The drilling industry in Norway faces new problems nowadays. The operators are reaching the capacity limits for existing installations and therefore need to re-use mature surface and intermediate casing sections of old wells. It is necessary to know the precise casing condition to make sure, that this worn casing can withstand the applied drilling loads. Therefore an extensive casing wear simulation has to be carried out before planning a new sidetrack. The scope of this master thesis was to develop a relevant casing wear model for the Gullfaks field on the Norwegian continental shelf. The work presented is split in three parts. The first chapters deal with general information about Gullfaks and different casing wear mechanisms and basics. This part also contains the results of extensive laboratory tests with several muds, additives and casing materials. An introduction to casing wear logging and in special to USIT logging, including detailed information on ultra sonic imaging, is also included. The following chapters describe the theoretical background of the used prediction software package Cwear and how it is operated. Special attention is put on the result output and how this is processed to compare it with existing USIT logs. The last part includes the simulation process workflow, the data acquisition and log processing. A detailed discussion about the presented outcome is included and possible software and procedure improvements and outlooks into the future presented.