Water Wave Impact on Ship Structures

Christine M. Ikeda, Ph.D.School of Naval Architecture and Marine Engineering

Carolyn Judge

UNO Engineering Forum September 19, 2014

Sponsored by:

Outline• A Hydrodynamics Point of View on the Slamming Impacts

experienced by High-Speed Craft: Dynamic Pressures• Classical Wedge Drop Study Revisited• Tow-Tank Experiments on Planing Craft in Regular Waves

• Structural Response of Slamming Loads on High-Speed Craft: Full-Field Strain and Deflection Measurements• Classical Wedge Drop Study Revisited• Tow-Tank Experiments on Semi-Planing Craft in Regular and

Irregular waves.

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Acknowledgements: Results shown from the United States Naval Academy were funded by ONR, and the project principal investigator was Carolyn Judge.

Special thanks to the Hydromechanics Laboratory Staff at the United States Naval Academy:Dan Rhodes, Mark Pavkov, Bill Beaver, and John Zselecsky

Ship Structure

Hydrodynamic loading

Structural Response

• Research Questions:• Is there a better way to design marine vehicles if we understand the physics of

slamming events?• What happens if the bottom of the hull deflects?• How does the use of composites or aluminum in ship-building affect the physics and

strength of the hull?

• Laboratory-scaled experimental studies seeking to provide insight into the physics of the slamming events• Slamming motions and forces are a function of wave

topography, impact angle, forward speed, and body orientation during impact

• Classic wedge drop experiment with new contributions:Spray Root and high-density pressure measurements

• Tow-tank experiments on model-scaled planing hull in different wave conditions

Slamming Impacts on High-Speed Marine Vessels

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Image courtesy of Combatant Craft Division (CCD) Little Creek

Experimental Details

• Acrylic wedge, constructed with 1.3-cm thick plates

• Deadrise angle, β = 20°• Dimensions in cm• Length in and out of

screen: 60 cm

Measurements:• Vertical Acceleration: Accelerometer• Vertical position: String Potentiometer• High-Speed Video:

Phantom Miro M320S• Pressure on bottom surface

PCB

Tekscan Map

Drop heights ranging from 8 to 64~cm

High-speed camera

• Point Measurements: PCB Piezotronics• Mapping System: Tekscan

Pressure Measurements• Bottom layout of pressure measurement locations

• 6 point pressure transducers

• Pressure mapping system (consists of an array of measurement points)

Field of View for High-speed Camera

Point-pressuresensors

Pressure mapping system

Single “sensel”or measurement point

High-Speed Video

Video recorded at a speed of 4,000 frames per second and played back at 3 frames per second(1333 times slower than real life)

New Contribution

What was measured?

C. Ikeda, and C. Judge. Impact Pressures and Spray Root

Propagation of a Free-Falling wedge. Submitted to

Experiments in Fluids, May 2014.

P1 P2 P3P4

P5

P6

Keel Impact

Chine impact measured from videos

Similarity Solution

7.9 cm 31.8 cm 63.5 cm

Similarity Solution

Pressure Contours

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Pressure Contours

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Pressure Contours

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Pressure Contours

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Pressure Contours

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Pressure Contours

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Spray Root and Pressure Correlations

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Increasing Drop Height

The lines show the computed spray root position versus time.

The symbols show the position of the peak pressure versus time

High-Speed CraftExperiments performed at the United States Naval Academy

Experimental Facilities• Dual flap, servo hydraulic control wavemaker

• Regular, irregular and transient waves; frequency range 0.3 to 1.4 Hz

• Tow-speed: 6.4 m/s

• Bretschneider Spectrum to develop irregular wave model of a Sea State 3 Condition• 9.4 cm significant wave height• 1.7 s modal period

• Regular Wave field based on the most probable waves from Bretschneider Spectrum• Wave Height: 6.1 cm

• Wave Period: 1.1 s

Wave Characteristics

Planing Hull Characteristics

Full-Scale Model-Scale I Model-Scale II

Overall Length (m [ft]) 13.0 [42.8] 1.2 [4] 2.4 [8]

Maximum Beam (m [ft]) 4.0 [13.1] 0.37 [1.2] 0.73 [2.4]

Displacement (metric tons [lb]) 15.9 [35000] 0.013 [27.9] 0.12 [223]

LCG (m [ft]) 4.6 [15.1] 0.42 [1.4] 0.85 [2.82]

KG (m [ft]) 1.5 [4.8] 0.14 [0.45] 0.27[0.9]

• Fixed degrees of Freedom:• Sway, roll, yaw,

surge (fixed to carriage)

• Free in• Heave, Pitch

Measurements

• Accelerations• A1 triaxial accelerometer (Heave, Sway and Surge)

• A2 & A3 Heave (vertical accelerations only)

• Heave and Pitch measured at the LCG

• Incoming water surface (encounter wave) at 52 cm in front of the bow

• Wave Height elevation at 30.5 m from the wavemaker (fixed in tow tank)

Pressure Measurements

• Point – Pressure Transducers:• PCB Piezotronics Model

113B28• Range: 344.7 kPa • Temperature effects

mitigated with dielectric grease

• No hydrostatic pressure reading

• Pressure Mapping System• Tekscan High-Speed

Pressure Mapping System

• Range: 690 kPa• Reads hydrostatic

pressure

Sensor Name #1 #2 #3 PCB

Tekscan Model Number 5051 9550 5570 N/A

Measurement Area (cm2) 31 108 175 N/A

Sensel Area (mm2) 0.64 50.4 22.9 24

Number of Sensels 1936 42 264 N/A

Sample Rate (kHz) 0.73 20.4 4.4 20

Tow-Speed: 6.4 m/s (12.4 knots), Regular WavesMovie Taken at 400 fps and played back at 10 fps

Run 44

Identification of Single Impact• Use of acceleration-time histories to identify a single slam event

• Free-fall or zero vertical acceleration followed by short duration high, upward acceleration from slam

• Heave, Pitch and Wave history • behavior consistent with the slam event characteristics

• Run 44

Single Impact Event (Run 44)Tow-Speed: 6.4 m/s (12.4 knots), Regular WavesMovie Taken at 400 fps and played back at 2 fps

Pressure Time History Run 44

• Point sensor measurement area: 24 mm2

• Sample Rate: 20kHz

• Sensel measurement area: 0.64 mm2

• Sample Rate: 730 HzMust assume that the planing motion is symmetric about the keel

Spatial Pressure Correlation Run 44

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P22

P23

P21

P22

P23

P21

P22

P23

P21

P22

P23

Spatial Pressure Correlation Run 44

P21

P22

P23

P21

P22

P23

P23

P21

P22

P21

P22

P23

Tow Speed: 9 m/s (17.5 knots), Regular waves2.4-m long model

Movie taken at 1400 fps and played back at 150 fps

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Conclusions and Future WorkWedge Drop Experiment

• Novel method of quantifying the spray root propagation

• Pressure measurements correlate well with measured spray root propagation

• Calculated maximum velocity at impact and verification of similarity solution

• Understanding of the basic physics of these impact events can allow for the development of design tools and can aid in computer model validation

High-Speed Planing Craft

• 146 total runs with about 15-20 impacts per run still a work in progress

• Analysis of pressure measurements show a discrepancy in pressure magnitudes between the two methods, but qualitatively look reasonable

• Isolating of individual slamming events using vertical acceleration data show there are different types of behavior based on how the ship hits the water surface, curvature of water surface

Structural ResponseExperiments to be performed at the University of New Orleans

Deflection of Ship Hull Bottoms• Why is this a concern?

• Wide-spread use of composite materials in ship-building that are more likely to deflect• High-Speed craft slamming into large waves can severely injury passengers; consider

an autonomous craft and focus shifts to not damaging the equipment on-board

• Research Questions:• How does the pressure-field in the fluid deform the structure?• How does the structure deformation affect the pressure field?

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Image courtesy of Combatant Craft Division (CCD) Little Creek

Ghavami, K. and Khedmati, M.R., Finite Element Analysis - Applications in Mechanical Engineering, 2012

Deflection of Ship Hull Bottoms

• How can a ship be designed to take into account composite materials or deflections in the hull bottom?

• Conduct experimental study to determine the strength of the a composite deformable hull • Wedge drop study • Semi-planing study• Use of Digital Image Correlation (DIC) as an non-intrusive way to measure the

full field strain on the structure• Use of Stereo DIC will allow for out-of-plane deflection• Continue to explore fluid dynamics of this problem in addition to the structural

motions and deformation (What does the spray root behavior look like on high-speed craft?)

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Classical Wedge Drop Study Revisited• Prismatic Wedges with thin-bottom plates: Aluminum Alloys and

Composites

• Traditional measurements such as pressure, acceleration, heave

• Full-field strain measurements taken with Stereo- Digital Image Correlation to compute out-of-plane deflections

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High-speed cameras

Spatial Correlation

Camera 1 Camera 2

t1 t1

t2 t2

Semi-Planing Craft in Waves

• Scale-model hulls with thin-bottoms: Aluminum Alloys and Composites

• Traditional measurements such as pressure, acceleration, heave, pitch, roll

• Full-field strain measurements taken with Stereo- Digital Image Correlation to compute out-of-plane deflections

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Final Remarks

• Fluid-Structure Interaction problems are present in many every-day applications.

• The physics of this interaction is interesting and can provide many new innovations/designs.

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Off-Shore Wind Turbine Farm, Press-Release Photo from Siemens

Image courtesy of Combatant Craft Division (CCD) Little Creek

Off-Shore Wind Turbine Farm, Press-Release Photo from Siemens

References

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Ikeda, C., Fluid–Structure Interactions: Implosions of Shell Structures and Wave Impact on a Flat Plate. PhD thesis, University of Maryland, College Park, August 2012.