COMPARATIVE ANALYSIS OF WAVE WEATHER WINDOWS IN OPERATION AND MAINTENANCE OF OFFSHORE WIND FARMS AT HSINCHU AND CHANGHUA, TAIWAN

Journal of Marne Scence and Technology, Vol. 5, No. 5, pp. 563-570 (07) 563 DOI: 0.69/JMST-07-0703- COMPARATIVE ANALYSIS OF WAVE WEATHER WINDOWS IN OPERATION AND MAINTENANCE OF OFFSHORE WIND FARMS AT HSINCHU AND CHANGHUA, TAIWAN Heng-Wen Chang,, Mng-Jun Maa, and Mng-Chung Ln Key words: weather wndow, operaton and mantenance, offshore wnd farm. ABSTRACT The explotaton of offshore wnd farms s an nexorable trend n the development of wnd power generaton n Tawan. The operaton and mantenance (O&M) perod at offshore wnd farms depends on the wave condtons that allow vessels to safely access wnd turbnes. Ths study used well-calbrated smulaton wave data for a 9-years perod for the sea areas of Hsnchu and Changhua to analyze the weather wndows and quantfy the accessblty of offshore wndfarms for O&M. Two factors, namely wave heght lmt and wndow length, were consdered. The results revealed a hgher wave dstrbuton and lower percentage of access hours at a wave heght below m for each month at Changhua. In addton, hgher levels of access were observed at Hsnchu than at Changhua. The annual number of wndows decreased as the wndow length ncreased. November was more accessble than the adjacent months n wnter for smaller wave heght lmts. The naccessblty analyss revealed that, for a wave heght lmt of m and a wndow length of at least 6 h, the longest watng tme for access s 7.6 days at Hsnchu and 3.9 days at Changhua; f the wave heght lmt s up to.5 m, the longest watng tmes are only. and 6.3 days, respectvely. At Hsnchu, the hghest possble number of watng perods s less than days, rrespectve of wave heght lmts and wndow length. At Changhua, most watng perods are also less than days. I. INTRODUCTION The development and applcaton of wnd electrcty have Paper submtted 0/3/7; revsed 05/03/7; accepted 07/03/7. Author for correspondence: Heng-Wen Chang (e-mal: BHWChang@tr.org.tw). Green Energy and Envronment Research Laboratores, Industral Technology Research Insttute, Hsnchu County,Tawan, R.O.C. Department of Engneerng Scence and Ocean Engneerng, Natonal Tawan Unversty, Tape County, Tawan, R.O.C. been postvely promoted n Tawan snce 000 (MOEA, 03). Through wnd resource exploraton, technology transfer, research and nvestgaton, and publcty and promoton by the government, Tawan Power Company (TPC) and other prvate companes have focused on successvely developng onshore wnd power. Untl June 05, 8 wnd farms were bult onshore, wth a total of 3 wnd turbnes and a cumulatve nstalled capacty of 637.5 MW. Apart from onshore wnd power, abundant wnd resources are avalable n Tawan s western sea. The area s estmated to have approxmately 00 MW of wnd power capacty at depths from 5 to 0 m and 5000 MW at depths from 0 to 50 m. On July 4, 0, the Mnstry of Economc Affars announced and launched the Offshore Demonstraton Incentve Program (DIP). Ths was the turnng pont n the shft from onshore to offshore wnd power generaton n Tawan. The Offshore DIP was formulated to encourage prvate companes to buld demonstraton offshore wnd farms wth a budget subsdy for both equpment and developng processes. Three cases were selected, and each case ncludes one wnd mast and two offshore wnd turbnes that are requred to be bult by the end of 07, except for the case nvolvng TPC, the schedule of whch could be postponed for reasons related to local ndustry. For three demonstraton wnd farms, 60 wnd turbnes wth a total of 300 MW capactes are expected to have been bult by the end of 09. One of the three offshore wnd farms s located near the Hsnchu sea area and the other two are located at the Changhua sea area. Subsequently, through zonal development at a commercal scale (e.g., at Changhua, Yunln, and Chay open sea areas) and an nstalled capacty of 300 MW per year, offshore wnd farms are expected to be promoted to gradually acheve an nstalled capacty of 4000 MW. From the perspectve of offshore wnd farm setup costs, wnd turbnes account for approxmately 30%-40% of total costs, whereas operaton and mantenance (O&M) and ocean engneerng (ncludng nstallaton and grd connecton) account for approxmately 55% (Chen et al., 04). Therefore, O&M and ocean engneerng costs substantally affect nvestment gans. Evdence from the development of offshore wnd farms n Europe

564 Journal of Marne Scence and Technology, Vol. 5, No. 5 (07) past years ndcates the cost of offshore wnd farms s stll hgh because of problems such as lmted weather wndows for operaton, the huge moblzaton cost of nstallaton vessels, harsh envronmental damage to equpment (ncludng wnd turbnes and submarne cables), unclear expectatons regardng completon schedules, avalablty of equpment, and O&M costs at the desgn stage. Consequently, n Europe, cost reducton s currently the man target n attempts to mprove aspects such as external condtons, wnd turbne systems, grd ntegraton, and offshore technology (European Wnd Energy Technology Platform, 04). The bggest dfference between offshore wnd farms and onshore wnd farms s the envronment. The envronment of offshore wnd farms s substantally more challengng than that of offshore wnd farms. The feasble mantenance perod on offshore wnd farms depends entrely on martme wnd and wave condtons. Therefore, more expensve, dffcult, and tme-consumng O&M actvtes are requred n offshore engneerng. The most mportant factor s the accessblty, whch nvolves the ablty of vessels to safely access a wnd farm for a perod suffcently long to perform mantenance actvtes. The wave weather wndow denotes the perod for whch waves are smaller than the threshold, whch s the lmt at whch wave heght vessels can access to complete specfc mantenance operatons. Avalablty and accessblty are two major concerns for offshore wnd farms (O Connor et al., 03). Avalablty s defned as the perod for whch wnd turbnes can generate electrcty. Wnd farm avalablty s dependent on numerous factors, such as falure rates, downtmes for recovery after falure, naccessblty, lack of spare parts, and logstcs problems nvolvng wnd turbnes. Accessblty s the percentage of tme for whch a wnd turbne can be accessed, and t nfluences the falure rate and, ultmately, the avalablty of wnd turbnes. Turbnes that are more accessble receve more regular mantenance and consequently tend to have a lower falure rate. In addton, accessblty affects downtmes after falure because of affectng the tme requred to perform repars. Evdently, a detaled analyss on the accessblty of wnd farms s necessary. Dfferent O&M strateges were adopted for dfferent weather wndows to ensure the hgh avalablty of wnd turbnes. A further reason for usng weather wndows analyss s that of economcs: Walker et al. (0) concluded that the prmary nfluencng factor for nstallaton captal expendture s the downtme due to weather wndows; thus, understandng weather wndows s essental n the plannng of operatons. Salzman et al. (007) stated that more than 90% of all mantenance actons requre only the transfer of personnel and parts, whch can be carred by man or lfted usng a turbne s permanent nternal crane. Therefore, the transportaton of personnel to wnd turbnes becomes the man problem. Boat-to-ladder transfer s a more conventonal access method and s commonly used. Each type of vessel has a specfc ablty under certan accessble wave condtons. The accessble sgnfcant wave heght s approxmately.5 m for ordnary vessels, approxmately.0 m for passenger yachts, and up to.5 m for largescale multfuncton workng shps. More advanced access vessel systems, whch enable people to walk stably to the ladder of wnd turbnes, are currently beng developed. More expensve vehcles such as helcopters, whch are not nfluenced by waves, could be used f the wnd turbne ncludes a landng pad. Unlke that of wnd turbnes wth fxed foundatons, the mantenance of floatng wnd turbnes nvolves not only access problems but also ther floatng behavor. Ths challenge s smlar to that of wave energy converters and s not consdered here. The present study focused only on analyzng the wave weather wndow of on-ste O&M excludng the trp to ste. Although wnd speed may also be an nfluencng factor when cranes are used, t was not nvestgated n ths study. Moreover, the day and nght cycle was not consdered. Conductng measurements n oceans s expensve, partcularly n the development of offshore wnd farms, whch also requres wnd measurements at the hub heght. When suffcent metocean data are not avalable from wnd farm stes, the use of numercal models s a feasble, cheap, and effcent means of smulatng metocean condtons after valdatng the numercal model by usng neghbor observatons. The man goal of ths study was to quantfy the levels of access of vessels to wnd turbnes for O&M actvtes and to analyze challenges related to O&M at the Hsnchu (Formosa) and Changhua (TPC and Fuha) sea areas. Due to the lack of observaton data at three demonstraton stes, wth only the Hsnchu buoy of Central Weather Bureau (CWB) close to the Hsnchu demonstraton ste, numercal wave model data was appled to the analyss after verfcaton by usng Hsnchu buoy observaton. The remander of ths paper s organzed as follows: The numercal model and verfcatons are explaned n Secton ; n Sectons 3 and 4, the wave heght analyss and weather wndow analyses are dscussed separately; and dscussons and conclusons are presented n Secton 5. II. NUMERICAL MODEL AND VERIFICATION. Numercal Model Setup NWW3 wave model verson 3.4, a thrd generaton wnd wave spectral model (Tolman, 009), was used for smulaton. The model solves the weakly nonlnear acton balance equaton by usng the explct numercal method. It employs the multgrd approach, featurng a two-way nestng wth grds wth varous resolutons n a sngle wave model. Ths model could be effcently appled to the parallel computng platform. Moreover, t s sutable for applcatons n trans-scale scopes, rangng from klometers to thousands of klometers. The smulaton comprsed three grds (0.5, 0.05, and 0.00 ) and used a hgher resoluton n the regon near the Hsnchu sea area. The numercal water depth was mplemented based on three sources. In the largest area, the depth was extracted from ETOPO of the Natonal Geophyscal Data Center, Natonal Oceanc and Atmospherc Admnstraton (NGDC, NOAA), whch s a arc-mnute global relef model of the Earthʼs surface that ntegrates land topography and ocean bathymetry. The area close to Tawan was replaced wth 500-m resoluton data

H.-W. Chang et al.: Weather Wndow Analyss n O&M of OWF at Tawan 565 m 40 5000 4600 400 3800 3400 3000 600 00 800 400 000 800 600 400 00-00 35 30 5 0 5 0 5 Hsnchu Changhua Fg.. Locatons used n the weather wndow analyss. 00 05 0 5 0 5 30 35 40 45 50 55 Fg.. Computaton domans and water depth. from the Tawan Ocean Research Insttute. At the Hsnchu sea area, 03 water depth measurements wth approxmately 50-m resoluton were used. The computaton domans of the wave model and water depth are presented n Fg.. Wnd forcng was obtaned from hndcast wnd of the CWB Nonhydrostatc Forecastng System (NFS) wth two spatal grd resolutons, RC (45 km) and MC (5 km). Ths nvolves the objectve analyss results beng ntegrated wth the observatons (soundng reports, soundng wnd reports, arcraft reports, satellte observatons, surface synoptc observatons, shp observatons, bogus CWB Global Forecast System (GFS) data, and dropsonde). Important parameters of numercal models contan 5 frequences from the lowest frequency (0.0478 Hz), frequency ncrement factor., and 5 drectonal resoluton. The bultn wnd nput source and dsspaton term proposed by Tolman and Chalkov (Tolman, 009) were used.. Methodology for Verfcaton For verfcaton, the study used the CWB hourly, qualtycontrolled observed buoy data from Hsnchu for the January 005 to December 03 perod, whch was the only long-perod observaton avalable at both stes. Fg. presents the locatons of the Hsnchu buoy and Changhua Offshore DIP. The study used the followng conventonal verfcaton metrcs to quanttatvely assess the magntude of errors, bas (BIAS), root mean square error (RMS), correlaton (CR), scatter ndex (SI), and performance score (Ps), whch are defned as follows (Chawla et al., 009): The nondrectonal error metrcs are gven by BIAS N RMS N CR (P O ) () (P O ) Ps (3) (4) Rˆ MS bˆ SˆI 3 (5) BIAS RMS ˆ ( SI ) (6) ˆ RMS ( ) bˆ ( ) SI ORMS ORMS where ORMS s the root mean square of the measurements. For drectonal data, the angular bas and crcular correlaton are defned as follows: S BIAS tan ( ) C for S 0, C 0s S tan ( ) for C 0 C (7) S tan ( ) for S 0,C 0 C where S sn( p o ) C cos( p o ) CR sn( ) sn( sn( ) sn( () where P and O refer to model and observaton parameters (sgnfcant wave heght, Hs; mean wave perod, Tm0; and peak perod, Tp). N s the number of data. To compute the performance score, error estmates must be normalzed. o o p p ) p p ) ( P P )(O O) ( P P ) (O O) ( P O BIAS ) N SI O BIAS ) where bˆa ( 80 o o Ps bˆa CR (8) (9)

566 Journal of Marne Scence and Technology, Vol. 5, No. 5 (07) Table. Performance metrcs of numercal wave model (Buoy Hsnchu, 005.-03.). Hs (m) T m0 (sec) Tp (sec) Dr ( ) BIAS -0.5-0. -0.37 6. RMS 0.39 0.79.04 43.5 CR 0.78 0.53 0.43 0.67 SI 0.40 0.6 0.3 Ps 0.70 0.87 0.78 0.8 The overall performance scores (Ps) are defned as the average of the normalzed error estmate that the best value of Ps s for the perfect model system, and the worst Ps s 0. Table presents the verfcaton metrcs. The negatve bas ndcates that the model s relatvely small compared wth the observaton. The score ndcates that the model performs better n forecastng the wave perod (T p and T m0 ) and wave drecton than the wave heght Hs. The valdaton result of the NWW3 model conducted by NOAA (Chawla et al., 009), whch compared wth buoy measurements, revealed that the performance scores averaged by regons (for Alaska, the US West Coast, and US East Coast) were 0.84-0.89 for Hs, 0.83-0.86 for T p, and 0.80-0.88 for wave drecton durng 007-008. The scores also ndcated that the model performs better n predctng the peak perod T p than the wave heght Hs. An examnaton of the water depth and dstance from the coast of buoys revealed that the nearest dstance from the coast and mnmum depth were 9 km and 35 m both at the US West Coast and Alaska. In general, the buoys n the two regons belong to deep water buoys. At the US East Coast, the water depths of buoys were between 8 and 47 m, and the nearest dstance was 4 km, except for buoy 4035, whch was located at a depth of 0 m and was 7.7 km from the coast. No observatons near Tawan were used n the study by Chawla et al. (009). Nevertheless, our model results for wave perod and wave drecton were close to ther results; however, the wave heght was slghtly smaller. One reason for ths fndng may be that the Hsnchu buoy was located only 4.5 km from the coast, the wnd was probably not very well represented (5-km resoluton), and the downscalng effect accountng for land sea transtons was not resolved (Chawla et al., 009). Another reason may be the use of dfferent wnd forcng. Chawla et al. (009) used the analyss wnds from the Global Data Assmlaton System (GDAS), whch s a system used by the Natonal Center for Envronmental Predcton (NCEP) Global Forecast System (GFS) model to place observatons nto a grdded model space for the purpose of startng or ntalzng weather forecasts wth observed data. GDAS adds the followng types of observatons to a grdded, 3-D, model space: surface observatons, balloon data, wnd profler data, arcraft reports, buoy observatons, radar observatons, and satellte observatons. Huang (006) compared the NFS wth the wnd measurement by usng the satellte QukSCAT from Aprl 004 to July 005. He noted the negatve bas of RC and MC. Chang (0) compared the NFS wth the wnd data from 005 to 009 calculated usng the Ku-band backscatter coeffcent from the satelltes JASON and JASON. The results revealed that the BIAS was approxmately 0.03 to -0.47 m/s, RMS was approxmately.95-.47 m/s, and CR was approxmately 0.8-0.80. In addton, Chang compared the NFS wth the wnd data obtaned from the European Center for Medum-range Weather Forecastng (ECMWF) from JASON and JASON. The results ndcated that the negatve BIAS was approxmately -0.7 to -0.8 m/s, RMS was.6 m/s, and CR was 0.88. The results of NFS appeared to be close to ECMWF. Rascle and Ardhun (03) concluded that the Clmate Forecast System Reanalyss (CFSR) and NCEP analyss yelded systematcally hgher wnd speed values than ECMWF analyss. Therefore, smaller wnd growth parameters should be used for NCEP wnds. CFSR, whch employs GDAS, was desgned and executed as a global, hgh-resoluton, coupled atmosphere-oceanland surface-sea ce system to provde the best estmate of the state. We can nfer that dfferent wnds resulted n the slghtly smaller performance score n the study. The data coverage rate at the Hsnchu buoy was only 88.8% over 9 years, ndcatng mssng or unrelable data. If weather wndows are to be analyzed on the bass of observatons, the data should be amended and supplemented frst. Ths s a huge task; moreover, data are stll mssng at the Changhua sea area. Therefore, nstead of usng only the observatons at Hsnchu, the numercal model results at both stes were used for consstency n the followng analyss. III. SIGNIFICANT WAVE HEIGHT ANALYSIS The analyss was conducted usng the model output at the stes of the Hsnchu buoy (E0.84, N4.76) and Changhua Offshore DIP (E0.5, N4.03). The water depths were 5 and 8 m, respectvely, whereas the offshore dstances were 4.5 and 9. km, respectvely. The data selecton perod was from January 005 to December 03. Fgs. 3 and 4 present the occurrence probablty dstrbuton and exceedance probablty dstrbuton of the sgnfcant wave heght, respectvely. A hgher percentage of large wave heghts were observed at Changhua when the wave heght was larger than. m. Ths fndng may be because the dstance from the coast s long. At Hsnchu, 7.% of the wave heght was lower than m and 98.6% was lower than m. At Changhua, 55.6% of the wave heght was lower than m and 85% was lower than m. In the Tawan Strat, the northeast monsoon prevals n wnter wth hgher waves, whereas the southwest monsoon prevals n summer wth smaller waves. June to October s the typhoon season, wth an annual average ht frequency of 3 to 4 tmes (CWB, 07). Ths type of weather pattern results n a prolonged perod of calm seas n summer and a contnuous perod of hgh waves n wnter. Ths ndcates more frequent and easer access n summer that could last for a longer perod. By contrast, the opportunty for wnd turbne access n wnter s lmted, and a longer watng tme s requred. The annual average hours of occurrence durng each month of wave heght below the lmts of,.5,, and.5 m are pre-

H.-W. Chang et al.: Weather Wndow Analyss n O&M of OWF at Tawan 567 Wave Hgh Frequency (%) 0 5 0 5 0 HsnChu 0.0 0. 0.4 0.6 0.8.0..4.6.8.0..4.6.8 3.0 3. 3.4 3.6 3.8 4.0 Hs (m) Fg. 3. Occurrence probablty of ndcatve wave heghts at Hsnchu and Changhua sea areas. Hours 800 700 600 500 400 300 00 00 HsnChu <=.0 m <=.5 m <=.0 m <=.5 m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fg. 5. Annual average hours for wave heghts lower than varous wave heght lmts n each month (Hsnchu). Fg. 4. Exceedance (%) 00 90 80 70 60 50 40 30 0 0 0 HsnChu 0.0 0. 0.4 0.6 0.8.0..4.6.8.0..4.6.8 3.0 3. 3.4 3.6 3.8 4.0 Hs (m) Exceedance probablty of wave heghts at Hsnchu and Changhua sea areas. Hours 800 700 600 500 400 300 00 00 <=.0 m <=.5 m <=.0 m <=.5 m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fg. 6 Annual average hours for wave heghts lower than varous wave heght lmts n each month (Changhua). sented n Fgs. 5 and 6. The greatest potental for access could occur at hgher wave heght lmts at Hsnchu for the entre year, but only from March to September at Changhua. In addton, low levels of access occurred durng wnter at low wave heght lmts. The occurrence hours of smaller wave heght lmts ( and.5 m at Changhua and m at Hsnchu) n November were evdently hgher than those n the adjacent months (Fg. 5). Mantenance actvtes may be performed n ths wnter month. At a wave heght lmt of m at the two stes, hgher percentages of access hours were observed at each month at Hsnchu as a result of more chances of access for O&M tasks, partcularly durng wnter (Fg. 7). Ths fndng s consstent wth the exceedance dstrbuton n Fg. 4, whch ndcated the presence of lower wave heghts at Hsnchu. Fg. 8 presents the accessblty of the two stes. Hgh accessblty was observed at Hsnchu at each wave heght lmt. IV. WEATHER WINDOW ANALYSIS The above analyss accounts only for the probablty of occurrence wthout consderng the tme seres of wave heght, such as persstence of wave heght relatng to one another. However, the mantenance n offshore wnd farms s tme-consumng and Percentage (%) 00 90 80 70 60 50 40 30 0 HsnChu Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fg. 7. Occurrence probablty of wave heghts below m n each month at Hsnchu and Changhua sea areas. mght requre several hours, days, or weeks. Varous operatng procedures have dfferent requrements regardng weather condtons and duratons. The duraton requred for mantenance s extremely mportant for O&M plannng. If a longer duraton s requred and vessels can only access wnd turbnes at the condtons of smaller waves, the watng tme ncreases due to the

568 Journal of Marne Scence and Technology, Vol. 5, No. 5 (07) Table. Occurrence percentages of wave heghts wthn specfc wndow lengths and wave heght lmts from 005 to 03 for Hsnchu. Hs (m).5 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 98 98 98 98 98 98 98 98 98 98 98 98 98 98 98 97.5 90 90 90 90 90 89 89 89 89 88 88 88 87 87 86 85 7 70 69 68 68 67 66 65 64 6 6 60 59 58 56 54 6 8 4 30 36 4 48 54 60 66 7 78 84 90 96 Mnmum Length of Wndows (Hours) Table 3. Occurrence percentages of wave heghts wthn specfc wndow lengths and wave heght lmts from 005 to 03 for Changhua. Hs (m).5 94 94 94 94 94 93 93 93 93 93 93 9 9 9 9 9 84 84 83 83 8 8 8 80 79 79 78 77 76 75 74 74.5 7 70 69 69 68 67 66 65 64 63 6 60 59 57 56 54 55 54 53 5 5 50 48 47 46 45 44 4 4 39 38 37 6 8 4 30 36 4 48 54 60 66 7 78 84 90 96 Mnmum Length of Wndows (Hours) Year round accessblty (%) 00 95 90 85 80 75 70 65 60 55 50 45 40 35 30 5 0 5 0 5 0 HsnChu 0.5 m.0 m.5 m.0 m.5 m Fg. 8. Accessblty of Hsnchu and Changhua stes. hgh probablty of naccessblty. Consequently, two parameters should frst be set before analyzng weather wndows, namely wave heght lmt and wndow length. The wave heght lmt s the lmt wthn whch vessels can access an offshore wnd turbne, and the wndow length s the requred operaton perod below the lmt (Chen et al., 008; O Connor, 0; O Connor et al., 03).Issues such as the longest watng tme, number of watng perod between weather wndows, the probablty of occurrence and annual number of weather wndows at least a certan length at each wave heght lmt were analyzed.. Probablty of Occurrence Tables and 3 present the occurrence percentages of wave heght wthn specfc wndow length and wave heght lmts of,.5,, and.5 m for Hsnchu and Changhua from 005 to 03. Occurrence probablty ndcates the levels of access to hgh probablty ndcates hgh accessblty. The probabltes of occurrence have been demonstrated to become low wth small wave heght lmts and a long wndow length. For example, at Hsnchu, access to wnd turbnes s avalable for 7% of the year at a wndow length of at least longer than 6 h and the % Occurrence % Occurrence Table 4. Mnmum, mean, and maxmum number of wndows for at least 6,, 4, 48, and 96 h wndow lengths and at varous wave heght lmts at Hsnchu and Changhua sea areas from 005 to 03. Hsnchu.0 m.5 m.0 m.5 m mn mean max mn mean max mn mean max mn mean max 6 h 50 60 66 4 9 35 6 0 4 3 5 h 4 5 59 8 34 4 9 3 5 4 h 37 40 44 0 5 30 4 9 3 5 48 h 5 3 38 8 8 4 8 3 5 96 h 9 3 3 7 4 8 3 5 Changhua.0 m.5 m.0 m.5 m mn mean max mean max max mn mean max mean max max 6 h 43 56 69 46 54 66 8 43 5 3 4 3 h 40 47 54 40 47 58 7 39 47 9 4 h 3 38 47 34 38 4 4 33 36 9 3 48 h 8 6 3 4 9 3 7 3 0 8 96 h 0 3 6 5 9 6 8 0 5 7 strctest wave heght lmt of m. If the wndow length becomes 96 h (4 days), the level of accessblty decreases to 54%. Nevertheless, at wave heght lmts of and.5 m, the level of accessblty s almost hgher than 98%, rrespectve of the wndow lengths. Ths s consstent wth the result presented n Fg. 5. At Changhua, the level of accessblty s 55% at a wave heght lmt of m and a wndow length of at least 6 h and only 37% at a wndow length of at least 96 h. Evdently, hgher levels of access were observed at Hsnchu than at Changhua. At the North Sea offshore wnd farm Col Rver Bar, 45 km off the Dutch coast, access to the devce s acheved for only 5% of the year at a wave heght lmt of m and wndow length of at least longer than 6 h and 6% at a wave heght lmt of.5 m at the same wndow length (O Connor et al., 03). In summary, the North Sea exhbted lower accessblty than the two stes n Tawan, and Changhua exhbted a lower level of access than Hsnchu. Accessblty s one of the key factors for O&M actvtes. Therefore, vessels that can sustan hgh wave lmts should be used at stes wth low accessblty.. Annual Number of Weather Wndows Table 4 presents the statstcal results of the annual number of weather wndows wthn 9 years at wave heght lmts of,.5,, and.5m and wndow lengths of at least 6,, 4, 48, and 96 h, ncludng the hghest number (max), mean number (mean), and lowest number (mn) over 9 years. The hghest number s the hghest number of wndows observed n any of the 9 years, and the lowest number s the lowest number of wn dows observed over the 9 years. The mean number of wndows s the average number of wndows that occurred n 9 years. The number of wndows decreased as the wndow length ncreased. The annual changes n number are also presented. Despte not ncludng clmate changes n the study, the annual change n the number of weather wndows s an mportant factor nfluencng future O&M and power producton of offshore wnd farms. Theoretcally, when the wave heght lmt ncreases, the number of weather wndows ncreases. That s, the probablty or opportunty for O&M ncreases. However, ths result s related to the dstrbuton of wave heghts. If 50% of wave heghts s

H.-W. Chang et al.: Weather Wndow Analyss n O&M of OWF at Tawan 569 Table 5. Longest watng tmes (days) at wave heght lmts of,.5,, and.5 m and wndow lengths of at least 6,, 4, 48, 7, and 96 h. Wave Heght Lmts Hsnchu Wndow Length (hours) 6 h h 4 h 48 h 7 h 96 h Wave Heght Lmts Changhua Wndow Length (hours) 6 h h 4 h 48 h 7 h 96 h.0 m 7.6 3.3 3.6 46.9 66.7 5.8.0m 3.9 40.4 63.8 75.3 4.6 07.7.5 m 6.9 6.9 6.9 4.4 3.0 45.9.5m 3.3 3.3 3.3 6.6 70.5 6..0 m 3. 5.5 5.5 5.5 5.5 8.5.0m 8.3 8.3 4.5.8 46. 46..5 m.......5m 6.3 6.5 6.5 7.0.3 0.5 small (e.g., < m), on the contrary, the number of weather wndows decreases at a hgher wave heght lmt because each wndow length s a very prolonged perod of contnuous access (> 96 h), as observed at Hsnchu. By contrast, because of the hgher wave heght dstrbuton at Changhua, the number of weather wndows ncreased when the wave heght lmt ncreased up to.5 m. 3. Watng Tme between Wndows The watng tme or perod between wndows represents the perods of naccessblty. In the plannng stage, the longest perod of naccessblty should be consdered. The longer the watng tme s, the hgher the cost becomes. Table 5 presents the longest watng tmes over 9 years at varous wave heght lmts and least wndow length condtons. The watng tme becomes longer at a small wave heght lmt and long wndow length. Smlarly, the shorter the watng tme s, the hgher the wave lmt s. In the wnter season, the perods of naccessblty tend to be more pronounced because wnter represents the least favorable scenaro n a year for most areas n the world. For example, when the wave heght lmt s m and the requred wndow length s at least 4 days (96 h), the longest watng tme s up to 5.8 days at Hsnchu and 07.7 days at Changhua. Ths ndcates that the watng tme are at most 4 months and over 6 months respectvely, durng whch the stes are naccessble. If only 6 h are requred, the longest watng tme for access wll be 7.6 and 3.9 days at Hsnchu and Changhua, respectvely. If the wave heght lmt s up to.5 m, the longest watng tme becomes only. days at Hsnchu and 6.3 days at Changhua. 4. Number of Watng Tme between Wndows The hghest possble number of watng tmes after the weather wndow s another concern for O&M actvtes. The number of watng tmes represents the possblty of the watng tmes between weather wndows. The hgher the number s, the hgher the potental watng tme s. Table 6 presents the number of watng tmes between wndows of at least 6,, 4, 48, 7, and 96 h at wave heght lmts of,.5,, and.5 m. In addton, the watng tmes are dvded nto three categores at each wave heght lmt: less than days, between days and a week, and longer than a week. The number of watng tme decreased as the wndow length ncreased at each wave heght and watng tme (Table 6). At Table 6. Number of watng perods wthn 9 years. Hsnchu Changhua Wave Watng Wndow Length (hours) Wave Watng Wndow Length(hours) Heght Tme Heght Tme 6 h h 4 h 48 h 7 h 96 h Lmts (days) Lmts (days) 6 h h 4 h 48 h 7 h 96 h wt < = d 373 308 95 36 94 68 wt < = d 95 46 80 50 35.0 m d < 37 3 35 04 76 44 d <.0 m 58 44 4 86 55 40 7d < wt 9 6 3 46 54 54 7d < wt 53 57 55 66 63 44 wt < = d 5 94 68 34 93 wt < = d 34 59 8 0 7 4.5 m d < d < 43 47 53 57 55 44.5 m 4 39 7 93 66 4 7d < wt 0 0 0 5 0 7 7d < wt 0 7 43 46 56 53 wt < = d 7 64 63 6 6 57 wt < = d 307 59 96 36 93 73.0 m d < d < 6 6 6 7 7 6.0 m 74 85 87 8 7 59 7d < wt 0 0 0 0 0 7d < wt 3 3 8 9 35 wt < = d 0 8 7 7 7 7 wt < = d 88 63 34 0 0 9.5 m d < d <.5 m 8 3 30 34 37 33 7d < wt 0 0 0 0 0 0 7d < wt 4 8 Hsnchu, rrespectve of wave heght lmts and wndow lengths, the hghest possble number of watng tmes s observed for the less than days category. Most stuatons occur at a wndow length of 6 h and a wave heght lmt of m. At Changhua, the hghest possble number of watng tmes s also observed for the less than days category; however, most stuatons occur at a wndow length of 6 h and a wave heght lmt of.5 m nstead of m due to the hgher wave dstrbuton. 5. Comprehensve Analyss In concluson, based on the wave heght analyss and weather wndows analyss, the accessblty at Hsnchu s evdently hgher than that at Changhua. Accordng to Salzman et al. (007), the accessblty could be up to 50% wth the wave heght lmt of m for a typcal offshore wnd farm n the North Sea. When an access system can tolerate sgnfcant wave heght of up to.5 m, t can be accessed for more than 90% of the year. The result reveals that accessblty at both Hsnchu (7%) and Changhua (55%) at a wave heght lmt of m s hgher than the typcal wnd farm n the North Sea. Fg. 8 shows that the accessblty s very close (approxmately 73%) wth a wave heght lmt of.5 m at Changhua and m at Hsnchu. O&M actvtes n offshore wnd farms are stll unavalable n Tawan. The number of offshore O&M teams requred n Tawan remans unclear. Assumng that the O&M actvtes at both stes are performed by one company and an accessblty of 73% s the typcal acceptance level, the mnmum access level s at a wave heght of.5 m. However, the longest watng tme s approxmately 3-63 days at Changhua at a wndow length of at least of 6-48 h, whereas t s 7-4 days at Hsnchu. If the longest watng tme s acceptable at both stes, t could be used as the O&M strategy. If not, the wave heght lmt should be hgher, whch s nevtable by usng vessels wth hgher seakeepng and advanced accessblty ablty. V. DISCUSSION AND CONCLUSION Mantenance at offshore wnd farms manly depends on wave condtons, thereby ncreasng the costs, tme, and dffcultes

570 Journal of Marne Scence and Technology, Vol. 5, No. 5 (07) relatve to onshore wnd farms. Increased accessblty to wnd turbnes ncreases the chances of regular mantenance and consequently decreases the falure rate of wnd turbnes, as well as the downtme after wnd turbne falure. Ths s a prelmnary study to nvestgate the accessblty of offshore wnd farms n Tawan for O&M actvtes and wth respect to both possblty of occurrence and persstence of wave heght. The study used results from well-verfed hgh-resoluton numercal smulatons from 005 to 03, applyng the NWW3 wave model to analyze sgnfcant wave heghts and weather wndows at the Hsnchu buoy locaton and Changhua Offshore DIP ste. Sgnfcant wave heght analyses reveal that the hghest access occurs at hgher wave lmts. A hgher wave dstrbuton and lower percentage of access hours wth a wave heght below m for each month was observed at Changhua. In addton, the seasonal varaton of the probablty of occurrence was below.5 m. For Hsnchu, wave heghts below m accounted for 7.% and wave heght below m accounted for 98.%. For wave heghts below and.5 m, the probablty of occurrence presented obvous seasonal dfferences and a hgher value n summer. November appears to be a monsoon transton perod, and the ste could be more accessble n the wnter season. The analyss of weather wndows reveals that the probabltes of occurrence decrease wth small wave heght lmts and long wndow lengths. Moreover, hgher levels of access were evdent at Hsnchu than at Changhua. The annual number of wndows decreased as the wndow length ncreased. The features of annual change were also dsplayed. Due to the smaller wave heght dstrbuton at the two stes, the number of weather wndows decreased at hgher wave heght lmts because each wndow length s a very prolonged perod of contnuous access. Inaccessblty or watng tme s another mportant concern. Watng tme between wndows represents the perods of naccessblty. If the wave heght lmt s m and the requred wndow length s at least 4 days (96 h), the longest watng tme s up to 5.8 days at Hsnchu and 07.7 days at Changhua. If only 6 h s requred, the longest watng tme for access wll be 7.6 and 3.9 days at Hsnchu and Changhua, respectvely. If the wave heght lmt s up to.5 m, the longest watng tme becomes only. days at Hsnchu and 6.3 days at Changhua. The number of watng perod represents the possble watng tme between weather wndows. The hgher the number s, the hgher possblty of the watng tme s. At Hsnchu, rrespectve of wave heght lmts and wndow lengths, the hghest possble number of watng perod s less than days. At Changhua, the most possble watng perod s also less than days. A thorough analyss of the access condtons of dfferent vessels for O&M actvtes could provde sutable strateges for dfferent weather wndows. A more effcent plan could then be proposed to utlze the duraton for mantenance. Good O&M strategy could thereby mprove the avalablty of wnd turbnes and thus beneft the performance of offshore wnd farms. However, the envronment n offshore wnd farms n Tawan appears to be more favorable than that n the North Sea. The experence of Europe must not be only consdered to determne a sutable strategy n Tawan. Ths study focused only on the sgnfcant wave heght. Apart from the sgnfcant wave heght, other envronmental factors that affect weather wndows nclude wave perod, wnd speed, and ocean current. Among these, wnd speed manly affects the safety of crane operatons, and ocean current affects cablng and dver actvtes. Further research should focus on the multvarate analyss of weather wndows to consder more other envronment factors. The advantage of studes based on numercal models s that contnuous tme seres are obtaned wthout any outlers or gaps n the data. However, the dsadvantage of ths approach s the uncertanty and error n the smulaton, whch makes assessng the mpact dffcult. 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