Cfd&nb mv08122010

Navigating through mud:beyond physical modelling

Marc Vantorre Maritime Technology Division, Ghent University

Knowledge Centre ‘Manoeuvring in Shallow and Confined Water (Flanders Hydaulics Reseach, Antwerp)

HSB Workshop, Antwerp, 8 December 2010

2

Definitie van de nautische bodem

• PIANC WG30 (1997):the level where physical characteristics of the bottom

reach a critical limit

beyond which contact with a ship’s keel causes

either damage

or unacceptable effects on controllability and manoeuvrability

• Principe: “Blijf van de bodem”

3

Definition of nautical bottomMud and ship behaviourPresent proceduresDevelopment of rheological criterionUseful configurations

4


5

Definition of nautical bottom




either damage


• Principle: “Don’t touch the bottom”

6





either damage


• Advantage: generally applicable:

?

7





either damage

or unacceptable effects on controllability and manoeuvrability :

• Difficulty: practical application:– Which physical characteristic?– How to determine critical limit?– Relevance for ship behaviour!

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9

Mud and ship behaviour

• Interface water – mud Internal wave generationRelative motion of ship with respect to water and mud layersMainly determined by DENSITY of mud layer Important for navigation above and through mud layers!

10

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-1 -0.5 0 0.5 1 1.5

inte

rface p

osit

ion

ab

ove s

olid

bo

tto

m (

m)

-12%

-7%+4%

+10%

Layer thickness: 3.0 m

Density: 1100 kg/m³

Ship’s speed: 5 knots

UKC to interface:


-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-1 -0.5 0 0.5 1 1.5

inte

rface p

osit

ion

ab

ove s

olid

bo

tto

m (

m)

-12%

-7%+4%

+10%

Layer thickness: 3.0 m

Density: 1100 kg/m³

Ship’s speed: 10 knots

UKC to interface:


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Onderzoek nautische bodem in WL

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• Interface water – mud Internal wave generationRelative motion of ship with respect to water and mud layersMainly determined by DENSITY of mud layer Important for navigation above and through mud layers!

• Rheological characteristics of mud Non-newtonian Thixotropic: relationship shear rate / shear stress depends

on recent history Mainly important for navigation through mud layers!

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15

Survey methods• Important parameters for ship behaviour:

– Rheologic characteristics– Density

• Required:

unambiguous, simple survey method

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Survey methods• Echosounding

• Density

• Rheology

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Survey methods• Echo sounding

– High frequency (210 kHz)

“top mud”– Low frequency (33 kHz)

consolidated mud

mostly lower than nautical

bottom

210 kHz

33 kHz

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Survey methods• Density

– Common practice in most harbours

with muddy bottoms– Relatively simple measurement– Continous or point measurements

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Survey methods• Density

– Mostly based on correlation with rheology– No universal relationship density – rheology

(mud/sand content, organic fraction, …)– Not always increasing with depth

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Survey methods• Densiteit

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Survey methods• Echo sounding

• Density

• Rheology

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Survey methods• rheology

– In principal, best suited as criterion for nautical bottom– Practical issues:

• Complex rheology

difficult to characterize by a limited number of parameters (at

least 4)

thixotropy: disturbing mud modifies characteristics

• Dependent on equipment and measuring procedure

– Criterion?• Relative (rheological transition)• Absolute

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• rheologie

200 kHz

rheological transition

1987

yield stress

de

pth

200 kHz

1997

rheological transition 1

rheological transition 2

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Present procedure in Zeebrugge• Till 2004: density 1150 kg/m³

(cf. rheological transition)

Rheological transition lower than 1.15 t/m³ horizon

Rheological transition higher than 1.15 t/m³ horizon

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Present procedure in Zeebrugge• Since 2004:

– Density 1200 kg/m³– Additional operational parameters:

• Minimum under keel clearance of 10% of draft relative to nautical bottom

• Maximum penetration of keel in upper mud layer of 7% (12%) of draft if sufficient tug assistance is available

– Based on simulations of arriving and departing container carriers by coastal pilots on the ship manoeuvring simulator at Flanders Hydraulics Research, with simulation models based on systematic captive model tests

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Captive model tests

Mathematical manoeuvring simulation model

• Test variables:

– ship models:

6000 TEU container carrier

bulk carrier

8000 TEU container carrier

• Test variables:

– ship models

– bottom conditions:

layer thickness: 0.5 m - 3.0 m

density: 1100 - 1250 kg/m³

viscosityWATER

MUD

HARD BOTTOM

• Mud simulating material = mixture of:

2 chlorinated paraffins

petroleum

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1.05 1.1 1.15 1.2 1.25 1.3

density (ton/m³)

dyna

mic

vis

cosi

ty (

Pa

s)

observations CDNBobservations Albert II Dockobservations swinging area IFHR 2004MARIN 1976FHR 1988FHR 1988 (natural)

3.0 m 1.5 m 0.75 m +10% +15% +26% +32% +10% +15% +26% +32% +10% +15% –12% –7% +4% +10% –1% +4% +15% +21% +4% +9% Mud layer thickness: 3.0 m

+10% +15% +26% +32%

-12% -7% +4% +10% +10% +15% +26% +32%

-1% +4% +15% +21%

Mud layer thickness: 1.5 m

+10% +15%

+4% +9%

Mud layer thickness: 0.75 m

Layer thickness

UKC relative to solid bottom

UKC relative to mud-water interface

Test section (44 m)

mud reservoir

water reservoir

EXPERIMENTAL PROGRAM

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Mathematical manoeuvring simulation models

Real-time simulations

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

und

er

kee

l cle

ara

nce

to w

ate

r-m

ud

inte

rfa

ce

density (t/m³)

extra tug assistanceOVERALL

Wind E, 6 Bf

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

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Present procedure in Zeebrugge+ Survey technique rather simple+ Accounts for behaviour and controllability of ship

- Rather pragmatic- Rheology is only implicitly taken into account, via density- Mathematical models are based on model tests

above/through a homogeneous “mud” layer- Survey with towed density probe not possible

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Development of rheological criterion• Characteristic of mud layer relevant for effect of contact

between keel and mud due to:– Damage (not probable) or– Uncontrollable behaviour

rheological properties probably dominant• Survey procedure should be based on:

– Mud characteristics with relevant effect on controllability of deep drafted vessels

– Survey methods to measure these characteristics in an unambiguous, simple way

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Development of rheological criterion• Controllability requirements (mainly container carriers):

– Controllability of forward speed

– Course stability

– Manoeuvrability at low speed

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– Controllability of forward speed• Deceleration at arrival

• Acceleration at departure

(cf. cross current)

• Dependent of:

Resistance

Propulsion characteristics

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– Course stability• Entering / leaving breakwaters

• Without tug assistance

• Rectilinear track without excessive use

of rudder

• Lateral force & yawing moment due to

forward speed + sway/yaw motion

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TUG ASSISTANCE

ARRIVAL/ DEPARTURE BERTH

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TUG ASSISTANCE:BEND AT OLD BREAKWATER

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TUG ASSISTANCE: TURNING MANOEUVRE

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– Manoeuvrability at low speed• With own rudder, propeller, bow/stern thrusters

• With tug assistance

• Cf. forces/moments due to pure sway and yaw

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Development of rheological criterion• Additional research

– By means of model tests: not (directly) feasible• Scale effects

• Selection of mud simulation material

– Numerical models• Complete CFD-modelling: very ambitious

• Simplified, relevant configurations

• More insight into relevance of mud characteristics with respect to ship behaviour

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Useful configurations: overview

• Forces due to direct contact mud – keel

direct effect of mud characteristics

selection of criterion for nautical bottom

• Ship manoeuvres in mud layer with depth dependent characteristics:

which part of mud layer will not be brought into motion?

indirect determination of nautical bottom

more direct link with practice

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Useful configurations: direct contact

• Forces on a flat plate: frictional resistance– Flow parallel to a flat plate with zero thickness, infinite width and

limited length (cf. Froude experiments)– Flow parallel to a flat keel shaped plate with zero thickness from

different inflow angles

in different fluids:– Water (reference)– Homogeneous mud layers with constant rheological

characteristics– Initially homogeneous mud layers with thixotropic characteristics

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Useful configurations: direct contact

• Forces on a keel structure: viscous pressure resistance (form resistance)– Flow along a simplified keel structure from different inflow angles

in different fluids:– Water (reference)– Homogeneous mud layer with constant rheological characteristics– Initially homogeneous mud layers with thixotropic characteristics

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Useful configurations: indirect effect

• Flow as a result of navigating/manoeuvring ship with certain under keel clearance above– Mud with characteristics in the nautical bottom range– Solid bottom

• Simplified configurations:– Complexity– Calculation time– More insight into parameters

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• Lateral motion of a ship section (2D)– Above solid bottom (reference)– Above/through homogeneous mud layers with constant rheological

characteristics– Above/through mud layers with thixotropic characteristics– Above/through mud layers with depth dependent density and

rheology

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• Longitudinal motion of a ship– Ship with forward speed above/through mud layer

wave generation in interface:

function of speed, depth to interface, mud density,

under keel clearance, hull form …

simplified calculations

boundary conditions for CFD calculations– Configurations: same as lateral motion

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Further research

• Rudder and propeller behaviour– Essential for manoeuvring and controllability– Essential for determining operational limits– Secundary effect on determination of nautical bottom– Later stage

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Conclusion: Required expertise

A. Rheology of sediments and mud: theoretical base and numerical modelling

B. CFD: Numerical fluid dynamics for non-newtonian fluids with time dependent characteristics (thixotropy)

C. Manoeuvring behaviour: Mathematical modelling and simulation of ship manoeuvres in muddy areas

D. Experimental research and measuring techniques with respect to mud characteristics (lab scale & in situ)

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Conclusion

Model tests: • Wave generation• Overall effect of mud layers on ship behaviour• Operational conditions• Rheology??

Numerical methods:• Account for all characteristics• Micro-scale• Calculation time?• Validation?

Navigating through mud:beyond physical modelling

Marc Vantorre Maritime Technology Division, Ghent University

Knowledge Centre ‘Manoeuvring in Shallow and Confined Water (Flanders Hydaulics Reseach, Antwerp)

HSB Workshop, Antwerp, 8 December 2010

Documents

Cfd&nb mv08122010