OMV Austria Exploration & Production GmbH (OMV AUT) conducts around 300 workover jobs in existing oil and gas wells every year. All data are collected and documented in a production database (Gewinnungsdatenbank, GDB). These data include information about the field, horizon, produced media, age of the well, job type, workover reports, treatment time and costs. The GDB is an OMV AUT intern database; therefore the information is limited to wells in Austria. Nevertheless it allows access to hundreds of well documented workover jobs over more than 10 years. Until now, OMV AUT used only a reservoir related economic approach to evaluate workover risks in the planning process. This method is unsuitable to take technical risks into account, which are especially high in mature oil fields. The objective of this thesis is to assess the technical risk associated with workover operations of oil and gas wells in Austria during the last years. In a first step a baseline study of documented workover incidents was done. It is shown that several cases had a massive impact on the workflow and consequently on the rigtime. In a second step well interventions and workovers were split into essential operational steps to analyze the deviations from the planned program to the actual job. This information is then used to generate multistage process trees to show all possible outcomes. In a third step the process trees are converted into a rigtime estimation tool that is able to predict the rigtime of the different workovers. The rigtime estimation tool, which is constructed by the use of historical data, has been tested on recent cases and has shown to predict the rigtime requirement very accurately. By means of this analysis, future workover operations can now be better planned, optimized and risk be assessed.