A Field Study of Reduced Axial Compressor Performance Deterioration through Online Washing and Air Intake Filtration Upgrade

Abstract

Background Oil and gas production at several North Sea offshore installations is limited by the gas turbine power available, and any deterioration in gas turbine performance directly affects production rates. Fouling in the compressor section of the gas turbines is the main cause of performance deterioration, and the fouling is removed by offline water wash, where the compressor performance is restored back to the baseline level. An offline wash requires the engine to stop (downtime), and it is therefore crucial to have as lengthy intervals as practically possible between offline washes to enhance production efficiency and the economy of the plant. In order to achieve lengthy intervals between offline washes, the deterioration rate must be as low as possible, by implementing the optimum combination of intake air filtration system and online water wash of the gas turbine compressor. Objectives Analyse gas turbine compressor deterioration mechanisms and water washing fundamentals in order to develop, validate and implement an optimum water washing system for offshore applications. In addition, analyse the effect of air intake filter system deterioration in order to develop, validate and implement optimum air intake solutions. Research approach, activities The basis for this work has been empirical data from eleven LM2500PE gas turbines at a North Sea offshore field. Testing and validation of online water wash at increased water rates were performed on the subject engines, as well as air intake filtration upgrades by increasing filter class and improving the filter housing design features. The PhD project work has also included onshore testing of a gas turbine in deteriorated condition, as well as gas turbine intake air filter testing in a test cell facility. The field work on this subject has spanned more than ten years and was initiated by the author prior to the PhD project. Conclusions, contributions An effective water wash of the axial compressor starts with an offline wash system that is capable of fully restoring the performance and baseline of the engine, i.e. restoring the recoverable deterioration (fouling). The non-recoverable deterioration rate (mechanical wear, increased tip clearance, etc.) is marginal compared with the recoverable deterioration rate. A typical figure for the recoverable deterioration rate is a 4 % drop in compressor efficiency over a 4-month operation period between offline washes, whereas the typical non-recoverable deterioration rate is only 0.5 % over 3 - 4 years (25,000 operating hours between gas generator overhaul). The main finding in this work is that a daily online wash at high flow rate is the key parameter for high effectiveness and a low deterioration rate, with a recommended water-to-air ratio for the LM2500PE of between 1.2 to 1.6 % (by mass). An effective air intake filtration system in an offshore environment consists of two main components. An upstream vane separator that knocks out the large droplets (coalescer) followed by a downstream filter section that stops the smaller droplets. In addition, other key elements for an effective filtration system are: drainage system, water/air locks, holding frame for filter elements, anti-icing system and access doors/windows for inspection and maintenance. The main challenge for an offshore filtration system is to handle water droplets with airborne salt particles. Other solid particles (soot, sand, etc.) are not the main challenge offshore. However, the filter system must occasionally handle such particles. In foggy/high humidity conditions, it is particularly challenging for the filtration system to operate effectively due to deterioration of the filter performance. This work has tested filters at different filter grades, both offshore and in an onshore test cell facility. The main findings are that increasing the filter grade from M6 to F7 has proven to be effective, combined with a shorter filter change interval, prior to the point where filter deterioration is severe. In addition, a CFD study of the air intake system has been performed, which resulted in flow optimization in the field. On the whole, the results of the optimized air intake filtration and online water wash at high flow rate have given longer operation intervals between offline washes and lower deterioration rates. The present standard offline wash interval is extended to six months, compared with two months in the past. The compressor efficiency deterioration rate is reduced to just 0.5 % over each 6-month operation period. This has a major impact on the production rates, production efficiency and economy of the plant. It entails lower fuel consumption and exhaust emissions and hence lower costs for fuel, CO2 and NOx taxes and reduced environmental impact. In addition, procedures and algorithms for gas turbine compressor efficiency correction and air flow analysis have been developed in the project. The major contributions of this work are presented in six papers included in the appendices. Proposed further work The shift in performance curves on the compressor map from clean to deteriorated condition should be further studied, as well as the effect on velocity diagrams and Reynolds number. The sensitivity in engine parameters for compressor efficiency vs. air flow should be further studied.