EXPERIMENTAL INVESTIGATION OF EXTREME

WAVE LOAD ON JACKET STRUCTURE

[ WIND TURBINE MODEL]

BY, AMRIT SHANKAR VERMA OE13M054

Under, Dr V SRIRAM, ASSISTANT PROFESSOR

TASK DESCRIPTION

• Wind turbine foundation structures in shallow water may be prone to slamming forces from breaking waves, typically plunging breaking waves.

• Calculations show that the forces from the plunging breaking waves are governing the responses of the structure and the foundations.

• Thesis involves experimental investigation ,analysis of the result data using MATLAB coding.

OBJECTIVE OF THESIS

• The main objective of the thesis project is to experimentally obtain the slamming forces coming on jacket [truss] structure from breaking waves at different location with respect to structure and calculate the slamming coffecient Cs and present a better method to determine slamming force.

METHODOLOGY EXPERIMENTAL: [total forces] •Designing a jacket model in the scale 1 : 17 from the existing wind turbine jacket model. •Designing Load cell and strain gauge arrangement for the defined objective •Numerical modeling of load cell in ANSYS MECHANICAL and ANSYS APDL and checking the VON MISES stress. •Fabrication of load cell and soldering of strain gauge on the load cell.. •Designing of Channel section [ L section] for hanging jacket in the flume. •Experimental set up in 2 m wave flume. •Running of breaking wave: pre structure, post structure and on the structure [artificial focusing waves].

ANALYSIS: •Filtering the total forces obtained by solving inverse Fourier transformation in MATLAB. •Obtaining the force time series and maximum wave slamming force.

PLANNING OF THE EXPERIMENT

EXPERIMENTAL VIEW OF FLUME WITH STRUCTURE

Final setup of structure

SETUP OF LOAD CELL

DATA ACQUISITION SYSTEM

WS4

LABORATORY GENERATION OF BREAKING

WAVE

Under each bandwidth of energy , 4 conditions are taken.So totally, there

are [2 x 4 =8 ] conditions written as W1C1, where w1 means wave packet

no 1 and C 1 means case 1.each cases are explained in the next slide.

CASE I CASE II

CASE III CASE IV

W2C1: Wave breaking at a distance plunging towards

legs

00:00:41 00:00:36

00:00:41.5 00:00:42

Time history of waves breaking before the

structure

Snapshots of a 15 m high breaking wave captured at FINO 1

on 4th October 2009 (Source: Germanischer Lloyd)

Hildebrandt and Sriram (2014), Pressure distribution and vortex shedding

around a cylinder due to a steep wave at the onset of breaking from physical and numerical modeling,

ISOPE 2014 , Korea

00:44:44 00:44:45

00:44:46 00:44:58

Total horizontal force v/s surface

elevation

Analysis of experimental data and obtain

total slamming force:

• As per the logic explained in the literature by ALF TORUM, a simple code was written in matlab as frf method, which finds the transfer function.

• Again with frf code ,a simple IFFT code was written in matlab with butter filter and this gives total slamming force.

• Thus it involves 4 steps:

• A)Finding total horizontal force on jacket structure.

• B) Finding transfer function using impulse hammer test and use filt-filt option to filter out hydrostatic component to get only the dynamic component.

• C)finding IFFT which filters out slamming forces,which is supposed to be the high frequency part of the time series.

• D)finding IFFT,using butter filter which gives required slamming force

Impulse hammer test

Analysis of wave data

Step 1 :Total horizontal force Step 2 :Decomposition of total response

Step 3 : Slamming forces using IFFT Step 4 :Final slamming forces

W2C2: Wave breaking directly on the structure

-100

-50

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0 10000 20000 30000 40000Fo

rc

e I

N N

EW

TO

N

TIME

Step 1 :Total horizontal force Step 2 :Decomposition of total response

Step 3 : Slamming forces using IFFT Step 4 :Final slamming forces

W2C2: Non breaking wave directly on the structure

Step 2 :Decomposition of total response

Step 3 : Slamming forces using IFFT Step 4 :Final slamming forces

Step 1 :Total horizontal force

Another case of moving the structure carriage and

fixing the focusing point

Water Depth Bandwidth GA POINT OF FOCUSSING

m ∆f m

W1C1 1 0.4352 0.0045 13 226.32 115.45

W1C2 1 0.4352 0.0045 15 194.13 82.3

W1C3 1 0.4352 0.0045 16 215.12 105.23

W1C4 1 0.4352 0.003 13 68.32 2.03

W2C1 1 0.569 0.0053 13 251.47 109.7

W2C2 1 0.569 0.0053 15 204.32 89.32

W2C3 1 0.569 0.0053 16 229.17 115.17

W2C4 1 0.569 0.0032 13 120.32 3.04

W3C1 1 0.569 0.0053 13 252.32 125.17

W3C2 1 0.569 0.0053 15 210.32 91.2

W3C3 1 0.569 0.0053 16 222.37 111.32

W3C4 1 0.569 0.0032 13 120.32 3.04

Test

Measured

Total Response

(N)

Filtered

Slamming

Force (N)

Filtered slamming forces for all cases

Theoretical calculation of slamming forces on

truss structure. [IEC GUIDELINES]

Slamming force is calculated

based on their formula and

results are compared.This

obviously yield higher values

because these are for prototype

due to air entraining effects.

Also using the filtered slamming

force ,slamming coeffecient is

also obtained ,which comes in

range

In literature ,a formula is given by

GODA,as well as AUNE [2011],to

calculate approximately wave

slamming force coming on the

prototype truss structure.

Where they used CS ,slamming

coeffecient as π as well as 2π in

weinker and omeraci.where was λ

is the curling factor obtained by lab

test.

;

Cb is the breaking wave celerity

L

λ

W & O W & O Goda

W1C1 226.32 115.45 0.2212 0.26 0.05750 3.461200 1.125 245.23 145.32

W1C2 194.13 82.3 0.2513 0.31 0.07790 3.503000 1.125 213.450 131.210

W1C3 215.12 105.23 0.24 0.32 0.07680 3.487700 1.125 237.23 124.97

W1C4 68.32 2.03 0.1632 0.23 0.03750 3.370000 0.56 NA NA

W2C1 251.47 109.7 0.3015 0.29 0.08749 3.573190 1.125 215.320 139.210

W2C2 204.32 89.32 0.361 0.32 0.08796 3.650000 1.125 203.030 147.210

W2C3 229.17 115.17 0.32 0.33 0.10560 3.598490 1.125 209.870 152.100

W2C4 120.32 3.04 0.21 0.27 0.05670 3.44530 0.56 NA NA

W3C1 252.32 125.17 0.297 0.28 0.08300 3.54000 1.125 230.120 139.210

W3C2 210.32 91.2 0.356 0.31 0.11036 3.58000 1.125 195.210 147.210

W3C3 222.37 111.32 0.32 0.32 0.10240 3.59000 1.125 205.76 152.1

W3C4 120.32 3.04 0.21 0.28 0.05880 3.44530 0.56 NA NA

ληb (m) Cb (m/s)

Calculated Slamming

Force (N)Test

Measured

Total

Response

Filtered

Slamming

Force (N)

ηb (m)

Theoretical calculation of slamming forces for all the cases

Case no

Filtered slamming force

Slamming coefficient

W1C1 115.45 4.49

W2C2 82.3 4.19

W1C3 105.23 4.371

W2C1 109.7 4.39

W2C2 89.32 4.21

W3C3 115.17 4.483

W3C1 125.17 4.59

W3C2 91.2 4.31

W3C3 111.32 4.41

Calculated slamming coefficient for all the cases

Best fit for slamming coefficient [test1]

SXY 71.91

SXX 3429.8

B 0.0021

XMEAN 94.25

YMEAN 4.9308

Cs 4.7328

X Y XY X2

59.32 3.27 193.9764 3518.8624

78.27 3.69 288.8163 6126.1929

82.3 3.898 320.8054 6773.29

83.2 4.01 332.2018 6922.24

89.13 4.05 360.9765 7944.1569

89.32 4.09 365.3188 7978.0624

96.23 4.15 399.3545 9260.2129

97.32 4.2 408.744 9471.1824

107.32 4.36 467.9152 11517.582

115.17 4.49 517.1133 13264.129

116.17 4.52 525.0884 13495.469

118.32 4.59 543.0888 13999.622

1132.07 49.308 4723.9976 110271

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Sla

mm

ing

co

eff

cie

nt

T/R/V

GODA EXPERIMENT

WIENKE AND OUMERACI

AUNE 2011 PAPER

EXPERIMENT (SWWF)

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Cases

Broad bandwidth

Narrow bandwidth

Thesis results compared with past research and variation of data

with frequency bandwidth

Conclusion :

• As per IEC-61400 guidelines , the value of slamming coefficient to be taken ranges between π to 2π for truss structure ,where as for the experiment conducted it came as close to 4.73 thus even coming in affirmatively with the AUNE 2011 paper in which he arrived at a value of 4.77 for truss structure.

• Slamming forces are higher for the case when the waves break at a distance and surges towards the legs then for the case wave breaking directly on the legs. The value is about 2 times.

• Goda and Oumeraci values of slamming forces yielded higher values theoritically because of the higher curling factor assumed which varies from case to case in the experiment as well as their experiment was done on large scale testing.

• Also since the focusing wave was generated for two wave packets of energy , one was for narrow band width and one for broad band width and it can be seen from figure the irrespective of the bandwidth the variation of slamming coefficient is only 2.91%.

REFERENCES

• Shankar Babu Karnam,2008,Scattering of long and short crested waves due to dual porous cylinders,IITM

• Grilli,Yt yal,Sriram,2009,Simulation of focussing waves and local line forces due to wave impact on tripod structure,NTNU

• Vonkorman,1979,wave slamming forces on vertical Cylinders.

• Alf Torum,2012,Analysis of force response data for test on model truss structure subjected to breaking waves.