Introduction
Maker : Jotun
Power Supply: AC 440V, 60Hz, 3ph
The vessel is provided with an impressed current cathodic protection system.
This method of corrosion protection automatically controls electrochemical corrosion of the ship’s hull structure below the water line. Cathodic protection can be compared to a simple battery cell, consisting of two plates in an electrolyte. One of the battery plates in the electrolyte will waste away through the action of the flow of electrical current, if the two battery electrodes are connected electrically. The metal to be protected, in this case, the ship’s hull, acts as the battery anode, the sea water being the electrolyte. If an external flow of current is impressed to reverse the normal flow in the battery, then the anode now acts as a cathode and ceases to waste away. In essence, this is how an impressed current cathodic protection system functions. When a vessel is fitted with ICCP (Impressed Current Cathodic Protection) the hull steel is maintained at an electrical potential more negative that the surrounding seawater.
For this reason, terminals normally comply with the ISGOTT Recommendation 20.6, Earthing, Bonding and Cathodic Protection, which states, referring to IMO recommendations for the safe transport, handling and storage of dangerous substances in port areas, that ship shore bonding cables should be discouraged. High currents that can occur in earthing cables and metallic connections are avoided. These are due to potential differences between ship and terminal structure particularly due to the residual potential difference that can exist for up to 24 hours after the shipboard ICCP has been switched off. These terminals usually utilise insulating flanges on hose connections to electrically isolate ship and terminal structure.
During preparations for berthing at terminals where such insulation is not employed, or where earth connections are mandatory by local regulation, or when bunker barges come alongside, the ICCP should be switched off at least 24 hours in advance.
Fresh Water Operation
When the vessel enters a river estuary, the fresh or brackish water may limit the spread of current from the anodes due to the higher resistivity of the water.
Normally this would cause the voltage output to increase to compensate for it, accompanied by very low current levels and the reference electrode potentials may indicate under protection. However, in this system this is taken care of by the computer and the system will automatically return the hull to optimum protective level on returning to sea water.
Operation
Protection is achieved by passing low voltage DC current between the hull metal and anodes, insulated from the hull, but in contact with the sea water.
The electrical potential of the hull is maintained in a more negative state than the anodes, i.e. cathodic, and in this condition corrosion is minimised. Careful control is necessary over the flow of impressed current, which will vary with the ship’s speed, salinity and temperature of the sea water, and the condition of the hull paint work. If the potential of the hull is made too negative with respect to the anode, then damage to the paint film can occur electrolytically or through the evolution of hydrogen gas between hull steel and paint. The system on this vessel controls the impressed electrical current automatically to ensure optimum protection. Current is fed through titanium electrodes situated forward and aft of the ship. The titanium prevents the anodes themselves from corroding and the anode surfaces are streamlined into the hull. Fixed zinc reference electrodes forward and aft are used to compare the potential of the hull to that normally found between unprotected steel and zinc electrodes.
Sufficient current is impressed via the anodes to reduce this to a level of between 150 and 250 mV.
Electrical Installation
The system consists of a Controller Power Unit, reference electrodes and anodes are installed, one forward and one aft. System status readings are available on an L.C.D. display at the control unit and these should be inspected and logged each day.
This control unit is also equipped with an alarm to give warning of any system abnormalities.
Aft System
The aft system consists of a power supply and control unit. Each control unit is connected to two hull mounted anodes and one hull mounted reference cell.
The aft unit is supplied from 25A circuit breaker Q03 in engine room 440V distribution board P6.
Forward System
The forward system consists of one hull mounted reference cell.
Propeller and Rudder Stock Earthing
To avoid electrolytic corrosion of shaft bearings and rudder stock, brushes are fitted and bonded to the ship's structure. In the case of the shaft, a slip ring is clamped to the shaft and is earthed to the hull via brushes. A second set of brushes, insulated from earth, monitors the shaft mV potential and this signal is fed to a millivolt meter. To ensure efficient bonding, the slip ring should be cleaned on a regular basis.
The rudder stock is earthed via a 70mm2flexible earth cable between the deck head and rudder stock to minimise any electrolytic potential across bearings and bushes.
Sacrificial Anodes
Sacrificial zinc anodes are provided in the water ballast tanks. Aluminium anodes are fitted to the sea chests and rudder.
Preparations for the Operation of the ICCP System a) Supply power to the control unit.
b) Switch to manual mode.
c) Check the voltage of each reference electrode.
d) Switch to automatic mode.
e) Set the control to the required level.
Routine checks
a) Record the total current on a daily basis.
Manual operation will only be required on the failure of the reference electrodes.
b) Check the reference electrode voltage on a daily basis.
c) Check and clean the shaft slip ring and brushes every month.
d) Inspect the rudder stock earth strap every month.
e) Inspect and clean control unit cooling fans and grills every three months.
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Illustration 2.13.9a Thrusters Control
Main Bridge Control Panel
Thruster and Hydraulic Pump Starter Panel Bridge Wing Panel
ECR Unit
Central Unit
Valve Control Signals
Pitch Feed back Signal Feed Back Unit
2.13.9 Thrusters
Maker: Kamewa
Type: 1650 K/BMS - CP
Overview
The vessel is equipped with a bow and stern thruster.
The Tunnel Thruster System consists of four main parts:
1. A tunnel with propeller unit, a driving motor, a hydraulic system, and an electric control system.
2. The propeller unit is driven by an electric motor at a constant speed and single direction of rotation. The propeller is provided with hydraulically adjustable propeller blades, which makes it possible to vary the magnitude and direction of thrust.
3. The tunnel thruster facilitates the manoeuvring of the vessel to a great extent when speeds are low or zero. The ship's tunnel thruster is also a useful complement to the ship's rudder even at higher speeds. The thruster and the rudder together give an increased steering effect.
4. The controllable pitch tunnel thruster runs at a constant shaft speed. Power and thrust are controlled by changing the pitch of the blades. The propeller always rotates in the same direction. As starboard and port thrust must be equal, the blades are designed with zero initial pitch and symmetrical blade section. The tunnel thruster has two purposes. One is to keep the vessel in position in a crosswind, the other one is to turn the vessel at zero or low ahead speed.
(Note ! When a stationary vessel is turned with a tunnel thruster, the vessel is also given a sideways motion. The simultaneous turning and crabbing results in a slow longitudinal motion of the vessel - ahead when the tunnel thruster is located in the bow - astern when it is located at the stern. This should be kept in mind when manoeuvring in narrow harbours.)
The propeller unit comprises a propeller tunnel in which a single stay gear housing is bolted A four bladed propeller and shaft assembly are mounted in bearings in the gear housing.
The main part of the tunnel thruster is the propeller hub with blades and the propeller shaft. The shaft is supported by one spherical roller bearing and two axial roller bearings. The shaft seal of rubber sleeves prevents water from penetrating and oil leakage.
Operating Principle
In the propeller hub there is a servomotor which turns the propeller blades. The servomotor consists of an integrated piston and an axially moving piston rod.
The movement is obtained by leading pressure oil to one side or the other of the piston.
The piston rod has a crosshead with four transverse slots for sliding shoes, one for each of the blades.
The eccentric crankpin fits into the hole of the sliding shoe. The crankpin ring is supported in a bearing lining, which is integrated within the hub body.
When the piston rod moves, the crankpin ring rotates with the circular movement transmitted via the piston rod slot sliding shoe and crankpin.
The propeller blade, which is fixed on the crankpin ring by screws, will then turn.
Each blade is provided with a sealing ring to prevent water entrance to the hub or oil leakage.
Remote Control System
The control system is a microprocessor based remote control system used to control the pitch setting of the tunnel thruster.
The system can order both port and starboard manoeuvres by changing the pitch setting while the propeller blades continue rotating in one direction.
The manoeuvring is performed from a control station equipped with a control lever. When ordering thrust with the control lever, the system applies the proper pitch setting according to a pitch curve which is pre-programmed in the computer, allowing the thrust to be proportional to the lever position.
When manoeuvring, the load of the drive motor is controlled by the system through automatic regulation of the pitch. The maximum allowed load is determined by the ‘load limit’.
When there is more than one control station, there is also a responsibility system included, which allows only one control station at a time to be ‘In Command’.
On each control station the actual pitch setting of the tunnel thruster(s) will be continuously indicated.
The driving motor can be started only when the propeller blades are in zero position, which reduces the starting torque to a minimum. This means low starting current.
Control of the system is generally from the main bridge or bridge wings, but can be controlled from the engine control room usually for pre-departure tests or due to control system failure.
The control panels have the following features:
Control of pitch with proportional thrust command Indication of pitch
Indication of drive motor current
Start/stop of drive motor and hydraulic pump motor Indication of alarm
Operating Procedures
Before Starting the Tunnel Thruster a) Check that power is available.
b) Start the electric driven hydraulic pump.
c) Check that no alarm exists.
d) The pitch will automatically go to zero.
Starting the Drive Motor a) Start the drive motor.
b) Check that the drive motor has started. A lamp indicates that the drive motor running.
c) The tunnel thruster is now ready for use.
Control Panel Selection
a) Select the control panel by pushing the ‘COMMAND REQUEST’ push-button. When the ‘IN COMMAND’ lamp lights, the control panel is in command.
b) The propeller thrust can now be manoeuvred in the desired direction by means of the control lever.
c) The propeller thrust is approximately proportional to the position of the control lever, via the pitch curve.
Stopping the Tunnel Thruster
a) Set the control lever in ‘0’ position.
b) Stop the drive motor.
c) Stop the electrically driven hydraulic pump.
Pitch Control Operation Control lever
The control lever can be rotated ±60°with click stop locations for the outputs 0-1/2-1. The propeller thrust is approximately proportional to the position of the control lever, via the pitch curve.
The control system controls the pitch. The lever movement is transmitted to the central unit and fed into a function generator (FG) where the required rela-tionship between lever position and pitch command can be adjusted.
Output from the F.G. is the pitch command, which is fed to the regulator where it is compared to the actual pitch position, (feedback signal). The pitch correction signal, from the load control process and the external thrust reduction, is also fed to the regulator.
If there is a difference between ordered and actual pitch, the hydraulic pitch control valve is activated in order to correct the actual pitch setting until the control error (difference) has disappeared.
Change from Main Bridge to Bridge Wing Control
Push the ‘Command Request/Test’ button for request of command and trans-ferring between main bridge and the bridge wing station(s). (When the drive motor is stopped, pitch testing is possible by pushing this button).
The ‘In Command’ lamp lights, indicating when the control station in question is ‘In Command’. (Can only be in command when the drive motor is started).
When the command is on ‘Bridge’ the command can be transferred between main bridge control station and the bridge wing control station(s).
When the push button ‘Command Request’ is pushed the command is directly transferred. The lamps ‘In Command’ indicates which station is in command.
Change from Main Bridge to ECR Control
When in Control Room control the lamp indicates ‘Control Room’ in command.
When in Bridge control the lamp indicates ‘Bridge’ in command.
The Switch BR/ECR is used for manoeuvre station change over. When Command request button is pressed on the bridge, the switch is changed to
‘Bridge.’
For switching over the control between Bridge and Engine Control Room there is a manoeuvre responsibility system.
The Engine Control Room is the master control station where the switch
‘BR/ECR’ is located.
Change of Control Station from Bridge to ECR
When the switch in the control room is switched from position ‘Bridge’ to position ‘ECR’, the command will be directly transferred from bridge to ECR.
Change of Control Station from ECR to Bridge
When the switch in control room is switched from position ‘ECR’ to position
‘Bridge’ the command will not be transferred until ‘Command request’ is pushed at any of the bridge control stations. Until then no station will be in command.
When set for ECR operation the pitch can be operated using the push buttons on the ECR panel. These act directly on the hydraulic control valves. The main control system is bypassed and the control failure alarm blocked.
Load Control
The load control system prevents the drive motor from being overloaded. The system measures the drive motor current, i.e. load on the drive motor. The load signal is compared to the Load limit parameter (Load limit 1 or Load limit 2).
If the drive motor current is too high, the pitch, as well as the drive motor load, will be reduced.
To prevent mechanical damage at high speed, pitch changes, caused for example by air in the hydraulic system, is protected by supervision of a pitch response overspeed.
Emergency Stop
The emergency stop push-button activates an opening contact which causes the drive motor to stop. The drive motor running information disappears. When the drive motor is stopped, the pitch is automatically reset to zero.
Drive Motor Start/Stop
In order to be able to start the drive motor, the pitch must be in zero position and the hydraulic pump motor has to be running.
When stopping the drive motor, the drive motor running information disappears, causing the control system to steer the pitch to zero.
In order to be able to start the drive motor, the hydraulic pump motor must first be started by using the ‘Hydr. start’ push button.
If the hydraulic pump motor is stopped by using the ‘Hydr. stop’ push button, the drive motor will be stopped as well, due to lack of hydraulic pressure.
Key
Fresh Water
Illustration 2.14.1a Domestic Fresh Water System
QG6
QG10 Fresh Water
Expansion To Inert GS and Vapour
Collection System
& Foam System
QG13
QG17QG16 QG15
To F.W Gen &
F.W Fill System To F.W.
Expansion Tank
Fresh Water Spray Fire Extinguishing
System Pump To Oily Water
Discharge &
Monitoring System
Stern Tube Cooling Water Tank
Aft Peak Tank
On Tank Top Aft
QG39
On Tank Top Forward
For Chemical Cleaning Tank Of M.E. Air Cooler Fresh Water
Domestic Hot Water
Air
2.14 Accommodation Systems