A Savelyev DYNAMIC POSITIONING SYSTEM
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Dynamic Positioning System Chapter 5 |
ENVIRONMENT SENSORS |
WINDSENSORS WITHIN A DP SYSTEM
Wind has the potential to blow a vessel off position. Therefore, DP systems need wind speed and direction data from windsensors:
To compute the effects of the wind on the vessel’s superstructure and hull. To determine thruster force necessary to counteract the effects of wind.
To calculate Weathervane or Minimum Power Heading.(Common in shutter tanker operations).
Several types of windsensors are fitted aboard vessels. Generally, a windsensor commonly consists of a rotating-cup type transmitting anemometer, with a separate windvane to show wind direction.
Another type of windsensor has the impeller attached to the windvane.
WIND SENSOR FEED FORWARD FUNCTION
The windsensor has an input to the mathematical model. However, the mathematical model takes time to evaluate and respond to changes in the vessel or environment.
The wind, on the other hand, can suddenly gust without warning. Hence, the windsensor is also connected to the DP system by a “feed forward” function to bypass the mathematical modeling process. This function enables the DP system to immediately react to a radical change in wind condition.
LIMITATIONS OF WINDSENSOR INPUT
The accuracy of windsensor input in the DP system is influenced by the following factors: Windshadowing resulting from masts, stacks, adjacent oil rig, platform or other vessel
obstructing the wind.
Malfunction in the windsensor (i.e. rotating cups or windvane becoming stuck). Reliability of wind data from the windsensor selected by the DPO.
Data from the windsensor is essential in DP operations. The speed and direction of the wind are important factors in the calculation of the weathervane or minimum power heading. Some vessels such as shuttletankers and FPSOs require the vital information in order to keep the correct attitude at all times.
The windsensors are coupled into the DP system by means of a “feed forward” function, which bypasses the mathmatical model, in addition to being included in the modeling process. This is also known as wind feed forward.
DESELECTING WINDSENSOR INPUT
Two or more windsensors positioned at opposite ends of the yardarm enable the DPO to select or deselect a windsensor, depending on the prevailing circumstances.
The disadvantage of deselecting windsensors is wind data to the mathematical model and feed forward function are discontinued. However, the DP system uses wind data stored in the mathematic mode.
ADVANTAGE OF DESELECTING WINDSENSOR
During helicopter operation, if the helideck is close to windsensor, the downwash of air from the helicopter rotor will trigger the feed forward function, Hence, the DP system will issue thruster command to react to an apparent gust.
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ENVIRONMENT SENSORS |
Deselecting windsensors during this critical operation will ensure that the vessel is not inadvertently reacting to a fictitious gust.
Caution: Keep in mind when reselecting windsensors that the DP system may interpret the difference between the constant wind value in t mathematical model and the prevailing wine condition as a gust. And, react accordingly.
OTHER SENSORS
INERTIAL NAVIGATION SYSTEM (INS)
Rate Gyroscopes (sensors used to measure rate velocity) and accelerometers (sensors used to measure acceleration in various axes) are combined to compute the vessel’s heading, altitude and position.
Note: Unlike a DGPS receiver which determines position relative to satellites, INS is self-contained. While INS cannot determine an initial position, it accurately computes position relative to an initial position. Hence, combining DGPS and INS enhances DP capabilities.
MESSAGES ON DP SYSTEM AND PRINTER
Dynamic Positioning systems are designed to consistently check for inconsistencies, faults, and warnings. Unique to the IVCS 2000, is a voice alarm in plain English.
When critical conditions are detected, messages (reports) are generated. These messages, are constantly displayed on the LCD monitor and/or printed in an abbreviated format. Most DP systems have a dedicated display area or facility for Messages. The type of information on display will consist of:
Date and time of message generation. Message text.
Message reference number.
Message type (Alarm, Warning or Information). Source of origin (e.g. computer A or B).
Status of message (acknowledged or not, active, inactive, etc.). Additional data.
All messages are printed out hard copy by a printer. I addition to the brief message text, the DPO may consult a message listing, either on paper or on-screen help file, to provide a much greater description of the causes and effects of the message.
Basically, DP systems issue three categories of messages: Alarm. Warning
Information.
In the IVCS 2000 DP system, the three categories of messages are: Error. Warnin
Informatio
Alarm messages are issued with a flashing lamp and audible alarm whenever the system discovers a situation which adversely affects DP operation. The DPO must acknowledge the alarm,
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check the contents of the alarm message, and determine a corrective course of action, in order to rectify the situation. The following messages qualify as Alarm:
Setpoint Alarm Limits Exceeded. Position Out of Limit. System Fault.
Thruster #2 Feedback Error.
Warning messages, appear on the alarm display and printer, are issued with flashing lamp alarm whenever the system discovers a situation which will adversely affect DP operation, but do not have any serious effect on the performance of the system. The DPO must also acknowledge warning alarms, and check the contents of the message in order to rectify the situation. The following messages qualify as Warning:
No windsensor selected. Wind Direction Difference. Thruster #1 High Force.
If system tests do not report the same message after a specific timeout period, the message becomes inactive. Generally, inactive Alarm and Warning messages need to be acknowledged by the DPO before they are removed from the active message display list, while Information messages are removed automatically when they become inactive.
Information messages are issued without a flashing lamp or audible alarm to inform the DPO of important issues that will not adversely affect DP operation. Reference Reject HPR 1 qualifies as information.
CATASTROPHIC FAILURE MESSAGE
Catastrophic Failure indicates and extremely harmful situation that would cause the vessel to loose DP capability, i.e. loosing heading (gyro compass) or position (DGPS) input. Alarms and Warnings associated with catastrophic failure must be check and acknowledged by the DPO.
CORRECTIVE ACTION FOR ALARMS/WARNINGS
The DPO must fully understand any alarm or warning message before acknowledging it. Regardless, the DPO may consult an on-screen help field or message listing to get a detail explanation of the abbreviated alarm or warning message.
If one of two gyros selected by the DPO experiences a catastrophic failure (signal loss) for example, the DPO must ensure that the alternate gyro is selected in order to maintain heading input into the DP systems.
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POWER GENERATION AND SUPPLY |
POWER GENERATION AND SUPPLY
Dynamic Positioning vessels, compared with conventional merchant vessels, have a much higher need for power due to all the systems and redundancy required for DP operation.
DIESEL ELECTRIC DP VESSEL
Generic power generation and distribution for a dive support vessel.
In a typical diesel-electric DP vessel, power may be generated as follows: Six diesel generators, fitted in two separate machinery spaces.
The generators send power to a split HT (high tension) switchboard.
The switchboard busbars are installed in separate spaces, also. And, are connected by a bus
switch.
The bus switch is opened to isolate the two halves of the switchboard so each can operate independent of the other. When the bus switch is closed, the two halves connect.
Each busbar provides power to one main propeller, and at least one thruster at the bow and stern. This provides redundancy should a fault develop in one busbar.
A Diesel Electric DP Vessel has a diesel engine connected to an electrical alternator/generator. Alternators/Generators provide power for the diesel electric engine by a bank of diesel driven alternators also called generators for this purpose. One of the advantages of this type of operation is the cost saving on fuel. Another advantage is the ability to take generator on and off line when they are not needed. Switchboard is an essential part of a diesel electric vessel. The alternators/generators feeds the switchboard at which time the switchboard distributes the power. The typical switchboard is 480 volt and split into two sections, the port board and starboard board. These two side are connected by bus tie breakers. Proper setup of this equipment is necessary in order to have a workable DP operation. DP technicians have simplified this process over the years with computer systems and more modern equipment. Different types of DP systems have different requirements from the bus-ties. An abnormal power condition, such as a generator taking on too big of a load and tripping, may cause a blackout. When this happens the other generators try and assume the remaining load and one of them may trip causing a blackout. This is why the bus-tie switches are important and should be monitored. A bus-tie on a DP 2 system can either be open or closed in order to fix any problems while maintaining position. The main purpose is so a single fault failure will not cause the vessel to lose DP. A DP 1 type vessel might only have a 280 volt switchboard but will be split like the 480 volt. In a typical diesel-electric DP vessel, power may be generated:
Six diesel alternators, fitted in two separate machinery spaces.
The alternators send power to a split HT (high tension)switchboard.
The switchboard busbars are installed in separate spaces also. And, are connected by a bus switch. The bus switch is opened to isolate the two halves of the switchboard so each can operate independent of the other. When the bus switch is closed, the bus bars connect the two halves.
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Each busbar provides power to one main propeller, and at least one thruster at the bow and stem. This provides redundancy should a fault develop in one busbar.
POWER REQUIREMENTS
Power is critical for the operation of various subsytems in the DP system.
The power generation system must be capable of rapid increase in production to met high power demands by the control system, while “scaling “back when power demand is low, in order to conserve fuel.
POWER MANAGEMENT SYSTEM
Power Management is the system that efficiently matches the level of power to the existing conditions and having adequate power for future conditions. Diesel-electric powered vessels generally have sufficient generators, connected to a switchboard driving the necessary motors.
In new modern vessels, the power management systems have the ability to start and stop generators, trip certain systems before others, distribute load sharing through the system.
Power Management is the process of producing enough power to meet the demand of the DP system, while economizing fuel consumption.
Redundancy level required determines the complexity of the power management system.
In a typical diesel-electric power vessel, enough alternators are connected to the switchboards to produce the required power. When power demand increases, more alternators come online. When, power demand decreases, the reverse occurs.
Power Management system is generally designed to prevent large motors from starting until enough alternators are online to produce the required power.
In order to have redundancy, the power generation system is divided into tow halves. The tow plants are fitted in separate machinery rooms, Moreover, switchboards are subdivided to isolate faults or prevent blackout when necessary.
A Power Management system included in the IVCS 2000 provides:
The power monitoring for each of thrusters, CPPs, Shaft Generators, Main Engines.
The actuators’ (thrusters and CPPs) power limiting in order to prevent Shaft Generator and Main Engine overload. Reduced power consumption of the C
PP and thruster connected to the same Shaft Generator with the thruster, which is being started, in order to start the thruster motor.
UNINTERRUPTED POWER SUPPLY
The electronic components of the DP system (console, computers, position measuring equipment, environmental sensors, etc.) need a stable power supply. Excessive power fluctuation may not only blow some fuses, it can also damage sensitive electronic equipment. Moreover, DP electronic components must have backup battery power in case the vessel experience a blackout. These battery backups systems are called Uninterruptible Power Supplies (UPS).
DP class 2 and 3 must have redundant UPS’s and have a minimum duration of 30 minutes of operation. There are other types of UPS’s on the market and many have longer durations. A better system would be to setup two systems for redundancy in case of a UPS failure. UPS’s should be tested on a regular basis as they do not last forever. They come in many different price ranges from inexpensive to very expensive. The vessel would be bettered served with a reliable
Provisions For Uninterrupted Power Supply: Case 1 - Various peripheral elements of the DP system are given dedicated individual UPS. For example, one independent UPS unit for each of the two DGPS receivers in a DP system. Case 2 - One large capacity UPS facility to provide power to several components in the DP system.
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POWER GENERATION AND SUPPLY |
SIMPLEX UPS SYSTEM
Two separate supplies Master and Alternative, are taken from individual busbars. These supplies go into charging rectifiers, which converts the ships a.c. to 120 v. d.c. The d.c. then supplies the inverters, and backup batteries. When the vessel loses power, the batteries provide power to essential DP electronic components for about 30 minutes.
NOTE: The batteries do not power the thruster and taut wire winch.
Inverters in the simplex UPS system convert the 120v. dc. into the a.c. voltage and frequency required by the DP electronic components. Outputs from the Master and Alternative Inverters are synchronized in phase. The static switch sends the power from Master or Alternative inverter to the DP electronic components. Although the static switch is dependable, it is not redundant. Hence, it is a source of singlepoint failure. Consequently, the simplex UPS system is limited to use in Equipment Class 1 DP vessels.
DUPLEX UPS SYSTEM
Each of two independent UPS systems is used to provide power to half of the DP system/ Each UPS has a backup battery for reduddancy.
TRIPLEX UPS SYSTEM
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A DP Equipment Class 3 vessel will have a third UPS system installed for triple redundancy.
WINDOWS
POWER LIMITS
In this window, the DPO can set maximum allowed power consumption for actuators (thrusters
and propellers) and maximum allowed power production for |
Main |
Engines and |
generators. |
|
Power limits |
set in this window are considered by the IVCS |
2000 |
during Thrust |
Allocation. |
When power |
consumption/production within these limits is not |
enough |
for system operation, an |
|
alarm appears.
Using "<" and ">" buttons it is possible to set required limit values for the following actuators: Bow #F Thruster.
Bow #A Thruster.
Stern #F Thruster.
Stern #A Thruster. Port Diesel.
Stbd Diesel. Port Generator. Stern Generator. Port Propeller. Stbd Propeller.
Power limits are set in percents from maximum consumed/produced power.
Set power limits are indicated in the Power Monitoring Window of the IVCS 2000.
GENERATOR LIMITS
In this window an operator can set low and high voltage and frequency limits for generators. When these limits are overstepped, an alarm appears.
Using "<" and ">" buttons it is possible to set required limit values for the following generators: Port Generator.
Stbd Generator.
Generator limits cannot exceed nnximum voltage and frequency values, defined by generators' specifications.
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SYSTEM DIAGNOSTIC
The System Diagnostic Window is used for:
The IVCS 2000 hardware monitoring.
Testing of the system operability.
Determination of the current system configuration.
The upper part of the System Diagnostic Window shows system structure diagram where all system hardware components are indicated: I/O boxes connected with
vessel actuators (upper row of rectangles):
Bow Forward Thruster. Bow Aft Thruster. Stern Forward Thruster. Stern Aft Thruster. Port propeller. Starboard Propeller.
Color of the rectangle indicates state of the respective I/O box:
Green color - I/O box is correctly operated.
Red color - I/O box is failed.
Genus Bases A and B are presented as two horizontal lines located under the I/O boxes. Color of the line indicates state of the bus:
Green - the bus is correct and in operation. This means that at least one of I/O boxes is operated through this bus. It is possible that both Genius buses are green.
White - bus is correct and Hot Standby. Red - bus is failed.
PLCs and Computers A and B states are determined by color of the respective rectangle: Light Green - PLC/Computer is correct and in operation (Master).
Dark Green - PLC/Computer is correct and Hot Standby. Red - PLC/Computer is failed.
Port and Stbd Steering Systems states are determined by color of the respective rectangle: Green - Correct connection between PLC and Steering Gear.
Red No connection between PLC and Steering Gear-Rudder control is not
available.
Ethernet connections between PLCs and Computers are presented with color lines, indicating the state of connection:
Green - connection line is correct. Red - connection line is failed.
Control panels (MCP A, MCP B, and PCP) are presented as rectangles located in the lower row of the diagram. Color of the rectangle indicates state of the respective Control Panel:
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Light Green - CP is active. Connection line between CP and Computer is light green. Dark Green - CP is not active. Connection line between CP and Computer is white. Red color - CP is failed or no connection with CP. At that connection line between
CP and Computer is red also.
Sensor sets are presented as two black boxes with sensor lists. Color of the sensor name indicates sensor state:
Green - sensor correctly sends data to the 1VCS 2000. Red - no data from sensor.
The following AC/DC Monitoring color circle indicators are presented in the lower part of the System Diagnostic Window;
Availability of I/O boxes energizing from 24 VDC A and B Power suppliers: Green color of indicator — power supply is available.
Red color - no energizing.
Availability of Main Housings A and B energizing from 24 VDC A and B Power suppliers:
Green color of indicator - power supply is available. Red color - no energizing.
Availability of Main Housings A and B energizing from internal 24 VDC Power suppliers:
Green color of indicator - power supply is available. Red color - no energizing.
UPS A and B failure indicators:
Line Failure (115 AC ship power failure). Low Battery.
Replace Battery.
Grey color of indicator - no failure signal. Red color - failure.
NOTE: In the case of Low Battery the Operator Console will be automatically shut down in 30 seconds and the following inscription will be displayed:
System is automatically shut down. Low battery.
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SYSTEM STRUCTURE DIAGRAM