Abstract The regulations in Norway require the mooring design check to be done for a 100-year storm. This requirement is also valid for short-term drilling operation. This typically leads to mooring systems with long mooring lines, poor operability for drilling operations and significant thruster usage to keep position. This paper provides an alternative design philosophy where the mooring system are optimized for drilling operations and released from the rig in severe sea states above a pre-defined environmental limit, while maintaining the required safety level. The required safety level is achieved by calibration of safety factors for mooring line tension which depends on weather restrictions. The calibration is done using structural reliability analysis (SRA), where three different rig designs have been analyzed. A joint distribution environmental model for wind, wave and current data has been used. The main results are a new design philosophy where calibrated mooring line safety factors for a return period limited to 10 years compared to the currently applied unrestricted case using 100 years return period. Analyses are performed for both ultimate limit state (ULS) and accidental limit state (ALS). A case study has been prepared using COSL Promoter at the Statfjord field, where the consequences of the new design philosophy is compared with current method. The study includes differences in required mooring equipment, anchor handling vessel requirements, fuel consumption and CO2 emissions The main finding from the case study is a 50% reduction in number of anchor handling vessels per mooring operation and a significant reduction in fuel and associated CO2 emissions for station keeping for the drilling unit. Given the expected shortage of anchor handling vessels in the coming years and push for greener operations, the new design philosophy is highly relevant for the future. This paper shows that if the environmental return period is reduced in the design analysis, the associated mooring line safety factor should for some cases be increased to maintain the required safety level. The study is considered relevant since it appears not to be normal practice to change (increase) the safety factor when limitations to the environmental return period is introduced.