Chapter 11Structure Weight Algorithms

11.1 Weight Algorithms

11.1.1 Tubular Weight Algorithm

Most of the material in a wind farm is composed of tubular steel elements, which isthe simplest component to estimate weight since the geometric shape is uniquelydescribed by its diameter, length, and thickness. For steel density of 7.85 g percubic cm, the formula:

W ¼ 24; 660ðD� tÞtL ð11:1Þ

gives the total weight W (kg) in terms of the outside diameter D (m) of themember, wall thickness t (m), and total length L (m). For tapered components andnon-uniform thickness, average diameters and thicknesses can be employed for amore accurate estimate.

In Table 11.1, unit weight in kg per linear meter is described for a tubularmember in terms of outer diameter (m) and thickness (mm). The most commonranges of weight for monopiles are identified in bold. In Table 11.2, monopileweight was computed via formula and compared to reported weight at selectedwind farms. On average, the estimated weight is within 10% of the reportedweight. Deviations are due to non-uniform taper and thicknesses, unknown/unre-ported thickness, and reporting error.

11.1.2 Foundations

Foundation lift weight during removal consists of the cut monopile section, thetransition piece and grout, and marine growth. We assume marine growth isnegligible or has been removed. The removed foundation is composed of a section

M. J. Kaiser and B. F. Snyder, Offshore Wind Energy Cost Modeling,Green Energy and Technology, DOI: 10.1007/978-1-4471-2488-7_11,� Springer-Verlag London 2012

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of ‘‘free’’ monopile, a section of monopile grouted to the transition piece (referredto as monopile/transition overlap), and a section of free transition piece. SeeFig. 11.1. The weight of each component must be determined because the pro-cessing and disposal costs of grouted tubular components are different from thecosts of free components.

11.1.2.1 Monopile

Monopile weight is frequently reported in public documents, but in decommis-sioning operations it is the weight of monopile removed that is the importantfactor. Federal regulations require monopiles to be cut 4.6 m (15 ft) below themudline and the total weight of the monopile removed is composed of themonopile below the mudline, the monopile from the mudline to the water level,and the monopile above the water line.

The portion of monopile grouted to the transition piece usually ranges from 1.3to 1.6 times the pile diameter [1, 2]. Given the length of monopile/transitionoverlap and the total length of the monopile, the length of free monopile is the totallength minus the length of monopile grouted to the transition piece. The weight of

Table 11.1 Steel weight per diameter and wall thickness (kg/m)

Wall thickness (mm)

Nominal size (m) 12.5 25.0 37.5 50.0 62.5 75.0

0.75 227.3 447.0 658.9 863.1 1059.6 1248.41.2 366.0 724.4 1075.0 1418.0 1753.2 2080.71.5 458.5 909.3 1352.4 1787.9 2215.5 2635.52.5 766.8 1525.8 2277.2 3020.9 3756.8 4485.03 920.9 1834.1 2739.6 3637.4 4527.4 5409.84 1229.1 2450.6 3664.3 4870.4 6068.7 7259.34.5 1383.3 2758.8 4126.7 5486.9 6839.3 8184.05 1537.4 3067.1 4589.1 6103.4 7609.9 9108.85.5 1691.5 3375.3 5051.4 6719.8 8380.5 10033.5

Note The most common range of weights for monopiles are identified in bold

Table 11.2 Specifications and estimated weights of monopiles at selected wind farms

Wall thickness Weight

Wind farm Length(m)

Outer diameter(m)

Reported(cm)

Assumed(cm)

Reported(ton)

Estimated(ton)

Kentish Flats 38–44 4.3 4.5 144–184 178–206Horns Rev 34 4 5 160 164Horns Rev 2 30–40 3.9 4.0–8.2 150–210 169–226North Hoyle 50 4 3.0–7.0 5 250 242OWEZ 45 4.6 4.0–6.0 5 230 250

202 11 Structure Weight Algorithms

free monopile and the weight of monopile grouted to the transition piece isdetermined by the length times the unit weight.

Table 11.3 summarizes the monopile removal calculations and assumes a 4.6 mbelow mudline cut, that the length of monopile grouted to the transition piece is1.3–1.6 times the pile diameter, and a wall thickness of 35–65 mm. The length ofthe monopile above water is assumed to range from 1 to 5 m and the unit weightranges from 2.7 to 7.6 ton/m.

To illustrate application of Table 11.3, consider a 4.5 m diameter monopile in10 m water depth with a wall thickness of 50 mm. Assume the pile extends 1.5 mabove the water line. The total length of the monopile removed is the water depth,plus the 4.6 m below mudline, plus the length of monopile above the waterline, or16 m. The weight of the total monopile removed is the length times the unit weight

Fig. 11.1 Components of a wind turbine foundation

Table 11.3 Monopile dimensional specification estimates

Specification Unit Calculation

Diameter (D_M) mLength of monopile above water (L_MW) mLength of total monopile removed (L_TMR) m WD ? 4.6 ? L_MWMonopile/transition overlap factor (OF_MT)Length of monopile/transition overlap (L_MT) m OF_MT * D_MLength of free monopile (L_FM) m L_TMR–L_MTWall thickness (WT_M) mUnit weight (UW_M) ton/mWeight of total monopile removed (W_TMR) ton (WD ? L_MW+4.6)*UW_MWeight of free monopile (W_FM) ton UW_M * L_FMWeight of monopile in mon/trans (W_MMT) ton L_MT * UW_M

11.1 Weight Algorithms 203

(5.48 ton/m) and total weight is therefore 16 m*5.48 ton/m = 87.8 ton. Thelength of monopile grouted to the transition piece is 4.5 m*(1.3 to 1.6) = 5.9 to7.2 m and the length of the free monopile is 16 m–(5.9 to 7.2 m) = 8.8 to 10.1 m.Weights are the length times the unit weight.

11.1.2.2 Transition Piece

If length, diameter and wall thickness of the transition piece is known, weight maybe estimated from the tubular steel weight formula. If unknown, values may beestimated as a function of monopile diameter and height above water. Figure 11.2illustrates the transition piece length estimation method and Table 11.4 gives thecalculations, assumptions, and nomenclature.

We consider a 4.5 diameter monopile that extends 1.5 m above the water forillustration. Assume the transition piece has a wall thickness of 50 mm, a heightabove the waterline of 15 m, and an annulus of 50 mm. The length of the transitionpiece is the length of the overlap, plus the length above water, minus the length of themonopile above water or: (1.3 to 1.6)*4.5 ? 15 - 1.5 m = 19.4 to 20.7 m. Thediameter of the transition piece is the diameter of the monopile, plus the annulus andthe wall thickness of the pile or: 4.5 m ? 2*(0.05 m) ? 2*(0.05 m) = 4.7 m. Weestimate the unit weight as 5.8 ton/m. Multiplying unit weight by the length gives5.8 ton/m*(19.4 to 20.7 m) = 112 to 120 ton.

Specifications of transition pieces for select wind farms and estimated weightsfrom the Table 11.4 formula are shown in Table 11.5. Wall thickness wasassumed to be 50 mm and the annulus 125 mm; if height above water wasunknown, it was assumed to be 15 m. The formula weight is generally lowerthan the reported weight, which may be due to the presence of secondary steel or

Fig. 11.2 Transition piece height estimation

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concrete in the boat landings, tidal differences in the measurement of heightabove water, or reporting differences. On average, the formula underestimatesreported weight by 33%.

11.1.2.3 Grout

Grout is used to secure the transition piece to the monopile.1 The weight of grout isthe volume of grout used multiplied by its unit weight. Figure 11.3 illustrates thegrout volume estimation. The volume of grout is computed as the volume dif-ference created by the outside diameter of the monopile and the inside diameter ofthe transition piece. The length of both cylinders is given by the overlap (1.3–1.6times the diameter); the inside diameter of the transition piece is given by the outerdiameter of the monopile plus twice the annulus.

Table 11.4 Transition piece dimensional specification estimation

Specification Unit Assumption Calculation

Height above water (H_T) m 8–20 (mean 15)Annulus (A_T) m 0.05–0.12Length (L_T) m O_MT ? H_T - L_MWDiameter (D_T) m D_M ? (A_T)*2 ? (WT_T)*2Wall thickness (WT_T) m 0.0375–0.0625Unit weight (UW_T) ton/m 2.7–7.6Weight (W_T) ton UW_T * L_T

Table 11.5 Weights of transition pieces at selected wind farms

Wind farm Outerdiameter (m)

Weight(ton)

Length abovewater (m)

Totallength (m)

Estimatedweight (ton)

Kentish Flats 4.5 90 8 92Burbo Bank 5 225 22 194Belwind 4.3 160 22 137Gunfleet Sands 5 212 18 169Lincs 5 250 6 86OWEZ 4.3 250 13 25 156Lynn/Inner Dowsing 5 181 21 188Rhyl Flats 5 220 23 200Robin Rig 4.5 160 20 177Horns Rev 2 4.2 170 139Baltic 1 4.4 250 27 169

1 In recent years, problems with grouting at some sites have emerged which have promptedredesign and costly intervention. This may lead to changes in grouting techniques and foundationdesign which could influence weight estimates.

11.1 Weight Algorithms 205

Table 11.6 shows estimates for grout weight given different assumptions onannulus size and pile-transition piece overlap. In all cases, the weight of grout issmall relative to other weights in the system and grout is not expected to exceed10% of the total weight removed.

11.1.3 Tower and Turbine

If tower height, diameter, and wall thickness are known or can be reasonablyapproximated, weight may be calculated via formula. If these variables are notknown, they may be estimated from the turbine specifications shown inTable 11.7. The ranges of tower weight for a given turbine are due to differences inhub height. In Table 11.8, the tower length and weight at selected offshore farmsare depicted. As the capacity of a turbine increases, weight and load increases, andalong with hub height, impacts tower weight [3]. Differences in technology anddesign account for the variation observed.

Fig. 11.3 Grout weight estimation

Table 11.6 Weight of cement grout used in turbine foundations

Pile diameter(m)

Pile overlap as afunction of pile diameter

Annuluswidth (mm)

Grout volume(m3)

Weight(ton)

4 1.3 50 1.7 3.75 1.3 50 2.6 64 1.6 125 4.9 115 1.6 125 7.8 17.5

Note The density of cement is assumed to be 2,500 kg/m3

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Turbine rotor and nacelle weight vary based on capacity, blade length, drivetype, and manufacturer. The most popular European offshore turbines are given inTable 11.9. In recent years, major turbine suppliers have developed direct driveturbines which eliminate the use of a gearbox in the turbine. This results insignificant weight savings in the turbine and may have impacts on the weights ofother farm components.

Much of the weight of the turbine is composed of steel, and if the turbine isdisassembled, these materials will be sold or scrapped. Processing costs willdetermine material disposition. Table 11.10 shows general material compositionby weight. Approximately 70% of the weight of the turbine is composed of steel,some of which may be recycled, depending on processing costs and scrap prices.Steel in the hub, gearbox, and frame is relatively accessible.

Table 11.7 Tower weights by turbine type

Turbine Capacity (MW) Tower (ton)

Vestas V80 2 130–200Vestas V90-3 3 100–150Siemens 3.6-107 3.6 180–200Repower 5 M 5 210–225

Table 11.8 Tower length and weight at selected offshore wind farms

Wind farm Turbine Tower weight (ton) Tower height (m)

Kentish Flats Vestas V90-3 108 62Horns Rev Vestas V80 160Gunfleet Sands Siemens 3.6-107 193 60Alpha Ventus Repower 5 M 210Burbo Bank Siemens 3.6-107 180 65Arklow Bank GE 3.6 160 70Beatrice Repower 5 M 225 59

Table 11.9 Weight of turbine components in commonly used offshore wind turbines

Turbine Capacity (MW) Diameter (m) Rotor (ton) Nacelle (ton) Total (ton)

Siemens 2.3-93 2.3 93 60 82 142Nordex N90 2.5 90 55 91 146Vestas V90-3 3 90 42 70 112Siemens 3.0-101 3 101 40 73 113Siemens 3.6-107* 3.6 107 95 125 220Repower 5 M 5 126 120 300 420Multibrid M5000 5 116 110 199 309Repower 6 M 6 126 135 325 460

Note * Cape Wind proposed wind turbinesSource Turbine specification sheets; industry press

11.1 Weight Algorithms 207

11.1.4 Cable

Total cable weight is a function of the weight per meter times the cable length inmeters. The weight of cable varies by size and capacity and inner-array cableweight may vary within a wind farm because as more turbines are connected inseries, the size of the power cable must increase to handle the load. Table 11.11describes export and inner-array cables used at selected wind farms according tohigh- and medium-voltage classes. Export cables generally weigh 50–100 kg/mand inner-array cables weigh 20–40 kg/m [4, 5]. Average medium-voltage cableweighs 30.5 kg/m; average high-voltage cable weighs 75.2 kg/m.

11.1.5 Substation

The wind farm substation will be supported on a monopile or jacket foundation.Jacket weight depends on framing configuration, the degree of batter, pilerequirements, and topsides load. Design philosophy and soil conditions also play arole in determining the amount of steel used in construction. Jacket weight forselect wind farms in different offshore regions are summarized in Table 11.12 interms of water depth and topsides load. Topsides for offshore wind substationsrange in weight from 500 to 2000 tons; the BorWin1 platform is an HVDCplatform and is especially heavy.

The data in Table 11.12 yields the functional relationship:

Weight ¼ 12:8WD0:19Load0:48 ð11:2Þ

where Weight is in metric tons, water depth (WD) is given in meters, and topside Loadis in metric tons. The equation predicts about half of the variation in jacket weight. Ifadditional data is available, subcategories can be defined and the model relationrefined.

Table 11.10 Material usage in large (4 MW) turbines

Component Steel (%) Copper (%) Other (%) Proportion of turbineweight* (%)

Blades 2 98 23Gearbox 96 2 2 30Generator 93 4 3 8Frame 85 3 12 20Hub 100 18Total 72 1.5 26 100

Note * Turbine weight includes the blades, nacelle and hubSource NREL 2008

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11.2 Example

The weight relations were used to estimate Cape Wind component weights. Fromthe Environmental Impact Statement [6], we assume the mean water depth at CapeWind is 9.5 m, pile diameters are 5.2 m and extends 6 m above the waterline, andthe transition piece extends 10 m above the water. Given these assumptions, theweights in Table 11.13 were calculated.

The total length of monopile removed is the length below the mudline (4.6 m)plus the water depth (9.5 m) plus the extension above the waterline (6 m).Approximately 6.8–8.3 m of overlap between the transition piece and monopole is

Table 11.11 Medium andhigh-voltage cable weights

Windfarm Voltage (kV) Weight (kg/m)

North Hoyle 33 37Kentish Flats 33 32Walney 33 34Cape Wind 34.5 22Cape Wind 34.5 31Cape Wind 34.5 41Lillgrund 36 16Lillgrund 36 19Lillgrund 36 22Lillgrund 36 56OWEZ 36 19OWEZ 36 33Burbo 36 35Cape Wind 115 83Nysted 132 72Sheringham Shoal 132 77Princess Amalia 150 69Horns Rev 150 70Belwind 150 80

Table 11.12 Jacket weightsat selected offshore windfarms

Wind farm Weight(ton)

Waterdepth (m)

Topsideload (ton)

Walney 990 30 1,030Alpha Ventus 750 30 700Horns Rev 2 800 13 1,230Generic UK 1,200 19 1,600Generic UK 1,400 24 1,800Lincs 950 10 2,250Thanet 695 21 1,200BorWin1 1,700 40 3,200Greater Gabbard 767 30 2,069

11.2 Example 209

required for a 5.2 m diameter pile, leaving 11.8–13.3 m of free monopile. Thetransition piece length is the monopile-transition overlap (6.8–8.3 m), plus thelength above water (10 m) minus the monopile length above water (6 m) or 10.8–12.3 m. Assuming a wall thickness of 50 mm, the unit weights are estimated as6,300 kg/m for monopiles and 6,400 kg/m for transition pieces. Assuming anannulus of 50 mm, approximately 14–17 ton of grout is required per foundation.

Tower and turbine weights are estimated from the data in Tables 11.7, 11.8 and11.9. Cape Wind has reported that they plan to use Siemens 3.6 MW turbines,which, based on previous installations, will weigh approximately 220 tons and besupported by 190 ton towers. Cape Wind has also made information on theweights of export and inner-array cables available [5]. The Cape Wind substationis to be placed in shallow water (8.5 m); assuming a topside load of 1,500 tons, thejacket is expected to weigh approximately 800 tons.

The vast majority of the weight of the decommissioned components at CapeWind will consist of steel in the turbine towers, turbines, and monopiles andtransition pieces. Turbines will be the heaviest component and will account forapproximately 30% of the material decommissioned. The towers and foundationswill each account for 25–30% of the disposal weight. If removed, export and inner-array cable will account for approximately 10% of the weight of removedcomponents.

References

1. Moller A (2008) Efficient offshore wind turbine foundations. Power Expo, Zaragoza, Spain,24–25 Sept 2008. Available at: http://www.aeeolica.es/doc/powerexpo2008/06-Anders-Moller-DENSIT.pdf

2. Schaumann P, Wilke F, Lochte-Holtgreven S (2008) Nonlinear structural dynamics of offshorewind energy converters with grouted transition piece. European Wind Energy Conference.Brussels. March 31–April 3

Table 11.13 Cape Wind farm components and disposal weights

Component Length (m) Weight (kg/m) Weight (ton) Total farmweight (ton)

Percentage(%)

Total monopile 20.1 6,300 127 16,510 17.9Free monopile 11.8–13.3 6,300 74–84 9,620–10,920Monopile/transition 6.8–8.3 12,700 86–105 11,180–13,650Total transition 10.8–12.3 6,400 69–79 8,970–10,270 10.4Grout 13.8–16.8 1,794–2,184 2.2Tower 68.5 190 24,700 26.8Turbine 220 28,600 31.1Export cable 80,400 83 6,030 6.6Inner-array cable 127,000 30 3,810 4.1Substation jacket 800 800 0.9

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3. Thresher RW, Dodge DM (1998) Trends in the evolution of wind turbine generatorconfigurations and systems. Wind Energy 1:75–85

4. Wright SD, Rogers AL, Manwell JF, Ellis A (2002) Transmission options for offshore windfarms in the United States. In: Proceedings of the American Wind Energy Association AnnualConference, pp 1–12

5. Pirelli ABB (2004) 115 kV Solid dielectric submarine cable. Available at http://www.boemre.gov/offshore/PDFs/CWFiles/120.pdf. Accessed 3 Jan 2012.

6. MMS (2008) Cape Wind final environmental impact statement. Minerals ManagementService, Herndon, VA. 2008-040

References 211