Power companies in the geothermal industry face well-known challenges when exploiting high temperature geothermal reservoirs. To extract the geothermal energy from the reservoirs, geothermal wells are drilled to retrieve the geothermal steam to produce both electricity and hot water for district heating. The geothermal steam can be both highly corrosive and cause high thermal expansions in the steel casings which can result in high repair costs. The Icelandic R&D company Gerosion is developing a protective casing for high temperature wells, called the Sacrificial Casing. The product is a multi-layered design and can protect the other casing strings from corrosion and high thermal expansions. This thesis project aims to design and optimize a new fastening mechanism for the Sacrificial Casing, a so called slip mechanism. The material selection and geometrical dimensions of the mechanism are explored based on computational results. The most feasible design for installation of the mechanism is then used to construct both 2D and 3D FE-models where the mechanism is subjected to high temperatures and pressure and the load-carrying capabilities of the design analyzed. The results indicate that thermal expansions cause extremely high stresses in the mechanism and as a result the S960 high strength steel grade does not offer sufficient strength to withstand the combined loading the mechanism is subjected to.