This paper describes the methodology behind the partial safety factors and design fatigue factors of the DNV GL standard for certification of tidal turbines. The standard follows a risk based approach that allows the adjustment of the design requirements to the identified risk level of the system.
The work undertaken in this study has involved the identification of the uncertainties during the design process. Tidal turbines are located in highly energetic sites which are very difficult to characterise, hence site conditions are one of the largest sources of uncertainty. Key parameters like turbine inflow conditions or predictions of extreme values are still grey areas. Load simulation tools are still quite uncertain and are often dependent on the experience of the people running them.
Both partial safety factors and design fatigue factors have been calibrated to different target safety levels with due account for the uncertainties introduced by load models as well as site characterisation. The different target safety levels have been selected in accordance to the risk associated to structural failures of tidal turbine support structures and aim for a more streamlined design from the safety requirements point of view.