Renewable energy sources are a hot topic, both when it comes to climate change and the constant increase in demand of electricity due to population growth and a more electrified society. One such energy source is wave energy - an energy source with great potential but still relatively new with the need for further development. Blekinge Institute of Technology (BTH) together with Ocean Harvesting Technologies (OHT) have made a collaboration to build a scaled Hardware-In-the-Loop (HIL) system of a power take-off (PTO) based on OHTs wave energy converter, InfinityWEC. The purpose is to teach the students at BTH about hydrodynamic and HIL simulations. A manual will also be written to help students perform the lab activities. A model of the HIL system will first be implemented in Matlab/Simulink, both with and without the WEC-Sim hydrodynamic simulation toolbox and simulations will be run to predict the system's behaviour. To parametrize the hydrodynamic model, the open-source Boundary Element Method (BEM) code, NEMOH, is used. The HIL system consists of electric motors, connected mechanically to each other with a coupling. One of the motors is the actuator, which applies torque to the second motor according to the simulated hydrodynamic loads on the buoy. The second motor on the other hand applies a torque according to the load connected to it or torque-controlled according to a selected control strategy. In this thesis two different types of loading is used: 1) resistive load without control of the generator drive, 2) resistive and capacitive load with reactive control of the generator drive. The load resistance can be changed within a limited range as well as the sea state. Data that can be collected are the position and angular velocity of the motors, the currents to and from the two motors and the voltage over the load capacitance. The project concluded that the compensation needed for the motors to get the true hydrodynamic force has little effect when using reactive control and that a protective capacitor is be needed between the actuator motor and the power supply to protect it from reverse current. Finally, this work demonstrated the effectiveness of HIL systems to execute simulations to test and validate PTO systems in wave energy converters. The advantages are that one can create representative wave loading without the presence of water and with ease test different sea states. / Förnybara energikällor är ett hett ämne, både när det gäller klimatförändringar och den ständiga ökningen av efterfrågan av el på grund av befolkningsökning och ett mer elektrifierat samhälle. En sådan energikälla är vågenergi - en energikälla med stor potential men fortfarande relativt ny med behov av vidareutveckling. Blekinge Tekniska Högskola (BTH) vill tillsammans med Ocean Harvesting Technologies (OHT) konstruera ett skalat Hardware-In-The-Loop (HIL) system av power take-off (PTO) baserat på OHT:s vågenergiomvandlare, InfinityWEC. Syftet är att lära eleverna på BTH om hydrodynamik och HIL-simuleringar. En manual kommer också att skrivas för att hjälpa eleverna att utföra labbaktiviteterna. En modell av HIL-systemet kommer först att konstrueras i Matlab/Simulink, både med och utan WEC-Sim hydrodynamisk simuleringsverktygslåda och simuleringar kommer att köras för att förutsäga systemens beteende. För att bestämma de nödvändiga parametrarna för hydrodynamiska modellen används Boundary Element Method koden NEMOH. HIL-systemet består av elmotorer, som är mekaniskt anslutna till varandra med en koppling. En av motorerna är ställdonet, som tillämpar vridmoment på den andra motorn enligt de simulerade hydrodynamiska belastningarna på bojen. Den andra motorn tillämpar ett vridmoment enligt belastningen som är kopplad till den eller är moment reglerad enligt en vald kontrollstrategi. I denna avhandling används två olika typ av belastning: 1) resistiv belastning utan kontroll av generatorndrivdonet, 2) resistiv och kapacitive belastning med reaktiv kontroll av generatorndrivdonet. Belastningsmotståndet kan ändras inom ett visst intervall och lika så havstillståndet. Data som kan samlas in är motorernas position och vinkelhastighet, strömmen till och från de två motorerna och spänningen över last kapasitatorn. I projektet drogs slutsatsen att den kompensation som behövs för motorerna för att få den riktiga hydrodynamiska kraften har liten påverkan reaktiv kontroll används och att det behövs en skyddande kondensator mellan ställdonsmotorn och strömförsörjningen för att skydda den mot bakström. Slutligen visade detta arbete hur effektiva HIL-system är för att utföra simuleringar för att testa och validera PTO-system i vågenergiomvandlare. Fördelarna är att man kan skapa representativ vågbelastning utan närvaro av vatten och med lätthet testa olika havstillstånd.

This report presents a master's thesis conducted within the machine design track at KTH Royal Institute of Technology. The research work was undertaken in collaboration with Novige AB, who commissioned the project. Novige AB is in the development and testing phases of a wave energy converter (WEC) of which exhibits a large area of unutilized potential. This led to the purpose of this thesis, conceptualizing a support structure and expanding mechanism for solar panels to be mounted on the float of the WEC. Since no previous work related to the subject had been conducted, the objectives were to explore different solutions and present a detailed final concept, including initial finite element calculations from expected load cases. The work consisted of several concept phases to ensure a thorough design process and to be able to accurately evaluate each concept. The outcome of the project yielded a conceptual design, featuring stackable solar panel modules. Each module consisted of four panels arranged horizontally and three panels vertically, resulting in a total of 288 solar panels, when incorporating eight modules on each float. The cumulative potential maximum power output of the configuration was estimated to be approximately 115 kW. When harsh conditions would be detected, the outer modules would retract under the fixed center module. To support the outer modules, a telescope beam was incorporated, spanning the outermost points of the structure, while roller guides were utilized at the inner end. Moreover, the movement of the outer modules was facilitated by a chain mechanism, housed within a U-profile. Most components in the design were proposed to be manufactured using steel, supplemented with protective measures such as paint or coating to ensure durability in the oceanic environment. / Denna rapport presenterar ett mastersarbete som utförts inom maskinkonstruktionsspåret vid KTH Kungliga Tekniska Högskolan. Arbetet genomfördes i samarbete med Novige AB, som beställde projektet. Novige AB befinner sig i utvecklings- och testfaserna av ett vågkraftverk (WEC) som har en stor outnyttjad yta med potential. Detta ledde till syftet med detta arbete, att konceptualisering en stödstruktur och en expanderingsmekanism för solpaneler som ska monteras på flotten av WEC. Eftersom ingen tidigare forskning hade utförts på området var målet att utforska olika lösningar och presentera ett detaljerat slutkoncept, inklusive initiala beräkningar med FEM under förväntade lastningsfall. Arbetet bestod av flera konceptuella faser för att säkerställa en noggrann designprocess och för att kunna utvärdera varje koncept på ett genomgående sätt. Projektet resulterade i en konceptuell design med stapelbara solpanelesmoduler. Varje modul höll 12 solpaneler, fyra horisontellt och tre vertikalt med 3 moduler per struktur. Varje WEC kunde bära totalt 8 strukturer vilket ger 288 solpaneler per WEC. Den sammanlagda potentiella effekten för konfigurationen uppskattades till cirka 115 kW. Vid svåra väderförhållanden, skulle de yttre modulerna dras tillbaka under den fasta mittmodulen för att minska vindfånget. För att stödja de yttre modulerna inkluderades en teleskopisk balk som bär de yttersta punkterna på modulen, medan rullstöd användes i den inre delen. Dessutom utfördes rörelsen hos de yttre modulerna av en kedja-kuggmekanism som placerades inuti en U-profil. De flesta komponenter i designen föreslogs tillverkas av stål, kompletterat med skyddsåtgärder såsom färg eller beläggning för att minimera risken för korrosion i den marina miljön.

As the world’s need for electricity increases, so does its demand for sustainable energy with low to no greenhouse gas emissions. One of these renewable sources of energy is the Ocean which is one of the world’s largest and most predictable energy source, where the extraction of energy can be from waves or tidal current, with zero greenhouse gas emissions during production. A company which works with wave energy is Waves4Power which has developed the wave energy buoy WaveEL.WaveEL is comprised of a buoy which is eight meters in diameter with a 36 meters long vertical cylinder which goes through the buoy. In the cylinder is a piston that oscillates in pace with the waves and generates electricity. Between the piston and the cylinder wall is a gap where the water can move from one side of the piston to the other in pace with the piston’s oscillations. The gap is called the leakage clearance. The leakage clearance effect on the flow resistance is the focus of this master thesis as something which has not been studied before in scientific articles for similar wave energy buoys or in other fields.The aim of the master thesis is to improve the understanding of how the water flow, because of the leakage clearance in the WaveEL buoy, affects the force which the piston is subjected to, and in turn how much electricity can be generated. As it is a complex system the focus will be to determine the dynamic flow resistance parameter because of the leakage clearance and the changes to the dynamic flow resistance parameter as the dimensions of the piston is varied.The leakage clearance effect on the flow resistance has been studied with the help of Computational Fluid Dynamics (CFD) in the software COMSOL Multiphysics 6.0 in two different models. In the first model, model A, the piston is locked in different positions in the cylinder and the pressure at the bottom of the cylinder varies to reflect the motion of the waves. For the second model, model B, the piston is allowed to move vertically in the cylinder due to a set force which reflects the motion of the waves.The dynamic flow resistance parameter for model A is lower at higher Reynolds number and within an interval between 0.4 and 1.6 within the working area. Outside of the working area the dynamic flow resistance parameter is lower at a higher Reynolds number and higher than in the working area at an interval between 0.7 and 45.For model B, the dynamic flow resistance parameter has only been calculated for the working area and is within an interval of 0.1 and 7, the value for the dynamic flow resistance parameter is low when the Reynolds number is high. Dissimilar to model A where the piston is locked into position, the piston oscillates in model B. There is a phase shift between the velocity of the piston and the velocity of the water, which leads to the piston being subjected to a larger force than in model A at lower water velocities. This is one of the reasons why the dynamic flow resistance parameter is higher in model B than model A at low Reynolds number. As model B calculates the dynamic flow resistance parameter based on the relative velocity between the piston and the water, the dynamic flow resistance parameter becomes lower than for model A at higher Reynolds number.For the performed sensitivity analysis, the results shows that a more advantageous value on the dynamic flow resistance parameter can be achieved by altering the dimensions of the piston. A more advantageous result was achieved for example when the rounded edge on the piston became sharper or when the leakage clearance width was increased by 10%. If this master thesis work is to be extended, the studies should focus on elaborating model B either more in depth or with values derived from experiments from the WaveEL buoy for a more realistic model and thus determine a more accurate dynamic flow resistance parameter.The results from the sensitivity analysis justifies a future study where the dynamic flow resistance parameter should be investigated with greater variations of the piston diameters, as this can increase the flow resistance and thus generate more electricity. However, it should be investigated in relation to the cost of manufacture, to obtain the ultimate design which generates maximum electricity for a reasonable manufacturing cost. / Allt eftersom värdens elektricitetsbehov ökar, ökar också efterfrågan på förnybar energi med låga eller inga växthusgasutsläpp. En av dessa förnybara energikällor är havet som anses vara en av världens största och mest förutsägbara energikällor, där utvinningen av energi kan ske från vågor eller tidvattenströmmar, med noll växthusgasutsläpp under produktion. Ett av företagen som arbetar med vågenergi är Waves4Power som har utvecklat vågenergibojen WaveEL.WaveEL består av en boj som är åtta meter i diameter med en 36 meter lång vertikal cylinder som går igenom bojen. I cylindern finns en kolv som oscillerar i takt med vågorna och genererar elektricitet. Mellan kolven och cylinderväggen finns en spalt där vatten kan förflytta sig från ena sidan av kolven till den andra i takt med att kolven oscillerar, denna spalt kallas läckspalten. Läckspaltens påverkan på flödesmotståndet är någonting som tidigare inte har studerats i andra vetenskapliga artiklar för likartade vågenergibojar eller inom andra branscher och är därför fokus för detta examensarbete.Syftet med examensarbetet är att förbättra förståelsen hur vattenflödet till följd av läckspalten i WaveEL-bojen påverkar kraften som kolven utsätts för, vilket i sin tur påverkar hur mycket elektricitet som genereras. Eftersom det är ett komplext system kommer focus vara att fastställa den dynamiska flödesmotstånds parametern till följd av läckspalten och förändringarna i den dynamiska flödesmotstånds parametern när dimensionerna på kolven varieras.Läckspaltens påverkan på flödesmotståndet har studerats med hjälp av Computational Fluid Dynamics (CFD) med programvaran COMSOL Multiphysics 6.0 i två olika modeller. I den första modellen, modell A, är kolven låst i olika positioner i cylindern och trycket vid botten av cylindern varierar för att efterlikna vågrörelser. För den andra modellen, modell B, tillåts kolven röra sig vertikalt i cylindern genom en bestämd kraft som efterliknar vågrörelserna.Resultatet från modell A visar att flödesmotståndet befinner sig mellan 0,4 och 1,6 inom arbetsområdet och utanför arbetsområdet är den dynamiska flödesmotståndsparametern större än i arbetsområdet, här är den dynamiska flödesmotstånds parametern mellan 0,7 och 45. Oberoende av kolvens position i bojen sjunker den dynamiska flödesmotståndsparametern i takt med att Reynolds talet ökar.För modell B har den dynamiska flödesmotstånds parametern endast beräknas för arbetsområdet och ligger inom intervallet 0,1 och 7, där ett lågt värde på den dynamiska flödesmotstånds parametern fås när Reynolds talet är högt. Tillskillnad mot modell A där kolven är låst i sin position, oscillerar kolven i modell B. Det finns en fasvridning mellan kolvens och vattnets hastighet, vilket innebär att kolven utsätts för en större kraft i modell B än i modell A vid låga vattenhastigheter. Det är en av anledningarna till att den dynamiska flödesmotstånds parametern är högre i modell B vid låga Reynolds tal. Eftersom modell B beräknar ut den dynamiska flödesmotstånds parametern med hjälp av den relativa hastigheten mellan kolven och vattnet blir den dynamiska flödesmotståndsparametern lägre än för modell A högre Reynolds tal.För den genomförda känslighetsanalysen visar resultatet att ett mer fördelaktigt värde på den dynamiska flödesmotsstånds parametern kan uppnås vid förändring av kolvens dimensioner. Exempelvis uppnåddes ett mer fördelaktigt resultat när den avrundade kanten på kolven blev skarpare eller när bredden på läckspalten ökades med 10%. Om detta examensarbete ska utökas bör studierna fokusera på att utveckla modell B antingen mer på djupet eller med värden härledda från experiment från WaveEL-bojen för en mer realistisk modell och därmed en mer exakt bestämning av den dynamiska flödesmotstånds parametern.Resultatet från känslighetsanalysen motiverar en framtida studie där den dynamiska flödesmotstånds parametern bör undersökas med kraftigare dimensions variationer på kolven, eftersom detta kan komma att öka flödesmotståndet och således generera mer elektricitet. Dock behöver de undersökas i relation med tillverkningskostnaderna, för att hitta den ultimata design som genererar maximalt med elektricitet vid en rimlig tillverkningskostnad.

This work is divided in to two distinct parts. In the first part a model is developed to assess the redistribution of wave energy about an offshore array of overtopping type wave energy converters. The model is based on a classical analytical solution for diffraction about a breakwater which is modified to consider an array of dissipating, reflecting and transmitting breakwater segments, which are used to approximate an overtopping type WEC array. The model is computationally efficient and phase resolving which allows the effect of wave scattering to be investigated for large domains with high resolution irregular wave distributions. It was found that the radial waves generated by the diffraction effect spreads and defocus wave energy away from the geometrical shadow of the array. This counteracts the rate of recovery of wave energy deficit from wave directional spreading. In the second part, short-term wave forecasting for pneumatic power regulation through relief valve control is investigated at the Pico oscillating water column power plant, located in the Azores. Operational data from the Pico OWC is used to develop and critically assess a number of univariate and multivariate short-term wave forecast modelling approaches. A number of relief valve control strategies, which utilise a short-term wave forecast, are also developed and assessed using a numerical time-domain wave to wire system model. A system model for the Pico OWC is developed and validated using operational data from the Pico plant. The absolute performance potential resulting from control utilising a perfect forecast is considered, in addition to the realistic potential where a forecast, realisable in real-time, is used to drive control actions. One of the proposed relief valve control strategies is within the mechanical limitations of the existing relief valve adjustment system at Pico and this strategy was deployed in real field tests. Field test results of the plant’s performance under this strategy closely matched the simulated performance and power enhancements of up to 29% were achieved in certain sea states and the expected annual power enhancement was projected to be around 10%. Simulations of the long term plant performance under the more advanced relief valve control strategies project far greater potential for enhanced power production although these could not be tested in the field due to the project limitations.

Malgré les innovations technologiques, les éoliennes restent sujettes à des défaillances prématurées de composants mécaniques imposants, ayant des conséquences considérables sur le coût de l’énergie. Parmi les défaillances majeures au sein des roulements d’éoliennes, un mode de fatigue de contact atypique se caractérise par de vastes réseaux de fissures ramifiées avec des phases microstructurales adjacentes d’apparence blanche à l’origine de la dénomination White Etching Cracks (WEC). Contrairement à la fatigue de contact classique, les WEC apparaissent pour un nombre de cycles et des charges relativement faibles, menant à une défaillance du composant imprévisible selon les modèles de durée de vie actuels. Les WEC ont été observés chez tous les roulementiers, dans diverses applications industrielles et pour différents types de roulements, éléments, lubrifiants, aciers et traitements thermiques. Ce manque de dénominateur commun rend les WEC difficilement reproductibles sur bancs d’essai sans chargement artificiel en hydrogène de l’acier. Ainsi, pour le moment, la formation des WEC ne fait pas l’objet d’un consensus. Une analyse des reproductions de WEC a été menée afin d’en comprendre les mécanismes tribologiques. Des protocoles expérimentaux ont été établis pour révéler les WEC, souvent situés à des positions inhabituelles par rapport au contact. Leur reproduction sur des roulements standards, chargés ou non en hydrogène, a permis de démontrer que le chargement artificiel en hydrogène, jusque-là couramment employé pour étudier la défaillance, reproduit des faciès identiques mais semble modifier l’initiation des WEC. Ainsi, des reproductions de WEC sans chargement en hydrogène et dans des configurations différentes ont été comparées afin d’appréhender les phénomènes tribologiques à l’origine des WEC. Les résultats suggèrent que l’initiation est principalement déclenchée par des phénomènes de surfaces avec l’absorption tribochimique d’hydrogène au niveau des surfaces métalliques fraîches sur la piste de roulement ou au niveau des flancs de microfissures superficielles. La propagation est ensuite assistée chimiquement par l’hydrogène concentré en pointe de fissure. Un arbre des causes étendu révèle que les WEC peuvent être associées à de multiples combinaisons de conditions opératoires qui semblent cependant conduire à des paramètres tribologiques similaires à l’échelle du contact avec, notamment, des cinématiques de glissement, des formulations de lubrifiants spécifiques et des paramètres tribochimiques catalyseurs comme la présence d’eau et/ou d’électricité. Une vaste campagne d’essai a alors été conduite sur un tribomètre bi-disques afin de simuler la fatigue de contact. Les résultats confirment que les facteurs influents identifiés ne sont pas pour autant auto-suffisants. La formation des WEC repose sur un équilibre instable entre aspects matériaux, mécaniques et tribochimiques, à maîtriser pour concevoir des solutions industrielles. / Despite constant expansion and engineering progress, wind turbines still present unexpected failures of heavy duty mechanical components drastically affecting the cost of energy. Among the most prevalent tribological failures in wind turbine rolling element bearings, a peculiar rolling contact fatigue mode has been associated to broad subsurface three-dimensional branching crack networks bordered by white etching microstructure, and thus named White Etching Cracks (WEC). Compared to conventional microstructural alterations, WECs tend to develop at moderate loads and cycles eventually leading to premature failures that remain unpredictable using fatigue life estimations. Far from being generic to specific manufacturers, WECs occur in various industrial applications, for various bearing types, components, lubricants, steels grades and heat treatments. As WEC occurrences present no common evident denominator, they remain delicate to reproduce on laboratory test rigs without prior artificial hydrogen charging, so that no consensus on WEC formation mechanisms have been confirmed yet. In this study, a thorough tribological analysis of WEC formation mechanisms has been led. Expertise protocols have been established to best reveal and observe WECs that commonly develop at unconventional locations versus the contact area. First analysis of WEC reproductions on standard rolling element bearings either hydrogen precharged or kept neutral have signified that artificial hydrogen charging, commonly employed to apprehend the failure mode, results in similar WEC morphologies but tends to alter WEC tribological initiation. In consequence, WEC reproductions in remarkably different configurations but without hydrogen charging have been compared in order to propose a better understanding of WEC surface-affected formation mechanisms: first, initiation via tribochemical hydrogen permeation at nascent steel surfaces formed either directly at the raceway or at surface microcracks flanks and second, propagation by local hydrogen embrittlement at crack tips function of the stress state. An extensive root cause analysis have then been led suggesting that WEC may be associated to various combinations of macroscopic operating conditions that often interact and come down to similar tribological parameters including high sliding energy thresholds, specific lubricant formulations and tribochemical drivers such as water contamination and/or electrical potentials. Further investigations on a minimalist twin-disc fatigue tribometer have provided additional evidence that WEC influent drivers are non-self-sufficient, supporting that WEC formation mechanisms rely on a subtle equilibrium between tribo-material, tribo-mechanical and tribo-chemical drivers that all should be mastered to design efficient and durable countermeasures.

Generation of electricity from ocean waves has seen increasing research and commercial interest in recent years. The development of projects of several hundred megawatts rated capacity is now being considered. There is a clear need for improved understanding of the environmental impact of large-scale wave energy extraction, particularly in nearshore regions where sediment transport and cliff erosion may be affected. This thesis investigates the change in nearshore wave conditions and sediment transport due to energy extraction by long strings of wave energy devices. The influence of wave energy converter (WEC) arrays has been studied using transmission coefficients implemented within a spectral wave model. It is shown that the breaking wave height nearshore is larger (5%) if transmission is defined as frequency dependent. This is due to the energy dissipation processes associated with different wave frequencies. Linear wave theory is employed to determine frequency dependent transmission and reflection coefficients across a line of wave energy devices based onthe amplitude of scattered and radiated waves. This approach is compared with experimental measurements of the wave field in the vicinity of an array of five heaving floats. The transmitted wave amplitude is predicted with reasonable accuracy but additional numerical damping is required to predict the measured float response amplitude. This comparison indicates that linear analysis is an acceptable approach for predicting float response and wave field in the vicinity of the array for a certain range of conditions. Linear wave analysis is subsequently applied to investigate the variation of transmission coefficients with distance inshore of a long array of heaving WECs undergoing free response and with damping specified to optimise power extraction. A method is presented for identifying representative transmission and reflection coefficients such that change in wave energy is equal to energy extraction by the devices. These coefficients are employed to quantify the change in nearshore conditions due to deployment of a long line of wave devices at a site near the East Anglian coastline. Wave conditions are modelled at 12 points along the shoreline over a 140 year period and significant wave height reductions up to 30% were obtained. Importantly, changes in nearshorewave direction are also observed. Analysis using the sediment transport model SCAPE (Soft Cliff and Platform Erosion model) indicates that the introduction of the array reduces both the sediment transport rate and cliff recession rate by an average of 50%.

This thesis studies wave energy converters developed at Uppsala University. The wave energy converters are of point absorbing type with direct driven linear generators. The aim has been to study generator design with closed stator slots as well as offshore experimental studies. By closing the stator slots, the harmonic content in the magnetic flux density is reduced and as a result the cogging forces in the generator are reduced as well. By reducing these forces, the noise and vibrations from the generator can be lowered. The studies have shown a significant reduction in the cogging forces in the generator. Moreover, by closing the slots, the magnetic flux finds a short-cut through the closed slots and will lower the magnetic flux linking the windings. The experimental studies have focused on the motion of the translator. The weight of the translator has a significant impact on the power absorption, especially in the downward motion. Two different experiments have been studied with two different translator weights. The results show that with a higher translator weight the power absorption is more evenly produced between the upward and downward motion as was expected from the simulation models. Furthermore, studies on the influence of the changing active area have been conducted which show some benefits with a changing active area during the downward motion. The experimental results also indicate snatch-loads for the wave energy converter with a lower translator weight. Within this thesis results from a comparative study between two WECs with almost identical properties have been presented. The generators electrical properties and the buoy volumes are the same, but with different buoy heights and diameters. Moreover, experimental studies including the conversion from AC to DC have been achieved. The work in this thesis is part of a larger wave power project at Uppsala University. The project studies the whole process from the energy absorption from the waves to the connection to the electrical grid. The project has a test-site at the west coast of Sweden near the town of Lysekil, where wave energy systems have been studied since 2004.

This thesis is concerned with the use of the open source computational fluid dynamics (CFD) software package, OpenFOAM® for predicting and analysing the behaviour of a near-shore oscillating wave surge converter (OWSC), when subject to various types of ocean wave conditions in a numerical wave tank (NWT). OpenFOAM® which utilises a Finite Volume Method (FVM) is used to solve the incompressible, Reynolds Averaged Navier-Stokes (RANS) equations for a two-phase fluid, based on a Volume of Fluid (VOF) phase-fraction approach to capture the interface between the air and water phases. Preliminary studies on classic wave-structure interaction benchmark cases, involving a fixed and a vertically oscillating semi-immersed horizontal cylinder are carried out. The gradual transition of the linear to non-linear behaviour of the horizontal and vertical forces induced on a fixed cylinder when subject to various regular waves, and the amplitude ratios of the surface waves elevations generated by the prescribed oscillatory motion of the cylinder, are shown to provide good overall agreement within the limitations of the relevant theory and the experimental data in the literature. The OWSC is modelled with the inclusion of a Power Take-Off (PTO) system, using a linear damping restraint, and simulated in two-dimensional (2D) and three-dimensional (3D) setups. The 2D and 3D numerical results, such as the surface wave elevations, flap angular velocity, PTO torque and flap angular displacement, compare well with one another and with the experimental data for operational regular head-on and oblique wave conditions. Small discrepancies between numerical results and experimental data are likely to be caused by non-linear behaviour of the PTO system. Pressure distributions on the flap surfaces and forces induced on the flap and hinge of the OWSC for various wave conditions are also presented. The effects between 2D and 3D wave-structure interactions become more significant when subject to large waves that break during impact. Comparison between the full scale and 1:24 scale numerical results of the OWSC shows no significant evidence of viscous and scaling effects. The validated 2D OWSC model is also subject to embedded focused waves, to predict the worse possible scenario of wave loading in extreme wave conditions. The delay of the focus event breaking is shown to affect the slamming behaviour for the larger focus event wave heights. Incorporation of a focused wave at different phase positions within a background of regular waves reveals that the focus event wave height has little effect on the peak tangential force on the flap during the slamming event, when a PTO cut-off mechanism is implemented to prevent excessive torque surges. In contrast, the peak radial force on the flap and the maximum resultant force on the hinge appear to respond more sensitively to the focus event wave height. It has been demonstrated that OpenFOAM® is able to provide a comprehensive understanding of the complex hydrodynamic analysis and prediction of highly non-linear wave-structure interactions for an OWSC, which give useful guidance and confidence to WEC developers on the design considerations relevant to the OWSC systems.

The waves traveling on the surface of the world’s oceans carry atremendous amount of energy. The ability to convert this energy forhuman use has the potential to help solves the worlds energy problem. Adirect-drive linear generator point absorber is a wave energy converter(WEC) that aims to reduce the complexity of the overall system andshelter the most vulnerable parts of the system by placing them on theseabed. This concept builds around the buoy moving up and down indifferent sea-states which leads to a correlating vertical movement of thestroke in the generator resulting in the conversion of mechanical energyto power. This report aims to explore the possibility to use the open codeWEC-Sim to model the Uppsala University direct-drive linear generatorWEC in extreme sea states and to identify the resulting extreme loads. Theconstructed WEC-Sim model constrained the buoys motion in heave andsurge and limited its range of motion by modeling the generators upperend-stop spring. Simulations were run for different sea-states and theresulting forces on the system were analyzed. The peak line force for thedifferent sea states was calculated and compared to previous studies. Theresults validated the model as they showed a good correlation for mostsea-states. It was only for larger significant wave heights that there was adivergence compared to the results in previous studies.

Wave energy converters of point absorber type have been developed and constructed. Full scale experiments have been carried out at sea and electricity has been successfully delivered. Linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been investigated theoretically and experimentally. The design has in large extent an electronic approach, keeping the mechanical part of it as simple as possible, due to the long life span and reliability of electric components. Because of the nature of a linear generator, the internal translator with permanent magnets has a limited stroke length which will be reached when the buoy is exposed to large wave heights. Internal springs at the top and bottom of the generator prevent the translator from hitting the generator hull. Inertial forces due to the mass and velocity of the translator and the buoy and its heave added mass compresses the spring. The added mass is a rather large part of the total moving mass. Simulations of a converter with a vertical cylindrical buoy and with a toroidal buoy were conducted, as well as real sea experiments with converters with cylindrical buoys of two different sizes and a toroidal buoy. The overloads are likely to affect the design and service life of the generator, the buoy and the wire which interconnects them. Buoy shapes with as much excitation force as possible and as little heave added mass as possible were sought. A toroidal buoy caused less overloads on the generator at sea states with short wave periods and relatively large wave height, but for sea states with very long wave periods or extremely high waves, the magnitude of the overloads was mainly determined by the maximum displacement of the buoy. Snap loads on the interconnecting wire, as the slack wire tensed up after a very deep wave trough, were found to be greater but of the same order of magnitude as forces during the rest of the wave cycle. During a 4 day period at various wave conditions, two converters with cylindrical buoys proved efficiency between 11.1 % and 24.4 %. The larger buoy had 78 % larger water plane area than the other buoy which resulted in 11 % more power production. Short wave period was beneficial for the power production. Infinite frequency heave added mass was measured for a cylindrical buoy at real sea and found to be greater than the linearly calculated theoretical added mass.