Numerical Study of Bilge Keel Length Variations of Floating

Breakwater to Optimize Transmission Coefficient

Haryo Dwito Armono

, Ketut Suastika

, Asfarur Ridlwan

and Tito Biaperi

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Sukolilo, Surabaya, Indonesia

Department of Naval Architecture, Institut Teknologi Sepuluh Nopember, Sukolilo, Surabaya, Indonesia

Keywords: Floating Breakwater, Bilge Keel, Wings Plate, Computational Fluid Dynamics, Transmission.

Abstract: Waves and winds that move dynamically often cause damage on the coast so a protective beach building is

needed. Breakwater is a coastal protection structure to destroy the incoming wave energy before reaching the

coast. One type of breakwater is floating breakwater, this structure has an advantage compared to the fixed

breakwater. Research on floating breakwater has been developed with the main goal of being the most

efficient structure and can absorb waves well. In this research, floating breakwater simulation modeling with

variations in bilge keel length was conducted. Validation is done by comparing the results of (GH Dong,

2008) with numerical results of the breakwater pontoon type. There are four variations of bilge keel length,

namely 0 m, 0.6 m, 0.9 m, and 1.2 m. So, four variations can be concluded that the longer the bilge keel, the

better absorb the waves.

1 INTRODUCTION

Coastal areas need to be considered for safety against

erosion and wave surges. One of the structures to

protect the coast from waves is a breakwater. There

are two types of breakwater, bottom-founded

breakwater, and floating breakwater. Floating

breakwater has the advantage of bottom-founded

breakwater: (1) The time of construction is shorter

because it has been done in fabrication, (2) floating

breakwater can be easily moved, reassembled with

different layouts, and can be moved to different

locations, (3) floating breakwater is suitable for

muddy soil, (4) floating breakwater is more

environmentally friendly because it does not cause

pollution and sedimentation.

Floating breakwater (FB) research began around

one century ago, many studies and model tests were

carried out to develop floating breakwater. As

technology develops, research on floating breakwater

is rapidly increasing. There have been many studies

exploring floating breakwater where exploration will

continue to be developed to obtain the most optimal

results.

Experimentally and numerically of floating

breakwaters have been studied. According to G.H

Dong (2008), floating breakwater can be used

effectively in coastal areas with relatively mild wave

conditions. His experiment was to find a simple and

relatively inexpensive type, by studying 3 types of

structures including the shape of a single box, double

box, and board net. (Wang and Sun, 2010) conducted

a study of porous breakwater where the structure was

fabricated with large numbers of diamond-shapes

blocks arranged to reduce transmitted wave height

and the mooring force. (Drimer et al., 1992)

conducted a study of the simplification of a floating

breakwater design where width and wavelength are

the greater than the gap between the breakwater

position and sea bed. (William and Abul-Azm, 1997)

conducted a study of the hydrodynamic

characteristics of a dual pontoon floating breakwater

consisting of a rectangular floating cylinder

connected by a rigid deck. (Liang et al., 2004)

conducted research on the reflection and wave

transmission of floating breakwater spar bouys as a

well mooring forces.

This paper presents numerical simulation to

analysis transmission coefficients using

computational fluid dynamics (CFD) method. The

basic geometry is pontoon floating breakwater that

modified in several length variations of bilge keel on

the bottom of structure. Data parameters and

numerical model verifications based on the result of

experiments that conducted by (G.H. Dong., 2008).

Armono, H., Suastika, K., Ridlwan, A. and Biaperi, T.

Numerical Study of Bilge Keel Length Variations of Floating Breakwater to Optimize Transmission Coefﬁcient.

DOI: 10.5220/0010060501910197

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 191-197

ISBN: 978-989-758-516-6

191

2 NUMERICAL SIMULATIONS

2.1 Dimension of Floating Breakwater

Floating breakwater dimension is obtained from

international journal data entitled “Experiments on

wave transmission coefficients of floating

breakwaters” by G.H Dong (2008). See figure 1 and

figure 2 for cross section of the prototype of floating

breakwater by GH Dong. The data will be used to

compare the G.H Dong experimental test with a

numerical test. The data that will be used in numerical

test modelling are as follows, see table 1.

2.2 Data Parameters

Research conducted by (G H Dong, 2008) uses

environmental data (wave height and wave period

data). The test uses a model scale, so the enviromental

data used data that has been scaled too. The data are

scaled by assuming Froude scaling to be valid. For

detail, see table 2.

Table 1: Pontoon Dimensions.

No Dimension

Prototype

[m]

Scale

Model

[m]

1 Length (L) 20 1:40 0.50

2 Width (B) 6 1:40 0.15

3 Height (H) 4.8 1:40 0.12

Table 2: The Wave Parameter Data (Froude Scaling).

No. Dimension Prototype Scale Model

1. Wave

Heigh

2.5 m 1:40 0.0625m

2 Wave

Period

6 s

√

0.949 s

7 s

√

1.107 s

8 s

√

1.265 s

9 s

√

1.423 s

10 s

√

1.581 s

2.3 The Wave Flume

Wave flume that is modeled is in the State Key

Laboratory of Coastal and Offshore Engineering,

Dalian University of Technology, China. The flume

is 20 m long, 3 m wide, 1 m high, and 0.5 m water

depth. The flume is filled water with density of 1000

kg/m

. The flume is equipped with a hydraulically

driven, piston-type irregular wave generator at one

end and a wave absorber at the other. The floating

breakwater model is located at 20 meters from the

wave maker. The waves come from the left toward

the right across the structure. The number of wave

probes used is two, probe 1 is located 3 meters in front

of the structure and probe 2 is located at 3 meters

behind the structure. See, Fig. 3 for illustration.

Figure 1: Basic Cross-section Pontoon FB (Top View).

Figure 2: Basic Cross-section Pontoon FB (Side View).

Figure 3: Sketch of Wave Flume (Side View).

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

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2.4 Developed Floating Breakwater

Developed floating breakwater is intended to

determine shapes of floating breakwater which is

effective in reducing wave energy. Developed shape

of floating breakwater refers to the single-box

floting breakwater by GH Dong. Therefore, this

research will be changed variations of bilge keel

length. The developed floating breakwater can be

seen on figure 4.

2.5 Wave Measurements and Analysis

The objective of this study was to obtain the

transmission coefficient (C

), the ratio of transmitted

wave height (H

) to incident wave height (H

). See

equation 1 below:

= H

/ H

(1)

Measurement of transmitted and incident wave

heights were represented by surface elevation using

wave gauges.

2.6 Mooring System

The structure is connected to the sea bed by mooring

chains. There are six mooring chains on each side of

floating breakwater. Each mooring chain is 0.1 m in

diameter, 60 m in length, and 230 kg/m in unit mass.

2.7 Boundary Conditions

The purpose of boundary conditions is to determine

model conditions that represented experiment

conditions. See figure 5. Boundary conditions used in

the model are as follow:

(1) Free surface (Wave): A wave boundary condition

was defined at left (X Min). A surface wave

entered the computational domain and

propagated in the direction normal to the

boundary. The wave was set 2

stokes as wave

generator represent the physical wave conditions

at the boundary.

(2) Outflow: The outflow boundary condition was

defined at right (X Max). It allowed users to

numerically investigate the effects of wave

interactions with structures. The capability

permited a reduction in the extent of the

computing mesh needed for accurate

computations. A wave-absorbing layer used to

reduce reflection of periodic wave at an open

boundary.

(3) Symmetry: The symmetry condition was defined

at front (Y Min), behind (Y Max), below (Z

Min), and up (Z Max). No-slip conditions were

imposed using the wall shear-stress options

described in the Prandtl Mixing Length model. A

symmetry condition can be specified as free-slip

conditions that have a non-zero wall shear-stress.

2.8 Meshing

Mesh block is used to determine the area that

modelled. The smaller mesh will be more detailed,

but the output files will be larger and simulations run

longer. Floating breakwater modelling used two mesh

block, (1) Block A with a meshing size of 0.06 m at

total length (X-axis) is 20 m, total width (Y-axis) is 3

m, and total height (Z-axis) is 1 m, (2) Block B with

a meshing size of 0.01 at length (X-axis) is 19.95-

20.19, width (Y-axis) is 1.2-1.8, and height (Z-axis)

is 0.4-0.6. See figure 6 for the illustration.

3 RESULTS AND DISCUSSION

3.1 Validation

Validation is done by comparing the experiment

result with the modelling result. The comparison uses

a transmission coefficient which is illustrated in one

graph (H/L Vs K

). Validated if the modeling results

show the similarity of trend curve with experimental

results. If not, redesign the geometry model and do

the modeling again until the results really match.

= 20, B= 6, S= 0 (m) L=20, B= 6, S=0.6 (m) L=20, B= 6, S=0.9 (m) L=20, B= 6, S=1.3 (m)

Figure 4: Developed of Floating Breakwater (S = Bilge Keel Length) in meter.

Numerical Study of Bilge Keel Length Variations of Floating Breakwater to Optimize Transmission Coefﬁcient

193

Figure 5: Boundary Conditions Model.

Figure 6: Mesh-size Model.

Here, the results of modeling validation with the

experiment are shown on tabel 3. More detail see

figure 7.

Table 3: The Validation of Transmission Coefficient.

H T H/L

Eror

Numeric Experiment

0.0625

0.949 0.0452 0.325 0.356 7%

1.107 0.0345 0.661 0.654 2%

1.265 0.0282 0.908 0.844 8%

1.425 0.0238 0.960 0.873 10%

1.581 0.0206 0.891 0.815 10%

The graph on figure 7 explain that the transmission

coefficient in numerical modeling and experiment

test has almost the same similarity and valid based on

Mean Absolute Percentage Error (MAPE) theory.

However, there are still difference in value of

transmission coefficient which may be caused by

differences in recording wave gauges or meshing

size.

3.2 Transmission Coefficient Result

3.2.1 Model 1 (Without Bilge Keel)

Data from modeling of floating breakwater in Model

1 (without bilge keel) were obtained from recording

of waves elevation on wave gauges in the form of

time series data. The data is then processed using

statistics Wave Analysis (WAVAN). The following

is transmission coefficient result in Model 1, see

table 4.

3.2.2 Model 2 (0.6 m Length of Bilge Keel)

Data from modeling of floating breakwater in Model

2 (0.6 m length of bilge keel) were obtained from

recording of waves elevation on wave gauges in the

from of time series data. The data is then processed

using statistics Wave Analysis (WAVAN). The

following is transmission coefficient result in Model

2, see tabel 5.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

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Figure 7: Validation graphic (K

Vs H/L).

3.2.3 Model 3 (0.9 m Length of Bilge Keel)

Data from modeling of floating breakwater in Model

3 (0.9 m length of bilge keel) were obtained from

recording of waves elevation on wave gauges in the

from of time series data. The data is then processed

using statistics Wave Analysis (WAVAN). The

following is transmission coefficient result in Model

3, see tabel 6.

3.2.4 Model 4 (1.3 m Length of Bilge Keel)

Data from modeling of floating breakwater in Model

4 (1.3 m length of bilge keel) were obtained from

recording of waves elevation on wave gauges in the

from of time series data. The data is then processed

using statistics Wave Analysis (WAVAN). The

following is transmission coefficient result in Model

4, see tabel 7.

3.3 Comparison Model Result

In order to know the results that are easily understood,

numerical modellings of Model 1 up to Model 4 are

compared in one graph as shown in figure 8. Figure 8

shows the correlation of transmission coefficient (K

)

with wave steepness (H/gT

) at depth of 0.5 meters

and variations in wave height (H) and wave period

(T). Transmission coefficient in model 1 ranges from

0.233-0.390, model 2 ranges between 0.221-0.377,

model 3 ranges between 0.210-0.370, and model 4

ranges between 0.205-0.365. The higher the wave

height the smaller the wave period is the steeper the

wave steepness. Figure 8 shows that each model has

a transmission coefficient that continues to decrease

with increasing wave steepness.

Table 4: Graphic of K

Vs H/gT

on Model 1.

No H

H/gT

1 0.0240 0.800 0.004 0.390

2 0.0380 0.684 0.009 0.298

3 0.0550 0.658 0.012 0.281

4 0.0550 0.575 0.016 0.233

Table 5: Graphic of K

Vs H/gT

on Model 2.

No H

H/gT

1 0.0240 0.800 0.004 0.377

2 0.0380 0.684 0.009 0.287

3 0.0550 0.658 0.012 0.270

4 0.0550 0.575 0.016 0.221

Table 6: Graphic of K

Vs H/gT

on Model 3.

No H

H/gT

1 0.0240 0.800 0.004 0.370

2 0.0380 0.684 0.009 0.278

3 0.0550 0.658 0.012 0.260

4 0.0550 0.575 0.016 0.210

Table 7: Graphic of K

Vs H/gT

on Model 4.

No H

H/gT

1 0.0240 0.800 0.004 0.365

2 0.0380 0.684 0.009 0.265

3 0.0550 0.658 0.012 0.250

4 0.0550 0.575 0.016 0.205

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.02 0.025 0.03 0.035 0.04 0.045 0.05

transmission coefficient

H/L

Numeric and Experiment Validation

Experiment Modeling

Numeric Modeling

Numerical Study of Bilge Keel Length Variations of Floating Breakwater to Optimize Transmission Coefﬁcient

195

Figure 8: Comparison of Four Models Results (K

Vs H/gT

4 CONCLUSIONS

In this study, a two-dimensional numerical simulation

was analysed for floating breakwater. Previously, the

results were validated by experiments that had been

carried out by G.H Dong. After that, the development

of floating breakwater shapes are analysed by

numerical simulation using Computational Fluid

Dynamics (CFD) method. There are four

development shapes of floating breakwater analysed

that are based on the length of bilge keel. The results

are that longer the bilge keel, the more efficient the

transmission coefficient can reduce wave energy.

ACKNOWLEDGEMENTS

This study was in part of founded Directorate General

of Resources for Science Research. The Author

would like thanks to the team research and the

Directorate General of Resources for Science,

Technology and Higher Education; Ministry of

Research, Technology and Higher Education, the

Republic of Indonesia, which funding this project of

research.

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