Shell Stabilizes Long Pipeline-Riser Gas/Liquid Flow

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

Industry:
Oil and Gas

Products:
Real-Time Module, PID and Fuzzy Logic, LabVIEW, FieldPoint, FP-2010

The Challenge:
Controlling and reducing the development of large liquid slugs forming in a flowline/riser system due to operational changes such as start-up, production increase, or flow conditions.

The Solution:
Using the NI LabVIEW Real-Time Module 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. Ideally, a pipeline would produce a constant amount of gas and liquid. However, in a single pipeline, 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, called slugs or riser-induced slugging and hydrodynamic slugs, may flow through the pipeline. Furthermore, operational changes, such as start-up and production increase, can create large liquid slugs, which may result in large oil and gas production losses. Production deferment results from poor use of downstream separators, process instabilities, time-consuming start-ups, especially for flowline/riser systems, and top-side 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 may result in more major upsets in top-side facilities.

Suppressing and Controlling Slugs

The S 3, developed by Shell Global Solutions and licensed to Dril-Quip for marketing, sales, and manufacturing, 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. The valves control both outlet flows, which receive their signals from a control system. We developed a control strategy using the LabVIEW Real-Time Module and FieldPoint distributed I/O to suppress severe slugging and control transient slugs without gas surges.

Accurate Control System

The circuitry used in the S 3 control system consists of a redundant section of NI FP-2010 intelligent controllers to provide 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 built-in redundancy, the availability of the system is 99.95 percent, assuming a four-hour repair period during downtime.

We programmed the entire application using LabVIEW. For the control algorithms, we applied standard LabVIEW PID Control Toolkit blocks plus additional algorithms to ensure correct and fast control of the slug suppression system when modes of control are changing. Implementation in existing programmable logic controllers (PLC) and digital cellular service (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 quickly and easily tested ideas and analyzed applicability with LabVIEW. This process significantly improved the research and development speed for the S 3. Additionally, instead of hiring a specialist, we built the applications using 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). The first test systems included standard LabVIEW software, data acquisition hardware, and SCXI, but we plan to use LabVIEW Real-Time and the FP-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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