Design of Scour Protections around Offshore Monopiles under combined
Wave and Current Loading
Leen De Vos, Julien De Rouck, Peter Troch
Ghent University, Department of Civil Engineering,
Technologiepark 904, Zwijnaarde, Belgium, tel: +3292645489, fax: +3292645837
e-mail: Leen.DeVos@UGent.be , Julien.DeRouck@UGent.be, Peter.Troch@UGent.be
Summary
Some of the best locations for wind-farms are in more or less shallow coastal waters where wind is unobstructed by onshore
buildings, trees, ... However, there are significant engineering challenges in placing the wind turbines in such environments
because, as well as having to withstand storm-force waves and tidal currents, the turbine towers have to remain stable on a
sea bed that may be continuously changing. One of the major challenges faced by designers is how to predict, and prevent,
scour of the seabed sediments around wind turbine foundations. A conservative way to design a scour protection is to allow
no or only little displacement of individual rock stones. A more economical solution can be to build a protection with
smaller elements, allowing movement of individual stones without failure of the protection. Under these conditions it is
important to formulate a clear damage definition, to define an acceptable damage criterion and to account for damage
development over time as important elements in the design of scour protections. This paper describes a physical model
study on a scour protection of dumped rock around a monopile foundation. For different stone sizes, damage initiation and
damage development during a storm is investigated, taking into account the influence of a combined wave and current
action. A suggestion for a damage definition for a scour protection around a monopile foundation is made. The test results
show that on average, movement of the stones is initiated for a bed shear stress equal to approximately half of the critical
bed shear stress (Shields). Damage development over time exhibits an exponential trend.
Keywords: scour protection, monopile, offshore wind turbine, rock armour, physical model, combined waves and current,
design
1. INTRODUCTION
When placing an offshore windturbine on an unprotected seabed with non cohesive sediments a scour hole will develop due
to the changes in the flow pattern, caused by the wind turbine's foundation. Sumer and Fredsøe (2002) suggest S/D = 1.3,
where S is the equilibrium scour depth and D is the pile diameter. For design purposes they recommend a standard deviation
of 0.6, implying that a scour depth of approximately two times the pile diameter has to be considered. The fore mentioned
design recommendations often lead to an unacceptable increase in the wind turbine's foundation/pile due to stability and
fatigue reasons. In order to circumvent this problem, scour must be prevented by means of an appropriate scour protection.
Most commonly rocks, large enough to be stable under these design conditions, are used as a scour protection around the
monopile. Recently several authors (Sumer and Fredsøe, 2002; Whitehouse, 1998 and Jensen et al., 2006) reported on scour
around monopiles. May et al. (2002) suggested guidelines for the design of scour protections for bridge piers in rivers (a
current only climate). However, little research is available that links the dimensions of the scour protection to the flow
parameters in wave-dominated flow or in a combined wave-current climate (den Boon et al., 2004). The key issue is to
determine the required stone size, the plan view extent and the thickness of the protection layer.
Shields criterion for the threshold of the motion at the surface of the stone layer determines the critical bed shear stress and
the required stone size. In practice the maximum bed shear stress around a monopile is calculated as the maximum
undisturbed bed shear stress multiplied by an amplification factor. The undisturbed bed shear stress can be calculated as
described by the CEM (2004) or by Liu (2001) for a combined wave and current climate. Measurements of the amplification
factor due to the presence of the pile can be found in Sumer and Fredsøe (2002) or Whitehouse (1998). The amplification
varies between 4 and 10 for wave and current situations. When no movement of the rocks is accepted in the design situation
(static design) this amplified bed shear stress should be smaller than the critical bed shear stress (Shields). This design
method leads to very conservative designs, mainly because the amplification of the bed shear stress due to the presence of
the pile is limited to a very small area close to the pile. A more economical solution consists of building a protection with
smaller elements, allowing movement of individual stones without failure of the protection. An important advantage of
using smaller stones is that one filter layer might be sufficient to avoid washing out of the underlying sand material through
the protection. As the correct placement of these filter layers is often difficult and expensive, reducing one filter layer will
cut the costs of the scour protection significantly. When using smaller stone sizes it is important to define damage, to
introduce an acceptable damage criterion and to account for damage development over time as important elements in the
design of a scour protection.
Sumer and Fredsøe (2002) suggest designing the plan-view extent of a scour protection based on the plan-view extent of a
fully developed scour hole and the angle of internal friction of the seabed sediment. When applying the design scour depth
(S/D = 1.9) and an angle of internal friction of 30° this leads to a scour protection diameter of 7.5D. Other suggestions
(Sumer and Fredsøe, 2002; May et al., 2002), based on scour protections for bridge piers in rivers (current only situations)
give a value of 3 to 5 times the pile diameter for the plan view diameter of the scour protection. The first method leads to an
overly conservative design since the amplification of the bed shear stress caused by the presence of the pile is limited to a
small region close to the pile. Sumer and Fredsøe (2002) also indicate that this method only gives a first and a conservative
estimate of the required extent of the protection. However, scour around the edge of the scour protection will occur,
degrading the edges of the scour protection into a "falling apron". This indicates that one should be careful to apply a very
small scour protection diameter as it might lead to a failure of the scour protection.
A commonly suggested value for the scour protection thickness ranges between 2 and 3 times the nominal diameter dn50
(Sumer and Fredsøe, 2002; May et al., 2002).
This paper describes experiments that have been conducted to investigate damage initiation and damage development of a
scour protection around a monopile foundation. Both wave load and a combined wave and current load are investigated. The
scour protection exists of loose rock material (commonly referred to as riprap) placed concentrically around the foundation.
Tests have been carried out for a range of values for water depth, wave height, wave period, current velocity and stone
diameters. A non-contact profiler measures changes in the protection surface, making it possible to quantify the damage
development.
2. EXPERIMENTAL SET-UP
Experiments are conducted in a wave flume (L x W x H = 30m x 1m x 1.2m) at Ghent University, Belgium. The model, the
test program and the measurements are shortly described in this section.
2.1 Description of set-up and model
A piston-type wave paddle generates waves at one end of the wave flume, where an absorption beach is installed at the other
end. An exterior pump-circuit generates currents in both directions.
A model of a monopile is built in the middle of the wave flume, centrally in a 4 m long sandbox (figure1), allowing the
development of a current in both directions. The offshore slope is fixed at 1:20, with a water depth varying between 0.2 and
0.4 m at the location of the monopile. The length of the sandbox is large enough to avoid influence of edge effects on the
results.
Figure 1: Experimental Set-Up
All tests are carried out with a monopile foundation and a pile diameter D = 0.10 m. Around the foundation, a scour
protection made of loose stones is applied. Stones are painted in different colours to allow visual observation of the amount
and direction of displacement. The coloured stones are placed in concentric circles around the pile, as shown in figure 2 and
have a thickness equal to the pile radius. The diameter of the applied scour protection is 5 times the pile diameter. The
thickness of the scour protection layer is 2.5d n50. The scour protection is placed on top of the sand bed.
5D
D
Figure 2: Plan view of the scour protection before a test showing the monopile foundation and the concentric coloured
circles on the sand bed. The top part of the monopile is removed for measurements.
Washing out of the fine bed material through the rocks might cause failure of the scour protection. This is avoided by
applying a filter layer. For offshore situations it is common to use one or two granular filters. However, during the tests a
geotextile was used as a filter. As the main interest of these experiments is the stability of and the damage to the scour
protection layer, no in depth research for an appropriate filter is made. For the filter, general guidelines should be followed
(e.g. May et al., 2002).
The model tests carried out to optimise the design of the scour protection include 3 different rock armour gradings: 2-80 kg,
2-300 kg and 80-300 kg. These result respectively in scaled nominal diameters dn50 of 3.61 mm, 5.22 mm and 7.3 mm. The
applied gradations are shown in figure 3.
100
lower limit
90
upper limit
80
applied gradation
Weight percentage [%]
(a)
70
60
50
40
30
20
10
0
1
weight percentage [%]
(b)
100
lower limit
90
upper limit
80
applied gradation
diameter [mm]
10
10
diameter [mm]
100
10
diameter [mm]
100
70
60
50
40
30
20
10
0
1
weight percentage [%]
(c)
100
lower limit
90
upper limit
80
applied gradation
70
60
50
40
30
20
10
0
1
Figure 3: scaling of applied gradation curves:
(a) 2-80 kg; (b) 2-300 kg; (c) 80-300 kg
2.2 Test program
Two types of tests are performed. First, initiation of movement under a combined wave and current loading is investigated.
For these tests, regular waves are superimposed on a current. Table 2 shows the detailed test program. For several current
velocities and wave periods, the wave height is increased until a stone in the protection layer moves over a significant
distance.
Secondly, damage development under different storm conditions is investigated for the different stone sizes. Table 1 shows
the variation of the parameters during the tests. The range of the parameters is roughly representative for the North Sea
climate on a scale 1/50. The target spectrum of the irregular waves used in the experiments is a JONSWAP spectrum, with a
peak enhancement factor  = 3.3. Damage is measured up to 5000 waves unless total destruction of the scour protection is
reached prior.
Table 1: Parameter range for tests
Parameter
water depth near pile
significant wave height
peak period
mean current velocity
50% value of sieve curve of
protection material
Symbol
d
Hs
Tp
u
Tested range
0.2 m – 0.4 m
0.05 m - 0.16 m
1.13 s - 1.7 s
0 - 0.36 m/s
dn50
3.61 mm, 5.22 mm and 7.3 mm
following waves and opposing
waves
current direction
2.3 Measurements and analysis of the results
A non-contact profiler is used to measure the stone surface, allowing a quantification of the volume of displaced stones. To
observe the changes in the profile of the scour protection, measurements are made before every test, after 1000 waves, 3000
waves and again after 5000 waves.
Figure 4 shows the profiler set-up as used for the flume tests. In figure 5, the resulting (severe) damaged scour protection
(after 5000 waves, for the test with Hs = 0.16m, Tp = 1.7s, U = 0.14m/s, dn50 = 3.61mm) and the measured stone surfaces
(before the storm and after 5000 waves) are presented as an example.
Figure 4: Set-up of non-contact profiler on wave flume
(a)
(b)
(c)
Figure 5: Hs = 0.16m, Tp = 1.7s, u = 0.14m/s, dn50 = 3.61 mm;
(a): Scour protection with severe damage after 5000 waves,
(b)-(c): Measurements of the profiler: before the storm (b) and after 5000 waves (c)
To analyse the measurements, the scour protection area is divided into 4 rings, corresponding with the 4 coloured rings in
the set-up (figure 2). These rings are further divided into sub-areas with a surface area equal to the pile surface area
(figure 6). This allows to define damage for the three-dimensional scour protection and to locate the damage.
Waves
Figure 6: Partitioning of the scour protection area into sub-areas with surface area equal to the pile surface area
For 2D damage and low mobility rocks (as for breakwaters) damage is commonly defined as S 2D = A/d²n50, with A the
average erosion area in a cross section (van der Meer, 1988). Due to the relatively small stone sizes used in the present tests,
higher mobility is expected. For this reason and because of the three dimensional situation, damage is defined as the
volume-percentage of eroded stones in one sub-area:
S 3 D %  100 
Ve
Vi
(1)
with Ve the volume of eroded stones in one sub-area and Vi the initial volume of stones in one sub-area. The highest damage
found in one sub-area represents the characteristic damage for a storm.
3. RESULTS AND DISCUSSION
3.1 Initiation of movement
When a static design is desired, no movement of stones is allowed. As mentioned before, the required stone size to fulfil this
condition is calculated by comparing an amplified bed shear stress with the critical bed shear stress. The amplification of the
bed shear stress is caused by the presence of the pile and varies for wave and current situations from 4 to 10 (Sumer and
Fredsøe, 2002; Whitehouse, 1998). For a combined wave - current situation it is not well known what this amplification is.
For this reason, a current is combined with regular waves to find the load at which the initial movement of a stone is
observed. Table 2 shows the test program, together with the wave height, required to initiate movement. In this table, a
negative value of the current velocity stands for a current which opposes the waves.
The main issue is to determine a definition of initial movement, as both rocking and displacement of a single stone or
different stones could be considered as initial movement. For each test, the distinction was made between initiation of
rocking and the first entrainment of one stone over a length larger than the stone size. It is the latter which is regarded as the
definition of initial movement, as it is the displacement of the stones which causes real damage to the scour protections. The
wave height which causes initial movement (Hdispl) is used for the further analysis. To find Hdispl, the wave height is
increased with a step of 0.01 m. This implies that initial movement occurs for a wave height between Hdispl and Hdispl –
0.01m.
The wave and current loads which initiate movement (shown in table 2) are used to calculate the undisturbed bed shear
stresses on the scour protection. Figure 7 compares this undisturbed bed shear stress (c for a current only situation; wc for a
combined current-wave situation) to the critical bed shear stress (cr,Shields). An adapted Shields criterion is used to calculate
the critical bed shear stress (Whitehouse, 1998):
(2)
 cr , Shields   cr  g  s     d 50
with the Shields parameter
 cr 
0.24
 0.0551  exp  0.02 D 
D
(3)
and
1
 g s  1  3
D  
  d 50
2
 

(4)
Table 2: Test program for initiation of motion and resulting wave height required for initiation of movement
d (m)
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.2
0.2
0.2
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.2
0.2
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.2
0.2
0.4
0.4
dn50 (m)
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
3.61
5.22
5.22
5.22
5.22
5.22
5.22
5.22
5.22
5.22
5.22
5.22
5.22
7.3
7.3
7.3
7.3
7.3
7.3
7.3
7.3
7.3
7.3
7.3
7.3
U (m/s)
0
0
0
0.07
0.07
0.14
0.14
0.21
0.21
0.21
0.28
0.28
0.36
-0.07
-0.07
-0.14
-0.14
0
0
0.07
0.07
0.14
0.14
0.21
0.21
0.28
0.28
-0.14
-0.14
0
0
0.07
0.07
0.14
0.14
0.21
0.21
0.28
0.28
-0.14
-0.14
T (s)
1.13
1.41
1.7
1.41
1.7
1.41
1.7
1.13
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
1.41
1.7
Hdispl (m)
> 0.12
0.107
0.095
0.108
0.097
0.106
0.087
0.078
0.040
0.027
0.011
0.008
0.000
0.100
0.086
0.094
0.077
0.150
0.114
0.129
0.114
0.127
0.117
0.112
0.070
0.031
0.030
0.134
0.116
0.146
0.126
0.136
0.140
0.110
0.120
0.100
0.080
0.031
0.025
0.103
0.109
The undisturbed bed shear stress c for current only situations is calculated according to Liu (2001) as:
 c    u 2
u 
(5)
U
g
 12d 

18 log 
 ks 
(6)
The undisturbed bed shear stress wc for waves and for combined wave and current loads is calculated according to Liu
(2001) as:
 wc 
1
 f wu 2
2
(7)
with
 k 
f w  exp  5.5 s 
  A

0.2

 6.3 


(8)
and
u  U m sin t   U 
(9)
where ks is the bed roughness equal to 2.5dn50 (nominal diameter of the stones), A is the horizontal amplitude of orbital
motion at the top of the scour protection and Um is the maximum orbital velocity at the top of the scour protection. Further
U  C  C 2  U 2
(10)
with

1  d
C  U  f wU m  6.2  ln 

  30

1

 

2
(11)
and
 A

 ks 
  0.26
0.70
ks
(12)
 = 0.4 is the Von Karman constant.
Figure 7 shows that the undisturbed bed shear stress (c or wc) at the moment of initial movement is smaller than the critical
bed shear stress derived from the Shields criterion, as expected. The value of
 cr , Shields
is a measure for the amplification
 wc
due to the presence of the pile and lies on average around 2.3 with a maximum of 5.1 for the current only situation (U = 0.36
m/s, dn50 = 3.61 mm). The average value of 2.3 for the amplification in a combined wave-current situation can have several
causes, but it implies that the waves have a large influence on the entrainment of the stones. Possible reasons could be the
method to calculate the undisturbed bed shear stress, the gradation of the stones or the use of the Shields criterion for
combined waves and currents while originally it is deduced for uniform currents.
9.000
measurements
8.000
perferct agreement with Shields
criterion
best fit with measurements
7.000
6.000
5.000
wc
y = 0.4567x
2
R = 0.8192
4.000
3.000
2.000
1.000
0.000
0.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
8.000
9.000
cr,shields
Figure 7: Comparison of the undisturbed bed shear stress at the moment of initial movement with the critical bed shear
stress (Shields)
The results (table 2) indicate that the flow direction has no significant influence on the initial stone entrainment as the wave
height required to initiate movement is almost equal in both situations. However, the current does have a large influence on
the location of the stones which have the first tendency to move: looking in the direction of the waves, the initial movement
occurs behind the pile when the waves follow the current. On the other hand, the location moves to the front side of the pile
when the waves oppose the current. So the direction of the current is predominant for the location of the "initial movement
area".
Another issue relates to the use of regular waves. On the one hand, the use of regular waves allows a precise and repeatable
definition of the initiation of movement. On the other hand, the initiation of motion due to regular waves has to be linked
with the initiation of movement caused by irregular waves. This can be done using the bottom orbital velocity Um at the top
of the scour protection and a statistical interpretation of the possibility that this orbital velocity will be exceeded by an
irregular wave train with a certain spectrum.
3.2 Storm duration
When a more economical solution for the design of a scour protection is sought by reducing the stone size, stone movement
(i.e. scour protection damage) will occur under storm conditions. The scour protection's shape will transform over time until
a stable state is reached, a situation which is often named dynamic stability. In this case, the development of the damage
during a storm is important.
In figure 8, the ratio S3D(N)/S3D(5000) is represented as a function of the number of waves N. The average damage after
1000, 3000 and 5000 waves for all tests is shown. On average, 70% of the damage has been reached after 1000 waves. After
3000 waves in average 93% of the damage level is reached.
1
0.9
0.8
S3D(N)/S3D(5000) [-]
0.7
0.6
0.5
0.4
0.3
0.2
average of all tests with N = 5000
f(S), equation (14)
0.1
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Number of waves N
Figure 8: Influence of number of waves on damage
A function describing the influence of the storm duration is:
f(S)  a1 - exp - bN
(13)
Based on simple curve fitting, the following equation is derived:

f(S)  1 - exp - 1.110 -3 N

(14)
It is this equation which is presented in figure 8. It has to be noted however that the damage development also depends on
the wave and current load. For high loads, the damage development reaches equilibrium much faster than for smaller loads.
4. FURTHER RESEARCH
The quantitative analysis on damage development under different storm conditions is still in progress. Results should show
the influence of gradation, current direction, water depth, applied filter layer and stone size on the damage and the shape of
the stable profile. A new test series investigating the influence of the stone density is planned.
5. CONCLUSIONS
A physical model study investigating a riprap scour protection around a monopile foundation is described in this paper. The
main idea of this study is to investigate what happens when smaller stone sizes than required for a static design are used.
Two main conclusions can be made so far. The first test series shows that for a combined current – and wave situation, the
undisturbed bed shear stress at the moment of initial movement is approximately half of the critical bed shear stress.
A second conclusion is that damage development over time follows an exponential trend. On average, 70% of the total
damage (after 5000 waves) is obtained after 1000 waves, while 93% of the damage is reached after 3000 waves.
ACKNOWLEDGEMENTS
The work presented in this paper is subsidised by the Research Foundation – Flanders (Project 1.5.160.05). The Research
Foundation – Flanders is gratefully acknowledged.
SYMBOLS
A: amplitude of orbital motion
d: water depth
D: pile diameter
d50: median particle size
dn50: size of equivalent cube having the same volume as median particle size
g: gravitational acceleration
Hdispl: regular wave height causing initiation of movement
ks: bed roughness
s: specific gravity of particles s   s

S: scour depth
S2D: two dimensional damage
S3D: three dimensional damage
U: average current velocity
Um: maximum orbital velocity at the bottom
: undisturbed boundary layer thickness
: Shields parameter,  

g  s   d 50
cr: critical Shields parameter
: von Karman constant: water density
: kinematic viscosity
s: density of sediment or stones
c: undisturbed bed shear stress for current
wc: undisturbed bed shear stress for combined waves and current
cr: critical bed shear stress, corresponding to the critical Shields parameter
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