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After a few weeks, underwater components of offshore structures are colonized by marine species and after few years this marine growth can be significant. It has been shown that it affects the hydrodynamic loading of cylinder components such as legs and braces for jackets, risers and mooring lines for floating units. Over a decade, the development of Floating Offshore Wind Turbines highlighted specific effects due to the smaller size of their components. The effect of the roughness of hard marine growth on cylinders with smaller diameter increased and the shape should be representative of a real pattern. This paper first describes the two realistic shapes of a mature colonization by mussels and then presents the tests of these roughnesses in a hydrodynamic tank where three conditions are analyzed: current, wave and current with wave. Results are compared to the literature with a similar roughness and other shapes. The results highlight the fact that, for these realistic roughnesses, the behavior of the rough cylinders is mainly governed by the flow and not by their motions.

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.

International audience; Due to the ever-increasing electricity demand, along with the need to reduce the dependence on fossil or nuclear resources, a growing amount of renewable energy is integrated in the energy mix of many countries. However, the cost-effective integration of wave energy remains difficult as its cost is still not competitive compared with other energy sources. This paper deals with the energy production management of a simulated wave energy converter farm based on point absorbers that could be installed in the vicinity of the SEM-REV site. The approach considers the electrothermal behaviour of the export cable in combination with techno-economic aspects. The method can be used to extend the power export capability of a test site at no cost, which could be very interesting to install more marine renewable energy (MRE) converters and/or of greater rated power. Our study shows that exploiting the thermal inertia of a wave farm submarine export cable, while also considering techno-economic aspects, may lead to an increase of 18% in the annual energy production without modifying this cable.

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.

Volume 8: Ocean Renewable EnergyNantes, France, June 9–14, 2013Conference Sponsors: Ocean, Offshore and Arctic Engineering DivisionISBN: 978-0-7918-5542-3; International audience; This paper synthesizes the technical feasibility study carried out for a hybrid ocean energy converter, with balanced wind and wave contributions. The solution envisaged involves a 100m diameter circular barge equipped with floating oscillating wave surge converters (OWSCs). This floating structure is mounted with a 5MW wind turbine. The present study covers power performance estimations, structural analysis and mooring design calculations. The first section describes the “Wave to Wire” model programmed in both frequency and time domain. The mathematical and hydrodynamic assumptions are highlighted together with the numerical model. The second part starts with the assessment of the performances of this device, carried out on in-house simulation codes. Based on combined wave and wind resources, the annual average absorbed power figures are compared with published results for existing ocean energy converters. The total rated power of the combined system reaches 10MW. Eventually, the last section approaches practical topics, directly related to the capital and operational costs inherent to an industrial development phase. The total steel mass is estimated first, from structural calculations carried out for a selection of 3D static loads cases. Then, a technical solution for the mooring system is presented together with the envisaged installation procedure.

Volume 7: Ocean Space Utilization; Ocean Renewable EnergyRio de Janeiro, Brazil, July 1–6, 2012Conference Sponsors: Ocean, Offshore and Arctic Engineering DivisionISBN: 978-0-7918-4494-6; International audience; This paper synthesizes the performance evaluation of a hybrid ocean wave energy converter. The case of interest represents a singular approach to combine a 5MW wind turbine with floating oscillating wave surge converters (OWSCs). The first section describes the comprehensive set of equations of motion in both frequency and time domain. The mathematical and hydrodynamic assumptions are highlighted together with the numerical model. The second part starts with the assessment of the initial performance of this device, carried out on in-house simulation codes. The first stage of numerical validation is based on the linear transfer functions (RAOs) results. The analysis of these RAOs also exposes interesting mechanical properties of this particular device. Eventually, the annual average absorbed power figures are extracted from the power matrices calculated for a few different sites.