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International audience; As current attention of the offshore industry is drawn by developing pilot farms of Floating Wind Turbines (FWTs) in shallow-water between 50m and 100m, the application of nylon as a mooring component can provide a more cost-effective design. Indeed, nylon is a preferred candidate over polyester for FWT mooring mainly because of its lower stiffness and a corresponding capacity of reducing maximum tensions in the mooring system. However, the nonlinear behaviors of nylon ropes (e.g. load-elongation properties, fatigue characteristics, etc.) complicate the design and modeling of such structures. Although previous studies on the mechanical properties and modeling of polyester may be very good references, those can not be applied directly for nylon both on testing and modeling methods. In this study, first, an empirical expression to determine the dynamic stiffness of a nylon rope is drawn from the testing data in the literature. Secondly, a practical modeling procedure is suggested by the authors in order to cope with the numerical mooring analysis for a semi-submersible type FWT taking into account the dynamic axial stiffness of nylon ropes. Both the experimental and numerical results show that the tension amplitude has an important impact on the dynamic stiffness of nylon ropes and, as a consequence, the tension responses of mooring lines. This effect can be captured by the present modeling procedure. Finally, time domain mooring analysis for both Ultimate Limit State (ULS) and Fatigue Limit State (FLS) is performed to illustrate the advantages and conservativeness of the present approach for nylon mooring modeling.

International audience; Dynamic rating is an approach which implies to operate an electrical network closer to its thermal limits. This approach may be very beneficial for wave farms, as they are expected to present a highly fluctuating electrical current profile while benefiting from the large thermal inertia of the soil where their export cable will be buried. However, as the implementation of this approach is still in its infancy for offshore wind farms, it may be expected that the first wave farms, under the form of small-scale test sites, will be sized with respect to electrical current limits at a first stage. This sizing may be upgraded at a second stage when design methods will have included dynamic rating. However, this raises the question of the economic feasibility of this two-step approach, which is studied in this paper. Also, performing such a techno-economic analysis requires developing an electrothermal model of the export cable able to represent its transient response in a sufficiently precise manner while requiring also a reasonable computing time. In this perspective, a comparative analysis between several electrothermal modelling methods is also described in this paper.

Bio-colonisation affects the ageing of materials and the behaviour of offshore structures. Mooring systems and umbilicals belong to the family of slender bodies which are components sensitive to bio-colonisation because of a change of dynamic behaviour due to shape, roughness and mass modifications. However, this stochastic process in time and space is hard to predict. The purpose is then twofold: first, to provide a stochastic spatial model of the bio-colonisation on a mooring line, second, to show that in some defined environmental conditions, such as low wave height, low wind and current velocities, the monitoring of mooring lines tension can help to assess and reduce uncertainty on this model. Therefore, a comprehensive stochastic modelling based on mussels colonisation was carried out using on-site videotapes, experimental campaigns and expert knowledge. We studied the efficiency of a virtual sensing network using this model and a conditional entropy metric. It is first shown that the spatial model fits well with experimental data, and second that a denser medium accuracy sensor network is to be preferred to a single high accuracy fairlead sensor to reduce the uncertainty on the model parameters. It is then worth updating bio-colonisation on mooring lines during the life-time of a floating wind turbine.

Nowadays, the service life monitoring of mooring lines attracts more and more attention from the marine renewable energy (MRE) industry where developers desire to have a closer look on the safety margin. Besides, current standards are also being improved in order to ensure safe and cost-effective design for MRE devices. In the present study, we believe that monitoring and modeling should be implemented in parallel in order to better estimate the actual state and the reliability of the system during the expected service life. Besides, it is crucial to clarify the mechanic of the mooring line responses, to understand why and how they are modified during deployment, and how those can be accounted for in order to update the actual fatigue damage and predict the remaining allowable service life of system. A comprehensive modeling and service life monitoring methodology for mooring lines of floating wind turbines (FWTs) is discussed by the authors based on the reliability approach. Critical factors that influence structural behaviors such as material deterioration and marine environmental factors involved are identified and accounted for in this reliability-monitoring basis. Ultimately, based on the reliability level estimated, decisions regarding required maintenance or replacement of lines can be made.

Marine renewable energy (MRE) is an interesting sustainable source of power. However, planning the cost-effective integration of MRE, and in particular of wave energy, in the energy mix remains a challenge as its levelized cost of energy (LCOE) is still greater than these of more conventional energy sources. This paper describes a preliminary study about the energy production management of a simulated wave energy converter (WEC) farm based on point absorbers that could be installed in the vicinity of the SEM-REV site, i.e. the French multi-technology open sea test site. The proposed approach is based on the temporal analysis of the electrothermal behavior of the export cable of the SEM-REV. The models developed under Matlab-Simulink®, are generic and can be easily extended to other WEC types and more sophisticated models and control strategies, as well as to other energy sources (e.g. offshore wind). It is demonstrated in this work that there is room to maximize the utilization of the export cable, which allows to increase wave electricity production during a predetermined sea-state sequence with limited to no curtailment.