Fouling in gas turbine compressor has proven to be inevitable, but online compressor washing has shown to be promising in mitigating the effects of fouling. Despite the several researches and experiments carried out in laboratories or actual engine operations as presented in literatures, the economic benefit is always very important. This research aim to present the optimum online compressor water washes frequency and determines the creep life of the high pressure turbine HPT, which in this case is the first and second stage of the rotor blades. A Siemens twin shaft industrial gas turbine (SGT200 Tornado) was used for the performance simulation and degradation model. The engine code for Tornado gas turbine was not available in the Turbomatch library, as such data provided by engine manufacturer (Siemens, Lincoln, UK), was applied at the design point and the Turbomatch engine program ran successfully for both design and off-design point with the supplied data. The engine model was deteriorated with knowledge of underlying fouling mechanism and the possibility to apply the design point data using Pythia software and the non-linear gas path analysis with measurable parameters. Larson-Miller parameter LMP approach was applied in determining the effect of increasing turbine entry temperature TET on high power turbine HPT creep life. Hence, the compressor wash optimisation was determined, and the optimum online compressor wash interval was found to be once in every four days. The sensitivity analysis for the price of electricity, shutdown cost, fuel price, and degradation rate was tested, and the results are presented.