seminar on dredgers
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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
THANK YOU
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