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Mitigating Excessive Pitch and Roll Motions for

Semi-Submersibles

Nils Albert JenssenKongsberg Maritime

/ 2 / 17-Oct-10

Background

• Observed large pitch and roll motions on (some) new 6. generation drilling rigs

• Natural pitch and roll periods significantly longer than for previous generation semis

• Resonance between hull and mooring in shallow waters

• Thrusters may amplify natural pitching and rolling

• Can problems be mitigated by using the thrusters?

• Is pitch and roll rate feedback control feasible?

/ 3 / 17-Oct-10

Recorded pitch motion

0 500 1000 1500 2000 2500-4

-2

0

2

4

6

8

Time (s)

(deg

) / (d

eg/s

)

Pitch

PitchPitch rateLF pitch rate

48 min

/ 4 / 17-Oct-10

Recorded pitch motion

Low frequency pitch rate

Pitch

Pitch rate

1,6 min

/ 5 / 17-Oct-10

0 0.1 0.2 0.3 0.4 0.5

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16Amplitude Spectrum of Pitch Rate

Frequency (Hz)

|Y(f)

|

Pitch rate control

• Observations:• Wave period ≈ 10 sec• Natural oscillation period ≈ 50 sec• Wave motion dominant

• Pitch rate signal can not be used for control directly

• Need to filter out wave frequency components

• Dual oscillator filter• Natural oscillation period• Wave period• Adaptive to changing wave

characteristics

Pitch rate

/ 6 / 17-Oct-10

Adaptive wave filtering

0 500 1000 1500 2000 2500-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (seconds)

Pitc

h ra

te

Time Series Plot:Pitch rate

Pitch rateFiltered rateLF rateEst. wave freq * 10

Measured pitch rate

Filtered pitch rate

Real low frequency pitch

rate

Wave frequency

/ 7 / 17-Oct-10

Vessel dynamics

• Pitch – surge motions are dynamically coupled

whereu surge velocityq pitch rateθ pitch anglem inertia parametersd damping parametersτ Thrust

ax

x

rgqdqmumudqmum

τθτ

=+++=++

55555551

111511

Hydro dynamical coupling

Mechanical coupling

/ 8 / 17-Oct-10

Vessel Standard DP Kalman filter

Pitch, rollPosition refsystems

GyroThruster feedbacks

Control

Position, headingVelocity, rate of turn

Residual force

Thruster commands

Control

Thrust command x

bu

Low frequency surge position

Low frequency surge velocity

Estimated residual forces

x

bu

xτ

bugxgxˆˆˆ 2221 +−−=τ

/ 9 / 17-Oct-10

Vessel

Standard DP Kalman filter

Wave frequency pitch & roll rate

filterPitch, roll rate

Control

Low frequency pitch, roll rate

Thruster commands

Control

Thrust command

q Low frequency pitch rate

q

qg ˆ51−x

bu

Low frequency surge position

Low frequency surge velocity

Estimated residual forces

xτ

bugxgxˆˆˆ 2221 +−−=τ

NEW

/ 10 / 17-Oct-10

Simulation example

• Vessel and DP system is simulated

where is a disturbance• slowly varying random value• pulse 50 tonnes for 100 sec

ax

x

rgqdqmumudqmum

)(55555551

111511

δτθδτ+=++++=++

qgbugxgx ˆˆˆˆ 552221 −+−−=τ

=

measured

measuredx

qx

qgbuxf

qb

ux

)ˆ(00),ˆ,ˆ,ˆ(

ˆ

ˆˆˆ

τ

δ

0 500 1000 1500 2000 2500-1500

-1000

-500

0

500

1000

1500

Time [s]

kN

/ 11 / 17-Oct-10

Normal DP control- Almost persistent pitching

0 500 1000 1500 2000 2500-2

-1

0

1

2

Time [s]

[deg

/s]

Pitch rateFiltered rateLF rateEst. wave freq * 10

0 500 1000 1500 2000 2500

-5

0

5

Time [s]

[deg

]

PitchEst LF PitchLF Pitch

0 500 1000 1500 2000 2500-20

-10

0

10

20

Time [s]

[m]

SurgeSurge Est

0 500 1000 1500 2000 2500-1500

-1000

-500

0

500

1000

1500

Time [s]

[kN

]

Demand ThrustObtained Thrust

Pitch rate

Pitch

Surge

Thrust

/ 12 / 17-Oct-10

DP with pitch rate feedback- Well damped pitching

0 500 1000 1500 2000 2500-2

-1

0

1

2

Time [s]

[deg

/s]

Pitch rateFiltered rateLF rateEst. wave freq * 10

Pitch rate

Pitch

Surge

Thrust

0 500 1000 1500 2000 2500

-5

0

5

Time [s]

[deg

]

PitchEst LF PitchLF Pitch

0 500 1000 1500 2000 2500-20

-10

0

10

20

Time [s]

[m]

SurgeSurge Est

0 500 1000 1500 2000 2500-1500

-1000

-500

0

500

1000

1500

Time [s]

[kN

]

Demand ThrustObtained Thrust

/ 13 / 17-Oct-10

Thruster command details

• Pitch rate control does not require significant additional thrust

0 500 1000 1500 2000 2500-1200

-1000

-800

-600

-400

-200

0

200

400

600

800

Time [s]

kN

No pitch rate controlPitch rate control

With pitch rate control

Normal control

/ 14 / 17-Oct-10

Conclusions

• The overall pitching and rolling can be reduced• A significant damping of the low frequency pitch and roll

motions may be expected• For sudden external excitation the pitch rate control can

not significantly improve the immediate transient pitching but will dampen the effect over time

• No improvements can be expected for the wave frequency components

0 500 1000 1500 2000 2500-3

-2

-1

0

1

2

3

4

Time [s]

LFpi

tch

[deg

]

Pitch rate controlNo pitch rate control

Low frequency

pitch

With pitch rate control

Normal control

WORLD CLASS – through people, technology and dedication

Thank you for your attention

Questions?

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