STAR European Conference

London, 22-23 March 2010

RANS Simulation of Tug in Escort Mode

Background Approach and method Results Conclusions

Claus Daniel Simonsen

FORCE Technologywww.force.dk

Background

FORCE Technologys Division for Maritime Industry does maneuvering simulations and provides consultancy services based on experimental model testing and CFD simulations to the maritime industry.

The Facilities:

6 Manoeuvring Simulators

2 Towing Tanks - 1 deep / 1 shallow

4 Wind Tunnels high speed, boundary layer, wide and climatic

Experimental Fluid Dynamics Lab for internal flows

PC Cluster - 80 cores in total

Background

In order to manoeuvre, large ships are typically escorted by tugs, which will help them turn or break under conditions where their own rudder and propeller are insufficient

Designing a tug is a multidisciplinary effort and traditionally the design process has been based on model testing:

Resistance and self-propulsion testing

Free sailing model testing to evaluate escort capabilities

PMM and free sailing model testing to evaluate manoeuvring capabilities inclusive course stability

Seakeeping test to evaluate performance in waves

Background

However, model testing is expensive, particularly if several design variants are tested or if the design at some point in the test series turns out to be inadequate and the model must be changed and re-tested

Today CFD has become a supplement to the test and many parameters can be checked or optimized in the numerical towing tank before building the physical model.

Consequently, poor designs can be discarded and testing can be limited to check of final design a cost saver

Background

As an example on a practical project, FORCE Technology has recently assisted Wrtsila Ship Design Norway with CFD services in connection with design of a new tug before model testing:

Hull and Skeg optimized to:

Minimise required power in transit

Position towing point on tug to maximize steering force and reduce heel angle

Have sufficient free board during escort and avoid water on deck when heeling

Have sufficient lateral skeg area to obtain large steering force without heeling too much

Check if the design is course stable

However, the results from the Wrtsila project is confidential, so results from previous validation work with a tug will be shown

Background

In connection with application of CFD, FORCE Technology has the advantage of having the towing tank. Therefore, for each ship type and test type to be simulated, validation against experimental data is always done before CFD simulations are used in production and offered to our clients

This was also done for the tug in order to simulate the escort mode and investigate the escort capabilities tug

The goal was to check how well the steering force ,which is one of the important parameters for tugs, can be predicted.

All simulations are done with STAR-CCM+

Meshing

Surface wrapper + re-mesher

Trimmed mesh approach

Prism layer meshing in boundary layer

Zonal refinements

Physics modeling

Segregated flow

VOF model for free surface modeling

Transient/steady calculations

6DOF used to predict motions of the ship

Spring model for towing line

k- SST turbulence model, all Y+ treatment

Propeller effects are included via body-forces

Method

The test case is the geometry of a 29m tug including bilge keels and thruster units

Computations are done in model scale for a 2.3m model and results are scaled to full scale later as is standard praxis

To model the fully appended ship with thruster units during escort is complicated, so a stepwise approach is followed, where a couple of checks have been made for simpler cases to gain experience with this ship type:

Simulation of straight-ahead condition to predict resistance and dynamic sinkageand trim for bare hull

Simulation of static drift with heel to predict steering force for bare hull

Simulation of escort mode condition to predict position and steering force for appended hull

Approach

First step covers prediction of resistance:

Only bare hull

4 speeds covered: 9 to 12 knots

Model is free to heave and pitch

Flow visualization is used to guide the hull designand make a hull form that performs well from ahydrodynamic point of view

Results: Straight ahead

Integral quantities are used to quantify the performance of the design

Results compare well with measurements (within 5%)

Good enough to rank design variants

Good enough for Speed and Power prediction

Results: Straight ahead

0.00

0.50

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Model Speed

Mo

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l T

rim

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eg

)

CFD

EFD

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90

120

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180

0.0 1.0 2.0 3.0

Model Speed

Mo

de

l R

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tan

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(N

)

CFD

EFD

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Model Speed

Mo

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e (

m)

CFD

EFD

Results: Static drift

The next step in the validation process covered static drift computations, i.e. oblique flow

During escort conditions the tug is typically oriented with an angle to the sailing direction

Using a setup where the model is fixed at a representative towing position is a good way to quantify how well the transverse hull force can be determined, because the static drift PMM model test is designed for this type of setup

Condition representative for escort:

Speed of 8 knots

Drift angle of 28 deg

Heel angle of 5.3 deg

Dynamic sinkage and trim taken from modeltest

Results: Static drift

Flow field is challenging, since it involves flow separation and large vortex structures

Results: Static drift

Check of steering force, Fs, prediction by comparison with PMM data

Forces again compare reasonably well with measured data, i.e. within 5-6% of measurement

Different skegs have been calculated and it turns out that the change in steering caused by different skegs can be picked up

Accurate enough to choose skeg design based on performance with respect to steering force

Results: Escort mode

Final step in the validation process covered escort mode computations, where the model is self propelled and free to move in all 6 DOF

PMM gives steering forces for given fixed orientation, but this may not be the equilibrium position equivalent to the one experienced in the free sailing condition

Therefore, escort mode simulations are required if for instance water on deck is investigated

Escort condition taken from model test:

Speed of 8 knots

Propeller settings taken from model test, i.e.measured thrust and torque are applied

Results: Escort mode

Setup of model

Towed in stiff spring, escorted ship not incl.

Fully appended

No real propellers, but body-forces

Free to move in 6DOF

Results: Escort mode

Flow field results

Qualitative agreement with measured flow features

View of simulation

Results: Escort mode

Reasonable agreement between EFD and CFD allows the tool be used for design purposes

Method has also shown to be able to pick up changes in forces and model orientation due to change in skeg geometry and towing point position. Important in the design phase.

Heave

[m]

Pitch

[deg]

Heel

[deg]

Yaw

[deg]

Phi

[deg]

Total F

[N]

Steering F

[N]

EFD -0.042 -2.43 -10.84 44.47 78.9 444 438

CFD -0.037 -1.44 -10.80 38.73 78.5 409 423

Diff % -12.5 -40.6 -0.4 -12.9 -0.5 -7.9 -3.3

Summary and conclusion

Conclusion/Summary

Studies of RANS based CFD simulations with tugs instraight-ahead and oblique flow conditions show promising results, when compared with data measured in the towing tank. Though, experience shows that level of validation can vary between applications i.e. for different propeller and hull geometries, which must be kept in mind when the tool is used.

When it comes to evaluation of the escort capabilities of tugs, CFD appears to be a strong tool, which can be applied for evaluation of design variants in the early design phase. Hereby, it supplements the physical model testing and helps to focus the testing on the final design.

Questions?

Thank you for your attention!

Questions?