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AN OVERVIEW OF DREDGING EQUIPMENT &

OPERATIONS

November 2019

Donald F. Hayes, PhD, PE, BCEE, F. ASCEEnvironmental Laboratory

US Army Engineer Research and Development Center

OUTLINEOverview of DredgingMechanical dredges

• Focus on clamshell bucket dredges• Characteristics of mechanically dredged sediments

Hydraulic Dredges• Cutterhead dredges• Hopper dredges• Other hydraulic dredge types• Characteristics of hydraulically dredged sediments

DREDGING = (RE)MOVINGSEDIMENTSWhy do we (re)move sediments?

Mining: we want to use the resource

Beach Restoration: we want to use the resource

Flood Control: provide more hydraulic capacity

Irrigation: provide more hydraulic capacity

Navigation: provide more depth for vessel passage

Environmental Remediation: remove contaminated sediments from the environment

DREDGING DEFINITIONSDredging - Process by which sediments are removed from the bottom

of streams, lakes, and coastal waters, transported via ship, barge, or pipeline, and discharged as dredged material to land or water. (Corps of Engineers)

Dredge - a machine for removing earth usually by buckets on an endless chain or suction tube. (Webster)

Sediment - material deposited by water, wind or glaciers; material existing on the bottom of a water body. (Palermo)

Dredged material - sediment that has been excavated by a dredge and has been or is being transported to a placement site.

Maintenance dredging - dredging that involves the removal of sediments that have accumulated since a previous dredging operation.

New work dredging - dredging that involves the removal of materials that have not been previously dredged. (Also called capital dredging)

DREDGING AND INFRASTRUCTURE

What would our society be without dredging?

Aggregate (sand, gravel, other) used for construction industry

Marine-related activities a $360B/year industry (2013)

13% of our population lives in areas subject to flooding along rivers

Over 14% of our agricultural production relies on irrigation

53% of US Imports and 38% of US Exports are by waterborne vessels ($1.6T/year)

Millions of cubic yards of contaminated sediments are removed from US water bodies each year

SEDIMENT CHARACTERISTICS Mining and beach restoration target “suitable

sediments” Dredging for flood control, irrigation or navigation

removes whichever sediments are “in the way”, regardless of their characteristics

Dredging for environmental remediation specifically targets “contaminated sediments”

SEDIMENT CHARACTERISTICS Sediment characteristics in a water body depend

on waterbody and its watershedo Mountain streams – gravelo Ocean waters – sando Lakes and rivers downstream from mountains - sando Rivers and streams through flatland valleys – silts and clays

“The rougher the terrain, the coarser the sediments”

The more developed (urban/ agriculture) the watershed, the more anthropogenic constituents will be present in sediments

DREDGING – UNIQUE CONSTRUCTION Typically, large heavy equipment Working surface is below water, so the operator

(and observers) have no direct visibility Floating equipment moves erratically in horizontal

and vertical directions Challenging safety environment Target is sediment of varying densities, but the

addition of water (sometimes, lots of water!) unavoidable

Sediment is heavy, difficulty to handle, and costly to transport

Dredged materials are a mix ofo Cobbleso Pebbleso Sando Silto Clayo Organic material

Mixture varies based upon dredging location

Sediment placement and management depend on the physical characteristics resulting from the mix of sediment types

WHAT ARE WE DREDGING?

SEDIMENT DENSITY DESCRIPTIONS

Sediment Density Descriptions for Saturated Conditions*Description Physical Representation Units Equation Settled

SandSoft

Muck

Dry Bulk Density, dMass of solids per unit

volume

kg/m3

(or lb/ft3)

1600 350

Wet Bulk Density, w

Total mass per unit volume

kg/m3 or lb/ft3 2000 1200

Solids Content, S Mass of solids as percent of total mass % 80% 30%

Water or Moisture Content,

Mass of water relative to mass of solids % 20% 320%

Void Ratio, e Volume of water relative to volume of solids unitless 0.6 8.6

Porosity, n Volume of water relative to total volume % 0.2 0.9

*Gs (specific gravity of the solids) ranges from ~ 2.30 to 2.75, with 2.65 being a commonly used estimate; sediments are typically saturated during all dredging processes.

DREDGE TYPES

Dredging = (re)moving sediments, so…dredges are categorized by “how they move sediment”

I. Hydraulic dredges move sediments by means of slurry and pumps

II. Mechanical dredges move sediments using mechanical methods (e.g. various types of buckets)

EXAMPLE MECHANICAL DREDGES

Bucket-ladder dredge - picks up sediment using many circulating buckets attached to a wheel or chain. Often used in mining operations.

Backhoe/dipper dredge – excavator that uses an open-faced bucket to pick up sediment. Fixed arm limits digging depth. Open bucket face can reduce efficiency and increase environmental impact.

Bed leveler - bar or blade pulled behind any suitable vessel. Operates similar to a (small) bulldozer on land. Can effectively reduce localized sediment waves, increasing the time between dredging operations.

Clamshell dredge - picks up sediment with a single “clamshell” bucket, operated by a crane or fixed-arm excavator. Crane-style is very versatile. Can be used in shallow or deep water, with deep or shallow cuts, and in close proximity to structures. Fixed arm styles have more limited depth capability.

Photo courtesy of IADC

CLAMSHELL DREDGESClamshell dredgesoOften referred to as “grab dredgers” outside of the USoUse a clamshell-type bucket to remove subaqueous sediment

“Boom-type” excavator oCables (wire rope) control bucket position and operation. oBoom rotates around a vertical axis and can be raised or lowered to

shorten or extend its reach. The

Continuous dredging (save downtime), but production is cyclic, i.e. “one scoop at a time”

Significantly less water entrainment (compared to hydraulic transport) o Reduces transport and placement volumes o Simplifies water management requirements

TYPICAL CLAMSHELL DREDGE OPERATION

CLAMSHELL BUCKET OPERATIONS

COMMON BUCKET TYPES

Conventional Clamshell Buckets• Digging buckets• Flat-cut buckets• Hydraulic buckets

Environmental Buckets• Retrofit enclosed bucket• Cable-Arm® bucket• Positive-closure buckets

Bucket sizes• Vary from < 1 yd3 to > 100 yd3

• 5 yd3 to 30 yd3 common

FIXED-ARM DREDGES

BUCKET OPERATION

CLAMSHELL DREDGE ADVANTAGES

Mobilization less complex (expensive)Can operate successfully in o stiff sediments (with appropriate bucket)o tight areas

Equipment and operators readily availableReduced carrier water volume

Clamshell Dredge

CLAMSHELL DREDGE DISADVANTAGES

Lack of precision control in all 3 dimensions

Debris can prevent closure, causing loss of entire bucket

Irregular bottom shape (most buckets) Potential for residual “ridges” Potential for overfill and loss Sediment rehandling may be necessary

CLAMSHELL DREDGE OPERATION

DREDGE CYCLEVaries with:

• Water Depth• Disposal barge location• Dredging purpose

Vertical bucket speeds• Rise: 1 – 3 ft/sec (avg ~ 2 fps)• Fall: 2 – 5 ft/sec (avg ~ 4 fps)

Cycle times• Constant throughout project• 30 sec to > 4 min • ~ 1 minute typical

Ascent

GrabFall

Swing

Bucket descent Slew to discharge point (barge)

Closure/sediment grab Return to dredging position.

Bucket ascent

BUCKET FOOTPRINT

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

Sedi

mne

tVol

ume

(cy)

Open Bucket Area (ft^2)

"1 ft"

"2 ft"

"3 ft"

"4 ft"

"5 ft"

Avg. Penetration

IDEALIZED BUCKET PATTERN

10

13 Buckets

ACTUAL BUCKET PATTERN

20 Buckets

WHAT SIZE DREDGE/BUCKET?Excavator must be capable of handling the

sediment volume of a fully-loaded bucket plus the weight of the bucket and cablesoWet dredged material weighs ~ 1.5 ton/yd3.oBuckets range from 3 to 30 tons (some more!)

Bucket size selection:o Sediment thickness to be removed (on each grab)o Total sediment volume and desired project

durationo Available excavators and platformso Sediment management capacity

BUCKET DREDGE PRODUCTION Instantaneous Production (cy/hr) =

𝑃 = ∗ (%)

∗

Bucket fill varies with• Sediment thickness relative to bucket size• Bucket characteristics• Loss during ascent

Bulking Factor

𝐵𝐹 = ∗

∗ (%)

• Conventional buckets -

𝐵𝐹 = ∗

∗ (%)

varies with sediment characteristics and bucket type

SEDIMENT PROPERTIES CRITICAL

Sediment properties govern the effectiveness of the dredging operation, influence bucket selection, and impact dredge operation.

Sub-bottom profiler image from Redwood Harbor channel

WATER CAPTURE/BULKINGBulking Factor

Conventional buckets:

oTypical BF values: 1.1 to 1.2

Enclosed/Environmental buckets

oTypical BF values: 1.3 to 1.5

vent

MANAGING MECHANICALLY DREDGED SEDIMENTS

SEDIMENT MANAGEMENT

Sidecasting• Redistribute sediment within “casting” distance of dredging operation

•AdvantagesoInexpensive/High production

•DisadvantagesoMay return to the dredging siteoIncreased turbidityoSediments must meet EPA/COE Open Water

Disposal Criteria

ON-BOARD HOPPERPlace sediment in on-board hopper

• Dredge until hopper is filled• Entire dredge moves to placement site• Unload hopper• Return to dredging site

Photo courtesy of Dredge Brokers LLC

ADJACENT HOPPER BARGEDredge until hopper is filledBarge exchanged for empty barge when filled

HOPPER BARGE LOADINGHopper filled with sediment and water in the same

proportion as in bucket In some areas, “spillage” is not allowed for water

quality protection• Clarification often begins in barge, resulting in a

supernatant layer (free water)

If “overflow” allowed, dredge can replace free water with additional sediment • Loading continues until an “economic load” is

reached• Displaced supernatant discharges directly into the

water

BARGE OVERFLOW

BARGE OVERFLOW Increases economic load of the barge.

“Carrier water” forms supernatant layer on top of sediment in barge.

Replacing that water with sediment increases the load that the barge can carry.

Increased load in each barge = fewer trips to placement area = fewer barges required = cost savings

BUT….• Supernatant overflow includes suspended solids that may result in

unacceptable water quality impacts. This is especially true if the dredged sediment contains a significant fraction of fines.

• Overflow is restricted in many areas due to potential WQ impacts.• If contaminants are present in significant concentration, overflow is not

typically allowed.

MANAGING BARGED SEDIMENTS

THE BARGE IS FULL, WHAT DO YOU DO WITH THE SEDIMENT NOW?

BOTTOM DUMP BARGESHaul direct to open water disposal

siteAdvantages

•Eliminates rehandling•Can be inexpensive

Disadvantages•Transport to disposal location•Sediments must meet EPA/COE Open Water Disposal Criteria

MECHANICAL REHANDLINGMechanically remove sediment from

bargeNo water addition necessaryRequires multiple staging areas

• Landside• Disposal Facility

Requires trucking or rail transportNoise, leakage, and cost are a concern

HYDRAULIC UNLOADINGPump sediment directly to CDF/UplandAdvantages

• Direct discharge to CDF• Avoids truck/rail transport• Efficient process• Lower cost

Disadvantages• Large volumes of carrier water required

EXAMPLE CLAMSHELL

DREDGING PROJECT DESIGN

TYPICAL CHANNEL LAYOUT

TYPICAL CHANNEL X-SECTIONS

NOTE DISTORTION FROM HORIZONTAL AND VERTICAL SCALES

DREDGING PROJECT SCALES

OVERDREDGE~3-4 FT

Required depth

Required width

400 ft

PROJECT CHARACTERISTICS

Sediment volume: 100,000 CYAssumptions:o 5 cy Environmental Bucketo 1 minute cycle timeo Avg. Bucket Fill = 80%o Bulking factor = 1.4o No barge limitationso Effective time = 60% (downtime, repositioning, shift

changes, etc.)o 800 cy barges

PRODUCTION CALCULATIONS

Instantaneous Production P ∗ %

∗

Project Duration = 100,000 cy/(240 cy/hr*0.6) = 694 hrs = 29 days

Total volume in each bucket:• Sediment = 5 cy*0.8 = 4.0 cy/bucket• Sediment + Water = 1.4*4.0 cy = 5.6 cy/bucket

Time to fill 1 barge (no overflow)• 800 cy/(5.6 cy/bucket)/(60 buckets/hr) = 2.4 hrs• Contains 571 cy in situ sediment + 229 cy (46,000 gal) water

Total water generated for project = 46,000 gal/barge*100,000 cy/571 cy = 8 MG

OPERATIONAL DETAILS

Open area for a 5-cy bucketo2 ft cut = 67.5 ft2; Square bucket dimensions: 8.2 ft x 8.2 fto4 ft cut = 33.75 ft2; Square bucket dimensions: 5.8 ft x 5.8 ft

Assume each dredge pass can cover 50 ft of width and 2 ft of deptho8 parallel paths to cover 400 fto2 passes for complete removal

BARGE LOADS REQUIRED:o100,000 CY/(571 CY/BARGE) 175 BARGE LOADSoCARRIER WATER = 175 BARGES((46,000 gal/BARGE) 8 MG

HYDRAULIC DREDGES

Typical Equipment Basic Operations Produced Sediment Characteristics

HYDRAULIC DREDGES

….move sediments by making it into a slurry, and then pumping that sediment slurry.

A hopper dredge is a ship that sucks up the sediment slurry and holds that slurry in the ship (hopper) until it gets to its destination

A pipeline dredge sucks up the sediment slurry and pumps it through a pipeline directly to its destination

HOPPER DREDGES

Hopper dredges are self-propelled, fully contained vessels

Hydraulic excavation using dragheads(usually 2)

Pump slurry directly into hoppers

Vary greatly in sizeoSmall and super-simple/basic mining hopper

(inland river/lake)oSmall ocean-going hopper dredgeoVery large ocean-going hopper dredge

Manson Construction

HOPPER DREDGES - CHARACTERISTICS

Hydraulic dredge system, so it pumps the sediments Vessels are ships (self-propelled) Require sufficient water depth and access Can travel long distances Dredging and unloading intermittentDraghead requires straight lines, even

bottom, loose sedimentsHopper meant to hold sediments; excess

water is typically discharged as overflow

HOPPER DREDGES - OVERFLOW

Pumped slurries 10 to 20% solids by mass

oInitial filling of hopper includes a large portion of water

If solids settle rapidly (sand), dredges often pump past the initial filling to increase economic load

HOPPER DREDGES - UNLOADINGOnce hopper is filled with

sediment, dredging stops and vessel sails to unloading site

Typical unloading methodsoBottom dumpobottom doors osplit hull

oDirect pump outoPipe to placement siteoPump into open water (rainbowing)

PIPELINE DREDGES

Vary greatly in sizeo Truckable dredges (inland

rivers/lakes)o Small to medium-sized

dredges (larger rivers, coastal channels)

o Very large, ocean-stable dredges

Pipeline dredges are typically platforms with spuds for stability

Most move side-to-side or forward, pulled by cables attached to anchors

Hydraulic system pumps 10 to 20% solids (by weight) sediment slurry; contrast that with in situ solids concentrations of 30% (muck) to 80% (sand)

PIPELINE DREDGES - EXAMPLES

PIPELINE DREDGE OPERATION

Considerable mobilization/demobilization effort and costo Transport dredge, pipe, ancillary vessels to siteo Construct pipeline to placement site

Hydraulic dredge, so it pumps the sediments

Dredging is continuous (save downtime)

Can dredge hard/compacted sediments with proper cutter

PIPELINE DREDGE CHARACTERISTICS

Pipeline can be physical barrier for traffic

Not very mobile Placement site must be reasonably

near dredging site Produces high discharge with dilute

suspension

NEW CUTTER SUCTION DREDGE

Callan Marine’s General MacArthur 24000 hp and 800 mm (31.5 inch) discharge line.

PRODUCED SEDIMENT CHARACTERISTICSHydraulic dredges add large volumes of water to

facilitate transport.Slurry density changes continuously during operation

based on feeding rate.Typical Avg Slurry density:

• 10 to 20% solids by weight• C (% solids) ~ 10 + 0.1*%Sand

Slurry velocity• 10 to 20 ft/sec• 15 ft/sec typical average

EXAMPLE CUTTERHEAD

DREDGING PROJECT DESIGN

TYPICAL CHANNEL LAYOUT

TYPICAL CHANNEL X-SECTIONS

NOTE DISTORTION FROM HORIZONTAL AND VERTICAL SCALES

DREDGING PROJECT SCALES

OVERDREDGE~3-4 FT

Required depth

Required width

400 ft

PROJECT CHARACTERISTICS

Sediment volume: 100,000 CYAssumptions:o 12-inch hydraulic cutterhead dredgeo Cutterhead: 30-inch diameter; 30-inches longo In situ sediment density = 625 kg/m3

o Dredged material slurry density = 125 kg/m3

o Effective dredging time = 60% (downtime, repositioning, shift changes, etc.)

PRODUCTION CALCULATIONS

Total Flow

Q = 12 ft3/sec

Instantaneous Production P 12 ft3/sec /

/ ft3

= 320 cy/hr

Project Duration = 100,000 cy/(320 cy/hr*0.6) = 521 hrs = 22 days

Total water pumped = 100,000 cy(625/125) = 500,000 cy = 100 MG

OPERATIONAL DETAILS

Assume each dredge pass can cover 100 ft of width and 2 ft of deptho4 parallel paths to cover 400 fto2 passes for complete removal

Swing speed to meet 320 cy/hr instantaneous production• 320 cy/hr = 5.33 cy/min = 144 ft3/min• Cut area = 2 ft deep X 2.5 ft long = 5 ft2

• Swing rate = 144 ft3/min/5 ft2 = 28.8 ft/min = 0.5 ft/sec

COMPARING MECHANICAL AND HYDRAULIC

DREDGING

WHICH IS BETTER?No Universal AnswerCarefully Analyze Each Application

• Will One Likely Be “More Effective” Than The Other?

Conventional Dredging• Transportation Considerations• Disposal Considerations

Environmental Dredging• Sediment Treatment Considerations• Water Treatment Considerations

QUESTIONS?