There is a trend for offshore wind farms to move further from the point of common coupling to access higher and more consistent wind speeds to reduce the levelised cost of energy. To accommodate these rising transmission distances, High Voltage Direct Current (HVDC) transmission has become increasingly popular. However, existing HVDC wind farm topologies and converter systems are ill suited to the demands of offshore operation. The HVDC and AC substations have been shown to contribute to more than 20% of the capital cost of the wind farm and provide a single point of failure. Therefore, many wind farms have experienced significant delays in construction and commissioning, or been brought off line until faults could be repaired. What is more, around 75% of the cost of the HVDC and AC substations can be attributed to structural and installation costs. Learning from earlier experiences, industry is now beginning to investigate the potential of a modular approach. In place of a single large converter, several converters are connected in series, reducing substation individual size and complexity. While such options somewhat reduce the capital costs, further reductions are possible through elimination of the offshore substations altogether. This thesis concerns the design and evaluation the Hybrid HVDC Transformer, a high power, high voltage, DC transformer. This forms part of the platform-less (i.e. without substations) offshore DC power collection and distribution concept first introduced by the Offshore Renewable Energy Catapult. By operating in the medium frequency range the proposed Hybrid HVDC Transformer can be located within each turbine’s nacelle or tower and remove the need for expensive offshore AC and DC substations. While solid state, non-isolating DC-DC transformers have been proposed in the literature, they are incapable of achieving the step up ratios required for the Hybrid HVDC transformer [1]– [3]. A magnetic transformer is therefore required, although medium frequency and non-sinusoidal operation does complicate the design somewhat. For example, inter-winding capacitances are more significant and core losses are increased due to the added harmonics injected by the primary and secondary converters [1], [2]. To mitigate the impact of these complications, an investigation into the optimal design was conducted, including all power converter topologies, core shapes and winding configurations. The modular multilevel converter in this case proved to be the most efficient and practical topology however, the number of voltage levels that could be generated on the primary converter was limited by the DC bus voltage. To avoid the use of pulse width modulation and hence large switching losses, a novel MMC control algorithm is proposed to reduce the magnitude of the converter generated harmonics while maintaining a high efficiency. The development and analysis of this High Definition Modular Multilevel Control algorithm forms the bulk of this thesis’ contribution. While the High Definition Modular Multilevel Control algorithm was developed initially for the Hybrid HVDC Transformer, analysis shows it has several other potential applications particularly in medium and low voltage ranges.

The sustainable development of the offshore wind and wave energy sectors requires optimising the exploitation of the resources, and it is in relation to this and the shared challenge for both industries to reduce their costs that the option of integrating offshore wind and wave energy arose during the past decade. The relevant aspects of this integration are addressed in this work, and in particular the evaluation of the Wave Energy Converter (WEC) sub-system of a hybrid wind-wave energy converter: the state of the art of combined technologies; the definition of a novel hybrid prototype, based on a preliminary feasibility analysis of a conceptual proposal; and the evaluation of a simplified version of this prototype by means of physical and numerical modelling as a mean to set the reference and define new tools and methods for future evaluation and optimisation of the prototype. Because of the novelty of combined wave and offshore wind systems, fundamental knowledge was lacking as, for example a comprehensive review and classification, which was published as a journal paper framed in the present work. In particular, the core of this PhD thesis deals with the WEC sub-system of a hybrid device that integrates an Oscillating Water Column (OWC) device into the typical monopile substructure of an offshore wind turbine. A new prototype of the hybrid energy converter has been proposed, and a patent application was filled. Furthermore, an experimental set-up was designed, built and tested at a wave flume. On the basis of this experimental campaign the performance of the device is analysed. Finally, a full 3D-numerical mirror of the experimental set-up, including the hybrid energy converter, is defined and validated, and the flume enclosure effects studied for regular waves.

The installed capacity of offshore wind turbines in Europe is increasing with the monopile being the most common type of foundation. During its lifetime an offshore wind turbine is exposed to high dynamic loads which eventually can result in the fatigue of the substructure. This thesis will show how the linear damage accumulation approach based on the Miner’s rule can be used to estimate the damage induced on the substructure of an offshore wind turbine using measurements from strain gauges. Furthermore, the most important environmental influences will be illustrated and the different stress concentration factors and the size effect introduced in the industry guideline DNVGL-RP-C203 will be analysed towards their effect on the calculated lifetime.

The installed capacity of European Offshore Wind Turbines (OWT) is likely to rise from the 2014 value of 7GW to 150GW in 2030. However maintenance of OWT is facing unprecedented challenges and cost 35% of lifetime costs. This will be equivalent to £14billion/year by 2030 if current OWT maintenance schemes are not changed. However the complexities around OWT operation require tools and systems to optimise OWT maintenance. The design of optimal OWT maintenance requires failure analysis of over 10,000 components in OWT for which there is little published work relating to performance and failure. In this work, inspection reports of over 400 wind turbine gearboxes (source: Stork Technical Services) and SCADA data (source: Shetland Aerogenerators Ltd) were studied to identify issues with performance and failures in wind turbines. A modified framework of Failure Mode Effects and Criticality Analysis (i.e. FMECA+) was designed to analyse failures according to the unique requirements of OWT maintenance planners. The FMECA+ framework enables analysis and prediction of failures for varied root causes, and determines their consequences over short and long periods of time. A software tool has been developed around FMECA+ framework that enables prediction of component level failures for varied root causes. The tool currently stores over 800 such instances. The need to develop a FMECA+ based Enterprise Resource Planning tool has been identified and preliminary results obtained from its development have been shown. Such a software package will routinely manage OWT data, predict failures in components, manage resources and plan an optimal maintenance. This will solve some big problems that OWT maintenance planners currently face. This will also support the use of SCADA and condition monitoring data in planning OWT maintenance, something which has been difficult to manage for a long time.

No description available.

Abstract   The permit application process for offshore wind farms (OWF) in Sweden conceivably requires a comprehensive and transparent complement within risk management. The NETEP framework (covering risks concerning navigation, economics, technology, environment and politics), based on a futures planning mechanism (STEEP) has consequently been brought forward as a structure for the application of FMECA (Failure Mode, Effects, and Criticality Analysis) methodology to the permit application process of the Swedish offshore wind farm sector. FMECA, originating from the aeronautical and automobile industries, presents a systematic method for the prediction of future failure in a product, part or process, to evaluate the consequences of that failure and to suggest possible measures for its mitigation or eradication. Its application to attitude and acceptance, safety and environmental effect remains, however, limited which creates the research gap for this thesis. Three Swedish offshore wind farm (OWF) projects in the Baltic Sea area (Lillgrund, Taggen and Trolleboda) were put forward as case studies for use in the evaluation of the proposed FMECA-NETEP methodology, which was approached in two stages. The first evaluation stage results showed that the model accommodates the precautionary principle, the consideration of stakeholder viewpoints, the mitigation of negative effects, the analysis of alternative sites, the observation of relevant legislation and the utilisation of contemporary research. In the subsequent stage of evaluation, the factor for incorporation into the adapted model was intra- and inter-sector cumulative impact. Results showed that positive cumulative impact cannot be illustrated by the model whereas neutral and negative cumulative impact can.  The model’s added value is that it facilitates decision making by providing a rigorous, transparent and structured methodology, the holistic approach of which provides a sound basis for the incorporation of contemporary research.

To impede the progress of climate change, many policy makers are considering avenues to decarbonize electricity production. In addition to decarbonization, policy makers must consider how the cost of electricity will impact various stakeholders, balancing cost and social benefits. Offshore wind farms have the potential to produce affordable, carbon-free electricity, but they are a relatively new technology. The relative juvenescence of offshore wind lends itself to an uncertain future, regarding production costs. In this thesis, we seek to understand cost drivers behind offshore wind electricity by analyzing historic trends in offshore wind levelized cost of electricity (LCOE) through learning curves, characterizing how learning from producing a technology can lead to decreases in production costs. Additionally, we explore how the maturity of component technologies can affect the learning rate, and consequently the benefits of learning, of offshore wind. Finally, we create a robust data set to inform decision makers and researchers by marrying historic data to forward-looking expert elicitations.

System specifications and testing model for increasing the rated power output, rotor diameter, hub height, and maximum tip speed of horizontal axis wind turbines is designed and implemented on the system advisor model simulator. Its performance is tested on offshore wind turbine’s direct-drive and single stage-low speed generators. Although this simulator produces impressive results, it has some limitations in the operation of wind turbines. The terrain and topography of wind turbines are not considered in the simulation process. It also does not assess the electrical transients and physical stress of wind turbine components. Despite its limitations, four large offshore wind turbines and wind farms have been successfully simulated. It is found that the 9 MW, 10 MW, 11 MW and their respective wind farms generate more energy and better capacity factor on the direct-drive than single stage-low speed generator. Furthermore, a rectangular layout of 20 wind turbines considerably impacted the excellent performance of this generator on the wind farms. Another notable outcome of the study is that higher system specifications do not always generate feasible results for wind turbines despite favourable weather conditions. For the Vestas 8 MW wind turbine, the viable percentages for increasing the size of its rated power output, rotor diameter, hub height and maximum tip speed is only 12.5%, 25% and 37.5%. The viability of these three upgrades has been confirmed by suitable graphs of power curves and feasible energy production results. Thus, these percentages confirm an 8 MW wind turbine’s attainable design limits for generating realistic energy production and capacity factor. On the contrary, a 50% increase in the above four system specifications yielded unviable capacity factor and energy production results. This is because this upgrade is too high to work successfully on the current wind turbine technology. Furthermore, the shape of the power curve from the 50% specifications is not the typical curve for wind turbines. It has been observed that increasing the value of maximum tip speed beyond 143 m/s and the rotor diameter beyond 246 m give rise to an unusual power curve. Concerning wind speed for high energy production, an average daily minimum and maximum wind speed of 4.58 m/s and 15.08 m/s yielded good results. Given the prevailing trend of designing large wind turbines, findings in this study are particularly helpful in understanding how capacity factor, energy production and energy losses are affected by the size of system specifications. Not only that, but these findings also have fundamental concepts that can be used to optimize the design of large offshore wind turbines. The study is equally valuable for determining suitable weather conditions and wind power potential for large offshore wind farm sites. / Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2020. / Electrical, Electronic and Computer Engineering / MEng (Electrical Engineering) / Unrestricted

Le gouvernement français a fixé pour objectif que 23% de la consommation énergétique proviendraitdes énergies renouvelables, y compris les énergies renouvelables marines pour 2020. Huit parcs éoliensen mer sont prévus le long des côtes françaises Atlantique-Manche, parmi lesquels le site de Dieppe-LeTréport dans la partie orientale de la Manche. Afin de mieux connaître la structure et le fonctionnementde cet écosystème avant l'installation du parc éolien, ma thèse est axée sur la construction de modèlesde réseau trophique à l’aide du logiciel Ecopath sur ce site d’implantation en comparaison avec deshabitats benthiques similaires en Manche. La collecte de nouvelles données sur les compartimentsbiologiques (zooplancton, suprabenthos, méiofaune, benthos et poissons démersaux) est essentielle.Ainsi, quatre campagnes (étés 2014/2015, hivers 2015/2016) ont permis d'estimer la contribution dechaque groupe zoologique dans deux habitats benthiques principaux : les graviers ensablés/sablesgraveleux et les sables moyens sur le site (environ 30 km²) correspondant à deux communautésbenthiques largement répandues en Manche. La macrofaune a été échantillonnée avec une benne VanVeen, le suprabenthos avec un traîneau Macer-Giroq modifié avec une station dans chaque habitatbenthique (échantillonnage jour et nuit) et les poissons avec un chalut à perche. Au total, 1 584 poissonsdémersaux ont été recueillis et leurs contenus stomacaux ont été examinés. Les résultats montrent quele site se distingue par la forte contribution des bivalves en termes de biomasses et que les crustacés sontdes proies préférentielles des poissons. Ce lien est discuté à travers l’analyse du réseau trophique avecun focus sur les espèces consommées et non consommées de communautés benthiques, et la notion decul de sac trophique. / The French government has set a target of 23% of energetic consumption derived from renewable energysources including Marine Renewable Energy for 2020. Eight offshore wind farms (OWF) are plannedalong the Atlantic-English Channel French coasts and among them Dieppe-Le Tréport site in the easternpart of the English Channel. In order to gain further knowledge on the ecosystem structure andfunctioning before the OWF installation, my PhD thesis is focused on the construction of an EcopathModel on the wind farm site in comparison with similar benthic habitats from the English Channel. Thecollection of new data on biological compartments (zooplankton, suprabenthos, meiofauna, benthos anddemersal fishes) are essential. Four cruises (summers 2014 /2015 and winters 2015 /2016) allowed toestimate the contribution of each zoological group in two main benthic habitats: i.e. sandy gravels andmedium clean sands, founded on the site (about 30 km²), corresponding to two microbenthiccommunities well represented in the English Channel. Macrofauna was sampled with a Van Veen grab,suprabenthos with a modified Macer-Giroq sledge in one station from each benthic habitat (day andnight sampling) and fish with a beam trawl in ten stations. A total of 1,584 demersal fishes has beencollected and their stomach contents has been examined. The results show the importance of largebivalves in the benthic biomass whereas the crustaceans are preferential preys of fishes. This link isdiscussed within the trophic network between consumed and non-consumed species of benthiccommunities, and the concept trophic ‘cul de sac’.

Renewable energy sources are in strong demand as the need for clean energy is increasing, driven by the need to reach the environmental targets. Offshore wind power is becoming an attractive source as the technology matures and cost decreases. With time all wind farms age and when the time comes one needs to consider end of life alternatives. The objective with this Thesis is to assess lifetime extension of offshore wind farms, with a focus on reliability, availability and maintenance. As of today there are few offshore wind farms reaching their end of life but one needs to be prepared. From the literature review it is clear that there is somewhat limited documentation around lifetime extensions. The reason for lifetime extension is to increase the return on investment (ROI) of the original project, with a limited additional investment. With capital expenditure (CAPEX) being a large part of the overall project cost it is beneficial to achieve more operational years to divide the cost over, one also need to consider increased operations and maintenance (O&M) cost with aging assets. In this Thesis, a Reliability block diagram (RBD) model was developed in ReliaSoft BlockSim to calculate wind turbine failures, their associated downtime and O&M costs to create an overview of the future behaviour and cost. To assess the wind farm economics a financial model was built, based on output from the RBD model, to evaluate the levelized cost of energy (LCOE) and internal rate of return (IRR) for several cases of lifetime extension. A case study was done for a generic wind farm of 30 wind turbines, assumed to be built around 2010 using 3.6MW rated turbines, to illustrate a relevant case. Focus was on O&M cost, and it was assumed the structural integrity of the wind turbines would be maintained for the lifetime extension for up to 10 years.