PRA and CAFTA Training

1
PRA and CAFTA Training

• Objectives
• Review basic PRA concepts
• Review CAFTA components and functionality
• Create a complete fault tree

2
What is PRA?

• Probabilistic Risk Assessment (PRA) is an
  organized method of determining the likelihood
  that combinations of events lead to an
  unacceptable situation.
• Risk is defined as the product of expected
  frequency of occurrence and anticipated
  consequence for an accident or activity.
• PRA can be used to ensure cost effective, safe
  operation/use of facilities.
• PRA is also used to allocate limited resources
  among competing needs.

3
What is a Fault Tree

• Fault Tree model
• Models component / support system
  interdependencies. Used to generate minimal
  cutsets.
• Provides for a logical display of intra-system
  interactions andcomponent interrelationships
• Describes ways system can fail
• Uses AND, OR and other gate types
• Can be linked together to explicitly model design
  dependencies
• Includes Common cause failures and operator
  errors

4
Developing Fault Trees

• Development of System Fault Trees
• Basic Logic of Fault Tree Models
• Component Failure Characteristics
• Testing and Maintenance
• Human Errors
• Dependent Failures

5
PRA Terms

• Top Event Frequency is the estimated likelihood
  of the top event, per year.
• Top Event Frequency is expressed in units of
  expected events per year
• A basic event is the lowest level in a fault tree
  
• Cutsets are the minimum combination of basic
  events required to occur in order to cause the
  top event. When sorted by probability, cutsets
  reveal most likely sequences of events, leading
  to a better understanding of vulnerabilities.

6
CAFTA

• CAFTA is
• Database Editor
• Fault Tree Editor
• Cutset Editor

7
CAFTA Topics

• Software Overview
• Reliability Database Editor
• Fault Tree Editor
• Model Quantification
• Cutset Editor
• General Information
• Case Study

8
CAFTA For Windows

• Software Overview

9
CAFTA For Windows

• Four Steps to Construct System Model
• Define Fault Tree Logic
• Build Reliability Database
• Evaluate Fault Tree to Obtain Cutsets
• Review and Analyze Cutset Results

10
CAFTA For Windows

• Fault Tree Model and Databases

11
Database Editor-- RDED

• CAFTA Database Structures
• Basic Event database
• Failure Rate (Type Code) database
• Gate database
• Three databases work together to define the
  attributes of all events

12
BE Naming Conventions

• Basic Event Naming Conventions
• (Example)

13
Calculation Formulas
Calculation Formulas
14
Fault Tree Editor-- FTED

• Movement Commands

15
Navigating Through a Fault Tree

• Navigation
• by Dragging Gates
• by searching Parents of a event/gate
• by viewing Top events
• Find Gate/Event by FIND menu function
• Jump to the Gate/Event if found
• View and Select Fault Tree Tops

16
Gate Types

• Gate and Basic Event Types ---Text Form

17
Fault Tree Editor-- FTED

• Gate and Basic Event Types ---Graphic Form

18
Fault Tree Editor-- FTED

• Examples Combination gate
• G COM3 A B C D
• Indicates a 3-out-4 combination
  gate, while
• F COM3 A B C D E
• Indicates a 3-out-5 combination
  gate

19
Example
Example

• System B Failure is the top event in the fault
  tree.
• Valve failure and Operator failure are basic
  events. A cutset can consist of a single basic
  event.
• Pump failure occurs if Test Maintenance make it
  unavailable OR if it fails to start OR if it
  fails to run.
• There are five possible combinations of these
  failures, or cutsets, that can lead to system B
  failing.
• Failure of System B results in the accident
  sequences TAB SAFE or TAB FAIL.

20
Fault Tree Editor-- FTED

• FTED Editing Features
• Loading Existing Fault Tree File
• Connect to Different Databases
• Editing Database Records
• Text and Graphical Input
• Extract Subtree
• Delete event locally and/or globally
• Changing Gate Type
• Prune Gate Gate -gt Event
• Merging Files
• Drag-Drop Editing
• Undo to reverse the previous editing

21
Fault Tree Editor-- FTED

• FTED Logic Modeling Features
• Circular Logic Checking
• A AND B C D
• B OR E F G
• G OR X A Z
• Compressing Equivalence and True/False Values
  to make fault tree size smaller
• Prune Modules Change all Modules to basic
  events
• Module Independent Subtree

22
Fault Tree Editor-- FTED

• FTED Logic Modeling Features
• Check Gate Independence
• To produce list of events which prevent the gate
  from being independent
• Check Module Independence
• To produce list of events which prevent the
  module from being independent
• Define physical and logical links

23
Model Evaluation

• Model Evaluation Quantification Process

24
Quantification

• Quantification Setup

25
Quantification

• Four Parameters need to be set for
  Quantification
• Quantification Method Cutset generation or gate
  probability calculation or other user-defined
  quantification
• Truncation Probability decide how many cutsets
  are generated and how long it takes to generate
• Truncation order remove any cutsets which has
  higher order than specified
• Module Operation this is additional processing
  option, if any module exists.

26
Fault Tree Editor-- FTED

• Options of Module Operations
• Expand Module module cutsets are included in the
  final cutsets of the fault tree top
• Prune Modules Module is declared as a basic
  event and its probability is used rather than
  module cutsets
• Only Module cutset generation is for all
  modules, rather than for all tops of the fault
  tree.

27
Printing

• Printing Fault Trees
• Use Properties -gt Group Pages menu function to
  turn on/off of group of page

28
Printing

• Printing Fault Trees
• Use Properties-gtAuto Page Breaks menu function to
  decide appearance of each page

29
Cutset Editor-- CSED

• CSED Provides Cutset Review Functions
• Do the cutsets make any sense?
• Are there single and double, and higher order
  cutsets?
• What are the dominant cutsets?
• What do you think should be the dominant cutsets?
• What recovery actions should be added to some of
  cutsets?

30
Cutset Editor-- CSED

• Cutset Editor Screen
• Class Name enabled by the display option
• Top Event and Probability
• Cutset Properties (In different font and colors)
• Generated original
• Deleted not counted for module probability
• Added added by user not generated
• Recovery added by user

31
Cutset Editor-- CSED

• Cutset Editor Screen
• Cutset Properties (In different font and colors)
• True prob. 1
• False Prob. 0
• Subsumed no longer contribute to top
  probability
• Truncated cutset probability below the
  cutoff
• Delete-term cutset not count to top
  probability

32
Cutset Editor-- CSED

• Reporting Cutsets Results

33
CAFTA Files

• CAFTA Files
• .CAF - Fault Tree Files
• .RR - Basic Event Database, Gate Description
  Data, Failure Rate (Type Code) Database
• .CUT - Cutset File
• .RAW - Temporary output from cutset generator
• .FTP - Temporary input to cutset generator

34
CAFTA Data Flow

• CAFTA Data Flow

35
Advanced Features

• Fault Tree Editor
• Drag a .caf file from explorer and drop it in
  FTED will open the file in the FTED window
• Copy and Paste a gate logic between different
  fault tree files
• DragDrop gate logic between different fault tree
  files
• Could open many windows to view different parts
  of one fault tree file
• CtrlDlbClick between events(gates) will
  automatically add new event to the gate logic

36
Advanced Features

• Fault Tree Editor
• Can set some event as Flag and use the event
  string in sequence quantification
• Seven types of links to other PSA project files,
  and go to those file directly from FTED
• Can load different formats of fault tree files,
  as long as filters for those files are supplied
  and specified in the CAFTA configuration

37
Begin Using FTED
38
Begin Using FTED

• Choose a standard template and standard Access
  database.
• Name your database. The database will have an
  extension .rr It will contain a basic event
  table, a gate table, and type code table.FTED
  will display one basic event as shown below.

39
Input Fault Tree Logic

• Next, Click and hold the left mouse button on
  the basic event symbol (the circle). A palette
  of gate types will pop up, as shown below As you
  drag the cursor across each button in the
  pallette, the gate type label will appear in the
  lower left of your screen (on the status line).
  Choose the OR gate symbol

40
Input Fault Tree Logic

• This will put two basic events below the new OR
  gate.

41
Input Fault Tree Logic

• While the top gate is highlighted (surrounded by
  a dashed box), choose Edit and then Add Event to
  place the third event under this gate

42
Input Fault Tree Logic

•  Fault tree logic may also be entered using the
  following input methods
• In the toolbar click on the yellow down arrow
  key (with plus sign) to convert an event to a
  gate
• Press the Ctrl key and double click between two
  existing gates (be careful not to click in a
  description box).
• A quick way to add a large number of gates is to
  use the Edit, Input option.
• After events are added, they can also be changed
  to gates by clicking on basic event circle to
  pull down the palette of gates and events.

43
Input Event Data

• Double click on a basic event to enter event data

44
Input Event Data

• Select the appropriate calculation method

45
Merging Two Fault Trees

• If two fault trees share the same database
  (.rr), then merging the two fault trees will
  include all of the basic event, gate, and type
  code data.
• If you are merging two fault trees with separate
  databases, then you must open the database editor
  and import the basic event, gate and type code
  data for the second tree.

46
Merging two fault trees

• Open tree1.rr
• Select File, Import
• In the file name box type .rr this will show
  you all files with the extension .rr

47
Merging two fault trees

• Find the database for the second tree and click
  ok. This will allow you to choose field within
  that database to import.

48
Merging two fault trees

• Select the double arrows to import all fields.
• Click Ok
• To import Gate data, select Window, GATE DATA.
  Repeat the import process for the gate data
• To import Type Code data, select Window, TYPECODE
  DATA. Repeat the import process for the type code
  data

49
Failure Modes

• Example of failure modes
• Fail to Run
• Fail to Start
• Fail to Open
• Transfers Open
• Transfers Closed
• Common Cause Failure
• Operator Error
• Test Maintenance