Case Handling of liquid slugs – Avoid reduction in production

Slugging in wells, pipelines and risers represents a major type of plant disturbance for several offshore facilities. The result will sometimes be throttled production from these specific sources, in order to avoid separator level alarms, with possible plant shut-down as a consequence. A significant production potential may therefore be lost. Improved slug handling is obtained by coordinated actions of several liquid level controllers (LICs).

Coordinated separator control

In a general case, the setpoints of all LIC controllers, marked with green arrows, might be manipulated by the model predictive controller. The number of manipulated variables is decided from operational requirements.

Multivariable control of a separator train

The red markers indicate possible controlled variables. As indicated, both level values and flow (calculated or measured) of oil and water out of each separator may be controlled explicitly. It may seem a bit strange that the MPC controller includes level control, as this is the primary task for each LIC. However, the individual LIC controllers are monovariable controller loops, only considering a single level transmitter and a single valve. When introducing multivariable control, we choose to keep the basic PI(D) controller loops for safety reasons, and the setpoint of the LIC is thus only an indirect way of manipulating the liquid flow valves. The basic controller loops, including the PI(D) tuning, is therefore included in the prediction models of the MPC application. This is similar to how an experienced operator may act if he becomes aware of an incoming liquid slug: He decreases the LIC setpoint in order to drain the separator, not because he wants a lower level, but because he realize that without his intervention, the LIC controller alone, with a fixed setpoint, will give an unacceptable high level.

An MPC application can handle multivariable level control, include prioritazions between different levels, and also handle max- and min-values together with setpoint values in a consistent way.

Scenario 1: A slug is estimated to enter first stage:

The first stage separator is probably the largest liquid volume, and the MPC application will challenge the high- and low levels by manipulating the LIC setpoints in order to “filter out” the flow pulse to the second stage or to the water treatment system. If the level predictions for the second stage reveal safe operation of this separator, also the level limits here may be utilized to further damp the flow to the third stage. If necessary, even the LIC setpoints of oil and water in the third stage may be manipulated accordingly to avoid level alarms, or even process trips.

Scenario 2: A slug is estimated to enter second stage only, from a satellite well:

The figure indicates a possible multiphase source entering between first and second stage, for example from a satellite field requiring a lower inlet separator pressure. If this particular flow is sluggish, we may also in this case use the first stage separator as a “compensator”: The separator LIC loops can be operated so that the outlet liquid flow oscillates with opposite phase compared to the satellite inlet flow. The second stage separator may then experience close to a non-slug behavior of the inlet flows. If necessary, we may still operate the second and third stage level controllers as explained in scenario 1.

Finally, if there is a distillation column downstream the separator train, we may regard the flow out of the third separator as a main variable for reduced oscillations.

Norne FPSO, photo by statoil, Anne-Mette Fjærli