Reliability of design approaches for axially loaded offshore piles and its consequences with respect to the North Sea

In the near future several offshore wind farms are planned to be built in the North Sea. For these jacket and tripod constructions with mainly axially loaded piles are suitable as support structures. The current design of axial bearing resistance of these piles leads to deviant results regarding the pile resistance when different design methods are adopted. Hence, also a strong deviation regarding the required pile length must be faced. The reliability of a design method can be evaluated on the basis of a model error which describes the quality of a considered design method by comparing measured and predicted pile bearing resistances. However, only few pile load tests exist with regard to the boundary conditions in the North Sea. This paper presents 6 large scale axial pile load tests which were incorporated within a new model error approach for the current used design methods for the axial bearing resistance, namely API Main Text method and CPT-based design methods, such as ICP-05, UWA-05, Fugro-05 and NGI-05. Based on these new model errors a reliability based study towards the safety was conducted by performing a Monte Carlo Simulation. In addition also consequences regarding the deterministic pile design were evaluated. It is shown that the required embedded pile length has to be increased compared to the deterministic design to satisfy the desired safety level by Eurocode 0 of v = 3.8. Hence, the current prescribed global safety factor and therefore the partial safety factors are only valid for the API Main Text and the UWA-05 design method where as for the ICP-05, Fugro-05 and NGI-05 design methods an increase from gLgR = 2.69 to gLgR = 3.27 for the global safety factor should be considered. Further it is also shown that the reliability based design leads to a more robust determination of the required pile length compared to the deterministic design.