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14 November 2016
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The Trailing Suction Hopper Dredger
Prof. Dr. ir. C. van Rhee
Sectie Offshore & Dredging Engineering
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Contents
• Intro• Application of TSHD• General lay-out• Dredging Cycle
• Loading• Draghead• Hopper Sedimentation, optimal loading time
• sailing• unloading
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TSHD Intro
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Application of TSHD
Before 1980 • Maintenance Dredging
• Deepening of harbours & entrance Channels• Maintenance due to siltation• Soft sediments (silt clay)• Not stationary (wires anchors), so less problems
with shipping
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Application of TSHD
• Capital Dredging (new projects)• Most Reclamation works• Less suitable:
• Reclamation in combination with deepening• Short distance between dredging &
reclamation.• Dredged material suitable for fill• Sediments in dredge area difficult for TSHD
• Increase in size and number -> shorter execution time
Maasvlakte 2
2008 - 2013
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TSHD Process Discription
Suction
ExcavationVertical transport
Vacuum limit !
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TSHD Process Discription
Loading
Hopper sedimentation
Loading (overflow)
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TSHD Process Discription
Sailing loaded
Shallow water
sediment
3
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TSHD Process Discription
Discharge
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TSHD Process Discription
Sailing empty
water
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start overflow
start dredging
sailing loaded unloading
hopper
sailing empty
HOPPERLOAD (m3)
TIME IN MINUTES
next trip
TSHD Load Graph
stop loading
Max. displacement
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Dredgemaster “Pijpenman”
4
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Sailing
• Increase in size of TSHD’s• Increase in sailing distance• Sailing becomes a dominant phase• Sailing speed important• Special for TSHD
• Shallow water : Squat effect• Manoeuvring
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The Dredging Phase
• Draghead excavation process • Erosion by jets• Erosion due to inflowing water• Cutting with teeth
• Vertical transportation through suction pipe(s)• Vacuum limit• Pumping power
• Discharge into hopper• Sedimentation
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Process Overview
erosion cutting14 November 2016 16
Draghead components
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Dragheads
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Different Dragheads
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Draghead visor control
• Loose visor• Fixed visor• Active control with hydraulic cylinders
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Draghead loose visorLoose visor follows seabedVariation:
excavation depthAngle of suction pipe
excavation
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draghead
Cutting force
Fixed visor
In case cutting is needed
Overloading prevention
Active visor control
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Draghead / suction pipe Equilibrium
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Process in Draghead
Jetting cutting erosion
Flow
knife
Rock
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Limiting Factors
• Jet production• Jetpower
• Erosion• Pressure difference Draghead• Discharge
• Cutting Production• Trail force, Draghead Equilibrium
• All affected by Soil Conditions
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Jets versus Erosion
• Jetpower• Jet pressure * Jet discharge• Unlimited (efficiency is a problem)
• Erosion power• Discharge * pressure drop Draghead• Limited by ‘Vacuum’ • Limited by trail power (ships propulsion)• Limited by draghead equilibrium
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Jetting versus cutting
• Trail power = trail force * trail speed• Relative low efficiency of ships propulsion• Draghead equilibrium
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Hopper Sedimentation
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Loading & Overflow system
water
Sand
Delivery pipelineDistribution valvesDiffusor
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Loading & Overflow system
• Loading system• Distribution of sediment
• Influence on overflow losses• Influence on hopper load• Influence on trim of the hopper
• Overflow system• Adjustable in height
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Overflow system
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Overflow losses
• A sediment water mixture is discharged in the hopper• Not all particles will settle. A certain fraction flows
overboard• Losses depend on (most important influences):
• Discharge Q [m3/s]• Hopper area L*B [m2]• Settling velocity of sediment
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Settling Velocity
36 s wG D g
2 2104 2w D wF D C w
wG F
0
4
3s w
w D
gDw
C
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0
4
3s w
w D
gDw
C
0D
w DC f
Viscosity is function of temperature
Solve by iteration or use
Empirical formulae
Additional effects on settling velocity:
Concentration, PSD 14 November 2016 34
Settling velocity
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Schematic Process Overview
u
inc inQ outc outQ
c
bc 0 , , ,....sed b bv f w c u
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Flow field in hopper
•
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ws
V0=Q/(B*L)
0*s s
v QH
w BLw
Ratio between vertical velocity and settling velocity:
ws is a function of the particle size distribution
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In case H*>>1
100 % overflow loss
No settling in the hopper
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0
20
40
60
80
100
0.01 0.1 1 10
Particle size [mm]
Cu
m. p
erce
nta
ge
0
20
40
60
80
100
% in
ove
rflo
w
PSD in PSD out perc. in overflow
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.01 0.1 1 10
D [mm]
p [
-]
PSD in
PSD settled
PSD in overflow
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Maximum Cycle production
33 /cycle
m unloadedP m s
cycle time
Hopper X
Unloaded 20000m3
Sailing Empty 300min
Loading 70minSailing Loaded 330min
Unloading 15min
Turning etc. 10min
Total cycle 725min
Cycl prod 27.6m3/min
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HOPPERLOAD (m3)
TIME IN MINUTES
sailing
unloading
sailing
Loading
Cycle time
m3 unloaded
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HOPPERLOAD (m3)
TIME IN MINUTES
3
3tan /cycle
m unloadedP m s
cycle time
Cycle time
m3 unloaded
Optim. load
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Long sailing distance
Short sailing distance
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Hopper
Hull
Hull
Hopper
Low density soils High density soils
Restriction by volumeRestriction by weight
Carrying Capacity
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Hopper volume & loading capacity
1.3 1.85 1.9 2Loading cap max. hopper Hopper load [m3] load [m3] load [m3]max. draugth volume density[ton] [m3]
Hopper X 23200 18000 1.29 17846 12541 12211 11600Hopper Y 41000 22000 1.86 22000 22000 21579 20500
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Discharging methods
• Discharging through bottom doors• Rainbowing• Pumping Ashore
Decreasingproduction
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Pumping ashore + Rainbowing
• Hopper fluidization• With jet system• Concentration control• Minimizing rest load
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Bow connection
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Rain bowing
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Jets
Hopper self-discharge system
Water
SandJets
Water inletDischarge
Dredge pump
Jets
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Jets
Water inletDischarge
Dredge pump
JetsJets
Hopper self-discharge system
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Jets
Water inletDischarge
Dredge pump
Jets Jets
Hopper self-discharge system
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Jets
Water inletDischarge
Dredge pump
Jets Jets
Hopper self-discharge system
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Power systems
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Diesel electric pump
Pump in fore ship
Diesel direct pump
Pump in aft ship
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Diesel electric with OWP
Diesel electric with OWP and Inboard pump
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Final Remarks
TSHD is the workhorse of the dredging industryThe increase in scale created new markets• Price per m3 decreased• Execution time decreased
Still a lot of development to be expectedProcess automationIncrease in production