Author(s): Rui Figueiredo; Tiago Fazeres Ferradosa; Joao Chambel; Paulo Rosa Santos; Francisco Taveira Pinto

Linked Author(s): Francisco Taveira Pinto

Keywords: Offshore wind; Hindcasts; Statistical modelling; Scour protection

Abstract: There is growing interest in marine renewable energy and in the role that it can play in the transition to a cleaner and more sustainable energy system. Among available marine renewable energy technologies, offshore wind is seen as the most promising, and in recent years there is been a significant increase in its installed capacity worldwide. Offshore wind structures typically use bottom-fixed foundations that are susceptible to scour phenomena and therefore require adequate protection, which can represent a significant part of their global cost. The optimization of such protections can therefore play a key role in reducing the cost of offshore wind structures and increase their competitiveness against other energy sources. However, modelling the scour process in bottom fixed foundations and designing adequate scour protections is a technically challenging problem that is associated with a number of uncertainties. Among them, a primary source of uncertainty concerns the estimation of the physical actions that an offshore wind structure and its foundations will be exposed to over their lifetime. This requires reliable and sufficiently long time series of environmental variables of interest. Ideally, in situ measurements (e.g. from buoys) would be used for this type of analysis, but in practical situations such data will often not be available. In this context hindcasts can play a crucial role, as they can provide multi-decadal, spatially continuous and consistent estimates for relevant metocean variables. However, the hindcast modelling of sea states is associated with its own array of uncertainties, and it is unclear to which extent this can affect the design of scour protections for offshore structures. Therefore, in this study we compare different available multi-decadal wave hindcasts for the North Sea and analyse how the selection of hindcast models can affect the design of scour protections, particularly in terms of the adopted median stone diameter of the armour layer. To achieve this, for each of the hindcast datasets, various statistical models are used to represent the joint distributions of significant wave height and peak wave period, ranging from the widely-used conditional modelling approach to more novel copula distributions. A state-of-the-art method for the dynamic stability design of the armour layer is then applied and the results are compared across the different hindcast and statistical models. Based on this, we explore and discuss the implications of model selection on the definition of the median stone diameter of the armour layer of offshore scour protections. The results are expected to contribute towards more informed decision-making in this field, where due to the very large scale of the structures, even slight differences in design assumptions can result in considerable variations in overall costs and, consequently, in the level of attractiveness of offshore energy solutions.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221808

Year: 2022