Offshore Scour And Scour Protection Lecture29nov2010 TU Delft

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A lecture on offshore scour and scour protection.

Offshore Scour And Scour Protection Lecture29nov2010 TU Delft

  1. 1. Scour & scour protectionin the marine environmentLecture “Bed, bank and shore protection”Tim Raaijmakers
  2. 2. Content of presentation• Introduction• Mechanics of scour in the marine environment• Applications for scour prediction• Mitigating measures & selected examples• Conclusions 29 November 2010
  3. 3. Introduction (I): What is scour?Scour is erosion of sediment around a structure,thus requires an imbalance in sediment transportIncrease of sediment transport capacity around astructure due to:1. flow contraction: increase in flow velocity2. vortex development3. increase of turbulence 29 November 2010
  4. 4. Introduction (I): What is scour?Scour is erosion of sediment around a structure,thus requires an imbalance in sediment transportIncrease of sediment transport capacity around astructure due to:1. flow contraction: increase in flow velocity2. vortex development3. increase of turbulence local scour around monopileTypes of scour• local scour = erosion of seabed material at a single foundation• global scour = wider erosion around a structure consisting of multiple foundations• (edge scour = scour around a scour protection) multiple piles global scour 29 November 2010
  5. 5. Introduction (II): riverine vs. marine scourDifferences between scour in a riverine and marine environment riverine scour marine scour waves, currents, governing load(s) current combinations of current and waves tidal current, changing governing direction mostly unidirectional multiple piles global scour wave directions etc. scour+backfilling equilibrium scour depth focus of Dpile development in time during design river design&research during lifetime / discharge Smax unprotected period Breusers, Melville, Sumer&Fredsøe, available formulae HEC-18, Sheppard etc. Raaijmakers&Rudolph 29 November 2010
  6. 6. Introduction (III): offshore wind parks Booming wind energy market Need for optimization to become independent of governmental funding A significant part of the total costs of wind park development concerns the foundation & scour protection “The European offshore wind energy market is booming. In 2009 a growth rate of 54% was achieved. For 2010, a market growth of 75% is expected.” (press release EWEA, 2010) source: www.ewea.org 29 November 2010
  7. 7. Introduction (IV): offshore oil&gas industry jackup drilling rig production platform = fixed structure 29 November 2010
  8. 8. Introduction (V): offshore oil&gas industry horizontal2/18 29 November 2010
  9. 9. Introduction (VI): offshore oil&gas industry undermining loss of overburden pressure: risk on settlement 29 November 2010
  10. 10. Content of presentation• Introduction• Mechanics of scour in the marine environment • scour in waves • scour in combined current and waves• Applications for scour prediction• Mitigating measures & selected examples• Conclusions 29 November 2010
  11. 11. Mechanics of marine scour (I): wave-induced scourWave-induced scourvortex regime:dependent on Keulegan-Carpenter number: U w,bed T 2 Aw,bedKC D Dhorseshoe vortex:occurs only for very large KC-numbers ->flow for each half period of the orbital motionresembles steady current for typical monopile dimensions horseshoedevelopment is not significant under waves(lee-wake) vortex shedding:typical KC-numbers for offshore monopiles inNorth Sea storms are between 1 and 7: in transition regime between “no separation”and full “vortex shedding” [source: Sumer&Fredsøe, 2002] 29 November 2010
  12. 12. Mechanics of marine scour (II): combined current & wavesIn marine environment seldomly waves-only conditions, but combinations of currents andwaveshydraulic regime described by relative velocity: uc U rel uc U w ,bed Urel = 0: waves-only0 < Urel < 1: combined current and waves Urel = 1: current-only lee-wake vortices only occur for very long waves moderate waves superimposed to a current tend tobreak down horseshoe vortex development moderate waves cause very limited scour depth [source: Sumer&Fredsøe, 2002] 29 November 2010
  13. 13. Mechanics of marine scour (III): combined current & waves formula for equilibrium scour depth in conditions with waves hw S eq 1.5 D tanh Kw Kh [source: Raaijmakers&Rudolph, 2008] D based on: continuous transition towards Breusers formula for current-only reduction factor for wave action (between 0 and 1) K w 1 exp( A) in which wave action is represented by KC-number A 0.012 KC 0.57 KC 1.77U rel 3.76 reduction factor for pile height to account for submerged piles (0-1) 0.67 hp Kh hw 29 November 2010
  14. 14. Mechanics of marine scour (IV): combined current & waves waves-only combined current and waves current-only 10.0 1.0Seq / (Dtanh(hw/D)) [-] KC = 1 KC = 4 0.1 KC = 8 KC = 11 KC = 18 KC = 26 KC = 100 0.0 0 0.1 0.2 relative velocity 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Urel [-] 29 November 2010
  15. 15. Content of presentation• Introduction• Mechanics of scour in the marine environment• Applications for scour prediction • scour development around monopile • validation against field measurements• Mitigating measures & selected examples• Conclusions 29 November 2010
  16. 16. Applications (I): Scour development around monopile Model test: transparent pile with camera and fisheye lens current before view from inside the pile after 29 November 2010
  17. 17. Applications (II): Scour development around monopile Scour (and backfilling) depending on conditions and time colour gradient interface detection per time step distance [pixels] scour prediction formulae scour depth time 29 November 2010
  18. 18. Applications (III): Validation against field measurementsCollection of metocean data between surveyssources: field measurements and numerical modelling significant wave height Hs peak period Tp tidal current velocities water depth 29 November 2010
  19. 19. Applications (IV): Validation against field measurements Scour prediction model = computer model equipped with empirical formulations for equilibrium scour depth and characteristic timescales Basic idea of model: every hydrodynamic condition has its own equilibrium scour depth and characteristic timescale S t t discretization dt 1- exp - Sn+1 Seq,n+1 ( Sn Seq,n+1 ) exp Seq Tchar Tchar3/18 29 November 2010
  20. 20. Content of presentation• Introduction• Mechanics of scour in the marine environment• Applications for scour prediction• Mitigating measures & selected examples • dynamic scour protection, loose rock • dynamic scour protection, gravel bags • scour protection with collars • scour protection with frond mats• Conclusions 29 November 2010
  21. 21. Mitigating measures: basic approaches splitter plateMitigating measures are required if:• predicted scour depth is unacceptable (check “normal” conditions as well as design event)• not cost-efficient to increase the foundation length• soil conditions limit penetration depth• varying fixation level is undesirable (e.g. fatigue@windmills) threaded pileMethods to ‘fight’ scour:I. structure modifications (e.g. splitter plates, threaded piles, slots, collars)II. protect/armour the seabed against scour (e.g. concrete block mattresses, rubber mats, Dey et al (2006) gravel bags, frond mats, collars, rock protection) 29 November 2010
  22. 22. Mitigating measures: loose rock (I)Types of rock protectionI. static protection rocks in the armour layer are stable during the design condition well-proven technique, little maintenance 29 November 2010
  23. 23. Mitigating measures: loose rock (I)Types of rock protectionII. dynamic protection some stone movement is allowed, as long as deformation remains within armour layer goal: reduction in stone size and number of filter layers 29 November 2010
  24. 24. Mitigating measures: loose rock (I)Types of rock protectionII. dynamic protection some stone movement is allowed, as long as deformation remains within armour layer goal: reduction in stone size and number of filter layers 29 November 2010
  25. 25. Mitigating measures: loose rock (II) Validation of design formulae against field measurements Case: hindcast of deformation @ OWEZ with formula of De Vos (2008): 2 Uc 2 3 2 Uc a4U m hw S3D U Tm m 1,0 ws b a0 3 a1 a2 a3 3 N w0 ghw s 1 2 2 D n 50 gDn 50 2 Cumulativewave height since installation until last survey date Significant deformation of scour protection @ OWEZ eq w 8 S(t) [m], wave on De H [m] 18-1-2007 9-11-2007Significantbased height Vos Failure Seq; DeVos 21-11-2008 1 1-11-2006 s 6 Some deformation, but no failure Scum;DeVos 0.8 deformation levels 0.6 4 0.4 2 0.2 No movement 0 17/04/06 07/08/06 28/11/06 21/03/07 12/07/07 02/11/07 23/02/08 15/06/08 06/10/08 27/01/09 19/05/09 Raaijmakers, T.C., Oeveren, C. van, Rudolph D., Leenders, V., Sinjou, W. (2010), Field performance of scour protection around offshore monopiles. ICSE-5 San Francisco 2010 29 November 2010
  26. 26. Mitigating measures: loose rock (III) total installed protection total bed level change difference averaged over all WTGs averaged over all WTGs averaged over all WTGs• fairly evenly distributed level drop in “armour area”• neglible deformation of filter layer• deformation profile is visible, but not pronounced 29 November 2010
  27. 27. Mitigating measures: loose rock (III) total installed protection total bed level change difference averaged over all WTGs averaged over all WTGs averaged over all WTGs 338°N 23°N 293°N 68°N 248°N 113°N 158°N 203°N• fairly evenly distributed level drop in “armour area”• neglible deformation of filter layer• deformation profile is visible, but not pronounced 29 November 2010
  28. 28. Mitigating measures: loose rock (IV) armour on top of filter only filter most of the rays show average bed level drop neglible deformation of filter layer, except for Ray 23°N onset of shape of in armour layerdynamic deformation flattening of side slope close to pile of armour layer 29 November 2010
  29. 29. Mitigating measures: loose rock (V) 2007 2008 2009 2010 29 November 2010
  30. 30. Mitigating measures: loose rock (V) 1. Where would you bury your electricity cables? 2. And at what depth? 3. Where do you have to account for the “falling apron effect” Normal current conditions appear to be important! 2007 2008 flood current tidal current axis ebb current 2010 2009 29 November 2010
  31. 31. Mitigating measures: loose rock (VI)Offshore wind industry: trend towards even more dynamic scour protections goal: • less different gradings, less total volume • decrease costs • reduce number of installation activities at sea (i.r.t. workability windows) trend towards deeper water -> different foundation conceptsOffshore drilling industry:goal:• omit installation of scour protection because of delay of drilling operation• small stones, because big stones cause damage to the spud cans and to future operations• good redistribution capacities, because protection can not be applied at all locations 29 November 2010
  32. 32. Mitigating measures: loose rock (VI) Camera with fish eye lens inside transparent waves + current monopile foundationbefore waves currentafter 29 November 2010
  33. 33. Mitigating measures: gravel bags (I) 29 November 2010
  34. 34. Mitigating measures: gravel bags (I) 29 November 2010
  35. 35. Mitigating measures: gravel bags (I) Advantages of gravel bags • weight (25kg) and density of filling: scour protection • jute: filter function • in case of damage to bags: loose rock movie_installation_gravel_bags • redistribution capacity Disadvantages • degradation of jute – only temporary protection • handling costs and potential damage to bags during installation Vunfilled percentage Vpores of filling 50 to 70% Vfilling 29 November 2010
  36. 36. Mitigating measures: gravel bags (II) Model set-up waves waves waves 29 November 2010
  37. 37. Mitigating measures: gravel bags (III) Model set-up 29 November 2010
  38. 38. Mitigating measures: gravel bags (IV) Model set-up model 1:20 prototype water depth h [m] 0.75 15 significant wave height Hs [m] 0.22 4.4 peak wave period Tp [s] 2.7 12.1 3 scour protection volume V [m ] 0.005 40 width of structure B [m] 0.55 11 height of structure Hobs [m] 0.38 7.6 penetration depth P [m] 0.17 3.4 29 November 2010
  39. 39. Mitigating measures: gravel bags (V) Questions: 1. Gravel bags more stable? • … because of higher mass under water? • … because of smooth surface of bags? • … because of filter function of the jute? • … because wave pressures can penetrate into the bag? 2. Same stability? • … because the stability parameter DN50 is the same? • … because the volume is the same? 3. Loose rock more stable? • … because of better interlocking properties? • … because of smaller surface area for wave attack? • … because of larger fall velocity? 29 November 2010
  40. 40. Mitigating measures: gravel bags (VI) waves 29 November 2010
  41. 41. Mitigating measures: collar (I) structure modification: circular disk at or near seabed level prevents current and wave action from acting on the seabed around pile large collar: Dc = 3DpB. de Sonneville, D. Rudolph and T.C. Raaijmakers (2010). Scour reductionby collars around offshore monopiles. ICSE-5 San Francisco 29 November 2010
  42. 42. Mitigating measures: collar (II)Effect of collar under current-only conditions (model scale: uc = 0.3m/s) normal monopile, without collar small collar (Dc = 2Dp) large collar (Dc = 3Dp) 29 November 2010
  43. 43. Mitigating measures: collar (III)Effect of collar under combined current and waves (uc = 0.3m/s, Hs = 0.27m) normal monopile, without collar: Seq = 0.8Dp small collar (Dc = 2Dp) large collar at fixed height Dc = 3Dp) Seq = 0.4Dp above seabed (0.5Dp): Seq = 0.8Dp 29 November 2010
  44. 44. Mitigating measures: collar (IV) Combined current and waves (uc = 0.3m/s & Hs = 0.27m) 0.80 Collar width 2.0D 0.70 Collar width 2.5D 0.60 Collar width 3.0D without collar: Unprotected pile scour depth S/D [-] 0.50 • Tchar = 22 min T char = 22 min 0.40 • t0 = 0: scour starts immediately 0.30 0.20 with collar: 0.10 • t0 = ± 50-60 min: delay! 0.00 0 10 20 30 40 50 60 70 80 90 100 time [min] 0.80 Collar width 3.0D 0.70 long-duration test with collar: 0.60 • Tchar = 92 min: slower scour ratescour depth S/D [-] 0.50 0.40 • t0 = ± 100 min: delay! 0.30 Tchar = 92 min 0.20 0.10 0.00 75 100 125 150 175 200 225 250 275 300 325 350 time [min]14/16 29 November 2010
  45. 45. Mitigating measures: collar (V)collars at seabed level have potentialstill, some questions to be answered: additional laboratory tests with longer time durations and varying conditions what is the effect of natural seabed variations (e.g. mega ripples, sand dunes) is a flexible collar more effective than a stiff collar? in cooperation with the industry, installation methods should be evaluated to provide insight into economic feasibility 29 November 2010
  46. 46. Mitigating measures: frond mats (I) “artificial seaweed” to mimic behaviour of natural vegetation buoyant fronds ( fronds < water) attached to a mesh, which is anchored to the seabed increase of drag -> effect on velocity profile -> reduction of near-bed velocities -> reduction of scour depth movie_fronds_around_monopile flow velocity TKE 29 November 2010
  47. 47. Knowledge transfer: OSCAR, the Scour ManagerOSCAR, the Scour Manager• engineering software tool to: • estimate scour depth • design scour protections• for each new structure shape: • laboratory test program to determine Seq and Tchar for varying conditions • fit scour formulae • implement formulae in software tool 29 November 2010
  48. 48. Conclusions (I): scour prediction each hydrodynamic condition has its own equilibrium scour depth and corresponding characteristic timescale most severe scour development does not necessarily occur during the most severe storm: depends on structure shape, hydrodynamics and soil conditions so, always check both normal conditions and design storm conditions! scour around monopiles can be predicted with reasonable accuracy for scour prediction around more complex shapes laboratory tests are necessary: setup database with C, W and C+W-conditions 29 November 2010
  49. 49. Conclusions (II): scour mitigating measures structure modifications to reduce vortices seem to be less usefull for offshore structures due to varying hydrodynamic regimes and governing directions scour protections consisting of loose rock are best understood a static scour protection is most stable, but requires relatively large stones, large volumes and often multiple filter layers a dynamic scour protection can provide a cost-effective alternative that can still guarantee a constant pile fixation level around a scour protection always edge scour holes will develop, which cause a “falling apron effect”: retreat of the scour protection edge scour development seems to be (tidal) current-driven gravel bags can be applied for temporary operations collars have potential to effectively reduce local scour all other systems (frond mats, block mattresses, rubber mats etc.) are not sufficiently validated 29 November 2010
  50. 50. ¿Questions?For more information, contact tim.raaijmakers@deltares.nl 29 November 2010

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