09 use of sensors

8/4/2019 09 Use of Sensors

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Use of sensors.

Sensor redundancy.Voting.


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The most critical sensor for positioning is

the compass, because the headingmeasurement is used to determine thevessels position.

Gyro 1


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Gyro drift - two gyros

When two compasses are used, one of them will beselected as preferred. This means that the DP-system

will use this compass until the operator select the othercompass as preferred.

If one of two enabled compasses starts to drift, the DP-system alarm a compass difference. It may be difficult

for the operator to decide which one is drifting. If the preferred compass drifts, the DP-system will

follow it. To the DP it looks like the drifting compass isstable, while the vessel is changing heading. While the

other compass reads to correct heading. If the preferred compass drifts, you will lose some or allposition reference systems. The DP-system may lock onto one of them and follow that. This could cause a drive-off.


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Some position reference systems uses compassmeasurements directly. If such measurements drifts, the

position reference system will drift, and the DP-systemwill either reject or follow it. This depends on how manyposition reference systems are selected and theirperformance.

If a compass difference, compass rejected or failuremessage is given, always check the performance of theposition reference systems thoroughly and make thenecessary changes.

If you have three compasses installed, makesure that all three are enabled.This will activate the voting process which willautomatically reject a drifting compass.

Gyro Drift


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VRS measurements are used to correct position

reference systems for pitching and rolling.

If you have two sensors, select both. This will enable the

DP-system to display messages when one is showing

erroneous values.

When you get a difference the DP-system will give you

a VRS difference message.

If the preferred sensors is failing you may lose some or

all of your position reference systems depending on the

pitch and roll of the vessel. The DP-system will thencontinue on dead reckoning. This will normally allow

you time to switch to the other VRS before the vessel

drifts out off position.

VRS/VRU/MRU 1


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Some position reference systems uses pitch and roll

measurements directly. If such a sensor fails, the

position reference system will get more noisy, and the

DP-system will normally reject it.

If a VRS difference, VRS rejected or VRS failure

message is displayed, always check the performance ofthe position reference systems thoroughly and make the

necessary changes.

If you have three sensors installed, make surethat all three are enabled.

VRS/VRU/MRU 2


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Wind sensors are installed in different locations

onboard and will therefore often register readdifferently.

More than one wind sensor enabled will often lead to a

lot of unnecessary wind difference messages.

Normal practice is to enable only one windsensor at the time.

Wind Sensors


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Windsensor problems - 1


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VESSELINLEEOF

PLATFORM

WIND

PLATFORM

FORCEOFWIND

EXAGGERATED

ATWINDSENSOR

EVERYGUSTCAUSES

VESSELTODRIVE

TOWARDPLATFORM

ANDLOSEHEADING

WINDSENSOREXPOSED

TOFULLFORCEOFWIND

WIND

WINDSENSOR

EXPOSED

VESSELHULLAND

SUPERSTRUCTUREINWINDSHADOW

VESSEL

EXCURSION

STRUCTURE



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DOWNDRAFTFROMHELICOPTER

ROTORBLADESMAYCAUSE

WINDSENSORTOINPUTFALSE

HIGHVALUESTHUSDESTABILISING

THEDPCAPABILITY



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Other Sensors

Other Sensors

If there are redundancy of other sensors, select all. If a difference, rejected or failure messages is given,

make sure that this sensor is not used by the DP-system.


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First line voting.Three enabled sensor show different values.

Gyro 1 Gyro 2 Gyro 3

050 051 049

050

The computer will select and use the median value.

Input Voting


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Voting of input signals is a great help for the operator.

When two sensors are used, the computer can onlyreport difference between them and the operator have to

decide which one to use.

When three sensors are used and one of them are

showing a different value from the two others, thefailing sensor will automatically be voted out, i.e.

rejected by the computer.

Whenever three sensors are available, they should all be

enabled if they are likely to give the same reading.

Voting is most useful for sensors where the operator will

have problems deciding which one is correct.

Why and when to use Voting?


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First line voting.

Each computer select the median value.

A-computer

052 049 051

B-computer

049 050 051

C-computer

050 048 049

051 050 049

Second line voting.The three computers report their selected value to the others, and all three

computers vote again by selecting the median value.

TMR (Triple Modular Redundant) Input Voting

09 use of sensors

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