Summary
With the recent developments in the oil and gas industry, in which the
number of satellite fields are increasing, the industry faces major challenges.
A mixture of gas and oil is transported in multiphase pipelines along the
bottom of the sea from the wells and up through the riser to the oil rig. The
development of unstable flow in multiphase pipelines is a major and expensive
problem. The irregular flow results in poor oil/water separation, limits the
production capacity, and causes flaring. Handling slug flow thus has become
very important with regard to increasing the production rate and avoiding
possible equipment damages.
In this paper, we have developed a model of a physical process that
generates slugs. The model has been verified against experimental data. Several
control strategies have been tested on the model, and the simulation results
are presented. It is shown by simulation of multiphase flow that the unstable
slug flow can be stabilized by feedback control. In addition, interesting and
important phenomena such as inverse response of the pressure at the top and
asymmetric response of the pressure at the bottom have been revealed and
explained.
Introduction
Multiphase pipelines connecting remote wellhead platforms and subsea wells
are a common feature of offshore oil production. Recent developments indicate
that this trend will be followed in the future. In addition, the feasibility of
using long-distance tieback pipelines to connect subsea processing units
directly to onshore processing plants makes it likely that these will also be
deployed in the future. These trends in offshore oil production face a
major challenge.
The slug phenomenon in multiphase flow lines with both gas and liquid
hydrocarbons has gained increased interest in recent years. Slugging can be
characterized as either hydrodynamic or terrain slugging. Hydrodynamic slugs
are built in horizontal parts of the pipeline when liquid and gas velocities
are different. These slugs are usually short with higher frequency. The inlet
separator will in most cases handle these slugs with fewer difficulties,
because the amount of liquid is small compared to the volume of the separator.
On the contrary, a terrain slug can contain a lot of liquid and represents a
great challenge to the downstream processing system. Level and pressure
variations in the first-stage separator can be propagated in the downstream
process. Variations in the separator level may cause poor separation and
occasionally fluid flooding. Oscillations in the separator pressure can result
in increased flaring, which is environmentally benign and is also costly
because of CO2 taxes.
The degree of slugging depends on various factors. The most important are
pipeline pressure, pipeline topology, and production rate. Terrain slugging is
most likely to occur at low rates, with a low pipeline pressure toward the end
of a well’s lifetime.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
28 March 2003
- Revised manuscript received:
28 February 2005
- Manuscript approved:
2 March 2005
- Version of record:
20 August 2006
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