Scientific Publications


Performance Evaluation of Dynamic HV Cables with AL Conductors for Floating Offshore Wind Turbines    

Presented at the IOWTC2019, 3rd-6th November of 2019, St. Julian’s, Malta.
Floating Offshore Wind turbine installations will require HV dynamic power cables to be connected to the longer length static export power cables. Experience from offshore wind installations has highlighted the criticality of power cables, underlining the need for high integrity, yet cost effective cable solutions. This paper will assess the mechanical performance and load parameters for an Aluminum power conductor cable. Whilst copper is the conventional choice due to its lower resistive losses, Aluminum cores are increasingly used for static power cables, due to their benefits regarding overall cable weight and material cost. The work presented adopts a coupled aero-elastic and hydrodynamic modelling approach to simulate the behavior of the well-documented OC4 semi-sub platform, together with the 5MW NREL wind turbine. The model allows a direct comparison between the two cable types, maintaining the overall system and environmental conditions. The results inform the design envelope for the ultimate load conditions. Two critical cable design parameters, the effective tension and bending radius, are quantified through global load estimates, to subsequently inform the local stress analysis. The results will form the basis for future physical demonstration and validation tests. This paper will be of interest to technology developers and practitioners concerned with submarine dynamic power cables. It offers a methodology to directly compare and evaluate different cable design options and provides some design guidance for aluminum conductor cables.  

 Load and Fatigue Evaluation for 66 kV Floating Offshore Wind Submarine Dynamic Power Cable

Presented at the JICABLE19, 23rd-27th June 2019, Paris, France 
Floting Offshore Wind technology has seen a number of prototype deployments around the world in recent years. One of the critical components that must maintain the highest possible integrity is the dynamic power cable. This paper presents the approach and applied methods for the design work that informs the development and qualification of a 66 kV submarine dynamic power cable. The design envelope in quantified through coupled aero-hydrodynamic modelling, determining the ultimate load conditions for different cable configurations. The model sensivity and convergence for an OC4 floating design are explored regarding metocean conditions, computational is chosen as most suitable design, providing a compromise between hang-off tensions and induced bending stresses. The numerical results form the basis for subsequent physical cable demonstration and validation tests.
Innovative, low cost, low weight and save floating wind technology optimized for deep water wind sites.    

Presented at the EERA DeepWind 2020, 15th-1th January of 2020, Tronheim, Norway.


FLOTAN is a collaborative project funded by EU Research and Innovation programme Horizon 2020, under the Grant Agreement nº 815289. It is composed of 17 partners from 8 different European countries and has recently started (April 2019). The main objective of FLOTANT is to develop the conceptual and basic engineering, including performance tests of the mooring and anchoring systems and the dynamic cable to improve cost-efficiency, increased flexibility and robustness to a hybrid concrete-plastic floating structure implemented for Deep Water Wind Farms (DWWF).


According to WindEurope report, offshore wind is expected to produce 7% to 11% of the EU’s electricity demand by 2030, as offshore wind energy could have an average cost of 54 €/MWh in the most favourable locations. Energy produced from turbines in deep waters could meet the EU’s electricity consumption four times over, according to estimates from WindEurope. In consequence, encouraging the development and deployment of offshore wind in deep waters is a key strategic issue for the EU. With floating solutions, wind power can expand into new deep-water areas, often further from shore, opening vast new areas and markets currently unavailable for offshore wind. However, many elements of an offshore wind farm become more expensive as depth increases: mooring, anchoring and dynamic cables are the most obvious. Far-shore sites also pose additional challenges for installation, and O&M manoeuvres.

The cost of the mooring system is a growing part of the whole foundation costs as depth increases. Traditional mooring solutions, based exclusively on chains, are not a satisfactory solution, due to the high weight associated with long tethers and the lack of flexibility in the very likely case of excursions of the structure, due to currents and/or changes in the wind. The speed and off-set of these excursions can be important in the case of deep-water moorings, giving place to very important stresses in the rigid chains to counteract the inertia of the structure. Innovations in mooring components and anchoring systems to reduce loads and actively control and minimise excursions are required.

New challenges arise in DWWF for electrical transmission, as the power cable must be able to accommodate all movement and loading from the ocean in relation to the platform, as well as its own weight, and therefore has different performance specifications from the cable used in shallow-water wind farms. Advances in light weight dynamic cables are needed to reduce loads and achieve reliable and cost-effective export systems in FOW farms. Cost reductions should be achieved through optimisation of the whole energy export system, including the dynamic cable, the inter-array cabling, the floating offshore substation and the export lines.

Other aspects as installation and O&M strategies in DWWF should be optimised, boosting port-based pre-assembly and installation, removing the need for expensive heavy-lift installation vessels that would increase the investment needed as distance to shore is higher. Optimal marine management and predictive O&M strategies will enable the reduction of major repair costs, hence minimising the number of marine operations.

Expected Results

FLOTANT Innovative solutions will be designed to be deployed in water depths from 100m to 600m, optimizing the LCOE of the floating solution (85-95 €/MWh by 2030). Prototypes testing of this offshore wind floating platform and its associated mooring, anchoring and dynamic cable systems are foreseen in relevant environment and real sea conditions within the scope of the project. Moreover, the assessment and optimisation of the construction, installation and decommissioning techniques will also contribute to bring down the current cost of offshore wind energy, as well as, increasing its deployment. An expected 60% reduction in CAPEX and 55% in the OPEX by 2030, will be directly motivated by FLOTANT novel developments and additional reductions due to external technology improvements. In addition, environmental, social and socio-economic impacts will be assessed, increasing social acceptance of FOW in deep waters. 


Informing components development innovations for floating offshore wind through applied FMEA framework.    

Presented at the ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering OMAE2020, June 28-July 3, 2020, fort laudardale, FL, USA (Virtual Conference-Online)

Future offshore wind technology solutions will be floating to facilitate deep water locations. The EUH2020 funded project FLOTANT (Innovative, low cost, low weight and safe floating wind technology optimized for deep water wind sites) aims to address the arising technical and economic challenges linked to this progress. In particular, innovative solutions in terms of mooring lines, power cable and floating platform, specifically designed for floating offshore wind devices, will be developed and tested, and the benefits provided by these components assessed. In this paper a purpose-built Failure Modes and Effect Analysis (FMEA) technique is presented, and applied to the novel floating offshore wind components. The aim is to determine the technology qualification, identify the key failure modes and assess the criticality of these components and their relative contributions to the reliability, availability and maintainability of the device. This will allow for the identification of suitable mitigation measures in the development lifecycle, as well as an assessment of potential cost savings and impacts of the specific innovations. The methodology takes into account inputs from the components developers and other project partners, as well as information extracted from existing literature and databases. Findings in terms of components innovations, their main criticalities and related mitigation measures, and impacts on preventive and corrective maintenance, will be presented in order to inform current and future developments for floating offshore wind devices. 


Current Status and Future Trends in the Operation and Maintenance of Offshore Wind Turbines: A Review 

Published in Energies 2021, 14, 2484. https://doi.org/10.3390/en14092484

Operation and maintenance constitute a substantial share of the lifecycle expenditures of an offshore renewable energy farm. A noteworthy number of methods and techniques have been developed to provide decision-making support in strategic planning and asset management. Condition monitoring instrumentation is commonly used, especially in offshore wind farms, due to the benefits it provides in terms of fault identification and performance evaluation and improvement. Incorporating technology advancements, a shift towards automation and digitalisation is taking place in the offshore maintenance sector. This paper reviews the existing literature and novel approaches in the operation and maintenance planning and the condition monitoring of offshore renewable energy farms, with an emphasis on the offshore wind sector, discussing their benefits and limitations. The state-of-the-art in industrial condition-based maintenance is reviewed, together with deterioration models and fault diagnosis and prognosis techniques. Future scenarios in robotics, artificial intelligence and data processing are investigated. The application challenges of these strategies and Industry 4.0 concepts in the offshore renewables sector are scrutinised, together with the potential implications of early-stage project integration. The identified technologies are ranked against a series of indicators, providing a reference for a range of industry stakeholders.

Incorporating stochastic operation and maintenance models into the techno-economic analysis of floating offshore wind farms

Published in Applied Energy Volume 301, 1 November 2021. https://doi.org/10.1016/j.apenergy.2021.117420

Floating offshore wind is rapidly gaining traction in deep water locations. As with all new technologies, to gain the confidence of developers and investors, the technical and economic feasibility of this technology must be proven and robust cost estimates are necessary. In this paper, the authors present a methodology to calculate the capital and operational indicators of a floating wind farm over its project lifetime. A set of computational models is used to reduce the uncertainties in the estimation of the technical and economical parameters. In particular, the effect of using detailed operation and maintenance models and strategies allows a better estimation of operational cost. The paper highlights the requirements and specific adjustments considered for floating offshore wind technology. The methodology is demonstrated for two case studies inspired by real floating wind installations in the United Kingdom, namely the Hywind and Kincardine projects. The related input data, gathered from publicly available sources, constitute a reference database for future studies in the floating offshore wind sector. Results are presented for the two case studies. These show that availability and energy production are in line with typical values for offshore wind projects, and highlight the substantial contribution of operational expenses to the cost of energy. Results are also compared against previous estimations for floating offshore wind projects, showing satisfactory agreement for the overall project costs but an underestimation of operation and maintenance costs in previous studies. This highlights the importance of using detailed operation and maintenance models to adequately capture operational expenses.

The economics of floating offshore wind – A comparison of differentmethods

Published in Developments in Renewable Energies Offshore https://zenodo.org/record/4681746#.YOMAfuhKiUk

 Different technologies for Floating Offshore Wind (FOW) systems have been developed and multiple studies exist that aim at comparing the techno-economic advantages and disadvantages of each technology. However, the assumptions and calculations used in these different studies vary largely resulting in high variation of the final cost estimates. This points to the need of developing a consistent method that allows for technology comparison at whole system but also sub-system level. In this paper, the sensitivity of the final results to the different assumptions and calculation set-ups is investigated. This study provides a discussion of the suitability of the applied methods, and as a result suggests the standardisation of the procedure, while defining a set of recommendations for good practices in techno-economic assessment of FOW technologies. The suggested common framework will facilitate the comparison of different FOW technologies based on costs and increase investor confidence.

FLOTANT concept: floater design, integrated modelling & global performance

Published in Developments Journal of Physics, Vol 2257 https://iopscience.iop.org/article/10.1088/1742-6596/2257/1/012007#references

This paper presents the final design of the FLOTANT concept developed within the framework of a Cooperation Research Project funded by the European Union’s Horizon 2020 research and innovation programme. The FLOTANT concept is a hybrid concrete-plastic barge-type floating offshore substructure holding a 12MW wind turbine; buoyancy is achieved using plastic material (foam) fitted within the floater substructure. The paper describes the FLOTANT project innovations and presents the principal dimensions of the floating wind turbine system. The floating system has been developed to fulfil strict requirements at two different locations: Gran Canarias (GC) and West of Barra (WoB) sites. Therefore, two different mooring solutions including innovative elastomers to reduce peak loads are presented. The floating system’s intact and damaged static stability is checked along with the hydrostatic and hydrodynamic properties, showing the low-cost barge-type system’s feasibility.

To ensure the desired behaviour of the floating system, a relevant subset of design load cases (DLCs) based on international standard requirements have been simulated checking the global performance of floater and moorings. Fully coupled hydro-servo-aero-elastic simulations have been performed to consider the wind and wave loading, the aerodynamic response of the rotor, the hydrodynamic response and structural dynamics of the turbine-floater-mooring assembly, and the control system actions. A summary of the results of the last design loop within the FLOTANT project is presented.

Dynamic HV cables with AL conductors for floating offshore wind turbines: a cost and behavior comparative study

Published in Developments in Renewable Energies Offshore     https://doi.org/10.5281/zenodo.6590158

 Floating Offshore Wind installations require high-voltage dynamic power cables to transmit the
electricity generated from the devices to the offshore substation, before being exported to the onshore grid. High integrity, yet cost effective cable solutions are needed for this purpose. Whilst copper is the conventional choice of material due to its lower resistive losses, aluminium cores are increasingly being proposed for static power cables applications due to their reduced cost and weight. In this work, a comparative analysis of these two options in terms of costs and performance is presented. A numerical model evaluating the expected cable effective tensions and bending stresses, coupled with an aero-elastic and hydrodynamic model of a floating wind platform, is used to define the ultimate load conditions for various configurations. Results show the feasibility of cables with aluminium conductors for low weight, low cost and deep water applications, highlighting the advantages for floating offshore wind projects.

Improvements in the O&M modelling of floating offshore wind farms



Operation and maintenance are widely recognized as major areas for cost reduction in offshore
wind installation. This paper explores improvements in the O&M planning capabilities for the floating offshore wind sector. Computational tools have been developed in the last decades to support the strategic planning of onshore and offshore wind O&M interventions. However, adaptations will be needed in order to capture the novel operational requirements of floating devices with respect to traditional bottom-fixed turbines. In this paper, specific adjustments to offshore O&M planning computational tools in order to capture floating farms capabilities are identified and implemented. The effects of these amendments on the overall maintenance strategy, as well as related key performance indicators, are tested for the same case study. The comparison between bottom-fixed and floating offshore wind projects allows to quantify the impacts of using floating platforms, and provides indications for future improvements in floating wind O&M planning.