Dynamic simulation solves process control problem in Oman

A dynamic simulation study solved the process control problems for a Saih Rawl, Oman, gas compressor station operated by Petroleum Development of Oman (PDO).

PDO encountered persistent compressor failure that caused frequent facility shutdowns, oil production deferment, and gas flaring. It commissioned MSE (Consultants) Ltd., U.K., to find a solution for the problem.

Saih Rawl

Saih Rawl, about 40 km from Qarn Alam, produces oil and associated gas from a large number of low and high-pressure wells. Oil and gas are separated in three separators.

The oil is pumped to Qarn Alam for treatment and export. Associated gas is compressed in two parallel trains. Train K-1115 is a 350,000 standard cu m/day, four-stage reciprocating compressor driven by a fixed-speed electric motor. Train K-1120 is a 1 million standard cu m/day, four-stage centrifugal compressor driven by a variable-speed motor.

Tripping, surging

MSE describes the problems as beginning after the installation of K-1120 in the early 1990s. The compressor tripped during the day on motor overload and during the night due to compressor surging. This caused extensive flaring.

According to MSE, prior to its study, the problems were attributed to:

Ambient/oil temperature changes between day and night
Very limited control band for suction and discharge pressure
Possible liquid carry over with the gas
Possible operational problem with the Voith coupling, etc.

PDO's internal investigations highlighted process instability and compressor surging as the root of the problem. Although PDO identified and modified some of the station control loops, it still decided to study the system dynamics as a whole.

The tender for the study was awarded to MSE.

MSE conducted several tests, collecting data for compressors, separators, knockout drums, etc. Plant operation was observed and records made of transient events in the operation before constructing the model.

The K-1120's performance was modeled using data from the compressor vendor. MSE said because the electric motor showed a continuous cyclic variation in current and absorbed power, it added a model for this and the fluid coupling to determine the compressor acceleration rates, power, and current variations. The manufacturer tested the compressor surge lines and control systems, and MSE included these data in its model.

MSE simulated a wide range of scenarios to fully understand the plant behavior. The study revealed that the K-1120 compressor train was operating at the maximum power limit of the electric motor, and power demand exceeded available power during transients. MSE said this explained the failure of the motor on overload or at high winding temperatures. The problem was exacerbated during the summer with ambient temperatures reaching 45-50° C.

In addition, MSE said it confirmed the process control system was inadequate and over-complex.

MSE concluded that the master controller could not achieve process stability because it was simply trying to control too many devices, all with very different response rates and time constants. For example, station recycle valves and flare valves are small and fast acting, while other devices such as pistons and centrifugal compressors are massive with large moments of inertia, making them slow to react.

MSE investigated a number of alternative process control strategies for stability.

Control simplifications

MSE advised that the control philosophy should be simplified and recommended that the master controller should be limited to fast-acting devices only, with the slow-acting devices decoupled (Table 1 [47,882 bytes]).

Fig. 1 [103,432 bytes] compares the original and revised control systems. The result indicates a relatively constant compressor speed, power, and separator pressure, leading to much smoother operation and improved process stability, according to MSE.

MSE decribes centrifugal compressors as being designed for specified operating flows and conditions and, therefore, they require a minimum and a maximum flow to operate normally. If flow falls below the minimum, the compressor will surge, causing a reversal of flow within the compressor that is very damaging to both the machine itself and the plant.

At the other end of the scale, if maximum flow is exceeded, the system will avoid shutdown by flaring.

To avoid surging in the original system, MSE said the operators were forced to bring on-line several additional low oil-producing wells with high GORs. Although contributing little to oil production, the associated gas did solve the surging problem, but only at the expense of excessive gas flaring.

Under the revised control system, according to PDO, these wells are no longer needed as the simplification of the master controller allows it to open the recycle valve to avoid compressor surging and maintain stability.