Reliability-Based Calibration of Partial Safety Factors for the Horizontal Axis Tidal Turbine Standard for Certification

Abstract

Tidal energy is nowadays one of the fastest growing types of marine renewable energy. In particular, horizontal axis tidal turbines (HATT) are the most advanced designs and the most appropriate for standardisation. It is however, in the interest of the industry to provide a set of standard practises in order to help in the process of designing this type of marine converters. DNV GL is producing this standard with the support of Alstom and as part of the ReDAPT project commissioned and co-funded by the ETI (Energy Technologies Institute). The work undertaken by DNV GL to produce this standard involves the identification of the uncertainties that designers need to address during the design process. Unlike other marine structures, HATTs are usually located in very energetic areas where no other marine structure has been before. Site characterisation is one of the largest sources of uncertainty e.g. turbulence. Key inputs like turbine inflow conditions or predictions of extreme values are still grey areas due to the limited site measurements and the uncertainty of the metocean models. Numerical models of HATTs are still quite uncertain often dependent on experience of the people running them. As part of ReDAPT project there is an ongoing effort in validation and evaluation of the accuracy of these numerical models and some of the results are used in this calibration study. The new standard for HATTs deals with the loading uncertainty in a whole new way by introducing a new parameter that is added to the traditional partial safety factors for loads. This new standard uses the traditional safety factors from the offshore structures standards and allows changing them based on the level of uncertainty that was introduced during the design process. This paper describes the process of calibration of the partial safety factors in ULS for loads of HATTs that was part of the work in the creation of the new standard. The reliability based calibration involved the formulation of failure criteria, the identification of stochastic variables in the failure criteria, calculation of reliability against failure and ultimately the new set of partial safety factors for loads and a methodology for adjustment of the factors in a case by case basis.