Shell Stabilizes Long Pipeline-Riser Gas/Liquid Flow

Author(s):
Gert Haandrikman - Shell Global Solutions International B.V.

Industry:
Oil and Gas/ Refining/ Chemicals

Products:
Compact FieldPoint, LabVIEW, FieldPoint

The Challenge:
Reducing development and controlling of large liquid slugs forming in a flowline/riser system because of operational changes such as start-up, production increase, or flow conditions.

The Solution:
Using LabVIEW Real-Time and FieldPoint distributed I/O to develop the Slug Suppression System (S 3 ) that consists of a miniseparator positioned between the riser top and the normal first stage separator.

Slugging Behavior of Pipelines
Pipelines or flowline/riser systems transport liquid hydrocarbons, gas, and water from satellite wells to a central production platform. We often prefer a single pipeline for economic reasons. Ideally, a pipeline would produce a constant amount of gas and liquid. In a single pipeline, however, segregated flow of liquid and gas may cause problems. The actual velocity of the gas phase is faster than the actual liquid velocity. The liquid phase has the tendency to accumulate in the dips and inclined pipe sections causing irregular flow behavior. As a result, large volumes of liquid may flow through the pipeline. These plugs of liquid are called slugs, or riser-induced slugging, and hydrodynamic slugs. Furthermore, operational changes, such as start-up and production increase, can create large liquid slugs. Liquid slugs at the outlet of pipelines or flowline/riser systems may result in large oil and gas production losses. Production deferment results from poor use of downstream separators, process instabilities, time-consuming start-ups and, especially for flowline/riser systems, topside choking to avoid slugging. An additional problem with slugging is the compression of the gas phase behind a slug. The transportation of a slug requires a larger pressure behind the slug to keep the plug moving through the pipeline. This pressure increase depends on the size of the liquid slug. After the slug arrives at the outlet of the pipeline or production platform, the compressed gas creates a large gas surge, which again may result in major upsets in topside facilities.

Suppressing and Controlling Slugs
The S 3 , developed by Shell Global Solutions and licensed to Dril-Quip for marketing, sales, and manufacture, consists of a miniseparator positioned between the riser top and the normal first-stage separator. The miniseparator has two outlets - one for the gas flow and one for the liquid flow. Valves control both outlet flows, which receive their signals from a control system. This control system uses LabVIEW Real-Time software and FieldPoint distributed I/O. We developed a control strategy that not only suppresses severe slugging, but also controls transient slugs without gas surges.

Accurate Control System
The circuitry, as used in the S 3 control system, comprises a redundant section of National Instruments FieldPoint 2010 real-time controllers. The FieldPoint 2010 provides information for gathering and control, which also includes serial interface communication to existing control systems (ECS). We can also use the serial interface communication to remotely control set points and modes of operation. With this redundancy built in, the availability of the system is 99.95 percent, assuming a 4 hr repair period for any downtime.
We programmed the entire application in National Instruments LabVIEW. For the control algorithms, we applied standard LabVIEW PID Control Toolset blocks, plus additional algorithms to ensure correct and fast control of the slug suppression system when modes of control are changing. Implementation in existing PLC and DCS tools is not straightforward because of the complexity of these additional control algorithms, but LabVIEW provided the correct set of tools and abstractions. During the development of the application, we could test ideas quickly and easily and analyze applicability with LabVIEW. This process significantly improved the research and development speed for the S 3 . Additionally, instead of hiring a specialist, we could build applications in LabVIEW with only two days of training.

From Test to Production
We performed the first tests with the system on an air-water test loop consisting of a 100 m pipeline and a 15 m riser. We developed the complete measurement and control system on this loop. After these tests, we evaluated the system successfully under field conditions on the Shell U.K. E&P Gannet-Alpha platform (Central North Sea). While the first test systems included standard LabVIEW software, data acquisition hardware, and SCXI, we plan to use LabVIEW Real-Time and the FieldPoint 2010 Series of distributed I/O in the first commercially available systems. These systems can download LabVIEW code directly to the FieldPoint modules, ensuring real-time and reliable operation. The control loops are independent of an OS on a PC, which further optimizes the control system.

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