SEMINAR ON DREDGERS

PREPAIRED BY JOSE PRASOBH .M.J

Migration towards the coastal zones

Uses of Dredgers

Capital Preparatory Maintenance Land reclamation Beach nourishment Harvesting materials Seabed mining Construction materials Anti- eutrophication Contaminant remediation Removing trash and debris

Types of Dredging Vessels

Mechanical a)Bucketb)Grabc)Backhoe/Dipperd)Water injection

Hydraulic Continuously &

Discontinuously Conveyor belts

&Pipeline transport Fixed &Moving Grab & ship ,car

transport

Suction a)Trailing suctionb)Plain suction c)Cutter suction d)Auger suctione)Jet-liftf)Air-lift

Types of Dredging Vessels

Pneumatic Bed leveler Krabbelaar Snag boat Amphibious Submercible Fishing dredgers Police drag

TYPES OF DREDGERS

Bucket Ladder Dredger Grab Dredger

Dipper and Back hoe Dredge Plain Suction Dredger

Cutter Suction Dredge

Trailing Suction Hopper Dredge

Back hoe Dredger

Bucket ladder dredger

Grab dredger Grab hopper dredger

Water injection dredger Dustpan

dredger

Terminology

Production/production cycle

Scows Dewatering Spud Swing over Pawl length Cut width Loading time

Turning time Sailing time loaded Breaching Rain bowing Dynopt

Bucket Ladder Dredger

Components of a Bucket Ladder Dredge Working method of a BLD

Videos

spud_carriage_stepping_forward.exe hopper_simulator.avi

Self propelled BLD Caterpillar Track Bucket Chain

TIP vs LW TIP vs BC

Types of Buckets Lay Out of a BLD

High Lights Bucket size varies from 30

to 1200lit

6wire positioning

Head line >1000m

Swing speed varies with soil condition

Maximum dredging depth30m & minimum is 6m or more

Shallow water dredging not recommended

Not used in offshore conditions

Extensively used before 2nd world war

Capacity expressed in bucket volume

Capacity varies between 50to 1200lit

Rock bucket (egg shaped)type and soft silt type(mud buckets)

Small & heavy rock buckets

High Lights

Diesel, Diesel-electric &Diesel-hydraulic type of power system

Stability is critical when ladder is raised

During towing chain &ladder is stowed on deck

In pan buckets ,filling rate is varies with angle of the bucket ladder

L/B varies slowly with Light weight

B/T varies between 3.5to5.5

No relationship between TIP and Light weight

Prod. capacity is increased by Bucket capacity

Prod. capacity normally 50,000 m3/week

High Lights

Q=Prod. capacity, V b=Bucket capacity,v b=Bucket speed

Q=V b Xv b

Q c=Cutting prod. , h=Bank height , s=Step size, v s=Hauling speed ,

Qc = h x s x v s m3/s F b= Filling rate = Soil volume/V b , B=Bulking Factor( insitu vol. of

bucket/bucket vol.),Q c=(Q x F b)/B For design , F b=1, SPE=Specific energy ,P cutt = Cutting power,

Pcutt=Q c x SPE SPE=Energy to cut 1m3or power to cut 1m3/s P lift=Power to lift sand &water,

P lift=Q cg[(r s-r w)H b w+(r s+(1-F b s)r w)H u w]F b s=F b/B, g= Acc.due to gravity, H b w=Lift height below water ,Huw

=Lift height above water , r s=Density of sand , r w=Density of water

P fr=F(W,v),P fr=Power to overcome friction in ladder ,rollers , tumblers taken from existing dredgers .

Total Power P tot=P cut+P lift+P fr ,P tot< TIP

GRAB or CLAMSHELL DREDGER

Grab Dredger

Self propelled Grab Hopper Dredger

Grab & Principle of Closing &Hoisting Operation

Orange Peel Grab Largest Grab (200m3) Clamshell

Grab

High Lights

Dredgers are Moored by Anchors or by Poles(Spuds)(2 fixed spuds and 1 walking spud at aft)

Boom Type or Overhead Cranes are used Capacity depends on Volume of Grab Grab size varies from 1m3 to 200m3

Grab control is by Hoisting or Hydraulic Taught Wire is connected between a

Clamshell & Winch to avoid Spinning Dredging depth depends on the Length of

Wire

High Lights

Dredging process is Discontinuous and Cyclic

a)Lowering of the Grab to bottom

b)Closing of the Grab by pulling the Hoisting Wire

c)Hoisting starts when Bucket is completely closed

d)Swinging to the Barge or Hopper

e)Lowering the filled Bucket into the Barge /Hopper

f)Opening the Bucket by releasing the Closing Wire

Orange Peel Grab:- Used for the removal of large rock/irregular pieces

Cactus Bucket:-Used to remove fine coarse and sand

Main winch drive:-mainly

electrical (dc motor) and have 4 quadrant system

TIP α (1/Light Ship Weight)

Special care to be taken for eccentric loads and free surface effect

High Lights

Vg=Grab Volume, w=Average Efficiency, T c=Closing Time, F=Average Fill rate of the

Grab, SPE=Specific Energy, Pc=Closing Power

required for the Grab, F c=Closing Force, v c=Closing Speed of the Closing Wire

Pc=(V g x SPE)/(F X w x T c)= F c x v c

F c <Force Available from Closing Winch

BACKHOE DREDGER

Back Hoe Dredger

GA of a BHD

Effective Dredging Area

BHD cut width & Depth

BUCKET CAPACITIES

BHD Major components

High Lights BHD is a stationary one resting

on 3 spuds (2 on P&S at the for’d and a movable at aft)

Hydraulic systems of 2 types namely backhoe & front shovel

Front shovel method used when water depth is restricted

Limited dredging depth 25m is maximum

Dredging area depends on swing angle

Small step → Large width Large step → Small width Crane is fitted on a Turning

table Engine room and

Accommodation at the aft

Back hoe’s are used for soils like, Clay , Soft rock ,Blasted rock , Large stones.

Struck capacity:-water volume or max. amount of water that can hold by the bucket when bucket rim is at horizontal

Heaped capacity:-[SAE volume](society of automotive engineers), water vol. with extra soil at embankment slope of 1:1

Heaped capacity:-[CECE volume](committee of European Construction Equipment)water vol. with extra soil when embankment slope is at 1:2

High Lights

TIP is varies with Light Weight

No relation with Bucket size and Light Weight

F cv c x SPE=Qs=V

b/td =(d l a x step x W b)/t d

if and only if v c x t e<

cylinder lengthF c=Cylinder Force for

Hard Materials

V c =Speed

V b=Bucket Capacity

t d=Digging Time

d l a =Thickness Layer

step=Step Size W b=Width of Bucket

t e=Excavating Time

Plain

• Standard Plain Suction Dredger :-Material Discharged via pipeline to reclamation area

• Barge Loading Suction Dredger:-material loaded to a alongside barge with a diffuser

• Deep Suction Dredger:-Equipped with an additional underwater pump when depth exceeds 30m

• Dustpan Dredger:-it has a wide suction mouth perpendicular to suction pipe

Cutter•STATIONARY •PROPULSIVE

Trailer Hopper •STAIONARY•PROPULSIVE

PLAIN SUCTION DREDGERS

BREACHING PROCESSDilatancy & Contractancy

Breaching process in practice

High Lights Suction dredgers are used

in non cohesive soils (sand to fine gravels)

Jet water is used to assist Breaching process

Breaching is a process which makes the mixture of water and sand at the suction mouth

Breaching is a process of soil shearing on a slope caused by local instabilities or by erosion of the density current running along the slope to the suction mouth

PARAMETERS AFFECTS THE BREACHING

a)Soil Conditions of the slope b)Permeabilityc)Relative density L/B varies between 3& 8 B/T varies between 7 &3.5 Hoisting and lowering of

suction tube is performed by a gantry crane

Suction pump is normally kept at aft

Breaching starts with forming a wall sided(vertical slopes) cylindrical hole

High Lights

Vwall is the velocity slope moves inside the hole

Dilatancy :-An increase in pore volume due to shearing of the sand causing the porosity increases ,in turn causes pore pressure to decrease

Due to low pressure , Slopes are steeper than angle of internal friction

Contractancy:-A decrease in pore volume due to increase in pressure and less porosity

Actual situation is complicated due to irregular soil

Factors Affecting Design

a)Production Capacityb)Suction Depthc)Transport Distancesd)Particle Size of Sand

High Lights

Qmix. = Qsand (1-n)/Cvd

Qmix= Mixture Capacity, Qsand= Production Capacity m3/s , Cvd=Delivered Concentration

n= Porosity of the Sand Common values for Cvd are 0.25 to 0.3 when

n=0.4Vacuum Formulae:- To find out the Density

that can be lifted by the Dredge Pump(Ref:SDC 51-26)

Cutter Suction Dredger

• Cutter Suction Dredger is a stationary Dredger with Cutter Head• Dredger Swings around the Spud called Working spud •Working spud is moved by a Carriage and Hydraulic cylinder •Spud stays in the same position and dredger makes STEPS of concentric circles to a maximum position of the cylinder pushed • Cutter Power Ranges from 50kW to 5000kW• Standard CSD and Custom Built CSD•Over Cutting :-Rotation in the direction of Swing Movement•Under Cutting :-Rotation in the opposite direction of Swing Movement•In Overcutting mode the cutter Head Drags the dredger in the direction of Pulling Winch

Cutting Modes

Cutter suction dredger

Cutter Suction Dredger

ALBZEM

Cutter Arm and Teeth ,Adapters

LOCKING PIN,CHISEL

SELF PROPELLED MEGA CUTTER

CUT WAY MODEL

HIGH LIGHTS

Procedure of Stepping Dredger is moved to a

small angle from CL

Aux. spud is lowered ,work spud is lifted

Dredger is moved by pivoting around the aux. spud to an angle on the opposite side of the CL

Spuds are at a new vertical position

Overlapping of cutting areas is the only problem

Design Aspects Minimum dredging

width Maximum dredging

width ,B=2Lsin45 Maximum swing

angle of the side winch is 450

L= Distance between the spud and cutter head

L depends upon the water depth and position of spud pole

High Lights Heavy duty cutter

suction dredgers can dredge all kind of soil

Dredging depths are limited to 30m

Light ship weight α TIP

Design Factorsa)Required production

capacityb)Dredging depthc)Transport distancesd)Particle size of sand

Cutter Head Selection Factors:-

a)Cutter Powerb)Side Winch Forcec)Side Winch SpeedPcutter = Qcutting/SPE

Where Qcutter=Sxhxvswing

Pcutter=Effective power to the cutter head

Qcutter=Production cut by the cutter head

S=Step sizeH=cut heightVswing=Swing Speed

TRAILER HOPPER SUCTION DREDGERS

THSD ‘s are Non stationary Dredgers Normal working cycle divided into 4 Basic

Functions

a)Winning material from below the water surface

b)Containment of the materialc)Transporting the material to another locationd)Discharging the materiale)Sailing back empty to the winning areaf)Repeating this cycle endlesslyWinning means the material extracted from

the seabed by a suction dredger

Trailing Suction Hopper Dredger

BREYDEL

HOPPER OF A LOADED TSHD SIMULATION IN DREDGING CONSOL

DREDGE CONSOLE DRAG HEAD DISCHARGE PUMP

High Lights Suction tube is

lowered at the side of ship using the gantries ,steel connection lines and winches

Suction head is kept near the bottom but not touching

Vacuum is created by suction pump and the mixture is sucked into the hopper tank

During suction , the ship is trailing

TSHD ‘s are used in all soils even up to water depths 40m

Production capacity is expressed in ,

a)Total time taken to complete one dredging cycle or

b)Volume of dredged material in hopper

High Lights

Ttotal= Tfill+Tsail to

discahrge+Tdischarge+Tsail

to win

Tfill=time to fill hopper with dredging spoil

Tsail to discahrge=time to sail to discharge area

Tdischarge=time to dump the contents of hopper

Tsail to win =time to sail back to the material winning area

Volume of dredged material =max. available hopper volume (Hvol) or max. allowable payload (L)

Time method is used when ρmaterial<(L/Hvol), ex : silt or soft clay

Volume method is used when ρmaterial>(L/Hvol), ex: sand

High Lights

Stability :Transverse and Longitudinal stability is evaluated for 4 conditions

1.Departure Fully Loaded (100%Cargo+100%Stores Onboard)

2.Arrival Fully Loaded(100%Cargo+10%Stores Onboard)3.Departure Ballast Conditions (0%Cargo+100%Stores

Onboard)4.Arrival Ballast Condition(0%Cargo+10%Stores Onboard)Intermediate conditions with 20%,40% hopper filling can

be found Closing the hopper door is done after the discharge and

pumping the seawater into hopper un till required draft &trim is achieved

Free Surface Effect on stability due to spoil water is treated

High Lights

Resistance :CFD is done for developing the flow lines and resistance is calculated

Propulsion : a)Single fixed or cpp

with or without nozzle b)Twin fixed or cpp with

or without nozzlec)Bulbous bow with

gondola mounted propellers (on Pallieter class TSHD)

Type of Discharge Bottom doors/conical

bottom valves at the CL of the hopper or on both P&S sides

Pre dumping doors to get the required draft at shallow water via partial discharge

Shore discharging via dredge pumps

Split hopper system (hopper can open full length of ship)

Earth moving equipment (grabs)with conveyor belts

High Lights

Hopper Layout1.Straight side (to

prevent the adhesion of spoil to walls )

2.Slanted centre cage in triangular form to avoid clogging at discharge hole

3.Overflows to allow water and spoil water mixture to discharge from upper region of hopper while loading

Automatic control systems

Integrated control systems

Dynamic positioning systems

Dynamic tracking systems –to track the position of suction head

Automatic visor control Automatic draught

control Automatic flow control

High Lights Drag head :-1st

mechanical contact of a TSHD with bottom

It is designed for optimal dredge efficiency

Dredge pump:-Most challenging and robust , reliable part of a dredger

a)Economy pumpsb)Heavy duty pumpsc)High efficiency

pumps(92%)d)Custom built pumps

Long working life , less wear, replaceable parts, easy disassembly ,high performance, low net positive suction head requirement(NPSHR)

Single walled pumps Double walled pumps-

harder inner wall , highly safe

Suction pipe :Available even up to 1000mm dia. in mega TSHD

Swell compensator:-It is a spring device integrated with hoisting wire to keep the drag head in touch with soil.

GONDOLA MOUNTED PROPELLERS

COMPARISON OF STERNS

BULBOUS BOW

NAVIGATION CONSLOE

GEOLOGICAL PROFILE

4 PAHSES OF CUTTING

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