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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
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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
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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
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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)
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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
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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”
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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
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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
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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?
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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.
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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)
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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
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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
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TYPICAL CLAMSHELL DREDGE OPERATION
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CLAMSHELL BUCKET OPERATIONS
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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
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FIXED-ARM DREDGES
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BUCKET OPERATION
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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
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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
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CLAMSHELL DREDGE OPERATION
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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
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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
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IDEALIZED BUCKET PATTERN
10
13 Buckets
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ACTUAL BUCKET PATTERN
20 Buckets
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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
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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
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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
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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
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MANAGING MECHANICALLY DREDGED SEDIMENTS
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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
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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
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ADJACENT HOPPER BARGEDredge until hopper is filledBarge exchanged for empty barge when filled
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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
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BARGE OVERFLOW
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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.
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MANAGING BARGED SEDIMENTS
THE BARGE IS FULL, WHAT DO YOU DO WITH THE SEDIMENT NOW?
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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
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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
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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
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EXAMPLE CLAMSHELL
DREDGING PROJECT DESIGN
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TYPICAL CHANNEL LAYOUT
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TYPICAL CHANNEL X-SECTIONS
NOTE DISTORTION FROM HORIZONTAL AND VERTICAL SCALES
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DREDGING PROJECT SCALES
OVERDREDGE~3-4 FT
Required depth
Required width
400 ft
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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
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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
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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
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HYDRAULIC DREDGES
Typical Equipment Basic Operations Produced Sediment Characteristics
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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
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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
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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
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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
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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)
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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)
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PIPELINE DREDGES - EXAMPLES
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PIPELINE DREDGE OPERATION
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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
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NEW CUTTER SUCTION DREDGE
Callan Marine’s General MacArthur 24000 hp and 800 mm (31.5 inch) discharge line.
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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
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EXAMPLE CUTTERHEAD
DREDGING PROJECT DESIGN
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TYPICAL CHANNEL LAYOUT
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TYPICAL CHANNEL X-SECTIONS
NOTE DISTORTION FROM HORIZONTAL AND VERTICAL SCALES
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DREDGING PROJECT SCALES
OVERDREDGE~3-4 FT
Required depth
Required width
400 ft
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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.)
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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
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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
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COMPARING MECHANICAL AND HYDRAULIC
DREDGING
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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
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QUESTIONS?