In Chapter 2, the procedure for designing a mooring line under Norwegian jurisdiction is discussed. The procedure is given by the Norwegian Maritime Directorate through the ANCHORING REGULATION 09 and ISO 19901-7 (2005). The difference in the required safety factors and the environment return period between ISO 19901-7 (2005) Annex B and the ANCHORING REGULATION 09 has been evaluated. The effect of this difference on the annual probability of mooring line failure is investigated Chapter 4. In Chapter 3, two ways of estimating the annual probability of mooring line failure is discussed. One of them is the environmental line contour method. The method is used for the probability estimates performed in the following chapters of the report. In Chapter 4, the annual probability of mooring line failure is estimated based on model test results for the Midgard platform model. In Chapter 5, SIMO is used to analyze the line tensions of the Midgard platform. From the line tensions found by SIMO, the annual probability of mooring line failure is estimated for various safety factors. The results are compared with the results found in Chapter 4. The chapter also discusses the joint occurrence of environmental storm values. In Chapter 6, the effect of water depth on the annual probability of mooring line failure is discussed by using SIMO. Chapter 7 addresses problems raised while working on the previous chapters; The determination of the sea state which produces the largest line tensions is further discussed after first raising the topic in Chapter 2. The effect of changing the 90 % fractiles used in the environmental contour line method for deciding the annual probability of mooring line failure is investigated by using the model test results. The effect of changing the wave heading on the annual probability estimates is investigated by using SIMO after not having enough model test results of the worst sea sate in Chapter 4. The SIMO results found in Chapter 5 for 100 year environment are not a match to the results found in Chapter 4. The increase in values found by increasing the return period to 10 000 years is also not a match. The last discussion of chapter 7 is therefore the effect of the lack of matching increase.