Automatic Control Systems


Automatic Control Systems

Automation helps the crew to operate the systems on board easier and safer. It will execute actions which are too complicated for the crew to handle in a given time. Automation allows automatic ob­servation of systems, registration of failures, registration of service time and planned maintenance. Detailed requirements for Auto­matic Control Systems on board of ships are described in IEC publication 60092-504 Control and instrumentation.

   1.  Automation

The level of automation depends on a number of factors:

-  requirements of the owner
-  function of the ship
-  cost
-  complexity of the installation
-  rules and regulations of the classification society and the Flag State (registry).

First of all a cost/availability anal­ysis has to be made before plan­ning automation.

Integration of systems and the in­troduction of distributed control systems is an ongoing process. It reduces cost of cabling and man­ning. The only problem is that the rules and regulations of both the classi­fication societies and those of the national authorities cannot keep up with this everchanging process.

Such a control system can consist of programmable logic control­lers with remote input and output modules, connected through a two wire bus system and operated/su- pervised from a PC type worksta­tion through an operator-friendly SCADA software package. Redundancy both in hardware and software is a logical requirement for automatic systems. Software must be well-structured and tested as per class standards.

Essential systems required for sail­ing and comfort of the crew must have sufficient back-up or emer­gency controls.

Automatic Control Systems

Field I/O panel of a distributed automation and control system

1.1. More advanced systems

An operator workstation makes more sophisticated systems possi­ble, including control and display of engine room systems with sophis­ticated graphics. Trends over a period of time can be captured. Analyses of relationships between figures can be calculated. Running hours and the required automatic logging of all figures can be stored, along with many other statistics.

Some examples of systems that can be part of an automatic control system are:

-     Tank gauging system

From the simple, such as pro­viding liquid heights to the more sophisticated, giving tank contents in m3 or even in tons.

-     Reefer monitoring system

From failure alarms to com­plete data logs of the reefer's temperature and CO2 content throughout the voyage, which can prove that cargo is not damaged due to transport.

-     Generator control and pow­er management system

From minimum automatic start­ing of a standby generator in case of generator failure and sequential restarting of all es­sentials to a complete load-de­pendent start-stop of the gen­erator plant. In this case, there is automatic power reduction in case of generator failure, until the standby generator is start­ed, has been synchronised, put on-line and has taken the load.

-     Propulsion remote control system

From straight forward remote control systems where each handle controls a single engine or propeller to state-of-the-art systems which can make a ship move 25 metres to port, rotate with the stern as rotating point over 90° to port, follow track or a link in location, adjust speed in accordance with available wa­ter depth.

In automation there are no techni­cal limits and therefore, a balance between expected results and cost has to be found.

Essential automation systems must be composed of type-approved equipment and are subject to an acceptance test at the manufac­turer's under conditions as real as possible.

Automatic Control Systems

Two automatic boilers

Automatic Control Systems

Governor controlling speed of auxiliary engine

Automatic Control Systems

Alternator junction box with AVR open cover

Automatic Control Systems

Automatic sewage plant

2  Local control systems

Some equipment has a dedicated local control system which is sepa­rated from the central automation system. Most of the time these local control systems exchange some param­eters with the central automation system. Examples are:

-  Basic engine room alarm and monitoring system, consisting of simple displays giving status and analogue values of essen­tial parameters as required by class.
-  Local self-contained small au­tomatic systems controlling lu­bricating oil temperatures and high and low water tempera­tures of propulsion and auxiliary diesel engines.
-  Local automatic voltage regula­tors for generators, controlling the voltage.
-  Local governors on engines, controlling engine speed.
-  Local standby starters for dupli­cated essential auxiliaries.
-  Local automatic boilers
-  Local automatic sewage plant.

3  Essential services

Essential services are those servic­es required for sailing and keeping the ship in a habitable condition.

Electric power required for propul­sion can be supplied by a single generator set or by more sets in parallel. When supplied by a single generator, failure of this generator set should start a second generator. This generator should automatical­ly be connected to the switchboard followed by automatic restart of all essential auxiliaries.

A sequential start system may be required to limit the step load to the diesel engine.

Essential services include:

-  Main and emergency lighting
-  Propulsion engine lubricating oil pumps (if not engine driven)
-  Propulsion engine freshwater pumps (if not engine driven)
-  Propulsion engines seawater pumps ( if not engine driven)
-  Fuel oil booster pumps
-  Gearbox lubricating oil pumps
-  Controllable pitch propeller hy­draulic pumps
-  Steering gear hydraulic pumps - Start air compressors
-  Engine room fans

On ships sailing on heavy fuel oil, fuel oil circulating pumps, thermal oil circulating pumps and a thermal oil boiler are essential and must au­tomatically restart.

When the electric power required for propulsion is supplied by more generators in parallel, an automatic load shedding system must be fit­ted. This system reduces the load im­mediately to the capacity of the re­maining generator(s) after failure of one generator.

When large motors with frequency drives are installed the control sys­tem can be programmed to reduce the speed of the motors when the generators are close to be over­loaded. A complete shutdown of these mo­tors is then not required and when enough power is available again the motors can be set to the original speed.

Sequential restart timing priority:

-  Instantaneous main and emer­gency lighting
-  After 5 secons, lubricating oil pumps, engines and gearboxes and fuel oil pumps and thermal oil system and pumps
-  Steering gear pumps and con­trollable pitch propeller pumps
-  Freshwater pumps and air com­pressors
-  Seawater pumps
 In about 30 seconds, all auxilia­ries are back in service and pro­pulsion engines can be restarted

When auxiliaries are engine driven and the engines can be started without lubricating oil pressure, this process is simpler.

 4  Failure mode and effect analysis

The Failure Mode and Effect Analy­sis is an appraisal of the result of a failure of equipment on the opera­tion of a ship (or any other type of equipment). This study is compulsory for units which have to fulfill the require­ments of the MODU Code.

The MODU Code is one of the IMO Codes, especially drawn up for off­shore equipment. MODU stands for Mobile Offshore Drilling Unit.

Originally for drilling equipment only, but later made a requirement for offshore equipment in general. FMEA is not limited to the automa­tion of electrical systems but covers all systems required for propulsion of a ship and all components.

The following example of an FMEA covers the layout, the auxiliary sys­tems and the electrical installation of a large pipe lay vessel with the following main characteristics:

 -  6 main generators each 3360kW 
 -  Thrusters forward, two retract­able azimuth thrusters each 2400kW, one tunnel thruster 2200kW
 -  Three azimuth thrusters each 2900kW aft
 -  Class notation Lloyd's Register + 100A1, +LMC, UMS, DP(AA) equal to class 2.

The class notation DP(AA) or class 2, requires that a single failure does not result in loss of position of the vessel. Flooding or fire of a space is not considered in this notation.

The ship is designed for dual fuel but marine gas oil is used during DP operation with heavy fuel only for long passages or between jobs.

An FMEA addresses the items:

1. Layout of the vessel, location of main components, such as die­sel generators, switchboards, transformers, converters and thrusters.
2. Compressed air systems
3. Cooling water systems
4. Fuel oil systems
5. Freshwater system
6. Seawater system
7. Thruster control system
8. Electric main distribution sys­tem

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