In Switzerland, most of the hydro power plants were installed between 1950 and 1970. These power plants play an important role for electrical power network stability through their operational flexibility and ability to provide ancillary services. These services lead to frequent start and stop sequences, as well as continuous power variations inducing transient pressures in the water conduits. Due to electricity market recent evolutions, existing hydropower plants are subject to new operating conditions and sequences which were not foreseen during the design phase. This significant increase of load variations enhances fatigue problems by soliciting the penstock faster than originally expected. While loading spectra are the fundamental input for any fatigue assessment procedure, they are often difficult, if not impossible, to quantify accurately. In this paper, we present how the implementation of a digital clone of the power plant, namely the Hydro-Clone real-time simulation monitoring system, can be used to fill this gap. By replicating the hydraulic transients of the powerplant, the digital clone enables real-time knowledge of the pressure variations throughout the water conduits. This feature is used to implement a fatigue module in Hydro-Clone by monitoring the penstock level of solicitation, based on the accumulated damage during its past and future operations. To validate this approach, stresses related to pressure variations are measured in situ by installing strain gages on the penstock of the 200 MW La Bâtiaz hydropower plant, owned by Electricité d'Emosson SA, and compared to the simulated values. Our results reveal the considerable impact of the supply of ancillary services on penstock fatigue wear.