Digitalisation through Advanced Process Control
Ivar Aasen exports oil to Edvard Grieg and needs to meet an absolute maximum constraint due to the oil export. With a simple level controller, which controls the oil level in the separator through an oil export pump, the oil level typically varies around a chosen set point. The result of this is variation in the speed of the oil export pump and thus variations in oil export to Edvard Grieg, which makes it challenging to export oil close to the maximum constraint.
The goal of the Cybernetica CENIT NMPC application implemented at Ivar Aasen, is to maximize the oil export rate from the inlet separator, as a result from minimized fluctuations in the flow rate. This is achieved by introducing production swing wells (chokes) for level control of the separator, in combination with a “floating” oil level, instead of a PID level controller.
One of the production chokes is operated automatically, instead of manually by the control room operator, to keep the oil level within predefined minimum and maximum levels in the separator. In addition, the controller may manipulate the pump speed of the oil export pumps, if necessary.
The level deviation from the desired average oil level setpoint in the separator may be larger than what is observed prior to installation of the NMPC application, as the application will utilize the separator buffer volume to a higher extent. The variation in the pump speed is less, close to zero, and thus the variation of the oil flow from the export pumps is minimum, which makes it possible to increase the oil flow export to a maximum without exceeding the maximum export constraint for oil export.
Control philosophy for the Oil Export NMPC implementation at Ivar Aasen
A simple illustration of the control philosophy is shown in the figure blow. The CENIT NMPC application is manipulating the production chokes and the oil pump (green arrows). Only one production choke is manipulated at a time together with the pump. The operator selects min- and max values for the different chokes, in addition to the active swing choke currently used. The operator also defines the target oil level and the corresponding min- and max values in the separator. The NMPC application should fulfil the target oil level on average and allow the level to drift between the limits. The main level control is achieved by manipulation of the swing choke. In addition, the NMPC may manipulate the speed of the oil pump, if necessary. However, the pump speed will mainly be used to achieve the operator specified production rate. In normal situations, the pump speed is thus close to constant.
Figure: Illustration of control philosophy for the IAA NMPC application for oil export
The operator can switch between the NMPC application and the existing level controller (LIC) for oil export as preferred from the HMI. The LIC then starts operating from the last NMPC calculated pump speed value. The swing choke position remains at the last calculated NMPC value, until the operator chooses any manual adjustment.
Using this NMPC application, Ivar Aasen is able to export oil at maximum rate to Edvard Grieg without violating the maximum constraint for oil export, while managing operational changes like flow compositions and disturbances. This resulting in Ivar Aasen being able to export up to 55 000 more barrels of oil per year.
(IAA photo by Aker BP)