Flexible Couplings (FC) as a solution to mitigate thermal straining of casings in geothermal wells have been tested, where its function to allow intermittent displacement of the production casing during warm-up has been verified both for straight and deviated wellbore. During warm-up, free thermal expansion of the steel used for casing in geothermal wells is blocked in conventional design, resulting in high stresses that are often above the yield strength of the material. This stress can both damage the casing as it will release the stress by permanently deforming the casing plastically. The proposed solution allows the casing to expand into the FC that replace conventional connections, and by that ensure that the stress level will remain below the yield strength of the material. Chance of generating plastic strains will therefore be lowered reducing the risk of casing failures. Testing in two, third party, laboratories have been performed within the EU funded projects, GeoWell (www.geowell.eu) and DEEPEGS (www.deepegs.eu) and the plans are to make the first full scale testing in a high-temperature geothermal well in the end of 2019.