University of Stavanger

uis.no

Breaking wave forces on an Offshore Wind turbine

foundation (Jacket type) in the Shallow water

Jithin JosePhD Student

1

15/09/2016

Introduction

Offshore structures installed in shallow water regions are subjected

to highly varying hydrodynamic loads.

In addition to these loads, there will be forces from the breaking

waves.

Breaking wave forces are impulsive forces acting for a short period of

time.

Design of such structures are governed by the breaking wave forces

acting on the structures.

Offshore wind turbines installed in shallow waters, the substructures

are subjected to these breaking wave forces.

2

Introduction

There have been lot of researches in the past to study the wave slamming loads onmonopile structures. Existing models predict only impact forces on monopiles.

𝐹𝑆 (𝑡=0)=𝐶𝑆 𝜌𝑤 𝑅𝐶𝑏2 𝜆𝜂𝑏 (Goda, 1966)

There is limited information available in several technical recipes regarding the designof the jacket structures for breaking waves.

3

Slamming Load Model / AuthorSlamming

Coefficient

Goda et al. (1966) π

Sarpkaya (1978) π or 5.5

Swaragi & Nochino (1986) π

Tanimoto et al. (1986) π

Wienke & Oumeraci (2005) 2π

IEC 61400-3 (2009), ISO 21650 (2007), GL

(2005), ABS (2010)2π

DNV-RP-C205 (2010) 5.15

API RP 2A-WSD (2007)

ISO 19902 (2007)0.5π~1.7π ABS, 2011

WaveSlam Experiment

Large scale tests were carried out in 2013 at the Large Wave Channel, University of

Hannover.

The main aim of the research was to investigate the wave slamming forces from plunging

breaking waves on a truss structure in shallow water.

The truss structure was modelled in 1:8 scale.

The truss structure was equipped with four local force transducers and 22 local force

transducers to measure the response of the structure.

There were eight wave gauges distributed along the wave flume, additionally one was

located at the front pile of the structure, one in the middle and at the back of the

structure.

4

Experimental set-up on Large Wave Flume FZK

Methodology

5

Force on Jacket Structure

Experimental Analysis Numerical Analysis

Slamming Force,

Coefficients, Impact

Duration

Study the variation in maximum

slamming coefficients along the

length of the jacket members

Effect breaking wave

parameters on

Slamming Force

Experimental Analysis

Measured forces.

Quasi static loads due to wave motion.

Dynamic forces due to structures vibration.

Slamming loads due to breaking waves.

Various methods to filter out the slamming forces from the measured forces

were reviewed.

Filtering of total measured force EMD method and for local forces FRF Method

is proposed.

Jose, J, Podrażka, O, Obhrai, C, Gudmestad, OT, and Cieślikiewicz, W (2015). “Methods for

Analysing Wave Slamming Loads on Truss Structures used in Offshore Wind Applications based

on Experimental Data,” Journal of Ocean and Wind Energy (JOWE).

6

Experimental Analysis

7

Slamming force separation using FRF method Slamming force separation using EMD method

Experimental Analysis

Wave

Period

Wave

Height

Wave Breaking Position

T(sec) H(m) Front Middle Back

4.6 1.4 1401

1.5 1402

1.6 1404

1.7 1407

4.9 1.4 1411

1.5 1412

1.6 1413

1.7 1414

1.8 1416

5.2 1.4 1417

1.5 1419

1.6 1420

1.7 1421

1.8 1422

5.55 1.4 1427

1.5 1426

1.6 1423

1.7 1424

1.8 1425

8

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

1.5 1.6 1.7 1.8

Sla

mm

ing C

oeff

icein

t

Wave Height [m]

Mean

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

1.5 1.6 1.7 1.8

Sla

mm

ing C

oeff

icein

t

Wave Height [m]

Mean

T=5.55s

Slamming Coefficients for the bracings

T=5.2s

Experimental Analysis

9

Numerical Analysis

10

Develop a 3D numerical model to calculate the wave breaking loads on the

Jacket structure in detail.

Validate the numerical model with the WaveSlam experimental measurements.

Estimate the slamming coefficients on the local members of the Jacket

structure.

Jose, J, Choi, SJ, Lee, KH, Gudmestad, OT (2016). “Breaking wave forces on an Offshore Wind turbine foundation (Jacket type)

in the Shallow water,” 26th International Ocean and Polar Engineering(ISOPE) Conference, Greece, Rhodes, 26 June - 2 July 2016.

Jose, J and Choi (2016). “Estimation of slamming coefficients on local members of offshore wind turbine foundation (jacket type)

under plunging breaker,” Journal of Naval Architecture and Ocean Engineering (submitted).

Numerical Model

11

Slope 1:10

Wave

z

x

15 m 23 m 2L2L

2m

4.3m

Slope 1:10

Wave

y

x

2L2L 38 m

5 m

a)

b)

VG1 ,VG2

WG3 WG1

WG2

WG4

WG5

12 m

12 m

Schematic representation of Numerical Wave tank. a) Cross section, b) Plane view.

Case Type Wave

Height

Wave

Period

Water

Depth

(m) (s) (m)

a1 Non-breaking 0.75 4.00 4.3

b1 1.50

b2 Breaking 1.60 5.55 4.3

b3 1.70

c1 1.50

c2 Breaking 1.60 5.20 4.3

c3 1.70

d1 1.50

d2 Breaking 1.60 4.90 4.3

d3 1.70

e1 1.50

e2 Breaking 1.60 4.60 4.3

e3 1.70

Non-Breaking Wave

12

24 26 28 30

Time (s)

-0.4

-0.2

0

0.2

0.4

0.6

(

m)

CFD

EXP

24 26 28 30

Time (s)

-0.4

-0.2

0

0.2

0.4

0.6

(

m)

CFD

EXP

24 26 28 30

Time (s)

-0.8

-0.4

0

0.4

0.8

Velo

cit

y (

m/s

)

CFD

EXP

24 26 28 30

Time (s)

-0.8

-0.4

0

0.4

0.8

Velo

cit

y (

m/s

)

CFD

EXP

Comparison of the free surface elevations between the CFD

and experimental results for Case 1.(a) Wave gauge WG1 ;

(b) Wave gauge WG2.

Comparison of the water particle velocities between the

CFD and experimental results for Case 1. (a) Velocity

gauge VG1 and (b) Velocity gauge VG2.

Wave Surface Elevation Wave particle Velocity

Non-Breaking Wave- Total Force

13

24 26 28 30

Time (s)

-1000

-500

0

500

1000

1500

2000

Fo

rce (

N)

CFD

EXP

Comparison of total wave force on the structure between

the CFD and experimental results for Case 1.

Breaking Wave

14

Comparison of the free surface elevations between the CFD

and experimental results for Case 3.(a) Wave gauge WG4 ;

(b) Wave gauge WG5.

Comparison of the water particle velocities between the

CFD and experimental results for Case 3. (a) Velocity

gauge VG1 and (b) Velocity gauge VG2.

22 24 26 28 30 32

Time (s)

-0.8

-0.4

0

0.4

0.8

1.2

1.6

(

m)

CFD

EXP

22 24 26 28 30 32

Time (s)

-0.8

-0.4

0

0.4

0.8

1.2

1.6

(

m)

CFD

EXP

24 26 28 30 32

Time (s)

-2

-1

0

1

2

Velo

cit

y (

m/s

)

CFD

EXP

24 26 28 30 32

Time (s)

-2

-1

0

1

2

Velo

cit

y (

m/s

)

CFD

EXP

Wave Surface Elevation Wave particle Velocity

Breaking Wave- Total Force

15

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

16000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-5000

0

5000

10000

15000

20000

Fo

rce (

N)

CFD

EXP

28 29 30 31

Time (s)

0

6000

12000

18000

24000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

16000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

16000

20000

Fo

rce (

N)

CFD

EXP

28 29 30 31

Time (s)

0

5000

10000

15000

20000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

16000

Fo

rce (

N)

CFD

EXP

28 29 30 31 32 33

Time (s)

-4000

0

4000

8000

12000

16000

Fo

rce (

N)

CFD

EXP

Case b1 Case b2 Case b3

Case c1 Case c2 Case c3

Case d1 Case d2 Case d3

Slamming Coefficients

• The local force transducers, which have the size of the grid cells are distributed along the

length of the members.

16Front vertical member Back vertical member

760

1180

1180

100

1180

140

2250

SWLSWL SWL

B6B5B4

B3

V2

B2

B1

V1

Front Side Back

Slamming Coefficients

17

0 1 2 3

Slamming Coefficient (Cs)

-0.3

0

0.3

0.6

0.9

1.2

1.5

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

0 1 2 3

Slamming Coefficient (Cs)

-0.3

0

0.3

0.6

0.9

1.2

1.5

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

0 1 2 3

Slamming Coefficient (Cs)

-0.3

0

0.3

0.6

0.9

1.2

1.5

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

0 2 4 6 8

Slamming Coefficient (Cs)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

0 2 4 6 8

Slamming Coefficient (Cs)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

2 4 6 8

Slamming Coefficient (Cs)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

H=1.5m H=1.6m H=1.7m

H=1.5m H=1.6m H=1.7m

V1

B2

Slamming Coefficients

Wave

Case

Slamming Coefficient, Cs

Breaking PositionB1 B2 B3 B4 B5 B6 V1 V2

b1 0.95 1.55 1.88 3.70 0.67 1.32 0.68 1.57 Behind the back leg

b2 1.63 3.03 3.06 5.33 0.88 1.45 1.72 2.21 Just behind the front leg

b3 2.40 7.87 0.39 5.90 0.74 0.13 2.81 1.15 At the front leg

c1 0.85 1.29 1.71 2.96 0.56 0.90 0.58 2.08 At the back leg

c2 1.79 4.12 0.41 4.22 1.00 0.24 2.19 2.31 Just in front of front leg

c3 1.90 5.17 0.30 4.53 0.82 0.13 2.09 1.10 Just in front of front leg

d1 0.95 1.16 2.36 3.59 0.57 0.41 0.86 2.63 Just in front of back leg

d2 1.36 2.58 1.96 3.87 0.64 1.11 1.50 1.92 Ahead of front leg

d3 1.71 3.87 0.39 4.13 0.45 0.19 2.15 1.40 Ahead of front leg

e1 0.70 0.81 0.92 2.22 0.45 0.26 0.51 0.93 Middle of the structure

e2 0.80 0.93 1.72 2.86 0.48 0.28 0.55 1.33 Ahead of front leg

e3 1.24 3.96 1.24 3.57 0.41 0.21 2.94 1.56 Ahead of front leg

18

Summary of Slamming Coefficients for different members

Conclusions

A 3D numerical model was used to calculate the free surface elevation, water particle

velocities and the breaking wave forces on the jacket structure. The model is validated

with the experimental results from the WaveSlam project, for both breaking and non-

breaking waves.

The distribution of the slamming coefficients on the front and back vertical and bracing

members of the structure is calculated for the selected wave cases. Based on the present

simulations, the maximum slamming coefficient for the bracing members of the jacket

structure in the wave impact zone is estimated as 7.87, which is similar to the value

suggested by Wienke and Oumeraci (2005). On the other hand, in the case of vertical

member, maximum slamming coefficient is obtained to be 2.96, which is slightly smaller

than the values suggested by Goda (1966).

In the design of OWT substructures, it is not advised to use the maximum value of

slamming coefficient along the entire member. A triangular distribution of force should be

adopted in the calculation of slamming forces on the members. 19

20