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 Technology’s 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 Wärtsila 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 Wärtsila 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-ω 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

1.00

1.50

2.00

2.50

0.0 1.0 2.0 3.0

Model Speed

Mo

de

l T

rim

(d

eg

)

CFD

EFD

0

30

60

90

120

150

180

0.0 1.0 2.0 3.0

Model Speed

Mo

de

l R

esis

tan

ce

(N

)

CFD

EFD

0.00

0.01

0.02

0.03

0.04

0.05

0.0 1.0 2.0 3.0

Model Speed

Mo

de

l S

ink

ag

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 in“straight-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?