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How to Design Dredger

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Designing Dredging Equipment OE4671/WB3408

Prof.ir. W.J.Vlasblm

12 September 2006

1

Section Dredging engineering

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!

September 12, 2006

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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)

September 12, 2006

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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 DredgerSeptember 12, 2006 4

Assignments for mechanical dredgers The backhoe dredger The grab dredger Bucket ladder dredger

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Trailing Suction Hopper Dredger

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Cutter Dredger

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Plain Suction Dredger

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Plain Suction Dredger

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Dustpan Dredger

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The Environmental Dredger

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The Backhoe Dredger

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The Grab or Clamshell Dredger

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Bucket Ladder Dredger

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

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Designing Dredging EquipmentProductions Dredging installation Propulsion (optional)

Prime movers (main engines) General layoutSeptember 12, 2006 16

Design productionsYearly production Soil type Working hours Delays Effficeincy Excavating production Losses Transport production Deposing Losses Situ productionSeptember 12, 2006 17

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/monthSeptember 12, 2006 18

Design Basis (2) Small scale: Wall speed of a breach Pump Cutter head Bucket ladder dredger Backhoe dredger Grab dredgerSeptember 12, 2006

mm/s output in m3/s Excavated output in m3/s Buckets/min Bucket volume/cycle Grab volume/cycle19

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 conditionsSeptember 12, 2006 20

Concentration (1/3) By volume

Us Volume sand = Cv = Mixture volume U m

By weight

sU s s Sand mass = = Cw = Cv Mixture mass mU m mU s / time Qs Cvd = = U m / time Qm21

Delivered

September 12, 2006

Concentrations (2/3)Ratio between Cvd & Cv follows from:

Qs vs ACv Cvd vs Cvd = = = Qm vm A Cv vm Ratio between Cw & Cv

s Cw s = Cw = Cv Cv m m

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Concentrations (2/3)

Cvd vs = Cv vmIn horizontal transport vs < vm slip In vertical transport vs vm the difference is the settling velocity

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Mixture Densities

Volumetric Concentration

Massmixture=massliquid+masssolids

Us mU m = f U f + sU s with Cv = Um

m = f (1 Cv ) + s Cv m f Cv = s fSeptember 12, 2006

" Note U is volume"24

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=1.7September 12, 2006 25

LossesEvery dredging process can have losses, called spillage More excavated than picked up by the flow or bucket Non removed loads in TSHDs, 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

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Excavating production

Mechanically

Hydraulically

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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

power SPE = power = SPE production productionSeptember 12, 2006 28

Mechanical Excavating

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Mechanical Excavating

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Mechanical Excavating

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Hydraulic excavation Momentum of flowA reasonable assumption is that the jet- production is linear with the total momentum flux of the jet system independent of the trail speed.

Qsand = (1 n ) Qdredged M sand = Qsand sand = (1 n ) Qdredged sand

M sand = I = w Qu = w QSeptember 12, 2006

2p

w

= 2 w

Ppower p pressure32

Excavation by dragheads is hydraulically

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Water injection dredger

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Transport production

Mechanically ship/barge conveyor

Hydraulically pipeline

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Mechanical transport Trailing suction hopper dredger Barges Be aware of the effective load, because the unloading is not always 100%

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Transport by barges

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By Trailing Suction Hopper Dredgers

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Pressure [kPa]

Hydraulic transport Pump-pipeline systemmixture 3 water 4 mixture 1 2

water

Flow [m3/s]September 12, 2006 39

Hydraulic transport

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Methods of deposing (1/2)

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Methods of deposing (2/2)

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disposing

September 12, 2006

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Rainbowing

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Mechanical Assistance

September 12, 2006

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Design examples

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TSHD

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Example 1Design 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 hours Bunkers will be taken in the weekend Overhaul 2 weeks Weather delays 3 weeks Workability 95% Christmas 1 weekSeptember 12, 2006 48

Designing TSHDMain dimensions

Dredging installation Propulsion

General layoutSeptember 12, 2006 49

Main dimensionsYearly production Soil type Overflow losses Hopper capacity and Payload Dead weight Displacement Bock coefficient Main dimensions ship LxBxTSeptember 12, 2006 50

Working hours Delays Effficeincy

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

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Required load/tripAvailable hours: (52-6)x5x24=5520 Effective hours: 0.95x 5520=5244 Number of trips per year: 5244/9= 582 Required 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 ton Hopper density: load/volume=1.72 t/m3.September 12, 2006 52

Deadweight & lightweightCrew and their possessions, consumer goods, spare parts, and ballast water and payload. Deadweight=1.05 x payloadLight weight as function of deadweight25,000 Light weight [t] 20,000 15,000 10,000 5,000 0 0 10,000 20,000 30,000 40,000 50,000 Deadweight [t[September 12, 2006 53

y = -3E-06x 2 + 0.5586x R2 = 0.9607

60,000 70,000

Displacement70,000 60,000 50,000 Weight [t] 40,000 30,000 20,000 10,000 0 0 20,000 40,000 60,000 80,000 100,000 Displacement [t]September 12, 2006 54

y = 0.6827x R2 = 0.9929 G Light weight Dead weight y = 0.3173x R2 = 0.9622

Block coefficient

T L B

Cb = LBTSeptember 12, 2006 55

Ship NumbersShips Numbers8 7 6 5 4 3 2 1 0 1965

L/B, B/H, B/T

L/B B/H B/T

1970

1975

1980

1985

1990

1995

2000

Year of Construction

September 12, 2006

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Dredging installation CvdRequired pump capacity

Overflow losses

No Main Dimensions

Overflow losses equal as assumed?

Yes Pumps and pipelinesSeptember 12, 2006 57

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.September 12, 2006 58

Excavation process

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Calculated the required jet pressureSand mass follows from production

Qsand = (1 n ) Qdredged M sand = Qsand sand = (1 n ) Qdredged sandMomentum fol