Measured Seafloor Penetrations of a Large, Free-Falling Cylinder and Comparison to Model Predictions by Philip Valent, Grant Bower, Stephen Theophanis,

Measured Seafloor Penetrations of a Large, Free-Falling Cylinder and Comparison to

Model Predictions

by

Philip Valent, Grant Bower, Stephen Theophanis, Charles King, Michael Richardson, all Naval Research Laboratory,

Andrei Abelev, University of Southern Mississippi, and

John Bradley, OMNI Technologies, Inc.

30 October 2002

OCEANS 2002 ConferenceBiloxi, Mississippi

Project Objective: To predict height, projected area, and volume of bottom mines protruding above mudline

Measured Seafloor Penetrations of a Large, Free-Falling Cylinder and Comparison to

Model Predictions

Existing Predictive Model…

• Solution is deterministic

• Neglects boundary layer effects

• 2-D, limits motion to vertical plane– disproven in at-sea experiments, September and

November 2000

• Overpredicts penetration

3-D Motion in Water Column Illustrated in 1/3rd Scale Model Tests at NSWC Carderock Division

Double click on the above images to start the video clip

To gather data describing impact burial event in muds we needed:

• Sensors and data storage internal to model mine shell– To contain fiber optic gyro needed minimum

shell outside diameter of 0.27 m

• Test site in lake or ocean

Designed and FabricatedFull-Size Instrumented Cylinder

• Diameter = 0.53 m

• Length = 2.40 m (L/D = 4.5)

• Weight in air = 10 kN (2,400 lbs)

• ‘Weight’ in seawater = 4.9 kN

• CM – CV = 0.104 m

• Three interchangeable noses: hemispherical, blunt, chamfered

Cylinder Instrumentation

• Fiber Optic Gyro

• Accelerometers– 2.5 g– 4 g– 10 g

• Magnetometer

Experiment Off Cocodrie, Louisiana,

January 2002• Water depth: 15 m• Distance above water surface: +1.0 or -0.5 m• Pitch: horizontal and 45o nose down

Processed Data from Cocodrie Depict

3-D, Chaotic TrajectoriesDrop #OrientationHeight (m)

1level-0.5

2level-0.5

345o

-0.5

445o

+1.0

545o

+0.5

6level+1.0

7level+0.2

8level-0.5

945o

-0.5

1045o

-0.5

1145o

+0.2

January 8, 2002

January 9, 2002Drop #OrientationHeight (m)

Water Depth = 15.5 meters

Processed Data from Two Cylinder Drops, Cocodrie, LA

Drop #1Released 0.5 mbelow water surface

Drop #5Released 0.5 mabove water surface

Distribution of vertical speed of cylinder approaching mudline, Cocodrie, LA

0

1

2

3

4

5

6

7

8

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Vertical velocity at mudline, m/s

Fre

qu

en

cy

0

1

2

3

4

5

6

10 20 30 40 50 60 70 80 90

Pitch at mudline, deg

Fre

qu

en

cy

Distribution of pitch, nose down, of cylinder approaching in mud, Cocodrie, LA

0

1

2

3

4

5

6

10 20 30 40 50 60 70 80 90

Pitch, deg

Fre

qu

en

cy

Distribution of pitch, nose down, of cylinder embedded in mud, Cocodrie, LA

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7 8 9 10 11

Test Number

He

igh

t a

bo

ve

mu

dlin

e (

m)

Measured

Predicted

Diver Measured and IMPACT28 Predicted Height Above Mudline,

Cocodrie Experiment

Accelerometer Measured and IMPACT28 Predicted Pitch Nose Down in Mud, Cocodrie Experiment

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11

Test Number

Pit

ch

No

se

Do

wn

(d

eg

)

Measured

Predicted

Prediction of Impact Burial

• A predictive model improvement in progress

• Cylinders in free-fall in water column display complex 3-D behavior

• For one site, sediment impact model:– underpredicts height proud– overpredicts change in pitch angle

Summary

Vertical Speed at Mudline Used to Initiate IMPACT28

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11

Test Number

Ve

rtic

al S

pe

ed

(m

/se

c)