Minimization of transportation, installation and maintenance operations costs for offshore wind turbines

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1 Lousana State Unversty LSU Dgtal Commons LSU Master's Theses Graduate School 204 Mnmzaton of transportaton, nstallaton and mantenance operatons costs for offshore wnd turbnes Tasnm Ibn Faz Lousana State Unversty and grcultural and Mechancal College Follow ths and addtonal works at: Part of the Mechancal Engneerng Commons Recommended Ctaton Faz, Tasnm Ibn, "Mnmzaton of transportaton, nstallaton and mantenance operatons costs for offshore wnd turbnes" (204). LSU Master's Theses Ths Thess s brought to you for free and open access by the Graduate School at LSU Dgtal Commons. It has been accepted for ncluson n LSU Master's Theses by an authorzed graduate school edtor of LSU Dgtal Commons. For more nformaton, please contact gcoste@lsu.edu.

2 MIIMIZTIO OF TRSPORTTIO, ISTLLTIO D MITECE OPERTIOS COSTS FOR OFFSHORE WID TURBIES Thess Submtted to the Graduate Faculty of the Lousana State Unversty and grcultural and Mechancal College n partal fulfllment of the requrements for the degree of Master of Scence n Industral Engneerng n The Department of Mechancal and Industral Engneerng by Tasnm Ibn Faz B.Sc.Eng., Bangladesh Unversty of Engneerng and Technology, 20 May 204

3 CKOWLEDGEMETS I would lke to express my sncere grattude to my supervsor Dr. Bhaba R. Sarker for hs contnuous gudance throughout the tme of ths research. Hs advce and crtcal comments were most helpful n shapng up my work. I would lke to thank Dr. Fereydoun ghazadeh and Dr. T. Warren Lao for servng on my thess commttee. I have been greatly beneftted by ther nsghtful comments and suggestons; t has certanly mproved the qualty of my research work. Ths research was funded by the PFUD grant enttled Procurement, Manufacture and Supply of Components and Materals for Off-shore Wnd Energy Facltes, Contract #LEQSF-EPS (202)-PFUD-29, LSU ccount # and Elton G. Yates Dstngushed Professorshp Support Fund (/C: ). I also would lke to take ths opportunty to thank my father Md. Fazur Rahman and my mother Salma khter for ther contnuous encouragement and support throughout my lfe.

4 TBLE OF COTETS CKOWLEDGEMETS... LIST OF TBLES... v LIST OF FIGURES... v. ITRODUCTIO.... Lterature revew Offshore wnd energy faclty development and nstallaton Mantenance of offshore wnd farms Lmtatons of prevous studes Studes concernng transportaton and nstallatons Studes concernng mantenance of offshore energy faclty Research goal Research obectve pplcaton and scope of the study Methodology Transportaton and nstallaton cost model Mantenance cost model OFFSHORE WID FRMS D SUB-SSEMBLY OPERTIOS Support structures Foundaton Transton pece Scour protecton Turbne Tower acelle Hub Blades Electrc cables Substaton Turbne nstallaton vessel Turbne transportaton and nstallaton process Pre-assembly at the port Loadng of turbne components and sub-assembles on the vessel Transportaton from port to farm ste Jackng up of vessel at nstallaton ste Installaton of turbnes TRSPORTTIO D ISTLLTIO COST The problem Classfcaton of offshore wnd turbne pre-assembly ssumptons and notatons... 36

5 3.2. ssumptons otaton The T&I model Transportaton and nstallaton tme estmaton Total tme requrement Transportaton and nstallaton cost calculaton Total cost of transportaton and nstallaton nnual cost of transportaton and nstallaton Soluton procedure lgorthm : Soluton procedure for T&I cost model Computatonal results Senstvty analyss Effect of learnng rate Effect of dstance from port to farm ste Effect of avalable vessel deck area Effect of ntal lftng rate Benefts of the model OPPORTUISTIC PREVETIVE MITECE COST The problem ssumpton and notaton ssumptons otaton The preventve mantenance model Calculaton of cost n a mantenance cycle ge groups and degrees of age reducton ge update of components Soluton procedure lgorthm B: Soluton procedure for mult level mantenance cost model Computatonal results Senstvty analyss Effect of maxmum age threshold Effect of mnmum age threshold Beneft of the model COCLUSIO Summary of the research General concluson Sgnfcance of the study Lmtatons of the study Future works... 9 REFERECES VIT v

6 LIST OF TBLES Table. - Modfed total captal cost and turbne nstallaton cost for wnd farms n Europe*. 6 Table 2. - Turbne lftng and assembly sequence for nstallaton usng dfferent methods Table 3. - Popular pre-assembly methods for offshore wnd turbnes Table Wnd farm and vessel Parameters... 5 Table Model parameters and ther values Table Input matrx of turbne class and pre-assembly method Table Transportaton and Installaton cost of turbnes for a wnd farm of 300 MW Table 3.6- Effect of learnng rate on turbne transportaton and nstallaton cost Table Effect of dstance from port on transportaton and nstallaton cost Table Effect of avalable deck area on tme requrement Table Effect of ntal lftng rate on transportaton and nstallaton cost Table 4. - Turbne components and ther propertes Table nnual cost of mantenance for dfferent number of age groups Table 4.3- ge groups and preventng actons resultng n the mnmum cost v

7 LIST OF FIGURES Fgure. - Worldwde offshore wnd energy capacty growths... 2 Fgure.2 - Proected growth of offshore wnd energy capacty... 3 Fgure.3 - Lfe cycle cost breakdown of an offshore wnd farm... 4 Fgure.4 - Captal cost breakdown for an offshore wnd farm... 5 Fgure 2. - Offshore wnd turbne transportaton and nstallaton cycle Fgure Sub-assembles done onshore followng pre-assembly Method Fgure Sub-assembles done onshore followng pre-assembly Method Fgure Sub-assembles done onshore followng pre-assembly Method Fgure Sub-assembles done onshore followng pre-assembly Method Fgure Sub-assembles done onshore followng pre-assembly Method Fgure Lftng and assemblng of turbne components at the port Fgure Transportaton of turbne components Fgure Vessel ackng up at the turbne ste... 3 Fgure Lftng and assemblng of turbne components at the farm ste Fgure 3. - Transportaton and nstallaton cost for a fxed turbne class Fgure Transportaton and nstallaton cost for dfferent turbne classes Fgure Effect of dstance from port to farm ste for dfferent pre-assembly method Fgure Effect of dstance to port on cost for dfferent turbne class Fgure Effect of vessel deck area on cost for dfferent pre-assembly method... 6 Fgure Effect of vessel deck area on cost for dfferent turbne class... 6 Fgure 3.7 Effect of ntal lftng rates and pre-assembly methods on cost Fgure Effect of lftng rate across turbne classes and pre-assembly methods Fgure 4. - Effect of number of age groups on annual mantenance cost... 8 Fgure Effect of maxmum age threshold on cost for varous number of age groups Fgure 4.3- Effect of maxmum age threshold on mantenance cost Fgure Effect of mnmum age threshold on cost for varous number of age groups v

8 BSTRCT lthough t s a sustanable source and there s abundant potental for energy, cost of energy generated from offshore wnd s stll hgh compared to other sustanable energy sources. part from the manufacturng cost of turbnes, cost of energy s sgnfcantly affected by costs of transportaton and nstallaton operatons of wnd turbnes and mantenance operatons of turbne components. Through optmum selecton of decson varables, such as turbne nstallaton method and rated power output of each turbne, cost of transportaton and nstallaton operatons can be mnmzed. The frst model n ths study nvestgates the mpact of these decson varables and effect of learnng on cost of transportaton and nstallaton and dentfes optmal combnaton of these varables that mnmze the total cost. Once the offshore wnd farm becomes operatonal, mantenance cost of the turbnes becomes the most sgnfcant contrbutor to the cost of energy. The second model developed n ths study put forward a mantenance cost model followng mult-level opportunstc preventve mantenance strategy. In ths strategy, opportunty for performng preventve actons on components s taken whle a faled component s replaced. Total cost assocated wth mantenance operatons depends on the settng of age groups that determne whch component should be preventvely mantaned and to what level of mantenance. Through optmum selecton of the number of age groups, cost of mantenance can be mnmzed. The methodologes for fndng optmal solutons for both models are provded, numercal study s performed and senstvty analyses are presented to llustrate the benefts of the models. v

9 . ITRODUCTIO In the face of growng prce of fossl fuels and ever ncreasng demand of energy, renewable energy sources have been receved a great deal of attenton as a vable alternatve to the tradtonal energy sources. bundance and nature frendlness make renewable energy sources attractve and undoubtedly these sources would be the prmary source of energy n the future. Harnessng energy from wnd flow s one of the most ancent feats of manknd. Wnd energy has been consdered as one of the most effcent clean energy source. From the begnnng of the 980s, generatng electrc power from wnd energy started frst at small levels, and snce then there have been many mprovements n desgn of wnd turbnes and nstallaton methods to make wnd farms a great source of sustanable energy. In many countres, land based wnd farms have been nstalled and they are connected to electrc grd lnes. t the begnnng of the 990s, strvng for harnessng offshore wnd energy and translatng t to electrc energy began. The frst few offshore farms were developed n a small scale, wth capactes rangng around megawatts. Later on, nstallaton of large offshore farms wth capactes around 00 megawatts started n an effort to utlze economes of scale. lthough energy generaton system from offshore wnd s stll consdered as an emergng feld, t s attractng more attenton due to the abundance of offshore space and wnd potental, and also due to the fact that offshore wnd farms are, unlke onshore farms located far away from human habtat to cause nose and aesthetc annoyance. There s a sgnfcant growth n nstalled offshore wnd power capacty worldwde as shown n Fgure..

10 Source: avgant Consultng, Inc (203) Fgure. - Worldwde offshore wnd energy capacty growths So far, almost all the offshore wnd farms are nstalled n Europe. European countres lke UK, Germany, etherlands and others have set target to acheve a great percentage of ther total energy demands from offshore wnd. In these countres, large (~500 MW) offshore wnd farms have been nstalled and operatng and many more are on the way of becomng operatonal. few offshore wnd farms have been nstalled n Chna and Japan. In Unted States, so far no offshore faclty has been constructed; but one proect has been approved and two others are watng to be approved. It s proected that n the comng years, many offshore wnd farms would be nstalled n varous regons of the world ncludng orth merca, South merca and sa Pacfc regons. Several offshore wnd farms are currently n the development phase and they would be operatonal n the comng years. In Fgure.2 the proected growth of offshore wnd energy n varous regons of the world s shown. Europe s predcted to be at the leadng of the offshore wnd energy sector, but orth merca wll soon on the race. 2

11 Source: avgant Consultng, Inc (203) Fgure.2 - Proected growth of offshore wnd energy capacty Due to the fact that offshore wnd energy s gong to meet a great porton of total energy demand n future, there s a keen nterest for detal analyses n varous areas lke captal cost structure, energy output, structural ssues, supply network, mantenance, nstallaton, transportaton, and other ssues pertanng to offshore wnd faclty. The goal of these studes s to make offshore wnd energy effcent so that cost of energy remans the mnmum. Energy generated from offshore wnd s yet to be consdered as the cheapest form of energy avalable. lthough offshore wnd farm stes ensures better wnd potental, energy generated from offshore wnd costs more than that from onshore wnd. ccordng to avgant Consultng, Inc. (203), Consderng the whole lfetme of a wnd farm, the exstng levelzed cost (average) of energy from offshore wnd ranges between $200 to 250 per Megawatt-hour (MWh), whereas for onshore wnd t vares on the range of $00-30/MWh. Because of the communty dsapproval for land-based wnd farms, the wnd energy undertakng offshore s stll a vable opton for constant wnd potental and envronment frendly area, for whch reason the 3

12 enterprses are search for better methods to mnmze the offshore wnd farm cost.. Fgure.3 shows the lfe cycle cost breakdown for an offshore wnd farm. Source: avgant Consultng, Inc (203) Fgure.3 - Lfe cycle cost breakdown of an offshore wnd farm In Fgure.3, varous cost segments of levelzed cost of energy and ther relatve contrbutons are shown. Several of these costs are ncurred as the captal costs durng the development phase of the wnd farm, for example, turbne cost, foundaton cost, nstallaton cost etc. Operatons and mantenance cost and other varable costs ncur throughout the operatonal lfetme of the wnd farm. In Fgure.4, the breakdown of captal cost of offshore wnd farm s shown. Hgh cost of energy from offshore wnd can be attrbuted to two most cost ncurrng aspects of offshore wnd energy, e.g., cost of nstallaton of the wnd energy faclty and cost of mantenance. 4

13 Source: avgant Consultng, Inc (203) Fgure.4 - Captal cost breakdown for an offshore wnd farm It s observed from Fgure.4 that turbne and foundaton are the most sgnfcant cost ncurrng elements. These costs depend on desgn, materals, manufacturng processes etc. part from these costs, nstallaton operatons costs make up a sgnfcant porton (9%) of the total captal cost. Installaton operatons nvolve transportaton and nstallaton of turbne components, foundaton components and electrc cables. ccordng to Kaser and Snyder (200), turbne transportaton and nstallaton costs make up almost 30% of total nstallaton costs. In other words, wnd turbne transportaton and nstallaton contrbutes almost 5% of total captal costs. In table. total captal costs and turbne nstallaton cost for varous offshore wnd farms are shown. 5

14 Table. - Modfed total captal cost and turbne nstallaton cost for wnd farms n Europe* Wnd farm Wnd farm capacty (MW) umber of turbnes Captal cost per turbne ($ Mllon) Total Turbne nstallaton cost ($ Mllon) Installaton cost per turbne ($) Total captal cost ($ Mllon) lpha Ventus ,44, Bard I ,2,875,939 Barrow , Belwnd , Burbo Bank , Global Tech I ,038,750,662 Greater Gabbard ,000 2,84 Gunfleet Sands , Horns Rev , Horns Rev II , Kentsh Flats , Lllgrund , Lncs ,000,28 London rray ,57 3,047 Lynn/Inner Downsng , orth Hoyle , ysted , OWEZ , Prncess mala , Rhyl Flats , Robn Rgg ,67 58 Rodsand II , Sherngham Shoal ,88,684 Thanet ,000,30 Walney ,000,66 * Converted to dollars from Euros and other currency gven n Kaser & Snyder (200)* From Table. t s observed that dependng on the wnd farm capacty and number of wnd turbnes, captal costs vary from $200 mllon to $2 bllon. Turbne transportaton and 6

15 nstallaton s taken as 5% of the total captal costs and can be as hgh as $.5 mllon per turbne. Due to the magntude of transportaton and nstallaton cost, even a small process mprovement can lead to the savngs of mllon dollars. Mantenance cost s ncurred throughout the operatonal lfetme of a wnd farm. Mantenance strategy and polcy controls the cost to a great extent. Besdes that, number of turbnes n the farm, number of components n each turbne, falure dstrbutons of the components and cost parameters affect cost of mantenance. ccordng to GWEC (202), turbne mantenance cost make up almost 25% of the levelzed cost of energy. Mantenance cost of an offshore wnd farm consstng of 50 turbnes can be as hgh as $4 mllon per year [Dng and Tan (200)]. So, a reducton n mantenance cost results n economcal energy generaton. The present study would try to dentfy economcal ways of energy generaton from offshore wnd through nvestgaton of two aspects of offshore wnd farms, turbne transportaton and nstallaton, and turbne mantenance. In order to do that, prevous studes on offshore wnd energy are revewed and analyzed as outlned n the next secton.. Lterature revew Ths secton brefly summarzes the prevous studes concernng offshore wnd energy. Ths revew s done n two sub sectons. The frst sub secton provdes a revew on studes n offshore wnd energy faclty development and the latter one gves a background on studes related to mantenance of offshore wnd facltes. 7

16 .. Offshore wnd energy faclty development and nstallaton Very few studes have been conducted pertanng to offshore wnd energy faclty development and nstallaton of wnd farm. In these studes, dfferent models have developed, whch covered dverse problems concernng offshore wnd energy,.e. development potental for wnd farm, cost of nstallaton, effect of desgn of the wnd farm and learnng effect on cost etc. Menz and Vachon (2006) proposed a model for wnd energy development ndex. They suggested that, development of a wnd farm n an area not only depends on the wnd potental, but also on the dfferent energy polces effectve n that area. Ther model took wnd potental ndex as the output and varous fnancal ncentves granted by governments n a partcular regon as the varables of concern. From ther model, t s suggested that, mandatory polces set by the authortes lead to ncreasng wnd power development whereas voluntary choces and fnancal ncentves fal to stmulate the development. Ther model only consdered ncentves granted for settng energy prces and to energy provders. Incentve n manufacturng or logstcs ndustres for offshore wnd farm components and ther effects on the development potental were not consdered. Heptonstall et al. (202) developed a levelsed cost model for electrcty generaton from wnd energy. ccordng to ther model, cost of energy depends on the maor cost drvers, such as nvestment and operatons cost, fuel costs etc, whch ndcates a gradual rse n cost of electrcty generaton. They predcted that fnancal ncentves from governments, scale of producton and enhancng the capablty of supply chan can encounter ths rse n cost. Ths cost model consdered the overall nvestment cost and mantenance cost and dd not consder the nstallaton methods and ts effect on levelsed cost. Hong and Moller (202) gave another levelsed cost of electrcty generaton, n whch rsk of storm/cyclone was ncluded. To counter the loss assocated wth rsk, they proposed a specal ncentve. 8

17 The most crtcal stage for offshore wnd energy faclty s the nstallaton of foundatons and turbnes. Kranouds et al. (200) proposed that nstallaton cost s a functon of maxmum power output, number of wnd turbnes and area of offshore farm. Ther model was developed for a partcular regon; emprcal relatons and cost coeffcents used n ths model s vald only for that partcular study. Pantaleo et al. (2005) developed a cost model where they defned the cost of nstallaton as a functon of water depth at the farm ste and turbne hub heght. The authors compared cost of energy at varous offshore stes n a regon usng dfferent turbne models. They developed a method for selectng optmum offshore ste and turbne model but for a regon specfc case. The most detaled study for offshore wnd turbne transportaton and nstallaton was conducted by Kaser and Snyder (203). In ther model cost of nstallaton s functon of wnd farm nameplate capacty, wnd turbne capacty and dstance from port to farm ste. The authors formulated the cost model as a product of tme to complete nstallaton and daly cost of nstallaton. The model dd not provde nsghts whether there s any effect of nstallaton method on nstallaton cost. Herman (2002) consdered the effect of nstallaton method of wnd turbnes on n hs nstallaton cost model. He also consdered the effect of delay n operaton due to bad weather. The effect of turbne model and ts sze was not consdered n the model. Uraz (20) also studed the effect of nstallaton method of turbnes on the nstallaton cost. Both of these works proposed a number of pre assembly types and formulated the tme and space requrements for transport and nstallaton. These models provded estmatons of nstallaton tme and cost of offshore wnd farms but dd not propose any optmum decson that would mnmze the nstallaton tme and cost. 9

18 ..2 Mantenance of offshore wnd farms Operatons and mantenance costs contrbute a sgnfcant porton (25-30 percent) of cost of energy from offshore wnd turbnes. Based on desgn of components, crtera for mantenance and mantenance strateges, there can be numerous possble decsons set whch can be employed for mantenance. Several studes have been conducted to fnd the optmal decson set to mnmze mantenance costs. elsen and Sørensen (20) compared two dfferent mantenance strateges, e.g. condton-based and correctve mantenance for a generc offshore wnd turbne wth sngle component. The model s formulated as a benefts maxmzaton problem of wth constrants of desgn, nspecton and decson rules. Influencng parameters of the model are mnmum damage level to ntate repar, nterval of nspecton, mean tme between falures of the component. case study s presented that compared two strateges of mantenance and nvestgated the effects of varous parameters. lsson and Bertlng (2006) presented the effect of condton montorng as the mantenance strategy on lfe cycle cost for two cases, a sngle turbne onshore and a wnd farm offshore. ccordng to ther study, condton montorng benefts mantenance management of offshore power systems and cost of ths strategy can be covered by 0.43% ncrease n avalablty n turbnes for power generaton. Besnard et al. (2009), proposed an optmzaton model for opportunstc mantenance of offshore wnd turbnes. Ther model suggested that, schedulng preventve mantenance when power generaton potental s low can lead to mnmzaton of cost of mantenance. In ther model, Besnard et al. (203) proposed a model for offshore wnd turbne mantenance support organzaton. Ther model consdered modes of transportaton for mantenance, locaton of mantenance team, servce hours, and number of teams as decson 0

19 varables. Backloggng of mantenance actvtes were presented through a queung model. case study llustrated the model and offshore accommodaton of teams on servce 24 hours a day, 7 days a week transported by crew transport vessel equpped wth moton compensated access system was found to be most cost effcent. Besnard and Bertlng (200) proposed a model for optmzng the condton based mantenance for wnd turbne components, for whch degradaton can be classfed accordng to the damage level. In ther work, they compared three mantenance strateges, vsual nspecton, condton-montorng and onlne condton-montorng. In ther work, Tan et al. (20) consdered the falure probablty of the whole turbne system to develop an optmum condton based mantenance polcy. In the optmum polcy, two falure probablty threshold values were defned at turbne system level, and from the falure probablty of the system, component falure probablty dstrbuton s obtaned. Optmal mantenance decson ncluded mantenance schedule, wnd turbnes to be mantaned and key components to be nspected. In ther study, Shrmohammad et al. (2007) consdered two knds of tme as decson varables; the frst one s the tme between two preventve replacement cycles and the other one s the tme from the startng of a cycle, whch determnes whether an emergency replacement should postpone the scheduled preventve replacement or not. nother decson varable s the cut-off age of the system to be consdered for replacement. The authors consdered the falure and replacement of the whole system nstead of consderng ndvdual components. Laggoune et al. (2009) consdered opportunstc replacement of components through groupng of components n such a way that replacement tmes for each component n a group s

20 an nteger multple of the least replacement tme. In ths case, although a system wde optmzaton s possble but component wde replacement may not be optmal. Dng and Tan (200) proposed an approach to compare three opportunstc mantenance optmzaton models. They consdered preventve mantenance as perfect, mperfect and two-level acton n these three mantenance models. Instead of ndvdual component, they consdered mantenance for entre wnd farm. They set age threshold values for wnd turbne components that would trgger the mantenance operaton. In ths model the same threshold values were set for components n a faled turbne and components n the runnng turbnes. In ther followng work, Dng and Tan (202), they proposed dfferent age thresholds for components n faled and runnng turbnes. In both studes, opportunstc mantenance followng mperfect two-level acton was found to be optmum..2 Lmtatons of prevous studes In ths secton, lmtatons n prevous research and possble extensons n two aspects of offshore wnd energy, nstallaton and mantenance have been dscussed. Frst, the lmtatons n studes pertanng to transportaton and nstallaton have been presented and methodology for developng a cost model s dscussed. Followng that, methodology for developng a mult-level opportunstc mantenance cost model for offshore wnd farm as an extenson from prevous works s dscussed..2. Studes concernng transportaton and nstallatons Transportaton and nstallaton of offshore wnd turbnes s the prmary cost ncurrng aspect n developng an offshore wnd farm. Very few studes ventured n developng a cost model for wnd turbne transportaton and nstallaton. In these studes, developed cost models only consdered a few of the mpactng factors. Cost of transportaton and nstallaton s 2

21 sgnfcantly affected by the wnd farm propertes, prmarly by rated power output of wnd turbnes. nother mportant factor to be consdered s the nstallaton procedure followed for wnd turbnes. These cost determnng factors have not been ncluded altogether n prevous studes. lso, effects of varous parameters of offshore wnd farm, nstallaton vessel and learnng capablty on cost need to be analyzed. Formulatng a model that present the relatonshp between transportaton and nstallaton cost and wnd farm propertes and nstallaton method would be a maor advancement n ths area. It s observed that hgher turbnes class (.e., wth hgher power output), the number of turbnes n the farm can be reduced but that wll result n ncreased deck space requrement for each turbne resultng n fewer number of turbnes to be transported n each vessel trps. Hgher-rated power turbnes also results n longer tme for lftng and assemble each turbne because the turbne wth larger dmenson needs more tme to be nstalled. Pre-assembly at the port area s another controllng varable of transportaton and nstallaton cost. More preassembly done on shore results n lower number of offshore lfts and assembly, but t also results n hgher number of vessel trps. So, tradeoffs need to be made optmally for selectng turbne s rated power output and pre-assembly method to mnmze turbne transportaton and nstallaton cost..2.2 Studes concernng mantenance of offshore energy faclty Opportunstc preventve mantenance strategy n turbne component level s consdered to be a cost effcent strategy for mantenance of offshore wnd farms. Due to the costly nature of mantenance n offshore wnd farms, t s desrable to mnmze falure replacement of turbne components by takng preventve mantenance acton whenever the opportunty arses. In prevous works two knds of opportunstc preventve mantenance strategy were consdered, 3

22 .e. preventve replacement and two-level mperfect opportunstc mantenance. In the former the runnng component was replaced wth a new one whenever a component reaches a fxed age threshold. In the latter strategy, two age thresholds were set where mantenance s trggered by a falure, and durng correctve replacement, runnng components reachng the upper threshold are replaced and components reachng the lower age threshold undergo mperfect mantenance. For scenaros where many levels of mantenance actons can be performed based on the age of the components, a maor contrbuton would be the formulaton of a multlevel mperfect mantenance strategy where multple age groups are formed optmally and cost of mantenance s a functon of the number of such age groups. In a mantenance polcy where many age groups are set for preventve repar, each age group s of smaller nterval, and so number of components fallng wthn a partcular age group s lower. In such a mantenance polcy, most of the components are preventvely repared to such degrees that ther ages are reduced by small percentages. Many of these components qualfy for preventve mantenance agan for the next mantenance cycle. Such recurrng preventve mantenance of the same components ncreases cost. On the other hand, n a mantenance polcy where a few age groups are created, each group encompasses a large nterval and many components fall wthn an age group. So, many components undergo hgher degree of preventve repar although ther ages are not so hgh. Ths leads to ncreased mantenance cost. Snce cost of preventve mantenance s a functon of percentage of age reducton, to mnmze mantenance cost a tradeoff needs to be made between these two scenaros. mantenance polcy wth optmum number of age group results n mnmum cost of mantenance. 4

23 .3 Research goal The goal of ths research s to overcome the hurdle n mnmzng the offshore wnd turbne nstallaton and mantenance costs,.e., to dentfy cheaper ways to generate electrc energy from offshore wnd, especally from large wnd-farms n the sea. To do that, two aspects of offshore wnd farms (e.g., turbne transportaton and nstallaton and turbne mantenance process) wll be analyzed and ther cost structures wll be examned to plan an economcally vable power generaton operaton..4 Research obectve In order to meet the goal of ths study, the specfc obectve of ths work s to develop two models, one for offshore wnd turbne transportaton and nstallaton cost and the other for offshore wnd turbne mantenance cost, and to mnmze the total system cost. To attan these obectves, two cost models to be consdered:. transportaton and nstallaton cost model of offshore wnd turbnes. (a) model wll be formulated n whch cost of transportaton and nstallaton wll be expressed as a functon of wnd turbne class and pre-assembly method of turbnes. (b) Soluton procedure of the model wll be developed. (c) To mnmze the cost, optmal pre-assembly method and turbne class wll be determned. 2. n opportunstc preventve mantenance cost model for offshore wnd turbnes. (a) mantenance cost model wll be developed for of offshore wnd turbnes where mantenance cost s the model output and decson varable s the number of age groups for components 5

24 (b) soluton procedure for fndng the optmal value of decson varable wll be developed. (c) Optmal number of age groups and assocated degree of mantenance wll be set so that total mantenance cost s mnmzed..5 pplcaton and scope of the study The frst model s developed for offshore wnd turbne transportaton and nstallaton. The complete operaton s segmented nto several tasks, and for each task tme requrements are determned. Total tme to complete transportaton and nstallaton s obtaned by summng these tme requrements. lso, daly cost of nstallaton s determned. From these two, cost of transportaton and nstallaton cost s determned. The methodology used n ths study can be appled to estmate cost of nstallng a new faclty. Method for optmum selecton of transportaton hub locaton and nstallaton method n mnmzng nstallaton cost s another applcaton of ths study. The second model s formulated for determnng cost of mantenance of offshore wnd turbnes followng mult-level mperfect opportunstc mantenance strategy. Ths study would be applcable n developng mantenance strateges for complex systems wth multple components n maxmzng servce levels and mnmzng mantenance cost..6 Methodology In ths secton the methodologes for formulatng the two models have been dscussed. Methodology for developng transportaton and nstallaton cost model has been descrbed frst, and then methodology for developng the mantenance cost model followng mult level opportunstc mantenance strategy s outlned. 6

25 .6. Transportaton and nstallaton cost model In the frst model, wnd farm characterstcs, e.g., turbne class or rated power output turbnes and nstallaton method or pre-assembly method are consdered as decson varables. From the model, optmum pre-assembly method and turbne class are chosen whch would mnmze cost for transportaton and nstallaton of turbnes n an offshore wnd farm wth fxed capacty. Decson varables consdered n the model are pre-assembly method and turbne s rated power output. Turbne s rated power output s chosen from a fxed set of commercally avalable models. Pre-assembly methods are chosen from methods that are commonly followed. The best combnaton of pre-assembly method and turbne s rated power output that results n least tme and cost to transport and nstall requred number of turbnes can be determned from the model. In formulatng the model, tme requrements to perform varous operatons are determned, whch nclude transportaton, ackng up, lftng, and assembly operatons. Total tme requrement can be found by summng of all these tme segments. Transportaton and nstallaton cost s obtaned consderng the vessel cost per unt tme and total tme requrement. Optmum selecton of turbne s rated power output and pre-assembly method would mnmze the tme requrement and thereby cost for transportaton and nstallaton..6.2 Mantenance cost model In the second model, a mantenance cost model s developed for offshore wnd turbnes followng mult level opportunstc preventve mantenance strategy. In ths strategy, durng falure of any component of any turbne, faled component s replaced, at the same tme multlevel preventve mantenance s done for other runnng components whch have reached 7

26 some predetermned age level. Varous age groups are determned, and when a component s age falls wthn a partcular age group, the component s gone through the level of preventve mantenance assocated wth that age group. Level of preventve mantenance s characterzed by the percentage reducton n age and cost of mantenance. Controllng varable s the number of age groups for components and optmal settng of age lmts and degree of age reducton assocated wth each group would lead to mnmzaton of total mantenance cost of the whole wnd farm. 8

27 2. OFFSHORE WID FRMS D SUB-SSEMBLY OPERTIOS The purpose of ths chapter s to provde some bref descrpton about the elements of an offshore wnd farm. detaled descrpton of wnd turbne components and ther transportaton and nstallaton process s provded thereafter. n offshore wnd farm s a power plant that conssts of a number of wnd turbnes connected wth nternal grd to one or more substatons and an export cable to transmt power to local grd. The prncpal components of an offshore wnd farm nclude support structures, turbnes, substatons and electrcal transmsson systems. 2. Support structures Support structures consst of foundaton, transton pece and scour protecton. Foundaton provdes support to the turbne, transton pece s attached to the foundaton to absorb vbraton and smplfy turbne attachment, and scour protecton ensures that envronmental condtons do not degrade the ntegrty of the support structure. 2.. Foundaton Foundatons provde support to the turbnes. Ste condtons, such as maxmum wnd speed, water depth, wave heghts currents, surf propertes and sze and weght of turbne affect the foundaton type and desgn. Four basc knds of foundatons are n use n offshore wnd farms: monoples, ackets, trpods and gravty foundatons. Foundatons are usually manufactured onshore n one pece, transported to farm ste by barge or towed and then pled or set at the sea bed by derrck barge or crane. (a) Monople Ths knd of foundaton s sutable for lower water depth (up to 30 Meters). Monolples are large dameter (4 to 6 meter outer dameter), steel tubular that are drlled nto the sea bed 9

28 (40%-50% of ther length s nserted nto sea bed). Dependng on the load and condtons of sol, water and envronment dameter, thckness and depth of drllng vares. Monoples are the most popular forms of foundaton due to lower cost and smplcty. (b) Jacket Jacket foundatons have a lattce structure consstng of welded frame of tubular members. Through each of ts four legs, plng s drven to secure t to the sea bed. Jackets are heavy and robust and transportng and lftng of them s expensve. They have been used very rarely so far and usually they are called for when the farm ste s n deep water and turbne sze s greater. (c) Trpods Trpods are used n deeper waters where more robust foundatons are requred. It has a central steel shaft connected to three steel tubular legs through whch plngs are drven to the sea bed. Trpods are costly to manufacture and to transport and so far, only one wnd farm has employed trpods as foundaton. (d) Gravty foundaton Ths type of foundatons unlke the others s concrete structures and they use ther weght to resst wnd and wave loadng. Ther materal cost s less but requre specal fabrcaton facltes due to ther heavy weght and ther transportaton and lftng operatons are costly. They are employed when the sea bed sol s unft for ple drvng Transton pece Transton peces are placed on top of the foundatons such that they cover the upper part of the foundaton and act as connectors between foundatons and turbnes. They also levels horzontal naccuraces. For monoples, transton peces contan boat fenders, access ladders, 20

29 access deck and handrals and the gap between transton pece and foundaton s flled wth cement grout. For acket and trpod foundatons, transton peces are nstalled at the port and do not contan varous access systems as they are nstalled elsewhere on the foundaton Scour protecton To avod removal of sedment around the base of foundatons, a layer of small rocks are placed followng ple drvng. fter the cable nstallaton, large cover stones are placed around the foundatons. 2.2 Turbne Wnd turbnes are the man components of a wnd farm. wnd turbne s an assembly of four prmary components: Tower, nacelle, hub and blades. In the followng secton these components are brefly dscussed Tower Towers are composed of two tubular sectons, fabrcated from rolled and welded steel plates. Tower provdes support to the upper turbne assembly (nacelle, hub and blades). Heght of tower ranges from 60 to 90 meters and vares wth rotor dameter and the clearance above water level. Dameter of the tower ranges from 4 to 6 meters and depends on the weght of nacelle and wnd loads acelle acelle s attached at the top of the turbne tower and houses generator, gearbox and other control and communcaton components. acelle s essentally the power house of the turbne and composed of a manframe and a cover. Gearbox, generator and brake are attached to the manframe whch transmts all the loads from the rotor and reacton loads from the 2

30 generator and brake to the tower. acelle s the heavest component of the turbne and t requres a large amount of deck area whle beng transported Hub Hub s attached to nacelle and contans motor for controllng three blades that are bolted to t. Hub transmts wnd loads from blades to nacelle and transmts rotatonal loads to the gearbox Blades Each turbne conssts of three blades that made from renforced plastcs. Length of each blade can as hgh as 60 meters. Due to ther sze and low rgdty aganst wnd, transportaton and nstallaton of blades requre great attenton. 2.3 Electrc cables Electrc cables connect the turbnes to the electrcal grds. There are two knds of cables; the frst s the nner-array cables whch connect the turbnes to each other and to the offshore substaton. The length of nner-array cable depends on the layout of the farm and number of turbnes n the farm. The other knd of cable s the export cable, whch connects the wnd farm to onshore transmsson system. The length of export cable depends on the dstance to shore, routng of cable and water depth. 2.4 Substaton In a wnd farm, substaton s connected to all the turbnes through nner array cables and transmts the generated electrcty to onshore grd. The purpose of substaton s to mnmze transmsson loss by transformng voltage of the electrcty generated at the wnd farm. The prmary components of a substaton are voltage transformers and hgh voltage cables. The poston of a substaton should be such that length of cables s mnmzed. 22

31 2.5 Turbne nstallaton vessel The same vessel s used for both transportaton and nstallaton of turbnes at offshore farm stes. There are two knds of vessel that have been used for offshore wnd farm development. Frst one s the ack up barge, n whch case a barge s used to carry the turbnes and a tug boat s requred to transport the barge to the desred locatons. Separate vessels for transportng crew members are also needed f ack up barge s used. The other knd of vessel s a self propelled one, whch has become more popular because of ther self suffcent nature. self propelled nstallaton vessel has the capablty to transport turbnes and people and equpped wth cranes to nstall the turbnes. The vessel has also the ack up mechansm and can lft tself up to the desred heght durng loadng and nstallaton. 2.6 Turbne transportaton and nstallaton process Generally, all the turbnes are transported and nstalled together after the foundatons and transton peces are n place. s mentoned earler, turbnes are assembles of components wth consderable large dmensons. Transportaton and nstallaton of these huge components s a challengng task and requres a long tme and therefore hgh cost. Transportaton and nstallaton task nvolves several sub tasks, whch requres analyss to nvestgate the opportunty for mnmzng assocated cost. In Fgure 2., turbne transportaton and nstallaton cycle s shown. Each cycle s conssts of several operatons, some of whch are needed to be done only once n each cycle, but others are requred to be done for every turbne nstallaton. The operatons nvolved n transportaton and nstallaton of turbnes are descrbed n detal n the followng secton. 23

32 o Step : Pre-assemble components onshore Have all the turbnes been nstalled? Yes Stop Step 2: Lft and load components on vessel Step 6: Empty vessel returns to port and ack up Step 3: Vessel ack down and travel to offshore ste Yes Is the vessel empty? o Step 4: Vessel ack up and nstall the turbne Step 5: Vessel ack down and travel to next turbne ste Fgure 2. - Offshore wnd turbne transportaton and nstallaton cycle 2.6. Pre-assembly at the port Pre-assembly of offshore turbnes ndcates the performng of assembly operaton to some degree pror to transport them to offshore stes. Typcally, a turbne conssts of seven parts, two tower sectons, one nacelle and hub and three blades. Turbne manufacturer delvers 24

33 the turbnes unassembled at the port due to the large dmensons of each component. These components need to be assembled together and then attached to the transton pece of the foundaton at the farm ste for complete nstallaton. For assemblng the components, crane aboard the nstallaton vessel s used whch lfts the components to the requred heghts durng assembly. For assembly purpose, onshore lftng s mnmal and therefore t can be neglected. For loadng purpose at the port, the crane has to lft components or subassembly to the vessel ack up heght. For both assembly and nstallaton purposes at the farm ste, the lftng heght s equal to the turbne hub heght. It s possble to perform all the assembly operatons of components onshore and then transport the fully assembled turbne to the farm ste. In ths case the crane of the vessel has to perform only one lftng operaton for loadng the fully assembled turbne on the vessel. Upon arrval to the nstallaton ste, the vessel stops and acks up for fnal lftng and assembly of the turbne to the transton pece of the foundaton. Followng ths method only one lftng and one assembly operaton need to be done offshore. But the dsadvantage of ths method s that a fully assembled turbne requres large deck area of the vessel durng transportaton and t would be dffcult to transport more than one turbne at a tme whch would ncrease travel tme and cost. nother shortcomng of ths method s that weght of a fully assembled turbne s very hgh whch would requre crane capacty that s not cost effcent. So, there are two aspects that need to be consdered to determne to what degree the components are pre-assembled onshore. The frst one s that t s preferable to keep the offshore operatons mnmal because of offshore wnd, wave and weather condtons. The second one s that t s desred to transport as many turbnes as possble durng each trp of the vessel from port to farm ste to mnmze the number of vessel trps. Hgher number of assembles onshore 25

34 results n lower number of offshore lfts and hgher number of vessel trps. tradeoff, therefore need to be made, so that, number of offshore operatons and number of turbnes that can be transported n each trp of nstallaton vessel remans optmum. There are several pre-assembly methods that have been used n varous wnd farms. The methods are classfed accordng to the number of lfts requred for each turbne. The methods are dscussed n the followng secton. (a) Method : In ths pre-assembly method, two tower sectons are assembled together onshore. acelle, hub and two blades are also assembled onshore. These two sub-assembles and the thrd blade are transported to the farm ste and remanng sub-assembles (tower and transton pece, tower and nacelle, and thrd blade and hub) are done there. Only three lfts are requred durng loadng and durng nstallaton n ths method. In Prncess mala and OWEZ wnd farms, ths method was employed. In Fgure 2.2, the sub-assembles and parts followng preassembly Method s shown. Source: Uraz (20) Fgure Sub-assembles done onshore followng pre-assembly Method 26

35 (b) Method 2: In pre-assembly Method 2, as shown n Fgure 2.3, nacelle, hub and two blades are assembled onshore. Ths sub-assembly, the thrd blade and two tower sectons are transported to the farm ste and remanng sub-assembles (lower tower and transton pece, lower and upper tower sectons, upper tower and nacelle, and thrd blade and hub) are done there. In ths method, for each turbne four lfts are requred durng loadng and durng nstallaton. Ths method was employed n Horns Rev, orth Hoyle, Kentsh Flats wnd farms. Source: Uraz (20) Fgure Sub-assembles done onshore followng pre-assembly Method 2 (c) Method 3: In Fgure 2.4, sub-assembles and parts followng pre-assembly Method 3 s shown. In ths method, hub and all three blades are assembled onshore. Remanng sub-assembles (lower tower and transton pece, lower and upper tower sectons, upper tower and nacelle, acelle and the hub wth three blades) are done offshore. Two tower sectons and the nacelle are transported separately. Four lfts are requred for each turbne durng loadng and durng nstallaton. In ysted, lpha Ventus and Lllgrund wnd farms ths method was used. 27

36 Source: Uraz (20) Fgure Sub-assembles done onshore followng pre-assembly Method 3 (d) Method 4: In ths method, two tower sectons are assembled onshore, also the nacelle and hub are assembled together; all the remanng components are transported separately. For each turbne, fve lfts are requred durng loadng and durng nstallaton. Durng nstallaton, frst the tower s assembled to the transton pece, then the nacelle and hub sub-assembly s attached to the tower, fnally three blades are lfted and assembled to the hub. Ths method was used n Rhyl flats and Burbo Banks wnd farms. The sub-assembles and parts are shown n Fgure 2.5. Source: Uraz (20) Fgure Sub-assembles done onshore followng pre-assembly Method 4 28

37 (e) Method 5: In pre-assembly Method 5, as shown n Fgure 2.6, the nacelle and hub are assembled together onshore, all the other components are transported to the farm ste separately. Sx lfts are needed for each turbne for loadng and for nstallaton. Durng nstallaton, frst the lower tower s assembled to the transton pece, then two tower sectons, after that the nacelle and hub sub-assembly s attached to the upper tower; fnally three blades are bolted to the hub one by one. In Lynn and Inner Dowsng wnd farm, ths method was used. Source: Uraz (20) Fgure Sub-assembles done onshore followng pre-assembly Method Loadng of turbne components and sub-assembles on the vessel Once, pre-assembles are done, turbne components and sub-assembles are loaded on the nstallaton vessel at the port. umber of turbnes loaded n each trp of the vessel depends on the vessel capacty, turbne class and pre-assembly method followed. Fgure 2.7 shows the loadng operaton of turbne parts. 29

38 Source: 4C Offshore (203) Fgure Lftng and assemblng of turbne components at the port Transportaton from port to farm ste fter all the turbne components that can be carred n a sngle trp of the vessel are loaded, the vessel departs from the port and travels to the wnd farm ste. Transportaton tme depends on the vessel speed and dstance between port and farm ste. Fgure 2.8 shows the transportaton of turbne components aboard an nstallaton vessel. Source: 4C Offshore (203) Fgure Transportaton of turbne components 30

39 2.6.4 Jackng up of vessel at nstallaton ste fter the nstallaton vessel reaches the nstallaton ste of a turbne, t performs ackng up operaton to elevate at the requred heght and stable tself for nstallaton. Jackng up tme depends on the ackng up speed of the vessel and ackng up heght. fter nstallaton s done, the vessel acks down to the water level and travels to the next turbne ste. In Fgure 2.9, vessel ackng up at the nstallaton ste s shown. Source: 4C Offshore (203) Fgure Vessel ackng up at the turbne ste Installaton of turbnes Installaton of a turbne begns after the nstallaton vessel reaches at the turbne ste and acks up to the requred heght. Installaton nvolves lftng and assembly operatons of turbne parts and sub-assembles and the vessel crane performs these operatons. Tme to nstall a turbne depends on the pre-assembly method followed as descrbed n the secton Fgure 2.0 shows an nstallaton vessel performng nstallaton by lftng a turbne sub-assembly. 3

40 Source: 4C Offshore (203) Fgure Lftng and assemblng of turbne components at the farm ste (a) Installaton sequence of turbnes at the farm ste In Secton 2.6., fve pre-assembly methods for turbne nstallaton were descrbed. Each of these methods s characterzed by number of sub-assembles for each turbne, whch n turn determnes number of lftng and assembly operatons requred for each turbne. Followng these pre-assembly method, the lftng and assembly sequences for nstallaton at the farm ste are summarzed n Table 2.. The table shows the sequental nstallaton of a turbne at the farm ste followng dfferent pre-assembly methods. Followng Method, only three sub-assembles are done for each turbne. So, for the loadng of a turbne on the vessel, three lftng operatons are done at the port. t the nstallaton ste, three lftng and assembly operatons are needed to be done for each turbne. The table shows the progresson of the nstallaton procedure after each lftng and assembly operaton. 32

41 Table 2. - Turbne lftng and assembly sequence for nstallaton usng dfferent methods Preassembly method st operaton 2 nd operaton 3 rd operaton 4 th operaton 5 th operaton 6 th operaton Method Method 2 Method 3 Method 4 Method 5 Source: Kaser and Snyder (200) 33

42 3. TRSPORTTIO D ISTLLTIO COST In ths chapter a model s formulated for offshore wnd turbne transportaton and nstallaton (T&I) cost as a functon of wnd turbne class and nstallaton method. Tme requrement for completon of transportaton and nstallaton of turbnes s determned from tme estmaton for the operaton segments and summng them together. Smultaneously, daly rate of the nstallaton vessel s determned. T&I cost s obtaned from transportaton and nstallaton tme n days and daly cost of the nstallaton vessel. Varous parameters assocated wth wnd farm, nstallaton vessel, nstallaton method are consdered and ther effect on cost s nvestgated. 3. The problem case s consdered where a new offshore wnd energy generaton faclty s beng developed. The wnd farm would be stuated n the open waters where wnd turbnes would be arranged n rows and columns. The farm would conssts of a number of wnd turbnes nstalled atop ther foundatons, one or more substaton(s), nner-array cables whch would connect the turbnes to the substaton, and export cables whch would connect the substaton to the onshore grds. Installaton of the farm ncludes transportaton and nstallaton of foundatons and turbnes, substaton(s) and layng cables. From the pont of vew of tme requrements and assocated costs, transportaton and nstallaton of turbnes are the tasks that control the overall proect duraton and total cost to the greatest extent. Mnmzng the tme to complete transportaton and nstallaton thereby mnmzng costs assocated wth these tasks therefore s the key to mnmze total costs of nstallng an offshore wnd farm. 34

43 n offshore wnd farm s consdered whch s located n open waters. The farm ste s D meters away from the nearest operatonal port, ths port acts as the loadng area for wnd turbne components. The nameplate capacty of the wnd farm s C megawatt (MW). The wnd farm s conssted of number of wnd turbnes; each wth P MW rated power output. s mentoned before, turbnes are arranged n rows and columns and spaced d meters apart from each other. In the next secton, turbne propertes pertanng to transportaton and nstallaton are dscussed n detal. 3.. Classfcaton of offshore wnd turbne pre-assembly Transportaton and nstallaton of turbnes begn after the foundatons are pled and grouted at the farm ste and transton peces (connectng pece between turbne and foundaton) are n place atop the foundatons. Each turbne s an assembly of several parts; turbne tower n two peces, one hub, one nacelle and three blades. Before transportng from port to offshore farm ste, turbne parts are pre-assembled onshore followng one of fve methods to optmze vessel space and to ease offshore nstallaton. These methods are summarzed n Table 3.. In each row, the frst column ndcates a partcular pre-assembly method followed and the second column ndcates the subassembles that are done onshore, thrd and fourth columns represent number of assembly operaton done onshore and number of separate segments for each turbne. s seen from the table, for pre-assembly Method 3, hub and three blades of a turbne make a sub assembly; ths sub-assembly s transported and nstalled as though t s a sngle part. acelle and two parts of the tower are transported and nstalled separately. For all turbne classes, these pre-assembly methods are used. 35

44 Pre-assembly method ( ) Table 3. - Popular pre-assembly methods for offshore wnd turbnes Sub-assembles umber of assembly operatons done onshore ( m ) umber of separate segments for each turbne ( ) Method (acelle+hub+2 blades)+tower n pece+3 rd blade 4 3 Method 2 (acelle+hub+2 blades)+tower n 2 peces+3 rd blade 3 4 Method 3 (Hub+3 blades)+tower n 2 peces+ nacelle 3 4 Method 4 (acelle+ hub)+ tower n pece+3 blades 2 5 Method 5 (acelle+ hub)+ tower n 2 peces+3 blades 6 L For transportaton of wnd turbnes from port to farm ste and nstallng them, specal purpose-bult self propelled nstallaton vessel(s) s used. Each vessel has avalable deck area of square meters to carry wnd turbne parts, has speed of V S meter/hour. Ths type of vessel has specal legs attached to t and s capable of steady tself on ths legs when requred. Ths operaton s called ackng up operaton. Each vessel has ackng up speed of V number of such vessel employed for transportaton and nstallaton. V JU meter/hour. To calculate the nstallaton and transportaton cost, tme requrement for transportng and nstallng turbnes are formulated and the daly rate of the vessel s calculated. Total cost was formulated by multplyng the daly rate wth the total tme for transportaton and nstallaton. 3.2 ssumptons and notatons Transportaton and nstallaton of wnd turbnes are complex tasks requrng a combnaton of varous sub tasks. To reduce the complexty of analyss of nstallaton process for an offshore wnd farm, several assumptons are taken nto account durng the model formulaton. 36

45 3.2. ssumptons Followng assumptons are necessary to formulate the model:. ll the vessel(s) and turbnes are dentcal (same geometrcal propertes). 2. For all turbnes, same pre-assembly method s used. 3. Vessel(s) are avalable throughout the transportaton and nstallaton perod. 4. Weght concentraton on the deck of a vessel does not exceed the lmt. 5. Crane on the vessel(s) s the only avalable opton for performng lftng operaton. 6. Crane capacty s suffcent to lft the components of turbnes otaton The followng notatons are used n the paper: (a) Indces: Index for type of turbne class used Index for type of turbne pre-assembly used (b) System parameters: C D d V S Rated capacty of the wnd farm (megawatt) Dstance from port to farm ste (meter) Dstance between two turbnes stes (meter) Vessel speed (meters/hour) V JU Vessel ack up speed (meters/hour) V umber of vessel used (unt) Deck area avalable for transportng foundaton (square meter) H H Turbne hub heght (meter) 37

46 H JU Jack up heght (meter) R L Rate of lftng (meter/hour) R Rate of assembly (assembly/hour) t PL Pre-loadng tme at the port (hour) t FS Pre-loadng tme at turbne ste (hour) L Learnng rate for the crane operaton 0 L R S Captal cost of a vessel ($) R P S Percentage of fnanced captal cost I Y U e Interest rate for fnanced captal cost Vessel lfe (years) Utlzaton rate of vessel R Return on nvestment of vessel O C Daly operatng cost of vessel ($) (c) Intermedate varables: umber of turbnes n the farm (unt) n umber of sub assembles done onshore (unt) (d) Decson varables: P Rated power output of one turbne (megawatt) L umber of lfts for each turbne durng loadng or nstallaton (unt) T rea requred for one turbne durng transport (square meter) 38

47 3.3 The T&I model mathematcal model for transportaton and nstallaton cost of offshore wnd turbnes s developed n ths secton. To do that, frst tme requrement for the whole transportaton and nstallaton s determned and also daly cost of the vessel s formulated. Followng that, total cost s obtaned from the product of the two. Ths total cost s the ntal captal nvestment for offshore wnd transportaton and nstallaton from whch equvalent annual cost s determned Transportaton and nstallaton tme estmaton The nameplate capacty of the offshore wnd farm s C. If turbne class of rated power output P s used, number of turbnes would be requred to reach the wnd farm capacty, snce C P. The whole process of transportaton and nstallaton s subdvded nto several operatonal segments. Tme requrements for these operatonal segments of the process are determned and then summed together to determne the total tme requrement for wnd turbne transportaton and nstallaton. In the sub sectons followed, tme requrements for varous operatons are developed. (a) Travel/transportaton tme occupes If the turbnes are pre-assembled followng pre-assembly method, then each turbne square meters of deck area on the nstallaton vessel. In ths confguraton, t s possble to transport / number of turbnes n a sngle trp of the vessel. For V (for now V s assumed to be ) number of dentcal vessels, each wth avalable deck area of square meters, number of requred trps for transportng number of turbnes s found by dvdng total requred area by turbnes by avalable deck area V n each trp. So, requred number of trp s / V. If the dstance between port and farm ste s meters and 39

48 dstance between two adacent turbnes are meters, n each trp the vessel has to travel D meters from port to farm ste rrespectve of the number of turbnes t s carryng. fter loadng of turbnes at the port t arrves to the frst turbne ste, nstall that turbne and travel d meters to the second turbne and so on untl all the turbnes t was carryng have been nstalled. Then the vessel has to travel another D meters to return to the port. For nstallng number of turbnes the travelled dstance wthn the farm s / V d. lso, the vessel has to spend t PL hours at the port durng each trp and t FS hours at each turbne ste. So total travel tme for transportng turbnes s: T v V V S ( ) d V 2 (3.) D t FS t PL VS V whch can be smplfed to Tv 2 D d t PLVS 2V d t FSVVS (3.2) VVS where s a functon of turbne s rated power output and pre-assembly method. For a wnd turbne, rotor dameter s the dmenson that commensurate turbne s rated power output, the relaton between the two can be approxmated as an exponental one. Deck area requrement for a turbne can also be approxmated to an exponental functon of the turbne s rated power output. So, deck area requrement of a turbne wth rated power output of P and followng preassembly method s gven by the followng: q P 2 T e (3.3) 40

49 where q s a constant coeffcent and T s the deck area requrement of a turbne wth nomnal (2 MW) rated power output and followng pre-assembly method. Combnng equatons (3.2) and (3.3), total travel tme s found as followng: T v V V S 2D d t PL V S T e q P 2 2V d t FS V VS (3.4) whch gves the total travel tme of the vessel for transportng and nstallng the turbne components. (b) Installaton and vessel loadng tme Besdes transportaton, nstallaton process nvolves several actvtes, and tme requrements for them need to be estmated. Tme requrements of these tasks can be descrbed as followng:. Lftng operaton tme: a. Tme to lft turbne parts/sub-assembles aboard the vessel onshore b. Tme to lft turbne parts/sub-assembles durng offshore nstallaton 2. ssembly operaton tme: a. Tme to pre-assemble the parts of the turbne onshore b. Tme to assemble the remanng un-assembled parts of the turbne offshore 3. Jackng up operaton tme Tme requrements for these tasks depend on vessel propertes as well as operator s effcency and expertse. Rates of performng the operatons,.e. rate of lftng and rate of assembly are functons of learnng rate, turbnes rated power output and number of operatons 4

50 performed. Cumulatve averages of these rates are calculated from the ntal rates of performng these operatons, turbne s rated power output and number of operatons as follows: RL R R L q2 P 2) and R b q P (3.5) 2 2) b e e O L O where b log( LR )/ log 2, LR, b 0 and q2 denotes a constant, L R s the learnng rate,, are total number of lftng and assembly operatons respectvely and R L, R are ntal OL O rates of lftng and assembly operatons for turbnes wth nomnal rated power output. Both of these rates decreases exponentally wth ncreasng rated power output of turbnes. Learnng rate s assumed to be fxed throughout the nstallaton perod. learnng rate of 90% ndcates tme to perform an operaton s reduced by 0% whenever the number of operaton s doubled. Vessel ackng up tme depends on the ackng up speed V JU, whch s a characterstc of the vessel. In the followng sectons, tme requrements for each of the tasks pertanng to nstallaton are developed n detal. (c) Lftng operaton tme s mentoned n Table 3., followng pre-assembly method, each turbne conssts of L number of separate turbne segments, and so each turbne requres L number of lfts (pckng up and placng at the desgnated area) durng loadng at the dock and agan durng nstallaton at the turbne ste. So, total number of requred lfts s 2 O L L. Lftng tme s proportonal to the heght to whch the components must be lfted. Durng loadng at the port, the lftng heght s equal to the vessel ack up heght, H JU meters, and durng nstallaton lftng heght s equal to the turbne hub heght, lftng heghts and rate of lftng and s gven by the followng: 42 H H meters. Total lftng tme s a functon of

51 T L L H H H JU (3.6) R L ow, hub heght H H s a functon of turbne s rated power output P, the relatonshp 2 between these two can be expressed as H ap b P c, where a, b, c are constant coeffcents. So, equaton (3.6) can be rewrtten as followng: H T L L 2 a P b P c H JU (3.7) R L Takng nto consderaton equaton (3.5) and 2 O L L, equaton (3.7) can be rewrtten as: T L 2 b b q2 P 2 L e R L 2 a P b P c H JU (3.8) whch gves total lftng tme requrements for all turbne components. (d) ssembly operaton tme Each turbne s conssted of M number of parts, so n total M number of subassembles need to be done rrespectve of the turbne s rated power output and pre-assembly method. Pre-assembly method determnes how many sub-assembles among M would be done onshore. The remanng sub-assembles are done at the farm ste durng nstallaton. nother sub-assembly s done between the transton pece of the foundaton and the lower part of the turbne, so n total, M number of sub-assembles are needed to be done for complete nstallaton of turbnes. It s assumed that all the sub-assembles are smlar n nature,.e. they requre same amount of tme to be done under the smlar condtons. If for a sngle turbne followng pre-assembly method, number of onshore assembly operaton s 43 m, then for complete nstallaton of a turbne, M m offshore assembly operatons are requred. Due to

52 the waves and wnd at the farm ste, t requres more tme to perform assembly operaton offshore compared to onshore (at the port). multpler W s ntroduced to consder offshore condtons. Total tme requrement for assembly operaton s expressed as followng: m W M m T (3.9) R R where s the cumulatve average rate of performng assembly operaton as mentoned n equaton (3.5). From Table 3., t s known that operatons done onshore s M be rewrtten as followng: L m and for offshore t s L M. So, number of assembly L. Then equaton (3.9) can T b q2 e R P 2 M L b W b L (3.0) whch gves the total tme requrement for assembly operatons perform for turbne nstallaton. (e) Jackng up operaton tme For stablty of the vessel and better crane operatons, the nstallaton vessel s elevated from the water level durng loadng at the port and also durng nstallaton at the farm ste. For ackng up, vessel s legs are reached and protruded to the ground below sea, and the vessel gradually lfts tself up to the requred heght. Once the task at that ste s fnshed, vessel legs are pulled up and the vessel comes down at sea level. t the begnnng of each trp, the vessel s loaded wth turbne parts or foundaton parts at the port. Durng each trp, the vessel performs ack up operaton at the port. fter a vessel s reached to the nstallaton ste carryng ether foundaton or turbne, t does the ack up operaton at each nstallaton ste. The vessel has to perform four operatons (rasng the platform up and down and extendng and pullng 44

53 45 back of vessel legs) at every turbne ste durng nstallaton. Total tme requred for ackng up operaton can be wrtten as: 4 V H T JU JU JU (3.) where JU V s the ack up speed of the vessel and JU H s the heght that the vessel needs to be elevated durng ack up. Takng nto consderaton equaton (3.3), the above equaton can be rewrtten as followng: 4 2 e V H T P q T JU JU JU (3.2) whch gves the total tme requrement for ackng up operaton of the vessel Total tme requrement Total tme requrement for transportaton and nstallaton of turbnes s found by summng the tme requrements for the operatons descrbed n the prevous secton. The expresson for total tme requrement whle only one vessel s employed s obtaned by summng equatons (3.4), (3.8), (3.0) and (3.2). Consderng V number of vessel s employed for the transportaton and nstallaton purpose, total tme requrement s found by dvdng the expresson by V. Total tme requrement for transportaton and nstallaton s found as followng: T S FS P q T S PL S V V t d V e V t d D V V e V V H P q T JU JU b L b L L JU b L b P q b R W M R H c P b P a V e (3.3) Replacng by P C /, the expresson becomes as followng:

54 46 T PL JU JU S P q T t V H V d D PV e C FS JU JU S t V H V d P V C 4 2 b L b L L JU b L b b P q R W M R H c P b P a P C V e (3.4) rea requrement on the vessel deck for each turbne of nomnal rated power output T and number of separate segments for each turbne L both are functons of pre-assembly method. If pre-assembly method s followed, each turbne requres T square meters on vessel deck durng transportaton and each turbne conssts of L number of separate segments. Thus total tme requrement for transportaton can be expressed as a functon of turbne s rated power output and pre-assembly method. So, equaton (3.4) can be wrtten as followng: L P T, P e P q T P q b b b L e P P P P M L (3.5) where, 4 2 FS JU JU S t V H V d V C, 2 U 2 PL JU J S t V H V d D V C b R V C,, 2 L JU b b R W R H c V C, 2 L b b R V a C

55 b b C 2 b and. V R L Transportaton and nstallaton cost calculaton It s assumed that transportaton and nstallaton of offshore wnd turbnes s done contnuously 24 hours a day, 7 days a week untl all the wnd turbnes are nstalled. Once the tme requrement s calculated as dscussed n the prevous sectons, cost of transportaton and nstallaton can be calculated by multplyng the tme n days wth daly rate of the vessel. (a) Daly rate of the vessel For offshore wnd farm nstallaton purposes, requred vessel(s) are usually leased from vessel owners for a perod of tme and the vessels are returned after proect completon. Self propelled nstallaton vessels are most popular for offshore wnd farm nstallaton purpose. ccordng to Kaser and Snyder (20), daly cost ncurred or daly rate of vessel D L s a functon of the owner s captal cost for the vessel S, proporton of the external fnanced nvestment P S, vessel s lfe Y, nterest rate for the fnanced nvestment I, utlzaton rate R s return on nvestment, daly operatng cost O C and s calculated as followng: U e, D L Vn 365U e SPS Y I 2 24 I Y S P Y S SR O C (3.6) Total cost of transportaton and nstallaton Expresson for total cost of transportaton and nstallaton s obtaned by multplyng the total tme requrement (days) for transportaton and nstallaton wth vessel day rate. So, from equatons (3.5) and (3.6), total cost can be expressed as: 47

56 48 L P TC, = C S S e n O R I P Y I P U SV P q b b b L P q T e P P P P M P e L (3.7) nnual cost of transportaton and nstallaton Total cost for transportaton and nstallaton of offshore turbne as determned above ncurs only once durng the nstallaton of the wnd farm. Ths cost acts as the nvestment cost whch s usually fnanced from varous fnancal sources and need to be repad. Therefore, determnaton of annual equvalent cost of transportaton and nstallaton would be benefcal. For the offshore wnd farm wth proected lfe of n years and nterest rate for fnanced nvestment, annual transportaton and nstallaton cost C s gven by the followng: L P C, = C S S e n O R I P Y I P U SV P q b b b L P q T e P P P P M P e L n P,, / (3.8) where annual cost C s converted from total cost TC followng the relatonshp between net present value and annual equvalence,, / n n P n P P. In ths formula, symbol P s used to ndcate the net present value (to dstngush t from turbnes rated power output, P ) whch n equaton (3.7) s represented by the total cost TC, and symbol s used to ndcate the annual equvalence (to dstngush t from vessel capacty, ) whch n the equaton s represented by annual cost C. Equaton (3.8) gves the expresson for annual cost for turbne transportaton and nstallaton for an offshore wnd farm wth farm capacty of C MW, each turbne s rated power

57 output s P MW and any one of fve pre-assembly methods s followed for nstallaton. From ths model, optmum rated power output P of each turbne and optmum pre-assembly method can be determned. By choosng hgher turbnes class, number of turbnes n the farm can be reduced but that would result n ncreased deck space requrement for each turbne and therefore less number of turbnes can be transported n each vessel trps. Hgher rated power turbnes also results n longer tme for lftng and assemble each turbne. Pre-assembly method s another controllng varable of transportaton and nstallaton cost model. Hgher degree of pre-assembly onshore results n lower number of offshore lfts and offshore assembly, but t also results n hgher number of vessel trps. So, tradeoffs need to be made optmally for selectng turbne s rated power output and pre-assembly method. 3.4 Soluton procedure In ths secton, the soluton procedure for the transportaton and nstallaton cost has been shown. n exhaustve search method s used due to the lmted soluton space. The algorthm for optmum soluton of turbne class and pre-assembly method s shown below lgorthm : Soluton procedure for T&I cost model Step : Intalze parameters,,,,,, other cost parameters andc. Step 2: Construct the sets of possble values for turbne s rated power output, P,,2, K, number of lfts for each turbne,,,2 Z P..., requred deck area for each turbne,,, 2 Z correspondng controllng varables P, T T,..., and L L,..., for each of the and L T. Step 3: Fndng the mnmum annual transportaton and nstallaton cost, C For,2,..., K 49

58 For, 2,..., Z (a) Calculate transportaton and nstallaton cost, (b) Fnd C Mnmum C. C usng equaton (3.8). (c) Identfy P, and L, correspondng to T C, obtaned n Step 3(b). Step 4: Stop 3.5 Computatonal results The obectve of ths secton s to provde a detaled analyss of the developed model through a case study. In the model for transportaton and nstallaton cost of offshore wnd turbnes, the decson varables are turbne class and pre-assembly method. For a farm wth fxed nameplate capacty, a numercal study s performed that provdes nsghts as to what combnaton of turbne s rated power output and pre-assembly method mnmzes the cost of transportaton and nstallaton of turbnes. The model parameters,,,,, are determned from wnd farm and vessel parameters. Table 3.2 provdes the values for varous parameters assocated wth wnd farm and nstallaton vessel that are used n calculatng the model parameters. The values of the parameters are collected from avalable data sheets and proect reports of operatonal offshore wnd farms and vessels and then averages of these values are taken [Thomsen (202), Uraz (20), Kaser and Snyder (200), 4C Offshore (203)]. Varous cost parameters are set accordng to the exstng market scenaro. 50

59 Table Wnd farm and vessel Parameters Parameter Descrpton Value C Wnd farm capacty 300 MW D Dstance from farm ste to port 200,000 meters d Dstance between two turbnes at the farm,000 meters Deck area of the vessel 2,000 square meters V S Vessel speed 5,000 meter/hour M umber of parts n each turbne 7 R L Lftng rate 40 meters/hour R Intal assembly operaton rate assembly/2 hours t Pre-loadng tme at port 5 hour PL t Pre-loadng tme at turbne ste hour FS W Multpler for offshore lft 2 V n umber of vessel H Jack up heght 35 meters JU V Jackng up speed 30 meters/hour JU q Constant 0.09 q Constant a Constant b Constant c Constant 77.2 LR Learnng rate 0.95 b log( LR ) / Log(2) Captal cost of vessel $ S U e Y P S I O R D L n Utlzaton rate of vessel 90% Vessel Lfe 20 years Fnanced percentage of vessel captal cost 80% Interest rate for fnanced captal for vessel 5% Daly operatng cost of vessel $35000 Return on nvestment 5% Daly rate of vessel $05066 Proected lfe of the wnd farm Interest rate of nvestment n wnd farm 5% 20 years 5

60 Table 3.3 provdes the model parameter values calculated from wnd farm and vessel parameters. In calculatng these values, nformaton from vessel operators and wnd farm developers has been used. Confdence lmts for the same parameters have been estmated from avalable data of seven offshore wnd proects. Consderng turbne class and pre-assembly method as the varables, from equaton (3.5), parameters have been estmated usng multvarate regresson analyss. It s found that parameters obtaned from the model fall wthn the specfcaton lmts for the parameters estmated from the proect data. Parameter Table Model parameters and ther values From the model From proect data (95% confdence lmt) Lower lmt Upper lmt , ,053.78, Parameters assocated wth offshore wnd farm, for example dstance from port, dstance between two turbne stes are assumed to be fxed. Parameters assocated wth vessel are also assumed to be fxed n nature. Effects of these parameters on cost are also nvestgated. nnual transportaton and nstallaton costs are calculated for an offshore wnd farm wth rated capacty 300 megawatt (MW) and turbnes wth rated power output of one of the fve avalable classes e.g. 2.0, 2.3, 3.0, 3.6, 4.0 and 5.0 megawatt followng fve dfferent pre-assembly methods usng equaton (3.8). Table 3.4 shows the nput matrx for varous combnatons of turbne class and pre-assembly method. Every pre-assembly method s characterzed by number of separate turbne segments for each turbne and area requrement (square meter) for each turbne. rea requrement for each turbne also depends on turbne class. 52

61 Table Input matrx of turbne class and pre-assembly method Turbne class (umber of turbne segments, area requrement) for pre-assembly methods (o. of turbnes) Method Method 2 Method 3 Method 4 Method MW (50 turbnes) (3, 550) (4, 630) (4, 500) (5, 360) (6, 480) 2.3 MW (3 turbnes) (3, 567) (4, 650) (4, 56) (5, 37) (6, 495) 3.0 MW (00 turbnes) (3, 609) (4, 698) (4, 554) (5, 399) (6, 532) 3.6 MW (84 turbnes) (3, 648) (4, 742) (4, 589) (5, 424) (6, 565) 5.0 MW (60 turbnes) (3, 747) (4, 856) (4, 679) (5, 489) (6, 652) In Table 3.4, the frst element n the parenthess ndcates number of separate segments for each turbne and the second element ndcates area requrement for each turbne for dfferent turbne classes and pre-assembly methods. In Table 3.5, turbne transportaton and nstallaton cost computed from the model for a wnd farm wth rated power output of 300 MW are summarzed. Costs are calculated for fve pre-assembly methods and fve turbne classes that are commercally avalable n the market. Learnng rate s set at 95%. Table Transportaton and Installaton cost of turbnes for a wnd farm of 300 MW Turbne class nnual transportaton and nstallaton cost ($) (o. of turbnes) Method Method 2 Method 3 Method 4 Method MW (50 turbnes),824,840,97,29,827,4,882,8 2,03, MW (3 turbnes),688,447,83,382,83,382,767,999,975, MW (00 turbnes),489,624,822,75,630,873,63,480,897, MW (84 turbnes),430,575,739,369,578,24,639,9,857, MW (60 turbnes),565,290,74,426,74,426,738,60,959,639 From Table 3.5, t s observed that, n general, ntally cost of transportaton and nstallaton decrease as the rated power output of each turbne ncreases, then tme requrements reach ther mnmum and start to ncrease agan wth ncreasng rated power output. From the Table 3.5, for a wnd farm wth 300 MW rated capacty, deployng 84 turbnes each wth 3.6 MW rated power output s optmal and results n mnmum cost when pre-assembly Method 53

62 s followed. For a fxed turbne class, e.g., 3.6 MW rated power output, and for learnng rate of 95%, a bar chart s shown n Fgure 3. representng costs for dfferent pre-assembly methods. Fgure 3. - Transportaton and nstallaton cost for a fxed turbne class In Fgure 3., costs of transportaton and nstallaton for dfferent pre-assembly methods are compared when 84 unts of 3.6 MW wnd turbnes are used. The fgure suggests that, under the condton and assumpton descrbed, pre-assembly Method s the most favorable one. 3.6 Senstvty analyss In ths secton effects of some parameters on transportaton and nstallaton cost are dscussed. Learnng rate of operators n lftng and assembly operaton has a sgnfcant effect on tme requrement and cost and has been dscussed here. Varous parameters related to offshore wnd farm and nstallaton vessel have mpact on the tme requrement and thereby cost. mong those, the most crtcal ones, namely, dstance from operatng port to farm ste, avalable vessel deck area and rate of lftng of vessel crane have been nvestgated and dscussed here. 54

63 3.6. Effect of learnng rate Learnng rate of lftng and assembly operatons mpacts transportaton and nstallaton cost. Hgher learnng rate results n lower cost of transportaton and nstallaton. It also affects the choce of turbne class and pre-assembly method. Table 3.6 summarzes the effect of learnng rate across turbne classes and pre-assembly methods. Table 3.6- Effect of learnng rate on turbne transportaton and nstallaton cost Turbne class (o. of turbnes) 2.0 MW (50 turbnes) 2.3 MW (3 turbnes) 3.0 MW (00 turbnes) 3.6 MW (84 turbnes) 5.0 MW (60 turbnes) Learnng rate nnual transportaton and nstallaton cost ($) Method Method 2 Method 3 Method 4 Method 5 o learnng 2,396,066 2,655,085 2,5,205 2,683,90 3,029,246 95%,824,840,97,29,827,4,882,8 2,03,972 90%,454,459,534,256,390,377,377,697,53,342 85%,220,876,262,668,8,788,069,224,87,048 o learnng 2,230,073 2,480,382 2,480,382 2,529,643 2,855,342 95%,688,447,83,382,83,382,767,999,975,695 90%,333,672,42,373,42,373,283,98,425,796 85%,07,525,49,98,49,98 984,823,09,652 o learnng,993,345 2,427,75 2,235,873 2,324,936 2,720,935 95%,489,624,822,75,630,873,63,480,897,769 90%,52,958,424,20,232,367,52,233,372,83 85% 933,74,68, ,72 86,02,047,064 o learnng,935,359 2,346,822 2,85,677 2,355,422 2,686,32 95%,430,575,739,369,578,24,639,9,857,303 90%,088,79,334,05,72,905,70,064,32,85 85% 863,84,070, , , ,032 o learnng 2,6,064 2,407,574 2,407,574 2,526,43 2,875,494 95%,565,290,74,426,74,426,738,60,959,643 90%,83,04,285,937,285,937,208,542,353,52 85% 924, , , , ,455 It s observed from the Table 3.6 that pre-assembly Method s most favorable n terms of transportaton and nstallaton cost when there s no learnng effect and farly large dfference exst between Method and other methods. Mnmum cost s acheved for turbne class wth rated power output of 3.6 MW. In case of 95% learnng rate, Method and Method 4 become almost the same effcent n term of cost for turbne classes 2.0 and 2.3 MW, but as 55

64 turbne s rated power output s ncreased further, Method becomes more favorable. Least cost s ncurred when pre-assembly method s used and turbnes wth rated power output of 3.6 MW are deployed. s learnng rate ncreases further to 90%, pre-assembly Method 4 becomes the preferable to Method, for all turbne classes except 3.6 MW turbne class, for whch, Method gves the least cost among all combnatons of turbne class and pre-assembly method. If learnng rate s as hgh as 85%, pre-assembly Method 4 s the best possble method for all turbne classes and transportaton and nstallaton s done n mnmum cost when 5.0 MW turbne class s deployed. Fgure 3.2 depcts the change n tme requrement due to change n turbne s rated power output for dfferent pre-assembly methods. Tme requrements have been calculated for four dfferent learnng rates to llustrate the effect of learnng on total tme requrement. Ths test case consders an offshore wnd farm of 300 MW rated capacty. For 90% learnng rate, the least cost s obtaned when followng pre-assembly Method and usng 84 unts of turbnes each wth rated power output of 3.6 MW. Fgure Transportaton and nstallaton cost for dfferent turbne classes 56

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