This document constitutes Deliverable ‘D6.1 Extreme Event LiftWEC ULS Assessment” of the LiftWEC project. LiftWEC is a collaborative research project funded by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No 851885. It is the intention of the project consortium that the LiftWEC project culminates in the development of one or more promising configurations of a Wave Energy Converter operating through the use of one or more rotating hydrofoils that generate lift as the primary interaction with the incident waves. 

In this report, a structural analysis methodology is developed to ensure the survivability of LiftWEC under extreme operating conditions. For a wave cyclorotor or the LiftWEC concept, extreme operating conditions are considered as the primary design conditions in a similar manner to the Atargis system’s approach [Siegel, 2019]. This is because first, the maximum lift force in LiftWEC occurs in operating conditions, and secondly, because it operates in a submerged manner, hence slamming and wave-impact loads are avoided. 

At this stage, seventeen concept designs were developed within the consortium. As such, this report presents an introductory classification of the major commonalities. Subsequently, two representative configurations are selected to perform the structural analysis. In the first configuration, the hydrofoils are supported at both ends, and in the second configurations, the hydrofoils are supported only in the middle. A bottom-fixed frame is assumed as the support structure for both configurations. 

Given that the purpose of this report is to detail the Ultimate Limit State (ULS) methodology, “typical” loading and material strength properties are applied here for illustrative purposes. Wave operating conditions are selected from literature and we compare these conditions to wave data from a point in the Atlantic coast of France. A moderate strength offshore structural steel is selected as the construction material, and the structural integrity of the device is assessed subject to hydrodynamic loading derived from irregular waves. 

It is worthy to note that a limit state is a condition beyond which a structure or a part of a structure exceeds a specified design requirement. For example, ULS can be defined as a condition where a loss of structural resistance occurs. Additionally, partial safety factors can be considered to account for abnormal operating conditions. As such, here we defined the threshold for the ULS as one third of the yield stress level. This threshold however can be refined to meet future design specifications.

The structural analysis is carried out on three substructures: the hydrofoils, the attachment structures between rotor and hydrofoils and the support structure. Python, Abaqus and SkyCiv are utilised to perform and validate the structural analysis. Shear forces, bending moments and deflections are computed, and maximum stresses are computed in the substructures. It is found that in LiftWEC, the substructures that undergo the highest bending stresses are the hydrofoils. However, for the conditions examined, the stress levels remain below the allowable stress level for a wide range of hydrofoil spans.