This paper describes a simple generic model for calculating the failure probability of a structural system (jacket), and evaluates the effect of updating the probability of failure based on the experienced wave load on this structure. An existing structure in harsh weather areas are often claimed to be a safe structure after having survived some years exposed to environmental loading. This paper evaluates this statement by a practical method to assess how large loading the structure must have experienced to achieve a detectable change in the calculated overload failure probability. Also the effect of model uncertainties, reserve strength is evaluated and a simplified modelling of a possible gross error is included. Based on the model presented here, the example structure, modelled without gross errors, must have experienced a load level in the order of 1 000 to 10 000 year loading in order to change the updated probability of failure significantly. If gross errors are included in the model, the experienced loading must exceed the 100 to 1 000-year loading in order to conclude that a gross error is unlikely.


When a structure has experienced a certain load level and succeeded in carrying this load, the degree of confidence in this structure is increased. It is a common understanding that when an offshore structure has been in operation for some years, it has proven its strength and robustness as a result of the wave loading the structure has experienced. The question to be raised against this statement is whether the experienced load level is sufficiently high to prove the safety of the structure, as very few of the installations on the Norwegian Continental shelf have experienced wave loading higher than a so called 10 year loading (a load level with a yearly probability of exceedance of 10–1).

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