It was demonstrated that the severe slugging region could be reduced by installing a helical pipe upstream of the riser base and even in severe slugging region the severity of the flow regime in terms of liquid/gas surges and pressure oscillations could also be reduced. The helical pipe investigated by Adedigba (2007) had a patented spiral geometry, SMAHT (Small Amplitude Helical Technology). The SMAHT tube is proposed to be made by intertwining a larger diameter pipe against a smaller diameter pipe. The ratio of the helix amplitude to the tube diameter is less than 0.5.

Encouraged by the effectiveness of the SMAHT tube on mitigating severe slugging (Adedigba, 2007), a new pipe device called PST (Pseudo Spiral Tube) was proposed (Yeung and Cao, 2007; Shen and Yeung, 2008). PST is constructed by connecting a series of standard piping elbows (or bends) together. The geometry of PST depends on the internal angle of elbow, the ratio of the elbow radius to the pipe diameter and the angle of twist between two adjacent elbows. The differences between PST and SMAHT tube are highlighted as below (Yeung and Cao, 2007):

 PST is made of standard piping elbows while SMAHT tube is made by intertwining a large diameter pipe against a smaller diameter pipe;

 PST always has a circular cross-sectional area while SMAHT tube may not have a circular cross-sectional area at certain parts;

 PST can have any amplitude to diameter ratios while SMAHT tube is defined as having an amplitude to diameter ratio less than 0.5;

 PST can be of non-helical shape when the twist angle between two adjacent elbows is 180°.

Two PST geometries as shown in Figure 2-14 were tested by Yeung and Cao (2007). PST1 was made of 26 elbows with the internal angle of 45° and with the radius/diameter ratio of 1.5 twisted by 90°; PST2 was made of 7 elbows of 90° internal angle and 2.2 radius/diameter ratio twisted by 180°. A series of tests to examine the effectiveness of PST1 and PST2 on severe slugging mitigation in pipeline/riser systems was carried out. The experimental results showed that PST2 of a wave shape was more effective than PST1 of a helical shape when located in the pipeline upstream of the riser.

Figure 2-14 PST geometries

This work focuses on the investigation o ‘wavy pipe’ is adopted in the rest of the a wavy pipe can be found in Chapter 3.