Slug-Control Strategy Proves Effective in New Angola Facility

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 31298, “Novel Active Slug Control in Angola: Development and Field Results,” by Lisa Ann Brenskelle, SPE, Martin Bermudez Morles, and Lauren Annette Flores, Chevron, prepared for the 2021 Offshore Technology Conference, Houston, 16–19 August. The paper has not been peer reviewed. Copyright 2021 Offshore Technology Conference. Reproduced by permission. Hydrodynamic slugging was anticipated during the design of a new facility in Angola. A simulation study demonstrated that a control scheme from the literature could be applied effectively to control the slugging. That solution was rejected, however, because of the use of a pseudovariable as the principal control point. A novel control scheme, therefore, was developed and tested in simulation for both hydrodynamic slugging and severe riser-induced slugging. Upon commissioning, slugging at the facility was found to be more severe than anticipated during design, but the novel active slug-control scheme was effective in controlling incoming slugs. Slugging-Control Approaches Various control schemes have been implemented to control slugging in hydrocarbon-processing systems, including subsea systems. The accepted control approaches to the various types of slugging differ because causes of slugging differ, although the effects on processing facilities are similar. For hydrodynamic slugging, the use of a pseudoflow controller, which uses a calculated value of flow, is the accepted conventional approach. The pseudoflow is calculated from the equation for volumetric liquid flow through a valve, which results in a value that is not physically meaningful for multiphase fluids. For terrain slugging, the accepted approach is the use of pressure control, wherein the pressure is upstream of the slug-forming area. For riser slugging, this is at the base of the riser. For both hydrodynamic and terrain slugging, the accepted control schemes usually modulate the control valve located upstream of the vessel first receiving produced fluids, normally depicted as a separator, although this vessel also could take other forms. Use of this valve in relation to slugging is common, whether used manually or in a control scheme. Maximum production occurs with the valve fully open, but this cannot control or prevent slugging. Known field-demonstrated control schemes include pseudoflow control, pressure control upstream of the slug-forming area, pressure control upstream of the slug-forming area cascaded to (i.e., determining the setpoint for) the pseudoflow control, and composite variable control. Each of these control schemes has practical disadvantages affecting usability in the field. The principal disadvantage of pseudoflow slug control is that setpoint determination is difficult because the pseudoflow is not an actual physical flow rate. Trial and error would be required to determine the pseudoflow setpoint each time it would need to be adjusted, which would be a frequent occurrence as operational conditions change. In the case of slug control through pressure control upstream of the slug-forming area, the principal disadvantage is the use of a subsea pressure sensor because the slug-forming area, the low point, frequently is subsea. Not only is subsea instrumentation expensive, but such instrumentation also is difficult to replace should it fail.