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[287d] - Stabilization of Desired Flow Regimes in Pipelines
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Abstract: Stabilization of desired flow regimes
in pipelines Kjetil Havre Stabilization of desired fluid flow regimes offers challenges of
immense potential value. The opportunities for control engineers in
this field are very large, because most fluid flow experts tend to
be "feedforward thinkers" with only limited insight into the
potential benefits of feedback control. They tend to believe that
the stability regions of a given steady-state flow pattern is fixed
by nature. However, these are open-loop stability regions, and with
feedback control one may change these boundaries.
The most well-known example is probably the transition from
laminar to turbulent flow in single phase pipelines which is known
to occur at a Re-number of about 2300. However, it is well known
that by carefully increasing the flow rate one may achieve laminar
flow at much larger Re-numbers, but that in this case a small knock
at the pipeline will immediately change the flow to turbulent. Some
attempts have been made in applying control to this problem (e.g.
see Bewley, 2001 for a survey), but the short time and length scales
make practical applications difficult.
Another unstable flow phenomenona occurs in multi-phase
pipelines, where pressure-flow fluctuations known as slug-flow can
be induced both by velocity difference between the gas and liquid
phase (hydrodynamic slugging) and by the pipe geometry (severe
slugging, terrain slugging, riser induced slugging). The latter
severe slugging phenomen occurs at a time and length scale that
males control a viable option. In addition, as a result of advances
in the petroleum industry, multi-phase pipelines are becoming both
longer and more common. In many cases severe slugging induced by the
terrain on the seabed or by the vertical distance to the platform is
a major problem for production, as it leads to large load
disturbances for the inlet separator causing compressor trip or
flooding of the separator. Slugging can also complicate future
transport of petroleum straight to shore, which would eliminate the
need for expensive processing platforms.
Elimination of slugging by changing the design or the operating
point (e.g. increasing the pressure, Taitel, 1986) has been suggsted
by many authors, e.g. see Sarica and Tengesdal (2000) for an
overview. The objective here is get to an operating point where the
desired flow regime is (open-loop) stable.
The objective of this paper is to study the use of feedback
control in order to operate in an operating point where the desired
flow-regime is (open-loop) unstable. The use of feedback control to
avoid severe slugging was proposed and applied on a test rig by
Hedne and Linga (1990). Later independent studies, including
simulations and actual implementations, are reported by Henriot et
al (1999) and Havre et al. (2000). The two-phase simulations were
performed using the industrial simulators TACITE and OLGA,
respectively, but these were essentially used as "black box"
simulators to test the proposed control strategies.
The objective of our work is to provide a theoretical basis and
understanding into how feedback control may be used to avoid severe
slugging. We use a simplified model based on the "two-fluid"
approach and from this investigate the existence of multiple
steady-state solutions, the stability properties and input-output
controllability. The model shows that, for a given total flow rate,
multiple steady-state solutions exist. One is the desired (but
unstable) stratified flow. The other is the undesired (but stable)
slug flow limit cycle.
The task of eliminating severe slugging in pipelines consists of
two sub-problems:
1) Break the limit cycle and bring the process to its desired
flow regime. A model of the slugging phenomena is obviously useful
here. The problem may be solved by either
(a) Directly applying feedback control. Some nonlinear strategy
is most likely needed, which, for example, could involve choosing
where in the cycle to activate the controller.
(b) Change the operating point to conditions where the desired
flow regime is open-loop stable, for example by increasing the
pressure (Taitel, 1986). In case (b) one must afterwards apply
feedback control to move to the desired conditions (e.g. lower the
pressure), and simple linear control may possibly be used for this.
2) Keep the process at it desired unstable operating point (flow
regime). Feedback control is the only possible option here since
feedforward control can not be used for stabilization. Linear
control theory is sufficient for this task.
In this paper the focus is on this second sub-problem. A model of
the desired stratified flow regime is used to study the best way of
stabilizing the process, including measurement selection and the
required speed of response (bandwith).
The full paper can be downloaded from:
http://www.chembio.ntnu.no/users/skoge/publications/2001/storkaas_reno/
References
Bewley, T.R.: "Flow Control: new challenges for a new
Renaissance", Progress in Aerospace Sciences, 37 (2000) p.21-58
Hedne, P and Linga, H.: "Suppression of Terrain Slugging whith
automatic and manual Riser choking" Advances in Gas-Liquid Flows,
1990, p.453-460
Henriot,V., Courbot,A., Heintzé, E. and Moyeux, L.: "Simulation
of Process to Control Severe Slugging: Application to the Dunbar
Pipeline" paper SPE56461 presented at the SPE Annual Technical
Conference and exibition held in Huston, Texas 3-6 October 1999
Havre, K., Stornes, K.O. and Stray, H.: "Taming slug flow in
pipelines" ABB Review 4/2000, p.55-63.
Sarica, C. and Tengesdal, J.Ř., "A new technique to eliminate
severe slugging in pipeline/riser systems", Paper SPE63185 presented
at 2000 SPE Annual Technical Conference and Exhibition, Dallas, 1-4
Oct. 2000
Taitel, Y. "Stability of severe slugging", Int. J. Multiphase
Flow, 1986, Vol. 12, No. 2,pp. 203-217
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