Control systems that prevent the development of riser slugging in pipeline-riser systems have in recent years been introduced on several offshore processing facilities. These control systems are based on active use of the topside choke valve to keep a pressure measured somewhere upstream of the riser constant. These control systems have been a great success, and are emerging as the standard method to avoid riser slugging in multiphase production pipelines.
The active use of the topside choke valve for control purposes opens the door for several other interesting applications as there are other multiphase phenomena occurring in a pipeline-riser system that can cause operational problems for the downstream production facilities. Among these are surge waves. When the production rate in a gas-condensate pipeline is increased, the liquid holdup in the pipeline will go down. This means that during the transition period, an excess amount of liquid needs to be transported out of the pipeline before a new steady state can be reached. The excess liquid can cause the liquid rate out of the pipeline to exceed the liquid handling capacity of the receiving facilities.
Start-up slugs occur when the pipeline is started up from shut-in conditions. These slugs are similar to surge waves as they are caused by an increase in the production rate. However, start-up slugs can be even more violent as they potentially can initiate riser slugs and thereby cause even larger peaks in the liquid production.
In this paper we introduce a novel control structure that extends the scope of pipeline control by including suppression of surge waves and start-up slugs. The control system consists of a slug controller and individual flow controllers for each phase combined seamlessly through a minimum select function. The performance of the control system is illustrated with simulations from an industrial case study.