17.17.9 Rotatable Thrusters
17.17.9.1 Main Control System Type tpc
The Control System Type TPC is a system assembled from modules, which can be combined in many ways, in order to obtain a flexible control system for an actual number of installed thrusters, type of prime movers, number of control stations and an interface wills other systems, etc. With the aid of the lever on the control head, both the thrust and direction of thrust (rotation) can be controlled by one hand. One control head can be used for a group of thrusters for parallel action.
Thrust control means that the load of the prime mover is sensed and that the load (approximately equal to thrust) is controlled by pitch changes, load being proportional to the lever position.
The control head is connected to the two controllers for thrust and rotation. The thrust controller thus compares the ordered thrust with actual load and pitch. The rotation controller compares the rotation order signal with the feedback rotation transmitter signal.
Both controllers act on the proportional hydraulic valves for pitch and rotation respectively. The terminal unit forms the central part for all connections. The complete system is supplied with 24V d.c. via a power supply unit (Refer Figure 17.22).
Additional Subsystems
Polar Joystick Control System
The Polar Joystick System is the solution, when control of several rotatable thrusters with one lever is required. With the joystick lever - similar to the lever for a single thruster - a number of thrusters can be manoeuvred with one hand. The joystick lever is located in conjunction with a Heading Control unit, which is connected to the gyro compass and by which an automatic heading control can be achieved both in positioning and transit control modes.
Manoeuvre Responsibility System
This is a common system for one or more rotatable thrusters and provides selection between control stations and also to be used if the Polar Joystick System or a Dynamic Positioning Control System is connected to the thruster controls.
Thrust Reduction Unit
This is an optional unit which monitors the generator plant (up to 8 generators) to prevent overload and a possible blackout. The generator load sensing transmitters consist of current transformers. The output from the thrust reduction unit is connected to all installed thrust controllers and proportionally reduces the ordered thrust command signals.
Features of the Electronic Control System for Rotatable Thrusters
Thruster and rotation can be controlled by one lever and the lever position gives information of the thrust magnitude and direction.
The built-in delay and load control functions permit fast handling of the command lever without risk of over loading the prime mover.
Independent back-up control system for both pitch and rotation allows full manoeuvrability in case the main system is out of order.
All modules of the system have front space dimensions of only 144 x 144 mm.
Thus each control station can be tailor-made to meet a wide variety of requirements
Each module has a cable connector and is therefore simple to install and replace.
The system is designed to accept a Polar Joystick System, an Automatic Anchor, Assic System and can easily be adopted for connection of a Dynamic Positioning System.
Description of the Main Control System
Transmitter Alarm Circuit
In order to obtain reliable supervision of the input-output signals connected to the control amplifiers, all important signals are of the usual proportional current type - (4 to 20 mA). Only two conductors are used for each transmitter. The transmitters are therefore equipped with an electronic circuit, which converts the measured value into a proportional current consumption.
The controllers are provided with an alarm detector for each transmitter signal. This detector senses whether the transmitter signal is within correct levels and in case of failure, activates an alarm to the common sensing alarm circuit, which also checks that all supply voltages are correct. Each alarm channel is equipped with light indicators on the alarm circuit board.
Thrust Controller
The command signal from the control head is converted into a proportional +10V d.c. signal after it has passed the alarm detector. Then the command signal proceeds via a delay circuit with an adjustable time from 0 to 100 seconds.
For an electric motor drive, the delay function is normally not used. After this possible delay the command signal is connected to a multiplier, which operates only when a load reduction unit is installed and produces a decreasing signal.
The command signal is then compared with the response signal in the control error comparator. The output signal from the comparator is now used for activation of the proportional value, controlling the pitch hub servomotor.
In order to avoid increasing the pitch above an allowed level, the control error signal is processed in a pitch-limiting circuit. The valve command signal is converted into a proportional value of current and when it has passed the alarm detector, it is connected to the valve interface in the thruster room.
As mentioned earlier, thrust control means that the load shall be proportional to the lever position. Therefore the response signal mainly consists of the load value.
When an electric motor drives the thruster, a current transformer is used as the load transmitter and when a diesel engine drives the thruster, the load-controlling circuits are more complicated. After passing the alarm circuits, the pitch and load signals are converted into proportional d.c. signals. In the load calculator block, the load signal is multiplied by the pitch-size signal, which is saturated at 25% pitch. Above this limit the load signal dominates as response. As the load signal does not change polarity for ahead-astern pitch, the response signal must be given correct polarity by sensing the pitch direction.
The output from the load calculator block representing thrust is then connected to the comparator.
As thrust is shown in the indication unit, the response signal is also connected to the thrust-pitch indication instrument.
By a switchover relay, pitch alone can be used as a response indication signal. Thus a pitch control mode is possible, if desired. However, in this mode the load of the prime mover is not controlled.
After the comparator a detector block is connected for sensing the control error. An error signal above a determined level gives a signal to the thrust deviation signal lamp in the indication unit (during manoeuvring, this lamp is therefore illuminated briefly when pitch changes).
Rotation Controller
The rotation controller utilizes a system with sine-cosine voltages and is used for representing the command and response signals.
For utilizing only two conductors for the sine-cosine transmitter signals, a multiplexing system is used, where the current pulse's magnitude and polarity represent the two signals.
When turning the lever head, the rotation transmitter in the lever unit feeds the command signals, which are first sensed by the alarm circuits and then go via the multiplexer -de-multiplexer blocks that are connected to the rotation error calculator inputs.
The error calculator then compares the sine-cosine signals with the response signals, which are received from the rotation feedback transmitter in a similar way to the command one. The calculation of the control error is made in such a way that the thruster always rotates in a direction to achieve the ordered position in the shortest possible time.
The error signal is now used for control of the proportional valves, which regulate the oil flow to the two hydraulic turning servomotors.
As supervision of the valve command signal is also desirable, converter and alarm circuits are used in this signal path.
The interface circuits for the control valve are normally placed in the thruster room. In order to obtain good control stability, the hydraulic valves are equipped with an inner loop feedback transmitter connected to the valve interface circuit.
A signal to the rotation control deviation signal lamp in the back-up indicating unit is available in the circuit, sensing the control error in a similar manner to that in the thrust controller.
Tunnel Thruster
The stern of a vessel is controlled by means of a propeller and rudder, but the bow is left to itself. In order to overcome this, a tunnel thruster (also known as a lateral or bow thruster | may be installed in a transverse tunnel in the bow (Refer Figure 17.24). The need for such a system has already been explained in article 17.3.3 (A Manoeuvring System).
Thus a tunnel-type side thruster can be defined as a system in which propellers are installed inside a tunnel that passes laterally through the hull of the ship both fore and aft. By taking water in from one side and discharging it out at the other side, the ship can be turned or moved sideways. Such thrusters are used on a wide variety of ships including freighters, cargo barges, passenger liners, ferries, fishing boats, and work barges.
The thruster may have fixed blades of a helical shape but with equal pressure and suction sides, with no section camber (arch / curve). If it is of the controllable pitch type, the blades are made "plane" with zero design pitch. As the blades have to operate equally well in either direction, they must be symmetrical.
Figure 17.24 - Tunnel Thruster
Thruster Applications
There is a wide spectrum of vessels equipped with thrusters of various types. The L-thruster is the most common and is used on almost every type of displacement merchant vessel, the largest being a 265,000-dwt tanker equipped with a 1620-kW (2172 hp) unit in the low and 1100-kW (1475 hp) unit in the stern. Several thousand L-thrusters have been installed and are standard on ferries making a relatively large number of port calls. The Highest L-thruster total power installed to date on a single vessel is on each of the two drill ships, Discoverer 534 and Discoverer Seven Seas, with 13,400 kW (18,000 hp approximately). While the use of L-thrusters on conventional vessels is quite well known, the application of L-thrusters as well as Ro-thrusters on vessels operating on offshore oil fields is still quite new.