Horizontal Transport of Dredged Mixtures Part 1_EN

Bernard Malherbe - Dredging Technology: horizontal transport of mixtures 1

Katholieke Universiteit LeuvenFaculty of Engineering SciencesDredging Technology

Ir. Bernard MalherbeProject Development DirectorJan De Nul Group

Hydraulic Dredging: horizontal transportPart 1: Transport of dredged mixtures

trough pipes


Dredging operation: disruption of aquatic soils (sediments, rocks,) + transport + disposal

Sediments: mainly 3 types

2 mm < d5063 m < d50 < 2 mmd50 < 63 m

Granular sediments: gravel, boulders,

Granular sediments: SandCohesive sediments: Clay & Silts

In hydraulic dredging the maximum allowable grain-size diameter of dredged soil is determined by the spherical aperture of the dredge pumps.


Dredging equipment: Hydraulic dredgers: Trailing Suction Hopper Dredger (TSHD)

Cutter Suction Dredger (CSD)Mechanical dredgers: Backhoe Dredger (BHD)

BHDCSDTSHD


Dredging Tools: mechanical loosening of soil + hydraulic jetting & erosion

3 methods

BucketCutterheadDragheadBHDCSDTSHD


Transport of dredged materialTransport over sea : 3

Side castingBargeHopperBHDCSDTSHD

Pipeline


Transport of dredged material

Transport over land : 3 methods

PipelineTrestlesDumptrucks


Hydraulisch Transport offers many advantages:

Continuous process, fit for huge quantities & productivities: 20.000 to 100.000 m3/day Swift mobilisation & readyness Limited maintenance Limited Personnel


Principles of hydraulic transport of dredged material:

Disrupted soil or rock Disrupted soil or rock individual particles, heavy suspensions or fragments individual particles, heavy suspensions or fragments are mixed with are mixed with water to form a slurry (typical densities: 1,15 to 1,50 t/mwater to form a slurry (typical densities: 1,15 to 1,50 t/m)) MixtureMixture--forming happens in forming happens in dragheaddraghead or or cutterheadcutterhead and is then sucked into the suction pipe, via and is then sucked into the suction pipe, via hydraulic depressionhydraulic depression Mixture velocity and turbulence (Re > 4.000) prevent the mixtuMixture velocity and turbulence (Re > 4.000) prevent the mixture from settling downre from settling down After discharge, turbulence decreases and particles are allowedAfter discharge, turbulence decreases and particles are allowed to settle downto settle down


Drawbacks of hydraulic transport

Limited transport-distances: 2 tot 5 km Differential settling: siltpockets Increase of suspension load of transport-water: visual impact of turbidity Wear of pipes


Other transport methods:

Backhoe / cable-crane: Ecological sensitive areas Small volumes, Big fragments

Trestles as used in Sand & Gravel miningUnload at fixed location

Dumptrucks: Large transport-distancesUpland disposal Limited Volumes


Working principles of a TSHD and a CSD


Hydraulic transport: practical application in dredgingShipborne pipeline systems


Hydraulic transport via shipborne & external pipeline systemsTSHD reclaiming


Hydraulic transport via shipborne & external pipeline systems

TSHD reclaiming


Hydraulic transport via external pipelines

Floating & Land pipeline


Hydraulic transport via floating/land pipelines

Cutter-dredging and direct upland reclamation

CSD Leonardo da Vinci in Port Hedland, australia


Hydraulic transport & reclamation

ReclamationArea: dredged mixture with high solids concentrations



Discharge of transport-water behind settling ponds


Beach restaurationdirect settling & open-water discharge

(Sylt, Germany),



Hydraulic Transport: Application in Deep Sea Mining


Hydraulic Transport: Application in rock-dumping


Hydraulic Transport: Application in rock-transport, ballasting of GBS


Hydraulic Transport: Hydraulic-production computations

cost-estimates for tender-preparation dimensioning of dredge-pumps and shipborne pipe systems for the design of dredgers Dimensioning of jet-devices and nozzles for fluidisation of soil prior tosuction Control of performance of dredge-pumps Development of simulators


Hydraulic Transport: physics of the system

1. Pipeline characteristic (Discharge (Q)/Head (H) relationship)- homogeneous fluid in straight pipe- Soil-water mixtures in straight pipes- special head-losses: bends, narrowings,- vertical and inclined pipes

2. Pump-characteristic (Discharge (Q)/ Head (H) relationship)- Pumptypes- Characteristic for homogeneous fluids- Characteristic for soil-water mixtures

3. Driving system- Modification of pump-characteristic- for diesel-elec, direct diesel, driving

4. Working area of whole system: driving system, pump, pipeline and mixture


Hydraulic Transport: the Pump Drive Pipeline FitDescription via Q H relationships


Hydraulic Transport: Pipeline Hydraulic Characteristic

Basic Assumptions: Horizontal cylindric pipeline: no bends, valves, Homogeneous incompressible fluid: perfect suspension, no

segregation, no gases, Newtonian fluid: (almost) linear realationship between shear stress

and strain Uniform flow: no velocity profiles between wall and center-line Constant flow velocity


Hydraulic Transport: Pipeline characteristic

outoutinin AvAv =

(Newtons 2nd law of motion)

(Navier-Stokes)

(Darcy-Weisbach friction coefficient)

with=f(Re, k/D) (Moody diagram)

21

. vDLpp outin +=

= externalFdtmomentumd )(

2

21

vDLp =

DLp 04

=

Law of mass-conservation:

Law of momentum-conservation:



Law of energy-conservation: Law of Bernouilli

Cstghvp =++ 221

Pressure EnergyKinetic Energy

Potential Energy

This physical law expresses the whole process: the pump-drive plant adds energyto the mixture by increasing velocity: this Kinetic Energy is then oscillatingconstantly within the system between Kinetic Energy (mixture velocity), Pressureenergy (pressure) and Potential energy (elevation). Velocity, pressure and elevation are thus the main parameters of the dredging process.



Integration of the 3 physical Laws yields:

Applied to:

Succession of pipes with various diameters:

Special losses with dedicated coefficient for bends, valves, etc..:

outoutinin ghvDL

vpghvp +++=++ 22221

.

21

21

222

12

21

.

21

21 ghv

DL

vpghvp iioutoutinin +++=++

2222

12

21

21

.

21

21 ghvv

DL

vpghvp iiioutoutinin ++++=++


Hydraulic Transport: pipeline characteristic

2222

12

21

21

.

21

21 ghvv

DL

vpghvp iiioutoutinin ++++=++

In the dredging process the geometry/elevation of the pipeline is generallyfixed and known, i.e. not variable during the process. Kinetic Energy and Pressure Energy are the components that can be controlled. They can betransformed into pressure, by dividing the terms by .g. dynamic pressure (head) and the static pressure (head)

These terms together express the HeadLosses,H, due to friction in the pipelineand in special pipe-components: note the same character as a dynamic pressure!


Special resistances for specific pipeline components:


Hydraulic Transport: Graphical representation of pipeline-characteristic

- Relationship is of the following kind: caQH +=

aQ = Head Lossesdue to friction

c = geometric elevation head


Suction characteristic for a horizontal pipeline

p0 pin

pz

pp

p0 = patm

pin = patm (1 + )0.5 m v

Pz = patm (1 + + L/D)0.5 m v

Pp = patm (1 + + L/D)0.5 m v + p


Suction characteristic for a dredge pipeline in operation:

p0 pinpz

pp

p0 = patm + w g hz

pin = patm + w g hz (1 + )0.5 m v

patm

hz

hp

pz = patm + w g hz m g (hz-hp) - (1 + + L/D)0.5 m v

pp = patm + w g hz m g (hz-hp) (1 + + L/D)0.5 m v + p


Hydraulic Transport: hydraulics of Water-Sand mixtures

The description runs over 4 different stages: Stage 1:

Water-Velocity = 0 One single particle Perfect spherical particle

Stage 2: Water-Velocity = 0 One single particle Non-spherical particles

Stage 3: Water-velocity = 0 Group of particles

Stage 4: Water has a velocity (e.g. typically between 1 and 7 m/sec) Group of particles


Hydraulic Transport: Stage 1 Spherical Particle in Still water

Forces exterted on particle: Gravitational forcet

Buoyancy force (Archimedes)

Flow-resistance forces Wall-friction Drag-resistance

gVF ssg =

gVF swB =

stwDD AvCF 21 =


Hydraulic Transport: Non spherical particles in Flowing water

Gravitational forces Buoyancy forces Flow-resistance forces Lift-forces due to velocity gradients, particle geometry,.

stwLL AvCF 21 =


Hydraulic Transport: free-fall velocity of particles in water


Practical application: dumping of particles trough a vertical pipe

Non-visquous fluids: only local water-displacement around the particle compensatesfor the volumetric passage of the particle. The total Head remains constant

Visquous fluids : the particle drags an added mass of water during its fall and the upward compensating current gets resistance from the wall of the pipeand the stone. The water-head decreases in the pipe.


HydraulicTransport: Group of particles in each others influence zone

At low concentrations is the group fall-velocitylower than the individual particle fall-velocity: the upwards compensation flow slows down the downward movement of the group.

At high concentrations, the mass of added water dragged by the group adds up to the mass of the particles and the whole group fall-velocity becomesgreater than the individual particle fall velocity.


Hydraulic Transport: Group of particles

Other forces acting on a group of particles:

Turbulent dispersion: the group of falling particles cause a lot of turbulences: these forces decrease the fall-velocity.

Intergranular forces between particles: When a bed is formed on the

bottom, particles may interlock witheach other

Particles may hit each other duringmvements


Hydraulic Transport: Particles in flowing water inside a dredge pipe

During the hydraulic transport-process of a dredger:- the dredge-pump & drive plant, adds energy to the fluid by increasing

its Kinetic Energy, which is soon transformed into a combination of Kinetic energy (fluid velocity) and into Potential energy (pressure)

- By increasing the velocity, the turbulence within the fluid will increase, hence facilitating the keeping of particles in suspension

- Energy will not really be transmitted to the sand-particles, but- Particles are kept into suspension by turbulences and

(omnidirectional) turbulent forces- They will be dragged by dragforces (actual friction resistance)

caused by the moving fluid- The velocity of sand-particles is lower than the one of the moving

fluid: this phenomenon is called slip and depends upon the sizeof the particles, the concentration of solids inside the suspension, the viscosity, etc


Hydraulic Transport: Hydraulic regimes of particles in flowing water Mainly, 3 flow-regimes:

Stationary bed Sliding bed with Homogeneouspartial suspension suspension

Governing factors- Increasing velocity: more drag, more turbulence- Increasing viscosity (increasing concentration): more drag- Decreasing grain-size: smaller fall velocity- Dcreasing pipe-diameter: higher velocity


Hydraulic Transport: Hydraulic regimes of particles in flowing water

Particular Case: Plug flow occurring mainly with cohesive sediments(mud-type) with

- High concentrations (> e.g. 450 kgds/m3)- Sliding bed occurs over the whole flow


Hydraulic Transport: Phenomenon and physics of slip

Slip-velocity is dependent upon hydraulic regime

- Particle in suspension:

- Particle in sliding bed:

- Particle in vertical flow:

Governing factors: Contact-surface between particle and fluid Specific density of particles wrt fluid

slpwp vvv =

0slipv

wslip vv


Hydraulic Transport: Inclined and vertical pipes

Inclined pipe: Slipvelocity is high

Vertical pipe: Slipvelocity is low


Hydraulic Transport: Density Density:

Granular material originates from a physical or chemicaldesintegration process of solid rock (Quartz, Feldspar, carbonates), shells and skeletons (carbonate) => The density of the base material of all soils is very constant

The density of dry granular material is lower because of cavities pores between the grains => the density of granular materialdepends on the grain shape, degree of compaction, uniformity of size distribution.

volumemass

m =

/7.265.2 mts =

sairsd nnnmt )1()1(/5.11.1 +==


Hydraulic Transport: Density

The density of water saturated granular material: the pores are filled with water=> Typical value for sand is 1.8 2.0 t/m

(depending on shell content, grain diameter, grain shape)

wssi nn += )1(

The density of granular material in suspension: the amount of water is larger than the porevolume: the material is oversaturated

simw


Hydraulic Transport: ConcentrationFirst method to describe the content of granular material in a fluid:

spatial or volumetric concentration

Spatial concentration:

mass balance

Mixturedensity

Spatial concentration

ws

sv VV

VC+

=

wwssmm VVV +=

)1( vwvsm CC +=

ws

wmvC

=


Hydraulic Transport: tdsA much used unit for dredged material is tdm (tonnes dry matter)

Tds is derived from the mass balance:

ws

wmmsvmsss VCVVtdm

=== .....

wwssmm VVV +=


Hydraulic Transport: ConcentrationSometimes it is more practical to describe the spatial concentration

as a function of the situ density of the material on the seabottom

Spatial concentration:

massbalance

Mixturedensity

Volumeconcentration

wsi

sivi VV

VC+

=

wwsisimm VVV +=

)1( viwvisim CC +=

wsi

wmviC

=


Hydraulic Transport: Notions of Delivered concentrationCvivwater

vsolid

Cvd

Delivered concentration Cvd

vwater = vsolid => Cvi=Cvd vsolid =0 => Cvd=0


Hydraulic Transport: ConcentrationSecond way: delivered concentration(Remark: delivered concentration is always a function of the situ-density

Delivered concentration:

massbalance

Mixturedensity

Delivered concentration

ws

svd QQ

QC+

=

wwssimmd QQQ +=

)1( vdwvdsimd CC +=

wsi

wmdvdC

=


Hydraulic Transport: Concentration

The correlation between the two concentrations is derived from the volumebalance

- Spatial concentration

- Delivered concentration

wwssmm

wsm

AvAvAvQQQ

+=

+=

AA

LALA

VVC ss

m

svi ===

.

.

LAvLAv

QQC

m

ss

m

svd

.

..

==

visvim

svd CfC

v

vC .==


Hydraulic Transport: Mass-ConcentrationMass-concentration :1 kg mixture contains Cw kg dry solids

and (1-Cw) kg water

Or:

Often used in process-technology

w

W

s

Wm CC

)1(1

+=

mws

swmwC

)()(

=


Hydraulic Transport: Sequence of Concentrations and volumes in adredging & reclamation project

Saturated (water) Dry


Hydraulic Transport: Hydraulic Process description

The hydraulic transport of sand-water mixtures is (for the time being) too complex and too poorly understood to be describedanalytically. Therefore, engineers have to rely on empiricalrelationships and formulae

The most relevant empirical formulae are based on closed-looplaboratory tests:

- PhDs theses uit 50-ies en 60-ies- R. Durand & E. Condolios (1952) R. Gibert (1960)- Alfred Fhrbter (1961)- Jufin-Lopatin (1966)- Wilson (1972-1996)

But the lab-tests had drawbacks: .too small diameters of pipes (excepted Durand and Wilson) too limited concentration ranges selected (near ideal) dredged materials (excepted Durand)


Hydraulic Transport: Two-layered Model cfr Wilson

Wilson (1992-1996)

A = Wet SurfaceC = ConcentrationV = Volume


Hydraulic Transport: Two-layered Model further elaborated

Vclav Matousek TU Delft (1997)


Hydraulic Transport: Real-scale tests on Two-Layered ModelVerification on real-scale reclamation works of the Two-Layered model Pusan Port Development (South-Korea, anno 2002) 0,300 mm sand


Hydraulic Transport: Practical engineering State of the Art

Concluding:- Only empirical formulae are used- Parameters are calibrated with site-specific measurements or

experience-data- Corrections to be applied for different pipe-diameters, cutter dredgers

or hopper dredgers,Careful:- Input-data are generally not precise (too little soil and soil-variability

data)- median grain-size is not easy to determine- effect of coarse materials (boulders,) is huge : stones> 5 cm diameter are removed from lab-tests- effect of fines on dynamic viscosity- effect of variations in grain-size distribution

- Type of dredging is importantTSHD: Segregation of material in hopper: coarser under

discharge pipeCSD: undercutting vs overcutting

- Variations in process-parameters are not smoothed out over long discharge-pipes


Hydraulic Transport: Hydraulic characteristic of sand-water mixture

- Relationship is of the following type:

- Minimum of curve: critical velocity/discharge

cQb

aQH ++=

vcrit

settlingsuspension


Hydraulic Transport: Influence of variable concentrations and grain-sizes (Fhrbter)

Effect of increasing concentration Effect of increasing median-grain-size


Hydraulic Transport:

END of PART 1


Hydraulic Transport: Concentration - Example

Given: Volume mud to be dredged and reclaimed: 10,000 ms=2.65 t/msi=1.5 t/mw=1.025 t/mm,zuig=1.20 t/mVhopper=3,800 mVrest=100 mCvi,recl=80%no overflow allowed during dredging

Required: what is the watercontent of the mud in situ?How many dredging cycles are required?what is the load (ton) of the ship after dredging?what is the amount of tdm each cycle? what is the weight concentration?How much volume does the mud occupy on the

reclamation area?

Documents

Horizontal Transport of Dredged Mixtures Part 1_EN