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Dr.K.Thiruvenkataswamy,Dr.Eng.,M.S., B.E.,MISTE.

Professor & Head

Department of Harbour Engineering & Offshore Technology

AMET UNIVERSITY

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Energy from sea waves is one of the most promising

sources of renewable energy which is also

environment-friendly.

The obvious energy content of the waves has long

attracted inventors. Several hundreds of patents for wave energy

conversion device have been registered worldwide.

But only few are developed as prototype

Of the several patents registered, Oscillating WaterColumn(OWC) wave power device is considered as

one of the most promising.

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An OWC (Oscillating Water Column) Device consists of achamber exposed to wave action through an opening in thefront.

Under wave action, air inside the chamber gets compressedand rarified, and energy from this bi-directional air flow is

absorbed using a pneumatic turbine. The performance of the device improves considerably by the

introduction of a pair of parallel guide walls (harbour walls) infront.

The OWC can be made to resonate to any incident wavefrequency by selecting the appropriate dimension of the

device. Thus, the choice of the dimensions is critical and they are

arrived through site specific theoretical and experimentalinvestigations.

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Present Study

Wave induced hydrodynamic forces on

1:25 scale model of the Indian 150kW

prototype Oscillating Water Column

(OWC) wave energy caisson have been

analyzed.

This paper shall give a detailed account

on stability analysis of OWC caissonagainst sliding and overturning.

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Falco & Sarmento(1980) - Hydrodynamic modeling of OWCdevices

Falnes & McIver (1985) - system composed of oscillatingbodies and oscillating pressure distributions

Takahashi (1988) has given a detailed account on the

development of a wave power extracting caisson breakwater inJapan. The device consists of an air chamber, attached to anordinary caisson. A vertical wall with slits is provided in orderto transmit wave energy into the chamber. The dynamicpressures measured inside the OWC chamber and on thesloping walls are found to compare well with the theory ofGoda (1985).

Ravindran et al, 1989, Joyce et al, 1993two-dimensions withsimplified geometries

Hotta et al (1986) - operational tests conducted on the plant

Coastal Structures2011, 5-9,September Yohohama,JAPAN


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Muller and Whittaker (1993) have measured thewave induced pressures on the lip wall of a 1:36

wave energy caisson model of the Isle of Islay, fordifferent lip wall inclinations. It is concluded thatthe highest pressure could be expected for a 5 to10 degree forward inclined lip wall.

Whittaker and Stewart (1993) - experimentalstudies on hydrodynamic efficiency of an OWC in afully reflecting coastline

Jayakumar (1994) - wave forces on an isolatedMOWC caisson

Sarmento and Brito-Melo (1995) - Azores OWCPico power plant

Coastal Structures 2011 Yohohama


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Clement (1996) used a two dimensional numerical wave tank tocompute the non-linear radiation step response of OWC wave

power plants.

Lee et al (1996) - to determine the hydrodynamic parametersaffecting the design and performance of the system.

Brito Melo et al (1999) - extended version of AQUADYN

Brito-Melo et al., (2000) - 3D Boundary Element Method (BEM) -code AQUADYN based on linear theory.

Ei-Hafid Tabet-Aoul and Eloi Lambert (2003) - maximum Horizontalwave forces acting on Perforated Caisson. Goda

Takahashi formula (Takahashi and Shimosako 1994) has beencompared, and found that the new formula is cost effective

Le Crom et al (2009) - the OWC Pico plant monitored by WavECsince 2005 - AQUADYN-OWC.

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The main aim of the study is to determine

Hydrodynamic stability of the device against

hostile wave climate.

It is achieved through a detailed

experimental investigation in the laboratory

on wave induced forces and moments due to

the action of regular waves.

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Based on the OWC principle, a 150 KW wave energy device

has been installed off the South West Coast of India near

Trivandrum, in the Arabian Sea.

It is a reinforced cement concrete (RCC) caisson of size 23.2m x 17.0 m in plan and 15.3 m high, consisting of a bottom

box, a back wall, two side wall, a lip and two harbour walls

in front.

The walls and lip are of cellular construction. On top is a

concrete dome, which supports the power module.

The caisson is installed at a mean water depth of 10.45 m in

front of a rubble mound breakwater of a harbour.

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Typical plots of normalized force, Fnwhich is defined

below, Kb = 0.95 and 1.37 under closed and opened

condition are given in Fig .2 and Fig.3.

Where Fmax, Fminare respectively the maximum and

minimum inline force, l and b are the length and breadth of

the caisson at SWL.

The force is compared with the theoretical prediction forrectangular caisson.



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In general, the normalized force increases with wave steepness.

Under closed condition, the increase in shoreward force is generally

non-linear.

While the shoreward force is higher than the theoretical prediction

for rectangular caisson, the seaward force correlates well with the

theory.

Similar correlation as discussed above is observed under opened

condition.

But the theoretical prediction for rectangular caisson over-estimates

the measured shoreward and seaward force.



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0

0.1

0.2

0.3

0.4

0 0.02 0.04 0.06 0.08 0.1

Fn

H / L

SHOREWARD

SEAWARD______ THEORY (RECTANGULAR)

ORIFICE CLOSED, Kb = 1.37

0

0.1

0.2

0.3

0.4

0 0.02 0.04 0.06 0.08 0.1

Fn

H / L

SHOREWARD

SEAWARD______ THEORY (RECTANGULAR)

ORIFICE OPENED, Kb = 1.37

Variation of Normalized Inline Force with wave steepness for

OWC(Closed and opened) Caisson Model

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For a structure symmetric about its transverse axis(for OWC), the

theoretical value of effective moment coefficient Cmm, which is defined

as below, and effective inline inertia coefficient, Cm based on linear

wave theory should be the same and verified experimentally for a

rectangular caisson.

Where, Mmaxand Mminare respectively the maximum and minimum

inline moments; H is the incident wave height; l and b are the lengthand breadth of Caisson; d is the water depth.

ksinhkd

1

ktanhkd

1dtanhkd0.5.5gMorMC

minmaxmm

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Fig.4. shows the plot of variation of Cmmwith kb under openedcondition of orifice.

In this condition, the values of effective moment coefficient are higherthan those derived from the total inline force.

It is found that under closed condition of orifice, the lever arm of theinline force is less when compared to opened condition.

This can be further explained from lever arm graph.

The lever arm is estimated from the ratio of the maximum moment tothe corresponding maximum inline force in a record.

Fig.5. shows the lever arm factor, lz/d, (where, lz, is the lever armdistance from the base of the structure) with relative water depth foran OWC caisson under opened condition of orifice.



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0

0.5

1

1.5

2

2.5

0.1 0.15 0.2 0.25 0.3 0.35

lz

/

d

d / L

SHOREWARD EXPTS.(ORIFICE OPENED)_____ THEORY (RECTANGULAR)

0

1

2

3

4

0 1 2 3 4

Cmm

Kb

SHOREWARD EXPTS.(ORIFICE OPENED)____ THEORY (RECTANGULAR)



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For the estimation of the stability of the OWC caisson model the Factor

of Safety (F.S) should be calculated. For the good stability of the

caisson model the value of Factor of Safety should be greater than

one. The factor of safety for the sliding failure can be calculated using

the formula,

(3)

Where W is the dry weight of the caisson, B is the buoyancy, is the

Permeability coefficient ( = 0.5), U is the uplift force and Fmax, Fminarerespectively the maximum and minimum inline force.

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In general, Factor of safety is higher in the seaward and lower in theshoreward for both the OWC Caisson and the Rectangular Caisson.

The Factor of Safety is compared with the orifice opened and closed

condition of the OWC caisson.

The stability is more when the orifice of the OWC Caisson is opened.

The full closing of OWC orifice should be avoided to enhance the

stability of the Caisson.

For higher values of H/L, the Factor of Safety is almost same for OWC(closed), OWC (opened) and for Rectangular Caisson.

However, this needs further investigations for non-linear wave impact.

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0

2

4

6

8

10

12

14

16

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

F.S

H / L

H/L vs F.S

SEAWARDSHOREWARD

(RECTANGULAR CAISSON)


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In general, Factor of Safety against sliding is higher in seaward and

lower in the shoreward for both the OWC Caisson and the Rectangular

Caisson.

The stability is more when the orifice of the OWC Caisson is opened.

The full closing of OWC orifice should be avoided to enhance thestability of the Caisson.

For higher values of H/L, the Factor of Safety against sliding is almost

same for OWC (closed), OWC (opened) and for Rectangular Caisson.

However, this needs further investigations for non-linear wave impact.

The Factor of Safety against overturning is found to be conservative.

More numerical and physical modeling are essentially recommended for

solitary waves and random waves.

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The authors wish to acknowledge the State KeyLaboratory of Hydraulics and Mountain River

Engineering, Sichuan University, China forfunding (Fund Ref.No: SKLH-OF-0902) to dothis research work. The first authoracknowledges the AMET University, Chennai,

India to provide the additional facilities.


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