Gas Turbine Deterioration and Recovery

Abstract

Gas turbines used in the platform industry are subjected to challenging operating conditions in the harsh offshore environment. Compressor fouling is reported to be the main reason for gas turbine performance loss and can be prevented by efficient inlet air filtration. The performance loss can also be recovered by offline and online water wash. Compressor performance is challenging to monitor as the operation is influenced by several factors, such as ambient conditions, gas turbine load and degradation. In the current monitoring systems, it can be a challenge to separate the various reasons for lost performance.

Gas turbines on an offshore installation in the North Sea are documented and operational data from six General Electric LM2500 compressors are collected. A performance analysis is conducted to validate monitored parameters in the monitoring software against operational data. The applied monitoring software are found to be capable of providing a sufficient display of real-time and historical operating data. The relative change in compressor efficiency is validated by applying simulation tools. The polytropic efficiency is considered better suited for condition monitoring due to smaller variations in the efficiency trends.

Water is daily injected to the compressors for two minutes during operation with a water-to-air mass ratio of approximately 0.9 %. This, in combination with high efficiency inlet filters, is reported to reduce the performance loss from 4.5 % after four months of operation to below 3 % after six months of operation. High efficiency inlet filters are tested and compared to existing filters and are proven to reduce the compressor performance loss by almost 1 %. The water-to-air ratio is increased to approximately 1.4 % on one compressor and no operational problems are revealed. The increased water rate is expected to further increase compressor performance recovery.

The monitored compressor efficiency currently has large variations in the long term trends and the monitoring software often do not display the compressor's conditions in a useful way. A procedure to correct for the influence of varying load and ambient temperature in the efficiency is established based on clean engine data. The former less applicable efficiency trends are transformed to display steady decreasing performance towards the approaching maintenance shut-downs. The procedure contributes to considerably improved monitored trends and has a great potential for further application in the industry.

The outcome of applying the developed procedure is that the compressor efficiency becomes a more realistic parameter for monitoring the performance. Additionally, abnormal compressor performance becomes more exposed. Foggy weather conditions, reduced pressure drop through the inlet filters and unexpected changes in the compressor efficiency are revealed to occur simultaneously on several compressors and calls for further investigations.