ABSTRACTWind turbinesare complex engineering systems, subjected to high fluctuations and irregularloading.

These mechanical systems are located in high demanding environmentwhich poses challenges in design of complete structure, including substructureand foundation. This paper reviews the fundamental aspects and major issuesrelated to the modern designs in practice now a days. The optimal design ofwind turbines, particularly its foundation structure is worthwhile. Due tolimited guidelines for design and analysis of foundations; insufficientstrategies and alternative techniques; and high construction and maintenancecost, offshore wind energy structures require more advance engineeringtechniques than for onshore power turbines. Different advance design approachesare discussed in terms of loading criteria, natural frequency and powergeneration. A review of design safety considerations and structural reliabilitystudies are mentioned.

The ultimate challenges and possible approaches arehighlighted. Some new recommendations are given for future work in this highrelevant field of research. Keywords: offshore wind turbines, structural analysis, design,components1.    Introduction:The windindustry is thriving worldwide, both offshore and onshore, for energyproduction as it requires a natural force (wind) for power generation.

Windturbines are categorized by axis of rotation of the main rotor shaft (eitherhorizontal or vertical axis) and whether they are located onshore or offshore(Tong, 2010). Due to less land availability, the offshore wind turbines sitesare of key interest to many countries. In the United States, roughly 50% oftotal population lives in coastal areas to include counties directly onshoreline and counties that drain to coastal watersheds. In 2016, 361 offshorewind turbines (OWTs) of an average capacity rating of 4.8 megawatts (MW) perturbine were constructed in Europe (Pineda and Tardieu, 2016). Abundantoffshore energy resources have the potential to supply immense amount ofrenewable energy to coastal areas.OWTs operate atthe same basic principle, wind blows and flows over the air foil blades of windturbines, causing the blades to spin.

The blades are connected to a drive shaftthat turns an electric generator to produce electricity.  Besides being characterized by reduced visualimpact since they are placed far from coast, OWT can take advantage of highvelocity wind with intensive force, which could increase the regularity andamount of power generated by each machine. From the general point of view, OWTis formed by mechanical and structural elements. As a consequences, it is not acommon “civil engineering structure”; it behaves differently according to itsvarious functional phases like (idle, power production etc), and it issubjected to highly variable loads like winds, waves, sea currents etc. Now adays, development of newer turbine and foundation technologies will allow windturbines to be built further offshore in deep waters.

For moderncommercial wind turbines, the main rotor shaft is aligned horizontally. Ratedpower generation capacity is largely dependent on rotor diameter and wind speed(IRENA, 2012); e.g.

, if wind speedincreases two fold, its energy content increases eight fold. Two key speedterms are ‘cut-in speed’ at which the wind turbine begins to produce power, and’cut-out speed’ at which the turbine must be shut down to protect the rotor anddrive train machinery from damage (Sørensen et al., 2009; Tong, 2010).In order togenerate more electricity, modern offshore wind turbines are built with largerotor diameter and at greater water depth, which significantly increases the costof an offshore project. A recent trend, however is the return of developmentinterest to new production lines for the size ranges most relevant to landbased turbines, from 800kW up to about 3MW. Of the other main components,larger rotor diameters have been introduced in order to enhance exploitation oflow wind speed sites. Reinforced structures, relatively short towers andsmaller rotor diameters in relation to rated power are employed on extremelyhigh wind speed sites.

Between 2000 and2011, global wind-power capacity approximately doubled every 3 years, with anestimated total power generation of 238 GW achieved by the end of 2011; China, theUSA and Germany are the top industry players (GWEC, 2011). Although the marketis still dominated by onshore, with significant onshore wind resources yet tobe explored, the offshore wind market is growing rapidly. Global totalinstalled capacity for offshore of 3.12 GW was generated by the end of 139 2010,with 1.16 GW added in 2010 alone – a 59.4% increase on the previous year (WWEA,2011). Total offshore wind-power capacity in Europe reached 2.90 GW by the endof 2010, with 0.

88 GW added in 2010; again this represents a significant increaseof 43.6% on the previous year. This occurred at the same time as onshorenew-capacity additions declined by 13% (WWEA, 2011). The size of offshore windfarms is also increasing, with 2010 data indicating that the average size of anoffshore wind farm in terms of power output was 155 MW – more than double theaverage wind farm size of 72 MW for 2009 (EWEA, 2011). Preliminary datafor 2011 suggest that offshore wind power capacity in Europe increased by 0.86GW (EWEA, 2012), with the offshore market likely to be driven by mainly the UKand Germany, although France and Sweden also have significant projects imminent.As the end of 2015, a cumulative total of at least 990000 wind turbines wereinstalled all over the world.

This is an increase of 5% (8.3% in 2014) comparedto previous year, when 945000 units were registered. The recorded small windcapacity installed worldwide has reached more than 945MW at the end of 2015.This is growth of 14% compared with 2014, when 830MW were registered.

(WWEA,2015) In 2016, 21GW of new installations took placeworldwide. Wind capacity has reached 456GW, where Germany, India and Brazilleading in market growth (WWEA, 2016). Interest in offshore wind power is alsoincreasing in other regions of the world, with, for example, China, the USA andSouth Korea planning to generate 6.0 and 3.

0, 2.5 GW, respectively, by 2020.Building on this, China and the USA have ambitious plans to generate 65 and 54GW, respectively, from offshore wind by 2030 (AWEA, 2012; Musial and Bonnie,2010). A significant hurdle for the offshore market,however, is the high initial capital investment costs of the project, which is relatedto: inadequate and (or) potentially unreliable design guidelines for offshorewind-turbine (OWT) installations, especially foundation structures; morestringent requirements for durable construction materials to withstand theharsh marine environment; high-tech equipment requirements for on-siteoperation and also shortage of trained manpower (Musial and Bonnie, 2010). 2.    Design considerations of modern offshore windturbines The main components of typical wind turbines (figure1) include foundation,support structure, tower, rotor blades and nacelle.

The foundation system andsupport structure, used to keep the turbine in its proper position while beingexposed to the forces of nature such as wind and sea waves, are now madestronger using materials such as reinforced concrete or steel. Supportstructures connect the transition piece or tower to foundation at sea bedlevel. The tower also provide a means to correct any misalignment of the foundationthat may have occurred during installation.Electric current generated throughwind power is converted to higher voltage via a transformer at the base of thetower. The power that can be harnessed from the wind is proportional to thecube of wind speed up to a theoretical maximum of about 59 percent. However,today’s wind turbines convert only a fraction of the available wind power toelectricity and are shutdown beyond a certain wind speed due to structurallimitations and concern for wear and tear (Malhotra, 2011).

Modern OWTs areinstalled with either pitch regulated blades or variable rotational speedsystems in order to allow optimization of power production over a wide range ofprevailing wind speeds. The rotational speed of main rotor shaft is typically10 to 20rpm (Alderlieste, 2010; Malhotra, 2011). The optimum tip speed depends on thenumber of blades and profile type used.Figure2: Effect of number of bladeson power performance (Burton, 2001) The fewer thenumber of blades, the faster the rotor needs to turn to extract maximum powerfrom the wind. Three bladed rotors have a higher achievable performancecoefficient which does not necessary mean that they are optimum. Two bladedrotors might be suitable alternative because although the maximum capacity is alittle lower, the width of peak is higher and that might result in largerenergy capture.

Reducing the number of blades reduces the weight of the rotorand subsequently the weight of the support structure. In addition it shortensthe time required for transportation and installation which directly decreasethe cost of energy.2.1      FOUNDATION SYSTEMS AND SUPPORT STRUCTURESThe offshore bottom mounted supportstructures are classified according to three basic properties that are:installation principle, structural configuration and foundation type (Fergusonet al.

1998). In this paper, classification based on water depth that eachconcept can be used economically, gravity base, monopile, tripod and floatingsupport structures are reviewed. Table 1: Concept for supportstructure (Ashuri and Zaaijer, 2007) Support structure concept Water depth (m) Gravity Base 0-10 Monopile 0-30 Tripod >20 Floating >50  2.1.1       Gravity base structures (GBS)Fromstructural point of view, a GBS is a monotower that is fixed at the top of agravity base foundation (figure 3).

The foundation has a flat base to resistoverturning loads imposed by the wind and wave, and a conical part at the watersurface level to break the ice and reduce ice load by causing the ice sheets tobend downwards and break-up as they contact the conical section (Zaaijer,2003). Figure3: A tupical Gravity basedsupport structure (Ashuri and Zaaijer, 2007)In order to keepthe attachment between the GBS and sea bed, ballasts are laid on the flat base.In this way, the foundation always remains in compression under allenvironmental conditions and cannot be detached from seabed.2.1.2         MonopileMonopile supportstructure consists of a steel pipe as a foundation which is driven or drilledin to the soil.

The monopile is equipped with a transition piece to absorbtolerances on the inclination of the monopile and to reduce the assembling timerequired at sea and the tower which is mounted offshore on the top of thetransition piece.The steel pipetransfers all the loads by means of vertical and lateral earth pressure to theground. Therefore, both uncertainties in the ground properties and scour holescan lead to a structure with different frequency than designed for. Designing amonopile support structure is a challenging task.Figure4: A monopile support structure(Ashuri and Zaaijer, 2007)2.1.3        TripodThe tripodconsists of a central steel shaft and three cylindrical steel tubes with drivensteel piles. The central part distributes the loads to the cylindrical tubesand acts as a transition piece for tower.

The cylindrical tubes give additionalstiffness and strength and increase the capacity of the structure to supportadditional overturning moments (Zaaijer, 2003).The foundationhas the advantage that it requires less protection against scour than themonopile, which generally has to be protected against scour in sandy sea beds. Figure5: A typical tripod structure (Ashuriand Zaaijer, 2007)2.1.4        FloatingCurrent fixed bottom technology has seen limited deployment to waterdepths of around 30-m thus far. A floating support structure increases theflexibility in locating the turbine in water depths of up to 200 meters and iswell known from oil and gas industry. The floating support structure consistsof a floating platform and a platform anchoring system.

The platform has atransition piece to install the tower on top of that. The platform can have several topologies such as single and multipleturbine floaters. The anchoring system fixes the platform and can be gravitybase drag embedded driven pile.suction anchor type (Musial, Butterfield andBoone, 2006)Figure 6: Candidate Floating supportstructure  (Ashuri and Zaaijer, 2007)3.    Environmental loading on offshore wind turbinesThe relationshipbetween the environment and offshore wind turbines is rather unique: offshorewind turbines are especially designed to catch as much wind load as possible.However, the economical aspect of wind loads on structural design must bestrictly separated.

The calculations for a successful project are critical andrely heavily on the quality of environmental data available. It is observed thatenvironmental loading is composed of a mean or slowly variable part and astochastic part. In the case of the aerodynamic and hydrodynamicactions, the first component is generated by the mean wind velocity and by thesea current, while the stochastic component is generated by the turbulence windvelocity and by the non-rotational (exception made for breaking waves) waves(Francesco, Hui and Franco, 2010). Aerodynamic loading resultsfrom interaction of the rotor and parts of the tower with the turbulent windfield, with generated wind power directly proportional to the cube power ofmean wind speed. However aerodynamic loading conditions for offshore andonshore scenarios are markedly different, with considerably lower fluctuationin loading experienced by the former on account of free-flow conditions andlower surface roughness, although advantages of reduced dynamic loading arepartly undone by higher mean wind speeds (Fischer, 2011). Since wind is the primaryenergy source for ocean waves, higher wind speed may produce marginal increasesin turbulence on account of ensuing increases in roughness of the ocean surface(Letchford andZachry,  2009).

Another aspect of fluctuating wind speed is turbulenceinduced in wake conditions. Ambient non-obstructed turbulence is the ‘normal’turbulence that would be experienced by a single stand-alone turbine at aparticular site (Frandsen and Thøgersen, 1999).Figure 7: wind and wave actionconfigurations (Francesco, Hui and Franco, 2010) 4.    Failure analysis of wind turbine componentsThere is alimited literature on failure analysis of wind turbines before 1990s. Veersnoticed the random and uncertain parameters involved in the component design ofwind turbines and first performed reliability analysis for a vertical axis windturbine blade. Around the 2000s, Ronald et al. applied reliability methods toanalyses of rotor blades of horizontal axis wind turbines.

Afterwards, moreprobabilistic models of wind turbine structural components were proposed, andadvanced wind turbine simulation tools came to use. In the past few years,interesting studies of reliability and failure analysis of different assembliesof wind turbines have been carried out. Thus we classify the survey intofollowing categories: rotor blades, bottom fixed spport structures, floatingsystems and mechanical and electrical components. Table 2summarizes representative failure modes of these components. The failure modesshould be interpreted in a broader sense. For example, large deformation ofblades does not necessarily cause damage on blade itself, but an interferencewith the tower should be deemed unacceptable. Suitable reliability methodsshould be used to avoid these failures.

Table 2: List of failure modes ofwind turbine components Category Component Failure Modes References Rotor Blades blade excessive bending stress, fatigue, buckling, large deformation (Veers,1990) and (Ronald and Larsen, 2000) Bottom-fixed support structures tower excessive deformation, fatigue, yielding and plastic collapse (Jin, Ju and Zhang, 2016) and (Philipidis and Bacharoudis, 2013) grouted connection loss of bearing capacity, soil failure Lee and Choi et al., 2014 gravity based foundation loss of bearing capacity, soil failure Vahdatirad et al., 2014 tubular structure fatigue, large displacement (Dong and Moan, 2012) and (Wei, Arwade and Myers, 2014) Mechanical components shaft fatigue Tarp, 2003 gear contact fatigue, bending fatigue (Dong and Moan, 2014) and (BSI, 2006) bearing rolling contact fatigue, white etching crack, skidding (Musial, Butterfield and McNiff, 2007) and (Jiang, Xing, Guo, Moan and Gao, 2015) Electrical components solder elements creep and fatigue, bond wire lift-off (Kostandyan and Sorenson, 2011) and (Blaabjerg, Ma and Zhou, 2012) Floating system mooring lines extreme load and line breakage (Wandji, Natarajan, Dimitrov, 2016) and (Hallowell et al., 2017)  Numerousreliability methods are suggested for each components such as response surfacemethodology (RSM), incremental wind-wave analysis (IWWA) and peak responsefactor (PRF) etc. (Ronal and Larsen, 2000), (Veers, 1990), (Teixeira, Connorand Nogal, 2016).

 5.    considerbale factors in designing owtThis paper also reviews the mainfactors on which modern designs of offshore wind turbines are based whichincludes, –       Capacity Factor-       Reliability-       Challenges in Offshore Production2.2  Capacity FactorThe previousfocus of the industry was increasing the total nameplate capacity of windturbines, the focus has now shifted to the capacity factor of the turbine,which helps keeps energy cost low by providing the most possible power.Keith Longtinsaid that about 10 years ago, the capacity factor of typical turbine was 25percent, today it is over 50 percent. This improvement in capacity factor alsoimproves the cost of energy which enables to go into more and more locationswhere wind is lower.One of thedeciding forces so far to increase capacity factor has been an increase in sizeof the rotors used on wind turbines. In U.S, a turbine with 1.

6MW capacitycomes with a 100 meter rotor, compared to 70 meter rotor in the past.Increasing the size of rotors creates new challenges for manufacturers, howeverrotors scale poorly with size, as a result the cost goes up faster than therevenue generated by the increased capacity factor.Turbine rotorsare affected by two different forces, torque and thrust.

Torque turns therotors and create energy while thrust pushes against the turbine. Dealing withthrust can be difficult when designing a rotor. There are tremendous loads ofup there, and it goes to great engineering technology to be able to createthese very reliable turbines.Astlom WindNorth America took its eco100, a 3MW turbine with a 100 meter rotor andupgraded it to 110 meter rotor in 2010. Last year, the company increased thatto 122 meter. The director of innovation for this company said that increasingnearly 40 percent the area of rotor will deliver more efficient wind turbine tothe costumers because this will tend to produce more energy at lower windspeed.

2.3  RELIABILITYWhile the focuson increasing the power produced from wind turbines may be on capacity factor,another way is to make sure wind turbines are operational and available.According to Keith Longtin, the availability of wind turbines 10 years ago wasabout 80-85 percent and the wind industry was doing fine with that because inpast it was about 70 percent. But for running steam, gas and nuclear plants, 98percent availability is required. Lot of investment is required to improve theoverall availability of a wind turbine and now it has increased to around 98percent.

To help achievethis kind of reliability and to continue improving on it, it is required toimprove individual components used in turbines both electronics and gearboxes.2.4  CHALLENGESIN OFFSHORE PRODUCTIONWhile theonshore wind turbine industry is going strong, the wind industry is lookingtoward the possibility of adding offshore wind capacity in the future. The needof offshore wind production require different solutions than onshore. The useof different technologies for onshore and offshore wind power projects isanother change that has occurred over the past 10 years, while companies usedto take the same wind turbine used on land and installed it offshore but now adifferent approach is used with current generation of offshore wind turbines.Floating windturbines are more significantly used which require floating structure insteadof requiring wind towers to be set into foundation under water. The modernoffshore wind turbines are now in the phase of demonstration which would resultin producing wind turbines with high capacity factor, more reliable andeconomical as well.

One of the immediate design challengesis the ability to accurately predict applied loads and resulting dynamicresponse of the coupled wind-turbine and support structure under the action ofcombined stochastic wave and wind loading (Musial and Butterfield, 2006). At present, analysis/design and installation of monopilefoundations for wind-turbine structures usually rely on general geotechnical standards, combined with morespecific guidelines and semi-empirical formulas developed by the offshoreoil/gas industry (API, 2000; DIN, 2005;DNV, 2011;GermanischerLloyd, 2005).  6.    recommendation for future prospectsWind power ismore efficient and affordable than it has ever been, it is an easy and quickenergy solution for increasing power generation. Technical modification andsystem upgrades are required for extensive energy and environmentaladaptability of offshore wind turbines which includes,-       Strengthening the tower to cope with loadingforces from waves and ice flows, pressurizing the nacelles to keep corrosivesea spray away from electrical components.-       Automatic greasing systems could be installed tolubricate bearings and blades as well as heating and cooling system within aspecific range.-       Use of more sophisticated electromechanicalparts in OWTs (e.g.

direct-drive units that eliminate the requirement for agearbox, thereby removing one of the key components prone to failure) willincrease the efficiency and hence energy yield and also reduce O costsfor the project.-       Computer simulation can be useful to companiesas they look to increase the capacity factor of turbines. These software allowscompanies to help design blades that allow for attached flow across a range offlow velocity without continuously make to rotors larger.-       The technology can be used efficiently by using othersimulations including manufacturing components along with monitoring thepotential performance of components, performing structural analysis and maintenanceof different parts. 7.    summary and conclusionOffshore windpower generation appears to be a promising solution to overcome the universaldemand for clean, cost-effective and sufficient amount of energy. There is aroom for improvement in all areas of wind farm development; in design, throughinnovative use of composite materials, support structures and foundations; andin construction processes, through improvement in installation techniques, fabricationand transportation.

General consideration in designs of modern offshore wind turbineshave been reviewed. Wind farm developers and engineers should identify variousissues that are likely to arise in the development phase of an offshore windturbines.In the designprocess, verification of design solution is mandatory.

Because of the complex behaviorof OWTs, design codes should be used to analyze the design solutions. Although someguidelines or recommendations for wind turbine design are scattered in theliterature, an accepted best practice that takes a new designer step by stepthrough process of designing a wind turbine is missing.  

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