design dredging design

112 September 200612 September 2006

Designing Dredging EquipmentOE4671/WB3408

Prof.ir. W.J.Vlasblm

Section Dredging engineering

September 12, 2006 2

Purpose of the lecture

• To design a particular type of dredger on basis of (simple) dredging processes.

• Such a method can be used for many design problems!


Course development

• Introduction lecture in the 5th quarter

• Assignment for one or two persons can be done the whole year around.

• Total 4 credits (ECT)


Assignments for hydraulic dredgers

• Trailing Suction Hopper Dredger (TSHD) for large reclamation works

• Multi Purpose TSHD for maintenance and beach nourishments

• Gravel Trailing Suction Hopper Dredgers• Cutter Suction Dredger• Environmental Dredger• Plain Suction Dredger• Dustpan Dredger


Assignments for mechanical dredgers

• The backhoe dredger• The grab dredger• Bucket ladder dredger


Trailing Suction Hopper Dredger


Cutter Dredger


Plain Suction Dredger


Dustpan Dredger


The Environmental Dredger


The Backhoe Dredger


The Grab orClamshell Dredger


Bucket Ladder Dredger


Work Load

The assignment for• the TSHD, CSD are for 2 students• The other ones are for 1 student


Designing DredgingEquipment

Dredging installation

Propulsion (optional)

General layout

Productions

Prime movers (main engines)


Design productions

Working hours

Delays

Effficeincy

Yearly production

Excavating production

Soil type

Losses

Transport production

Losses

Deposing

Situ production


Design basis = yearly output in m3

(production)

• To be translated to the significant design parameters. • Depends on the scale (or cycle) of the process.• Large scales

• Hopper dredger ⇒ volume and load per trip• Barge unloader dredger ⇒ Barge volume• Backhoe dredger ⇒ the volume per cycle

• Continuous operating dredgers or equipment m3/week or m3/month


Design Basis (2)

• Small scale:• “Wall” speed of a breach mm/s• Pump output in m3/s• Cutter head Excavated output in

m3/s• Bucket ladder dredger Buckets/min• Backhoe dredger Bucket volume/cycle• Grab dredger Grab volume/cycle


Problems during translation

• Change of volumes • In many cases the contractor is paid in volumes

removed, but many processes are based on mass.• Working hours per week (168 or 84 or 40)• Down time• Overhaul & Maintenance • Bunkering, crew changes, etc• Delays due to weather conditions


Concentration (1/3)

• By volume

• By weight

• Delivered

sv

m

UVolume sandCMixture volume U

= =

s s sw v

m m m

USand massC CMixture mass U

ρ ρρ ρ

= = =

//

s svd

m m

U time QCU time Q

= =


Concentrations (2/3)

• Ratio between Cvd & Cv follows from:

• Ratio between Cw & Cv

s s v vd svd

m m v m

Q v AC C vCQ v A C v

= = ⇒ =

s w sw v

m v m

CC CC

ρ ρρ ρ

= ⇒ =


Concentrations (2/3)

•In horizontal transport vs < vm → slip•In vertical transport vs ≈ vm the difference is the settling velocity

vd s

v m

C vC v

=


Mixture Volumetric Densities Concentration

• Massmixture=massliquid+masssolids

(1 )

"Note U is volume"

sm m f f s s v

m

m f v s v

m fv

s f

UU U U with CU

C C

C

ρ ρ ρ

ρ ρ ρ

ρ ρρ ρ

= + =

= − +

−=

−

⇔


Volume changes

• When removing soil the insitu density will change; mostly from a dense to a loose state⇒Increase in porosity; f.I. From 40 to 50%⇒Porosity n is ratio pore volume over total volume⇒Condition: V1(1-n1)=V2(1-n2)

Examples:⇒Sand; n1=0.4 and n2=.5 gives V2/ V1=0.6/0.5=1.2⇒Rock; n1=0 and n2=.4 gives V2/ V1=1/0.6=1.7


Losses

Every dredging process can have losses, called spillage• More excavated than picked up by the flow or bucket• Non removed loads in TSHD’s, particular when the

loads is pumped ashore or rainbowed.• Unstable slopes after dredging (plain suction dredgers)• In accurate placing of material• Losses due to current and waves


Excavating production

Mechanically Hydraulically


Mechanical excavation Specific Energy Concept (SPE)

Energy required to excavated 1m3 of soilDimension is Joule/m3

or per unit of time J/s/m3/s=W/m3/s,

That equals a power over production

powerSPE power SPE productionproduction

= ⇒ = ×


Mechanical Excavating


Hydraulic excavationMomentum of flow

A reasonable assumption is that the jet- production is linear with the total momentum flux of the jet system independent of the trail speed.

M I Qu Q p P

psand w ww

wpower

pressure

= ⋅ = ⋅ = ⋅ =α αρ αρρ

α ρ2 2

( )( )

1

1sand dredged

sand sand sand dredged sand

Q n Q

M Q n Qρ ρ

= −

= = −


Excavation by dragheads is hydraulically


Water injection dredger


Transport production

Mechanicallyship/barge conveyor

Hydraulicallypipeline


Mechanical transport

• Trailing suction hopper dredger• Barges

• Be aware of the effective load, because the unloading is not always 100%


Transport by barges


By Trailing Suction Hopper Dredgers


Hydraulic transportPump-pipeline system

mixture

water

mixture

water

1

2

3

4

Flow [m3/s]

Pressure [kPa]


Hydraulic transport


Methods of deposing (1/2)


Methods of deposing (2/2)


disposing


Rainbowing


Mechanical Assistance


Design examples


TSHD


Example 1

Design a Trailing Suction Hopper Dredger that can dredge yearly 5 Mm3 coarse sand & gravel at 75 nautical miles from a port.

The dredger works 5 days at 24 hoursBunkers will be taken in the weekendOverhaul 2 weeksWeather delays 3 weeksWorkability 95%Christmas 1 week


Designing TSHD

Main dimensions


Propulsion

General layout


Main dimensions

Working hours

Delays

Effficeincy

Yearly production

Hopper capacity and Payload

Soil type

Dead weight

Displacement

Main dimensions ship LxBxT

Bock coefficient

Overflow losses


Cycle time

First estimate of dredge cycle:Sailing to the dredging area: 75/15=3.0 hrLoading =1.5Sailing to the unloading area: =3.0Unloading =1.5Total =9.0 hr


Required load/trip

Available hours: (52-6)x5x24=5520Effective hours: 0.95x 5520=5244Number of trips per year: 5244/9= 582Required volume per trip: 5,000,000/582=8591 m3

In coarse sand & gravel max.filling hopper is 90%Required hopper volume: 8591/.9=9546 10000 m3

Density of sand & gravel in hopper 2000 kg/m3

PayLoad is: 8600x2=17200 tonHopper density: load/volume=1.72 t/m3.

⇒


Deadweight & lightweight

Crew and their possessions, consumer goods, spare parts, and ballast water and payload.

Deadweight=1.05 x payload Light weight as function of deadweight

y = -3E-06x2 + 0.5586xR2 = 0.9607

0

5,000

10,000

15,000

20,000

25,000

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000

Deadweight [t[

Ligh

t wei

ght [

t]


Displacement

y = 0.6827xR2 = 0.9929

y = 0.3173xR2 = 0.9622

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

0 20,000 40,000 60,000 80,000 100,000

Displacement [t]

Wei

ght [

t]

G Light weightDead weight


Block coefficient

CLBTb =∇

T

BL

Cb =


Ship Numbers

Ships Numbers

012345678

1965 1970 1975 1980 1985 1990 1995 2000

Year of Construction

L/B

, B/H

, B/T

L/BB/HB/T



Cvd

Required pump capacity

Overflow losses

Overflowlosses equal as assumed?

Yes

Main DimensionsNo

Pumps and pipelines


Pump capacities

• 10000 m3 in 90 min=1.85 m3/s including pores or 1.85x0.6=1.11 m3/s excluding pores

• Assume Cvd=0.2 → capacity Q=1.11/0.25=5.55 m3/s or per suction tube 2.8 m3/s

• Critical velocity for course sand is 5 m/s, so pipe diameter is 0.85 m → 0.85 m

• In coarse sand and gravel there are no overflow losses to account for.


Excavation process


Calculated the required jet pressure

•Sand mass follows from production

•Momentum follows from:With α=0.1

2 2 powerw jet w w

w pressure

PpI Q u Qp

α αρ αρ α ρρ

⋅ = ⋅ = ⋅ =

( )( )

1

1sand dredged

sand sand sand dredged sand

Q n Q

M Q n Qρ ρ

= −

= = −

sandM Iα= ⋅


Relation between Qmix, Qjet and Qerosion

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Qjet / Qmixture

Cvd/(1-n)=0Cvd/(1-n)=0.2Cvd/(1-n)=0.4Cvd/(1-n)=0.6Cvd/(1-n)=0.8


Pumps and pipelines

Submerged pump required?

Yes

No

Depth of submerged pump

Determine headlossesand power(s)

Choose pump(s)

Determine jet capacityand pressure


k

hs

Hρwater

ρmixture

Non cohesive material

Submerged pump required?

( )ρ ρ ξ ρ ρ ξ ρwater mixture z mixture mixture mixturegH Vac gh v g H k v+ = + = − +12

12

2 2


Hydraulic transport

• From seabed into the hopper• From hopper to the shore

• Mostly empirical relations (Matousek)

• For gravel dredgers this is mostly be mechanically


Pump characteristics


Propulsion

For dredging & sailing

Bow trust power

Total Installed

Power balance

General layout


Propulsion power

y = 0.4641x - 510.11R2 = 0.8741

0

5000

10000

15000

20000

25000

0 10000 20000 30000 40000 50000

Displacement [t]

Pp [k

W]


Bow trust power

Bow trust power

y = 0.1758x - 19.495R2 = 0.8036

0500

1,0001,5002,0002,5003,0003,500

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Propulsion power during trailing [kW]

Bow

trus

t pow

er [k


General Arrangement


General Arrangement of gravel dredger


Simple general arrangment

Lhopper = 62 mLship= 165 m

Bship= 33

Bhopper= 25 m

Tship= 13,2

Hhopper= 13

Hship= 14,7

G H ICA KFB ED MJ L N

See figureB7.1


The Cutter Suction Dredger


Example 2

Design a cutter dredger that can dredge dredge yearly 5 Mm3 rock with a unconfined compressive strength of 5 MPa. The tensile strength is 1 Mpa

The dredgers have to work 168 hrs a week.Yearly overhaul 4 weeksChristmas leave 1 weekGeneral delays 10%Dredging delays 20SPE~qu


Required cut production

Available hours (52-5)x168=7896Non dredging hours:0.3x7896=2369Dredging hours 7896-2369=5527Estimated spillage 25%Required hourly output: 1.25x5000000/5527=±1130 m3.Qdredged=1130/3600=0.314 m3/sTime losses due to stepping, spud changes 15%Qcut=0.31/0.85=0.37 m3/sSPE=5MJ/m3. Required mean cutter power 0.37x5=1.85 MW


Cutter head productions c.q. Spillage

•The rotational speed of the cutter head causes spillage.

•The productivity c.q. spillage depends on the ratio:

•For sand the productivity is:

•For rock the productivity is much lower

32.5 pumpr

cutter

QP

Rω≈


Cutter head productions c.q. Spillage

RELATIVE PRODUCTION

01020304050607080

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Flow number [-]

Pr [%

]

Gravel 10 mm Gravel 15 mm Ladder 25 deg. Sand


Cutter head production processin rock or gravel


Cutter head dimensions for rock with 25 % spillage

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Cutter diameter [m]

Cut

ter

head

spe

ed [r

pm]

0

2

4

6

8

10

12

Pum

p ca

paci

ty [m

3/s]

Vm=2.67 m/s Vm=3.5 m/s Vm=5 m/s

Capacities

cutter head speeds


Pump capacity and concentrations

Qdredged=0.314 m3/sQmixture=3.5 m3/s

( )1dredgedsandvd

mixture mixture

Q nQCQ Q

−= =


Pumping distances and installed pump power

• Knowledge of hydraulic losses can be found in the lecture notes of Matousek c.q. Talmon

• Knowledge of dredge pump can be found on our website and is downloadable.


Lightweight of pontoon

y = 0.3485xR2 = 0.925

0

2,000

4,000

6,000

8,000

10,000

0 5,000 10,000 15,000 20,000 25,000

Total installed power [kW]

Lig

ht w

eigh

t [t]


Pontoon dimensions (1/2)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Light weight [t]

L/B

& B

/T

L/B B/T


Pontoon dimensions (2/2)

y = 0.4664xR2 = 0.9597

01,000

2,0003,000

4,0005,000

6,0007,000

8,0009,000

10,000

0 5,000 10,000 15,000 20,000 25,000

BLD [m3]

Lig

ht w

eigh

t [t]


Simple Plan


Backhoe dredger


Example 3

• A backhoe dredger have to dredge 500 m3/in fine sand with a SPE of .7MJ/m3.

• Calculated the Bucket size and cylinder forces


Fill Degree & Bulk factor

Soil type Filling degree Bulking factor Soft clay 1.5 1.1 Hard clay 1.1 1.3 Sand & Gravel 1 1.05 Rock; well blasted 0.7 1.5 Rock, unblasted 0.5 1.7


Dredge Cycle

• Cycle times of the bucket depends on the dredging depth and soil type, but are in the order between 20 and 40 seconds.

• The cycle consists of:• • Digging• • Lifting and swinging• • Dumping• • Swinging and lowering• • Positioning.


Crane weight versus bucket size for soft soil

y = -7E-06x2 + 0.0494x + 1.5486R2 = 0.9778

0

5

10

15

20

25

30

0 100 200 300 400 500 600

Crane weight [ton]

Buc

ket s

ize

[m3]

bucket2


Required power

Liebherr Excavators

y = 4.4679xR2 = 0.9936

0

500

1000

1500

2000

2500

0 100 200 300 400 500 600

Crane weight [tons]

Pow

er [k

W]


Relation for existing dredgers

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

0.00 5.00 10.00 15.00 20.00

Bucket size [m3]

Inst

alle

d po

wer

[kW

]

Rock buckets


Light weight pontoon

0

200

400

600

800

1000

1200

1400

1600

1800

0 250 500 750 1,000 1,250

Total installed power [kW]

Lig

ht w

eigh

t [t]


Pontoon volume

y = 0.4713xR2 = 0.6122

0200

400600

8001000

12001400

16001800

0 500 1000 1500 2000 2500 3000

LBD [m3]

Lig

ht w

eigh

t [t]


Ships numbers for BHD

0.001.002.003.004.005.006.007.008.009.00

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800

Light weight [t]

L/B

& B

/t

L/B B/T


Simple Plan


Newer ideas can be discussed


The shallow draught TSHD

design dredging design

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