Push the limit a catchwords to describe todays and tomorrows drilling industry. The need to reach deeper, more remote and complex reservoirs with super extended reach, multi laterals and designer wells has driven operators to change traditional drilling programs. Also the requirements on the drill string and the casing had to be adapted: Internal wear of the casing during drilling is an increasing concern nowadays. The tensioned or compressed drill string is the main cause of casing wear, as it is pulled firmly against the casing while rotating to drive the bit. Often the wear penetration into the casing is acceptable, but in some fields, such as the Troll field on the Norwegian continental shelf, excessive casing wear is reported. The reduced pressure integrity of the casing represents a safety hazard; completion problems and additional costs for repairing or replacing worn casing arise. The work presented here is split in two parts. The first part covers general information on casing wear. The second part will be a field analysis to identify parameters affecting excessive casing wear in Troll field. As an introduction to the first part some information on the Troll field and the drilling strategy shall be given before basic wear mechanisms shall be presented. Models to describe and predict casing wear shall be outlined. Furthermore, it shall be discussed how operational parameters, drilling mud composition, drill string and tool joint design and formation parameters affect casing wear. Next, logging tools and mathematical models to calculate worn casing burst and collapse rating shall be presented. The last chapter of the first part will deal with general and Troll specific CO2 corrosion facts. In the second part some general information on Troll wells shall be given and problems occurring due to excessive casing wear shall be outlined. The analysis approach shall be presented, listing all the assumptions made for the field analysis. Available data for the field study shall be presented before identifying parameters affecting casing wear on Troll and presenting the results of sensitivity analysis on side force. Finally, the results of the casing wear analysis shall be presented and reasons for occurring deviation discussed. In the last chapter possible solutions to avoid excessive casing wear on Troll shall be given taking into consideration the operators planned drilling activities.