Seabed and wind farm interaction. A Research Program. B. Mutlu Sumer DTU Mekanik

Transcription

1 Seabed and wind farm interaction. A Research Program B. Mutlu Sumer DTU Mekanik

2 Will tell you about a current research program on Interaction between seabed and offshore wind farms

3 Start off with A small terminology check: Sea Seabed Foundation: Part of pile in seabed A wind turbine is supported with a tower structure, e.g., a simple monopile Foundation is the part of monopile in seabed, 20m, 30m,.. deep, that underpins tower structure

4 What happens to seabed around foundation? When flowing under tides, or under storm waves, Water will move seabed sediment around foundation End result: Scour (= Erosion of sediment around foundation)

5 What happens to seabed around foundation? A conceptual picture When flowing under tides, or under storm waves, Water will move seabed sediment around foundation End result: Scour (= Erosion of sediment around foundation)

6 Why is scour important? Scour is a threat to stability of structure DNV Rules (2004, 2nd edition 2007) requires: risk for scour around foundation of OWT should be taken into account

7 Why is scour important? Scour is a threat to stability of structure DNV Rules (2004, 2nd edition 2007) requires: Risk for scour around foundation of OWT should be taken into account

8 Other processes involved in seabedwindxfarm interaction?

9 Other processes Processes related to mitigation measures, e.g., scour protection (rock protection) is another issue

10 An improperly designed protection Rock protection Image from a survey at Scroby Sands, UK Red thick line: 4.2 m diameter monopile Rock protection Image illustrates scour around rock protection. Failing protection! Scroby Sands Offshore Wind Farm Coastal Processes Monitoring. Cefas, UK,

11 Other processes Sand waves and wind farm interaction is yet another process

12 Sand waves from a survey Sand Waves, O(20 m) wave length Scroby Sands (UK) wind farm survey: Each vertical line represents monopile with D=4.2 m, 375 m apart. From: Scroby Sands Offshore Wind Farm Coastal Processes Monitoring. Cefas, UK,

13 Other processes Sand waves is a threat for stability of rock protection

14 Other processes Backfilling of scour holes Catch phrase: Current scours and waves backfill Prediction of scour/backfilling sequence over large times (weeks, months, years) is a challenge!

15 Other processes Monopile sways under waves (like tall buildings sway under wind!) Processes involved in interaction between swaying (rocking) tower and seabed soil is also a challenge

16 These are some benchmark process examples involving seabed and wind farm interaction

17 Seabed Wind Farm Interaction. A DSF research program Financed under DSF (Danish Strategic Research) funding of Danish Council for Strategic Research

18 Research program Objective: To address these and several other issues; and To establish effective methods industry needs in design of wind farms Duration: 4.5 years,

19 Justification of program? Foundation of offshore wind turbines makes up to about one third of overall investment! Speaks for itself! See, e.g., The Middelgrunden Wind Turbine Co-operative web site: nomy/budget.htm

20 Partners of our research program DTU Mekanik (Coordinator) Danish Hydraulic Institute Aalborg University LICengineering, Copenhagen Our foreign, collaborating partner University of Dundee, UK

21 Organizations that follow project DONG Energy Vattenfall DNV

22 Ph.D. Projects under Program Five Ph.D. projects: Three at DTU. Funding: DSF (15/12) along with DTU (12/12), DHI (3/12), Statkraft (6/12) Two at Aalborg. Funding: Only one is funded by DSF

23 Staff at DTU Mekanik under DSF p. B. Mutlu Sumer (coordinator) Jørgen Fredsøe Erik Damgaard Christensen David Fuhrman Ole Hededal (DTU Byg) 3 Ph.D. students; 2 Post Doc; 1 Research engineer; and 7 Master s students (expected to increase until closure)

24 Links to other research programs Future-Generation Marine Structures, a research program ( ), undertaken jointly by DTU Mekanik; DTU Byg; and DHI Innovative Multi-purpose Offshore Platforms: Planning Design and Operation, a 4-year EU FP-7 research program recently awarded. Coordinator: Erik Damgaard Christensen, DTU Mekanik

25 Research challenges (shared with DONG) Behaviour of 1-layer rock scour protection (without filter layer). Effect of applied gradings, especially wide gradation Scour development in shallow water depths (h/d ~ 1-2) Scour development around gravity base foundations

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27 Offshore Wind Farms..\MyWINDFARMProject\Offshore Wind Farms_OFFSHORE CENTER.mht Reference: arms.asp

28 Horns Rev, Denmark..\MyWINDFARMProject\Horns Rev.mht Reference: arms_detail.asp?id=36965&t=horns Rev Cost per turbine: 3.4 million Euro (2002)

29 OWT foundation. Dimensions Windfarms in Danish waters are, for most part, erected in shallow water depths, up to 20 m or so Typical dimensions for monopiles: Diameter O(5 m); Length of part in seabed O(20-40 m); Length of part above seabed up to deck O(20-30 m).

30 Several foundation types Monopile foundation Gravity foundation Tripod foundation Reference: ON OFF, Offshore Center Denmark

31 Monopile Gravity base Suction bucket Reference: Musial, Butterfield and Ram, OTC, 2006, Paper (OER)

32 0-30 m m m Reference: Musial, Butterfield and Ram, OTC, 2006, Paper (OER)

33 OWT foundation/tower construction Schematic Part driven into seabed is a simple, steel pipe Second part is essentially also a steel pipe, grouted to the first part, with an overlapping extent of O(10 m)

34 OWT foundation/tower construction Schematic Part driven into seabed is a simple, steel pipe Second part is essentially also a steel pipe, grouted to the first part, with an overlapping extent of O(10 m)

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39 Why is scour important? Three factors Kazik temelin design inda 3 faktor: Max yuk; Yorulma yuku; ve Eigen frek. Turbine gelen Max yuk, kazik temelin boyutlarinin belirlenmesinde Maximum loading to determine size and depth of foundation of monopile; Fatigue load to determine wall thickness; Eigen frequencies (to determine operation of turbine) They are all heavily influenced by depth of scour hole around foundation Bu 3 faktor, Max yuk, Yorulma yuku, ve Eigen frekansi, dogrudan dogruya oyulma derinligi ile alakalidir!

40 Time evolution of scour. Numerical S/D=1.5

41 Depth of scour hole S/D=1.5! However, scour depth can be as much as times diameter! Is like height of a 3-4 storey apartment building!

42 Why are sand waves important? Rock dumping is most popular counter measure against scour; Just dump rock With this, monopile is protected against scour If sand waves exist in vicinity of monopile, they may be a threat to stability of rock p.!

43 Budget (million DKK) Danish Council for Strategic Research, 9.4 DSF Co-financing of partners 4.7 DTU Ph.D. grants 1.6 Statkraft (Norway) 0.8 DHI 0.4 Total 16.9

44 Workpackages WP 1. Fundamental knowledge (DTU, DHI, AAU, LIC) WP 2. Effective methods to predict scour over long time spans (DTU, DHI, AAU) WP 3. Interaction between structure, flow and soil conditions (DTU, AAU, LIC, Dundee) WP 4. Natural seabed changes and interaction with seabed forms (DHI)

45 Dissemination Plan to organize an End User Conference in 2012 spring to disseminate results Open to industry, Danish consulting companies

46 Den Norske Veritas DNV, a global provider of services for managing risk, to help safely and responsibly improve business performance