5.6.1 Thruster Manufacturer’s Control Systems
Most thruster manufacturers provide a closed loop control system for their thrusters. For azimuthing thrusters with controllable pitch propellers the closed loop control system will accept both steering and pitch commands from the DP control system and operate the hydraulic control system to achieve the desired blade pitch and steering angles. In the case of azimuthing thrusters with fixed pitch propellers, the closed loop control of propeller speed is provided within the variable speed drive. Only the closed loop steering control is provided by the thruster control unit.
Figure 101 shows a typical thruster control unit for a fixed pitch azimuthing thruster.
The unit accepts steering commands from the DP control system in the form of an analogue 4-20mA loop representing the required angle. The control unit then interprets the command and operates the clockwise (CW) and counter clockwise (CCW) solenoid valves to bring the thruster azimuth to the required angle by the shortest possible route. The hydraulic system is typically capable of turning the thruster at 2 rpm.
There are three angle measuring devices mounted on the thrusters. One device provides feedback to the control unit for closed loop control, the other provides feedback direct to the DP control system and the third is used for indication at the manual thruster control levers. The closed loop feedback device may be a shaft encored in some applications.
Feedback to the DP control system is usually provided by a sine/cosine potentiometer driven from an ±10V supply as shown in Figure 100. This device has resistance elements which produce a sinusoidal distribution of voltage. Wipers displaced by 90° allow a cosine voltage to be produced. The angle of the thruster is then computed from :
if V_sin > 0 and V_cos > 0 angle = arctan(V_sin / V_cos) / (2 x π) x 360°;
if V_cos = 0 and V_sin > 0 angle = 90°;
if V_cos < 0 angle = 180° + arctan(V_sin / V_cos) / (2 x π) x 360°;
if V_cos = 0 and V_sin < 0 angle = 270°;
if V_cos > 0 and V_sin < 0 angle = 360 + arctan(V_sin / V_cos) / (2 x π) x 360°.
Where V_sin and V_cos are the voltage signals from the sine output and cosine output in Figure 100.
In other devices a 4-20mA interface is used to indicate the sine and cosine feedback. Loop monitoring is provided to indicate that one channel has failed. Some control system suppliers also carry out a check that sin2 A+cos2 A = 1.
POWER SUPPLY EARTH
-Ve SUPPLY
+Ve SUPPLY
INSTRUMENT EARTH
SINE OUTPUT COSINE OUTPUT
Figure 100 – Sine/cosine potentiometer
Although the DP control system only provides open loop control of the thruster (closed by vessel position) it does monitor the thruster speed and azimuth and will issue a prediction error if either variable deviates from the required value by more than a defined amount in a specified time. Note that at least one type of thruster has a single mechanical drive for all three angle indicators. If this drive slips, the thruster may be pointing in the wrong direction with no indication that this is the case.
The thruster control unit provides one of several inputs to the DP control signal. This signal indicates to the DP system that the thruster is ready for DP commands. Other inputs may include the variable speed drive indicating that it is ready for speed or torque commands.
Some thruster failures will cause the DP ready signal to indicate „not ready‟. As soon as the DP control system detects the change in status it will automatically deselect the thruster and reduce the command to zero.
The thruster control unit will also provide some of the interlocks and protection associated with the thruster. Typical interfaces for this purpose include:
shaft brake applied;
air pressure available;
HPU pressure;
main and backup power supply present;
lub oil pressure;
thruster control unit healthy – warning/fault;
DP ready;
local/remote;
main motor start allowed;
main motor running/stopped;
stop main motor.
BRAKE
Figure 101 – Thruster control unit
5.6.2 Direct Control by Vessel Automation System
In some applications there is no thruster manufacturer‟s control unit and the hydraulics are interfaced directly to a vessel management system field station. The thruster control algorithms for steering and pitch control reside within the field station. This is a popular solution for vessel upgrades where the thruster mechanical part is to be retained but the obsolete control system is absorbed into a new vessel automation system.
5.6.3 Thruster Emergency Stops – Line Monitoring
Classification societies normally require that remote thruster emergency stops are located at the main DP station. Emergency stops for safety purposes may be located at other points in the thruster or drive machinery space. The thruster emergency stop needs to be independent of the normal drive control system. Ideally the emergency stop would act directly on the drive main circuit breaker but very few variable speed manufacturers adopt this because shutting down the drive in this way carries a significant risk of damage.
Therefore most emergency stop functions act on the drive control system in some way, usually as an input to the safety shutdown chain part of the drive controller electronics.
An alternative scheme has been proposed in which the drive will be shut down gracefully by the initial action of the E stop, with a time delay circuit to open the drive circuit breaker directly if the drive controller fails to open it in a few seconds.
DP rules and guidelines require that thrusters fail safe but there will always be some circumstances where the DPO needs to shut down a thruster quickly using a control other than the normal stop function. In the case of a run-away thruster it may be difficult to tell which thruster is faulty as all thrusters may load up to oppose the faulty one. Prediction alarms or other alarms indicting thruster faults may help to identify the faulty unit.
Classification society requirements vary but for DP Class 2 it is not unusual to require that all propulsion related emergency stops use normally open contacts to prevent spurious loss of a thruster. At least one classification society requires line monitoring to prevent shutdown of the thruster on emergency stop cable faults such as open circuit or short circuit. To achieve this, isolated switch amplifiers are used in combination with stop buttons having the necessary series and parallel resistors, as shown in Figure 102. The amplifiers will only respond to the correct change in line current caused by closing the stop button across the parallel resistor.
Alarms are provided to indicate a cable fault or loss of the emergency stop power supply.
LOCAL
24Vdc
0Vdc
E-STOP
REMOTE (BRIDGE)
PWR PWR
THRUSTER STOP
CCT
STOP
FAULT E-STOP FAULT
CABLE FAULT ALARM PSU ALARM
Figure 102 – Thruster emergency stops
In DP Class 3 vessels, the thruster emergency stops may form a common point connecting all thrusters, which should be taken into account when the effects of fire are considered. Line monitoring is generally accepted as mitigation of this potential failure. Other methods have been accepted as reducing the risk of thrusters responding to emergency stop cable faults.
One possible alternative is to arrange the emergency stop with two control circuits one using normally open contacts and the other using normally closed. The thruster will only shut down if both circuit change state, alarm will be given if the two circuits ever indicate the same status. This arrangement is similar to the logic used in the DP control system‟s fire back-up switch for DP Class 3 vessels.
To provide more information on the nature of a fault, the alarms for emergency stop cable faults should reset automatically at the switch amplifier if the fault clears. However, the vessel management system will retain the alarm until acknowledged so that the fault can be investigated.
6 Safety Systems