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On relative positioning of vesselsTheory

Practical implementationReference systems

byNils Albert Jenssen

Kongsberg Maritime, Norway

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Relative PositioningApplications

Free floating vessels

ROV following

Loading/ offloading Platform Supply Vessels

Shuttle tanker loading

Accommodation vessels

Vessels connected

Heavy lifting

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Heavy lifting

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0

1000

2000

3000

4000

5000

23:15 23:20 23:25 23:30 23:35 23:40 23:45

Time

Hoo

kLoa

d SB

[t]

-5

-4

-3

-2

-1

0

1

2

Surg

e po

sitio

n de

viat

ion

[m]

Oscillations due to interconnection between bodies

– Too large DP bandwidth(In fact must be almost zero)

– Relative position measurements do not help

– Special severe when lifting to SPAR buoy

Very different from free floating bodies

Load

Position

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Thrust

Relative PositioningControl theory aspect

Standard servo problem

Control problem

Absoluteposition

Relativeposition

Relative position

HRController-

HVessel -

Positionother vessel

HRController-

HVessel

Absolute position

Positionother

vessel

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Reference Tracking

Result:– Bad response– Possible instability

Bode Diagram

Frequency (rad/s ec)

Phas

e (d

eg)

Mag

nitu

de (d

B)

10-4

10-3

10-2

10-1

100

-180

-135

-90

-45

0-30

-20

-10

0

10

Badcombination

Relative measurements only

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Control theory aspect

Classical servo problem– Feedback loop– Feed forward

HRFeedback-

HVessel

Absolute position

Positionothervessel

+

HFFeed forward

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Feed forward

“Optimal” feed forward– Invert vessel dynamics

– Frequency independent response

)1()(

TssKsH+

=

No relative speed / acceleration measurements available

Differentiate relative position– Speed– Acceleration

Not Feasible

Bode Diagram

Frequency (rad/s ec)

Phase (deg)

Magnitude (dB)

-100

-50

0

50

100

10-3

10-2

10-1

-180

-90

0

90

180

Feed forwardVessel

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VesselRelativeposition

+

Acceleration

Position

Speed

- Positionfeedback

Setpointgenerator

- Speedfeedback

Speedfeed forward

Accelerationfeed forward

Position

Speed

Hysteresis

A practical solution

Relative position outside boundary

↓Trig a move

Calculate trajectory &feed forward Run trajectory

Necessary feedbacks/ feedforwardsprovided synthetically

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The Follow Target Mode

Reaction Circle– Operator defined circle within which the

target can move without causing the vessel to follow

– When target at edge of reaction circle, position setpoint is updated automatically to restore the vessel position relative to the target

– Reaction circle is redrawn around the new position of the target

Multi targets– Follow target position– Follow target heading

Target

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Operational safety issues

Need good absolute reference systems

– Takes care of the stability of the DP control

Need good relative reference systems

– Takes care of correct distance to target

No good idea to mix relative and absolute reference systems

– Relative systems may “through out” absolute or vice versa

What happens if all absolute reference systems are out of service?

– Make relative measurements absolute!

Bode Diagram

Frequency (rad/s ec)

Phas

e (d

eg)

Mag

nitu

de (d

B)

10-4

10-3

10-2

10-1

100

-180

-135

-90

-45

0-30

-20

-10

0

10

Anything to do with the control?

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Operational safety issues cont.

Anything to do with he control?– Parametric changes not adequate– Structural problem– DP to be tuned to cope with

typical target motions

Bode Diagram

Frequency (rad/s ec)

Phase (deg)

Magnitude (dB)

-60

-40

-20

0

20

10-4

10-3

10-2

10-1

100

101

-180

-135

-90

-45

0

Effect of gain level

Class 2/3 requirement– Absolute reference system

redundancy- DGPS/ DARPS , Acoustics, Taut wire

– Relative reference system redundancy

- RADius, Fanbeam, DARPS, Gangway

– Both types crucial for safe operation– “Counting exercise” not sufficient

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Conclusions

Special control algorithms needed– Possible stability problems– Feed forward from target motion necessary

Safety of operation– Emphasis on both relative AND absolute reference systems

Thank you for your attention

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