Structural Dynamic Analysis of Semi-Submersible Floating Vertical Axis Wind Turbines

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1 energies Article Structural Dynamic Analysis of Semi-Submersible Floating Vertical Axis Wind Turbines Jeremiah Ishie 1, Kai Wang 2, * Muk Chen Ong 1 1 Department of Mechanical Structural Engineering Material Science, University of Stavanger, 4036 Stavanger, Norway; j.ishie@stud.uis.no (J.I.); muk.c.ong@uis.no (M.C.O.) 2 Aker Solutions AS, 1366 Lysaker, Norway * Correspondence: wangkai.ntnu@gmail.com; Tel.: Academic Editors: Lance Manuel Rupp Carriveau Received: 5 August 2016; Accepted: 5 December 2016; Published: 13 December 2016 Abstract: The strong stable at offshore locations increasing dem energy have made application of turbines in deeper water surge. A novel concept of a 5 MW baseline Floating Vertical Axis Wind Turbine (FVAWT) a 5 MW optimised FVAWT with DeepWind Darrieus rotor optimised DeepWind Darrieus rotor, respectively, were studied extensively. The structural responses, fatigue damages, platm global motions mooring line dynamics of FVAWTs were investigated comprehensively during normal operating conditions under steady turbulent conditions, using a coupled non-linear aero-hydro-servo-elastic code ( Simo-Riflex-DMS code) which was developed by Wang et al. modeling FVAWTs. This coupled code incorporates models turbulent field, aerodynamics, hydrodynamics, structural dynamics, generator controller. The simulation is permed in a fully coupled manner in time domain. The comparison of responses under different conditions were used to demonstrate effect of turbulence on both FVAWTs dynamic responses. The turbulent condition has advantage of reducing 2P effects. Furrmore, comparative studies of FVAWTs responses were undertaken to explore advantages of adopting optimised 5 MW DeepWind Darrieus rotor over baseline model. The results identified 5 MW optimised FVAWT to having: lower Fore-Aft (FA) but higher lower Side-Side (SS) bending moments of structural components; lower motions amplitude; lower short-term fatigue equivalent loads a furr reduced 2P effects. Keywords: turbine; Simo-Riflex-DMS; fatigue analysis; coupled non-linear time domain simulation; finite element method; semi-submersible substructure; structural dynamics 1. Introduction The risk, price inconsistency environmental impact associated with oil gas exploration production have driven a focus on renewable energy. Wind power, fastest growing source of renewable power generation in Europe, is a major competitor to oil gas. The strong stable at offshore locations increasing dem energy have surged application of turbines in deep water. The Floating Horizontal Axis Wind Turbine (FHAWT) has been a research focus in deep water power production due to its commercial success in onshore applications. However, application of Floating Vertical Axis Wind Turbines (FVAWT) in deep offshore waters has potential because of ir economic advantages in installation maintenance. The application of Vertical Axis Wind Turbines (VAWT) rotor technology in large turbines could reduce cost-of-energy (COE) by over 20% [1]. Furrmore, due to higher maintainability of FVAWTs, ir ability to capture energy irrespective of direction without a yaw control mechanism, lower center of gravity, economies of installation amongst or designs as compared with Energies 2016, 9, 1047; doi: /en

2 Energies 2016, 9, of 36 FHAWTs [2], FVAWTs could be a better concept in deeper water applications. These aementioned advantages have steered resurgence of interest in research on application of FVAWTs in deep waters [3,4]. Studies have focused on design, structural integrity, dynamic response installation methodologies to better underst permance of various concepts to provide basis detailed structural designs [5 10]. Hence, various concepts were developed investigated use as floating substructures to support turbines. These concepts include spar [11], semi-submersibles [7,10] Tension Leg Platm (TLP) concept [12]. The dynamic response of a floating turbine differs significantly from that of a bottom-supported substructure or a l-based turbine. Floating turbines will experience large aerodynamic hydrodynamic loads, including loads due to rotating blades. Theree, it is pertinent to analyze numerous substructure concepts in order to underst ir respective responses suitability a particular sea state. Various FVAWT concepts have been evaluated, such as DeepWind concept [13 15], VertiWind concept [16] Aerogenerator X concept [17], conceptual designs technical feasibility. The DeepWind concept VertiWind concept were studied, but AeroGenerator-X concept was only proposed without any detailed report or relevant publication. A novel concept combining DeepWind 5 MW rotor [18] DeepC floater from Offshore Code Comparison Collaboration Continuation (OC4) project [19], were extensively investigated as well. Furrmore, a variety of studies applied different simulation tools to investigate response characteristics of FVAWTs to provide conceptual design descriptions detailed evaluation of technical feasibilities of various concepts. Limited state-of-art simulation codes have been developed over years conceptual evaluation of FVAWTs. These include (Horizontal Axis Wind turbine simulation Code 2nd generation) HAWC2, (Floating Vertical Axis Wind Turbine) FloVAWT, Simo-Riflex-DMS, etc. HAWC2 [20 22] was developed at Risø, Technical University of Denmark (DTU). In order to equip HAWC2 with ability to model FVAWTs in time domain DeepWind FVAWT an improved aero-elastic code based on Actuator Cylinder (AC) model was linked to it using a Dynamic Link Library (DLL). HAWC2 is a multi-body simulation tool which allows separate modelling of individual components of a turbine. However, hydrodynamic model employed is more accurate slender substructures like spars (Morrison-like structures), thus, it is considered unsuitable analysing structures such as semi-submersibles. The coupled aero-hydro-servo-elastic code, Simo-Riflex-Aerodyn was also developed [23] to fully integrate complete features of Aerodyn [24] into Simo-Riflex coupled code. However, this is used modeling FHAWT is not suitable FVAWT. At Cranfield University in UK, Borg et al. [25] developed a simplified coupled code designed using MATLAB programming preliminary design analysis of FVAWTs. This code is called FloVAWT. The code has been in continual development verification by Collu et al. [26]. However, code was criticized lacking a structural model, mooring line model dynamic control model, hence, a rigid rotor is assumed to rotate at constant speed while relationship between ce-displacement is linearized. A non-linear aero-hydro-servo-elastic, Simo-Riflex-DMS was developed by Wang et al. [27] modeling FVAWTs with semi-submersible substructure. The Double Multiple Streamtube (DMS) code [28] is an aero-elastic code used to model aerodynamic loads on turbine rotor. The Riflex code is finite element solver as well, serve as a link to external modules/codes while SIMO code is used to model floater hydrodynamics. Theree, Simo-Riflex-DMS code coupled aerodynamic, hydrodynamic, structural dynamic generator controller modules. The Simo-Riflex-DMS has been used to study a novel concept that combined DeepWind 5 MW rotor [18] DeepC floater from OC4 project [19], in a fully coupled time domain simulation [27]. The Simo-Riflex-DMS model, as developed by Wang et al. [27], is adopted this work. The dynamic analysis comparative studies of different FVAWT concepts have been permed. These include dynamic analysis of a 5 MW Darrieus curved bladed VAWT with rotating foundation [29]. However, VAWT was coupled to a spar platm. A furr analysis of this concept has been carried out at DTU, to reduce VAWT s weight bending loads on rotor [30].

3 Energies 2016, 9, of 36 A novel concept that combines 5 MW Darrieus curved blade VAWT with a semi-submersible platm has been extensively studied by Wang et al. [2,4,31]. Additionally, a comparative study of FVAWT concept by Wang with two similar FVAWT concepts (a combination of 5 MW Darrieus curved bladed VAWT with: 1) a spar platm 2) a TLP, has been permed by Cheng et al. [32]. He concluded that semi-submersible spar abated 2P effects on structural loads mooring line tensions as compared to TLP concept, at expense of larger platm motions. A furr study has been carried out to evaluate effect of difference-frequency ces on a FVAWT with semi-submersible platm under misaligned -wave conditions [33]. The effect of -wave misalignment second order difference frequency ce on FVAWT were more significant at extreme values of responses, especially at speeds above rated speed. In this work, a baseline semi-submersible FVAWT concept [34] is adopted. This concept has been studied in a systematic manner: a method analysis has been developed; global motion floating support structure was comprehensively analyzed considering normal operating condition emergency shutdown; a comparative analysis between proposed model an equivalent model of an Horizontal Axis Wind Turbine (HAWT) rotor has been carried out including a stochastic analysis of model [2,4,25,27,31,33,35,36]. Furrmore, response of concept in combination with an hydrodynamic brake under emergency condition has been investigated [31]. However, extensive investigations of structural responses of flexible components such as blades, tower mooring lines, include blade bending moment distributions fatigue responses have not been carried out. For FVAWTs, continuously varying aerodynamic loads on rotor lead to a considerably higher load level increasing number of load cycles. Theree, it is significant to evaluate fatigue damage based on time history of FVAWTs responses. The rainflow counting technique is used fatigue cycle counting while Mlife tool from National Renewable Energy Laboratory (NREL) is used to calculate short-term fatigue damage equivalent loads FVAWT. The dynamic responses of 5 MW baseline FVAWT were investigated by analyzing platm global motion, analysis of structural loads of flexible components, estimation of short-term fatigue damage equivalent loads of structural components. Furrmore, this work is extended to evaluate permance of a 5 MW optimized FVAWT in terms of power production, structural dynamic response, global motion short-term fatigue damage on structural components. The 5 MW optimized FVAWT is a concept combining optimized 5 MW DeepWind rotor [30] from Technical University of Denmark (DTU) DeepC semi-submersible from OC4 project [19]. The 5 MW DeepWind rotor was optimised by a group of researchers at DTU. The tower of rotor is optimized to weighs 67% less than tower baseline 5 MW DeepWind rotor with lower bending moments [37]. The two FVAWT models were evaluated under same environmental conditions. A comparative analysis of response results of both FVAWT models are presented to explore advantages of optimized 5 MW DeepWind rotor. 2. Methodology 2.1. The Floating Vertical Axis Wind Turbine Models This study considers two turbine models: a 5 MW baseline FVAWT shown in 1a a 5 MW optimized FVAWT illustrated in 1b. The 5 MW baseline FVAWT concept is fully described in [34]. The design consists of a semi-submersible substructure [38] with a curved Darrieus VAWT [29], consisting of a generator assumed to be positioned at tower bottom in central column of semi-submersible. The platm global motion is limited by anchoring semi-submersible to sea bottom by means of catenary mooring system in 1c. Furrmore, sea water dampens platm dynamics. This novel FVAWT concept has been studied by Wang [34].

4 Energies 2016, 9, of 36 semi-submersible to sea bottom by means of catenary mooring system in 1c. Energies Furrmore, 2016, 9, 1047 sea water dampens platm dynamics. This novel FVAWT concept has been 4 of 36 studied by Wang [34]. 1. The FVAWT concept: 5 MW baseline FVAWT; 5 MW optimized FVAWT; 1. The FVAWT concept: 5 MW baseline FVAWT; 5 MW optimized FVAWT; Mooring Mooring lines arrangement [34]. lines arrangement [34]. The 5 MW optimized FVAWT is similar to 5 MW baseline FVAWT, except turbine rotor is The 5 MW optimizeddeepwind FVAWT isrotor similar from todtu 5[30]. MWThe baseline Simo-Riflex-DMS FVAWT, except code developed turbine rotor by is Wang 5 MW et al. optimized [27], which has been validated from extensively [30]. The used Simo-Riflex-DMS by Wang [33,34] code developed or by professionals Wang et al. [27], [32,35], which is adopted has been validated numerical simulations extensively of used FVAWTs. by Wang [33,34] or professionals [32,35], is adopted numerical simulations of FVAWTs A Method Platm Adaptation 2.2. A Method Platm Adaptation The 5 MW optimised DeepWind rotor weighs about 41% less than 5 MW VAWT baseline rotor. The 5Hence, MW optimised if optimised DeepWind DeepWind rotor weighs rotor is about to be 41% coupled less than directly, 5 MWoriginal VAWT baseline draft of rotor. Hence, platm if will optimised reduce. DeepWind This will cause rotor is to mooring be coupled lines directly, to be pre-tensioned original draft at ir of undisplaced platm will reduce. position This because will cause length mooring of mooring lines to be lines pre-tensioned position at of ir fairleads undisplaced are unchanged. position The because idea is length to maintain of mooring platm lines properties. position Theree, of fairleads semi-submersible are unchanged. from The DeepC idea is to maintain phase II platm of OC4 properties. Project [19] Theree, must be slightly semi-submersible modified to from maintain DeepC same draft phase II oiginal OC4 Project platm. [19] must This be is achieved slightly modified by increasing to maintain mass of same ballast draft water. as The original following platm. assumption, This is achieved approximation by increasing notation mass were ofapplied ballastto water. successfully The following modify assumption, platm: approximation notation The were ballast applied water tois successfully sea water with modify density ρ platm: = t/m 3. From platm detailed specifications [19], 25% 75% of ballast mass were placed in The ballast water is sea water with density ρ = t/m 3. Upper Column (UC) Base Column (BC) of offset columns respectively. No ballast in Centre From Column platm CM. detailed specifications [19], 25% 75% of ballast mass were placed in Upper At Column platm (UC) undisplaced Base Column position, (BC) of ballast offsetmass columns geometry respectively. can be No assumed ballastas in solid Centre Column cylinder CM. with mass. At The platm FVAWT undisplaced assumed position, to be symmetrically ballast mass loaded geometry thus, can be assumed method asof solid buoyancy cylinder with compensating mass. ce [39] is applied in SIMO. The The FVAWT distance isfrom assumed centre to be of symmetrically mass of ballast water loadedto CM thus, equals method distance of from buoyancy compensating centre of mass ce of UC [39] or isbc applied to CM. in SIMO. TheTo distance annul from inconsistency centre of between mass ofriflex ballast water SIMO to in CMinterpretation equals distance of body fromces centre in ofcoupled mass ofmodel, UC or BC to method CM. of buoyancy compensation ce [39] is applied to ensure a correct static configuration. This furr reduce instability that may be associated with running To annul inconsistency between Riflex SIMO in interpretation of body ces in simulation. coupled model, method of buoyancy compensation ce [39] is applied to ensure a correct static The following methodology is used to modify platm slightly in order to support configuration. This furr reduce instability that may be associated with running simulation. 5 MW optimised FVAWT. Furrmore, this approach can be adapted similar scenario, where a slight The modification following methodology of platm is used weight to is modify required to platm maintain slightly platm in order draft to to support support a 5 MW optimised turbine of FVAWT. different Furrmore, weight a topside. this approach However, can some beof adapted steps would similar need scenario, to be applied where a slight appropriately modification to reflect of platm uniqueness weight of case isunder required consideration: to maintain platm draft to support a turbine of different weight or a topside. However, some of steps would need to be applied 1. Calculate original FVAWT submerged mass. To maintain same draft, submerged appropriately to reflect uniqueness of case under consideration: mass of both original FVAWT modified FVAWT must be equal. 1. Calculate original FVAWT submerged mass. To maintain same draft, submerged mass of both original FVAWT modified FVAWT must be equal. 2. Subtract respective total mass of flexible elements total platm metal mass from submerged masses to obtain masses of ballast water respective FVAWTs.

5 Energies 2016, 9, of Calculate mass moment of inertia due to mass of ballast water about three axis of rotation taking third assumption into consideration both original modified platm. 4. Subtract inertias original platm obtained in step 3 from original total platm inertias n add inertias due to mass of ballast of modified platm obtained in step 3. The moment of inertias were estimated by applying basic definition of moment of inertia a cylindrical object about its centre, considering assumptions. The empty portion of offset columns were considered as open cylinders while portions filled with ballast water were considered as solid cylinder. The parallel axis orem is used to compute inertias about platm s centre of mass (X g, Y g, Z g ). However, calculated moment of inertias are with respect to center of gravity of local coordinates. To apply this into symmetrical structural mass coefficient in Simo code, y must be transmed to global coordinate system as described in (1): RIXX = I XX + M B (Y 2 g + Z 2 g ) RIYY = I YY + M B (X 2 g + Z 2 g ) RIZZ = I ZZ + M B (X 2 g + Y 2 g, where I M B are moment of inertia (with respect to center of gravity of local coordinates) structural mass respectively. For more details, see Simo manual [40]. The change in platm weight as a result of change in ballast mass must be accounted in calculations of hydrostatic stiffness data. Hence, part of restoring terms C(4, 4), C(5, 5) in hydrodynamic stiffness data is estimated as given in (2) to obtain modified C(4, 4) New C(5, 5) New [41]: C(4, 4) New = C(4, 4) M O gz go + M P gz g C(5, 5) New (2) = C(5, 5) M O gz go + M P gz g, where M O Z go are total mass center of mass along z-axis of original platm, g is acceleration due to gravity. M P Z g are total mass center of mass along Z-axis of modified platm. The buoyancy ce center of buoyancy (COB) of modified platm are indifferent from that of original platm, hence, ir contributions to new restoring terms in (2) are zero. The main specifications of 5 MW baseline FVAWT 5 MW optimized FVAWT are respectively summarized in Table 1. Table 1. Main specifications of two FVAWT models. ) (1) Geometric Undistributed Properties of Blade 5 MW Baseline FVAWT 5 MW Optimized FVAWT Properties Unit Magnitude Magnitude Rotor radius m Rotor height m Blade chord m Airfoil type NACA0018 NACA0018 Solidity Length of one blade m Rotor mass kg Distance of hub height above mean sea level m

6 Energies 2016, 9, of 36 Table 1. Cont. Geometric Undistributed Properties of Blade Operational Permance Data 5 MW Baseline FVAWT 5 MW Optimized FVAWT Rated Power MW Rated rotation speed rpm Number of blades 2 2 Rated speed m s Cut in speed m s Cut out speed m s Floater Properties Depth of platm base below sea level (total draft) m Spacing between offset columns m Platm mass, including ballast generator kg Total mass of ballast water kg CM location of total concept below SWL m Buoyancy in undisplaced position kg CB location below SWL m Hydrostatic restoring stiffness in heave C33 N m Hydrostatic restoring stiffness in roll C44 N m rad Hydrostatic restoring stiffness in pitch C55 N m rad Moment of inertia in roll kg m Moment of inertia in pitch kg m Moment of inertia in yaw kg m Coupled Modelling Tool FVAWTs A fully coupled simulation tool, Simo-Riflex-DMS, developed by Wang et al. [27] is adopted time domain simulations of FVAWTs dynamics. The Simo-Riflex-DMS code consist of separate models flowfield, aerodynamics, hydrodynamics, structural dynamics controller dynamics integrated toger to m a coupled whole. The Simo code estimates rigid body hydrodynamic ces moments on floater; Riflex is a nonlinear finite element solver used to model flexible elements such as blades, tower, shaft mooring system, it also provides link to Double Multiple Streamtubes (DMS) code an external controller; DMS code calculates aerodynamic loads on blades using an external aerodynamic module. The generator torque control characteristics was written in Java. Simo computes hydrodynamic loads at actually displaced position of floater, DMS calculates aerodynamic loads on blades Riflex carries out full equilibrium iteration at each time step [34,35]. Simo-Riflex-DMS is a sophisticated aero-hydro-servo-elastic simulation tool which coupled a comprehensive hydrodynamic model, a stable non-linear finite element solver, a well-known aerodynamic code a user-defined controller model. This coupled code has been presented verified [27]. The computational analysis flow chart shown in 2 illustrating how adopted Simo-Riflex-DMS simulation tool works in relation to or codes used as inputs corresponding analysis tool employed in this work. In structural model, rotating shaft of rotor is coupled to platm through a very short tower. The arbitrary riser system in Riflex are used to model flexible components. Flexible axisymmetric beam elements are used to model tower shaft, while flexible beam elements with two symmetric planes are used to model blades to distinguish stiffness in edgewise flapwise direction to chord of blade. Thus, 75 elements are used to model each blade length with two symmetry axes while effects due gyroscopy geometric stiffening are considered.

7 Energies 2016, 9, of 36 Energies 2016, 9, of 36 DMS Model Dynamic Link Library Riflex Simo Turbine Permance Dynamic structural response Global motion MATLAB NREL MLife tool Statistical Results Wafo tool Fatigue Analysis Results Spectral Analysis Results Computation analysis flow chart coupled model. The dynamic equilibrium equations are solved by applying newmark-β numerical The dynamic equilibrium equations are solved by applying newmark-β numerical integration integration (β = 0.256, γ = 0.505) at each s time step. However, structural damping is included (β = 0.256, γ = 0.505) at each s time step. However, structural damping is included through through global proportional Rayleigh damping terms all beam elements to ensure numerical global stability proportional a global Rayleigh stiffness damping proportional terms damping all beam factor elements is set to to ensure all numerical structures. stability a globalthe stiffness rotor proportional structural dynamics damping is factor verified is setby to comparing all structures. results obtained from eigenanalysis The rotor structural of an equivalent dynamics l-based is verifiedvawt by comparing [27]. The first results 10 natural obtained frequencies from of eigenanalysis VAWT of an were equivalent investigated l-based using both VAWT Riflex [27]. TheAbaqus, first 10 natural adopting frequencies Lanczos s of method VAWT were both investigated models. using The both results Riflex shown in Abaqus, Table 2 indicated adoptingan agreement Lanczos sbetween method two bothstructural models. codes. The results shown in Table 2 indicated an agreement between two structural codes. Table 2. Natural frequencies of VAWT in Hertz [27]. Mode Table 2. Natural frequencies of VAWT in Hertz [27]. Description Riflex Abaqus Relative Error 1 1st tower SS % Mode Description Riflex Abaqus Relative Error 2 1st tower FA % 3 1 1st blade collective 1st tower SS flatwise % 0.556% 2 1st tower FA % 4 1st blade asymmetric flatwise % 3 1st blade collective flatwise % 5 4 1st blade 1st blade asymmetric twist flatwise % 2.821% 6 5 1st blade butterfly 1st blade (Edgewise) twist % 1.457% 7 6 2nd 1st bladeasymmetric butterfly (Edgewise) flatwise % 0.556% 8 7 2nd 2nd bladecollective asymmetric flatwise flatwise % 0.139% 9 8 1st 2nd blade blade asymmetric flatwise % 0.172% 9 1st blade asymmetric flatwise % 10 3rd blade collective flatwise % 10 3rd blade collective flatwise % 2.4. Design Load Cases 2.4. Design Load Cases Realistic environmental conditions, combining wave conditions are considered proper Realistic evaluation environmental of FVAWT conditions, models. combining Furrmore, or wave special conditions conditions are such considered as current, proper tidal evaluation conditions of ice FVAWT could models. be significant Furrmore, at some or offshore special location conditions like in such asartic current, region. tidal conditions However, such ice conditions could be significant are considered at some out-of-scope offshore location this like work. in Additionally, artic region. However, stochastic such conditions nature of are loading considered responses out-of-scope FVAWTs this work. are assumed Additionally, due to aleatoric stochastic uncertainty nature of in loading responses wave conditions. FVAWTs are assumed due to aleatoric uncertainty in wave conditions. Although, speed varies longitudinally, laterally vertically, longitudinal component is is considered dominant henceit it ms basis basis direction direction at which at which mean speedis is described periods 10 10min minor or 1 1h. h. Furrmore, mean lateral vertical speed speed are are assumed assumed to beto zero. be zero. The instantaneous The instantaneous speed speed at certain at certain direction direction a particular a point in space is combination of mean speed fluctuating or turbulent component.

8 Energies 2016, 9, of 36 In this paper, Normal Turbulence Model (NTM) according to International Electrotechnical Commission (IEC) stard is used. The NREL s TurbSim code [42] is used to generate a 3D turbulent field, based on Kaimal turbulence model IEC Class C. Consequently, variation of mean speed with altitude called shear due to viscous boundary layer effects generates a profile. This profile is modelled by power law described in [43], it estimates mean speed at any height above mean sea level Normal Wind Profile model (NWP) proposed in IEC stard. A real sea state can be represented by non-linear stochastic wave models. The wave time series are produced using inverse fast Fourier transmation (IFFT) with assumption that surface elevation of an offshore site is a Gaussian process wave phase is unimly distributed within interval [0, 2π]. Theree, assumed narrow-bed short term sea state, wave energy can be represented by a wave spectrum. The long-crested irregular waves in developing sea state were generated using Joint North Sea Wave Project (JONSWAP) spectrum [44] with a peakedness parameter of 3.3. To depict a realistic offshore environmental condition, a combination of -wave environmental state is considered. Hence, relationship between waves can be expressed by a joint distribution of 1-h mean speed at 10 m above sea water level, significant wave height (H s ) spectral peak period (T p ). Johannessen et al. [45] proposed a joint probability density distribution with respect to wave measurements from 1973 to The joint distribution presented in (3) is a product of marginal distribution f U10 (u), conditional distribution of H s given U 10, conditional distribution of T p given H s U 10 : f U10 H S T p (u, h, t) = f U10 (u) f HS /U 10 (h/u) f Tp /H S U 10 (t/h, u), (3) A contour surface can be generated using (3), it ensures various wear parameters with a certain return period can be jointly modelled. The conditional average values of H s T p a given U 10 corresponding to speeds U w (Cut-in to cut-out rotor speed) at hub height are calculated by applying joint distribution function. Theree, a set of Design Load Cases (DLC) normal operating condition as in Table 3 were selected simulating FVAWT response with respect to estimated correlated wave conditions at Statfjord site in North Sea. Table 3. Combined wave environment normal operating condition. DLC U w (m/s) H s (m) T p (s) Turbulent Model NTM NTM NTM NTM NTM NTM 3. Results The fully coupled nonlinear time domain simulation tool, Simo-Riflex-DMS developed by Wang et al. [27] described in Section 2 is used to study dynamic responses of FVAWTs, under various environmental conditions described in Table 2. The wave direction are in alignment. This Design Load Cases (DLC) were used both 5 MW baseline FVAWT 5 MW optimised FVAWT dynamic simulations. The JONSWAP irregular wave conditions were used in combination with conditions. The analysis FVAWTs were focused on effect of turbulence on FVAWTs dynamic responses under normal operating conditions by comparing responses under steady condition with that under turbulent condition. The dynamic responses of FVAWTs include global motions of platm, structural responses of flexible components, mooring lines dynamics. The bending moments on

9 Energies 2016, 9, of 36 Energies Energies 2016, 2016, 9, , of 936 of 36 responses of flexible components, mooring lines dynamics. The bending moments on responses of flexible components, mooring lines dynamics. The bending moments on structural components mooring lines tension were selected as primary structural response structural components mooring lines tension were selected as as primary structural response parameters used to investigate effect of turbulence on FVAWTs structural responses. parameters used used to to investigate effect of of turbulence on FVAWTs structural responses Dynamic Response Analysis of MW Baseline FVAWT Dynamic Response Analysis of of 5 MW Baseline FVAWT Validation of Response Results Validation of of Response Results The baseline 5 MW FVAWT was modelled exactly as it was described by Wang [34]. The The The baseline 5 MW 5 MW analysis of FVAWT s FVAWT FVAWT global was was motion modelled modelled has been exactly exactly carried as out it was it by described was described Wang [34], by hence Wang by Wang focus [34]. will The [34]. shift analysis The to of analysis ensuring FVAWT s of correctness global FVAWT s motion global of has motion model beenin carried has been this paper out carried by bycomparing Wang out [34], Wang hence [34], turbine focus hence permance will focus shift will to shift in ensuring to terms ensuring of correctness power generated correctness of model to that of inpresented this model paper in by this by Wang comparing paper by his Ph.D. turbine sis. The permance turbine power permance curve in terms is shown in of terms power in generated of power 3. togenerated that presented to that bypresented Wang in his by Wang Ph.D. sis. in his Ph.D. The power sis. curve The power is shown curve in is shown 3. in Power 3. Power generated generated under under steady steady turbulent conditions: conditions: Results Results of this of this sis sis simulations; From Ph.D. presentation by Wang [34]. simulations; 3. Power From generated Ph.D. presentation under steady by Wang turbulent [34]. conditions: Results of this sis simulations; From Ph.D. presentation by Wang [34]. The power curve in 3a from this work showed same permance as that presented by The Wang The power power [34] curve in curve in in 3b both 3a from from steady this this work work turbulent showed showed same conditions. same permance permance Hence, as as that model that presented presented proved by Wang by to Wang be [34] correctly in[34] in modelled 3b both 3b both steady furr steady analysis turbulent turbulent can be permed conditions. conditions. as discussed Hence, Hence, in model model subsequent proved proved to beto correctly sections. be correctly The modelled error modelled bars shown furr furr in analysis analysis 3 can indicate becan permed be stard permed as discussed deviation as discussed in from mean in subsequent values. subsequent sections. This The sections. meaning error bars The is implied shown error bars in error shown bars 3in indicate present in 3 indicate subsequent stard deviation figures stard in from this deviation work. meanfrom values. mean This values. meaning This is implied meaning is error implied bars present error in bars subsequent present in figures subsequent in this figures work. in this work Effect of Turbulence on Blade Bending Moments Effect Effect of on Blade In evaluating of Turbulence effect on of turbulence Blade Bending on Moments distribution of bending moments along blade, 20 Inpoints evaluating were selected, effect spanning of of turbulence from on bottom distribution to top root of ofbending nodes of moments blade. along The along plot of blade, blade, points distribution points were were of selected, bending moments spanning along from bottom blade is to as shown top root in nodes 4. of of blade. The The plot plot of of distribution of of bending moments along blade is as shown in Cont.

10 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) (d) 4. Blade Fore-Aft (FA) Side-Side (SS) Bending Moment (BM) distribution 4. Blade Fore-Aft (FA) Side-Side (SS) Bending Moment (BM) distribution DLC 1, 4. 3, Blade 6. Fore-Aft Mean (FA) FA BM; Side-Side Stard (SS) Bending deviation Moment of FA BM; (BM) distribution Mean SS BM; DLC (d) Stard 1, 3, DLC 1, 3, 6. Mean FA BM; Stard deviation of FA BM; Mean SS BM; (d) Stard 6. deviation Meanof FASS BM; BM. Stard deviation of FA BM; Mean SS BM; (d) Stard deviation of SS BM. deviation of SS BM. 4 showed 4 showed turbulent turbulent effects effects effects is is is minimal minimal minimal FA FA FA bending bending bending moment moment as as compared as compared to to to SS bending SS bending SS bending moment. moment. moment. Furrmore, Furrmore, Furrmore, variation variation variation in terms in terms in terms of stard of stard of stard deviation deviation deviation (std) (std) from (std) from from mean bending mean mean bending bending moment moment moment both FAboth both FA SS FA bending SS SS bending bending moment moment moment is negligible is is negligible negligible at locations locations locations closer to closer closer center to to of center center blade of of but blade increases blade but but as increases increases both extremes as as both both of extremes blade of are approached. blade blade are are approached. approached. Moreover, Moreover, both Moreover, plotsboth showed both plots plots blade showed showed experienced blade blade higher experienced bending higher at its bending extremes, at including its extremes, a slight including increase a a atslight increase center increase at at FA bending center center moment. FA FA However, bending moment. SS bending However, moment under SS bending both turbulent moment under steady both both turbulent condition approached steady steady zero condition at approached center unlikezero at FA bending center moment. unlike FA bending moment. The The distribution of of bending moments along blade presented in in in 4 4 showed that that bending moments are are are more more significant significant at at blade blade roots roots or extremes or extremes at at at centre. centre. Theree, Theree, se points se se points of higher of of higher bending bending moments moments were selected were selected furr analysis furr analysis as shown as as inshown in 5a d. The 5a d. 5a d. plot showed The The plot plot showed bending moment bending at moment lastat first last nodal first point nodal along point blade, along called blade, called blade called root blade root (top) BMs (top) blade root blade (top) root (bottom) blade root respectively. (bottom) respectively. In 5a,b In mean 5a,b BMs increases mean BMs as increases speed as speed constant increases speed except increases FA except BM at blade FA root BM (top) at which blade remained root (top) constant which between remained constant m/s between m/s in mean BM between speed but decreases m/s afterwards. speed but Also, decreases variation afterwards. in Also, mean BMvariation values increases mean as BM values increases as speed SS BMs at both blade extremes speed values increases as FAspeed increases SS BMs at both FA blade extremes SS BMs under at both both blade conditions. extremes under both conditions. The excitation due to turbulence on FA SS BMs at The under excitation both dueconditions. to turbulence The onexcitation FA due to SSturbulence BMs at on blade extremes FA remain SS BMs negligible at blade extremes remain negligible up to 22 m/s speed. However, above 22 m/s speed, up blade turbulent to 22 extremes m/s effects remain speed. negligible become However, up to more noticeable above 22 m/s 22 m/s with speed. mean speed, However, SS bending turbulent above 22 moment effects m/s become speed, increasing more noticeable turbulent tremendously with effects as seen mean become in SS bending more noticeable maximum moment value increasing with under turbulent tremendously mean SS bending conditions. as seen inmoment maximum increasing value under tremendously turbulent as seen conditions. in maximum value under turbulent conditions. 5. Cont.

11 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) (d) 5. Blade bending moments: Top root FA; Top root SS; Bottom root FA; (d) Bottom 5. Blade 5. Blade bending moments: Top root FA; Top root SS; SS; Bottom root root FA; FA; (d) (d) Bottom Bottom root root root SS. SS. SS. The The largest largest effect effect of of turbulence on on mean values were experienced at at m/s m/s speed speed The largest effect of turbulence on mean values were experienced at 25 m/s speed from from both both FA FA SS SS BMs. BMs. The The difference between stard deviation under under turbulent from both FA SS BMs. The difference between stard deviation under turbulent condition stard deviation under steady condition increases at at each each higher higher condition stard deviation under steady condition increases at each higher speeds. speeds. This This implied that that effect of turbulence on load variation increases as as speeds. This implied that effect of turbulence on load variation increases as speed increases. speed speed increases. increases. 6 presents plots of FA SS at of blade. presents 6 presents plots plots of of FA FA SS bending moments at at center center of of blade. blade. 6. Blade center: FA BM; SS BM. 6. Blade center: FA BM; SS BM. The effect of turbulence on 6. mean Blade values center: is only FA BM; significant SS BM. above 18 m/s speed with higher The effect values ofunder turbulence turbulent on mean condition values isboth only significant FA above SS bending 18 m/smoments. speed Also, with higher The values turbulent effect under of effect turbulence turbulent resulted on increasing mean condition values load variability is both only significant as FA speed above SSincreases bending 18 m/s above moments. speed 18 m/s, Also, with higher turbulent with values a maximum effect under resulted effect turbulent at in 25 increasing m/s condition speed. load variability both as FA speed increases SS bending above moments. 18 m/s, Also, with a maximum turbulent effect effect at resulted 25 m/s increasing speed. load variability as speed increases above 18 m/s, with a maximum Effect of Turbulence effect at 25 on m/s Tower speed. Base Bending Moments Effect The of tower Turbulence base is a onpoint Tower of large Base bending Bending moment, Moments hence, its selection furr analysis Plots Effect of Turbulence on Tower Base Bending Moments The of tower bending base ismoments a point of in large bending 7 showed moment, mean FA hence, bending its selection moment increases furr while analysis. mean The tower SS bending base moment is a point increases of large as well bending but in moment, opposite hence, bending its (negative) selection as furr speed Plots of bending moments in 7 showed mean FA bending moment increases while analysis. increases. The plot also revealed that as speed increases, effect of turbulence on both mean Plots of SS bending moments increases as well 7 showed but in opposite mean FA bending (negative) moment increases while speed mean FA. The mean SS bending moment is significant at speeds above 18 m/s with a larger increases. mean SS bending The plot moment also revealed increases that as as well but speed opposite increases, bending (negative) effect of turbulence as onspeed both bending moment under turbulent conditions. Furrmore, effect of turbulence on increases. mean The plot also revealed that as speed increases, effect of turbulence on both mean FA. values The increases mean SS as bending speed moment increases. is significant The effect at of turbulence speeds above had its 18 largest m/s with effects a larger at bending mean 25 FA. m/s moment The mean speed. under SS turbulent bending moment conditions. is significant Furrmore, at speeds effect above of turbulence 18 m/s with on a larger mean bending moment under turbulent conditions. Furrmore, effect of turbulence on mean values increases as speed increases. The effect of turbulence had its largest effects at 25 m/s speed.

12 Energies 2016, 9, of 36 values increases as speed increases. The effect of turbulence had its largest effects at 25 m/s speed. Energies 2016, 9, of Tower Base: FA BM; SS BM. The excitations created by turbulence in terms of load variation are described by stard The excitations created by turbulence in terms of load variation are described by stard deviation from mean values of respective bending moments are as depicted in 7. The deviation from mean values of respective bending moments are as depicted in 7. The plot plot showed effect of turbulence on load variability is negligible up to 18 m/s speed showed effect of turbulence on load variability is negligible up to 18 m/s speed both both FA SS bending moments. Furrmore, effect of turbulence only resulted in an FA SS bending moments. Furrmore, effect of turbulence only resulted in an increased increased load variation above 22 m/s speed both FA SS bending moments. load variation above 22 m/s speed both FA SS bending moments. Anor approach to investigate effect of turbulence on loads at tower base is to Anor approach to investigate effect of turbulence on loads at tower base is to study Bending Moment (BM) time history spectral plots. The environmental condition study Bending Moment (BM) time history spectral plots. The environmental condition DLC 6 DLC 6 (V = 25 m/s, Hs = 6.02 m Tp = s) was chosen this study because statistical (V = 25 m/s, H s = 6.02 m T p = s) was chosen this study because statistical analysis analysis of Bending Moments (BMs) in 7 showed turbulent effects was largest at 25 of Bending Moments (BMs) in 7 showed turbulent effects was largest at 25 m/s m/s speed. The BMs time history power spectra plots DLC 6 are as shown in speed. The BMs time history power spectra plots DLC 6 are as shown in 8. The effect 8. The effect of turbulence is depicted in plots of BM time history both FA SS by of turbulence is depicted in plots of BM time history both FA SS by slightly higher slightly higher spikes under turbulent condition within denser region. The spikes or BM spikes under turbulent condition within denser region. The spikes or BM amplitudes are amplitudes are indication of deviations from mean values under respective conditions. indication of deviations from mean values under respective conditions. The plot showed The plot showed a larger deviation under steady outside dense region. However, larger a larger deviation under steady outside dense region. However, larger portion of less portion of less dense region is situated within areas where BMs are negative. Furrmore, dense region is situated within areas where BMs are negative. Furrmore, less dense region less dense region has fewer data. Theree, mean BM is higher under turbulent has fewer data. Theree, mean BM is higher under turbulent condition. This turbulent condition. This turbulent effect is furr elaborated in spectral plots both FA SS BMs. effect is furr elaborated in spectral plots both FA SS BMs. The wave frequency excitation The wave frequency excitation dominates response under turbulent condition while 2P dominates response under turbulent condition while 2P frequency excitation proved frequency excitation proved dominant under steady condition as shown in spectral dominant under steady condition as shown in spectral plots. The spectral plot plots. The spectral plot FA BM revealed wave excitation under turbulent condition FA BM revealed wave excitation under turbulent condition indicated by first peak (red) indicated by first peak (red) is about 80% higher than under steady condition (blue). is about 80% higher than under steady condition (blue). Similarly, spectral plot SS BM Similarly, spectral plot SS BM showed similar response as that of FA BM. The lower showed similar response as that of FA BM. The lower second peak (red) due to turbulent second peak (red) due to turbulent condition at 2P frequency (at high speeds, condition at 2P frequency (at high speeds, turbine experiences a shift from original turbine experiences a shift from original 2P frequency of 1.1 rad/s) on spectral plot indicated 2P frequency of 1.1 rad/s) on spectral plot indicated that effect turbulence resulted in a lower that effect turbulence resulted in a lower excitation at 2P frequency. This effect is more than excitation at 2P frequency. This effect is more than 300% 600% reduction in excitation when 300% 600% reduction in excitation when compared with response under steady compared with response under steady condition at 2P frequency FA SS condition at 2P frequency FA SS BMs, respectively. BMs, respectively.

13 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) Tower Tower Base Base BMs BMs DLC DLC 6: 6: FA FA time time series; series; FA FA power power spectral spectral density density (PSD); (PSD); SS SS time time series; series; (d) (d) SS SS power power spectral spectral density density (PSD). (PSD) Effect of Turbulence on Mooring Lines Tension Effect of Turbulence on Mooring Lines Tension shows mean tension in mooring line increases as speed increases except 9 shows mean tension in mooring line 2 increases as speed increases except at 25 m/s speed where it decreased under steady conditions, unlike in mooring lines at 25 m/s speed where it decreased under steady conditions, unlike in mooring lines 1 3. Mooring line is positioned in negative x-axis (see 1c) in alignment with wave 3. Mooring line 2 is positioned in negative x-axis (see 1c) in alignment with wave. Theree, as speed increases, platm drift faster towards positive. Theree, as speed increases, platm drift faster towards positive x-axis. x-axis. As platm continues to drift, mooring line 2 will be continuously stretched while As platm continues to drift, mooring line 2 will be continuously stretched while mooring lines 1 mooring lines 1 3 are continuously relaxed simultaneously. This results in increasing tension of 3 are continuously relaxed simultaneously. This results in increasing tension of mooring line 2 mooring line 2 decreasing tension in mooring lines 1 3. However, as speed decreasing tension in mooring lines 1 3. However, as speed increases, platm is increases, platm is believed to drift in between positive X-Y direction, re exist load believed to drift in between positive X-Y direction, re exist load interaction among moorings interaction among moorings such that mooring line 3 becomes slightly stretched while mooring such that mooring line 3 becomes slightly stretched while mooring line 1 is continuously relaxed. line 1 is continuously relaxed. This phenomenon is believed to have been initiated at 18 m/s This phenomenon is believed to have been initiated at 18 m/s speed it continued up to speed it continued up to 25 m/s speed as shown in 9. However, this effect is 25 m/s speed as shown in 9. However, this effect is cushioned by unsteady platm drift cushioned by unsteady platm drift within identified speed region as observed in lower within identified speed region as observed in lower mean tension of mooring line 3 under mean tension of mooring line 3 under turbulent condition. The effect of turbulence is as well turbulent condition. The effect of turbulence is as well obvious in increasing disparities obvious in increasing disparities between load variability under turbulent condition between load variability under turbulent condition under steady condition as under steady condition as speed increases all mooring lines except speed increases all mooring lines except mooring line 3 whose response characteristic is mooring line 3 whose response characteristic is unlike ors. The load varies higher from unlike ors. The load varies higher from mean values under turbulent condition than mean values under turbulent condition than under steady condition at all speeds under steady condition at all speeds mooring lines 1 2. mooring lines 1 2.

14 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of Mooring lines tension: Line 1; Line Line Mooring Mooring lines lines tension: tension: Line Line 1; 1; Line Line 2 ; ; Line Line The time series plot of 10 showed turbulent effects is high at DLC 6 load condition The time series plot of 10 showed turbulent effects is high at DLC 6 load condition both mean values variations from mean. Furrmore, spectral plot is observed to both mean values variations from mean. Furrmore, spectral plot is observed to have higher peaks under turbulent condition at wave excitation frequency than under have higher peaks under turbulent condition at at wave wave excitation excitation frequency frequency than under than under steady steady condition. The last peaks were observed at higher frequencies (about 1.7 rad/s), which steady condition. condition. The lastthe peaks last were peaks observed were observed at higherat frequencies higher frequencies (about 1.7(about rad/s), 1.7 which rad/s), should which be should be induced by eigen frequency of blade, showed about 500% reduction on should inducedbe byinduced eigenby frequency eigen offrequency blade, showed of blade, about showed 500% reduction about 500% on reduction excitement on under excitement under turbulent condition. This means mooring lines 2 experienced reduced excitement turbulent under condition. turbulent This means condition. mooring This means lines 2 experienced mooring reduced lines 2 experienced load excitation reduced under load excitation under turbulent at this frequency. load turbulent excitation under at thisturbulent frequency. at this frequency. 10. Mooring lines Tension 2 DLC 6: Time series plot; Power spectral density Mooring Mooring lines lines Tension Tension 2 DLC DLC 6: 6: Time Time series series plot; plot; Power Power spectral spectral density. density.

15 Energies 2016, 9, of 36 Energies 2016, 9, of Effect of Turbulence on Fatigue Damage of Flexible Components Effect of Turbulence on Fatigue Damage of Flexible Components The fatigue analysis of flexible components is focused on effect of turbulence on Short-Term Damage The Equivalent fatigue analysis Loads (STFDEL). of flexible To components investigateis focused effecton of turbulence effect of onturbulence STFDELs of on flexible components Short-Term Damage 5 MW Equivalent baseline FVAWT, Loads (STFDEL). following To investigate positions of high effect load concentration of turbulence shown on STFDELs of flexible components 5 MW baseline FVAWT, following positions of high in Table 4 were selected fatigue analysis. load concentration shown in Table 4 were selected fatigue analysis. Table 4. Description of selected positions fatigue analysis. Table 4. Description of selected positions fatigue analysis. Nodal Points Coordinates (Y, Z) Length LengthSegment (m) (m) Description 11 (0.00, 0.00) - - Tower base base 22 (0.10, ) 0 0 Blade Blade root root (bottom) 33 (62.80, (62.80, 87.35) Blade Blade center center 4 (0.00, ) Blade root (top) 4 (0.00, ) Blade root (top) Mooring Mooringline line1 2 6 Mooring line 2 The The plots plots of of STFDELs STFDELs at at tower tower base base blade blade under both conditions are shown in in 11. The 11. The abbreviations abbreviations used used in in plots plots are are described described in in Abbreviations list. list. (d) 11. Fatigue STFDEL of selected areas: Tower base fatigue; Blade root (bottom); 11. Fatigue STFDEL of selected areas: Tower base fatigue; Blade root (bottom); Blade Blade center; (d) Blade root (top). center; (d) Blade root (top). The plot reveals increasing STFDELs as speed increases under both conditions with The exception plot reveals of increasing STFDELs due STFDELs to blade as SS BMs speed which increases experienced underrapid both decreasing conditions value at with exception bottom of STFDELs center from due18 tom/s blade 22 SS m/s BMs which speed experienced steady rapid decreasing turbulent value at bottom condition respectively. center from The 18 effect m/sof turbulence 22 m/s was observed speed at all steady points selected turbulent fatigue condition analysis. respectively. Furrmore, The effect effect of turbulence of turbulence wasis observed seen as higher at all STFDELs points selected under turbulent fatigue analysis. condition Furrmore, than under effect steady of turbulence condition. is seen The asdifference higher STFDELs between under STFDELs turbulent under condition turbulent than under steady conditions condition. widens The difference as between speed increases STFDELs both under SS turbulent FA bending steady moments but conditions experience widens a decrease at speed 25 m/s increases SS bending both SSmoments. FA bending This showed moments

16 Energies 2016, 9, of 36 Energies 2016, 9, of 36 butthat experience a decrease at 25 m/s SS bending moments. This showed that SS bending Energies 2016, SS bending 9, 1047 moment at blade under turbulent condition could have 16 a of lower 36 moment damaging at effect bladeas under bending turbulent moment condition under steady could have acondition lower damaging at effect speeds as of 25 bending m/s moment that speed under SS at steady bending blade moment top condition node. at The at blade plot under of speeds turbulent STFDELs of 25 m/s condition mooring speedcould lines at under have blade a both lower top node. Theconditions plot damaging of are effect STFDELs shown as in bending mooring 12. moment lines under under steady both conditions at are shown speeds inof 25 m/s 12. speed at blade top node. The plot of STFDELs mooring lines under both conditions are shown in Fatigue STFDEL of selected areas: Mooring Line 1; Mooring Line Fatigue STFDEL of selected areas: Mooring Line 1; Mooring Line Fatigue STFDEL of selected areas: Mooring Line 1; Mooring Line 2. The effect of turbulence on STFDELs due to tension in mooring lines created large disparity The effect of turbulence on STFDELs due to tension in mooring lines created large disparity between The effect STFDELs of turbulence at each on STFDELs speed except due to at tension 25 m/s in mooring speed lines created mooring large line disparity 1 where a between greater between damaging STFDELs STFDELs effect at at was each each experienced speed speed under except except steady at at25 25m/s m/s conditions. speed speed mooring mooring line 1 where line 1 a where a greater greater damaging effect was experienced under steady steady conditions. conditions Dynamic Response Analysis of 5 MW Optimised FVAWT 3.2. Dynamic 3.2. Dynamic Response Response Analysis Analysis of of 55 MW Optimised FVAWT Effect of Turbulence on Power Generation Effect Effect of of Turbulence on on Power Generation In 13, generator power produced by FVAWT under steady is compared with In In that obtained 13, 13, under generator turbulent power power produced conditions. by by FVAWT under steady is compared with that obtained with that obtained under turbulent under turbulent conditions. conditions Power generated under steady turbulent conditions. 13. Power generated under steady turbulent conditions. The The plot plot is is based based on on averaged values stard stard deviations deviations of of simulated simulated power power each The plot 3600 is s simulation based after averaged start-up values transient stard five different deviations seeds were of removed. simulated The power error each 3600 s simulation after start-up transient five different seeds were removed. The error eachbars 3600indicate s simulation stard after deviation start-upfrom transient averaged five different values. Although seeds were same removed. environmental The error bars bars indicate stard deviation from averaged values. Although same environmental indicate condition stard is used deviation steady fromturbulent averaged values. conditions, Although averaged same power environmental is found to differ condition condition is used steady turbulent conditions, averaged power is found to differ is used above steady rated turbulent speed of 14 m/s, conditions, disparity averaged widens power as is found speed to differ increases. above The rated above rated speed of 14 m/s, disparity widens as speed increases. The power produced under turbulent condition is higher than under steady condition power speed produced of 14 m/s, under turbulent disparity widens condition as is higher than speed under increases. steady The power condition produced under speeds above rated speed. Furrmore, error bars are longer turbulent turbulent speeds above condition rated speed. is higher Furrmore, than under steady error bars condition are longer under turbulent speeds above rated speed. Furrmore, error bars are longer under turbulent condition as speed increases. This demonstrates that effect of turbulence resulted into a higher power variability.

17 Energies 2016, 9, of 36 condition as speed increases. This demonstrates that effect of turbulence resulted into a higher power variability. Energies 2016, 9, of Effect of Turbulence on Platm Global Motion The effect of turbulence on platm global motion is investigated based on statistical analysis spectral analysis of simulation results, including inspection of plots of time series of motions 6 degree of freedom motion. The results of statistical analysis of global motion in terms of mean value of 5 seeds realisation at each DLC stard deviation from from mean mean values values are are as shown as shown in in surge, surge, roll, roll, pitch motion. pitch In motion. general, In itgeneral, can be said it can that re be said wasthat negligible re was turbulent negligible effect on turbulent mean effect values on especially mean atvalues speeds especially belowat ratedspeeds speed. below However, rated as speed. increases, However, this as effectspeed becomes increases, significant this widens effect becomes especiallysignificant pitch widens motion especially at speeds above pitch motion ratedat speed. speeds The above effect ofrated turbulence speed. is morethe obvious effect inof turbulence variationis ofmore motion obvious amplitudes in from variation mean of motion valuesamplitudes as represented from by mean stard values deviation as represented from mean by values. stard The difference deviation between from mean stard values. deviation The difference under turbulent between condition stard deviation steady under condition turbulent narrows as condition speed steady increases condition with turbulent narrows as condition speed having increases a higher with value turbulent all degree of freedom condition motion. having a higher value all degree of freedom motion Statistical Statistical results results all all DLC: DLC: Surge motion; Roll Roll motion; motion; Pitch Pitch motion. motion. As speed increases, variation in terms of stard deviation (std) from mean As speed increases, variation in terms of stard deviation (std) from mean values is observed to follow a fairly linear behaviour under steady condition. However, values is observed to follow a fairly linear behaviour under steady condition. However, stard deviation under turbulent condition increases up to rated speed n stard deviation under turbulent condition increases up to rated speed n decreases at speeds above rated speed. This showed that under turbulent condition, decreases at speeds above rated speed. This showed that under turbulent condition, platm do not experience a steady drift on any direction but rar exhibit a secondary effect. platm do not experience a steady drift on any direction but rar exhibit a secondary effect. This secondary effect could be greater interaction among all degree of freedom motion due to This secondary effect could be a greater interaction among all degree of freedom motion due to platm stiffness property with exception of heave motion. Two environmental conditions were considered critical time series inspection spectra analysis:

18 Energies 2016, 9, of 36 Energies platm 2016, stiffness 9, 1047 property with exception of heave motion. Two environmental conditions 18 of 36 were considered critical time series inspection spectra analysis; Environmental condition at at rated rated speed speed of of m/s m/s DLC 3 (V (V = 14 m/s, Hs H s = m Tp T p = s). s). Environmental Environmental condition condition at at cut-out cut-out speed speed of of m/s m/s DLC DLC 6 (V (V = m/s, m/s, H s = m Tp T p = s). s). The time series power spectra all six degree of freedom motion were investigated under The both time steady series turbulent power spectra conditions all six DLC degree 3 of freedom 6. However, motion only were plots investigated under surge, both steady roll turbulent pitch motion are conditions presented as DLC shown 3 in 6. However, 15a l. only The plots plots of motion surge, time history roll reveal pitch amplitudes motion are of presented motions are as shown clearly in higher under 15a l. turbulent The plots of motion condition time history than under reveal steady amplitudes condition of motions are clearly both design higher load undercases turbulent (DLC) condition all six degrees than under of steady freedom motion. condition both design load cases (DLC) in all six degrees of freedom motion. (d) (e) (f) 15. Cont.

19 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (g) (h) (i) (j) (k) (l) (a,b) (a,b) Time Time series series power power spectrum spectrum of of surge surge motion motion DLC DLC 3 respectively; respectively; (c,d) (c,d) Time Time series series power power spectrum spectrum of of surge surge motion motion DLC DLC 6 respectively; respectively; (e,f) (e,f) Time Time series series power power spectrum of roll motion DLC respectively; (g,h) Time series power spectrum of roll motion spectrum of roll motion DLC 3 respectively; (g,h) Time series power spectrum of roll motion DLC 6 respectively; (i,j) Time series power spectrum of pitch motion DLC 3 respectively; DLC 6 respectively; (i,j) Time series power spectrum of pitch motion DLC 3 respectively; (k,l) Time series power spectrum of pitch motion DLC respectively. (k,l) Time series power spectrum of pitch motion DLC 6 respectively. The power spectrum of motions in individual degree of freedom motion two conditions The power can be spectrum compared of motions to furr in evaluate individual effect degree of turbulence. of freedom In motion power spectra two plots, conditions natural canresponses be compared all to furr degree evaluate of freedom effect motion of turbulence. experienced Ingreater power excitation spectraunder plots, turbulent natural responses condition. allfurrmore, degree of freedom peaks motion are observed experienced at greater 2P frequency excitation (a characteristic under turbulent of VAWTs condition. with two Furrmore, blades) peaks both are steady observed turbulent at 2P frequency conditions. (a characteristic In VAWTs, of VAWTs axis with of two rotor blades) rotation is both not steady in line to turbulent direction, conditions. angle In VAWTs, of attack varies axis ofwith rotor rotation azimuthal is not inposition line to of blades direction, during operation. angle These of attack causes varies variations with azimuthal in aerodynamic position loads of within blades a during cycle of operation. rotation. These If causes VAWT variations has two blades, in aerodynamic each blade loads cuts within in a cycledirection of rotation. once If in VAWT every has cycle. twotheree, blades, each a two blade bladed cuts VAWT in will cut-in direction twice once in every in every cycle cycle. of Theree, rotor, se a two causes bladed a variation in torque twice in every cycle. The 2P effect creates 2P frequency. When

20 Energies 2016, 9, of 36 VAWT will cut-in twice in every cycle of rotor, se causes a variation in torque twice in every cycle. The 2P effect creates 2P frequency. When turbine rotates at rated rotational Energies 2016, 9, of 36 speed of 0.62 rad/s, 2P frequency is approximately 1.24 rad/s. The 2P effect is prominent in roll turbine pitch rotates motions. at rated rotational speed of 0.62 rad/s, 2P frequency is approximately 1.24 The rad/s. powerthe spectra 2P effect is prominent roll in pitch roll motions pitch respectively motions. revealed turbulence effect on se motions The power reduces spectra 2P effect roll as seen inpitch lower motions peaks respectively at 2P revealed frequency turbulence turbulent effect condition. se Furrmore, motions reduces as environmental 2P effect as seen condition in lower becomes peaks severe at in 2P DLC frequency 6, difference turbulent in height of condition. responsefurrmore, peaks at as 2P frequency environmental under steady condition turbulent becomes severe conditions in DLC 6, narrows difference in height of response peaks at 2P frequency under steady turbulent in roll motion but widens in pitch motion. Additionally, 2P frequency is observed to have conditions narrows in roll motion but widens in pitch motion. Additionally, 2P changed to about 0.55 rad/s steady condition 0.56 rad/s turbulent condition frequency is observed to have changed to about 0.55 rad/s steady condition 0.56 rad/s DLC 6 in both roll pitch motions as shown in 15d,l, respectively. This is due to turbulent condition DLC 6 in both roll pitch motions as shown in effect of15d,l, turbulence respectively. on This controller is due to as effect rotational of turbulence speedson under controller steady as turbulent rotational conditions speeds now under differ. steady turbulent conditions now differ Effect Effect of Turbulence of Turbulence on on Blade Blade Bending Moments To analyze To analyze effect effect of of turbulence on on blade blade bending bending moments, points points were were selected along blade along length. blade The length. distribution The distribution of bending of bending moments moments along along blade are blade as shown are as shown in in 16. The plot of 16. The distribution plot of of distribution bendingof moments bending alongmoments blade along revealed blade turbulence revealed effect caused turbulence extra excitations effect caused in extra bending excitations moments in bending especially moments in especially SS BM. The in mean SS BM. FA bending The moment mean attained FA bending higher moment values attained at blade higher center values at blade blade root center (top) positions. blade root The (top) plot positions. of mean The plot of mean SS bending moment showed higher values at some position slightly farr SS bending moment showed higher values at some position slightly farr from both blade extremes from both blade extremes with highest value near blade root (bottom) position. In general, it with highest value near blade root (bottom) position. In general, it was clear in 16 that was clear in 16 that effect of turbulence on mean FA mean SS bending effect moments of turbulence is negligible. on mean FA mean SS bending moments is negligible. (d) 16. Blade FA SS BM distribution DLC 1, 3, 6. Mean FA BM; Stard 16. Blade FA SS BM distribution DLC 1, 3, 6. Mean FA BM; Stard deviation of FA BM; Mean SS BM; (d) Stard deviation of SS BM. deviation of FA BM; Mean SS BM; (d) Stard deviation of SS BM.

21 Energies 2016, 9, of 36 Energies 2016, 9, of 36 The plots of bending moments at blade root (top), blade root (bottom) blade center are shown inthe plots 17a f. of bending moments 17 shows at an increasing blade root (top), ablade decreasing root (bottom) mean FA bending blade center moment are at blade shown root in (top) under 17a f. both 17 shows conditions an increasing at speeds a decreasing below mean above FA bending rated moment speed at respectively. blade Furrmore, root (top) under both 17a dconditions show re at were increasing speeds below decreasing above load rated variations about speed respectively. mean FA Furrmore, mean SS bending 17a d moments show re at both were blade increasing extremes decreasing under both load conditions variations at about speeds mean below FA above mean SS bending rated moments speed. at both The effect blade extremes of turbulence under isboth almost zero fromconditions cut-in at speed speeds up tobelow 18 m/s above speed. rated However, speed. above The 18 effect m/s of turbulence speed, is turbulent almost effects zero from on cut-in load variability speed up isto slightly 18 m/s significant. speed. However, this above effect 18 m/s remained speed, constant as turbulent speed increases effects on both load variability FA is slightly SS BMs. significant. 17e,f However, presents this plots effect of remained FA constant as speed increases both FA SS BMs. 17e,f presents plots of SS bending moments at center of blade. The effect of turbulence on mean values is FA SS bending moments at center of blade. The effect of turbulence on mean insignificant at all speeds. In addition, turbulence effect resulted in increased load variability values is insignificant at all speeds. In addition, turbulence effect resulted in increased at 18 m/s speed. Above 18 m/s speed, effect of turbulence remained constant. load variability at 18 m/s speed. Above 18 m/s speed, effect of turbulence However, remained constant. effect of turbulence However, is clearly effect of seen turbulence in higher is maximum clearly seen values in higher under maximum turbulent values condition under than turbulent steady condition conditions. than steady conditions. (d) (e) (f) 17. (a,b) Blade root (top) FA SS bending moments respectively; (c,d) Blade root (bottom) 17. (a,b) Blade root (top) FA SS bending moments respectively; (c,d) Blade root (bottom) FA FA SS bending moments respectively; (e,f) Blade center FA SS bending moments respectively. SS bending moments respectively; (e,f) Blade center FA SS bending moments respectively.

22 Energies 2016, 9, of 36 Energies 2016, 9, of Effect of Turbulence on Tower Base Bending Moments Effect of Turbulence on Tower Base Bending Moments The results of statistical analysis of FA SS BMs at tower base are as shown in 18. The The results plotof showed statistical that as analysis of speed FA increases, SS BMs at effect tower of turbulence base are as on shown mean in values remains 18. The insignificant plot showed at that as speeds below speed increases, rated effect speed of turbulence both on FA mean SS values remains insignificant at speeds below rated speed both FA SS BMs. Above rated speed, effect of turbulence mean FA BM is obvious with larger BMs. Above rated speed, effect of turbulence mean FA BM is obvious with larger value under turbulent condition than under steady condition. This effect continues to value under turbulent condition than under steady condition. This effect continues to increase as speed increases above rated speed with maximum effect at 25 m/s increase as speed increases above rated speed with a maximum effect at 25 m/s speed speed especially especially FA FA BMs. BMs. The The variations variations from from mean mean values values FA FA BM BM increase increase as as speed speed increases increases under under both both steady steady turbulent conditions. Furrmore, Furrmore, effect effect of of turbulence turbulence on on load load variations caused an an increasing load variation at at speeds speeds above above rated rated speed speed FA FA BM. BM. This This effect is is largest at at 25 m/s speed. Consequently, effect effect of of turbulence on on both both mean load variation SS BM is negligible. 18. Tower Base: FA BM; SS BM. 18. Tower Base: FA BM; SS BM. The Bending Moment (BM) time history spectral plots are alternative approaches to investigate The Bending effect Moment of turbulence (BM) timeon history tower base spectral BMs. The plots environmental are alternative condition approaches DLC 6 to investigate (V = 25 m/s, Hs effect = 6.02 ofm turbulence Tp = ons) based tower on base discussions BMs. The in Section environmental 3.1.3; turbulent conditioneffect DLC 6 (V = is 25 largest m/s, at H25 s = m/s 6.02 m speed. T p The = time s) series based on spectra discussions plots of in Section FA 3.1.3; SS BMs turbulent at effect tower is largest base are at as 25shown m/s speed. 19. Theplots timeof series BM time history spectra plots both of FA FASS showed SS BMshigher at amplitudes tower baseor are spikes as shown under turbulent in 19. The condition plots of than under BM time steady history condition. both FA The SS showed spectra plots higher embellished amplitudes orturbulent spikes under effects turbulent both FA condition SS BMs. than The under wave steady frequency condition. excitation The dominates spectra plots responses embellished both turbulent FA BM effects SS BM both as shown FA in SS BMs. spectra The plots wave frequency of excitation 19b,d. The dominates wave excitation responses under turbulent both FAcondition BM (red) SS is higher BM as than shown under in spectra steady plots of conditions 19b,d. (blue). The Similarly, wave excitation SS BM under showed turbulent a higher peak condition at wave (red) excitation is higher frequency under turbulent conditions than under steady conditions. The second peak at than under steady conditions (blue). Similarly, SS BM showed a higher peak at wave 2P frequency (observe shift of original 2P frequency of about 1.24 rad/s to 1.5 rad/s excitation frequency under turbulent conditions than under steady conditions. The second 1.6 rad/s at high speed (25 m/s) under steady conditions under turbulent peak at 2P frequency (observe shift of original 2P frequency of about 1.24 rad/s to 1.5 rad/s conditions, respectively) on spectra plots denote reduced 2P effect under turbulent conditions. 1.6 rad/s at The high effect of turbulence speed (25 on m/s) under 2P effect steady is negligible conditions FA BM. under The plots turbulent of BM conditions, time history respectively) both on FA spectra SS showed plotshigher denoteamplitudes reduced or 2P spikes effect under under turbulent turbulent conditions. conditions The than effect under of turbulence steady onconditions. 2P effect The isspectra negligible plots embellished FA BM. The turbulent plots of effects BM time history both FA bothss FA BMs. The SSwave showed frequency higherexcitation amplitudes dominates or spikes under responses turbulent both conditions FA BM than under SS steady BM as shown conditions. in spectra The spectra plots of plots embellished 19b,d. The wave turbulent excitation effects under turbulent both FA SS BMs. conditions The wave (red) frequency is higher than excitation under dominates steady conditions responses (blue). both Similarly, FA BMSS BM SS BM showed as shown higher in peak at spectra wave plots excitation of frequency 19b,d. under The wave turbulent excitation conditions under turbulent than under conditions steady (red) conditions. is higher than The second under steady peak at 2P conditions frequency (blue). (observe Similarly, shift of SSoriginal BM showed 2P higher frequency peak at of about wave 1.24 excitation rad/s to 1.5 frequency rad/s under 1.6 rad/s turbulent at high speed conditions (25 m/s) than under under steady steady conditions. conditions The second under peak turbulent at 2P frequency conditions, (observe respectively) shift on of spectra original plots 2P denote frequency of about 1.24 rad/s to 1.5 rad/s 1.6 rad/s at high speed (25 m/s) under steady conditions

23 Energies 2016, 9, of 36 Energies 2016, 9, 9, of 36 reduced under turbulent 2P effect under conditions, turbulent respectively) conditions. The spectra effect of plots turbulence denoteon reduced 2P effect 2P effect is under negligible turbulent FA conditions. BM. The effect of turbulence on 2P effect is negligible FA BM. (d) 19. Tower Base: (a,b) FA BM time series PSD respectively DLC6; (c,d) SS BM time 19. Tower Base: (a,b) FA BM time series PSD respectively DLC6; (c,d) SS BM time series series PSD respectively DLC 6. PSD respectively DLC Effect of Turbulence on Mooring Lines Tension Effect of Turbulence on Mooring Lines Tension In 20, mean tension in mooring line 2 increases up to rated speed n decreases In slightly, 20, mean but tension continues in mooring to increase line above 2 increases 18 m/s up to rated speed speed unlike in n mooring decreases slightly, lines but 1 continues 3. This is to due increase to above same 18 explanation m/s already speed unlike discussed in mooring Section lines The 3. effect This is of due to turbulence same explanation mean already load manifested discussedas inhigher Section mean loads Theunder effectturbulent of turbulence condition mean than load manifested under steady as higher mean condition loadsespecially under turbulent at speeds condition above than rated under steady speed except condition especially mooring atline 3. speeds above rated speed except mooring line Cont.

24 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of Mooring lines tension: Line 1; 1; Line 2; 2; Line Mooring lines tension: Line 1; Line 2; Line 3. The effect of turbulence is obvious in disparities between load variability under The effect of of turbulence is obvious is obvious in in disparities disparities between between load variability load variability under turbulent under turbulent conditions that under steady conditions at each mean speed all turbulent conditions conditions that under that steady under steady conditions conditions at each mean at each mean speed speed all mooring all mooring lines. However, effect of turbulence on load variation can be relatively constant as lines. mooring However, lines. However, effect of turbulence effect of turbulence on load variation load can variation be relatively can be constant relatively as constant speed as speed increases all mooring lines. increases speed increases all mooring lines. all mooring lines. The time series spectra plot DLC in 21 showed amplitudes of mooring line The time series spectra plot DLC 6 in 21 showed amplitudes of mooring line 2 tension is higher under turbulent conditions than under effects of steady conditions. tension is higher under turbulent conditions than under effects of steady conditions. 21. Mooring lines Tension 2 DLC 6: Time series plot; Power spectrum Mooring Mooring lines lines Tension Tension 2 DLC DLC 6: 6: Time Time series series plot; plot; Power Power spectrum. spectrum. The spectra plot revealed higher peaks at wave excitation frequency under turbulent The spectra plot revealed higher peaks at wave excitation frequency under turbulent conditions The spectra (red) plot than revealed under higher steady peaks at conditions wave excitation (blue). This frequency means under turbulent mooring lines conditions (red) than under steady conditions (blue). This means mooring lines conditions experienced (red) higher than excitation under steady due to wave conditions loads (blue). under This turbulent means mooring conditions lines experienced than under experienced higher excitation due to wave loads under turbulent conditions than under higher steady excitation conditions. due to wave Furrmore, loads under turbulent responses at conditions 2P frequency than under steady both steady conditions. Furrmore, responses at 2P frequency under both conditions. seem Furrmore, to have died out. responses at 2P frequency under both conditions seem to conditions seem to have died out. have died out Effect of Turbulence on Fatigue Damage on Flexible Components Effect of Turbulence on Fatigue Damage on Flexible Components Effect of Turbulence on Fatigue Damage on Flexible Components Similar to discussion in Section 3.1.5, following areas were selected fatigue analysis Similar to discussion in Section 3.1.5, following areas were selected fatigue analysis Similar MW to optimized discussion FVAWT in Section in order 3.1.5, to ease following comparison areas were of results selected with fatigue MW analysis baseline 5 MW optimized FVAWT in order to ease comparison of results with 5 MW baseline FVAWT. 5 MWThe optimized selected FVAWT points are in order as detailed to easein comparison Table 4. of results with 5 MW baseline FVAWT. FVAWT. The selected points are as detailed in Table 4. The selected points are as detailed in Table 5.

25 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Table Table Description of of selected positions fatigue analysis. Nodal Nodal Points Points Coordinates (Y, (Y, Z) Z) Length LengthSegment (m) (m) Description Description 1 (0.00, 0.00) - Tower base 1 (0.00, 0.00) - Tower base 2 2 (0.10, (0.10, ) ) 0 0 Blade Blade root root (bottom) (bottom) 3 3 (54.80, 93.17) Blade Blade center center 4 4 (0.00, ) Blade Blade root root (top) (top) 5 5 Mooring line 1 6 Mooring line 6 Mooring line 2 The The comparison of of estimated STFDELs STFDELs under under steady steady turbulent turbulent condition condition are shown are inshown in 22a f. 22a f. (d) (e) (f) 22. Fatigue STFDEL of selected areas, Tower base fatigue; Blade root (bottom); 22. Fatigue STFDEL of selected areas, Tower base fatigue; Blade root (bottom); Blade Blade center; (d) Blade root (top); (e) Mooring Line 1; (f) Mooring Line 2. center; (d) Blade root (top); (e) Mooring Line 1; (f) Mooring Line 2.

26 Energies 2016, 9, of 36 Energies 2016, 9, of 36 At tower base, fatigue STFDEL values are negligibly affected by turbulence at speeds below rated speed but significant both STFDELs due to FA SS BM at At speeds tower above base, rated fatigue STFDEL speed values with its aregreatest negligibly influence affectedat by25 turbulence m/s at STDEL speeds due to below FA BM. rated speed but significant both STFDELs due to FA SS BM at speeds above The rated plots of STFDELs speed with at itsblade greatest revealed influence increasing at 25 m/s decreasing STDEL STDEL due to values FA BM. speeds The below plots of STFDELs above at rated blade revealed speed, increasing respectively, with decreasing STDEL STDEL value values turbulent speeds below conditions above attaining rated higher values speed, than respectively, steady with STDEL conditions value all turbulent speeds. The conditions plots furrmore attainingdemonstrate higher values that than speeds steady farr from conditions rated allspeed speeds. (14 m/s) The could plotshave furrmore less damaging demonstrate effects that than speeds speeds farr closer from to rated speed (14 m/s) this FVAWT could have model. less The damaging turbulent effects effect than on blade speeds STFDELs closer toare only rated significant speed at this speeds FVAWT higher model. than The turbulent rated effect speed. on blade STFDELs are only significant at speeds higher than rated speed. The plot of STFDELs due to mooring lines tension revealed effect of turbulence at Thespeeds plot of below STFDELs rated due tospeed. mooring Above this lines tension speed, revealed disparity effect between of turbulence STFDEL at values speeds between below ratedconditions speed. narrows. AboveThis thismeans speed, tension disparity at speeds betweenabove STFDEL rated values between speed under turbulent conditions narrows. conditions This could means have similar tensiondamaging at speeds effects above as under rated steady speed condition under turbulent mooring lines conditions at lee position could (Line have 1). similar damaging effects as under steady condition mooring lines at lee position (Line 1) Comparison of Responses of 5 MW Optimised FVAWT with 5 MW Baseline FVAWT 3.3. Comparison of Responses of 5 MW Optimised FVAWT with 5 MW Baseline FVAWT The dynamic responses of baseline 5 MW FVAWT were compared with that of optimized The dynamic 5 MW responses FVAWT of to explore baseline 5advantages MW FVAWT of were optimized compared5 with MW that DeepWind of optimized rotor. The 5 primary MW FVAWT parameters to explore comparison advantages are ofpower optimized production, 5 MWbending DeepWind moment rotor. distribution The primary parameters fatigue damage comparison on flexible are components. power production, bending moment distribution fatigue damage on flexible components Comparing Power Production Comparing Power Production The plot in 23 shows a comparison between powers generated by 5 MW baseline FVAWT The plot in 5 MW 23 optimized shows a comparison FVAWT. As between discussed powers in Section generated 3.2.1, bycontrol 5 MW model baseline FVAWT 5 MW baseline 5FVAWT MW optimized was adopted FVAWT. As5 discussed MW optimized in Section FVAWT, 3.2.1, hence, control a significant modeldisparity 5 MW between baseline mean FVAWT power wasgenerated adopted by 5 MW optimized baseline FVAWT FVAWT, hence, that aby significant 5 MW disparity optimized between FVAWT is mean observed power at generated speeds byabove 5 MWrated baseline FVAWT speed. that by 5 MW optimized FVAWT is observed at speeds above rated speed Comparing Comparing Power Power generation. generation. The mean power increases as speed increases both FVAWT models, a value greater The mean power increases as speed increases both FVAWT models, a value than 5 MW rated power at rated speed was produced due to application of greater than 5 MW rated power at rated speed was produced due to application of Beddoes-Leishman (BL) dynamic stall model. Below rated speed, 5 MW optimized Beddoes-Leishman (BL) dynamic stall model. Below rated speed, 5 MW optimized FVAWT produced higher power than 5 MW baseline FVAWT. A value of about 3 MW 2.5 FVAWT produced higher power than 5 MW baseline FVAWT. A value of about 3 MW 2.5 MW MW were reached 5 MW optimized FVAWT 5 MW baseline FVAWT respectively at were reached 5 MW optimized FVAWT 5 MW baseline FVAWT respectively at 10 m/s 10 m/s speed. speed.

27 Energies 2016, 9, of 36 Energies 2016, 9, of 36 In terms of variability of produced power, both FVAWTs experienced increasing variability in Ingenerated terms of variability power as of speed produced increases power, both FVAWTs region below experienced rated increasing speed variability of 14 m/s. in However, generated variation power as power speed increases produced continued region to increase below rated 5 MW baseline speed offvawt 14 m/s. However, but decreases variation 5 MW in optimized power produced FVAWT continued as tospeed increase increases above 5 MW18 baseline m/s. This FVAWT could but be attributed decreases to controller, 5 MW optimized which responds FVAWT as differently at speed different increases rotational abovespeed. 18 m/s. This could be attributed to controller, which responds differently at different rotational speed Comparing Blade Bending Moments Distribution Comparing Blade Bending Moments Distribution In comparing structural responses of both FVAWT models, distribution of FA SS In comparing bending moments structural along responses blade of both FVAWT at tower models, base distribution were selected of as FA yardsticks SS bending comparison. moments along blade at tower base were selected as yardsticks comparison. 24 presents FA SS BM distributions along blade both FVAWTs under turbulent condition. The turbulent condition was used this comparison because most of cases cases discussed in in previous previous sections sections experienced experienced higher excitation higher excitation or larger or loadlarger variations load under variations turbulent under turbulent conditions. conditions. (d) Comparing Comparing distribution distribution of bending of bending moment moment along along blade blade DLC 1, 3, DLC 1, 6. 3, Mean 6. FA BM; Mean FA Stard BM; deviation Stard of deviation FA BM; of Mean FA BM; SS BM; Mean (d) Stard SS BM; deviation (d) Stard of SS deviation BM. of SS BM. The bending moment at blade roots or extremes 5 MW optimized FVAWT is expected The bending moment at blade roots or extremes 5 MW optimized FVAWT is to be larger than that of 5 MW baseline FVAWT since rotor 5 MW optimized FVAWT expected to be larger than that of 5 MW baseline FVAWT since rotor 5 MW optimized has longer blades both FVAWT models were simulated under same environmental conditions. FVAWT has longer blades both FVAWT models were simulated under same environmental However, plot of mean FA BMs showed 5 MW optimized FVAWT reached its lowest mean conditions. However, plot of mean FA BMs showed 5 MW optimized FVAWT reached its value of approximately zero, while 5 MW baseline FVAWT attained a value of about 2 knm at lowest mean value of approximately zero, while 5 MW baseline FVAWT attained a value of blade root (bottom) (point zero in plots along blade axis). However, blade 5 MW about 2 knm at blade root (bottom) (point zero in plots along blade axis). However, optimized FVAWT reached its highest bending at top node with about 2.25 knm at its rated blade 5 MW optimized FVAWT reached its highest bending at top node with about

28 Energies 2016, 9, of 36 Energies 2016, 9, of 36 speed unlike that of 5 MW baseline FVAWT that reached its highest bending at blade base knm at its rated speed unlike that of 5 MW baseline FVAWT that reached its highest At centre of blade, 5 MW baseline FVAWT experienced negligible bending both FA bending at blade base. At centre of blade, 5 MW baseline FVAWT experienced negligible SS BMs bending while both 5 MW optimized FA FVAWT SS BMs showed while significant 5 MW sagging optimized up to FVAWT 1.55 knm showed FA significant BM. sagging up to 1.55 knm FA BM. The The mean mean SS SS bending bending moment moment both both models models at at same same speed speed seemed seemed to follow to follow similar trend. similar However, trend. However, 5 MW optimized 5 MW optimized FVAWT FVAWT experienced experienced lower lower bending bending at at blade blade base base a higher a higher bending bending at at blade blade top than top than 5 MW5 MW baseline baseline FVAWT FVAWT at each at each speed. speed. The The 55 MW MW optimized FVAWT showed excessive load variation than 5 5MW MWbaseline FVAWT at at blade blade center as as shown in in plot of of stard deviation (STD) both FA FA SS SSBMs especially at at rated rated speed. Furrmore, disparity in load variation between 5 MW 5 MW baseline FVAWT 5 MW 5 MW optimized FVAWT FVAWT narrows narrows at at speeds speeds furr furr away away from from rated rated speed. speed Comparing Tower Base Base Bending Moments The The plots plots in in show show value value of of mean mean of of FA, FA, SS bending SS bending moments moments at at tower base tower with base ir with respective ir respective variation variation from from mean values mean invalues termsin ofterms maximum of maximum values values stard deviation stard (std) deviation both (std) FVAWT both models FVAWT under models turbulent under turbulent conditions. conditions Comparing tower base: FA FA BM; BM; SS SSBM. The plot of FA BM showed that as speed increases, mean values both FVAWT The plot models of increase. FA BM showed Furrmore, that as difference speed between increases, mean mean values values of both both FVAWTs FVAWT models increase increase. as Furrmore, speed increases. difference The 5 MW between baseline FVAWT mean values experienced of both higher FVAWTs FA increase bending as moment speedthan increases. 5 MW Theoptimized 5 MW baseline FVAWT. FVAWT In terms experienced of load variation, higher FA bending load variability moment(std than 5 MW max in optimized plot) FVAWT. 5 In MW terms baseline of load FVAWT variation, is higher load than variability 5 MW (stdoptimized max in FVAWT. plot) The largest 5 MWdisparity baseline in FVAWT FA isbm higher between than both FVAWT 5 MW optimized models occurred FVAWT. at The largest cut-out disparity speed in FA(25 BMm/s) between where both5 MW FVAWT baseline models FVAWT occurred experienced at cut-out more than 100% speed higher (25 m/s) mean where maximum 5 MW baseline FA BM FVAWT than experienced 5 MW optimized more than FVAWT. 100% higher mean maximum FA BM than 5 MW optimized The SS FVAWT. BM showed similar trend just as FA BM. The variations from mean values follow The same SS trend BM showed as discussed similar trend just FA asbm. FA Moreover, BM. The variations largest from disparity mean in values statistical follow parameters same trendbetween as discussed both models occurred FA BM. Moreover, at 25 m/s largest speed disparity as well. However, in statistical mean parameters SS BM between values both both models FVAWT occurred models atare 25very m/sclose. speed as well. However, mean SS BM values both FVAWT Alternatively, models arebending very close. Moment (BM) time history spectral plots can be used to compare Alternatively, tower base Bending BMs between Moment both (BM) FVAWT time history models. The environmental spectral plots condition can be DLC used 6 to compare (V = 25 m/s, tower Hs = 6.02 basem BMs between Tp = both s) based FVAWT on models. discussions The environmental in Section 3.1.3; condition turbulent DLC 6 (V effects = 25 m/s, are largest H s = 6.02 at 25 mm/s T p = speed s) The based time on series discussions spectral in Section plots of 3.1.3; FA turbulent SS effects BMs at are largest tower at base 25 m/s are as shown speed. in The time 26. series spectral plots of FA SS BMs at tower base are as shown in 26.

29 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) 26. Comparing tower base. (a,b) FA BM time series PSD respectively DLC6; (c,d) SS 26. Comparing tower base. (a,b) FA BM time series PSD respectively (d) DLC6; (c,d) SS BM BM time series PSD respectively DLC 6. time series 26. Comparing PSD respectively tower base. DLC (a,b) 6. FA BM time series PSD respectively DLC6; (c,d) SS The BM plots time series of BM PSD time respectively history DLC both 6. FA SS showed higher amplitude 5 MW baseline The plots FVAWT of than BM time history 5 MW optimized both FVAWT. FA SS This showed furr higher explained amplitude excitations at 5 MW baseline wave FVAWT The plots excitation than of frequency BM time 5 MW history optimized both 2P frequency FVAWT. FA SS experienced This showed furr higher by explained amplitude 5 MW baseline excitations 5 MW FVAWT atis baseline FVAWT than 5 MW optimized FVAWT. This furr explained excitations at wave higher excitation than that frequency experienced by 2P5 frequency MW optimized experienced FVAWT. by This implies 5 MW baseline 2P effect FVAWT at is tower higher wave excitation frequency 2P frequency experienced by 5 MW baseline FVAWT is than base that is experienced furr reduced by by 5 MW application optimized of FVAWT. optimized This5 implies MW DeepWind 2P effect rotor. at tower base is higher than that experienced by 5 MW optimized FVAWT. This implies 2P effect at tower furr reduced The plots by of bending application moments of at optimized blade root 5 MW (bottom) DeepWind rotor. 5 MW baseline FVAWT base is furr reduced by application of optimized 5 MW DeepWind rotor. The 5 MW plots optimized of bending FVAWT moments respectively at blade are represented root (bottom) in 27. The plots of bending moments at blade root (bottom) 55 MW MW baseline FVAWT 5 MW optimized 5 MW optimized FVAWT FVAWT respectively respectively are represented are represented in in Blade root (bottom): FA BM; SS BM. 27. Blade root (bottom): FA BM; SS BM. 27. Blade root (bottom): FA BM; SS BM.

30 Energies Energies 2016, 2016, 9, , of 3036 of shows that mean FA mean SS BMs at blade root (bottom) under both FVAWTs 27 model shows remained that fairly mean constant FA as mean SS speed BMs increases. at blade Furrmore, root (bottom) mean under FA both FVAWTs BM model 5 MW remained baseline fairly FVAWT constant is twice as as large speed as that increases. 5 MW Furrmore, optimized FVAWT meanat FAall BM 5speeds. MW baseline However, FVAWT 5 MW is twice optimized as large FVAWT as thatexperienced 5 MW higher optimized load variation FVAWTin at terms all of speeds. load However, maximum values 5 MW optimized stard FVAWTdeviation experienced than higher 5 MW loadbaseline variation FVAWT. in termshowever, of load maximum difference valuesin load variability stard deviation between thanfvawts 5 MWnarrows baseline at FVAWT. speeds However, above difference rated in load variability speed. between FVAWTs narrows at speeds above rated speed Comparing Mooring Lines Tension compares load variability mean load on mooring lines between 5 MW 5 MW baseline FVAWT 5 5 MW optimized FVAWT Mooring lines tension: Line Line1; 1; Line Line2; 2; Line Line3. 3. The mean load on mooring line 1 2 indicates a higher load 5 MW baseline FVAWT than The mean 5 load MW on optimized mooring FVAWT line 1 while 2 indicates opposite a higher is load case mooring 5 MW line baseline 3. As FVAWT discussed than earlier, 5 MW loads optimized in FVAWT three mooring while lines opposite interact. istheree, case it is mooring difficult line to conclude 3. As discussed which of earlier, FVAWTs loads in permed three mooring better lines terms interact. of mooring Theree, line ittension. is difficult However, to conclude if which difference of in FVAWTs permed magnitude better of inmean termsloads of mooring of all mooring line tension. lines However, respective if difference FVAWT are insummed magnitude at each of mean speed, loads of5 all MW mooring optimized lines FVAWT could respective be said FVAWT to have are experienced summeda at slightly each lower speed, tension 5 MW than optimized 5 MW baseline FVAWTFVAWT. could be said to have experienced a slightly lower tension than 5 MW baseline In FVAWT. terms of load variability, error bars 5 MW baseline FVAWT seems to be longer, hence In terms tension of load in all variability, mooring lines error varies bars higher in 5 5 MW baseline FVAWT seems than in to be5 MW longer, hence optimized tension FVAWT. in all mooring lines varies higher in 5 MW baseline FVAWT than in 5 MW optimized The FVAWT. time series plot DLC 6 in 29 showed that both mean value amplitudes The time of series mooring plot line DLC 2 tension 6 in is higher 29 showed that 5 MW both baseline mean FVAWT valuethan amplitudes 5 MW of optimized mooring line FVAWT. 2 tension The isspectral higher plot illustrated 5 MW baseline a higher FVAWT excitation than at wave 5 MW excitation optimized frequency FVAWT. The spectral plot illustrated a higher excitation at wave excitation frequency 5 MW baseline

31 Energies 2016, 9, of 36 Energies 2016, 9, of 36 5 MW baseline FVAWT than 5 MW optimized FVAWT. This means mooring lines experienced FVAWT thanhigher xcitation 5 MW optimized due to wave FVAWT. loads This means 5 MW baseline mooring FVAWT lines experienced than higher 5 MW Energies 2016, 9, of 36 optimized excitation due FVAWT. to wave Furrmore, loads 5response MW baseline at FVAWT 2P frequency than 5 MW optimized FVAWT. died Furrmore, out. This 5 MW furr baseline response proved FVAWT at 2P 5 than MW frequency optimized 5 MW FVAWT optimized 5 MW experienced optimized FVAWT. This FVAWT a reduced means died 2P out. mooring effect This than lines furr 5 proved MW experienced baseline 5 MWFVAWT. higher optimized excitation FVAWT due to experienced wave loads a reduced 5 MW 2P effect baseline than FVAWT 5 MW than baseline 5 FVAWT. MW optimized FVAWT. Furrmore, response at 2P frequency 5 MW optimized FVAWT died out. This furr proved 5 MW optimized FVAWT experienced a reduced 2P effect than 5 MW baseline FVAWT Mooring Mooring lines tension DLC6: Time series; Power spectral density. lines 2 tension DLC6: Time series; Power spectral density Comparing Platm 29. Mooring Global lines Motion 2 tension DLC6: Time series; Power spectral density Comparing Platm Global Motion The roll pitch motion of DLC (V = 14 m/s, Hs = 3.62 m Tp = s) has been selected as The roll Comparing pitch Platm motion Global of DLC Motion 3 (V = 14 m/s, H s = 3.62 m T p = s) has been selected parameters comparing global motion of both models because it is interesting to investigate as parameters The roll comparing pitch motion global of DLC motion 3 (V of = 14 both m/s, models Hs = 3.62 because m Tp it is = interesting s) has been to investigate selected as permance of both FVAWT model under rated speed. The roll pitch motions were permance parameters of both comparing FVAWTglobal model motion underof both rated models because speed. it The is interesting roll pitch to investigate motions were selected based on ir importance in turbine safety. selected permance based onof ir both importance FVAWT model in under turbine rated safety. speed. The roll pitch motions were selected The plots roll pitch motion time history in 30a,c, respectively, showed a The plots based on ir rollimportance pitch in motion turbine time safety. history in 30a,c, respectively, showed lower motion amplitude 5 MW optimized FVAWT than 5 MW baseline FVAWT. a lower The motion plots amplitude roll 5pitch MWmotion optimized time history FVAWT in than 30a,c, respectively, 5 MW baseline showed FVAWT. a Furrmore, last two peaks on spectral plots represent intensity of 2P effects both Furrmore, lower motion amplitude last two peaks on 5 MW spectral optimized plots FVAWT represent than intensity 5 MW of baseline 2P FVAWT. effects FVAWT Furrmore, models. In last terms two of peaks wave on excitation, spectral plots 5 represent MW optimized intensity FVAWT, of 2P effects spectral plots both in both FVAWT models. In terms of wave excitation, 5 MW optimized FVAWT, spectral plots FVAWT 30b,d models. demonstrated In terms a of lower wave excitation, with 5 higher MW optimized pitch FVAWT, roll natural spectral frequency plots when in in 30b,d demonstrated lower excitation with higher pitch roll natural frequency when compared with 30b,d demonstrated MW baseline a lower FVAWT excitation as indicated with higher by pitch first two roll peaks. natural frequency when compared with 5 MW baseline FVAWT as indicated by first two peaks. compared The peaks with at 2P 5 MW frequency baseline FVAWT both roll as indicated pitch by motion first two peaks. 5 MW optimized FVAWT The peaks at 2P frequency both roll pitch motion 5 MW optimized FVAWT (2P frequency The peaks 1.24 at rad/s) 2P frequency is observed to both be roll lower than pitch motion peaks 5 MW 5 MW optimized baseline FVAWT FVAWT (2P frequency = 1.24 rad/s) is observed to than peaks 5 MW baseline FVAWT (2P frequency (2P frequency 1.1 = rad/s) rad/s) This is demonstrates observed to be that lower than 5 MW optimized peaks FVAWT 5 MW experienced baseline FVAWT lower (2P frequency (2P frequency = 1.1 = 1.1 rad/s). rad/s). This This demonstrates that 5 MW optimized FVAWT FVAWTexperienced a lower a lower excitation at 2P frequency than MW baseline FVAWT. excitation excitation at 2P 2P frequency than than 55 MW baseline FVAWT. 30. Cont.

32 Energies 2016, 9, of 36 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) 30. (a,b) Comparing time series power spectrum of roll motion DLC3; (c,d) 30. (a,b) Comparing time series power spectrum of roll motion DLC3; (c,d) Comparing Comparing time series power spectrum of pitch motion DLC3. time series power spectrum of pitch motion DLC3. (d) Comparing 30. (a,b) Short Comparing Term Fatigue time Damage series Equivalent power spectrum Loads of roll motion DLC3; (c,d) Comparing Short time series Term Fatigue power spectrum Damageof Equivalent pitch motion Loads DLC3. In this subsection, STFDELs at some selected positions were compared between 5 MW optimized In this subsection, FVAWT STFDELs 5 MW baseline at some FVAWT. selected The positions tower were base compared blade between root (bottom) 5 MW Comparing Short Term Fatigue Damage Equivalent Loads optimized positions FVAWT proved to have higher 5 MWSTFDELs baselinevalues FVAWT. than The or tower points base selected fatigue blade root analysis (bottom) in positionsin previous proved this subsection, sections. to have Theree, higher STFDELs STFDELs at some two points selected were thanpositions selected or points were compared STFDEL selected comparison between between analysis 5 MW both in optimized FVAWT 5 MW baseline FVAWT. The tower base blade root (bottom) models previous under sections. turbulent Theree, condition. two points Furrmore, were selected fatigue damage STFDELdue comparison to tension in between mooring both positions proved to have higher STFDELs values than or points selected fatigue analysis in models line 1 under was selected turbulent comparison condition. as well. Furrmore, fatigue damage due to tension in mooring previous sections. Theree, two points were selected STFDEL comparison between both line 1 was The selected plots in comparison 31a d compare as well. STFDELs tower base, blade root (bottom), models under turbulent condition. Furrmore, fatigue damage due to tension in mooring mooring line 1 mooring line 2 respectively. The description of legends as used in plots line The1 plots was selected in comparison 31a d compare as well. STFDELs tower base, blade root (bottom), are mooring explained in Abbreviations list. In terms of STFDELs due to FA bending moment in The plots line in 1 31a d mooring compare line 2 respectively. STFDELs The tower description base, blade of legends root (bottom), as used in plots 31a, 5 MW baseline FVAWT experienced higher at all speeds than 5 mooring are explained line 1 in Abbreviations mooring line 2 respectively. list. In termsthe of description STFDELs of legends due to FA as used bending moment plots in MW are 31a, explained optimized 5 MW in FVAWT baseline Abbreviations at FVAWT tower list. experienced base. In terms However, of higher it experienced STFDELs due at a all lower to FA value bending speeds than moment than 5 MW in 5 MW optimized FVAWT at blade root (bottom) as shown in 31b. 31b indicates higher optimized FVAWT 31a, 5 atmw tower baseline base. FVAWT However, experienced it experienced higher STFDELs a lowerat value all thanspeeds 5 MW than optimized 5 STFDELs closer to rated speeds. This showed, blade of 5 MW optimized FVAWT FVAWT MW at optimized bladefvawt root (bottom) at tower as shown base. inhowever, 31b. it experienced 31ba indicates lower value higher than STFDELs 5 MW closer experienced lower damaging effects due to bending loads at speeds farr away from to optimized rated FVAWT speeds. at This blade showed, root (bottom) blade as of shown 5in MW optimized 31b. FVAWT 31b indicates experienced higher lower rated STFDELs closer speeds. to In terms rated of STFDELs speeds. due This to showed, mooring lines blade tension of in 5 MW optimized 31c,d, an increasing FVAWT damaging disparity effects is observed due to between bending STFDELs loads at 5 speeds MW baseline farr FVAWT away from 5 rated MW optimized speeds. experienced lower damaging effects due to bending loads at speeds farr away from In terms FVAWT of STFDELs as due speed to mooring increases linesfrom tension cut-in. Furrmore, 31c,d, an increasing 5 MW baseline disparityfvawt s observed rated speeds. In terms of STFDELs due to mooring lines tension in 31c,d, an increasing between STFDELs STFDELs doubled that of 5 5 MW MW baseline optimized FVAWT FVAWT at 5cut-out MW optimized speed FVAWT of 25 m/s as disparity is observed between STFDELs 5 MW baseline FVAWT 5 MW optimized speed tower FVAWT increases base bending from cut-in. moments, speed Furrmore, increases mooring lines from 1 5cut-in. MW2 baseline loads. Furrmore, FVAWT s 5 STFDELs MW baseline doubled FVAWT s that of 5 MWSTFDELs optimized doubled FVAWT that of at 5 cut-out MW optimized speed FVAWT of 25 at m/s cut-out tower speed baseof bending 25 m/s moments, mooring tower lines base 1bending 2 loads. moments, mooring lines 1 2 loads. 31. Cont.

33 Energies 2016, 9, of 36 Energies 2016, 9, of 36 (d) 31. Fatigue STFDEL of selected areas: Tower base; Blade root (bottom); Mooring 31. Fatigue STFDEL of selected areas: Tower base; Blade root (bottom); Mooring Line 1; (d) Mooring Line 2. Line 1; (d) Mooring Line Conclusions 4. Conclusions This work centers on structural dynamic analysis of FVAWTs with major contribution on Thisdistribution work centers of bending on structural loads along dynamic blade analysis fatigue of FVAWTs damage on with flexible major components contribution which on distribution has not been of bending fully explored loads along rotor blade in fatigue literature. damage Furrmore, on flexible components work is extended which to has not been compare fully explored permance of two rotor FVAWTs blade inmodels, literature. 5 MW Furrmore, optimised FVAWT work with is extended 5 MW to compare baseline permance FVAWT, in terms of two of FVAWTs power production, models, dynamic 5 MW optimised structural FVAWT response, with global motion 5 MW baseline FVAWT, short-term in terms fatigue of power damage production, of structural dynamic components structural to scrutinize response, global advantages motionof short-term 5 MW DeepWind rotor. fatigue damage of structural components to scrutinize advantages of 5 MW DeepWind rotor. The dynamic response characteristics of FVAWTs were comprehensively using a coupled The dynamic response characteristics of FVAWTs were comprehensively using a coupled non-linear aero-hydro-servo-elastic code ( Simo-Riflex-DMS code), developed by Wang non-linear aero-hydro-servo-elastic code ( Simo-Riflex-DMS code), developed by Wang modeling modeling FVAWTs. This coupled code incorporates models turbulent field, FVAWTs. aerodynamics, This coupled hydrodynamics, code incorporates structural dynamics, models controller turbulent dynamics. The field, simulations aerodynamics, were hydrodynamics, permed in structural a fully coupled dynamics, manner in time controller domain. dynamics. The simulations were permed in a fully coupled The platm mannerglobal in time motions domain. structural responses of flexible components of FVAWTs The platm were global studied motions in a systematic manner structural under responses normal operating of flexible condition. components Furrmore, of FVAWTs werefatigue studiedanalysis in a systematic of flexible manner components under such normal as blades, operating tower condition. mooring Furrmore, lines were permed fatigue analysis based ofon flexible time components history of calculated such as blades, responses tower considering mooring continuously lines were varying permed aerodynamic based on time loads history on of calculated rotor which responses leads to higher considering load level of continuously fatigue loads varying aerodynamic number of load loads cycles. on The rainflow counting technique is used short-term fatigue cycle counting. The Mlife tool from rotor which leads to higher load level of fatigue loads number of load cycles. The rainflow NREL is used to calculate short-term fatigue damage equivalent loads FVAWT. counting technique is used short-term fatigue cycle counting. The Mlife tool from NREL is used to The two FVAWT concepts were evaluated under same environmental conditions. The calculate short-term fatigue damage equivalent loads FVAWT. stochastic response characteristics of both FVAWTs were studied under steady turbulent The two conditions FVAWT based concepts statistical were evaluated analysis under spectral same analysis environmental of ir responses conditions. to The stochastic explore response effects of characteristics turbulence. The of both results FVAWTs identified were that studied effect under of steady turbulence on turbulent both FVAWTs conditions resulted basedin onan increased statistical power analysis generation, higher spectral power analysis variation, of ir higher responses excitation to explore effects low frequency of turbulence. motions, The greater results fatigue identified damage thatbut a effect reduction of turbulence in 2P effects. on However, both FVAWTs effect resulted in anof increased turbulence power is negligible generation, higher mean power bending variation, moment higher at blade excitation extremes low but frequency leads to higher motions, greater loads fatigue level damage in mooring butlines a reduction at in tower 2Pbase effects. especially However, at effect speeds ofabove turbulence rated is negligible on speed. meanfurrmore, bending moment results at of blade fatigue extremes analysis but showed leads to that higher at loads speeds level farr in mooring from lines rated speed (14 m/s), internal loads could have lower damaging effects than at speeds at tower base especially at speeds above rated speed. Furrmore, results closer to rated speed 5 MW optimised FVAWT. of fatigue analysis showed that at speeds farr from rated speed (14 m/s), Finally, a comparison of dynamic responses between 5 MW baseline FVAWT 5 internal loads could have lower damaging effects than at speeds closer to rated speed MW optimised FVAWT identified 5 MW optimised FVAWT to have lower Fore-Aft (FA) bending 5 MWmoments optimised but FVAWT. higher lower Side-Side (SS) bending moment in flexible components, lower motions Finally, aamplitude, comparison lower of dynamic short-term responses fatigue damage between equivalent 5 MWloads baseline a FVAWT furr reduced 2P 5 MW optimised effects FVAWT under turbulent identified condition. 5 MW optimised FVAWT to have lower Fore-Aft (FA) bending moments but higher lower Side-Side (SS) bending moment in flexible components, lower motions amplitude, lower short-term fatigue damage equivalent loads a furr reduced 2P effects under turbulent condition.

34 Energies 2016, 9, of 36 Author Contributions: Kai Wang Muk Chen Ong conceived designed numerical study; Kai Wang Jeremiah Ishie contributed analysis tools; Jeremiah Ishie permed numerical simulations under supervision of Kai Wang Muk Chen Ong; Jeremiah Ishie analyzed data with comments from Kai Wang Muk Chen Ong; Jeremiah Ishie wrote paper under supervision of Kai Wang Muk Chen Ong; Kai Wang Muk Chen Ong gave comments modified paper. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations FA BM SS BM STFDEL FA BM steady FA BM turbulent SS BM steady SS BM turbulent steady turbulent Optimized FA BM Baseline FA BM Optimized SS BM Baseline SS BM Optimized Baseline Fore-Aft bending moment (bending parallel to direction) Side-side bending moment (along a direction at 90 to direction) Short-Term Fatigue Damage Equivalent Load STFDEL due to FA BM under steady condition STFDEL due to FA BM under turbulence condition STFDEL due to SS BM under steady condition STFDEL due to SS BM under turbulence condition STFDEL due to axial ce or tension under steady condition STFDEL due to axial ce or tension under turbulent condition STFDEL due to FA BM 5 MW optimized FVAWT STFDEL due to FA BM 5 MW baseline FVAWT STFDEL due to SS BM 5 MW optimized FVAWT STFDEL due to SS BM 5 MW baseline FVAWT STFDEL due to axial ce 5 MW optimized FVAWT STFDEL due to axial ce 5 MW baseline FVAWT References 1. Paquette, J.; Barone, M.; Paquette, J.; Barone, M. Innovative offshore vertical-axis turbine rotor project. In Proceedings of EWEA 2012 Annual Event, Copenhagen, Denmark, April Wang, K.; Luan, C.; Moan, T.; Hansen, M.O.L. Comparative Study of a FVAWT a FHAWT with a Semi-submersible Floater. In Proceedings of Twenty-fourth International Offshore Polar Engineering Conference, Busan, Korea, June Anagnostopoulou, C.; Kagemoto, H.; Sao, K.; Mizuno, A. Concept design dynamic analyses of a floating vertical-axis turbine: Case study of power supply to offshore Greek isls. Ocean Eng. Mar. Energy 2015, 2, [CrossRef] 4. Wang, K.; Moan, T.; Hansen, M.O.L. Stochastic dynamic response analysis of a floating vertical-axis turbine with a semi-submersible floater. Wind Energy 2016, 19, [CrossRef] 5. Jiang, Z.; Karimirad, M.; Moan, T. Dynamic response analysis of turbines under blade pitch system fault, grid loss, shutdown events. Wind Energy 2014, 17, [CrossRef] 6. Karimirad, M.; Moan, T. Stochastic dynamic response analysis of a tension leg spar-type offshore turbine. Wind Energy 2013, 16, [CrossRef] 7. Kvittem, M.I.; Bachynski, E.E.; Moan, T. Effects of Hydrodynamic Modelling in Fully Coupled Simulations of a Semi-submersible Wind Turbine. Energy Procedia 2012, 24, [CrossRef] 8. Kvittem, M.I.; Moan, T. Effect of Mooring Line Modelling on Motions Structural Fatigue Damage a Semisubmersible Wind Turbine. In Proceedings of Twenty-second International Offshore Polar Engineering Conference, Rhodes, Greece, June Kvittem, M.I.; Moan, T. Frequency versus time domain fatigue analysis of a semi-submersible turbine tower. In Proceedings of ASME 33rd International Conference on Ocean, Offshore Arctic Engineering, San Francisco, CA, USA, 8 13 June Luan, C.; Gao, Z.; Moan, T. Modelling analysis of a semi-submersible turbine with a central tower with emphasis on brace system. In Proceedings of 32nd International Conference on Ocean, Offshore Arctic Engineering, Nantes, France, 9 14 June Karimirad, M.; Moan, T. Wave- Wind-Induced Dynamic Response of a Spar-Type Offshore Wind Turbine. J. Waterw. Port Coast. Ocean Eng. 2012, 138, [CrossRef] 12. Bachynski, E.E.; Moan, T. Design considerations tension leg platm turbines. Mar. Struct. 2012, 29, [CrossRef]

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