Compressor Performance Assessment and Debottlenecking

A UK upstream operator with assets in the North Sea was concerned about the performance degradation of one of its Gas Lift Compressor and about the capability of the machine to contribute to the platform’s oil production. MACH10 has been asked to review the actual compressor thermodynamic performance and assess whether the machine is fit for purpose until the planned end of production in 2018. The 1×100% compressor is composed by two stages driven by a fixed-speed electric motor. The pressure delivered by the compressor was 1 bar above the minimum required pressure to sustain the oil production. The compressor vendor advised that the compressor would have been unfit-for-purpose in less than 6 months due to the ongoing performance degradation. The vendor’s advice was to replace the entire compressor internal bundles (rotors and diaphragms). Due to insufficient historical data, MACH10 planned a monitoring campaign to periodically asses the compressor’s behavior. The analysis revealed that the compressor’s under-performance was caused by the low-pressure stage (head -5%, efficiency -7%). To compensate for it, the high-pressure stage was overloaded, resulting in gas discharge temperature often in the 170°C range, which was of concern for the aluminium-based alloy components. In addition, the monitoring campaign proved that the equipment performance degradation was not progressing.

As a performance recovery initiative, MACH10 recommended a control system tuning to rebalance the load between the two compression stages. Thanks to the operating point adjustment on the LP section, the head gained by the LP stage increased the final discharge pressure by 2 bar above the minimum requirement. This was enough to reduce the load on the HP stage and its gas discharge temperature to safe levels.

MACH10’s support allowed the client to:

• Quantify the compressor’s inefficiency and identify the major contributors;

• Have solid proof that a performance degradation occurred in the past but it was not taking place anymore, so the costly intervention recommended by the compressor vendor was not necessary;

• Increase the overall performance of the compressor (+1 bar of discharge pressure at reference flow);

• Reduce the thermal stress on the HP stage to a comfort area;

• Rebalance the overall process and increase the platform’s oil production by 7.5%.