A Comparison of Two Methods for Determining Wave Heights...
Transcript of A Comparison of Two Methods for Determining Wave Heights...
A Comparison of Two Methods for Determining Wave Heights from a Discus Buoy with a Strapped-Down Accelerometer
L. C. Bender III, N. L. Guinasso Jr., & J. N. WalpertGeochemical & Environmental Research Group
Texas A&M UniversityCollege Station, TX 77845 USA
S. D. HowdenUniversity of Southern Mississippi
Stennis Space Center, MS USA
Motivation
How does one get wave heights and directions from a heave/pitch/roll buoy equipped with an accelerometer and a tilt sensor?
Why should the buoy heel make a difference in the wave height calculations made from a heave/pitch/roll buoy?
MethodologyInstrumentation
Crossbow 3D accelerometer => ax, ay & azHoneywell HMR compass => heading, pitch & rollAll the raw data was saved on board.
Platforms3-m discus buoy2.25-m discus buoy
HurricanesKatrina – August 29, 2005Ike – September 12, 2008
TestOWIF at NDBC
Summary of Conclusions•Wave heights from a strapped-down 1D accelerometer can be overestimated during storm events
•if the buoy is small: 3m discus •if the buoy is heeled: >6o
•if the buoy is in shallow water: <100m
•A strapped-down 3D accelerometer can give comparable wave heights to an internally gimbaled accelerometer if the orientation of the strapped-down accelerometer is properly accounted for.
•Using a heave/pitch/roll buoy with a strapped-down 1D accelerometer to report on individual, large wave events in shallow water is not for the faint of heart.
What does an accelerometer measure?•Acceleration?•Force!•=> A Gravity Offset•Orientation is important•“Contamination” of g into three axes•Buoy heel becomes an issue for a strapped-down1D accelerometer.
•Two buoys and two hurricanes•One test•A theoretical model
Confirmation
INSTRUMENTATION
• Crossbow IMU400/440 accelerometer3-axis linear accelerationPitch, roll & yaw rate
• Honeywell HMR 3300 compassHeading, pitch and roll
Instrument Well
Crossbow Accelerometer
30' 95oW 30' 94oW 30' 93oW 30' 92oW 30'
27oN
30'
28oN
30'
29oN
30'
30oN
Buoy N Buoy V
Buoy FBuoy B
Buoy RHouston
1100
1700
2300
0500
2130
PLATFORMSHurricane Katrina
Hurricane Ike
Remove the Gravity Offset
I. Strapped-down 1D accelerometer only.Assume the tilt is small.Subtract 1g from the measurements,Instead of subtracting the component of gravity.
II. Strapped-down 3D accelerometer with pitch, roll & heading.Mathematically gimbal to get earth-referenced vertical acceleration.orz-axis only => 1D
26 27 28 29 30 31 010123456789
1011
August 2005
Sig
nific
ant W
ave
Hei
ght,
m
1DGimbaled
0 6 120
3
6
9
12
Hsig, m
Hsi
g, m
0 5 10 150
5
10
15
Peak Period, s
Pea
k P
erio
d, s
3 7 113
5
7
9
11
Mean Period, s
Mea
n P
erio
d, s
09 10 11 12 13 14 15 16 170123456789
10
September 2008
Sig
nific
ant W
ave
Hei
ght,
m
1DGimbaled
0 5 100
2.5
5
7.5
10
Hsig, m
Hsi
g, m
4 11 184
7.5
11
14.5
18
Peak Period, s
Pea
k P
erio
d, s
2 6 102
4
6
8
10
Mean Period, s
Mea
n P
erio
d, s
Hurricane Katrina
3m discus19m depth
Hurricane Ike
2.25m discus89m depth
Horizontal axes – 1DVertical axes – Mathematically
Gimbaled
Horizontal axes – 1DVertical axes – Mathematically
Gimbaled
0 5 10 15 20 25 30 350
2
4
6
8
10
12
14
16
18
Buo
y H
eel,
deg.
Wind Speed ms-1
0 5 10 15 20 250
2
4
6
8
10
12
14
16
18
Buo
y H
eel,
deg.
Wind Speed ms-1
Hurricane Katrina
Hurricane Ike
3m discus
2.25m discus
0 2 4 6 8 10 12 14 160
20
40
60
Buoy Heel, deg.
Hei
ght O
vere
stim
atio
n, %
0 5 10 15 20 25 300
20
40
60
Wind Speed ms-1
Hei
ght O
vere
stim
atio
n, %
Hurricane Katrina Hurricane Ike
0 2 4 6 8 10 12 14 16 18 200
10
20
30
40
Buoy Heel, deg.
Hei
ght O
vere
stim
atio
n, %
0 5 10 15 20 250
10
20
30
40
Wind Speed ms-1
Hei
ght O
vere
stim
atio
n, %
1m arm => HS = 2.83 m
A series of 38 runsPeriods: 6, 9, 12, 15, 18, 21 sTilts: 0, 8, 24 deg.Heel: 0, 20 deg.
Hippy 40SchaevitzInclinometer
Microstrain 3DM-GX1
Crossbow 400CC IMU
0 5 10 15 20 25 30 35 40 45-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
Time, s
Ver
tical
Acc
eler
atio
n, m
s-2
Hippy 40
Z-axis Crossbow
Mathematically GimbaledCrossbow
Hippy 40
15s period, 8o tilt, 20o heel
HS = 7.36m
HS = 2.75m
HS = 2.75m
HS = 2.72m
Summary of Conclusions•Be a discriminating user of buoy wave heights.
•Shallow water, smaller diameter buoys with a strapped-down 1D accelerometer can be significantly affected if the buoy is heeled.
•Deep water discus buoys with large diameter hulls and strapped-down 1D accelerometers are largely unaffected if the buoy is heeled.
•A strapped-down 3D accelerometer with pitch, roll and heading data can give comparable wave heights to an internally gimbaled accelerometer.
•Using a heave/pitch/roll buoy with a strapped-down 1D accelerometer to measure individual, large wave events in shallow water can lead to overestimated wave heights.
Questions?