DP Sensors Input Mangement

DP Operator Course DP Sensors Input Management

Training Manual

July 2010 Kongsberg Maritime AS Page 4.5.1

Rev. 11 Training

DP Sensors

Input Mangement

Introduction

The Kongsberg Dynamic Positioning (K-Pos DP) systems need several different sensors to

determine the position, heading and to calculate the necessary thrust output to position the

vessel.

In this chapter we will describe the different sensors and give some advice on how to handle

the different sensors.

DP Sensors Input Management DP Operator Course

Training Manual

Page 4.5.2 Kongsberg Maritime AS July 2010

Training Rev. 11

Heading sensors

There are several different types of heading sensors in use on DP vessels. The most common

type is the traditional gyro compass. More accurate and reliable but also more costly heading

sensors are the ring laser gyro and the fibre optic gyro. The GPS compass has precise

accuracy but is dependent on reliable GPS signals.

Sperry Gyro compass Octans Fibre Optic Gyro SEAPATH GPS compass

The gyrocompass finds north by using an electrically powered fast spinning wheel and

friction forces in order to exploit the rotation of the earth.

Laser gyro or fibre optic gyro uses interference of laser light within a bulk optic ring or a coil

of fibre optic cable to detect changes in orientation and spin and thus to find north.

The GPS compass precise heading is derived from the fixed-distance dual GPS antenna

arrangement to generate heading information.

Minimum one heading sensor must be enabled at all times to provide heading information to

the DP system for automatic control of heading.

Most DP vessels use traditional gyrocompasses. More accurate heading references than gyro

may be needed for operations like survey, rock dumping and construction work.

For DP operations in DP class 2 or 3 it is required to use three gyros. DnV is specifying that

“One of the three required gyros may be replaced by a heading device based upon another

principle, as long as this heading device is type approved as a TDH (Transmitting Heading

Device) as specified in IMO Res. MSC.116 (73). For notation DYNPOS-AUTRO this is not to be

the gyro placed in the alternate control station.”


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Gyro in position calculations

The heading input is first of all used by the DP system for automatic

heading control, but the heading is also used to find the vessels

position from the position measurements for each individual

position reference system.

Conversion from antenna position to vessel’s centre

The position-reference systems are measuring the position of an

antenna, transducer or gimbal’s head. The DP system needs to

convert those antenna positions to the vessel’s centre in order to

have one common point where all the position-reference systems

are compared. To do this we need to know the antennas’ position on

the vessel and the vessel’s heading.

Position calculation for relative position-reference systems

Most position-reference systems measure the position relative

to the vessel. For instance a Fanbeam laser system, measures a

Range and a Relative Bearing. For the DP to be able to

calculate a position, the DP needs to find the True Bearing;

True bearing = Heading + Relative Bearing.

The examples show that two different headings will give two

totally different positions with the same Range and Relative

Bearing. The examples are to illustrate how the gyro is used in

position calculations for relative position-reference systems,

but one can also understand that errors in the gyro/heading can

have impact on the position calculation.

Gyro input to HPR/HiPAP

For the HPR systems the conversion to centre is normally made by the HPR system itself. The

main reason is that there are often other users of the measurements that need the

measurements converted to the centre of the vessel.

This transformation is not only from the transducer to the centre of the vessel, but for the

whole measurement transducer->transponder->transducer->centre.

Where both range and direction to the transponder are measured, (SSBL) an error in the gyro

to HPR can give a significant position error, depending on the magnitude of the error and the

distance to the transponder. If only ranges to transponders are measured, (LBL) an error in the

gyro will only affect the transducer to centre of vessel conversion.

Which and how many gyros that are interfaced to a HPR/HiPAP system vary from vessel to

vessel. Some vessels have only one gyro interfaced to the HPR/HiPAP while others can have

up to three. If more gyros are interfaced, the operator can select which gyro to use on the

HPR/HiPAP operator station or by means of a dedicated gyro switch.

REMEMBER

• The heading input is used to calculate position for relative position reference systems.

• The DP system is converting all position-reference systems to the centre except for

the HPR/HiPAP where this is handled by the HPR-system itself.

• Gyro accuracy is of importance to get an accurate position calculation.

Antenna

position

Vessel

position

Antenna

position

Vessel

position

Range 100 m

Rel. dir 90°

Ra

ng

e 1

00

m

Re

l. dir 9

0°

Platform

Laser reflector

Heading 000°

Hea

din

g 0

90

°

Ex 1 Position

Ex 2 Position

True Bearing = Heading + Relative bearing

True Bearing = 000° + 90° = 090°

True Bearing = 090° + 90° = 180°


Training Manual

.5.4 Kongsberg Maritime AS July 2010

Training Rev. 11

Latitude and speed corrections in gyros.

The gyro compass settles with its North-South axis at right angles to the resultant movement

of the Earth's rotation and the ship's movement. The error is westerly when sailing north and

easterly when sailing south. Sailing east or west gives no error. The magnitude of the error is

related to the vessels speed, course and latitude.

This error can be corrected for in most gyros. Traditionally one has set the latitude and speed

by means of knobs on the gyros panel. If the vessel had a speed log, this was normally

connected to the ships gyro, automatically compensating for the speed.

Today you will often find that a GPS is connected to the gyro in order to get an automatic

compensation for both speed and latitude. You need to be aware of that this connection

creates a new failure mode for DP where a position calculation error in the GPS will give a

wrong correction both for speed and latitude resulting in a sudden change in the gyro reading.

This error can easily become more than 10° because of high calculated speed in the GPS.

As described earlier, the vessel heading is also used to calculate measured positions from the

different position reference systems to the centre of the vessel, so a gyro error will also effect

the position of the vessel.

Exactly what will happen if the speed and latitude correction to gyros becomes wrong

depends on the vessel installations.

• If all gyros are corrected from different GPSes, a problem with satellites can still give

the same error in all of them. In this case you might get a heading out of limit message

and possibly a position out of limit which will be corrected by the DP by changing the

vessels heading and position. This again might effect the operation, and if close to

another object, possibly cause a collision.

• If the different GPSes is giving different speeds and latitudes, the gyros will be

corrected differently. A gyro might then be rejected, and if this leads to a change of

used gyro in the system, and there is a difference between the gyros, this again could

lead to a change in position and heading.

The only way to avoid this problem is to switch off the automatic speed and latitude

compensation in the gyros when on DP. Note that this problem will also affect the auto pilot

when sailing.

When operating on DP the vessel will be stationary or move slowly, and thus the speed error

in the gyros will be so small that they can be ignored and the automatic correction is not

needed.

RECOMMENDATION!

Kongsberg Maritime recommend that when on DP you should change the automatic

correction mode in the gyros to manual mode and set the correct latitude and the speed

to zero on the gyros own operator panel.


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Operation/selection

The gyro dialogue is opened from the Sensor menu or by pushing the Gyro button on

the operator panel.

Sensor OK is a status signal coming

from the sensor itself and makes the

heading sensor available for selection.

Sensor Enable is a status set by the

operator to give the DP system access to

use the sensor.

The operator will normally enable all

available and reliable sensors.

There can be a maximum of three

sensors enabled at one time.

Sensor Preference is a status set by the

operator to tell the DP system which

sensor to use.

This sensor will be used as the DP

systems heading reference as long as the

sensor is reported OK and it is not rejected by any of the tests.

The operator should set Preference on the most reliable sensor.

In Use is a status signal coming from the DP controller indicating which sensor is used by the

system.

Gyro Heading is showing the heading received from each of the sensors.

Added Correction is a value the operator can set when a sensor is not enabled to correct the

sensors reading if it is not correct. Added correction can be used to compensate for an offset

of the ships gyrocompass compared with a surveyor’s gyrocompass. To ensure consistent data

for all users of a gyrocompass, it is recommended to adjust the gyrocompass itself.

Used Heading is the heading the DP system is using. This is the Gyro Heading + Added

Correction

Sensor view

The sensor view is showing each sensor reading, deviation between sensors and a historical

graph.

Gyro status lamp

The Gyro button on the operator panel has a status lamp that has three different statuses:

ON: at least one gyro enabled and accepted by the system

FLASHING: problems with either measurements (if more than one gyro selected), or

problems with the signals from the gyro.

OFF: no gyrocompasses are enabled.


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Tests, limits and messages

Message Description Message explain

Gyro difference

When 2 heading sensors are used and the

difference between them is more than 2° the

Gyro difference alarm will be given.

The DP operator needs to decide which of

the sensors are correct and then set

Preference on this sensor. The other sensor

should be disabled.

NOTE! If you switch to a sensor which has a

difference higher than the gyro prediction

error limit for the system you will get a Gyro

Prediction Error message and the sensor will

be rejected. (See the description of Gyro

Prediction Error). This implies that action has

to be taken immediately if the sensor that is

set as Preference starts to drift.

Description This message is reported if the difference between the

values read from two sensors exceeds a limit. The

reason for the message may be an error in one of the

sensors.

This message can only be given if two sensors are

enabled.

Additional information

Channel

Sensor number 1

Sensor number 2

Possible consequences

The system may use data from the sensor that has an

error.

Corrective actions

No action is necessary if the error message is only

reported occasionally. If reported frequently, check

which of the sensors that have the error, and if needed,

disable and repair the sensor.

Gyro rejected

When 3 sensors are used and one of them

deviates more than 2° from the median (the

one in the middle) it will be rejected.

If it is the sensor that is set as Preference

which is rejected, the DP system will switch

to the median sensor.

(The Preference will not switch, but the In

Use sensor will switch.)

The Gyro Rejected alarm will be given.


value that is read from the sensor and the median value

of all the enabled sensors of this type exceeds a limit.

The data from the sensor is not used by the DP system.

The reason for the message may be a faulty sensor.

This message can be reported when three or more

sensors of this type is enabled.


Channel


The DP system will automatically use data from the

other enabled sensors.

Corrective actions

No actions are necessary if the error message is only

reported occasionally. If reported frequently, disable

and repair the faulty sensor.

Gyro Prediction Error

If there is a difference of more than 6°

between the model and the measurement the

sensor is rejected by the prediction test. The

limit may be different for different vessels

and is set between 4° to 10°.

Note!

This message is given if two sensors are

enabled. If one sensor is enabled the system

will go directly to Heading Dropout and if

three sensors are enabled the failed sensor

will be rejected by the median test (voting).

Description

This message is reported if the difference between the

value read from the sensor and the Vessel Model value

exceeds a limit. This message is only given if two

sensors are enabled.

The data from the erroneous sensor is not used by the

system.

The reason for the fault may be an error in the sensor.


The DP system will automatically use data from the

other sensor (if enabled).

Corrective actions

Check the sensor - DP interface.

Consider to disable the sensor.


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Description Message explain

Heading dropout

If all sensors are rejected or lost (not ready)

the system will give the Heading Dropout

alarm.

If all gyros are rejected, this is due to a test

that will reject the inputs if there is a

difference of more than 6° between the DP

Vessel Model and the measurements. This

limit may be different for different vessels

and is set between 4° to 10°.

NOTE!

In a Heading dropout-situation all position

reference systems are lost. This will lead to

Position Dropout alarm 30 seconds later.

Description

This message is reported if no Gyro readings have

been received for the past 2 seconds. The Gyro may

not be ready, there may be an error in the connected

Gyro or in the interface between the Gyro and the DP

system. This is a serious alarm if the vessel is in DP

mode.


The system uses the Vessel Model to estimate the

vessel's heading. If the weather conditions change, the

vessel may have a position drift-off after a few

minutes. When the "HEADING DROPOUT" message

is reported, the system will stop any vessel movement

and automatically set the wanted position and heading

equal to actual position and heading. If only one of the

DP PSes (Process Stations) in a dual redundant system

reports heading dropout, an automatic change of

master will occur.

Corrective actions Observe the deviation data and go to Joystick mode if

the vessel starts drifting off. Check if the Gyro is

ready/drifting or if other error messages indicate

interface errors.

Gyro not ready

One of the enabled heading sensors has lost

its OK signal. If this was the sensor set as

Preference, the system has automatically

switched to the next OK and enabled sensor.



Description This message is reported if the sensor is not Ready for

DP or an error is detected by the sensors own BITE

(Built In Test Equipment).

The method of detection is by readings of signals from

the sensor's "health circuits".

Corrective actions Enable another sensor.

Gyro not enabled This message is given if no sensors are

enabled.

Description

This message is reported if no sensor of this type is

enabled by the operator.


The sensor value will be set to zero.

Corrective actions Enable a sensor.

Types of input

For redundant DP systems the heading sensors will be directly interfaced to the DP controller.

For simpler systems the heading sensor may come from the Gyro switch belonging to the

navigation system.

In modern systems the heading sensor is using a serial communication interface. In older

systems the gyro signals was of synchro signal type.


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Notes on gyro alignment

Accurate alignment of the gyro axis to the ships axis is crucial to the accuracy of the DP.

A gyro alignment error will cause the following problems. 1. The vessels heading is wrong

2. The internal coordinate system in the DP is incorrect

3. The models position displayed on the DP screen is wrong.

4. The position reference systems will drift apart and the vessel will move when the vessel is changing

heading.

5. The position reference systems will drift apart when the vessel is moved.

The following example illustrates this.

Start situation:

Gyro alignment error 1° to

starboard.

All three PRS calibrated OK

• The GPS antenna is 10 meter ahead and 0.5 meter to port of the centre of the vessel.

• The Fanbeam unit is placed 11.6 meter ahead and 3.5 meter to port of the centre of the vessel and the

Fanbeam reflector is 50 meter to the port of the vessel.

• The HPR transponder B01 is 112 meter to the port of the vessel.

Situation 1:

The vessel has rotated 90° to

port from the initial heading.

Situation 2:

The vessel has moved 64

meter NW from the initial

position.


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Gyro deviation monitoring

The data from each gyro can be monitored and evaluated using Gyro Deviation

Calculation. This function is based on the fact that the vessel heading can be derived

from the relative positions between two GPS antennas. The error for each gyro is

estimated from the filtered difference between the GPS derived heading and the gyro

data. Recommended minimum antenna separation is 10 m, and the accuracy of the

computed heading increases with distance between the antennas.

To display the Gyro Deviation dialog box, select Sensors→Gyro Deviation.

The Gyro Deviation Calculation uses the Used

Heading values (see Sensors dialog box - Gyro

page) as input for the calculations.

Active enables the Gyro Deviation Calculation.

Filter Time: The default value is the recommended

minimum value for your vessel. The shorter the

distance between the GPS antennas, the longer the

time required for data filtering.

Correction: For each gyro the difference between

the computed heading and the used heading from the

gyro is displayed.

Std.Dev for each estimate.

If the Correction is one degree or more, and the

Std.Dev is significantly smaller, you should correct

the error on the gyro.


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Vertical Reference Sensors

At least one vertical reference sensor must be available to provide the system with roll and

pitch information. This information is used to adjust the measurements received from the

position-reference systems for the vessel’s roll and pitch motions.

From the two examples above we see how the position measurements are effected by the

vessels motions. These effects are removed by referring the position reference systems to a

fixed horizontal plane generated by the Vertical Reference Sensor.

There are different types of sensors in use. The common term is VRS – Vertical Reference

Sensor. We are distinguishing between different types of sensors by giving then different

names.

VRU Datawell VRS MRU

VRU – Vertical Reference Unit. This term was used for a vertical gyro type used in the first

DP systems. The VRU is a unit used in the aviation industry to establish the artificial

horizon for the pilot.

VRS – Vertical Reference Sensor is the term we used for an analogue sensor made by

Datawell. A sphere half filled with a fluid is giving a horizontal plane. The motion of

the outer housing which is fixed to the vessels hull is measured by means of


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induction between a coil floating on the surface of the fluid and coils fixed to the

housing of the unit.

MRU – Motion Reference Unit is the modern sensor used today based on inclinometers and

acceleration sensors.

If the VRS is equipped with a heave sensor, the heave information is used for monitoring

purposes only.

If VRS information is lost, the system will be unable to compensate the received position

measurements for vessel motion. The positioning performance of the system can then be

severely degraded.

VRS input to HPR

As for gyros, the VRS pitch and roll measurements are fed to the HPR/HiPAP and are used in

the transformation of the position measurements to the centre of the vessel.

Be aware that both for gyro and VRS, this transformation is not only from the transducer to

the centre of the vessel, but for the whole measurement transducer->transponder->transducer-

>centre.

Where both range and direction to the transponder are measured, (SSBL) an error in the VRS

to HPR can give a significant position error, depending on the magnitude of the error and the

distance to the transponder. If only ranges to transponders are measured, (LBL) an error in the

VRS will only affect the transducer to centre of vessel conversion.

If more VRS’s are interfaced, the operator can select which sensor to use from a dialogue box

in the HPR/HiPAP operator station.

Operation/selection

Vertical reference sensors are enabled and controlled using the Sensors dialog box - VRS

page. Normally, all the available VRS’s will be enabled for use. The system then receives and

compares the signals from all the VRS’s, but uses only one of them. You can specify which

VRS is preferred for use by the system. If no errors are detected in the VRS measurements,

the system will always use the operator-preferred sensor (for which Preference is selected on

the Sensors dialog box - VRS page).


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sensor available for selection.



use the sensor.

The operator will normally enable all

available and reliable sensors.

There can be a maximum of three sensors

enabled at one time.

Sensor Preference is a status set by the

operator to tell the DP system which

sensor to use.

This sensor will be used as long as the

sensor is reported OK and it is not

rejected by any of the tests.



system.

Pitch Roll and Heave is showing the pitch roll and heave received from each of the sensors.

Sensor view

The sensor view is showing each sensor reading for pitch, roll and heave, indicating which

sensor are used (white background) and the root mean square (RMS) values of the sensor

used.

VRS status lamp

The VRS button on the operator panel has a status lamp which shows the status of the VRS:

ON At least one VRS is enabled and accepted by the system.

FLASHING The measurements from one of the enabled VRSs are not accepted by the

system or an error situation exists in at least one of the VRS channels if only a

single VRS has been enabled.

OFF No VRS’s are enabled.

Types of input

The VRS sensors are directly interfaced to the DP controller.

The old VRUs were using synchro signals.

The Datawell VRS is using analogue signals ± 10 volt.

The MRUs can be interfaced either by serial communication or analogue ± 10 volt signals


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VRS Difference

When 2 sensors are used and the difference

between them is more than 2° the VRS

difference alarm will be given.




should be disabled.

Description This message is reported if the difference between

the values read from two sensors exceeds a limit. The


sensors.


enabled.


Channel, Sensor number 1, Sensor number 2



error.

Corrective actions No action is necessary if the error message is only


which of the sensors that have the error, and if needed,

disable and repair the sensor.

VRS Rejected

When 3 sensors are used and one of them

deviates more than 2° from the median (the

one in the middle) it will be rejected.

If it is the sensor set as Preference which is

rejected, the DP system will switch to the

median sensor.



The VRS Rejected alarm will be given.


value that is read from the sensor and the median value

of all the enabled sensors of this type exceeds a limit.

The data from the sensor is not used by the DP system.

The reason for the message may be a faulty sensor.

This message can be reported when three or more

sensors of this type is enabled.

Additional information Channel (e.g. for VRS; Roll Pitch or Heave)

Possible consequences The DP system will automatically use data from the

other enabled sensors.

Corrective actions No actions are necessary if the error message is only

reported occasionally. If reported frequently, disable

and repair the faulty sensor.

VRS not ready

One of the enabled sensors has lost its OK

signal. If this was the sensor set as



Description

This message is reported if the sensor is not Ready for





Corrective actions Enable another sensor

VRS not enabled

This message is given if no sensors are

enabled.

Description




The sensor value will be set to zero.

Corrective actions

Enable a sensor.


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Wind sensor

At least one wind sensor should normally be enabled to provide the system with wind speed

and direction information.

Normally, all available wind sensors will be enabled. The system then receives and compares

the signals from all the sensors, and if there are differences exceeding certain limits between

the sensors, a Wind difference message will be reported. Only one of the sensors is In Use to

calculate the wind force acting on the vessel. That is normally the sensor that the operator has

set as Preference.

Wind Feed-Forward

In order to counteract the wind forces as quickly as possible, the feed-forward concept is used.

This means that the DP system will not allow the vessel to drift away from the required

position, but automatically compensates for the wind forces acting on the vessel by providing

the thrusters with the necessary extra thrust in the appropriate direction as soon as the wind

forces are detected.

Operating without wind sensor input

There can be situations where you want to temporarily stop using the input from the wind

sensors, such as during helicopter operations or when operating close to another large

structure where there may be sudden disturbances of the measured wind. If you disable all the

wind sensors, the system continues to use the wind speed and direction values that were

measured just before the most recent wind sensor was disabled.


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Types of wind sensors

There are different types of wind sensors in use.

Gill Wind observer Lambrecht wind sensor

Ultra sonic sensor Mechanical sensor Mechanical sensor

The ultra sonic wind sensor is using the Doppler Effect to measure the wind speed in X and Y

directions. Two sensors opposite of each other are sending an ultrasound pulse to each other.

By comparing the time difference for the sound to travel one direction versus the other, the air

speed is determined. Measuring the air speed in the two directions X and Y gives sufficient

information to calculate the wind speed and direction.

The mechanical wind sensors have a form of propeller driving a small generator. The current

produced is a measure of the wind speed. The wind direction is measured by means of a

weather vane connected to a potentiometer fed with a fixed voltage at the end terminals. The

voltage measured from the slide of the potentiometer is then a measurement for the wind

direction.

Definitions

Relative wind is the speed and relative direction from which the wind appears to blow, with

reference to the moving vessel.

True wind is the speed and direction from which the wind is blowing. The vessel’s speed and

direction of motion are removed from the relative wind measurements.

Wind sensors position on the vessel

A vessel needs to have several wind sensors

placed in different places onboard the vessel

to make sure to have at least one sensor

reading the correct wind at all times. As the

wind direction changes, the DP operator

must set Preference on the best sensor. If

several sensors are enabled and they show

considerable difference, the DP system will

notify the operator by giving a Wind

difference message.


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Wind sensor problems

Different situations might necessitate wind sensors to be de-selected. If all wind sensors are

de-selected, the DP system will continue to use the wind from the last period.

Wind sensor in turbulent area while the vessel is hit

by the wind coming under the platform. This gives

erroneous wind compensation from the DP system

and thus unstable positioning.

Wind sensors in the mast are measuring full wind

while the vessels hull is in sheltered area. This

gives over-compensation from the DP system and

as a result the positioning will be unstable.

A helicopter can disturb the wind measurements

resulting in unwanted reactions from the DP system

When sitting close to another installation you must

be aware of possible hazards like exhaust outlets or

water spray that may hit the wind sensors.


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Operation/selection

Wind sensors are enabled and controlled using the Sensors dialog box -Wind page.

You can specify which wind sensor is preferred for use by the system. If no errors are

detected in the wind sensor measurements, the system will always use the operator-preferred

sensor (for which Preference is selected in the Sensors dialog box Wind page).



sensor available for selection.



use the sensor.

The operator can enable all available and

reliable sensors, but since the sensors are

placed indifferent positions onboard the

vessel they might show very different

readings. In such situation it will be

normal to deselect the sensors which are

not giving a correct reading to avoid

having wind difference messages

repeated continuously from the DP

system.

Sensor Preference is a status set by the operator to tell the DP system which sensor to use.

This sensor will be used by the DP systems as long as the sensor is reported OK.



system.

Relative Speed is showing the wind speed received from each of the sensors.

Relative direction is showing the wind direction received from each of the sensors.


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Sensor view

The sensor view is showing each sensor’s

relative reading for wind speed and

direction, measured at the wind sensor. The

height for each sensor is given. The view is

indicating which sensor is used with a

white background in the speed and

direction fields.

The raw measurements of wind speed and

direction are filtered internally, to estimate

the most reasonable speed and direction

values to be used by the K-Pos DP system.

The present mode and whether or not one

or more sensor is enabled, determine which

values are displayed in these fields:

• In Standby mode with one or more wind

sensor enabled, the True Speed and True

Dir fields display the same values as

Relative Speed and Relative Dir.

• In any mode other than Standby with one or more wind sensor enabled, the True Speed and

True Dir fields display the true wind speed and direction values (filtered values).

• In any mode with no wind sensors enabled, the True Speed and True Dir fields contain the

manually-entered values for the true wind speed and direction.

Wind status lamp

The Wind button on the operator panel has a status lamp which shows the status of the Wind

sensors:

ON At least one wind sensor is enabled and accepted by the system.

FLASHING The measurements from one of the enabled wind sensors are not accepted by

the system or an error situation exists in at least one of the wind sensor

channels if only a single wind sensor has been enabled.

OFF No wind sensors are enabled.

Types of input

The Wind sensors are directly interfaced to the DP controller.

Older Wind Sensors are using analogue signals 0 to +10 volt.

Newer Wind Sensors are communicating the data over serial communication lines.


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Wind difference

When more than one sensor is enabled and

the difference in speed is more than 6 m/s or

direction more than 45.8° the Wind

difference alarm will be given.




should be disabled.


values read from two sensors exceeds a limit. The


sensors.


enabled.


Channel, Sensor number 1, Sensor number 2



error.

Corrective actions No action is necessary if the error message is only


which of the sensors that have the error, and if

needed, disable and repair the sensor.

Wind sensor rejected

Faulty jumps in readings from a wind sensor

would lead to an unwanted increase in thrust

setpoint.

If the wind speed is above a predefined limit

a wind sensor will be rejected if the wind

measurement increases by more than a certain

limit over a certain interval or changes

direction of more than a certain limit over a

certain interval.

The alarm Wind sensor rejected is given:

The input data to the test is the difference

between new measurements, and low pass

filtered speed data with a small time constant.

A rejected sensor will be accepted again if it

is deselected and then selected again or if the

DP system is set to Standby mode

Description

The difference between the value received from the

sensor and previous values received exceeds fatal limit

and the sensor is rejected.

The reason for this fault may be an error in the sensor,

or its interface to the PS (Process Station).


Failed measurement (Speed or Direction)

Measured value (m/s or degrees)

Filtered value (m/s or degrees)


Wind data are frozen if only one wind sensor is

enabled. DP will loose dynamic update of wind data.

Corrective actions

Disable failed sensor.

Check failed sensor and its interface to the PS.

Wind Sensor not ready

One of the enabled sensors has lost its OK

signal. If this was the sensor set as





Description

This message is reported if the sensor is not Ready for





Corrective actions Enable another sensor.

Wind Sensor not enabled

This message is given if no sensors are

enabled.

Description



Possible consequences The sensor value will be set to zero.

Corrective actions

Enable a sensor.


Training Manual


Training Rev. 11

REMEMBER

Remember the following rules for automatic and manual switching of sensors!

With 3 Gyros

The DP system will use the gyro set as Preference as long as it is inside the median limit of

2°.

If the sensor set as Preference is voted out, the median gyro will be used.

The rejected gyro should be disabled.

With 2 Gyros

The DP system will normally always use the gyro set as Preference.

If the difference between the two gyros is more than 2° the Gyro difference alarm will be

given.

The DP operator has to decide which Gyro is correct and thus set Preference on that Gyro.

The faulty gyro should be disabled.

The DP Vessel Model will follow the gyro set as Preference. The DP operator should switch

to the correct gyro before the difference is more than 6° between the DP Vessel Model and

the correct gyro. If the difference is more than 6° when the switching is performed, the system

might go into Heading Dropout. (Limit is normally 6°, but can be configured from 4° to 10°)

If the Heading Dropout situation persists, to resolve the situation, the DP system must be set

to Standby mode before going back in with correct inputs.

Gyro OK

If the gyro set as Preference looses the OK signal the “next” gyro will be used.

The automatic switching will be from sensor 1 to sensor 2, from sensor 2 to sensor 3 and from

sensor 3 to sensor 1.

VRS

The same principles for switching apply as for Gyro.

Wind

DP system will always use sensor set as Preference if sensor is OK.

The operator should set Preference on the most correct sensor according to the prevailing

conditions and disable faulty sensors if necessary.

Switching as for Gyro, but there is no voting for wind sensors.


Training Manual


Rev. 11 Training

OTHER SENSORS

Draught sensor

For optimum positioning performance, the system must have accurate information regarding

the vessel’s draught at all times.

The vessel draught can either be specified by the operator or measured by a draught sensor.

The source of draught information is selected and controlled using the Sensors dialog box -

Draught page.

If the information from the draught sensors is correct and reliable, then this should be used in

preference to manually-entered or fixed values.

Sensor: When Sensor is selected, you

can enable the draught sensor that is to

be used. If more than one sensor is

enabled, the system uses the average of

all the enabled sensors. If Sensor is

selected but no sensors are enabled, the

Manual value is used by the system.

Manual: When manual is selected, the

draught value entered under Fixed

Draught is used by the system. If you

try to enter a value that is too high or

too low, the value is rejected by the

system and a message informing you

about the legal range for draught is

displayed.

Operation: When Operation is

selected, the predefined operational

draught value is used by the system.

Transit: When Transit is selected, the predefined transit draught value is used by the system.

Used Draught shows the draught value that is currently used by the system. This field is for

information only.


Training Manual


Training Rev. 11

Speed sensor

Speed measurements can be used as an addition to position measurements to improve the

vessel speed control in high speed modes (Auto Pilot, Auto Track High Speed and Seismic

Track). The speed information can be from different sources, e.g. Doppler Log sensors and

GPS speed sensors. You can enable available speed sensors or enter a manual alongships

vessel speed value. In high speed operations all enabled sensors will be used by the system

and an average is calculated. It is recommended to also enable Manual speed input since the

Manual value will be updated based on the In Use sensors. Should all sensors be lost, you then

have an initial Manual value that reflects the real situation.

Using the Sensors dialog box - Speed page:

Doppler Log Enable enables the

Doppler Log speed sensor.

In Use is showing that the sensor is

used by the system.

Along shows the measured alongships

vessel speed.

Athwart shows the measured

athwartships vessel speed.

The Doppler Log speed is marked with

G or W depending on the type of speed

output:

• G — Speed over ground

• W — Speed through water

Gps Speed Enable enables the GPS

speed sensor.

In Use is showing that the sensor is used by the system.

Speed shows the measured vessel speed.

Course shows the measured vessel course.

Note: When a failure occurs in the GPS speed and/or course signals, the Speed and

Course boxes are shown empty.

Manual enables the Manual speed. It is recommended to enable Manual when using speed

sensors to have an initial Manual value that reflects the real situation.

In Use is showing that the manually entered value is used by the system.

Along When no speed sensors are enabled, you can enter a value in this field.

Used Speed shows the vessel speed used by the system. This is an average, filtered value of

the combined measurements from the In Use speed sensors or manual input.

Along shows the used alongships vessel speed.

Athwart shows the used athwartships vessel speed. This will be zero when no speed sensors

are enabled.

Note: Since the Used speed is an average between all selected speed sensors, it is

important to monitor the sensors to detect errors and to deselect sensors that are not

correct.


Training Manual


Rev. 11 Training

Rate of turn sensor

Rate Of Turn (ROT) measurements can be used to improve the heading control of the vessel.

This is useful when a very accurate heading control is required during high-speed sailing, or

when a vessel has a hull shape that makes it difficult to control the heading. Measurements of

the vessel’s Rate Of Turn are received from the gyrocompasses, or a dedicated ROT sensor.

The ROT sensors should be enabled to allow this information to be used by the K-Pos DP

system. ROT sensors are enabled and controlled using the Sensors dialog box - Rate Of

Turn page.

OK is a status signal coming from the

sensor.

Enable is a status set by the operator to

give the DP system access to use the

sensor.

In Use is a status signal coming from

the DP controller indicating which

sensor is used by the system.

Rate Of Turn shows the measured

Rate Of Turn.


Training Manual


Training Rev. 11

Tension sensors

Fore operations where forces are applied to the vessel these needs to be measured for the DP

system to work in a safe way. The forces measured will be added to the feed-forward and thus

directly compensated by the thrusters. Typical operations are ploughing, cable lay, pipe lay.

Force Surge: The force acting on the

vessel in the alongship direction.

Force Sway: The force acting on the

vessel in the athwartship direction.

Moment: The moment of the force

acting on the vessel.

None: The sensor is not used.

Measured: shows the measured

values from the tension sensors.

Manual: You can enter manual

tension values to be used in the

Manual text boxes

Calculated: The Moment is calculated

based on the selected Force Surge and

Force Sway.

Details: Click this button to show the

Details part of the dialog box.

Hide: Click this button to hide the

Details part of the dialog box.

Scale/Bias: Additional sensor scaling

can be entered. Used sensor value =

measured value*Scale/100 + Bias

Max value: Measurements will be

limited upwards to this value. To

avoid increasing tension until

something breaks, the maximum value

should be set to a safe level. This is

for example used for ploughing to

avoid breaking the towing wire if the

plough gets stuck on the seabed

Min value: Measurements will be

limited to this value. This function is

needed to hold a minimum tension in

the pipe, during pipe lay, to avoid the

vessel moving astern which would

result in a buckled or broken pipe.

Filter Constant: Because tension

measurements can be noisy it might be

necessary to filter them and to set a

time delay before the sensor is

reported erroneous.

Point of Attack: You can enter values

in the Along and Athwart text boxes to inform the system about the position of the point at

which the force is acting on the vessel.

DP Sensors Input Mangement

Documents

Transcript of DP Sensors Input Mangement