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Captain Yevgen
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Understanding the Role of Controllers in the DP System
While speaking about a controller, the word “computer” as well as the word “brain” are often used, as all calculations and control of the DP system are done there. The word “controller” will be used to avoid confusion as there is a computer in the base of the DP console either.
There is one controller on DP class 1 vessel and there are two controllers on DP class 2 vessels respectively. There are some requirements for the latter: in case of a failure of one controller, another one shall take over the control and shall not be affected. To provide this, in other words redundancy, a communication of two units needs to be established and changing from an online controller to a back-up one shall be made automatically.
For equipment class 1, the DP control system need not be redundant.
MSC.1/Circ.1580
For equipment class 2, the DP control system should consist of at least two computer systems so that, in case of any single failure, automatic position keeping ability will be maintained. Common facilities such as self-checking routines, alignment facilities, data transfer arrangements and plant interfaces should not be capable of causing failure of more than one computer system. An alarm should be initiated if any computer fails or is not ready to take control.
MSC.1/Circ.1580
All of the above shall be explained: there is no idea of worse-better, main-auxiliary or master-slave – both controllers are equal as per their characteristics. One can encounter the following practice on board, when the use of controllers is done using a schedule one week on-line, the other one is back-up. Thus, one controller is active (controlling the operations) and the second one is back-up (ready to take over the control at any moment).
DP class 3 vessel shall have at least two controllers complying with the DP class 2 requirements and one additional back-up controller, which is separated with a fireproof bulkhead (A.60 class). During DP operation this back-up controller shall constantly receive information from sensors, reference systems, thruster feedback, etc.
For equipment class 3, the main DP control system should consist of at least two computer systems arranged so that, in case of any single failure, automatic position keeping ability will be maintained. Common facilities such as self-checking routines, alignment facilities, data transfer arrangements and plant interfaces should not be capable of causing failure of more than one computer system. The two or more computer systems mentioned above do not include the backup computer system; thus, in addition, one separate backup DP control system should be arranged, see paragraph below. An alarm should be initiated if any computer fails or is not ready to take control.
For equipment class 3, the backup DP control system should be in a room separated by an A-60 class division from the main DP control station. During DP operation, this backup control system should be continuously updated by input from at least one of the required sets of sensors, position reference system, thruster feedback, etc. and be ready to take over control. The switchover of control to the backup system should be manual, situated on the backup computer, and should not be affected by a failure of the main DP control system. Main and backup DP control systems should be so arranged that at least one system will be able to perform automatic position keeping after any single failure.
MSC.1/Circ. 1580
The third controller is mentioned as a back-up one, though it is able to control position of the vessel by itself. Switching over to it is performed manually. Based on the above, any failure, whatever it is, shall not affect the back-up controller. If any of the controllers are not able to take over the control, the DP system shall initiate corresponding alarm.
Also, controllers communicate and verify each other, e.g. the alarm goes off, if one of the controllers works slower, i.e. the rate of information processing of such a controller differs - it is not synchronized with another one or other ones and is not ready to take over the control.
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