Safety-Critical Systems 2Requirement Engineering

T- 79.5303 Spring 2006

Ilkka Herttua

Safety Context Diagram

HUMAN PROCESS

SYSTEM

- Hardware - Software

- Operating Rules

Critical Applications

• Computer based systems used in avionics, chemical process and nuclear power plants.

• A failure in the system endangers human lives directly or through environment pollution. Large scale economic influence.

Safety Definition

• Safety: Safety is a property of a system that it

will not endanger human life or the environment.

• Safety-Critical System: A system that is intended to achieve, on

its own, the necessary level of safety integrity for the implementation of the required safety functions.

Developing safety-related systems

• To achieve safety: - safety requirements (avoiding hazards, risks) - quality management (follow up process) - design / system architecture (reliability) - defined design/manufacture processes - certification and approval processes - known behaviour of the system in all conditions

 

Overall safety lifecycle

 

Concept1

System Acceptance10

System Validation (including Safety Acceptance and

Commissioning)

9

Installation8

Design and Implementation

6

Apportionment of System Requirements

5

Performance Monitoring12

Modification and Retrofit13

System Definition and Application Conditions

2

Re-apply Lifecycle(See note)

Risk Analysis3

Operation and Maintenance11

System Requirements4

Manufacture7

Decommissioning and Disposal

14

Note: The phase at which a modification enters the life-cycle will be dependent upon both the systembeing modified and the specific modification under consideration.

Risk Analysis

• Risk is a combination of the severity (class) and frequency (probability) of the hazardous event.

• Risk Analysis is a process of evaluating the probability of hazardous events.

• The Value of life??Value of life is estimated between 0.75M –2M GBP.

USA numbers higher.

Risk Analysis

• Classes: - Catastrophic – multiple deaths >10 - Critical – a death or severe injuries- Marginal – a severe injury

- Insignificant – a minor injury

• Frequency Categories:Frequent 0,1 events/year Probable 0,01Occasional 0,001Remote 0,0001Improbable 0,00001Incredible 0,000001

Hazard Analysis

• A Hazard is situation in which there is actual or potential danger to people or to environment.

• Analytical techniques: - Failure modes and effects analysis (FMEA) - Failure modes, effects and criticality analysis (FMECA) - Hazard and operability studies (HAZOP) - Event tree analysis (ETA) - Fault tree analysis (FTA)

 

Fault Tree Analysis 1

The diagram shows a heater controller for a tank of toxic liquid. The computer controls the heater using a power switch on the basis of information obtained from a temperature sensor. The sensor is connected to the computer via an electronic interface that supplies a binary signal indicating when the liquid is up to its required temperature. The top event of the fault tree is the liquid being heated above its required temperature.

Fault event notfully traced to its source

Basic event, input

Fault event resultingfrom other events

OR connection

Risk acceptability• National/international decision – level of an acceptable loss (ethical,

political and economical) Risk Analysis Evaluation:

ALARP – as low as reasonable practical (UK, USA)“Societal risk has to be examined when there is a possibility of a catastrophe involving a large number of casualties”

GAMAB – Globalement Au Moins Aussi Bon = not greater than before (France)“All new systems must offer a level of risk globally at least as good as the one offered by any equivalent existing system”

MEM – minimum endogenous mortality “Hazard due to a new system would not significantly augment the figure of the minimum endogenous mortality for an individual”

 

Risk acceptabilityTolerable hazard rate (THR) – A hazard rate which guarantees that the

resulting risk does not exceed a target individual risk

SIL 4 = 10-9 < THR < 10-8 per hour and per function

SIL 3 = 10-8 < THR < 10-7

SIL 2 = 10-7 < THR < 10-6

SIL 1 = 10-6 < THR < 10-5

Potential Loss of Life (PLL) expected number of casualties per year

SIL = safety integrity level

 

Current situation / critical systems

• Based on the data on recent failures of critical systems, the following can be concluded:

a) Failures become more and more distributed and often nation-wide (e.g. air traffic control and commercial systems like credit card denial of authorisation)

b) The source of failure is more rarely in hardware (physical faults), and more frequently in system design or end-user operation / interaction (software).

c) The harm caused by failures is mostly economical, but sometimes health and safety concerns are also involved.

d) Failures can impact many different aspects of dependability (dependability = ability to deliver service that can justifiably be trusted).

Examples of computer failures in critical systems

V - Lifecycle model

SystemAcceptance

System Integration & Test

Module Integration & Test

Requirements Analysis

Requirements Model

Test Scenarios Test Scenarios

SoftwareImplementation

& Unit Test

SoftwareDesign

Requirements Document

Systems Analysis &

Design

Functional / Architechural - Model

Specification Document K

now

led

ge B

ase

** Configuration controlled Knowledge that is increasing in Understanding until Completion of the System:

• Requirements Documentation• Requirements Traceability• Model Data/Parameters• Test Definition/Vectors

Safety Requirements

• Requirements are stakeholders (customer) demands – what they want the system to do. Not defining how !!! => specification

• Safety requirements are defining what the system must do and must not do in order to ensure safety. Both positive and negative functionality.

 

Specification

• Supplier instructions how to build the system. Derived from the required functionality = Requirements.

Requirements R + Domain Knowledge D => Specification S

Where do we go wrong?• Many system failures are not failures to

understand R requirements ; they are mistakes in D domain knowledge– A NYC subway train crashed into the rear end

of another train on 5th June 1995. The motorman ran through a red light. The safety system did apply the emergency brakes. However the ...signal spacing was set in 1918, when trains were shorter, lighter and slower, and the emergency brake system could not stop the train in time.

• Are you sure?

Requirement Engineering Right Requirements

• Ways to refine Requirements

- complete – linking to hazards (possible dangerous events)

- correct – testing & modelling

- consistent – semi/formal language

- unambiguous – text in real English

 

Requirement Engineering

Tools – Doors (Telelogic)- Data base and configuration management- History, traceability and linking

  

Requirements Managementwith DOORS

Slides provided by Telelogic/ Quality Systems & Software

Dynamic Object Oriented Requirements System

Effizienz

InterfacesRequirements

Links

ReportsAnalysis

Change Proposal SystemFilter, Views

Multiuser-DatabankUser Accounts

Configuration-management

Text ProcessingTemplates, Standards

DOORS

Capture, Link, Trace, Analyse, Administer

Terminology in DOORS

One Document, Group of related Information

Requirements, Tests, Specifications,Change Requests, etc

Consists of numerous ModulesProject

Module

Object

Object

Object

Object Attribute

Attribute

Attribute

Data of a Module

Characteristics of Objects or Links

Date of last Change, Priority, Status, Costs, ...

Relation betweentwo Objects

Links

Traceability in DOORSRequirement Specification Architectur

alDesign

TestPlan

Follow Customer Ammendments through all the Documentation

Traceability - Requirements from Scenarios

Goal hierarchy

user requirements

traceability

Two people shall be able to lift the boat onto the

roof of the average saloon car.

The sailor shall be able to contact the coastguard

when the boat is capsized.

The sailor shall be able to perform a tacking

manoeuvre.

To have sailedand

survived

Ready to sail

Sailed

Returnedhome

Boatloaded

Boat lifted

Boatunloaded

Boatrigged

Boat on car

Mast rigged

Center-plate rigged

Rudder rigged

Gibed

Boatmanoeuvred

Tacked

Cruised

Boatcapsized

Gone ashore

Boat righted

Coast guardcontacted

V - Lifecycle model

SystemAcceptance

System Integration & Test

Module Integration & Test

Requirements Analysis

Requirements Model

Test Scenarios Test Scenarios

SoftwareImplementation

& Unit Test

SoftwareDesign

Requirements Document

Systems Analysis &

Design

Functional / Architechural - Model

Specification Document K

now

led

ge B

ase

** Configuration controlled Knowledge that is increasing in Understanding until Completion of the System:

• Requirements Documentation• Requirements Traceability• Model Data/Parameters• Test Definition/Vectors

Additional home assignments

• From Neil Storey’s book Safety Critical Computer Systems

• 1.12 (Please define primary, functional and indirect safety)

• 2.4 (Please define unavailability)

Email by 1 March to herttua@eurolock.org