Scour protection for 6MW OWEC with monopile foundation in North ...

Scour protection for 6MW OWEC with monopile foundation in North Sea

January 2003

1 Revision 23/01/03 Eot/RDu WodV/CJS WodV 0 For information 04/04/02 RH/GS WodV HL D DRAFT 30/01/02 RH

Rev Reason for issue

Date Prepared by Checked by Approved by

Client: DOWEC project team Van Oord ACZ B.V. 2, Jan Blankenweg P.O. Box 458 4200 AL Gorinchem The Netherlands tel: +31 183 642 200 Project No: 98.0678-CC Document No: DOWEC-F1W1-RH-02-050/01

Technical Report Project: DOWEC Page: 2 of 21 98.0678-CC Date: 23/01/03 Rev. No.: F1W1-RH-02-050/01

Prep.: Eot/RDu Title: Scour protection for 6MW OWEC with monopile foundation in North Sea Chkd./App.: CJS/WodV

Van Oord ACZ, Engineering Department I:\Projects Eng\1998\98.0678cc - DOWEC\Eng\Engineering\Scour berekening\SCOUR voor 6MW fundatie\F1W1-RH-02-050-01-final.doc

TABLE OF CONTENTS 1 INTRODUCTION.................................................................................................................3 2 AVAILABLE INFORMATION ................................................................................................4

2.1 Design Condition.........................................................................................................4 2.2 Bathymetry..................................................................................................................4 2.3 Current Data................................................................................................................5 2.4 Design Wave...............................................................................................................5 2.5 Wave climate...............................................................................................................5 2.6 Soil Conditions ............................................................................................................6

3 DESIGN PHILOSOPHY.......................................................................................................7 4 CALCULATION METHODS.................................................................................................9

4.1 Armour grading ...........................................................................................................9 4.1.1 Oscillatory flow ........................................................................................................9 4.1.2 Uni-directional flow ................................................................................................10

4.2 Grading of filter layers ...............................................................................................10 4.3 Thickness of layers ................................................................................................... 11 4.4 Length of scour protection......................................................................................... 11

4.4.1 Critical velocities.................................................................................................... 11 4.4.2 Disturbance factor α .............................................................................................. 11 4.4.3 Clear Water Scour Depth.......................................................................................12 4.4.4 Slope Stability........................................................................................................12 4.4.5 Calculation of Length.............................................................................................13

5 DESIGN OF SCOUR PROTECTION.................................................................................14 5.1 Rock Grading and Layer Thickness...........................................................................14 5.2 Dimensions and Rock Volumes.................................................................................15

6 INSTALLATION ASPECTS ................................................................................................16 6.1 Introduction ...............................................................................................................16 6.2 Workability ................................................................................................................16 6.3 Shape of scour protection .........................................................................................17 6.4 Influence of cable on scour protection of 6 MW OWEC.............................................17 6.5 Installation schedule..................................................................................................17

7 BUDGETARY COSTS .......................................................................................................18 7.1 Introduction ...............................................................................................................18 7.2 Costs Calculations.....................................................................................................18

References: ..............................................................................................................................19 Appendix A: Side stone dumping vessel Frans .........................................................................20




1 INTRODUCTION

Within the framework of the DOWEC project Van Oord ACZ have performed studies to determine the influences of the layout and location of an offshore wind park on the price of scour protections around a monopile foundation of any structure. A previous study was carried out for a 3 MW OWEC [ref. 4]. This study was based on conditions at the Horns Rev park in Denmark as the environmental conditions of the DOWEC sites were not yet available. Furthermore, the present 6 MW OWEC study is based on other insights for the design of dimensions of the scour protection resulting in a more economic design compared with the 3 MW OWEC study. Foundations for offshore wind turbines are designed to withstand the combined dynamic loading of wind and waves. The resistance against these pre-dominantly horizontal loads must be derived from the soil – foundation interaction. When a piled foundation is used, the fixation length along the pile determines the dynamic response of the turbine. Under offshore circumstances the seabed surrounding the foundation is subject to scouring induced by the presence of the foundation. Steady currents and wave induced water velocities are accelerated by the obstruction and exceed the critical velocity of the seabed particles. The result of this scouring is that the fixation length of the foundation pile changes, and thus the dynamic response of the foundation. The scour depth that can be anticipated around a piled construction is in the order of 2 times the diameter of the pile. When a monopile foundation is used, this scour hole will lead to a significant change in dynamic response that may not be tolerable. In that case some form of protection around the monopile foundation is required.




2 AVAILABLE INFORMATION

In this paragraph the relevant data of the two DOWEC locations, site III and site VII, is given as well as the sources of the data. These two locations are given in the Terms of Reference [Ref.2].

Location Longitude Latitude Site III 4°25’ 52°50’ Site VII 3°30’ 52°15’

2.1 DESIGN CONDITION

Survival conditions for the scour protection are defined for the 50 year return period. The technical and economic lifetime of the OWEC is 20 years, see Terms of Reference of DOWEC [Ref.2].

2.2 BATHYMETRY

The bathymetry of the area around the two locations must be known as well. The Dutch part of the North Sea is surveyed on a regular basis by the “Dienst der Hydrografie” of the Ministry of Defence, Royal Dutch Navy. Information on the bathymetry in the DOWEC areas was requested by VOACZ. For the two DOWEC locations a total of three surveys were carried out from which the data was extracted by the “Dienst der Hydrografie”. The maps did not cover the complete envisaged DOWEC areas. About 45% of the site area at location III were covered during two surveys at 1993 and 1999. The survey map of 1986 for location VII covered about 65% of the total site area. The maximum and minimum seabed levels at the two locations are read from the maps.

Location Lowest seabed level

Highest seabed level

Site III CD - 24.9 m CD - 19.4 m Site VII CD - 34.2 m CD - 20.7 m

The tidal range at the two locations is taken from the British Admiralty Chart 1408 (latest update: 1988).

Location MLWS MSL MHWS Site III CD + 0.2 m CD + 1.0 m CD + 1.9 m Site VII CD + 0.2 m CD + 1.0 m CD + 2.0 m




2.3 CURRENT DATA

The depth averaged current velocity with a return period of 50 years are taken from the DOWEC report F1W1-WB-01-048/01-C [Ref.1]. In this report the results of the analysis of the NEXT database are given.

Location Current velocity (50 year)

Site III 0.77 m/s Site VII 0.95 m/s

2.4 DESIGN WAVE

The significant wave height and mean zero crossing period with a return period of 50 year are taken from the DOWEC report F1W1-WB-01-048/01-C [Ref.1]. In this report the results of the analysis of the NEXT database are given.

Location Hs 50 year Tz 50 year Site III 6.83 m 7.8 s Site VII 6.35 m 7.5 s

2.5 WAVE CLIMATE

The governing wave climates at Site III and Site VII are subtracted from the NEXT database as well.

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

Occ

urr

ence

(%

)

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR

Wave climate NL3

< 0.50

< 0.75

< 1.00

< 1.25

< 1.50

< 2.00

< 2.50

<10.00

< 0.50 7.6% 12.1% 9.2% 12.2% 19.5% 22.4% 20.3% 21.6% 11.7% 14.7% 7.1% 10.1% 14.1%

< 0.75 21.0% 31.7% 27.5% 35.6% 47.6% 51.8% 49.2% 47.1% 33.0% 33.1% 24.1% 24.1% 35.5%

< 1.00 36.5% 50.8% 45.0% 57.7% 71.6% 70.0% 70.7% 67.9% 51.6% 48.3% 37.6% 37.9% 53.8%

< 1.25 49.3% 63.6% 57.4% 75.1% 84.2% 83.1% 84.3% 79.8% 66.9% 61.0% 50.6% 49.1% 67.0%

< 1.50 59.5% 71.9% 67.7% 83.9% 91.5% 92.2% 92.1% 85.6% 78.7% 72.9% 60.4% 59.1% 76.3%

< 2.00 75.0% 85.4% 82.8% 93.3% 97.4% 98.8% 97.8% 94.1% 91.3% 87.3% 76.5% 75.0% 87.9%

< 2.50 85.4% 92.1% 92.2% 97.6% 99.8% 99.5% 99.6% 97.8% 96.2% 94.4% 88.6% 85.5% 94.1%

<10.00 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%





0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

Occ

urr

ence

(%

)


Wave climate NL7

< 0.50

< 0.75

< 1.00

< 1.25

< 1.50

< 2.00

< 2.50

<10.00

< 0.50 7.1% 11.1% 10.6% 12.5% 18.6% 21.0% 20.7% 22.0% 13.4% 15.0% 7.9% 9.0% 14.1%

< 0.75 20.3% 31.3% 28.7% 33.6% 43.5% 48.5% 46.5% 45.2% 31.9% 33.6% 24.0% 21.4% 34.1%

< 1.00 35.2% 49.7% 45.0% 55.1% 65.9% 67.8% 68.6% 63.8% 51.2% 46.8% 37.9% 36.0% 51.9%

< 1.25 49.3% 62.6% 58.4% 70.1% 81.3% 81.2% 83.2% 76.3% 68.2% 59.1% 49.3% 47.8% 65.6%

< 1.50 59.5% 70.9% 67.9% 80.4% 89.2% 89.5% 90.5% 84.5% 78.1% 71.0% 59.5% 56.8% 74.8%

< 2.00 74.6% 83.1% 83.6% 91.8% 95.5% 97.9% 96.9% 93.1% 89.9% 84.5% 74.8% 72.5% 86.5%

< 2.50 85.0% 90.9% 92.1% 96.6% 99.3% 99.3% 99.4% 97.8% 95.4% 92.3% 86.6% 84.2% 93.2%

<10.00 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%


2.6 SOIL CONDITIONS

A description of the top layer of the seabed soil is relevant for the scour protection design. The composition of the seabed soil at the two locations in the North Sea is taken from the available soil information report [Ref.3]. At site III the soil is found to consist of fine to medium sand, medium dense to dense. This top sand layer is typically up to 7m thick over finer material. At site VII the soil consists of fine loose sand. From these descriptions d50 and d90 are estimated.

Location d50 d90

Site III 200µm 1000µm Site VII 100µm 800µm




3 DESIGN PHILOSOPHY

Typically the scour protection will be realised using layers of natural, crushed rock. This form of scour protection is relatively simple to install and provides sufficient flexibility to adapt to changes over longer time. The rock itself is an environmentally friendly material that will endure without degradation for the economic lifetime of the turbines. The design of the scour protection can be separated into the following issues: 1) Grading of armour rock (to get a stable top layer under design condition) 2) Grading of filter layer(s) (to avoid washing out of the seabed soils through the rock

layers) 3) Thickness of different rock layers (to avoid washing out of the seabed soils through

the rock layers) 4) Horizontal dimension of the scour protection (to secure the soil at sufficient distance

to the turbine, required for stability of the foundation) For the design of the armour grading the combined shear stresses from currents and waves is established. The effect of the acceleration induced by the monopile is discussed and taken into account. This results in a required minimum average rocksize (D50). A standard rock grading that fulfils the requirement of this average rocksize is then taken for the armour grading. The required number and grading of the filter layers depend on the dimension of the rock armour and the seabed soil. The standard filter rules are being used for these calculations. The layer thickness of the armour and filter layer(s) is then determined with a standard formula depending on the average rocksize of the grading used in that layer. The introduction of the scour protection on the seabed results in increased turbulence at the downstream side of the scour protection. This increased turbulence introduces scour of seabed material at the edge of the scour protection. The resulting scour hole partly undermines the edge of the scour protection. Some of the rock will relocate thereby stabilising the scour slope. The depth of the scour hole that will form at the edge of the scour protection system, as well as the resulting slope influences the soil strength along the pile with increasing depth. This resulting strength variation must be used as input in the P-Y curves used to calculate the dynamic response of the turbine foundation. By extending the scour protection farther away from the monopile, the effect of the scour hole is reduced. This interaction is the driving parameter to determine the required horizontal dimension of the scour protection. To determine the depth of the scour hole, the critical situation can be defined as “clear water scour”. Clear water scour is defined as the situation when the current and wave




induced velocities in the undisturbed situation would not result in sediment transport. This implicates that the effects of the bed protection generate scour and thus an outflow of sediment at the downstream side of the bed protection, without any incoming sediment from the upstream side. Under these circumstances the largest scour depth will occur. However, when current velocities or wave action further increase from such a state, upstream inflow of sediment will occur, so that the scour hole will not deepen further. Finally an equilibrium scour depth will be reached.




4 CALCULATION METHODS

4.1 ARMOUR GRADING

Flow approaching a monopile sets up a pressure gradient on the upstream face of the pile, between the low pressure in the near seabed flow and the high pressure in the flow above, which drives a flow down the face of the pile. When no scour is tolerated, the top layer of the scour protection will have to be statically stable against the combined action of waves and currents. The theory used for the stability calculations is the Bijker/Shields theory.

4.1.1 Oscillatory flow

A recirculating eddy (primary vortex) is formed when the flow impinges with the rock protection or seabed; this wraps around the monopile (horse shoe vortex) and causes the fluid to accelerate to approximately twice the initial flow velocity. This horse shoe vortex reaches its maximum at a distance of 0.2*D from the face of the monopile and may continue to have an influence of approximately a distance of 4*D for oscillatory flow (waves). This horseshoe vortex occurs at values of the Keulegan-Carpenter number higher than 6 [Ref.7]:

6≥=D

T*UKC ww for existence of horseshoe vortex

where: Uw : amplitude of the bottom orbital velocity.




Tw :corresponding wave period D :pile diameter The 1:50 year design waves correspond to amplitudes of the bottom orbital velocity of 3.5 m for Site III and 3.0 m for Site VII. This results in a Keulegan-Carpenter number smaller than 6. Therefore it can be concluded that the horseshoe vortex does not develop due to oscillatory (wave) flow. The horseshoe vortex will only exist due to the steady state current.

4.1.2 Uni-directional flow

From Ref. 8 it was found that the potential flow theory around a vertical cylindrical pile indicates that the flow amplification factor due to the pile equals 2. Assuming bed shear stress is proportional to the square of the flow speed (Ref.8, Appendix 2) this produces an amplification factor of 4 on the shear stress close to the cylinder. Baker [Ref.9] summarised the following conclusions: • The maximum amplification occurs directly adjacent to the structure • The amplification becomes smaller with increasing distance from the structure. • At a distance of 1*D of the face of the monopile the amplification on the current

velocity is reduced to 1.0 When calculating the required rock grading for the scour protection, the amplification in the current velocity will have to be taken into account. Waves have an insignificant part in the development of the horseshoe vortex due to the small amplitude of the orbital velocity. Because of the rapid decrease in velocity at a distance away from the cylinder an average amplification of 1.5 on the current velocity is proposed to calculate the required rock size for scour protection.

4.2 GRADING OF FILTER LAYERS

The bed protection will be built using a granular filter. Granular filters are designed in successively coarser layers proceeding upward from the underlying finer soil. The first layer should hold the base material (i.e. subsoil), while each following layer has to be able to hold the underlying one (stability or piping criteria). The criteria for a granular filter are the following:

585

15 ≤bD

fD stability (piping) criteria

60550

50 ≤≤bD

fD segregation criteria

40515

15 ≤≤bD

fD permeability criteria




4.3 THICKNESS OF LAYERS

The minimum thickness of each rock layer is 1.7*D50 of the grading of that layer. From the pile to a distance 1*D of the pile, the thickness of the layer is advised to be thicker than above values. This is to account for the loss of smaller rocks that may be displaced by wave and current action. The recommended thickness near the pile is 2.5*D50.

4.4 LENGTH OF SCOUR PROTECTION

In paragraphs 4.4.1 through 4.4.4 the prior conditions for the final calculation of the length of the scour protection are determined. Calculating the scour hole at the end of a bed protection the following parameters should be determined: 1) Critical velocity according to Van Rijn 2) Turbulence factor determined by the roughness and the length of the bed protection

4.4.1 Critical velocities

The formula for the critical depth averaged flow velocity [Ref.6] is given for seabed material as:

)12ln(*****5.2 050

scc k

hDgu ∆Ψ=

where:

cΨ = Stability parameter calculated through the empirically found formula of van Rijn

[Ref6]

∆ = (ρsandgrains/ρ - 1) = 1.6 g = 9.81 m/s2

sk = 3*D90

Site III:

cΨ = 0.053; Filling in the governing values for the soil parameters and water depth

yields a critical velocity of uc = 0.37 m/s. Site VII:

cΨ = 0.1; Filling in the governing values for the soil parameters and water depth yields

a critical velocity of uc = 0.37 m/s.

4.4.2 Disturbance factor �

For preliminary design purpose it is necessary to have insight in the relation between the length of the bed protection and the disturbance in flow expressed in α. The parameter α is an amplification factor for the velocity and may greatly be reduced by increasing the length of the bed protection or by making it more rough. A smooth bed




protection results in a faster scour process because the flow above a smooth bed has a greater momentum than the flow above a rough bed. From Ref.6 it can be concluded that α=2.5. This value should be used in combination with calculations for the determination of the clear water scour hole depth.

4.4.3 Clear Water Scour Depth

Clear water scour means that no upstream sediment supply is present. Clear water scour calculations determine the maximum scour depth possible. In reality an upstream supply of sediment is often present and in such a case the development of a scour hole is reduced. The calculations are therefore conservative in this approach. The formula for determination of the scour hole depth (ym;e) is dependent on the water depth [Ref.6].

c

coem u

uuhy

]*5.0[*;

−=

α EQ.1

As this formula is for clear scour, the occurring water velocity u in the formula may not exceed the critical velocity. As in our case it does for both locations, u is limited to uc. This results in a minimum scour hole depth ym;e = 4.9 m for Site III (water depth 20 m) and ym;e = 5.2 m for Site VII (water depth 25 m).

4.4.4 Slope Stability

The stability of the upstream slope of the scour hole is the result of the interaction between fluid motion and the seabed material. The equilibrium situation of upstream scour slopes for non-cohesive material is based upon a consideration of the downward acting gravity force and the upward acting wave and current induced forces. As described on page 57 of Ref.6 Hoffmans and Pilarczyk found a semi-empirical relation for the equilibrium slope angle β.

)*)*75.011.0(**

*10*9.2arcsin(50

24

co frdg

u ++∆

= −β EQ. 2

where:

C

gr *2.10 =

40

45

0

==CC

fc

C is the coefficient of Chezy for the scour protection and C0 for the sea bed. Bijker and De Bruyn [Ref.6] found that the downstream slope is between the 1.5 and 2 times gentler than the upstream slope.




The equilibrium upstream slope is calculated with Hoffmans and Pilarczyk to be 14o for Site III and 21o for Site VII. With these slopes, failure due to shear is likely to occur. Wilderom [Ref.11] studied approximately 200 instabilities due to shear failure. From these cases an extreme slope of approximately 1:8 was derived in 1992 by Konter et al [Ref. 10].

4.4.5 Calculation of Length

A minimum length of the scour protection is set at 2*D. We assume that in this area the current is accelerated and a scour protection is needed. Although in literature a value of 1*D is found, we believe this has to be verified with tests. Since the pile diameter is defined by Ballast Nedam to be 6.0m, the minimum length is 12.0m. It is assumed that the seabed directly next to the monopile must remain at the same level if shear failure occurs. The seabed under the scour protection will in the worst case have a slope of 1:8. Stability calculations for the monopile must be performed based on the geometry after failure, i.e. a slope of 1:8 from the monopile to infinity with the rock weight of the scour protection taken into account. This will determine the length of the monopile in the seabed. This method is not the standard method used for offshore structures where the entire soil triangle will be protected. Where the proposed method results in lower costs for the scour protection, it may increase the costs for the pile foundation. An optimum in costs can be found. The minimum distance between the monopile and the scour hole can now be calculated from the equilibrium scour slope and the extreme slope after shear, which will result in a minimum length for the scour protection. This formula is as follows [page 38 Ref.6]:

)cot(cotyL ms −= 221 EQ. 3

where: γ2 = extreme slope after shear failure This results in a minimum length of scour protection for Site III of 10m (� so the minimum of 12.0m must be taken) and for Site VII of 14m. This difference is caused by the difference in water depth, current velocity and particle size of the seabed soil at the two sites.




5 DESIGN OF SCOUR PROTECTION

5.1 ROCK GRADING AND LAYER THICKNESS

The calculations for the stability of the armour grading have been performed for a range of water depths to be expected at Site III and Site IV. The influence of shoaling on the shape of the design wave is taken into account. The other design conditions of the two locations are kept as they were. For these two different locations, the scour protections are calculated per water depth. The filter layer design is based on the smallest possible standard gradings under each armour grading.

Armour layer Filter layer TOTAL Water depth Required D50 Grading Layer

thickness Grading Layer

thickness Layer thickness

20.0 m 411 mm 60–300 kg 0.80 m 10-80 mm 0.30 m 1.10 m 22.5 m 362 mm 40–200 kg 0.70 m 10-80 mm 0.30 m 1.00 m 25.0 m 309 mm 40–200 kg 0.70 m 10-80 mm 0.30 m 1.00 m 27.5 m 276 mm 10–100 kg 0.50 m 10-80 mm 0.30 m 0.80 m

Table 5.1: Scour protection for Site III

Armour layer Filter layer TOTAL Water

depth Required D50 Grading Layer thickness

Grading Layer thickness

Layer thickness

20.0 m 395 mm 60–300 kg 0.80 m 10-80 mm 0.30 m 1.10 m 22.5 m 347 mm 40–200 kg 0.70 m 10-80 mm 0.30 m 1.00 m 25.0 m 307 mm 40–200 kg 0.70 m 10-80 mm 0.30 m 1.00 m 27.5 m 273 mm 10–100 kg 0.50 m 10-80 mm 0.30 m 0.80 m 30.0 m 234 mm 10–60 kg 0.45 m 10-80 mm 0.30 m 0.75 m 35.0 m 132 mm 5–40 kg 0.40 m 10-80 mm 0.30 m 0.70 m

Table 5.2: Scour protection for Site VII

From the above results it can be concluded that although the required D50 is larger for Site III the resulting standard rock gradings and layer thicknesses are identical for both sites. For an overview of the thickness of the rock layers calculated above, the value of 1.7*D50 is used. The resulting number is rounded up to the next multitude of 5 cm in the table. For the filter layer a minimum layer thickness of 30 cm is used because of practical matters during installation. In addition to the layer thicknesses as presented in above tables, the additional rock next to the monopile (thicker layer of armour rock, see Section 4.3) is determined by the length (1*Dp) and the thickness (0.5* thickness of armour layer).




5.2 DIMENSIONS AND ROCK VOLUMES

In combination with the length of the scour protection the theoretically required volumes of armour and filter rock can be determined. The length can be calculated by filling out equations 1 and 2 into equation 3 (see Section 4.4). The variables are water depth, current velocity and D50 of the seabed soil. The required lengths of the scour protections and the resulting theoretical volumes are presented in the following tables: Water depth

Required Length

Volume armour rock Volume filter rock Total rock volume

20.0 m 12.0 m 633 m3 204 m3 837 m3 22.5 m 12.0 m 554 m3 204 m3 758 m3 25.0 m 12.5 m 585 m3 217 m3 802 m3 27.5 m 13.7 m 481 m3 255 m3 736 m3

Table 5.3: Rock volumes Site III

Water depth

Required Length

Volume armour rock Volume filter rock Total rock volume

20.0 m 13.5 751 m3 248 m3 999 m3 22.5 m 15.2 786 m3 303 m3 1088 m3 25.0 m 16.9 927 m3 363 m3 1290 m3 27.5 m 18.5 772 m3 429 m3 1201 m3 30.0 m 20.2 801 m3 500 m3 1301 m3 35.0 m 23.6 923 m3 659 m3 1582 m3

Table 5.4: Rock volumes Site VII

From the above tables can be concluded that for Site III no trend can be found between the water depth and the total rock volume, whereas for Site VII the rock volumes appear to increase with increasing water depth. However, for both sites the relations are too obscure to make unambiguous trendlines. The reason that it is not possible to make a trendline between rock volume and depth is that deeper water leads to deeper scour hole depths and to longer scour protection lengths, but it also leads to smaller sizes of armour stone and smaller layers. In the end it does not have to mean that a clear relation between depth and rock volume is present.




6 INSTALLATION ASPECTS

6.1 INTRODUCTION

Next to the rock volumes, gradings and lengths of the scour protections, other parameters like the water depth, distance to shore and workability are important for the cost estimates. It must be kept in mind that the costs may fluctuate enormously for small variations. If for example the required rock volume increases with 100 m3, this could mean that the volume is too large to fit in one shipload and an additional tidal cycle is required for installation of each scour protection.

6.2 WORKABILITY

It is expected that the scour protection will be placed by a side stone dumping vessel. This vessel can be positioned close enough to the pile for the rock of the scour protection to be placed in contact with the pile, thereby giving a good scour protection. The limiting wave height for a side stone dumping vessel, when working so close to a structure, is 1.0m. The resulting workability in the different months is given in Table 6.1. Month Site III Site VII Month Site III Site VII January 36.5% 35.2% July 70.7% 68.6% February 50.8% 49.7% August 67.9% 63.8% March 45.0% 45.0% September 51.6% 51.2% April 57.7% 55.1% October 48.3% 46.8% May 71.6% 65.9% November 37.6% 37.9% June 70.0% 67.8% December 37.9% 36.0%

Table 6.1: Workability during installation of scour protection (Hs < 1.0m)

It can be concluded that the workability is more or less the same for both sites. The major difference is the distance of the sites to shore. For Site VII the probability of a continuous period with wave heights smaller than 1.0 m during a rock dumping cycle (loading � sailing � positioning � dumping � sailing) is less than for Site III. However, the wave height limit of 1.0 m is only applicable for positioning and rock dumping. For loading of rock no wave height limit is applicable, since loading will take place in sheltered harbours. For sailing to and from the sites the wave height limit is approximately 2 m. Based on weather forecasts it will be decided by the Captain and Superintendent whether the port is left when waves are smaller than 2 m. Only when based on the forecasts the predicted wave height at the time and location of the rock dumping is less than 1.0 m, the vessel sets sail to the rock dumping location. Although only a limited period of the rock dumping cycle the wave height has to be smaller than 1 m, it can not be concluded that the workability percentages are higher




than those shown in Table 6.1. It may be possible that the vessel stays in the harbour even if the waves are smaller than 1.0 m and less favourable wave conditions are expected during the rock dumping cycle. Therefore, the wave height limit for rock dumping is commonly used for the determination of the workability of the entire rock dumping cycle.

6.3 SHAPE OF SCOUR PROTECTION

The theoretical shape of the scour protection is circular. However, to be able to make a uniform layer thickness, the rock dumping vessel has to move in straight lines. The minimum size of the resulting rectangular dumps depends on the dimensions of the equipment. Although the pattern of rectangular dumps is optimised so that the circular shape is best fitted, an additional amount of rock is required compared to the theoretical quantities of the circular shape. The degree of optimisation possible for a certain rock dumping vessel, depends on the required dimensions of the scour protection. Appendix A shows rock side stone dumping vessel Frans in action.

6.4 INFLUENCE OF CABLE ON SCOUR PROTECTION OF 6 MW OWEC

The offshore cable will only reach to the top of the foundation. A special end connection of the sea cable is therefore required at that level. A working platform and a winch will be required above the end connection of the sea cable in order to pull the cable from the seabed up to that level. A more detailed description of the cable trenching and tie-in to the monopile will not be given in this document since it will probably not have an influence on the design of the scour protection of the 6MW OWEC. This has been dealt with in the report on the cable installation [Ref. 5].

6.5 INSTALLATION SCHEDULE

The following working schedule is used for the cost calculation: 1 Install filter layer scour protection 2 Drive monopile 3 Attach transition section 4 Install J-tube and cable; make tie-in and lay to the next OWEC 5 Install armour layer scour protection 6 Place tower (and possibly nacelle plus rotorblades) on foundation The time between installation of the foundation and that of the scour protection should be as limited as possible to prevent the developing of a scour hole in the meantime. Therefore, the driving of the monopile and the construction of the scour protection occur as successive actions. In between the transition section needs to be installed so that the J-tube can be attached to the monopile. The interconnecting cable needs to be covered by the scour protection.




Measures should be taken to avoid spanning of the cable over a future scour hole at the end of the scour protection. For example, some additional cable length can be placed in a loop on the seabed.

7 BUDGETARY COSTS

7.1 INTRODUCTION

For the scour protection design the budgetary installation costs have been calculated. The costs are divided into: • fixed initial cost per installation period, for mobilisation and demobilisation of

equipment and personnel, regardless of the number of OWECs; • unit cost per OWEC, for equipment, personnel, fuel, material purchase and

transport, to be multiplied by the number of OWECs. Cost listed below is based on material cost, fuel cost, personnel cost and exchange rates at a 2003 price level. For the cost calculations of the rock transport also the distances to shore are required. In order to be able to use the installation costs in a DOWEC cost model, these data are based on Sites III and VII as defined in the DOWEC terms of requirements.

7.2 COSTS CALCULATIONS

In the following tables the costs are presented for different water depths. The costs are based on 80 OWECs per site. The total costs include the fixed initial costs of ��for mobilisation and demobilisation. In case the installation of the scour protection for all OWECs can not be carried out in a continuous operation, the costs for additionally required mob/demob have to be incorporated. Water depth 20 m 27.5 m Total/OWEC � 150,000 � 175,000 Total ��12,000,000 ��14,000,000

Table 7.1: Budgetary costs per OWEC for scour protections at Site III

Water depth 20 m 25 m 35 m Total/OWEC �� Total ��

Table 7.2: Budgetary costs per OWEC for scour protections at Site VII




REFERENCES:

Ref. 1: Wind and wave conditions, DOWEC-F1W1-WB-01-048/01-C, W. Bierbooms Delft University of Technology; Section Wind Energy, August 2002. Ref. 2: Terms of reference DOWEC, 176-FG-R0300-V1,F. Goezinne, September 2001. Ref. 3: Soil Data ,A. Kooistra, Ballast Nedam Engineering B.V., June 2001. Ref. 4: Concept study bottom protection around pile foundation of 3MW turbine, DOWEC-F1W1-RH-01-023/02, Van Oord ACZ, November 2001. Ref. 5: Cable installation study for DOWEC, DOWEC-F1W1-RH-01-033/00, Van Oord ACZ, November 2001. Ref. 6: Scour manual, C.J Hoffmans and H.J. Verheij, 1997. Ref. 7: Scour around vertical piles in waves, Summer et al., 1992. Ref. 8: Scour at marine structures, Richard Whitehouse, Hr Wallingford, 1998. Ref. 9: The turbulent horsehoe vortex, Baker. J Wind Eng. Ind Earodyn., 1980. Ref. 10: Konter, J.L.M., R.E. Jorissen & H. E. Klatter, 1992, Afsluitdammen regels voor het ontwerp, Ministry of Transport, Public Works and Water Management, Road and Hydraulic Engineering Division, Delft. Ref. 11: Wilderom, M.H., 1979, Resultaten van het vooronderzoek langs de Zeeuwse stromen, Report 75.2, Rijkswaterstaat, Directie Waterhuishouding en Waterbeweging Studiedienst Vlissingen.




APPENDIX A: SIDE STONE DUMPING VESSEL FRANS

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