Bombardem breakwaters
-
Upload
ajit-kumar -
Category
Engineering
-
view
137 -
download
1
Transcript of Bombardem breakwaters
The ‘Bombardon’ Floating Breakwater
Rahul Jindal 11NA10028
Ajit Kumar 11NA30024
Under the guidance of
Prof. T SahooOcean Engineering and Naval Architecture
IIT Kharagpur1
2
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
3
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
4
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
6
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.)
7
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.)
8
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.
10
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.
11
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.
12
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.
13
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.
14
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
15
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
18
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
19
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
20
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
21
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.
22
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)
23
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.
51
201
24
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).