A lecture on offshore scour and scour protection.

1. Scour & scour protection
in the marine environment
Lecture “Bed, bank and shore protection”
Tim Raaijmakers

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. Introduction (I): What is scour?
Scour is erosion of sediment around a structure,
thus requires an imbalance in sediment transport
Increase of sediment transport capacity around a
structure due to:
1. flow contraction: increase in flow velocity
2. vortex development
3. increase of turbulence
29 November 2010

4. Introduction (I): What is scour?
Scour is erosion of sediment around a structure,
thus requires an imbalance in sediment transport
Increase of sediment transport capacity around a
structure due to:
1. flow contraction: increase in flow velocity
2. vortex development
3. increase of turbulence local scour around monopile
Types 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. Introduction (II): riverine vs. marine scour
Differences 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. 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. Introduction (IV): offshore oil&gas industry
jackup drilling rig
production platform = fixed structure
29 November 2010

8. Introduction (V): offshore oil&gas industry
horizontal
2/18 29 November 2010

9. Introduction (VI): offshore oil&gas industry
undermining
loss of overburden pressure: risk on settlement
29 November 2010

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. Mechanics of marine scour (I): wave-induced scour
Wave-induced scour
vortex regime:
dependent on Keulegan-Carpenter number:
U w,bed T 2 Aw,bed
KC
D D
horseshoe vortex:
occurs only for very large KC-numbers ->
flow for each half period of the orbital motion
resembles steady current
for typical monopile dimensions horseshoe
development is not significant under waves
(lee-wake) vortex shedding:
typical KC-numbers for offshore monopiles in
North 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. Mechanics of marine scour (II): combined current & waves
In marine environment seldomly waves-only conditions, but combinations of currents and
waves
hydraulic regime described by relative velocity:
uc
U rel
uc U w ,bed
Urel = 0: waves-only
0 < 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 to
break down horseshoe vortex development
moderate waves cause very limited scour depth
[source: Sumer&Fredsøe, 2002]
29 November 2010

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. Mechanics of marine scour (IV): combined current & waves
waves-only combined current and waves current-only
10.0
1.0
Seq / (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. 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. 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. 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. Applications (III): Validation against field measurements
Collection of metocean data between surveys
sources: field measurements and numerical modelling
significant wave height Hs
peak period Tp
tidal current velocities
water depth
29 November 2010

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 Tchar
3/18 29 November 2010

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. Mitigating measures: basic approaches
splitter plate
Mitigating 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 pile
Methods 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. Mitigating measures: loose rock (I)
Types of rock protection
I. static protection
rocks in the armour layer are stable during the design condition
well-proven technique, little maintenance
29 November 2010

23. Mitigating measures: loose rock (I)
Types of rock protection
II. 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. Mitigating measures: loose rock (I)
Types of rock protection
II. 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. 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
9-11-2007
Significantbased 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. 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. 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. 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 layer
dynamic deformation flattening of side slope
close to pile of armour layer
29 November 2010

29. Mitigating measures: loose rock (V)
2007 2008
2009 2010
29 November 2010

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. 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 concepts
Offshore 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. Mitigating measures: loose rock (VI)
Camera with fish eye
lens inside transparent waves + current
monopile foundation
before
waves
current
after
29 November 2010

33. Mitigating measures: gravel bags (I)
29 November 2010

34. Mitigating measures: gravel bags (I)
29 November 2010

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. Mitigating measures: gravel bags (II)
Model set-up
waves
waves waves
29 November 2010

37. Mitigating measures: gravel bags (III)
Model set-up
29 November 2010

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. 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. Mitigating measures: gravel bags (VI)
waves
29 November 2010

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 = 3Dp
B. de Sonneville, D. Rudolph and T.C. Raaijmakers (2010). Scour reduction
by collars around offshore monopiles. ICSE-5 San Francisco
29 November 2010

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. 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. 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 rate
scour 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. Mitigating measures: collar (V)
collars at seabed level have potential
still, 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. 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. Knowledge transfer: OSCAR, the Scour Manager
OSCAR, 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. 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. 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. ¿Questions?
For more information, contact tim.raaijmakers@deltares.nl
29 November 2010