On Relative Positioning of Vessels

WORLD CLASS – through people, technology and dedication© KONGSBERG1

On relative positioning of vesselsTheoryPractical implementationReference systemsbyNils Albert JenssenKongsberg Maritime, Norway© KONGSBERG2

Relative PositioningApplicationsFree floating vessels• ROV following• Loading/ offloading PlatformSupply Vessels• Shuttle tanker loading• Accommodation vesselsVessels connected• Heavy lifting© KONGSBERG3

Heavy liftingLoad50002• Oscillations due to interconnectionbetween bodies40001– Too large DP bandwidthHookLoad SB [t]3000200010000Position0-1-2-3Surg e p osition deviation [m ](In fact must be almost zero)– Relative position measurements donot help– Special severe when lifting toSPAR buoy-1000-4-2000-523:15 23:20 23:25 23:30 23:35 23:40 23:45Time• Very different from free floatingbodies© KONGSBERG4

Relative PositioningControl theory aspect• Control problemThrustAbsolutepositionPositionother vessel-H RControllerHVessel -• Standard servo problemRelative positionRelativepositionPositionothervessel-H RControllerHVesselAbsolute position© KONGSBERG5

Reference TrackingPhase (deg) Magnitude (dB)100-10-20-300-45-90-135Bode DiagramRelativemeasurements onlyBadcombination• Result:– Bad response– Possible instability-18010 -4 10 -3 10 -2 10 -1 10 0Frequency (rad/sec)© KONGSBERG6

Control theory aspect• Classical servo problem– Feedback loop– Feed forwardH FFeed forwardPositionothervessel-H RFeedback+Absolute positionHVessel© KONGSBERG7

Feed forward• “Optimal” feed forward– Invert vessel dynamicsKH ( s)=s(1+ Ts)– Frequency independentresponse• No relative speed / accelerationmeasurements available• Differentiate relative position– Speed– Acceleration• Not FeasibleP hase (d eg) M agni t ude (d10050B ) 0-50-100180900-90-180Feed forwardBode Diagram10 -3 10 -2 10 -1Frequency (rad/sec)Vessel© KONGSBERG8

A practical solutionHysteresisSetpointgeneratorAccelerationAccelerationfeed forwardSpeedfeed forwardNecessaryfeedbacks/feedforwardsprovidedsyntheticallyRelativepositionSpeed-Speedfeedback+VesselPositionRelative positionoutside boundary↓Trig a moveCalculatetrajectory &feed forward-PositionfeedbackRun trajectoryPositionSpeed© KONGSBERG9

The Follow Target Mode• Reaction Circle– Operator defined circle within which thetarget can move without causing the vesselto follow– When target at edge of reaction circle,position setpoint is updated automatically torestore the vessel position relative to thetarget– Reaction circle is redrawn around the newposition of the target• Multi targets– Follow target position– Follow target headingTarget© KONGSBERG10

Operational safety issues• Need good absolute referencesystems– Takes care of the stability of theDP control• Need good relative referencesystems– Takes care of correct distance totarget• No good idea to mix relative andabsolute reference systems– Relative systems may “throughout” absolute or vice versa• What happens if all absolutereference systems are out ofservice?– Make relative measurementsabsolute!• Anything to do with the control?Phase (deg) Magnitude (dB)100-10-20-300-45-90-135Bode Diagram-18010 -4 10 -3 10 -2 10 -1 10 0Frequency (rad/sec)© KONGSBERG11

Operational safety issues cont.• Anything to do with he control?– Parametric changes not adequate– Structural problem– DP to be tuned to cope withtypical target motions• Class 2/3 requirement– Absolute reference systemredundancy- DGPS/ DARPS , Acoustics, Taut wire– Relative reference systemredundancy- RADius, Fanbeam, DARPS, Gangway– Both types crucial for safe operation– “Counting exercise” not sufficientB ) -20-40-60P hase (d eg) M agni t ude (d2000-45-90-135-180Effect of gain levelBode Diagram10 -4 10 -3 10 -2 10 -1 10 0 10 1Frequency (rad/sec)© KONGSBERG12

Conclusions• Special control algorithms needed– Possible stability problems– Feed forward from target motion necessary• Safety of operation– Emphasis on both relative AND absolute reference systemsThank you for your attention© KONGSBERG13

WORLD CLASS – through people, technology and dedication© KONGSBERG14