An Overview of Wave Impact Forces on Offshore Wind Turbine Substructures

Abstract

Offshore wind turbines are always subjected to highly varying aerodynamic and hydrodynamic loads which dictate the design phase of the wind turbine substructures. The breaking wave forces yield the highest hydrodynamic loads on substructures in shallow water, particularly plunging breaking waves. Due to the complex and transient nature of the impact forces, the description requires more details concerning the physical properties of breaking waves and the response of the structure. The objective of this paper is to give an overview of the previous and recent research on wave impact forces and the key issues pertaining to these forces on offshore wind turbine substructures.

Full text

Energy Procedia 20 ( 2012 ) 217 – 226
1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Centre for Renewable Energy.
doi: 10.1016/j.egypro.2012.03.022
Technoport RERC Research 2012
An Overview of Wave Impact Forces on Offshore Wind
Turbine Substructures
Mayilvahanan Alagan Chella
a
*, Alf Tøruma and Dag Myrhauga
Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
Abstract
Offshore wind turbines are always subjected to highly varying aerodynamic and hydrodynamic loads which dictate
the design phase of the wind turbine substructures. The breaking wave forces yield the highest hydrodynamic loads
on substructures in shallow water, particularly plunging breaking waves. Due to the complex and transient nature of
the impact forces, the description requires more details concerning the physical properties of breaking waves and the
response of the structure. The objective of this paper is to give an overview of the previous and recent research on
wave impact forces and the key issues pertaining to these forces on offshore wind turbine substructures.
© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of Technoport and the
Centre for Renewable Energy
Keywords: Breaking waves; slamming force; offshore wind turbine substructures; wave forces.
1. Introduction
The offshore wind turbine structures are slender and wave and wind loads act on the lower and the upper
part of the tower. Near to the free surface zone, the wave forces may obtain their maximum values. Most
of the recent substructures for wind turbines are monopiles, truss structures, tripods, gravity based
structures etc. The substructures exposed to the harsh sea environment, experience the extreme impact
force, run-up, scour etc. Breaking waves exert very high impact forces in very short duration on the
substructures and the analysis is extremely intricate. Due to the impact force on the substructures, the
*Corresponding author. Tel.: +47-73594667, Mobile: +47-48348518
E-mail addresses: acm@ntnu.no.(Mayilvhanan, Alagan Chella), alf.torum@ntnu.no (Alf Tørum), dag.myrhaug@ntnu.no (Dag
Myrhaug)
Available online at www.sciencedirect.com
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Centre for
Renewable Energy.

218 Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226
performance and fatigue life of the offshore wind turbine is affected [1]. Wave run-up affects the design
of boat landing and platform facilities of the offshore wind turbine structures.
Many laboratory and numerical studies have investigated the impact forces caused by breaking waves for
oil and gas structures. The offshore wind structure is a long slender member, extending high above the
mean water level and it carries the mass at the tip (rotor and nacelle). Hence, it is obvious that the
dynamic characteristics of the wind turbine substructures are completely different from fixed oil and gas
structures. Hence the effect of breaking wave forces on an offshore wind turbine needs to be investigated
in more detail to improve the current design methods. The aim of this paper is to discuss the previous and
recent research on the experimental results, numerical modeling, theoretical description of wave impact
forces, design guidelines and the key issues concerning the wave impact forces on offshore wind turbine
substructures.
2. Breaking wave force characteristics
2.1 Breaking waves
Breaking is initiated when the wave gains more energy, becomes unstable and dissipating the energy in
the form of turbulence. During the wave breaking process, the energy of the wave system is focused close
to the crest of the wave and a spatial spread of wave energy occurs [2].According to the Stokes criterion
for wave breaking the particle velocity at the crest of the wave reaches the celerity. The common ratio of
the wave height to the water depth at breaking is between 0.8 and 1.2. Breaking waves may occur at the
site depending on the water depth, wave height, sea bed slope, wave period and steepness. Breaking
waves are classified as spilling, plunging, surging and collapsing where the latter is the combination of
plunging and surging [3]. The spilling and surging wave forces can be approximated as a quasi-static
force. The breaking waves most relevant to offshore wind turbine structures are spilling and plunging
breakers [4]. The energy from the plunging breakers is dissipated over a relatively small area, and high
impulsive loads and high local pressures are exerted. Breaking wave properties are also depending on the
wind-wave interaction, wave-wave interaction and wave-current interaction. There are two major
uncertainties in breaking wave forces: the kinematics of the flow and the relationship between the flow
and the breaking wave forces [5].
2.2 Breaking wave forces
The non-breaking wave force is normally calculated using Morison equation as the sum of the quasi static
inertia and drag force, and the values of the inertia and drag coefficients are dependent on Keulegan-
Carpenter number, Reynolds number, roughness parameters and interaction parameters [6]
(1)
where FD is the drag force, FM is the inertia force, CD is the drag coefficient, CM is the inertia coefficient,
w
is the mass density of water, D is the pile diameter, u is the water particle velocity and t is time. The
Morison equation is generally valid for small diameter members that do not considerably modify the
incident waves, and it depends on the ratio of the wave length to the member diameter. The Morison
equation is applicable when this ratio is larger than 5.0.
For design purposes, the impact force is previously approximated by considering only the drag force
component and multiplying by a factor of 2.5 [7]. The total wave force on a sub-structure due to breaking
waves can be divided into a quasi-static force and an impact force called slamming force. The quasi-static
2
1
24
DM wD wM
dd
Du
FF F CDuudz C dz
t

Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226 219
force can be well described by the Morison equation and the impact force component must be added with
the Morison equation to determine the total wave force due to the breaking waves.
Figure 1 (a) Breaking wave parameters (SWL= Sea water level) and (b) The nature of the slamming force.
Three different approaches are used to account for the impact forces due to breaking waves in the
structural design. First, a simple approach to estimate the impact force by applying the non-linear wave
kinematics (non-linear wave theory) in the breaking zone to the structural members using Morison
equation with conventional force coefficients [8].
Second, the impact force can be
velocity of the member to the water particle and with a suitable drag coefficient, because of the
uncertainty involved in the prediction of accelerations caused by breaking waves [5, 9, 10]. In the splash
zone, submerged structural members are vulnerable to wave impact due to the action of breaking waves.
Moreover, the influence of the change in the momentum (inertia forces) is very important to account for
the impact forces.
Third, if the wave breaks against the structure the Morison equation ought to be modified or expanded to
include the wave breaking effect, especially due to plunging breakers on the slender structure. The nature
of the slamming force is indicated in Fig. 1(a). However, the force coefficients in the Morison equation
cannot describe the impact force of very short duration typically of the order of milliseconds. Hence it is
imperative to add an extra term in Eqn. (1) to include the impact force effect (slamming force) in the total
wave force [5, 11],
(2)
(3)
Here Fs is the slamming force, Cs is the slamming force factor, Cb is the breaking wave celerity (the water
particle velocity is set equal to the wave celerity at breaking), and is the curling factor which indicates
how much of the wave crest is active in the slamming force as shown in Fig. 1(a).
2.3 Impact force characteristics
Basically, the impact force is caused by the collision of the upright wave front with a structure leading to
a change in the forward momentum which yields a force of large magnitude in a short duration [11]. A
2
0.5
Swsbb
FCDC
DM S
FF F F
b
Hb
hb
Total force
Time (s)
Slamming force, FS
SWL

220 Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226
particular characteristic of plunging wave impacts is the considerable variation of the peak between
different impacts. Fig. 2 shows a circular cylinder exposed to a breaking wave and the various parameters
of the breaking process. The wave breaking is always associated with extreme velocities and accelerations
with high surface elevations.
Figure 2 Breaking wave impact force on a circular cylinder [13]
Hence the structural members in the splash zone experience the severe loading due to breaking waves
[5].The rising time is the time at which the impact force reaches its maximum value and it plays a vital
role in the dynamic response of the structure. In fact, the rising time distribution affects the slamming
force amplification since it is nondeterministic. Further, the maximum impact force response may be
driven by the dynamic response of the structur
zero as in the case of vertical wall. Later, the importance of the wave front inclination is addressed by
Sawaragi et al. [12]. The angle of inclined wave front is an important parameter to find the rising time and
the initial sudden rise in the impact force [12]. In addition to that the surface roughness of the structure
also tends to increase the rise time and reduces the magnitude of the amplification. The factors affecting
the impact forces are the compressibility of the air between the cylinder and the water surface, water
depth, curling factor, entrapped gases in the water, cylinder surface irregularities, rise time etc. [10].
3. Wave impact models
The wave impact acts for a very short time relative to the wave period and with high amplification. In
general various factors affect the wave impact force such as irregular sea, the compressibility of air
between the structural member, the compressibility at the beginning of impact, three-dimensional shape
of the sea surface, size and shape of the air bubbles near the free surface and sea bed slope [13]. Hence
the description of the wave impact model becomes complex. One of the first attempts to investigate the
impact force on a body during landing on the water was performed by von Karman [14]. The impact force
on the cylinder is approximated as a flat plate with a width equal to the immersed width of the cylinder
and integrating the force over the height of the impact area results the impact force. In his theoretical
model, the raise of the free surface elevation during the impact, the so called pile-up effect, is neglected,
which affects the duration and magnitude of the impact force [2]. Later, the model developed by Wagner
[2] includes the pile-
icient is also higher in the former case. The maximum
inline force at the beginning of the impact can be obtained by applying the approach of von Karman and
Wagner. The von Karman model is implemented by Goda et al. [11] and Tanimoto et al. [15] to calculate
the wave impact forces on vertical cylinders. The theoretical model presented by Wienke and Oumerachi,
-linear velocity terms in the Bernoulli equation are
C
is the wave celerity
Hb
is the wave height at the breaking location
b
is the maximum free surface elevation
R
is the radius of the cylinder
is the curling factor

Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226 221
considered in order to account for the temporal development of the impact. The description of the shape
of the body is very important to predict the immersed width of the cylinder and is approximated as an
ellipse by Fabula [7] and a parabolic shape by Cointe and Armand [7].
Figure 3 Comparison of time histories of the inline force, (t=time, R=cylinder radius, V=cylinder
velocity) [7]
For the direct impact force on the upright cylinder, the quadratic parabola representation is applicable at
the beginning of the impact; it is not valid for the total duration of the impact [7]. To improve the
approximation, Wienke and Oumerachi [7] described it as a circular shape and introduced a polynomial
stepwise function to describe the wetted surface of the circular cylinder. In the case of impact with an
angle (oblique), then the shape of the body has to be described as an elliptical shape instead of a circular
shape. The comparison of time histories of inline force for the different theoretical models is shown in
Fig. 3.
There are two theoretical models based on the study of the penetration of a horizontal circular cylinder
entering into calm water at various constant down ward velocities: Sarpkaya [10] and Campbell-
Weynberg [16]. The theoretical model by Sarpkaya [10] predicts the design forces on a horizontal
cylinder subjected to impact, but this model does not describe the impact area and the curling factor [13].
Though, the wave slamming coefficient depends on the rising time and the natural period of the structure.
The impact model by Campbell and Weynberg [16] recommends that the slamming coefficient of the
fully submerged cylinder is 0.8, but the model does not define the curling factor [16].
4. Experimental investigations of impact forces due to breaking waves
4.1 Investigations on cylindrical structures
Goda et al. [11] investigated the impact forces on the circular and triangular vertical cylinders and the
study includes the information of force-time relationship. They assumed that the impact force is the result
of the change in momentum of the water mass of a vertical wave front and they have not considered the
rising time of the impact force [12]. The experiments by Sawaragi and Nochino [12] revealed that the
wave front is not always vertical and that the front shape of breaking wave determines the rising time of
the impact force. Moreover, its magnitude depends on the wave breaking pattern and the wave breaking
point. The vertical distribution of the peak values was found to be a triangular shape whose peak appears
at the height about 70% of the wave crest above the still water level. They defined the total force as the
Weinke and Oumerachi

222 Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226
the difference of water levels between the leeward and the seaward sides of the cylinder, and the largest
observed that the phase difference between the accelerations of the water particles and the inertia forces
must be considered for the estimation of both the drag and the inertia coefficient.
All the previous tests, except those by Wienke and Oumeraci [7], have been carried out at a fairly small
scale with cylinder diameters typically 5 10 cm. They carried out tests in a large wave flume with a
cylinder with diameter 0.70 m, water depths approximately 4 m and with wave heights up to 2.8 m. They
found that the pile-up effect considerably affects both the duration and the magnitude of the impact force.
Further, they observed that the distance between wave breaking and cylinder greatly influences the
magnitude of the impact force, and the impact force is proportional to the curling factor, which depends
on inclination angle of the cylinder and on the angle of the wave front inclination. Ros [17] and Arntsen
et al. [18] carried out tests on the wave slamming forces on a single pile where local force responses were
measured at different elevations.
4.2 Investigations on truss structures
The wave forces on a truss structure on this scale are subjected to scale effects, especially the Morison
type forces. However the results obtained are nevertheless of interest and suggest that tests on a larger
scale are needed before any final conclusion on wave slamming forces on truss structures can be made.
There has not been carried out any major investigation on the wave impact forces on truss structures.
Results from an introductory experimental study carried out to find impact forces on truss structures in
scale1:50 by Aune [19] are shown in Fig. 4(a). Aune [19] made a brief analysis of the wave forces and
used these forces to calculate the response of a full scale structure. Tørum [20] has made some additional
analysis of the responses measured by Aune [19]. As seen in the Fig.4 (b), there is a low frequency part
and a high frequency part of the response. The low frequency part is the Morison force part, while the
high frequency part is from the wave impact. The recorded response force is corrupted from dynamic
effects on the model from the impacts. The challenge is to extract the wave impact force from the force
response signal, taking the dynamic effects of the model-measuring system into account. This is normally
done by using a convolution technique, e.g. similar to what Ros [17] did for a monopile. However, this
has not been pursued so far on the truss structure model. As aforesaid, the wave slamming force on a
monopile occurs when the crest region of the wave hits the pile.
00.2 0.4 0.6 0.8 11.2 1. 4 1.6 1.8 2
-10
-5
0
5
10
15
20
25
30
Time, s
Force, N. Waveheight, cm
Lars5. Model truss structure. Wave forces and wave heights
Force at bott om
Force at bott om
Force at top
Wave at structure
Waves ahead of struct ure

Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226 223
Figure 4 (a) Wave impact test at NTNU (b) General appearanse of the wave and response force
recordings.
Table 1 Comparison of different wave impact models
Authors
Theory
CS
Vertical force
distribution
Goda et al. [11]
von Karman
Uniform
Sarpkaya [10]
A method by
Kaplan [16]
For dynamic
otherwise 5.5
Depends on the rise time
and natural period
Sawaragi and
Nochino [12]
Experimental study
Triangular
Tanimoto et al. [15]
Von Karman and
Wagner
Triangular
Weinke and
Oumerachi [7]
Wagner
Uniform
Ros [17]
Experimental study
4.3
Triangular
In the case of truss structures, there are apparently some impacts caused by low wave surface elevations
from the mean water level (approximately) as shown in Fig. 4(b). A truss structure has been designed for
the Thornton bank outside the Belgian coast, where plunging breakers have been specified, and that the
wave slamming forces from plunging breakers are governing the stresses in this structure [22]. Table 1
provides a comparison of different experimental and theoretical wave impact models for single circular
cylindrical structures.
5. Numerical simulations of impact loads due to breaking waves
The estimation of the total wave force using Morison equation with the von Karman or the Wagner
impact models requires the input of wave kinematics. Nevertheless, there are many uncertainties in the
application of the wave theories to describe steep and breaking waves in shallow water [21]. Hence the
numerical simulation may be an alternative to the exact description of the shallow water impact forces.
The numerical description requires the modeling of wave-structure-air interaction during the impact [22].
The most destructive impact occurs when a breaking wave approaches the structure with almost vertical
front and entrapping a small air pocket at the wall [23]. Numerical simulations of offshore wind turbines
should include a fully non-linear model to account for breaking wave impact loads on offshore wind
turbines.
Wu et al. [24] simulated the impact wave force due to breaking waves without entrapped air on a vertical
wall by describing the complex free surface and splashing, and breaking by the Volume of Fluid (VOF)
technique. Zhang et al. [25] studied the impact of a two-dimensional plunging wave on a rigid vertical
wall using a Boundary Element Method (BEM) and scaled the maximum impact pressure by the breaker
parameters. BEM has some limitations in modeling the post-breaking and the extreme turbulent impacts.
Hence the model must include the complete flow physics based on the solution of Navier-Stokes equation
[26]. Christensen et al. [22] demonstrated the coupling of a Boussinesq wave model with a Computational
Fluid Dynamics (CFD) solver for the wave-structure interaction problems. This model is applied to
calculate the wave loads on the wind turbine substructures and the new model reduced the computational
time. Mokrani et al. [26] investigated the impact force and the overtopping flow generated by plunging
breaking waves on a vertical wall by combining Navier-Stokes equations and VOF technique (NS-VOF).

224 Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226
Bredmose and Jacobsen [23] studied the extreme spilling breaking wave loads on a monopile foundation
of an offshore wind turbine using Open Field Operation and Manipulation (OpenFOAM).
Christensen et al. [22] studied the extreme wave run-up and wave forces on monopile for offshore wind
turbines using the NS-VOF. It was observed that the run-up caused by nearly breaking waves is higher
than the run-up due to periodic waves. Corte and Grilli [27] modeled the extreme wave slamming on
monopile offshore structures using a NS-VOF for two-phase flow. Nielsen et al. [28] studied
experimentally and numerically the effect of three-dimensional waves on the wave run-up and predicted
the maximum run-up using a fully non-linear NS-VOF technique. Bredmose and Jacobsen [29]
investigated the vertical wave impact force and subsequent run-up on a monopile sub-structure using a
VOF method.
6. Recommendations from standards for the wave impact forces
There are several design guidelines for the prediction of design wave impact forces from wave breaking
on vertical cylinders. Though, there are limited guidelines for design impact forces on truss structures.
The IEC 61400-3 [4] standard recommends that extreme events for the design load phase should account
for the stochastic nature of both wind and wave loading, the flexibility of the structure and the non-linear
nature of waves simultaneously. The load due to the wave run-up should be considered to the design of
the low level platforms. If an offshore turbine is located near a coastal breaking wave zone, the coupled
wave and current model should take into account the surf currents generated by the breaking waves. API
RP 2A-WSD [30] suggests the slamming coeff
time and the natural period according to Sarpkaya [10]. Slamming forces affect the local structural
member design. According to DNV- OS-J101 [15] and DNV-RP-C205 [16], slamming on horizontal
cylinders can be predicted using a method described by Kaplan [16] and slamming on vertical cylinders
can be represented by the Campbell and Weynberg [16] impact model.
The air entrainment increases the rise time and reduces the maximum impact forces. In sea water, the
bubbles are smaller and disappear slowly where as in fresh water, the bubbles are larger and disappear
quickly. Hence it is recommended that the water properties should be considered for the slamming
experiments [16]. Table 2 shows the comparison of design guidelines for the impact loads.
7. The key issues in the performance of an offshore wind turbine under the influence of wave
impact forces
First, in the case of impact forces, the reaction forces are highly important and do distinctly depend on the
structural response and the shape of the structure [32]. Breaking waves may potentially cause significant
dynamic amplifications of the structural response on substructures.
Table. 2 Comparison of design guidelines for the impact loads [14]
Design
standards
IEC 61400-3 [4]
GL [31]
ABS [21]
DNV-OS-J101[15]
DNV-RP-C205[16]
API RP 2A-WSD [30]
Theoretical
model
Wienke and
Oumerachi model [7]
For Horizontal cylinders-Kaplan
[16]
For Vertical cylinders-Campbell
& Weynberg [16]
Sarpkaya [10]
Slamming
Coefficient (CS)
2 at t=0 for force per
unit length
5.15 at t=0 for force per unit
length
length

Mayilvahanan Alagan Chella et al. / Energy Procedia 20 ( 2012 ) 217 – 226 225
Time invariant
Time invariant
For dynamic analysis-
Otherwise 5.5
The consequence of large breaking wave forces would increase the probability of fatigue failure and
affect the design of the offshore wind turbine structures, which will result in large stiff structures that are
more expensive [1]. Moreover, breaking waves affect the global dynamic load and responses [13].
Second, wave run up indicates a complex process that is dependent on a number of wave characteristics,
structure conditions and local effects. The strong wave run-up induces an additional inline force and
overturning moment on the lower level platforms. Moreover, the short duration vertical impact forces
may excite structural ringing at very high frequencies [23]. The report [28] has shown that wave run-up
has removed the grating at the access platforms located 9m above mean sea level and affected the access
ladders at the Horns Rev offshore wind farm [28]. Experiments have shown that the long waves with
higher crest velocities have large influence on the wave run-up [33]. The important design parameters of
these platforms are maximum wave run-up height and the associated forces.
Third, the scour process around the base of the sub-structure is due to erosion of the bed soil due to the
combined wave and current induced flow velocities and it is the complex interaction between the
incoming flow, the base of the sub-structure and the sea bed. The depth of this scour is in the order of 1.5
times the pile diameter. However, combination of a current with waves in the same direction is relative
[34]. It is clear that the scour affects the stability and the dynamic behavior of the offshore substructures.
Hence the substructure design should consider the wave and current induced scour.
8. Summary
A detailed literature review is carried out to study the influences of the breaking waves and the associated
effects on offshore wind turbine structures. The considerable uncertainties in the estimation of
hydrodynamic loads, fatigue life and the extreme loads are caused by the breaking waves. The design
loads of offshore wind turbine are more sensitive to the dynamic characteristics than the offshore oil and
gas structures [21]. Hence the design methods and guidelines need to be investigated in detail for offshore
wind turbine substructures.
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