General Operation
LOGAN for Windows is a program for the construction, evaluation and printing of Fault and Event Trees in the Microsoft Windows™ environment. Fault trees can be linked to event trees by specifying the fault tree which calculates a branch probability. If the fault trees contain common events, the effects of the resulting non-independence are fully accounted for. The system uses the Windows Graphical User Interface for the construction of Fault (or Success) Trees and Event Trees. The system lays out the diagrams automatically for optimum presentational clarity.
Editor
The graphical editor allows the Fault Tree or Event Tree structure to be built up on the screen and the associated event or branch descriptions and numerical data to be entered. Some of the main features are:
- Create transfer events or rejoin transfer events to allow the trees to be split onto several pages
- Move up and down between the transfer events
- Option to automatically number the pages to your specified format
- Copy and paste data between events
Evaluation
LOGAN performs a full cutset analysis for Fault Trees and for Event Trees which are linked to fault trees. The cutset generator generates the minimal cutsets in terms of basic events. Cutsets containing more than the specified number of basic events are discarded. The system average failure probability or failure rate and unavailability is calculated from the probability of failure or failure rate and repair time or proof test interval of the basic events. The program identifies the use of inconsistent data (e.g. attempting to AND two events entered only as failure rates) helping to prevent errors in Fault Tree construction.
Sensitivity Analysis
A number of different Sensitivity Analyses can be performed to determine the dependency of the system results on the failure rate or probability of each event and on repair times and proof test intervals. The calculations that can be performed are as follows:
- Fussel-Vesely importance
- The effect on the top event of setting all probabilities to 1 (Fault Tree only)
- The effect on the top event of increasing failure rates by a factor of 10 (Fault Tree only)
- The effect on the top event of increasing all events by a factor entered by the user (Fault Tree only)
Graphical Output
LOGAN for Windows generates high quality Fault and Event Tree diagrams. Two formats of Fault Tree are available:
- A horizontal format which presents all the data for the Fault Tree in a convenient data table.
- A traditional top-down format.
Proof Test Interval Optimisation
An additional feature is available which calculates the optimum interval between maintenance of protective systems. This function finds an optimum set of proof test intervals which minimises the test requirements whilst maintaining the required system failure frequency or probability of failure. For every event in the Fault Tree which represents unrevealed failure of a protective component, the system requires entry of the maintenance man-hours required to carry out the proof test. Alternatively, some other measure such as the cost of the proof test could be used.
The program takes as its input the target top event frequency (which may be, for example, equal to the originally calculated top event or the required top event frequency). Using the results of the sensitivity analysis, the program carries out an iterative Gaussian minimisation process and calculates a set of proof test intervals which minimises the total proof test effort (or cost) whilst still achieving the target failure rate. This often results in very long proof test intervals for the least sensitive items. It is possible, therefore, to enter a 'cap' or limit value on the proof test intervals, which is the maximum time period which is considered acceptable between the tests.
Link to Reliability Database
This feature allows the fault tree data to be imported from and exported to an Microsoft Access™ database according to the data reference field entered. This facility can be used to create a central databank.
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