Probabilistic design and reliability analysis of scour protections for offshore windfarms

Abstract

Scour protection is an important component of fixed bottom foundations for offshore wind turbines. Depending on the hydrodynamic conditions, they might be indispensable to avoid the structural collapse of the foundation due to scour phenomena. The design of scour protections is typically deterministic, which often results in overestimated mean diameters of the armour layer. Moreover, the design methodologies currently applied do not provide a measure of safety associated with the proposed design. The present research proposes a novel methodology to assess the safety of the protection and to perform the probabilistic design of static and dynamic scour protections. A case study based on Horns Rev 3 offshore wind farm is used to show how to select the mean stone diameter according to a pre-defined probability of failure of the protection. The results show that a dynamic scour protection could be safely designed with a reduction of the mean stone diameter up to 15 cm, when compared with the statically stable protection.

Probabilistic Comparison of Static and Dynamic Failure Criteria of Scour Protections

“…The results respectively showed that P f increases with N, that is, P f = 2.5 × 10 −4 for N = 1000, P f = 4.2 × 10 −4 for N = 3000 waves, P f = 6.9 × 10 −4 for 5000 waves and P f = 7.8 × 10 −4 for 7000 waves (Figure 6). This aspect was identified in References [9,17] as potentially having an effect on the probability of failure, which is hereby shown as being the case. For the present case study, although varying within a considerable range, it is interesting to note that the magnitude of P f remained within the order of 10 −4 .…”

“…Note however, that such event, does not necessarily means that there is an actual failure in terms of the filter layer exposure. There is a failure in the sense that the design criterion is not being respected as it should [9].…”

“…A reason for this is the fact that the majority of the design techniques for scour in waves and current environment have a remarked empirical nature [8]. The literature shows a lack of studies performed on the probabilistic design and reliability (safety) assessment of scour protections at marine environments [9] but several works have been performed for scour under current alone, for example, References [10,11]. Recently, reliability design and analysis of scour protections for offshore wind foundations has been addressed in References [9,12].…”

“…The literature shows a lack of studies performed on the probabilistic design and reliability (safety) assessment of scour protections at marine environments [9] but several works have been performed for scour under current alone, for example, References [10,11]. Recently, reliability design and analysis of scour protections for offshore wind foundations has been addressed in References [9,12]. Both researches, outline the fact that the design choices on empirical variables, such as the failure criteria or the wave friction factor, have an influence on the evaluation of the reliability.…”

“…Moreover, this could be extended to design the scour protection based on a pre-determined probability of failure. This was recently proposed in Reference [9], which concluded that more knowledge and a deeper discussion was required on the impact of the design choices on the output of the probabilities of failure. In Reference [9], preliminary conclusions indicate that reliability analysis could be used to optimise the mean diameter of the armour stones, thus providing a potential contribution to cost savings without compromising the systems' safety.This paper performs the reliability study of a scour protection inspired in the case study of Horns Rev 3 offshore wind farm [16], with the sea-state data being modelled with non-parametric bi-variate version of the Kernel Density Estimation Method.…”