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
0
50
100
150
200
250
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
0
1
2
3
4
5
6
7
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Sla
mm
ing
co
eff
cie
nt
T/R/V
GODA EXPERIMENT
WIENKE AND OUMERACI
AUNE 2011 PAPER
EXPERIMENT (SWWF)
0
1
2
3
4
5
6
-0.2 0 0.2 0.4 0.6
Sla
mm
ing
co
eff
cie
nt
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.
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