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Abstract

The ConocoPhillips Alpine facility, on the Alaskan North Slope, has experienced slugging problems severe enough to trip the high-high inlet separator level, causing frequent plant shutdowns, and loss of production of 110 kbbl/d.  A slugging study was commissioned to investigate the cause of the existing CD-2 pipeline slugging, and possible mitigation procedures, which could alleviate and/or eliminate slugging.  Further, the Alpine expansion called for an additional two pipelines (CD-3 and CD-4) to be brought into Alpine inlet separator. 

Slugging mechanism and instability analysis were performed. The instability is due to combination of its low flow rate, overly-sized pipeline ID and unfavorable pipeline profile.  Flow pattern transition exists at the low spots and liquid accumulates and blocks the flow. In the low pressure system, once gas blows out and system pressure drops, the pipeline inlet gas increases velocity and picks up a new hydrodynamic slug.  This slug moves through the road crossing and the pipe rack riser, becoming a long slug which arrives at the separator.

In this study, a slug-tracking model with separator gas/liquid PID controllers was built to reproduce the field SCADA data.  A remarkably good match of pressure variations, slugging frequency and liquid level was achieved.  A sensitivity study was performed to investigate the effective and practical ways to suppress slugging in the existing CD-1 and CD-2 pipeline.  Finally, a combined control was recommended by installing a by-pass control valve at the butterfly valve location.  The by-pass inlet control valve before the separator acquires separator liquid level signal, and actuates when the separator liquid level exceeds the set value.  This significantly reduces slugging effect on separator performance.

The slugging model and results based on the existing CD-2 pipeline were applied to the future expanded CD-3 and CD-4 pipeline study.  Some conclusions were drawn from the slugging behavior.

Introduction

In many oil and gas developments with multiphase flowlines, production instability due to slugging is a major flow assurance concern. Slugging initiates oscillations, puts excessive demands upon the separation and operation, and increases the wear and tear of equipment.  Large liquid surges can cause poor performance, separator shut down, high pressure trips, or flaring.

Slugging can be characterized by periodical change of pressure and gas/liquid flow.  The slugging severity depends on slugging types.  There are three types of slugging:

• Hydrodynamic slugs: a feature of the slug flow regime where slugs are continuously formed due to instability of waves at certain gas-liquid flow rates. Generally, hydrodynamic slugs do not exceed 20 times of pipe diameters if there is no obvious inclination change. 
• Operationally induced surges: generated by changing the flow conditions from one steady state to another, such as restart, flow rate ramp-up or pigging operations.  The generated liquid surge can upset the system.
• Terrain induced slugs: also called severe slugs, caused by accumulation and periodic purging of liquid in flowline dips at low flow rates;