Gas Turbine Fouling Offshore: Effective Online Water Wash Through High Water-to-Air Ratio

Abstract

Optimized operation of gas turbines is discussed for a fleet of eleven GE LM2500PE engines at a Statoil North Sea offshore field in Norway. Three engines are generator drivers and eight engines are compressor drivers. Several of the compressor drive engines are running at peak load (T5.4 control), hence production rate is limited by the available power from these engines. The majority of the engines discussed run continuously without redundancy, hence gas turbine uptime is critical for the field’s production and economy. The performance and operational experience with online water wash at high water-to-air ratio, as well as successful operation at longer maintenance intervals and higher average engine performance are described. This work is based on long-term operation with online washing, where operational data are collected and performance is analyzed over a 10-year period. Today, all engines are operated with 6-month intervals between maintenance stops, where offline crank wash is performed as well as other necessary maintenance and repairs. Online washing is performed daily between the maintenance stops at full load (i.e. normal operating load for the subject engine). To keep the engine as clean as possible and reduce degradation between maintenance stops, both an effective online water wash system and an effective air intake filter system are critical factors. The overall target is to maintain high engine performance, and extend the interval between maintenance stops through effective online washing. Water-to-air ratio is significantly increased compared to the OEM limit (OEM limit is 17 l/min which yields approx. 0.5% water-to-air ratio). Today the engines are operated at a water rate of 50 l/min (3 times the OEM limit) which yields a 1.4% water-to-air ratio. Such a high water-to-air ratio has been proven to be the key parameter for obtaining good online water wash effectiveness. Possible downsides of high water-to-air ratio have been thoroughly studied. The effect of optimized online water wash for the subject engines is longer intervals between maintenance stops, higher power availability, lower engine performance deterioration and reduced emissions (CO2 and NOx). The operating intervals are now extended to six months (4,000 hours), from initially two months (1,500 hours, early 1990s) followed by four months (3,000 hours, mid-2000s). Other installations operated as low as 750 hours between offline washes in the 1980s and 1990s. Of a total efficiency deterioration improvement of 6% over each 6-month operating period, the deterioration is reduced by an estimated 3% related to online water wash.