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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

2

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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

9

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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

•

10

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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