The ‘Bombardon’ Floating Breakwater

Rahul Jindal 11NA10028

Ajit Kumar 11NA30024

Under the guidance of

Prof. T SahooOcean Engineering and Naval Architecture

IIT Kharagpur1

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Contents1. Introduction

1. Types of breakwaters

2. Development of Bombardon breakwater

3. Full scale floating breakwater

2. Fundamental aspects of floating breakwater design

3. Principal Particulars

4. Base Principals

5. Mathematical Theory1. Wave energy reflection

2. Pressure on barrier

3. Breakwater with gaps

6. Final Impact

7. Conclusion

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Breakwaters are structures constructed on coasts as part of coastal defense or to protect an anchorage from the effects of both weather and longshore drift.

Purpose of Breakwater

A breakwater structure is designed to absorb the energy of the waves that hit it, either by using mass (e.g. with caissons), or by using a revetment slope (e.g. with rock or concrete armour units).

To manipulate the littoral transport conditions and thereby to trap some sand

Introduction

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Introduction (Contd…) Types of breakwaters

• Detached breakwater ( Breakwater be completely isolated from shore)Headland breakwaterNearshore breakwater

• Attached breakwater ( Breakwater can be connected to the shoreline)Low crested structure Rubble mound structureHigh crested structure Composite structure

• Using mass (Caissons)• Using a revetment slope ( e.g. with rock or concrete)• Emerged breakwaters• Submerged breakwaters• Floating breakwaters

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Introduction (Contd…) Types of breakwaters

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Floating breakwaters can be made where bottom connected breakwaters is unfavourable due to poor foundation, deep water, other environmental problems like water circulation and fish migration.

The engineering and subsequent construction of the "Bombardon" floating breakwaters was an important episode in the historical development of floating breakwater technology.

It was build along the coast of Mulberry harbor for the D-day invasion of France in June 1944.

Conclusions drawn from "Bombardon" floating breakwater development still hold true today and virtually all subsequent floating breakwater development uses its finding till now.

Introduction (Contd.)

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Normandy harbour componentsGooseberry : Corn cub block shipsMulberry : Floating outer breakwater ( Bombardons)

Static breakwater ( Gooseberries) Concrete caissons (Phoenix) Floating piers or roadways ( Whales) Pier spuds ( Spuds)

Introduction (Contd.)

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Phoenix:• Reinforced concrete caissons• Once refloated, were towed across channel to

form Mulberry breakwater along with Gooseberries• Made on 6 different scales for various depths• 146 caissons were built in 9 months

Introduction (Contd.)

Gooseberries:

• Ships were sunk in order to form interior breakwater type to save the harbour from the stormy sea.

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Introduction (Contd.)

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Fundamental aspects of floating breakwater design

Buoyancy and floating stability Wave transmission Mooring forces Breakwater unit structural design

Floating breakwater design is a complicated and iterative process due to the interdependency of each design factor.

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Development of Bombardon Breakwater• 1st model was tested in May, 1943, equipped with flexible sides.• Proved floating breakwaters efficiency as good as fixed ones.• Due to flexible sides, reflection of wave energy took place at antinodes.• Largest floating structures ever built at that time.• Number : 3• Length : 200 ft• Beam : 12 ft• Draft : 16.5 ft

This breakwater was not adopted because of the vulnerability of its fabric sides.

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Development of Bombardon Breakwater• The first rigid wall Bombardon model was tested in June,1943 and by the

end of August , sufficient data was assembled to establish the correctness of theories applying to rigid sided type.

• Over 300 model tests of the rigid type were made before full scale design were put in hand

Results:• It was possible to construct a breakwater which would surpasses wave of

maximum size anticipated in operation ‘Overlord’ for an expenditure of about 1.25-2.5 tones of steel per foot of breakwater frontage.

• Expenditure of one-tenth of that required by any other possible method.

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Full scale floating breakwater• First test of full scale harbour took place at the beginning of April 1944 in

Weymouth bay.• On the 1st and 2nd April an onshore gale was recorded with a wind of force 7

gusting up to force 8 resulting in a sea up to 170 feet long and 8 feet high.• Sea corresponds to stress on the breakwater of nearly double the estimated

value.• The height of waves reduced by approx. 2 feet in lee of breakwater.

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Principal ParticularsSteel weight : 250 tons

Length/unit : 200 ft

Beam : 25 ft 1 inch

Depth : 25 ft 1.75 inch

Draft : 19 ft

Gap : 50 ft

Effective beam : <5 ft

at Waterline

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Base PrinciplesThe operation of the breakwater depends on four well known

principles :• The maximum height, length and period of the waves in any given

locality are determined by its geographical configuration.• The waves of sea are relatively skin deep.• The amplitude of oscillation in an oscillatory system having a long natural

periodicity is small when subjected to a forced oscillation of relatively short periodicity.

• The floating object may, under suitable circumstances, be design to have long natural periods in each of its three modes of oscillation: Heave, Pitch and Roll.

Mathematical Theory• Wave Energy Reflection

• Marine or gravity waves were investigated on the assumption that the motion of particles in system of uniform travelling waves is either circular or elliptical.

• Coordinates of particles acted upon by system of travelling waves given as:

• From these , trajectory equation is determined , which found to be elliptical in nature with major and minor axis represented as:

X )cos(sinh

)(coshwtkx

kH

HykaX

)sin(

sinh

)(sinhwtkx

kH

HykaY

,sinh

)(cosh2

kH

Hyka

kH

Hyka

sinh

)(cosh2

Mathematical Theory• Wave Energy Reflection

The breakwater must be designed in order to meet maximum period and the depth must be enough in order to reflect the desired quantity of wave energy

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Mathematical Theory (Contd…)• As length to beam ratio of Bombardon breakwater was very small i.e it can

be considered as a thin beam.• The GDE for beam vibration and its corresponding amplification factor is:

• ‘m’ be mass of breakwater and external force is train of gravity waves.

• is natural time period• By making ‘m’ large, R small and increasing Q as much as possible, the

amplification factor can be made considerably less than unity.• This results in very less motion of breakwater, the train of waves on

reaching wall will suffer total reflection and any water on lee side of wall will remain unaffected by the passage and reflection of wave.

,2

cos2

2

tP

aRst

sQ

t

sm

E

222

1

1

E

N

E

N

P

P

mRQ

P

P

AF

R

mPN 2

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Mathematical Theory (Contd…)• The pressure on barrier

– Hydrostatic : due to water at rest– Dynamic : due to motion of water

If incident wave is considered to be sinusoidal, • The horizontal force exerted by waves has same frequency as that of incident waves.• Only the reflected waves contribute to the external force.• The natural frequency of barrier must not match that of incident wave frequency to

avoid resonance.

Resonance must be avoided in all three modes of vibration: Heave, Pitch & Roll

Done by:

Increasing mass by using ballast waterReduction in restoring force by using flexible sides

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Mathematical Theory (Contd…)

Here,D : DraftP : Wave pressureY : 0 at surface

• Pivoted at by mooring lines

•The total linear amplitude at Y=0 is a3.

2

1KD

Ke

y

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Mathematical Theory (Contd…)Breakwater with gaps• Bombardon breakwater was constructed as combination of several floating units.• Units are positioned by using ships, which leads to certain gaps ( approx. 50 ft)• A diffraction pattern of wave will immediately formed behind breakwater, which

will ultimately rebuild itself into transmitted wave.• The energy passed through gaps must be included in calculating transmitted wave

amplitude.

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Final Impact• The ‘Bombardon’ floating breakwaters were designed for waves of

• Maximum wave height of 3.3 m • Wave length of 45.7 m (5.6 s period)

• Unfortunately, the breakwaters were destroyed by an unexpected storm in June 1944, ( worst storm in 40 years), just after 2 weeks of D-Day with• Wave height of order of 4.6 m• Wave length of 91m ( 8 s period)

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Conclusion• The floating harbour principle is one of considerable importance.• There are many advantages of floating over fixed harbours

– The cost is to of fixed type.

– The area of sheltered eater may be readily enlarged by the addition of units or mere re-sitting

of moorings.– No interference with underwater currents, results in freedom from silting and scour.– Erection is very faster– Floating harbour may be arranged to cater with temporary or seasonal requirement and then

units can be moved on or stored away in off season.– Temporary floating breakwater can be utilized to protect fixed harbour works during its

erection phase

Although, at first floating breakwater appeared as rather startling innovation, sceptics found many reasons before its trials why it could not work, yet, in fact, it did work and it successfully accomplished its allotted task in the invasion and liberation of Europe.

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References

• Lochner, R., Faber, O. and Pennry, W.G., “ The ‘Bombardon’ Floating Breakwater”, The Civil Engineer of war, Vol 2, (1948), Docks and Harbours, The Institution of Civil Engineers, London, England.

• Tsinker, G.P., “ Marine Structures Engineering : Specialized Applications”, published by Chapman & Hall, (1995).

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Thank You!