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Captain Yevgen
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Understanding System Sensors in maritime vessels
First of all, the basic requirements shall be reviewed.
Vessel sensors should at least measure vessel heading, vessel motions, and wind speed and direction.
MSC.1/Circ.1580
The DP system shall have at least 3 sensors: Gyro (gyrocompass), Vertical Reference Sensor (VRS) and Wind sensor.
When an equipment class 2 or 3 DP control system is fully dependent on correct signals from vessel sensors, these signals should be based on three systems serving the same purpose (i.e. this will result in at least three heading reference sensors being installed).
MSC.1/Circ. 1580
According to this requirement, DP class 2 and DP class 3 vessels shall be fitted with at least 3 gyrocompasses.
Sensors for the same purpose which are connected to redundant systems should be arranged independently so that failure of one will not affect the others.
For equipment class 3, one of each type of sensor should be connected directly to the backup DP control system, and should be separated by an A-60 class division from the other sensors. If the data from these sensors is passed to the main DP control system for their use, this system should be arranged so that a failure in the main DP control system cannot affect the integrity of the signals to the backup DP control system.
MSC.1/Circ. 1580
Failure of one sensor shall not affect serviceability of the other ones. For DP class 3 vessels, sensors for the back-up controller shall be connected to it directly and shall be separated from the other sensors (of the same type) with the bulkhead class A.60.
The main sensor is Gyro – it shows the heading and is responsible for yaw axis respectively. Without Gyro the system is not able to take yaw axis under control (auto-heading) and, as a result, will not be able to switch to Auto Positioning Mode (AutoPos). That is why, there is the following requirement – not less than three gyro compasses on DP class 2 vessels shall be installed.
Having three gyro compasses on board and when one of them starts to feed incorrect data, the system or personnel is able to compare the data from all gyro compasses and identify which one transmits inaccurate information. As for DP class 1 vessels, there are two gyro compasses installed only and it is impossible to find out which one failed by means of such comparison. That is why gyro compasses are compared with the magnetic compass and conclusions are drawn.
After determining the faulty gyro compass, it is necessary to ensure if it is not set as a preferable one (Preference) in the DP system. If so, a serviceable gyrocompass shall be set as “Preference”.
Vertical Reference Sensor (VRS) or Motion Reference Unit (MRU) – is a sensor, enabling the DP system to measure deviation angles of the vessel in relation to her vertical position. The difference is that VRS is a previous generation sensor, measuring Pitch and Roll only. MRU – is a modern one and is able to measure Pitch, Roll and Heave.
Wind Sensor – is a sensor of the wind direction and speed. One of the forces that influence the vessel (wind, waves and current) but that can be accurately measured is the wind force. The rest of the forces, affecting the vessel, cannot be read and the DP system calculates them by means of mathematical modelling. The overall influence of these forces, received by the calculation, is called DP Current or Residual force.
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- DP
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