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Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller Energinet.dk Endelig Rapport 7

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Page 1: Afsluttende rapport for Erosionsbeskyttelse omkring ...€¦ · Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller, FU5102 . Projekt nr. 06-80117 . Dato . 19. december

Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller

Energinet.dk Endelig Rapport December 2007

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Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller December 2007

Agern Allé 5 2970 Hørsholm Tlf: 4516 9200 Fax: 4516 9292 [email protected]< www.dhigroup.com CVR nr: 37057819

Klient

Energinet.dk

Klientens repræsentant

Niels Einer Helstrup

Projekt

Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller, FU5102

Projekt nr.

06-80117

Dato

19. december 2007

Forfattere

Anders Wedel Nielsen

Godkendt af

Henrik Kofoed-Hansen

0 Endelig Rapport AWN VJ HKH 19.12.07

Revision Beskrivelse Udført Kontrolleret Godkendt Dato

Nøgleord

Klassifikation

Åben

Intern

Tilhører klienten

Distribution Antal kopier

Energinet.dk: DHI:

Niels Einer Helstrup AWN, JF, EDC, VJ, Bibliotek

PDF 1+PDF

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INDHOLDSFORTEGNELSE

1 RESUMÉ ........................................................................................................................ 1 1.1 Opmålinger af havbunden og erosionsbeskyttelser........................................................ 1 1.2 Model til bestemmelse af den tidslige erosionsudvikling ................................................ 2

2 SUMMARY ..................................................................................................................... 4 2.1 Surveys of the Seabed and Scour Protections ............................................................... 4 2.2 Model to Determine the Time Dependent Scour Development ...................................... 5

3 END REPORT ................................................................................................................ 7 3.1 “Scour Protection around Offshore Wind Turbine Foundations, Full-scale

Measurements” – Paper from EWEC 2007 .................................................................... 8 3.2 “Time-varying Wave and Current-induced Scour around Offshore Wind Turbines” –

Paper from OMAE 2007 ................................................................................................. 9 BILAG A Technical Note: “Damage on the Scour Protection near the Turbine Foundations in

Horns Rev Wind Farm” B Technical Note: “Analysis of Scour Protection of Offshore Wind Turbine, Horns Rev,

Denmark” C Survey and Hydrographic Data (DVD)

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1 RESUMÉ

Formålet med projektet var at:

1. Sammenligne fuldskalamålinger af havbunden og erosionsbeskyttelsen omkring Horns Rev I fundamenterne med resultaterne af forskellige teoretiske modeller samt skalaforsøg.

2. Opdatere eksisterende teoretiske modeller vha. disse fuldskalamålinger.

3. Udvikle ingeniørmæssigt designværktøj til brug for fremtidige mølleparker.

1.1 Opmålinger af havbunden og erosionsbeskyttelser

Før og efter installationen af vindmøllerne er der foretaget 11 uafhængige opmålinger af erosionsbeskyttelsen og havbunden omkring fundamenterne på Horns Rev I. Af disse er opmålingerne fra 2002 (udført umiddelbart efter opsætningen og installation af dæksten) og fra 2005 blevet analyseret i detaljer.

Sammenligning af bundniveauet i henholdsvis 2002 og 2005 viser en sænkning af bun-den langs yderkanten af erosionsbeskyttelsen. Dette er forventeligt, da erosionsbeskyt-telsen vil medføre øgede hastigheder over bunden langs kanten af stenlaget og dermed give lokal erosion.

Desuden viste det sig, at der flere steder var en betydelig sænkning på op til 2 m af ero-sionsbeskyttelsen nær fundamentet. Denne type sænkning var i modsætning til erosio-nen langs kanten ikke forventet. Sænkningen kan skyldes tre forskellige forhold:

• Dækstenene er ustabile og transporteres væk.

• Filterlaget er for fint og transporteres op gennem dæklaget, hvorefter den samlede erosionsbeskyttelse synker sammen.

• Filterlaget er for groft og det underliggende sand transporteres op gennem filterlag og dæksten, med samme konsekvens som ovenfor.

Der er ikke fundet dæksten uden for den oprindelige erosionsbeskyttelse, og der er hel-ler ikke sket nogen signifikant forhøjelse af erosionsbeskyttelsen længere væk fra fun-damentet. Det er derfor sandsynligt, at dækstenene ikke er blevet transporteret væk. Dette stemmer overens med de modelforsøg, der blev udført forud for installationen af Horns Rev I Vindmøllepark.

Filterlaget kunne have bevæget sig op igennem dæklaget. Simple beregninger, erfa-ringstal og modelforsøg tyder dog ikke på, at det er det, der er sket i noget betydeligt omfang. Det er dog meget sandsynligt, at de fineste komponenter i filterlaget er blevet skyllet ud, men det kan langt fra forklare hele sænkningen af erosionsbeskyttelsen langs fundamentet. Derimod tyder erfaringstal fra bølgebrydere og simple beregninger på, at sandet kan være blevet transporteret væk gennem filterlag og dæksten. I modelforsøge-ne har dette fænomen ikke været studeret, idet sandet ikke nedskaleres (det vil blive så

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fint, at man kommer over i ler med helt andre egenskaber). Filter design skal sikre mod denne udskylning af sand.

Det har ikke været muligt at udvikle et definitivt ingeniørmæssigt designværktøj, da processerne er for komplekse. Det er dog muligt, at de simple beregningsmetoder, der er anvendt, kan vise sig at være mere generelt brugbare. Det vil dog kræve en bedre verifi-kation.

DHI arbejder videre med en avanceret numerisk model til at simulere strømningerne mellem stenene i erosionsbeskyttelsen. Det forventes, at resultaterne fra dette arbejde kan offentliggøres på OMAE konferencen i 2008. Resultatet af dette arbejde vil også give et bedre billede af muligheden for en generel anvendelse af de simple metoder, der er udviklet i projektet indtil nu.

Erosion omkring vindmøllefundamentet kan føre til reduceret stabilitet og ændret egensvingningsfrekvens af møllen, hvilket potentielt kan medføre skader på eller kol-laps af vindmøllen. Ifølge ejerne af Horns Rev I Vindmøllepark (Vattenfall og DONG Energy) er fundamenterne designet for et erosionshul, der er op til ca. 5,5 m dybt i for-hold til den oprindelige havbund. På baggrund af dette må det konkluderes, at de obser-verede sænkninger af erosionsbeskyttelsen ikke udgør en væsentlig forøget risiko for møllerne. Erosionsbeskyttelsen er efterfølgende blevet tilført flere sten.

1.2 Model til bestemmelse af den tidslige erosionsudvikling

Sideløbende med behandlingen af opmålingerne fra Horns Rev er der blevet arbejdet med at programmere en numerisk model til simulering af den tidslige udvikling af ero-sionen omkring en vindmølle funderet på en monopæl. Programmet har fået en forelø-big titel Wind Turbine Scour, WiTuS.

Med udgangspunkt i data som fundamentsdiameter, kornstørrelsen på bundmaterialet og vanddybde samt tidsserier for bølger, strøm og tidevand kan WiTuS beregne erosions-udviklingen omkring pælen.

WiTuS er bygget op på basis af empiriske formler, der er udviklet på baggrund af mo-delforsøg. De enkelte formler har som oftest et relativt snævert anvendelsesområde, som for eksempel: ”Erosion i strøm” og ”erosion i bølger”. Ved at kombinere disse formler opnås der et klarere billede af, hvor stort et erosionshul der rent faktisk optræder om-kring en monopæl, hvis en erosionsbeskyttelse udelades.

Afprøvninger af WiTuS med realistiske tidsserier, pæle og geotekniske data viser, at erosionshullet i lange perioder er meget mindre end 1,3 gange fundamentsdiameteren, en størrelse der ofte bliver anvendt i designet af monopæle. 1,3 gange fundamentsdia-meteren er middelværdien af erosionsdybden for ren strøm, hvor de største erosionshul-ler opstår.

Specielt er erosionshullet meget lille i perioder, der er domineret af bølgelaster. Det må forventes, at der er et sammenfald mellem store bølgelaster og store vindlaster, som er den primære last på havvindmøller. Det er derfor rimeligt at antage, at der kan anvendes et betydeligt mindre erosionshul for ekstremlaster, end der gøres i dag. Dette resultat er dog temmelig afhængigt af, hvor hurtigt erosionshullet fyldes op efter en strømdomine-

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ret periode. Da der kun er et meget begrænset antal forsøgsresultater til rådighed på det-te område, er der en vis usikkerhed med hensyn til resultatet. Det må forventes, at de ak-tuelle miljøforhold har en betydelig indflydelse på resultatet.

WiTuS kan i sin nuværende form bruges til at bestemme erosionen omkring monopæle uden erosionsbeskyttelse. DHI forventer imidlertid at fortsætte udviklingen i de kom-mende år, så det bliver muligt at inkludere erosionsbeskyttelse.

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2 SUMMARY

The objective of the project was to:

1. Compare full-scale measurements of the seabed and scour protection around the Horns Rev I foundations with the results of different theoretical models and scale tests.

2. Update existing theoretical models by use of full-scale measurements.

3. Develop engineering tools for the use of designing future offshore wind farms.

2.1 Surveys of the Seabed and Scour Protections

Before and after the installation of the wind turbines 11 independent surveys have been performed of the scour protection and the seabed around the foundations on Horns Rev. Of these the surveys from 2002 (made immediately after the installation and the armour stones) and from 2005 have been analysed in detail.

Comparison of the bed level in 2002 and 2005, respectively, shows a lowering of the seabed around the edge of the scour protection. This is to be expected as the scour pro-tection will cause increased velocities over the seabed along the edge of the stone cover, leading to local scouring.

In addition a significant lowering of the scour protection of up to 2 m near the founda-tion can be seen from the surveys. Contrary to the edge scour this type of lowering was not expected. The lowering may be caused by three different reasons:

• The armour stones are unstable and are transported away.

• The filter layer is too fine and is transported away through the armour stones, caus-ing the entire scour protection to settle.

• The filter layer is too coarse and the underlying sand is transported away through the filter and armour layers, with the same consequence as above.

Armour stones have not been found outside the original scour protection, and there is no significant increase of the scour protection layer further away from the foundation. For this reason it is most likely that the armour stones are stable. This is in good agreement with the model tests conducted prior to the installation of the Horns Rev I Wind Farm.

The filter stones may have moved through the armour layer. However, simple calcula-tions, common practice, and model tests indicate that this has not happened in any sig-nificant way. It is likely that the finest components in the filter layer have been trans-ported away through the rest of the filter layer and the armour stones, but this cannot explain the total lowering of the scour protection along the foundation. Common prac-tice used for breakwaters and simple calculations indicate that the sand may have been transported away through the filter and armour layers. In the model tests this phenome-non has not been studied, as the sand cannot be scaled down (it will be so fine that it

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will become clay, with completely different properties). The filter design shall prevent this washout of sand.

It has not been possible to develop a general engineering design tool because the proc-esses are too complex. However, the simple calculations applied in the project might be useful for a more general purpose. Before these simple calculations can be used it will, however, require more extensive verification.

DHI continues working with an advanced numerical model in order to simulate the flow between the stones in the scour protection. The results of this work are planned to be published at the OMAE conference in 2008. The result of this work will also give a more clear picture of the possible general use of the simple calculations developed in the project.

Scour around wind turbine foundations may lead to reduced stability and changed natu-ral frequency of the wind turbine. This may cause damages and failure of the wind tur-bine. According to the owners of the Horns Rev I Wind Farm (Vattenfall and DONG Energy) the foundations are designed for a scour hole up to approximately 5.5 m deep relative to the original seabed. Based on this it can be concluded that the observed low-erings of the scour protections do not result in any significant increased risk for the tur-bines. Additional stones have been applied for the scour protections afterwards.

2.2 Model to Determine the Time Dependent Scour Development

Parallel to the analysis of the surveys from Horns Rev a numerical model to determine the time varying scour around a monopile has been programmed. The programme has the working title WiTuS (Wind Turbine Scour).

Based on data such as diameter of the foundation, grain size of the seabed material, and water depth in addition to time series for waves, current, and tide WiTuS can calculate the development of the scour hole around the pile.

WiTuS is based on empirical formulas which are developed on the basis of model tests. Each of the formulas often applies to a relatively narrow part of the subject, like: “Scouring by current” and “scouring by waves”. By combining these formulas a clearer picture of the actual size of the scour hole can be achieved.

Tests of WiTuS with realistic time series, piles, and geotechnical data show that during long periods the scour hole is much smaller than 1.3 times the diameter of the founda-tion, a scour depth which is often used in the design of monopiles. 1.3 times the diame-ter of the foundation is the mean value of the scour depth for current, where the largest scour holes are developed.

Especially during periods dominated by wave loads the scour hole is very small. It is expected that there is a direct correlation between high wave loads and high wind loads, which is the largest load on an offshore wind turbine. On this basis it is justified to as-sume that a significantly smaller scour depth can be applied for the design for extreme loads, compared to the depths applied today. This result is, however, depending on the time scale of the backfilling of the hole, after a current dominated period. As the num-ber of available test results for backfilling is very limited there are some uncertainties

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for this part of the result. It is expected that the actual environmental conditions have a significant influence on the results.

At the present stage WiTuS can be used for determining the scour development around monopiles without scour protection. DHI plans to continue the development of the pro-gramme during the coming years to include e.g. scour protection.

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3 END REPORT

The project consists of three parts: • Analyses of the full-scale measurements from the Horns Rev I Wind Farm. This

work is described in the section “Scour Protection around Offshore Wind Turbine Foundations, Full-scale Measurements”.

• Programme for determination of the time-varying scour development around a wind turbine foundation. This part is described in the section “Time-varying Wave and Current-induced Scour around Offshore Wind Turbines”.

• Detailed calculations of the flow in the scour protection. The module for the computational fluid dynamics, CFD, program to be used for the de-tailed calculations of the flow in the scour protection is still under development. The module will make it possible to calculate the flow in a porous media like a scour protec-tion or rubble mound breakwater. The module can be used for steady currents as well as oscillating flows due to waves. At the moment the program has not been used to calculate the flow in a scour protec-tion, but has been tested for a dam break where a porous media is placed in front of a dam, see Figure 1. The results from the simulation compare very favourably with the measured data. It is expected that the results of calculations of the flow in a scour pro-tection will be presented at the OMAE 2008 conference.

0 0.2 0.4 0.6 0.80

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Figure 1 Dam break through a porous media. The porous media is placed between x=0.3 and x=0.6. The dam breaks at time=0. The red dots are experimental data while the fully drawn blue line is the calculated data.

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3.1 “Scour Protection around Offshore Wind Turbine Foundations, Full-scale Measurements” – Paper from EWEC 2007

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Scour Protection around Offshore Wind Turbine Foundations, Full-Scale Measurements

Erik Asp Hansen DHI Water • Environment • Health

[email protected]

Anders Wedel Nielsen DHI Water • Environment • Health

[email protected]

Michael Høgedal Vestas Wind Systems

[email protected]

Hans Jacob Simonsen DHI Water • Environment • Health

[email protected]

Jan Pedersen DONG Energy

[email protected]

Abstract: The major challenge of ensuring the stability of the seabed around offshore wind turbine foundations is addressed in the present paper.

Full-scale, high-resolution measurements of the seabed and the scour protection around the monopile foundations in Horns Rev offshore wind farm are analysed and presented. The measured developments in the scour protection are related to wave and current conditions.

The wind turbines at Horns Rev wind farm are each placed on circular monopiles at water depths ranging from 6-13 m at MSL. The seabed consists of fine to coarse sand (D50 = 0.15-1 mm). The wave conditions are severe – the largest waves are in the order of 8 m high. The max current in the wind farm area is approximately 0.88 m/s (50 year return period).

The stability of the seabed around the wind turbine foundations is one of the major challenges in offshore wind farm design. Each foundation at Horns Rev is protected by a 50 cm thick filter layer (D50 = 10 cm) and a 1 m thick armour layer (D50 = 40 cm) placed within a radius of 9.5 m from the centre of the monopole.

5 high-resolution surveys of the scour protection around all the wind turbine units have been carried out before and after installation of the turbine foundations. The paper presents the development of the surrounding seabed and the scour protection themselves.

The surveys around all 80 wind turbines consistently show a lowering of the scour protection and a net transport away from the scour protection area. The measurements also show that up to 1.5 m local lowering of the armour layer have taken place

during the time period from 2002 to 2005. The largest lowerings have occurred close to the wind turbines.

The history of the wave current climate in the wind farm area has been established from a combination of numerical simulations and measurements. From the wave current climate the movement of the underlying sand bed has been assessed.

Simple flow analysis and the surveys indicate that the sand has been transported upwards through the filter and armour layers at most of the wind turbine locations. This has resulted in subsidence of the filter and armour layers into the sand bed. The top level of the scour protection layer is, however, still above the surrounding seabed level, which the wind turbines have been designed for. It is further concluded that some erosion up to 0.5 m (0.1 tower diameter) deep has occurred outside the scour protection in the distance 15 m away from centre of the wind turbines on the east side of the turbines (opposite the main wave direction (coming from)). Keywords: Full-scale measurements, scour, scour protection, wind turbine foundations.

1 Introduction

All today’s offshore wind farms producing energy (start 2007) are located in shallow waters; they are either foundated on monopiles or on gravity based caissons. The design of the offshore turbines has to include an assessment of the possible long-term morphological development occurring during the design period, and an assessment of the local erosion caused by the presence of the piles themselves. For unprotected monopiles placed on erodible sand scour depths deeper than 2.0 times the pile diameter have

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been observed in experiments, see [1]. For a typical pile this means that the piles have to be designed for extra 8-10 m tower length. It shall be noted that the largest local scour depths generally occur in a current dominating environment, in wave dominating environment the scour depth typically becomes much smaller.

In order to avoid the extra costs, which are associated to having a longer tower, and required to dredge or protect the power cables from turbines, the seabed around many energy producing offshore wind turbines is today protected by stones and gravel. Two approaches have been used, the so-called static design, where the seabed is protected before or immediately after the pile is installed (see [2]) or dynamic design, where a scour is allowed to be developed before the scour potection is installed.

The Horns Rev offshore wind farm is located 15 km offshore in a harsh environment of the North Sea. To protect the foundation from scouring and thus ensuring the stability of the pile, each foundation has been protected by a rock berm consisting of 50 cm single filter layer (D = 10 cm) and an approximately 1 m thick armour layer (D = 40 cm). The armour layer was designed in a desk study, and stability was verified in completed experiments in scale 1:25 in co-directional waves and current for the design wave-current seastate. The filter layer was designed to withstand a summer seastate, and was also tested in co-directional waves and current.

Almost all of today’s knowledge about scour development and the stability of scour protection is based on physical experiments carried out in flumes in co-directional waves and current, with two-dimensional waves, see for example [3]. Each test typically has a limited duration, focussing on the extreme events. The transformation of these physical experimental results into full-scale conditions is based on the assumption that the Froude scaling law is valid. This means that all Reynolds number effects are considered to be negligible. In most cases, it has not been possible to scale the seabed material according to Froude law because the grain sizes, in model scale, would have become so small that they no longer would have acted as sand (cohesionless material). For example at Horns Rev the seabed grain size is typically between 0.1 mm and 1.0 mm. Making physical experiments in scale 1:25, the grain size gives grains between 0.004 and 0.04 mm. Such fine material does not act as sand. Instead, physical experiments are carried out using as fine sand as possible, app. 0.1 mm.

Some full-scale measurements/observations have been reported. It is costly and difficult to collect all information required to determine the environmental conditions that the seabed/scour protection has experienced after being installed. Full-scale measurements have therefore only been used to a limited degree for the design of scour and scour protection for the offshore wind turbines installed during the last years. In the future it is expected that more and more elements of the design of scour protection will be based on numerical modelling.

The present study was initiated with the purpose of analysing and using the full-scale data collected during 3 surveys prior to installation of the piles and 6 surveys completed after installation of the piles at Horns Rev wind farm. And finally compare the results by the existing theories, and with the model test results carried out during the design phase. The surveys indicate that the armour layers have been stable, however, they have moved downwards due to movement of the underlying filter layer and seabed away from the piles. So the purpose of the study was directed towards a study of the stability of the filter layer and the seabed. A simple model for the movement of the filter layer and seabed has been set up in order to explain the lowering of the armour stones.

2 Survey

The location of the wind turbines is shown in Figure 1. The water depth in the farm is between 6.5 m and 13 m. The survey was conducted by divers or as multibeam survey from a vessel (2005). The accuracy of the measurements is in the order of 0.2 m.

Turbine number 07 is selected as an example and the results are presented and interpreted in the following. The survey of 2002 and the first time evolution surveys of 2005 (2005 survey) are compared. The location of turbine number 07 is shown in Figure 1. Figure 2 shows the difference between the 2002 survey (after installation of armour layer) and the 2005 survey. The blue colour indicates an increase in depth, e.g. a scour hole, whereas the red colour indicates a decrease in depth, e.g. deposition of sand. As seen on the figure there are numerous red and blue spots with an approximate diameter in the order of 0.2 m. This indicates a relocation of stones.

Close to the pile on the south western side, a scour hole has apparently been developed. However, this scour hole might be caused by uncertainties in the measurements and/or in the conversion of the survey of 2002.

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Figure 1: Overview of Horns Rev wind farm. Turbine number 07 is marked with distinct colours. The depth contours are based on the survey carried out in 2001.

Wes

t cro

ss

ti

East

cro

ss

ti

E-W cross ti

N-S cross ti

Figure 2: Difference between 2005 survey and cover 2002 survey. (’05 minus ’02). The blue colour indicates erosion, whereas the red colour indicates deposition. Cross-section lines are indicated with black dashed lines.

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Figure 3: Cross-section going N-S approximately 25 m east (upper figure) and cross-section going E-W approximately 25 m west (lower figure) of turbine 07, see Figure 2. Black is the cover 2002 survey and

cyan is the 2005 survey.

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From Figure 3 it is seen how the sand bed east of

turbine number 07 is approximately 0.3 m eroded, whereas backfilling in the order of 0.2 m has occurred on the western side.

Cross-sections going through the pile in E-W and N-S directions are shown in Figure 4. A lowering of the scour protection layer of up to 0.5 m north of the pile is observed. Just west of the pile erosion is indicated.

Figure 4: Cross-section through the foundation pipe of turbine 07, see Figure 2. Top: going E-W, bottom: going N-S. Black is the cover 2002 survey and cyan is the 2005 survey.

In the entire area, see Figure 2, 66 m3 was deposited and

520 m3 eroded, which gives a net erosion of 454 m3, corresponding to a 0.20 m average lowering of the seabed.

2.1 Lowering in the Entire Park The lowering around all piles has been determined as done for turbine 7. The maximum change between the level of the scour protection in 2002 and 2005 is shown for the entire wind farm in Figure 5. The 20% fractal for the maximum lowering is -2.0 m, 50% fractal is -2.2 m, and 80% fractal is -2.5 m. For each pile the averaged bed level changes are determined inside the following three areas:

• 2.5-3.5 m, which is close to the pile and therefore the results may be affected by inaccuracies in the measurements

• 3.5-6.0 m • 6.0-10 m which is relatively close to the outer

perimeter of the scour protection

Figure 6 shows the results of the net transport analysis. As seen, the majority of the scour protections are subjected to a large negative net transport, i.e. material is transported away from the area.

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Figure 5: Maximum scouring. The values are calculated as the maximum change in scour protection level from 2002 to 2005 up to 10 m from the centre of the foundation. Negative (red colour) is erosion.

Figure 6: Overview of the net transport from the entire wind farm. Most of the scour protections are subjected to erosion. All values are in m3.

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Column number (from West to East) 0x 1x 2x 3x 4x 5x 6x 7x 8x 9x

X1 -51.9 -128.4 -41.7 -6.4 6.4 -33.3 -80.3 -86.7 -86.7 -154.6 X2 -25.0 -125.6 39.7 -92.4 -41.9 -63.5 -117.6 -68.8 -34.9 -119.4 X3 -36.3 -88.5 -71.4 -136.8 17.1 -26.7 - -91.9 -54.1 -87.6 X4 -72.4 -134.0 -17.7 - -62.7 -90.2 -47.1 -42.3 -152.0 -78.3 X5 -49.6 -77.4 -98.6 -269.5 -72.3 -67.5 -130.8 -80.8 -117.1 -86.2 X6 -142.4 -274.7 -57.6 -147.9 -65.9 -152.0 -56.3 -96.9 -37.6 -135.0 X7 -83.5 -117.8 -146.9 -55.8 -127.7 -68.0 -40.0 -125.3 -146.5 -75.7

Row

num

ber

(fro

m N

orth

to S

outh

)

X8 -100.0 -54.3 -81.3 -113.4 -114.5 -1.1 -96.0 -58.6 -144.5 -65.9

Table 1: Net transport for the entire wind farm from 2002 to 2005??. The net transport is calculated in a circular band going from r = 2.1 m to R= 10 m measured from the centre of the wind turbine. All values are in m3.

2.2 Movement of Armour Layer The survey clearly shows that the level of the armour layer close to the piles has been lowered. This may either be caused by movement of armour stones away from the piles or movement downwards.

There is no indication in the surveys that armour stones have been moved away from pile centre to a position on top of the armour layer away from the pile centre nor have armour stones been observed outside the scour protection. From the survey it has therefore been concluded that most likely the armour stones have moved downwards. This is only possible if parts

of the underlying filter layers or seabed have been transported up through the armour layer. This is considered in the following.

3 Environmental Conditions

The waves have continuously been measured by a wave rider, see Figure 7. The waves have in the entire period been simulated by a wave spectral model that includes tidal and wind generated water level changes, see [4]. Current velocities up to 1m/s can occur at the Horns Rev.

Figure 7: Sample of the calculated significant wave height (Hs). The location of the wave rider and the ADCP

(Acoustic Doppler Current Profiler) (in the period before summer ’05) is shown in the figure.

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Figure 8: Comparison in a 25 days period in 2005 between Hs from the numerical spectral wave model (red line) and Hs (Hs = 4•std(η)) calculated from the wave rider measurements (blue line).

0x 1x 2x 3x 4x 5x 6x 7x 8x 9x x1 4.19 4.07 3.96 3.85 3.71 3.57 3.47 3.36 3.24 3.12 x2 4.32 4.22 4.13 4.03 3.88 3.69 3.56 3.44 3.32 3.20 x3 4.49 4.40 4.30 4.21 4.03 3.83 3.66 3.53 3.41 3.29 x4 4.73 4.60 4.47 4.34 4.16 3.97 3.77 3.64 3.53 3.42 x5 4.97 4.80 4.63 4.47 4.29 4.10 3.92 3.77 3.66 3.56 x6 5.22 5.01 4.80 4.59 4.42 4.25 4.09 3.97 3.85 3.73 x7 5.43 5.26 5.08 4.90 4.71 4.52 4.35 4.21 4.07 3.94 x8 5.65 5.51 5.37 5.21 5.00 4.80 4.61 4.45 4.30 4.14

Table 2: 1 hour maximum significant wave height at each of the wind turbines in the period 1 January 2004 –

1 January 2006. All values are in m. Figure 8 shows a comparison between the simulated and measured significant wave heights. The 1 hour maximum significant wave height at each of the wind turbines in the period 1 January 2004 – 1 January 2006 is presented in Table 2.

4 Filter Criteria

The Horns Rev scour protection consists of: 1. armour stones: 350-550 mm, with mean diameter

= 400 mm 50d2. filter stones: 30-200 mm with mean diameter

= 100 mm 50d3. the underlying seabed consists of sand with

= 0.1-1 mm 50d

15d 50d 85d Armour 370 mm 400 mm 550 mm Filter 20 mm 100 mm 200 mm Seabed 0.1 mm 0.5 mm 1.0 mm

Table 3 Grain sizes at Horns Rev.

The grain sizes of the armour stones, the filter stones, and the seabed have been compared with often used filter criteria for rubble mound breakwaters, the Thompson & Shuttler Criteria and U.S. Army Criteria. These filter criteria can e.g. be found in [5]. The filter criteria for the soil conditions as stated above are summarised in Table 4 together with often used filter criteria, see Table 4. Both the Thompson & Shuttler Criteria and U.S. Army Criteria consist of three sub-

criteria: f,

a,

dd

85

15 ensure that the smallest stones in the

armour layer would not subside into the filter layer,

f,

a,

dd

15

15 ensure that the smallest filter stones will not

move out through the armour layer, and f,

a,

dd

50

50 ensure

the overall stability of the main part of the filter stones.

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Ratio between over and lower layer

low

up

dd

,85

,15

low

up

dd

,50

,50

low

up

dd

,15

,15

Horns Rev filter to armour layer

1.85 4 18.5

Horns Rev seabed to filter layer

20 200 200

Thompson & Shuttler Criteria

≤4 ≤7 ≤7

U.S. Army Coastal Engineering Research Center Criteria (Shore protection manual)

~2.2 ~2.3 ~2.5

Table 4 Filter criteria for typical soil conditions.

As seen in Table 4, the Thompson and Shuttler criterion is nearly fulfilled for the interface between the armour layer and the filter layer. However, some of the smallest stones in the upper part of the filter layer might be lost through the armour layer. The filter criteria for the interface between the seabed and the filter layer are far from being fulfilled. Transport of sand through the filter and armour layer may take place if the flow between the stones becomes high enough to move the sand. This is investigated in the following. 4.1 A Simple Model for Vertical Transport of Sand and Filter Stones upwards through the Armour Layer In this section the flow inside the scour protection is estimated, using a simple engineering approach: 4.1.1 Step 1, near bed velocity The undisturbed (not influenced by the presence of the piles or scour protection) near bed velocities, amplitudes, and excess pressures are estimated using first order sinusoidal wave theory. The near bed flow conditions are used to evaluate the time varying flow in the armour layer, in the filter layer, and in the underlying sand.

The first order near bed wave velocity is related to the wave height, H, the period, T, the wave length, L, and the water depth, h, as:

( ) )tsin()L

hsinh(THtU bed ω

ππ

21

=

The first order near bed pressure height (in metre):

)tsin()L

hcosh(H

gPbed ωπρ 2

12

=

)tsin()L

hcosh(LH

gxPbed ωπ

πρ 2

11=

∂∂

The wave length, wave period, and water depth are related through the linear dispersion relation:

)Lhtanh(gTL π

π2

2

2

=

Using the above relations the near bed velocity and pressure spectra can be determined from a given wave spectrum. For the Horns Rev wind turbines, assuming a JONSWAP spectrum:

( ) ( )a

ffexp

fgS γ

παω

⎥⎥⎦

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−=

4

054

2

45

2

where α=0.0081 and f is the frequency and f0 is the peak frequency.

33.=γ

( )⎥⎥⎦

⎢⎢⎣

⎡ −−= 2

02

20

2 fff

expaσ

where σ=0.07 for f≤f0 and σ=0.09 for f>f0. Using a peak period of the wave of 8 s, and a water depth of 10 m the significant near bed velocity and the significant pressure gradient are related to the significant wave height as:

1350 −⋅= s.Hu stsignifican

10320 −⋅=∂

∂m.H

xg

P

stsignifican

bedρ

4.1.2 Step 2, effect from pile and protection The pile and scour protection changes locally the near bed velocities and pressure gradients. In the general case the near flow and pressure field can be determined using CFD modelling, see [6] or [7]. For a circular cylinder on a plane bed exposed to small waves, these changes can be estimated from Mc-Cammy Fucks analytical solution. For shallow waves both the velocities and the pressure gradient will be increased by a factor of 2. This factor is used in the present simple analysis, although the authors are aware that this may be too crude to use in a general case. 4.1.3 Step 3, flow inside the scour protection The flow occurring in the armour layer is partly due to the shear from the flow above the armour layer and partly due to pressure gradients. The importance of the first part decreases moving downwards through the layer, whereas the pressure part can be taken to be constant in the first few metres below the surface. The flow is here evaluated using only the horizontal gradients.

The horizontal filter velocity, V (defined as the volume flux per area) through the armour layer can be estimated by use of a modification of Darcy’s

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formulae, where the effects of turbulence and inertia are included, see [8]:

tVcVV'baV

dxg

Pd

∂∂⋅++=

ρ

where a, b’, and c are rock specific constants. The coefficients a, b’ are taken from [9].

] and ( )[ ]22 m/s gd/b 35=[s/m gd/a 22000ν=and c is found from [8] for grains in

consideration. m/s.c 250=

For known time variation of the pressure gradient, the filter velocity, V, can be found by solving the first order differential equation numerically.

The flow velocity (the water velocity between the grains) is related to the filter velocity through the porosity n as:

nVU =

4.1.4 Step 4, stability of the interfaces If the flow is sufficiently high above a certain layer, the upper grains will start moving. In the present simple analysis the onset of grain movement is determined using the Shields parameter concept.

The Shields parameter at the interface between an upper layer and a lower layer is defined:

50,ff

f

gd)('

'ρρτ

θ−

=

where: fρ is the specific gravity of the lower layer

ρ is the water density

50,fd is the mean grain diameter of the lower layer The shear stresses fτ ’ on the top of the filter layer

are assumed to be determined by the same relation normally used to determine the wave induced bed shear stresses:

221

awf Uf' ρτ = where:

aU is the horizontal velocity just above the lower boundary in the upper layer

wf is the friction factor in oscillatory flow, depending on the Nikuradse equivalent roughness

taken equal Nk 5052 ,fN d.k = and the amplitude of the

water movement in the armour layerπ

paTUa = (Tp is

the peak period). The following relation, see [10], is

used: 5004041

>⎟⎟⎠

⎞⎜⎜⎝

⎛=

NNw k

a,ka.f

5040750

<⎟⎟⎠

⎞⎜⎜⎝

⎛=

N

.

Nw k

a,ka.f

The method presented has been used for a typical storm situation: significant wave height to equal 3.5 m, water depth 10 m, and peak wave period 10 s. The associated orbital velocities are listed in Table 5.

At seabed far from the pile 1.5 m/s In the armour layer far from the pile 0.34 m/s In the filter layer far from the pile 0.17 m/s Above scour protection close to the pile

3.0 m/s

In the armour layer close to the pile 0.49 m/s In the filter layer close to the pile 0.24 m/s

Table 5: Orbital velocities. The orbital velocities from Table 5 have been used to estimate the Shields parameter inside the armour and filter layer caused by the wave orbital motion. The authors are fully aware that the use of this approach may be questionable, as the flow in the layers is far from being uniform which was the case in the experiments on which the formulas are based.

The authors are also aware that Hatipoglu et al. in 2007 will present an experimental study “Suction Removal of Sediment from between Armour Blocks in Waves”, see [11]. These results may be more applicable for the present purpose than using simple wave friction factors and it will be interesting to see how their results match with the observations made on Horns Rev.

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0 0.05 0.1 0.15 0.20.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

Filter size [m]

Shi

elds

par

amet

er

Near pileFar from pile

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Grain size [mm]

Shi

elds

par

amet

er

Near pileFar from pile

Figure 9: Shields parameter on the upper part of the filter layer (upper figure) and the seabed (lower figure), due to

oscillatory flow in the armour layer. Upper layer D50 = 0.4 m, Hs = 3.5 m, Tp = 10 s, and h = 10 m.

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Figure 10: Definition for the calculation of the lift of

sediment

4.1.5 Step 5, evaluation of sand movement during one wave cycle Vertical displacement of the sediments will take place if the vertical water velocities are larger than the fall velocity of the sediment. The fall velocity is here found from Stokes by:

νρ

ρ

18

1 250gd)(

ws

s

−=

where ν is the viscosity. The vertical distance a sediment grain will move

during one cycle can be calculated as:

0for )(2

1

≥−−= ∫ s

t

tsv wvdtwvl

See Figure 10 for the definitions of v, t1 and t2. The vertical flow velocities are related to the

vertical pressure gradients. Far from the pile the vertical gradients are significantly lower than the horizontal gradients, whereas the vertical pressure gradients close to the pile are expected to be equal the horizontal gradients.

The vertical velocities are here taken to be proportional to the horizontal velocity. α=UW . In this simple analysis, α=0.5 and α=1 is used near the pile and far from the pile 20.=α is used. The vertical movement using these values is presented in Figure 11. This figure indicates that the theory presented in this paper explains why the most severe lowering has taken place close to the piles. Furthermore, the model predicts that sand finer than 0.35-0.45 mm close to the pile can move 0.5 m upwards (all the way through the filter layer) during one wave cycle, for wave height equal 3.5 m.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.5

1

1.5

2

2.5

3

Grain size [mm]

Ver

tical

dis

plac

emen

t dur

ing

one

wav

e cy

cle

[m]

Near pile, α=1.0Near pile, α=0.5Far from pile, α=0.2

Figure 11: Vertical movement of sand during one wave cycle. Upper layer D50 = 0.1 m, Hs = 3.5 m, Tp = 10 s, and h = 10 m.

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8. Conclusion

The surveys indicate that the armour layers have been stable, even though they have moved downwards due to movement of the underlying filter layer and seabed away from the piles. A simple model for the movement of the filter layer and seabed has been set up, which indicates that the lowering of the armour stones is mainly caused by a sand transport through the filter and the armour stones. To avoid movements of the seabed material a finer filter layer or a geotextile could be applied. However, due to the strong current it is impractical to apply a geotextile and an extra finer filter layer would easily be lost during construction. For this reason these solutions were disregarded in the design of the Horns Rev Wind Farm. A possible solution would be to apply a thicker filter layer with a higher gradation. This will of course result in some loss of filter material through the armour layer, but over time it will be stable. Another option will be, as at Horns Rev, to make frequent surveys and then apply new stones if necessary. Acknowledgements

The financial support (1.3 mio. DKK) of the Danish Public Service Program (PSO) under Contract PSO: 6508 (FU5102) is gratefully acknowledged. References

[1]. Sumer, B.M. and Fredsøe, J. (2001). Scour around pile in combined waves and current. J. Hydraulic Eng., Vol. 127, No. 5, 403-411. [2]. J.H. den Boon, J. Sutherland, R. Whitehouse, R. Soulsby, C.J.M. Stam, K. Verhoeven, M. Høgedal, T. Hald. Scour Behaviour and Scour Protection for Monopile Foundations of Offshore Wind Turbines. [3]. Sumer, B.M. and Fredsøe, J. (2002). The Mechanics of Scour in the Marine Environment, World Scientific, Singapore. [4]. Jacobsen, V., Rugbjerg, M. (2005). Offshore Wind Farms – the Need for Metocean Data. Copenhagen Offshore Wind 2005, Copenhagen, 26-28 October 2005. [5]. Jensen, O. J. (1984). A Monograph on Rubble Mound Breakwaters, Danish Hydraulic Institute. [6]. Bredmose, H., Skourup, J., Hansen, E.A., Christensen, E.D., Pedersen, L.M. and Mitzlaff, A. (2006). Numerical reproduction of extreme wave loads on a gravity wind turbine foundation. 25th Int. Conf. Offshore Mech. Arctic Engng. Hamburg 2006. ASME. [7]. Christensen, E.D., Bredmose, H. and Hansen E.A. (2005). Extreme wave forces and wave run-up on offshore windturbine foundations. Proceedings of Copenhagen Offshore Wind 2005, Copenhagen October 2005. [8]. Andersen, O. H. (1994). Flow in Porous Media with Special Reference to Breakwater Structures. Hydraulics and Coastal Engineering Laboratory Aalborg University in collaboration with Danish Hydraulic Institute.

[9]. Engelund, F. (1953). On the Laminar and Turbulent Flows of Ground Water trough Homogeneous Sand. Transactions of the Danish Academy of Technical Sciences, No 3. [10]. Fredsøe, F and Deigaard, R (1992) Mechanics of Coastal Sediment Transport, World Scientific, Singapore. 11. Hatipoglu, F. Sumer, B. M. and Fredsøe, J. Suction Removal of Sediment from between Armor Blocks in Waves. 30th International Conference on Coastal Engineering, San Diego 2006, Book of abstracts, paper no. 275

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3.2 “Time-varying Wave and Current-induced Scour around Off-shore Wind Turbines” – Paper from OMAE 2007

19.12.07/AWN/hec/pot/rapport/80117-afsluttende rapport 9 DHI

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Proceedings of OMAE2007 26th International Conference on Offshore and Arctic Engineering

June 10-15, 2007, San Diego, California USA

OMAE2007-29028

TIME-VARYING WAVE AND CURRENT-INDUCED SCOUR AROUND OFFSHORE WIND TURBINES

A.W. Nielsen DHI Water & Environment

Agern Allé 5, DK-2970 Hørsholm, Denmark [email protected]

E.A. Hansen DHI Water & Environment

Agern Allé 5, DK-2970 Hørsholm, Denmark [email protected]

ABSTRACT The paper presents the engineering model, WiTuS (Wind

Turbine Scour), which uses the results of several previous published physical experiments with scour around cylinders to determine the time development of the scour hole around the pile due to the actual and the previous wave and current climate.

The WiTuS includes: • Time-varying water level, sea states and current • Seabed material properties • Description of the scour geometry around the pile

Simulations presented in the paper show that for typical North Sea conditions the scour depth will be around 0.3 times the pile diameter in periods with larger waves, which is a significant reduction from 1.3 times the pile diameter which is often seen as the industry standard.

INTRODUCTION During the last years, the interest of developing offshore wind turbines has increased significantly as the wind turbines are increased in size and numbers and it has become more difficult to find new good locations onshore. The offshore wind turbines are often located in relatively shallow water, where the foundations are often exposed to large breaking and near breaking waves and strong currents. Today industry standard for calculating the scour depth around a circular pile is using a fixed number (often 1.3 is used) times the diameter of the pile, corresponding to the equilibrium scour in pure current. However, it has been shown by model tests (for example by Sumer and Fredsøe (2001a)) that the equilibrium scour depth in combined waves and current and in waves alone is significantly smaller than 1.3 times the pile diameter.

NOMENCLATURE a: wave amplitude d50: mean diameter of sediment dt: time step D: diameter of the pile fw: wave friction factor g: acceleration due to gravity h: water depth Hs: significant wave height k: wave number kw: apparent wave-current roughness ks: bed roughness (2.5d50) KC: Keulegan-Carpenter number (based on RMS values

(Sumer and Fredsøe (2001a)) RScour: radius of scour hole s: relative density of sediment S: scour depth Sc: scour depth induced by steady current alone Seq: equilibrium scour depth T: time T: time scale T*: non-dimensional time scale Tp: peak period Uc: current velocity D/2 above the seabed Ucw: velocity ratio (Uc/(Uc+Um)) Uf: friction velocity Ufw: friction velocity for waves only Ufc: friction velocity for current only Um: maximum wave velocity at seabed Uδ: current velocity outside the oscillating boundary

layer Vc: mean current velocity W: scour width Weq: equilibrium scour width y: distance from seabed δ: wave boundary layer thickness α: slope of scour hole

1 Copyright © 2007 by ASME

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φ: angle of internal friction for sediment ρ: density of water θ: Shields parameter τ0c: mean bed shear stress due to current

DESCRIPTION OF THE MODEL The WiTuS model integrates the processes leading to scour

and backfilling on an erodible, non-cohesive seabed dependent on the environmental conditions, the pile diameter and seabed parameters. The governing input parameters to the model are listed below:

Environmental conditions (time series): • Water level variation • Depth-averaged current speed • Current direction • Significant wave height • Peak period • Wave direction Pile and seabed parameters: • Pile diameter • Water depth • Mean grain size of sediment • Angle of internal friction for sediment • Relative density of sediment WiTuS is a time-domain model determining both the

scour depth as well as the horizontal extension of the scour hole.

The equilibrium scour depth is determined by use of different models for current, waves and combined waves and current. For pure current an equilibrium scour of 1.3 times the diameter of the pile is used, see Sumer and Fredsøe (2002). For pure waves and combined waves and current, the model proposed by Sumer and Fredsøe (2001a) is used. The time scale for combined waves and current is found using the sum of the time scales for pure waves and for current relative to actual Ucw. Where Ucw is defined as:

mc

ccw UU

UU+

=

Backfilling takes place if the equilibrium scour depth is shallower than the actual scour depth and the undisturbed Shields parameter is above 0.06 (live bed condition).

Cables from the turbine shall either be protected or buried. The required burial depth is directly governed by the horizontal extension of the scour hole. Therefore determination of the extent of the scour hole is important.

DESCRIPTION OF INDIVIDUAL PROCESSES The model is based on the assumption that the seabed near

the pile always develops towards an equilibrium scour depth determined by the actual environmental conditions.

It is generally accepted that a scour hole under constant environmental conditions, starting with a plane bed, develops as

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛−−=

TtStS eq exp1)( [1]

where T is the time scale for the erosion process. Using equation [1], a general equation including erosion

and backfilling can be developed:

( ) ( )( ) ⎟⎠⎞

⎜⎝⎛−−=+=

TtStSStS eqeq exp0 [2]

Using equation [2] the development in the time interval dt from a given time step n-1 to the next time step n, the scour can be found as:

( ) ⎟⎠⎞

⎜⎝⎛−−+= − T

dtSSSS neqnneqn exp,1, [3]

WiTuS is based on the assumption that both erosion and deposition can be described by the equation above. The time scale, T, depends, among other parameters, on the type of flow. Time scales for scour development by current and waves are suggested by e.g. Sumer and Fredsøe (2002), while, to the knowledge of the authors, no time scales for combined waves and current and for deposition processes are developed. It is assumed that the seabed will never raise above the original undisturbed seabed.

A number of equations for the equilibrium scour in waves, current and combined waves and current have been developed over the years. In the present study, the equations presented in Sumer and Fredsøe (2002) are used.

Together with the scour depth, the scour is characterized by the scour hole width. The equilibrium scour hole is characterized by four widths: Two widths in-line of the resulting flow (upstream and downstream of the pile) and two widths perpendicular to the resulting flow, as shown in Figure 1. The equilibrium scour width is assumed to vary smoothly between those four values.

Figure 1. Definition sketch of the width of the scour

hole.

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( ) ( ) ( ) ( )( ) ⎟⎠⎞

⎜⎝⎛−−+= − T

dtWWWW neqnneqn exp,1, αααα [4]

The widths of the scour hole are calculated by use of equation [4]. It is assumed that the inclination, α, of the side of the scour hole is constant for a specific position relative to the flow, but will change depending on the position relative to the flow direction. Figure 2 shows the development of the scour width over a time step dt.

Figure 2. Equilibrium and time variation of scour

width. The time scales and equilibrium properties are described

in the following.

Near Bed Conditions The near bed conditions are separated in two situations:

wave or current dominated.

Near Bed Conditions in combined Waves and Current dominated by Waves The depth-integrated current is input to the model, while

the maximum orbital velocity is found by linear transformation of the significant wave height using the peak period.

khTH

Up

sm sinh

1π= [5]

The mean velocity profile in the combined wave-current flow is determined using a method developed by Fredsøe (1981). This model assumes the waves and current to be co-directional. The model has the advantages compared with more advanced models that the equations are simple and easy to solve, and involves no solution of differential equation and turbulence modeling:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+=

wfcc k

yUyU ln5.26.8)( [6]

where kw is the “apparent wave-current roughness” determined by a method developed by Fredsøe (1981):

⎥⎥⎥⎥⎥

⎢⎢⎢⎢⎢

⎡ −=

5.2

6.8

exp 21

δ

δ

δ

UUfU

k

mw

w [7]

The wave boundary layer, δ, and the wave friction factor, fw, are taken according to Fredsøe and Deigaard (1992).

The wave boundary layer thickness is taken as: 82.0

09.0 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

ss ka

[8]

The wave friction factor is related to the amplitude of the orbital motion as:

Equilibrium

W(t)

W(t+dt) 50 04.041

>⎟⎟⎠

⎞⎜⎜⎝

⎛=

ssw k

akaf [9]

and

50 4.075.0

<⎟⎟⎠

⎞⎜⎜⎝

⎛=

ssw k

akaf [10]

Uδ is the current velocity at the top of the oscillating wave boundary layer.

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+=

wfc k

UU δδ ln5.26.8 [11]

The mean current velocity is:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+=

wfcc k

hUV ln5.22.6 [12]

According to Fredsøe (1981), Uδ is related to mean bed shear stress:

mmwfcc UUUUfU <<== δδρ

τ 2

120 [13]

kw, Uδ and Ufc, can now be found when combining equations [6], [7] and [12]. It shall be noted that the method presented above is a simplified approach. However, considering the general uncertainties in the predictions of the scour hole, the applied method is fully suitable for this engineering approach.

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Near Bed Conditions in Current-dominated Environment In the current-dominated situations (large values of Uδ/Um),

the friction velocity is related to the depth-integrated mean current as:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

=

s

cfc

kh

VUln5.22.6

[14]

Transition between Wave and Current-dominated Environment

The model by Fredsøe is not valid for pure current environment. In order to be able to model also a current-dominated environment, the transition between the wave and current-dominated environment is taken as the largest value of the friction velocity, see Figure 3. The input parameters for the figure are: h=11.0m, d50=0.5mm, Hs=3m, Tp=7.8s and Vc increases from 0.0m/s to 3.0m/s. The red circles represent the friction velocity determined by the expression for current alone. The black plusses represent the friction velocity determined by the Fredsøe-1981 theory. The blue line shows the actual shear stresses used in WiTuS.

0 0.5 1 1.5 2 2.5 30

0.02

0.04

0.06

0.08

0.1

0.12

Vc [m/s]

Uf [m

/s]

U

f,res

Ufc,c

Ufc,w

Figure 3. Current friction velocity as function of Vc.

Near Bed Conditions due to Waves The friction velocity due to the waves is found as:

mmwfw UUfU 212 = [15]

see e.g. Fredsøe (1981). fw and Um are found as in the previous section.

Total Shields Number The Shields parameter can now be found as:

( )( ) 50

2

1 gdsUU fwfc

+=θ [16]

Equilibrium Scour Depth for Uni-directional Current The scouring process in pure current is dominated by the

horseshoe vortex and contraction of streamlines at the sides of the pile. The effect of the horseshoe vortex in relation to scour around piles in steady current has been studied extensively, for references see e.g. Sumer and Fredsøe (2002).

The equilibrium scour depth for steady current is here set equal to 1.3 times the pile diameter, see Sumer and Fredsøe (2002):

3.1, =D

S eqc [17]

It should be noted that the equilibrium scour depth of 1.3 is an average value found from a set of data, and the standard deviation corresponding to this set of data is 0.7.

Time Scale for Erosion in Current The time scale of the scour is taken from Sumer and

Fredsøe (2002) as well. The non-dimensional time scale for the scour development is described by Sumer and Fredsøe (2001a):

2.2

20001* −= θ

DhT [18]

where the non-dimensional time scale is defined as:

( )( ) TD

dsgT 2

3 21

1* −= [19]

The onset of the scour process will take place when the undisturbed Shield’s number become larger than ¼ of the critical Shield’s number, Olsson (2000).

Equilibrium Scour Depth in Waves and Combined Waves and Current

In case of waves, the vortex shedding becomes important for the scour process. The applied model for the equilibrium scour depth in the case of waves and combined waves and current are developed by Sumer and Fredsøe (2001a). This model is applied when the following criteria are fulfilled:

• KC<100 • KC>6 • Ucw<0.7 For cases with KC>100 or Ucw>0.8, the model for steady

current is applied. A linear transition from the combined wave and current scour depth to the steady current scour is applied in the interval 90<KC<100 or 0.7<Ucw<0.8. The equilibrium scour depth for waves and combined waves and current is taken as:

( ){ }[ ] BKCBKCAD

SD

S eqceqwc ≥−−−= ;exp1,, [20]

where: 6.2

4303.0 cwUA +=

( )cwUB 7.4exp6 −= If B becomes larger than the KC-number, the value of B is

set equal to the value of the KC-number.

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For KC<6, the flow regime around the pile will be steady streaming or transition to steady streaming in the case of pure waves. The maximum scour caused by steady streaming is small (S/D<0.05), see Sumer and Fredsøe (2001b). Because of the small maximum scour the steady streaming is not included in the model.

Time Scale for Erosion in Waves The non-dimensional time scale for the scour development

in waves is taken from Sumer and Fredsøe (2002): 3

610* ⎟⎠⎞

⎜⎝⎛= −

θKCT [21]

where the non-dimensional time scale is defined as for a current, see equation [19].

The onset of the scour process will take place when the undisturbed Shield’s number becomes larger than ¼ of the critical Shield’s number as for the steady current.

Time Scale for Erosion in combined Waves and Current

As no expression for the time scale for the combined waves and current is known to the authors, a first approach using the weighted mean of the time scale for current and the time for waves is used:

( cwwavescwcurrentcombined UTUTT −+= 1*** ) [22]

Effects of Breaking Waves To the knowledge of the authors, only a few sets of

experiments with scouring around a pile exposed to breaking waves have been conducted. Bijker and de Bruyn (1988) reported heavy scour (S/D=1.46) around piles placed on a sediment slope exposed to breaking waves. On the other hand, Carreiras et al. (2000) reported that no or only a minor scour developed during a similar experiments.

The reason for the different results can be the difficulties to distinguish between the development in the bed shape in the undisturbed case and the scour caused by the pile. The authors of this paper find that the lowering of the seabed in the experiments of Bijker and de Bruyn may rather be caused by general changes in the seabed profile. In the WiTuS model, no special effects of breaking waves have been implemented as the effects seem to be small. However, the effects of breaking waves in relation to scour around piles should be subject to further investigation.

Effects of Shallow Water The scour depth decreases with the decreasing water depth.

The equilibrium scour depth for current in shallow water can be estimated using data from Sumer and Fredsøe (2002):

6.2)/(

)/(21

,

<=∞= D

hfor

Dh

DhSDhS

eqc

eq [23]

For the waves and combined waves and current, the horseshoe vortex (which governs the scouring process) mainly depends on the KC-number and will therefore for practical purposes be independent of the water depth, although the wave height is limited by the water depth.

Equilibrium Shape of Scour Hole To the knowledge of the authors, there is a lack of

information about the shape of the scour hole. In Roulund et al. (2005), it is stated that the upstream profile has an inclination equal to the angle of internal friction, with a variation of ±2°. The downstream side of the scour hole is less steep around half of the angle of internal friction or less. On the basis of this profile, the equilibrium shape of the scour hole is calculated in the model: The angle of the upstream slope is equal to the internal angle of friction for the sediment and the downstream angle is 2/3 of the internal angle of friction, while the sides are 5/6 of the internal angle of friction. These values are rough estimates and may differ from the actual situation, especially in the cases where waves are dominating.

Of special interest is the profile obtained after backfilling of larger scour holes. Will the seabed obtain the original level in most of the area or will a major area of the maximum scour hole continue to be significantly below the original seabed? This might have an effect on how deep the scour hole will be.

Backfilling Process in Waves and combined Waves and Current

Backfilling of a scour hole will take place if the actual scour depth is larger than the equilibrium scour depth and the undisturbed Shields number is larger than the critical Shields number (live bed). The backfilling process will take place in waves and combined waves and current. In all cases of current, the equilibrium scour depth is set to S/D=1.3, with possible reduction in accordance with equation [23], due to shallow water.

To the knowledge of the authors, only a few tests with backfilling have been conducted. Thomsen (2006) reported results obtained in backfilling experiments. All the tests were carried out using the same wave parameters and sediment; during some of the experiments a current was added. Due to the limited amount of data, it has not been possible to make a general time scale equation for backfilling. The authors believe that the backfilling process will be slower than the similar erosion process. Based on this speculation, ten times the time scale for waves/combined waves and current scouring is applied.

Transition between Scour Depth caused by Waves and combined Waves and Current

The equation used to find the equilibrium scour depth for the combined waves and current does in general not approach the value of the current-induced scour depth when Ucw is going to 1 or the KC-number becomes large. For this reason, two transition conditions are introduced: The interval Ucw=0.7 to Ucw=0.8 and the interval KC=90 to KC=100.

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The equilibrium scour depth in the transition intervals is found by linear interpolation. The equilibrium scour depth for the lower bound (Ucw=0.7) is calculated by use of equation [20], where the actual KC-number is applied. The equilibrium scour depth for the upper bound is set to the steady current scour depth. Figure 4 shows the effect of the transition interval. The fully-drawn lines are for the wave-dominated part and for current dominated part, while the dashed line is the transition. The following parameters were applied: D=4.0m, h=11m, d50=0.5mm, Hs=1.5m and Tp=5.5s. The current was linearly increasing from 0.0m/s to 2.0m/s.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

1.2

1.4

Ucw

S/D

Figure 4. Effect of the linear interpolation from steady current scour depth to combined waves and current

scour depth in the transition interval.

MODEL VALIDATION The model is set up for a number of cases: • Steady current • Backfilling by waves • Waves • Waves and current, dominated by waves (Ucw<0.7) • Waves and current, dominated by current (Ucw>0.8) • Low water level and current

Scour in steady Current and Backfilling by Waves The model is set up for steady current and backfilling by

waves with the following input parameters: D=0.5m, h=4.6m, d50=0.5mm, s=2.65 and Vc=2.0m/s. After 30 minutes, the following parameters were changed Vc=0m/s, Hs=2.0m and Tp=6.4s. The result of the test is shown on Figure 5. The scour is fully developed after approximately five minutes (the scour depth is more than 99 percent of the equilibrium scour), which is in good agreement with the results found in Sumer and Fredsøe (2002). When the current is changed to 0 and the waves are added after 30 minutes, the equilibrium scour depth reduces to S/D=0.19 and the developing time is approximately 27 minutes. These results agree with the equations reported in

Sumer and Fredsøe (2002). The Shields number is kept almost constant at approximately 0.7 in both situations. The KC-number for the waves is 11.2.

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

1.2

1.4

Time [hour]

S/D

Actual scourEquilibrium scour

Figure 5. Scour development in steady current

continued by pure waves.

Scour due to Waves The model is set up for waves only and the following input

parameters are used: D=0.5m, h=11m, d50=0.5mm, s=2.65, Hs=2.5m and Tp=7s. The near bed orbital velocity is determined to be 0.8m/s, the Shields parameter 0.3 and the KC–number is 8.1. The equilibrium scour is found to be S/D=0.081 and the scour development time to be approximately nine minutes, see Figure 6. These values are equal to those reported by Sumer and Fredsøe (2001a) and Sumer and Fredsøe (2002), respectively.

0 0.05 0.1 0.15 0.20

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Time [hour]

S/D

Actual scourEquilibrium scour

Figure 6. Scour development in pure waves, D=0.5m,

h=11m, d50=0.5mm, s=2.65, Hs=2.5m and Tp=7s.

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Scour due to combined Waves and Current The model is set up for a combined wave and current case

with the following input parameters: D=0.5m, h=11m, d50=0.5mm, s=2.65, Vc=0.1m/s, Hs=4m and Tp=9s. After half an hour when the equilibrium is reached, the wave and current parameters are changed to: Vc=1.2m/s, Hs=1m and Tp=4.5s. For the first half hour, the Shields parameter is found to be 1.1 and the KC-number is found to be 19.6. For the rest of the time, the Shields parameter is found to be 0.4 and the KC-number is found to be 1.0.

The first half hour is wave-dominated, Ucw=0.05, while the rest of the time is current-dominated, Ucw=0.86. The scour during the wave-dominated period is smaller than during the current-dominated period, even though the Shields number is smaller during the current-dominated period (0.91 and 0.23 for the wave-dominated period and the current-dominated period respectively).

The development time of the scour is shown in Figure 7 and found to be approximately four minutes for the wave-dominated part and 1.9 hours for the current-dominated part. This is in accordance with the values obtained from Sumer and Fredsøe (2002). The equilibrium scour depth for the wave-dominated part is S/D=0.47, see Figure 7. This is in agreement with Sumer and Fredsøe (2001a).

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

1.2

1.4

Time [hour]

S/D

Actual scourEquilibrium scour

Figure 7. Scour development in combined waves and current. First half hour is wave-dominated, while the

rest is current-dominated.

Scour due to combined shallow Water and Current The model is set up for a constant mean current and a

decreasing water level. The following input parameters are applied: D=2.0m, d50=0.5mm, s=2.65, Vc=1.2m/s and the water level is changing linearly from 5m to 3m. After 12 hours, the water level is kept constant at 3m.

The scour development is dominated by the current, so the equilibrium scour is 1.3D as long as the water level is higher than 2D. This is the case the first six hours here after the water level is less than 2D and the scour decreases. The new

equilibrium of S/D=0.98 corresponding to a water level of 1.5D is reached.

0 5 10 15 20 250

0.2

0.4

0.6

0.8

1

1.2

1.4

Time [hour]

S/D

Actual scourEquilibrium scour

Figure 8. Scour development in constant mean

current and decreasing water level.

HORNS REV 1 WIND FARM Horns Rev 1 Wind Farm is located in the harsh

environment of the west coast of Denmark. The wind farm consists of 80 turbines. All the turbines are founded on monopiles and the seabed around the piles are protected by a conventional scour protection. The turbines are located in between 6.5m and 13m water depth; for the actual case 11m water depth is chosen. The monopiles have a diameter of approximately 4m. The seabed consists of sand d50=0.1 to 1mm. here 0.5mm is used for d50. The relative density is taken to be s=2.65.

The model has been set up to determine the scour depth if the scour protection was omitted.

This case is based on wave and current data generated for the Horns Rev Offshore Wind Farm. The numerical data are calibrated with data measured at Horns Rev Offshore Wind Farm. Wave, current as well as tide and surge data are plotted in Figure 9. It is seen that the first approximately 2000 hours and the last 2500 hours are dominated by waves, while the current become more dominating during the rest of the time.

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0 1000 2000 3000 4000 5000 6000 7000 8000 90000

5

Hs

[s]

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

10

20

Tp

[s]

0 1000 2000 3000 4000 5000 6000 7000 8000 9000−1

012

Tid

e [m

]

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

1

2

m/s

Time [hour]

VcUw

Figure 9. Typical environmental conditions for Horns

Rev Offshore Wind Farm. Figure 10 shows the relative scour depth together with the

KC-number and the Shields parameter. It can be seen from the figure that the environmental conditions are dominated by waves in the beginning and end of the time series, resulting in small scour depths, S/D<0.1, with exceptions around 1700, 7000 and 7400 hours where a scour depth of approximately 0.25 is reached. In the middle of the time series, the contribution from the current is higher resulting in larger scour depths almost up to S/D=1.3, even though the Shields parameter in general is smaller during this part of the time series. From Figure 10, it can also be seen that there is a good correlation between the wave activity and the scour depth. During periods with high wave activity, the scour depth is in general small and the maximum relative scour depth in the wave-dominated periods is below 0.25.

The most important load on a wind turbine foundation is the wind load transferred from the turbine. This load will of course increase with increasing wind speed. The wind will on the other hand also generate larger waves, which will reduce the scour. This means that during periods with high loads on the foundation the scour will be small. In Figure 10, it is seen that for the time series the scour depth is most of the time smaller than S/D=1.3 that is often used as industry standard. By comparing Figure 9 and Figure 10, it is clear that the backfilling occur so fast in wave-dominated conditions that the pile will not be exposed to large waves (and consequently heavy wind) with a scour depth larger than S/D=0.3. It should be noted that the reduction of the relative scour depth, among other parameters depends on the diameter of the pile.

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

0.5

1

1.5

S/D

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

1

2

3

Kc

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

1

2

Shi

elds

Par

amet

er

Time [hour]

Figure 10. Relative scour depth if the scour protection is omitted. KC number and Shields

parameter are plotted below. The width of the scour hole in four directions is shown in

Figure 11 for the period between 4500 hours and 6100 hours. The shape of the hole is based on data from experiments with pure current. This may give some uncertainty for the wave-dominated periods. However, these periods are characterized by the very limited scour. The extent of the scour is of course determined by the scour depth, but the maximum width will change direction with respect to the current and wave direction. It should be noted that the equation for the scour width is based on a relatively small amount of data.

4600 4800 5000 5200 5400 5600 5800 6000 62000

0.5

1

1.5

2

2.5

3

3.5

Time [hour]

W/D

090180270

Figure 11. Width of the scour around the pile in four

different directions if the scour protection is omitted.

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EFFECT OF TIDAL CURRENT For waves alone, the width of the scour hole is rather

constant around the pile. In case of pure current, the downstream width is typically much wider than the upstream width. In tidal current areas, there are some indications that the scour hole may become deeper than for uni-directional current, see Noormets et al. (2006). It should be noted that the case study by Noormets et al. (2006) also includes variations caused by moving bedforms.

Almost all physical scour model tests in current have been carried out in uni-directional flow, and most of them in relatively narrow flumes. For a pile exposed to current, not only the seabed in near downstream area (2-5 diameters) is affected, but also further downstream (10-30 diameter) maybe effected by the presence of a pile lower the seabed level. If one considers a situation where the current changes direction with 180 degrees, the upstream water depth is now increased and the seabed level decreased seen from a current point of view. In this case, the scour develops towards an equilibrium hole corresponding to a lower seabed level, resulting in a deeper scour hole than for uni-directional current, as illustrated in Figure 12.

Figure 12. Three states of the assumed scour process

in an idealized tidal current. The development of the tidal-induced scour is as follow: In

State 1, the scour has reached the equilibrium scour. In State 2, the current has just reversed. The dashed line shows the starting scour profile (equal to the equilibrium scour profile in State 1). The full-drawn scour profile is the equilibrium scour profile corresponding to the actual depth in the distance R0 upstream of

the pile. However, the scour further upstream than Wupstream of the pile will subsequently be backfilled. This will give the equilibrium scour in State 3, which is the same as in State 1, but mirrored.

In the program Pipesin, see Hansen et al. (1995), for predictions of scour and self-lowering of pipelines, the scour development is also described by equilibrium properties and time scales. However, in that program the scour width was divided into a local width and a leeside width. By using two widths, the model was both able to describe uni-directional flow where the scour depth typically becomes one pipe diameter deep and tidal flow where the scour hole can be more than two diameters deep.

Based on the best available data, the present version of WiTuS includes the effect of wide scour as follows: The reference level for the calculation of the scour is taken equal to the actual seabed level in the distance, R0=3D, upstream of the pile (relative to the pile centre). The authors have not been able to find any experimental or full scale measurements for piles placed in areas with elliptical tidal variations, whether this de or increases the possible impact on the scour is therefore unknown. It is recommended to address this subject before turbines are installed in such areas.

This means that the scour will be deeper than 1.3D if R0 is smaller than the actual width of the scour hole upstream of the pile.

A test set-up was applied to the model with the effect of a tidal current included. The set-up was: D=1.0m, h=11m, d50=0.5mm, s=2.65, Vc=1.8m/s. After six hours, the current was turned 180°. When the current turns, the scour increases up to approximately 1.75D or approximately 35 percent more than without the effect of tidal current, see Figure 13.

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time [hour]

S/D

Actual scourEquilibrium scour

Current Current

Figure 13. Effect of a tidal current for the actual

set-up.

9 Copyright © 2007 by ASME

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ACKNOWLEDGEMENTS Effect of Tidal Current in Horns Rev Case The financial support (1.3 mio. DKK) of the Danish Public

Service Program (PSO) under Contract PSO: 6508 (FU5102) is gratefully acknowledged.

The authors of this paper are aware that the stated effect of a tidal current is mainly based on an assumption, as no experimental data have been available. However, it might be interesting to study the effect of the tidal current in the Horns Rev case, although it might differ significantly from the actual scour situation for a unprotected pile at Horns Rev. The scour depth shown in Figure 14 is calculated using the same input data as used in the section “Horns Rev 1 Wind Farm”, but by assigning the effect of a varying water depth as shown in Figure 12. In Figure 14, it is seen that the effect of the tidal current in the Horns Rev case is relatively small. During the critical wave-dominated periods, there are no significant changes. The scour depth during the current-dominated periods has increased slightly most significantly for the peak around 5400 hours, which now exceeds 1.3D.

REFERENCES Bijker, E.W. and de Bruyn, C.A. (1988). Erosion around a

pile due to current and breaking waves. Proc. 21st Coastal Engineering Conference, Costa del Sol, Malaga, Spain, Vol. 2, 1368-1381.

Carreiras, J., Larroudé, Ph., Santos, F.J. Seabra and Mory, M. (2000). Wave scour around piles. Proc. 27th Coastal Engineering Conference, Sydney, Australia, Vol. 2, 1860-1870.

Fredsøe, J (1981). Mean current velocity distribution in combined waves and current. Progress Report 53, pp 21-26, April 1981, Institute for Hydrodynamics and Hydraulic Engineering, Technical University of Denmark.

4800 5000 5200 5400 5600 58000

0.5

1

1.5

Time [hour]

S/D

Including effect of tidal currentExcluding effect of tidal current

Fredsøe, J and Deigaard, R (1992). Mechanics of coastal sediment transport. World Scientific.

Hansen, E A, Smed, P F, Bryndum, M B, Klomp, W H G, Chen, Z and Bijker, R (1995). Free span development and self-lowering of pipelines. OMAE 1995 – Vol. V, Pipeline Technology, ASME 1995.

Noormets, R, Ernstsen, V B, Bartholomä, A, Flemming, B W and Hebbeln, D (2006). Implications of bedform dimensions for the prediction of local scour in tidal inlets: a case study from the southern North Sea. Geo-Marine Letters, Springer Verlag, Vol. 26, No 3, 165-176.

Olsson, P. (2000). Influence of ice-cover on local scour at circular bridge piers. Licentiate Thesis. Luleå University of Technology.

Roulund, A. Sumer, B.M. Fredsøe, J. and Michelsen, J. (2005). Numerical and experimental investigation of flow and scour around a circular pile. Journal of Fluid Mechanics, Vol. 534, 351-401.

Figure 14. Relative scour depth for the Horns Rev case when the effect of tidal current is included (full

blue line) and when the effect of tidal current is excluded (dashed red line). The effect is small and

only present from around 5300 to 5600 hours. Sumer, B.M. and Fredsøe, J. (2001a). Scour around a pile

in combined waves and current. J. Hydraulic Engineering, ASCE, Vol. 127, No. 5, 403-411.

Sumer, B.M. and Fredsøe, J. (2001b). Wave scour around a large vertical circular cylinder. J. Waterway, Port, Coastal, and Ocean Engineering, ASCE, Vol. 127, No. 3, 125-134.

CONCLUSION It is shown that the actual scour depth around a pile can

vary significantly depending on the actual environmental conditions and that the scour depth is often much smaller than the often used industry standard of S/D=1.3. This is especially the case when waves are dominating the near bed conditions. A study of the effects of a tidal current showed that the scour might be larger in the case of a tidal current-dominated environment, but this is mainly based on assumptions.

Sumer, B.M. and Fredsøe, J. (2002). The mechanics of scour in the marine environment. World Scientific.

Thomsen, J.M. (2006). Scour in a marine environment characterized by current and waves. MSc thesis, Aalborg University.

A number of important topics are still in need of further investigations, in particular in relation to data:

• Time scale for erosion in combined waves and current • Time scale for backfilling • Equilibrium for backfilling • Shape of scour hole/effect of tidal current

10 Copyright © 2007 by ASME

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B I L A G

19.12.07/AWN/hec/pot/rapport/80117-afsluttende rapport DHI

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B I L A G A

Technical Note: “Damage on the Scour Protection near the Turbine Foundations in Horns Rev Wind Farm”

19.12.07/AWN/hec/pot/rapport/80117-afsluttende rapport DHI

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PSO-FU5102 Technical Note

November 2006

Damage on the Scour Protection near the Turbine Foundations in Horns Rev Wind Farm

Page 39: Afsluttende rapport for Erosionsbeskyttelse omkring ...€¦ · Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller, FU5102 . Projekt nr. 06-80117 . Dato . 19. december

Damage on the Scour Protection near the Turbine Foundations in Horns Rev Wind Farm November 2006

Agern Allé 5 DK-2970 Hørsholm, Denmark Tel: +45 4516 9200 Fax: +45 4516 9292 e-mail: [email protected] Web: www.dhigroup.com

Client

PSO-FU5102

Client’s representative

Project

Damage on the Scour Protection near the Turbine Foundations in Horns Rev Wind Farm

Project No

06-80117

Date 27 November 2006

Authors

Jacob Simonsen

Approved by

Erik Asp Hansen

0 Technical Note HJS EAH EAH 27.11.06

Revision Description By Checked Approved Date

Key words

Horns Rev Scour Protection Full-scale Measurements Filter Layer

Classification

Open

Internal

Proprietary

Distribution No of copies

Elsam/DONG: Vestas: DHI:

EAH, JAO

PDF PDF

2

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CONTENTS

1 INTRODUCTION ............................................................................................................ 1

2 RESULTS FOR THE ENTIRE WIND FARM................................................................... 2

3 SELECTED RESULTS FROM SINGLE TURBINE......................................................... 5 3.1 Turbine 58....................................................................................................................... 5 3.2 Turbine 12....................................................................................................................... 8

4 DISCUSSION ............................................................................................................... 12

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1 INTRODUCTION

Severe damage on the scour protection is observed near a large part of the turbine foundations in Horns Rev. The observations are based on the survey conducted by Fugro Denmark, spring 2005, and the converted pdf files, based on surveys by Jan De Nul, fall 2002. The present note describes the investigation of the damage on the scour protection near the turbine foundations.

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2 RESULTS FOR THE ENTIRE WIND FARM

The tendencies of the 2005 survey are evident and very consistent. The wind farm is subjected to severe damage on the scour protection. The minimum thickness of the stone protection in a radius of 10 m from the centre of the turbine foundation is calculated for the entire wind farm. The level is calculated as the minimum level above the surrounding seabed within the 10 m radius of the turbine foundation (mean seabed being the mean of three of four corners, see note “analysis of surveys”). The results of the calculations are shown in Fig 2.1 and Table 2.1. In the near area of 50% of the turbine foundations (50% fractal), the scour protection has a minimum thickness of 0.16 m. Near 80% of the turbine foundations (80% fractal), the scour protection has a minimum thickness of 0.27 m. (According to the 2005 survey).

Fig 2.1 Maximum thickness of scour protection layer. Negative (red colour) values indicate possible

penetration of the filter layer. The values are calculated based on the 2005 survey results in an area within 10 m from the centre of the foundation.

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Table 2.1 Minimum thickness of scour protection layer. Negative values indicate possible penetration of the filter layer. The values are calculated based on the 2005 survey results up to 10 m from the centre of the foundation

Column number (from west to east) 0x 1x 2x 3x 4x 5x 6x 7x 8x 9x

x1 - -0.18 0.05 0.27 0.43 0.48 0.08 0.38 0.17 -0.1 x2 0.19 0.13 0.14 0.06 0.29 0.21 0.21 0.07 0.09 0.12 x3 0.54 0.04 0.32 0.05 0.33 0.17 - 0.11 -0.19 0.12 x4 0.22 0.15 0.48 - 0.6 0.27 0.12 -0.16 0.32 -0.27 x5 0.1 -0.07 0.05 0.24 0.33 0.2 0.22 0.45 0.2 0.24 x6 0.21 0.1 -0.32 0.17 0.27 0.16 0 -0.01 0.41 0.21 x7 -0.46 0.14 0.16 0.04 0.2 0.18 0.21 0.15 0.19 0.19

Row

num

ber

(from

nor

th to

sou

th)

x8 -0.1 -0.1 -0.19 0.05 -0.01 -0.43 0.3 0.04 0.18 0.19 The maximum change between the level of the scour protection in 2002 and 2005 is shown for the entire wind farm in Fig 2.2. The 20% fractal for the maximum scouring is -2.0 m, 50% fractal is -2.2, and 80% fractal is -2.5. These results may be distorted by erroneous 2002 measurements or conversion.

Fig 2.2 Maximum scouring. The values are calculated as the maximum change in scour protection

level from 2002 to 2005 up to 10 m from the centre of the foundation. Negative (red colour) is erosion.

The net transport for each wind turbine foundation is calculated as the sum of the changes in level in calculation square multiplied by the area of the calculation square. The results are calculated back-wise: • 2.5-3.5 m, which is close to the turbine foundation and therefore the results may be

affected by inaccuracies in the measurements • 3.5-6.0 m • 6.0-10 m which is relatively close to the outer perimeter of the scour protection

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Figure 2.3 shows the results of the net transport analysis. As seen, the majority of the scour protections are subjected to a large negative net transport, ie material is transported away from the area

Fig 2.3 Overview over the net transport from the entire wind farm. Most of the turbine scour

protections are subjected to erosion.

Table 2.2 Net transport for the entire wind farm. The net transport is calculated in a circular band going from r = 2.1 m to 2 = 10 m measured from the centre of the wind turbine. All values are in m3.

Column number (from west to east) 0x 1x 2x 3x 4x 5x 6x 7x 8x 9x

x1 -51.88 -128.38 -41.68 -6.43 6.39 -33.33 -80.34 -86.73 -86.7 -154.57 x2 -25.03 -125.63 39.7 -92.4 -41.92 -63.49 -117.6 -68.84 -34.91 -119.37 x3 -36.25 -88.54 -71.36 -136.76 17.12 -26.68 - -91.9 -54.07 -87.62 x4 -72.35 -134.02 -17.66 - -62.65 -90.16 -47.13 -42.27 -151.96 -78.3 x5 -49.64 -77.36 -98.55 -269.51 -72.32 -67.45 -130.78 -80.82 -117.1 -86.21 x6 -142.42 -274.66 -57.55 -147.85 -65.92 -151.96 -56.34 -96.85 -37.62 -135.04 x7 -83.49 -117.83 -146.94 -55.79 -127.68 -67.96 -40.04 -125.26 -146.49 -75.69

Row

num

ber

(fh

h)

x8 -100 -54.31 -81.28 -113.41 -114.46 -1.06 -96.01 -58.58 -144.47 -65.87

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3 SELECTED RESULTS FROM SINGLE TURBINE

3.1 Turbine 58

According to Table 2.2 the erosion has penetrated the sand bed by 0.43 m. In Fig 3.1 the result of the survey of 2005 is shown. The figure shows that the scour protection layer has a reduced thickness in some areas near the foundation. Figure 3.2 shows cross-sections of turbine number 58, survey 2005 and 2002.

Fig 3.1 Results of survey 2005 near turbine number 58. The contour colours are given in m above

the surrounding seabed.

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Fig 3.2 Cross-section in E-W direction (top) and N-S direction (bottom) through the turbine

foundation number 58. The locations of the cross-sections are shown as dashed lines in Fig 3.1. Red line is the 2002 survey and blue line is the 2005 survey.

Figure 3.3 shows the change in level above mean seabed for the 2002 survey compared to the 2005 survey. Positive values indicate a deposition of material, while negative values indicate erosion. The most severe erosion is in the order of 1 m. As the thickness of the scour protection layer was small according to the 2002 surveys, the erosion apparently has penetrated into the filter layer.

Sec

Sec

Fig 3.3 Changes in levels above seabed for turbine number 58 for the 2002 survey compared to the 2005 survey. Positive values (red colour) indicate deposition and negative values (blue colour) indicate erosion.

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Turb

ine

foun

datio

n

Fig 3.4 Levels above the surrounding seabed in section 1, turbine number 58. Blue line is the 2005 survey and red line is the 2002 survey. The location of section 1 is shown in Fig 3.3.

Turb

ine

foun

datio

n

Fig 3.5 Levels above the surrounding seabed in section 2, turbine number 58. Blue line is the 2005 survey and red line is the 2002 survey. The location of section 2 is shown in Fig 3.3.

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3.2 Turbine 12

Figure 3.6 shows the result of the 2005 survey given in level above the surrounding seabed. Figure 3.7 shows the EW and NS cross-section, indicated by dashed lines in Fig 3.6. The thickness of the scour protection layer, just south of the foundation is reduced to approximately 0.5 m-1 m.

Fig 3.6 Result of survey 2005 near turbine number 12. The contour colors are given in m above the

surrounding seabed.

Fig 3.7 EW cross-section (top) and NS cross-section through turbine foundation number 12. Red

line is the 2002 survey and blue line is the 2005 survey.

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Figure 3.8 shows the changes in the seabed level between the 2002 survey and the 2005 survey. Positive values (red colour) indicate deposition and blue line erosion. It is noted that erosion occurs in a rather large area some 15 m SSE of the foundation. 3 cross-sectional variations have been calculated. These are shown in Figs 3.9-3.11. The figures show erosion close to the foundation. However, the figures also show very large levels in the 2002 survey close to the foundation. These large levels might be caused by erroneous measurements in the 2002 survey or in the conversion of the 2002 surveys. Therefore, the maximum values of the changes in level cannot always be trusted in the region close to the mill.

Sec

Sec

Sec

Fig 3.8 Changes in levels above seabed for turbine number 12 for the 2002 survey compared to the 2005 survey. Positive values (red colour) indicate deposition and negative values (blue colour) indicate erosion.

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Turb

ine

foun

datio

n

Fig 3.9 Levels above the surrounding seabed in section 1, turbine number 12. Blue line is the 2005 survey and red line is the 2002 survey. The location of section 1 is shown in Fig 3.8.

Turb

ine

foun

datio

n

Fig 3.10 Levels above the surrounding seabed in section 2, turbine number 12. Blue line is the 2005 survey and red line is the 2002 survey. The location of section 2 is shown in Fig 3.8.

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Turb

ine

foun

datio

n

Fig 3.11 Levels above the surrounding seabed in section 3, turbine number 12. Blue line is the 2005 survey and red line is the 2002 survey. The location of section 3 is shown in Fig 3.8.

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4 DISCUSSION

In the survey of 2002 there may be noise near the foundation, which results in larger thickness of the scour protection layer, than what is really the case. In Fig 4.1, a close-up of the survey 2002 result (pdf format) turbine number 64 is shown. The blue area near the foundation indicates according to the colour legend a level of -8.5 below z-reference (unknown). Just next to the blue area are areas with a lighter colour, indicating levels of approximately -10.25 m below z-reference. Thus, according to the surveys provided by Jan De Nul fall 2002, very large gradients in the scour protection exist near the foundation, which is rather questionable.

Fig 4.1 Example of the raw image file from turbine 64. The image above is converted into xyz-

values based on the colour. Blue colour indicates that there may be some uncertainties in the bed profiling.

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B I L A G B

Technical Note: “Analysis of Scour Protection of Offshore Wind Turbine, Horns Rev, Denmark”

19.12.07/AWN/hec/pot/rapport/80117-afsluttende rapport DHI

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PSO-FU5102 Technical Note

November 2006

Analysis of Scour Protection of Offshore Wind Turbine, Horns Rev, Denmark

Page 55: Afsluttende rapport for Erosionsbeskyttelse omkring ...€¦ · Afsluttende rapport for Erosionsbeskyttelse omkring havvindmøller, FU5102 . Projekt nr. 06-80117 . Dato . 19. december

Analysis of Scour Protection of Offshore Wind Turbine, Horns Rev, Denmark November 2006

Agern Allé 5 DK-2970 Hørsholm, Denmark Tel: +45 4516 9200 Fax: +45 4516 9292 e-mail: [email protected] Web: www.dhigroup.com

Client

PSO-FU5102

Client’s representative

Project

Analysis of Scour Protection of Offshore Wind Turbine, Horns Rev, Denmark

Project No

06-80117

Date 27 November 2006

Authors

Jacob Simonsen

Approved by

Erik Asp Hansen

0 Technical Note HJS EAH EAH 27.11.06

Revision Description By Checked Approved Date

Key words

Horns Rev Scour Protection Full-Scale Measurements Filter Layer

Classification

Open

Internal

Proprietary

Distribution No of copies

Elsam/DONG: Vestas: DHI:

EAH, JAO

PDF PDF

2

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CONTENTS

1 INTRODUCTION ............................................................................................................ 1

2 SURVEYS....................................................................................................................... 2

3 METHODS...................................................................................................................... 3 3.1 Survey 2002: PDF2XYZ Analysis ................................................................................... 3 3.2 Survey 2005: Import of XYZ Data................................................................................... 5 3.3 Setting the Vertical Reference ........................................................................................ 9

4 RESULTS ..................................................................................................................... 10

5 REQUESTS IN FUTURE SURVEYS............................................................................ 15

6 CONCLUSION.............................................................................................................. 16

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1 INTRODUCTION

The present note presents an example of how the measurements of the seabed and scour protection around a single wind turbine foundation at the Horns Rev Offshore Wind Farm are analysed.

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2 SURVEYS

Three groups of surveys were performed on Horns Rev: • Pre-installed survey. Before the foundation was installed. This group consists of

four independent surveys, which should assess the global variation of the seabed in the area. These were carried out in: o 1998 (20th - 30th August 1998, ASCII/xyz format) o 1999 (25th - 26th June 1998, ASCII/xyz format) o 2000 (28th April 2000, ASCII/xyz format) o 2001 (?, ASCII/xyz format).

• Post-installed survey. After the foundation was installed. This group consists of five surveys carried out in 2002: o Before the filter layer was installed (12th March 2002 - 18th May 2002, printouts

of Matlab plots) o After the filter layer was installed (17th March 2002 - 15th June, 2002 printouts

of Matlab plots) o After the cover layer was installed (2nd - 15th October 2002, pdf format) o After extra filter layer was placed (15th October - 20th November 2002, pdf

format) o After extra cover was placed and reparations were carried out (30th October -

30th November 2002, pdf format) (hereafter named cover 2002 survey) • Time evolution monitoring survey.

o Spring 2005 (20th-– 28th April, ASCII/xyz format) (hereafter named 2005 survey)

o Fall 2006

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3 METHODS

3.1 Survey 2002: PDF2XYZ Analysis

The 2002 surveys were given as vector pdf files, where the depth contours were embedded jpeg object. An example of a pdf file is seen in Fig 3.1.

Fig 3.1 Example of pdf file for the 2002 survey, turbine 07. Step 1: Objects, such as position of turbine foundation and scour protection, summary of survey results (depth etc) were removed from the pdf file using the commercial program Corel Draw. Step 2: The files, including contour map, legend and UTM grid overlay, were saved as bitmap and imported into Matlab. A Matlab routine was constructed, which read the RGB colour value in each pixel and compared the value of each pixel to the colour values of the legend. The variations of the colour components in the legend were not smooth, see Figs 3.2 and 3.3.

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Fig 3.2 Colour legend for the 07 turbine, close-up from Fig 3.1.

Fig 3.3 Colour variation of the legend (from Fig 3.2). The y-axis holds the colour value of each

colour component (0-255) and the x-axis holds the corresponding depth. A band interpolation algorithm was deployed (see Fig 3.3): For each of the colour components red (R), green (G), and blue (B) a band was found, where the colour component matched the one of the legend. The intersection of the sets is found and the depth for that particular colour combination is found as the average of the endpoints of the intersection of the sets. A tolerance of 10 colour points was given, corresponding to 0.1 m. However, the tolerance in the colour values is not directly comparable to the accuracy of the z-coordinate, because it enters the calculation along with two other colour components. The result of the band interpolation is a structured grid, where part of the data is missing, due to mismatch with the colour legend. Figure 3.4 shows the positions of the apparent measurements, according to the pdf file and the positions of the apparent measurements, which matched the colour legend within the given tolerance.

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Fig 3.4 Position of the valid and non-valid data points near turbine 07, cover survey. The red points

are the positions of the pixels in the bmp version of the pdf file. They are in a quadratic grid, with a grid spacing of 0.012 m. The blue points mark the position of the data points, in which the colour value matched the colours of the colour legend within the given tolerance.

The data were smooth/interpolated (gridded) into a quadratic structured grid with grid spacing in both dimensions of 0.12 m. The interpolations/gridding were done by a linear interpolation, based on a Delaunay triangulation of the valid bmp pixel values. There are a number of uncertainties with the conversion from pdf: • The jpeg compression distorted the colour values in the picture. Using the band

interpolation, many of the distorted pixels were removed • For depths above a certain maximum level a constant depth is used (see eg Fig 3.1) • For depths below a certain minimum level a constant depth is used

3.2 Survey 2005: Import of XYZ Data

The XYZ data from the 2005 survey have been stored in three different files: (1) Raw measured xyz data including all errors (2) Edited xyz file including small errors. The data are unstructured and the density of

the data is approximately 55 points per square metre, ie the average distance between the point is 0.13 m

(3) Gridded data. Small errors in the above file (2) were corrected and the data were smoothed and interpolated into a quadratic grid with grid spacing 0.3 m

The gridded data were found to be smoothened too much for the present purpose. This is illustrated by comparing Figs 3.5 and 3.6, where it is seen how the small area of only 2 by 2 m2 is very flat for the gridded data (3), whereas patterns of stones can be observed in the edited data (2). For the purpose of the present PSO project, it is important that the stone structures are noticeable.

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Fig 3.5 Gridded data in a 2 by 2 m2 section some 8 m SSE of turbine number 07, 2005 survey. The

red circles mark the locations of the gridded data (3).

Fig 3.6 Raw edited data in a 2 by 2 m2 section some 8 m SSE of turbine number 07, 2005 survey.

The red circles mark the locations of the unstructured, edited data (2). Figure 3.7 shows the difference between Fig 3.6 and Fig 3.5 and illustrates the effect of the smoothing. It can be seen that the smoothing has changed the level with more than ±0.3 m.

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Fig 3.7 Difference between gridded and smoothened data (3), Fig 3.5 and raw, edited data (2), Fig

3.6. The edited data (2) contained some obvious errors, as illustrated in Fig 3.8. The points were therefore sorted out by calculating a temporary gridding of the edited data (2). From the temporary gridded data an acceptance band of ±0.55 m was constructed, ie points from the unstructured data (2) which differed more than 0.5 m from the temporary gridded data were sorted out. The principle is explained in Fig 3.9. The sorted data were interpolated onto the same grid as the survey 2002, namely quadratic 0.12 m grid. The described method of correcting errors (referred to as PSO error correction) maintains the stone structures, which were observed in the raw ungridded data (2), while still correcting obvious errors.

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Fig 3.8 Seabed variation near foundation of turbine number 07 seen at an angle of 90 degree to the

mill. The blue points mark the measurement points, while the red marks the erroneous points.

Fig 3.9 Seabed variation near foundation of turbine number 07 seen at an angle of 90 degree to the

mill. The blue points mark the measurement points, while the red marks the erroneous points. Points above the top surface (red and cyan) and below the lowest surface (blue) were categorised as erroneous and thus sorted out.

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3.3 Setting the Vertical Reference

To unify the plots, the vertical coordinate - the z-coordinate - was reset to be level above mean seabed level of the surrounding seabed. The reference level, mean seabed level, was calculated as being the mean of three of the four corners of the 50 by 50 m2 seabed surrounding the turbine. The level in each corner was calculated as the mean depth inside an area of 5 by 5 m2. Of the four calculated mean corner levels, the corner, which differs the most from the mean of all four corners, was sorted out.

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4 RESULTS

Turbine number 07 was selected and the results are presented and interpreted in the following. The last of the post-installed surveys of 2002 (cover 2002 survey) and the first time evolution surveys of 2005 (2005 survey) are compared. The location of turbine number 07 is shown in Fig 4.1.

Fig 4.1 Overview of Horns Rev Wind Farm. Turbine number 07 is marked with distinct colours. The

contours are based on the survey carried out in 2001. Figure 4.2 shows the difference between cover 2002 and 2005 survey. The blue colour indicates an increase in depth, eg a scour hole, whereas the red colours indicate a decrease in depth, eg deposition of sand. As seen on the figure there are numerous red and blue spots with an approximate diameter in the order of 0.2 m. This indicates a relocation of stones. Close to the mill on the south western side, a scour hole has apparently been developed. However, this scour hole might be caused by uncertainties in the measurements and/or in the conversion of the survey of 2002.

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Fig 4.2 Difference between 2005 survey and cover 2002 survey. (’05 minus ’02). The blue colour

indicates erosion, whereas the red colour indicates deposition. Cross-section lines are indicated with black dashed lines.

On Figs 4.3 and 4.4 it is seen how the sand bed east of turbine number 07 is approximately 0.3 m eroded, whereas in the order of 0.2 m backfilling has occurred on the west side.

Wes

t cro

ss

ti

East

cro

ss

ti

E-W cross ti

N-S cross ti

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Fig 4.3 Cross-section going N-S approximately 25 m east of turbine 07, see Fig 4.2. Black is the

cover 2002 survey and cyan is the 2005 survey.

Fig 4.4 Cross-section going N-S approximately 25 m west of turbine 07, see Fig 4.2. Black is the

cover 2002 survey and cyan is the 2005 survey.

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Cross-sections going through the mill in E-W and N-S directions are shown in Fig 4.2. A lowering of the scour protection layer of up to 0.5 m north of the mill is observed. Just west of the mill erosion is indicated.

Fig 4.5 Cross-section through the foundation pipe of turbine 07, see Fig 4.2. Top: going E-W,

bottom: going N-S. Black is the cover 2002 survey and cyan is the 2005 survey.

Fig 4.6 Cross-section through the foundation pipe of turbine 07, see Fig 4.2. Top: going E-W,

bottom: going N-S. Black is the cover 2002 survey and cyan is the 2005 survey. Same as Fig 4.2, only the spacing on the axis is equal.

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In the entire area, 66 m3 was deposed and 520 m3 eroded, which net gives an erosion of 454 m3, corresponding to a 0.20 m average lowering of the seabed.

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5 REQUESTS IN FUTURE SURVEYS

It would be very interesting and helpful to have measurements of the sand bed elevation in between the turbine foundations. The measurements would assist in determining where the sediment settles and the assessment of the effect of the wind farm on the global erosion or deposition.

Fig 5.1 Sketch of approximate position of measurement points, which would come in hand in the

further analysis of the wind farm.

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6 CONCLUSION

The development of the sand bed and scour protection around turbine number 07 has been analysed. Based on the data near turbine number 07, the data seem to be of extraordinary good quality.

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B I L A G C

Survey and Hydrographic Data (DVD)

19.12.07/AWN/hec/pot/rapport/80117-afsluttende rapport DHI