Experimental work has been performed at to investigate the effect of viscosity on flow regimes and slug characteristics. The studies, performed on a 2” air water/glycerol rig, showed that the slug flow region of the expands with increasing viscosity. This increase in the area of the slug flow region is in close agreement with the predictions of the Taitel-Dukler
No data has been collected on the effect of viscosity on slug characteristics. However, the BP model correctly predicts a higher equilibrium liquid hold-up with increasing viscosity. As a higher hold-up in practice results in more frequent slugging, we can confidently predict that increasing liquid viscosity will give rise to smaller, more frequent slugs.
Recommended
In the BPE Report “Summary of Field Data on Slugging Multiphase Systems” it was commented that flow regime data collected by BP had shown that the original (T-D)
was “unsafe” in that in many instances it predicted the flow to be annular when in fact slug flow was observed. The Barnea modified T-D predicts that the annular to slug transition occurs at higher gas velocities than the original T-D and so has a larger slug flow region. All the tests where slug flow was observed at Prudhoe Bay were predicted to be in slug flow by the Barnea modified T-D On some occasions, however, annular flow was experienced when slug flow was predicted. The Barnea modified T-D can thus be thought of as providing a safe, or conservative, prediction of flow regime.
The Barnea modified T-D is now available in and is the recommended method of evaluating flow regime in crude oil/gas multiphase pipelines.
BP Multiphase Design Manual Section 3. Gas Pipeline Design
3.3.4 Effect of Pipeline Geometry and Risers on Flow Regime
The type of slug flow indicated by flow regime maps arises as a result of fluid dynamic insta-bility. It is frequently described as normal slug flow since it can occur in perfectly horizontal lines and does not depend for its generation on line topography or throughput changes.
Slugs may, however, be generated by other mechanisms. In particular when stratified flow occurs in a pipeline which undergoes changes in inclination, liquid may accumulate at low points to form a temporary blockage. Eventually the pressure builds up behind the blockage causing the liquid to be expelled and transported through the line as a slug. This type of flow is termed terrain, or geometry dependent slug flow.
when a terminates in a riser a particular type of terrain slugging may occur where some, or all, of the riser fills with liquid. This phenomenon, known as severe slugging, imparts greater perturbations on the process plant, and larger forces on pipe bends, than does normal slug flow. Severe slugging occurs when stratified flow occurs in the flowline, and the ratio of gas to liquid is below some critical value such that the rate of pressure build-up upstream of the blockage at the riser base is insufficient to overcome the increase in hydro-static head as the slug grows in the riser. (Severe slugging is discussed in detail in Section 3.4.2 and in Appendix 3C).
The conditions leading to the occurrence of severe slugging have been determined from experi-mental tests at
The Multiphase Flow Group at have developed a mechanistic model which describes the conditions leading to the occurrence of severe slugging. This model has been validated with data obtained from the experimental facility. The mathematical expression devel-oped defines a critical liquid velocity above which severe slugging will occur:
where:
= separation pressure
L = line length upstream of the riser(m) = liquid density
= gas density
average liquid hold-up in line g = acceleration due to gravity (9.81
inclination of riser to horizontal (i.e. sin is usually 1)
This expression, in a slightly different form, was later published by Shell following studies carried out on an experimental facility at the KSLA Laboratories. Shell defined a dimensionless number
as:
i
Section 3. Gas Design BP Multiphase Design Manual
Hence when severe slugging is not experienced.
The expression developed by BP for severe slugging can be drawn on the T-D in MULTIFLO, and this then provides a flow regime map for the complete flowline-riser system.
When the flow regime is predicted to be stratified-wavy, but lies to the right of the severe slug-ging line (i.e. at gas velocities in excess of that required for severe slugslug-ging), then wavy flow will occur in the flowline, and a churn or annular type flow will be experienced in the riser.
When the flow regime is anything other than stratified or stratified-wavy flow in the flowline, then that type of flow will persist through the riser. In particular if normal slug flow occurs in the line, then the slugs will continue on through the riser. Field data gathered from the Forties Echo-Alpha inter-field pipelines have shown that at moderate flowrates, slugs, once formed in the flowline, do persist essentially unaltered through the riser. However, at lower flowrates it was observed that although slug flow was maintained through the riser, the slugs underwent significant deceleration in the riser. This type of flow, termed slug deceleration, is discussed further in Section 3.4.1 below.
BP Multiphase Design Manual Section Oil Gas Pipeline Design