Reliability Evaluation of a Structure at Sea

Conventional design practice for coastal structures is deterministic in nature and is based on the concept of a design load, which should not exceed the resistance (carrying capacity) of the structure. The design load is usually defined on a probabilistic basis as a characteristic value of the load, e.g. the expectation (mean) value of the 100-year return period event, however, often without consideration of the involved uncertainties. The resistance is in most cases defined in terms of the load which causes a certain design impact or damage to the structure and is not given as an ultimate force or deformation. This is because most of the available design formulae only give the relationship between wave characteristics and structural response, e.g. in terms of run-up, overtopping, armour layer damage etc. An example is the Hudson formula for armour layer stability. Almost all such design formulae are semi-empirical being based mainly on central fitting to model test results. The often considerable scatter in test results is not considered in general because the formulae normally express only the mean values. Consequently, the applied characteristic value of the resistance is then the mean value and not a lower fractile as is usually the case in other civil engineering fields. The only contribution to a safety margin in the design is then the one inherent in the choice of the return period for the design load. It is now more common to choose the return period with due consideration of the encounter probability, i.e. the probability that the design load value is exceeded during the structure lifetime. This is an important step towards a consistent probabilistic approach.