Lecture 5Safety Analysis

FHA, HAZOP

Introduction• While designing a safety-critical system usually

several safety analysis techniques are applied• The idea is to achieve completeness of safety

requirements, spot defects in the design and amend it as early as possible in the development process

• Each safety technique gives a different perspective on system behaviour

• Potentially each of them brings a new insight on system behaviour

Today

Functional Hazard Assessment (FHA)

Hazard and Operability Study (HAZOP)

Functional Hazard Assessment (FHA)

• The FHA allows us to identify hazards at a functional level and correspondingly derive safety requirements.

• The FHA process consists of five steps:– Identification of all functions associated with the level

under study.– Identification and description of failure conditions

associated with these functions.– Determination of the effects of the failure condition. – Classification of failure effects on the system.– Assignment of requirements to the failure conditions

to be considered at the lower level.

Identification of failure conditions

• An identification of failure conditions can be done systematically by applying the following guidewords:

• - Loss of function• - Function provided when not required • - Incorrect operation of function.

Issues to be addressed

• How to describe functionality so that it will be easy to use in FHA?

• What is the proper level for the analysis?• Often FHA results in derivation of new

functional requirements? How to integrate them into already existing requirements?

Use case model

• We identify users of the system and the tasks which they must undertake with the system

• Actor (= User in UML notation )– is a user in a particular role. – is external to the system. – interacts and places demands on the

system.• A use case is a task which an actor needs to

perform with the help of the system

Use cases as input for FHA

• use cases clearly define system’s functions

• use case diagrams explicitly show interdependencies between use cases by means of associations

Documenting details of use cases

• Borrow copy of book A BookBorrower presents a book. The system check that the potential borrower is a member of the library, and that s/he does not already have the maximum permitted number of books on loan. This maximum is 6 unless the member is a staff member, in which case it is 12. If both checks succeed, the system records that this library member has this copy of the book on loan. Otherwise it refuses the loan.

• Note: description is in third-person, active-voice English

• Use case: Borrow copy of book• Actors: Book borrower (BB)• Purpose: Capture book borrowing • Overview: BB arrives at the checkpoint.The

system check that the potential borrower is a member of the library, and that s/he does not already have the maximum permitted number of books on loan. If both checks succeed then loan is allowed, otherwise it is refused.

• Type: primary and essential• Cross References (other resources which are

needed to implement the use case. e.g. some system functions)

Example

Use case diagram

• Use case Aspirate

• Brief description This use case defines system’s reaction on the operator’s command “aspirate l units of liquid from plate p”. It includes positioning of the operating head over the required plate, pumping the liquid in the pipette and reporting success or failure of the execution

• Includes use cases “Move to X position”, “Move to Y position”, “Move to Z position”, “Pump”

• Preconditions Operator chooses command Aspirate l units from plate p, the system is fault free

• Postconditions The amount of liquid in the pipette is increased by l units, the head is positioned over the plate p and success is reported. Otherwise failure is reported

• Typical flow of events• 1. Verify that p is a valid plate ID. If the verification fails then A_Failure1 in

alternative cause of events, else calculate X, Y- coordinates of plate p.• 2. Execute use cases “Move to X position”, “Move to Y position”.• 3. If execution of use cases “Move to X position”, “Move to Y position” failed

then A_Failure 2 in alternative flow of events else if execution of use cases “Move to X position” and “Move to Y position” succeeded then execute use case “Move to Z position”.

• 4. If use case “Move to Z position” failed then A_Failure 3 in alternative flow of events else if execution of the use case “Move to Z position” succeeded then execute use case “Pump”

• 5. …• Alternative flow of events • A_ Failure1: Prompt message “Incorrect plate ID p.” Cease automatic

execution mode.• A_Failure2: If the execution of the use case “Move to X position” failed then

cease automatic mode of execution, revert to the operator’s control, prompt message “Moving to X position has failed”.

Use case Move to X positionBrief description This use case defines reaction on the command “Move to X

position”. As a result of the execution of the use case either the operating head is brought to X position and success is reported or failure is reported.

Includes nonePreconditions None Postconditions The operating head is placed at the position X or failure is

reported

Typical flow of eventsCheck that xmin ≤X ≤ xmax, if not X_Failure1 in alternative flow of events Check current x-position.If current x-position equals X then report

success of execution. Otherwise move operatinghead to X position.

Check current x-position. If current x-position equals X then report success of execution else X_Failure2 in alternative flow of events

Alternative flow of eventsX_Failure1. Prompt message “Input parameter X is outside of valid range” X_Failure2. Prompt message “Loss of precision of X movement”

Conducting FHA

• Each element of use case description– Pre-conditions,– Guard-conditions,– System responses – Post-conditions

• Is identifies and recorded in the analysis table

• For each element we apply the guide words

Example of FHA

• Example from domain of engine control.• Deceleration is a core aircraft function. • Control of reverse thrust is a part of it.• It is decomposed and allocated to sub-systems • of aircraft• As a result identification of failure of a single

function (reverse thrust direction) result in discovery a new functional requirement

System level use case diagram

Decelerate on landing scenario

System level scenario

Example of extracting new functional requirements

• Element Airframe status =on ground (precondition)• Guideword Commission • Deviation On ground detected when not true• Possible Causes System failure, invalid airframe data; data

transmission failure• Use Case Effect Reverse thrust implemented when

precondition not satisfied• Real World Effect Thrust reverser deployed when not on

ground; loss of controlled flight• Severity Catastrophic• Integrity Constraints Assign on ground detection reliability;

validate airframe data; specify data sampling rate• New Safety Requirements Disallow thrust reverser when airframe

not on ground; detect inadvertent deploy…

Example of extracting integrity constraints (guide word omission)

• Element Thrust reverser state = in transit (guard condition)• Guideword Omission • Deviation Thrust reverser state= in transit not detected when

true• Possible Causes System failure, invalid thrust reverser data;

data transmission failure• Use Case Effect Engine thrust demand not commanded to

thrust limit when guard condition satisfied • Real World Effect Engine thrust exceeds thrust limit; structural

damage to thrust reverser; loss of controlled deceleration on landing

• Severity Catastrophic• Integrity Constraints Assign thrust reverser state detection

reliability; validate thrust reverser data; specify data sampling rate

Example of extracting integrity constraints (guide word value)

• Element Thrust reverser state = in transit (guard condition)• Guideword Value• Deviation Thrust reverser state= in transit detected as thrust

reverser = deployed• Possible Causes System failure, invalid thrust reverser data;

data transmission failure• Use Case Effect Engine thrust demand not commanded to

thrust limit when guard condition satisfied • Real World Effect Engine thrust exceeds thrust limit; structural

damage to thrust reverser; loss of controlled deceleration on landing

• Severity Catastrophic• Integrity Constraints Assign thrust reverser state detection

reliability; validate thrust reverser data; specify data sampling rate

FHA: conclusions

• FHA provides a systematic way to identify hazards caused by incorrect provision of system functions

• FHA can be applied at different levels of design, e.g., you can try to apply FHA to overall use cases, e.g., to investigate what happens when use case provided incorrectly, or when not expected, or not provided when expected

Hazard and operability studies (HAZOP)

• Why: to identify all possible derivations from the design’s expected operations and all hazards associated with these deviations

• How: apply a list of guide words to designed functions and identify invoked deviations. Next determine the consequences of such deviations

• Information analyzed: The design intention of the plant, potential deviations from the design intention, the causes of these deviations from the design intention, the consequences of such deviations

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Possible guide words interpretation

Examples of HAZOP use (hardware oriented)

Evaluation of HAZOP• “+”• A simple, systematic approach which is easy to apply• Encourages cross-disciplinary communication and

allows to find the problems overlooked by functional teams working alone

• “-”• Time consuming and labor-intensive approach• Relies heavily on personal judgment • Depends on the level of expertise of the team-

members

Example: HAZOP, use cases and security requirements

• The idea is similar to example of FHA: an application of guide words to use cases to derive security requirements

• The idea is to study use, abuse and misuse (of use cases) for analysis of security requirements

• Tailor HAZOP guide words to security perspective

HAZOP, use cases, security

• Actor: anything (anybody) that needs to exchange information with the system

• Deviations from intent (e.g., it should not allowed to process and approve payment) may result in threats

• Consider potential or resources

HAZOP: Actors Guidewordsin security

• Action/Intent• NO : The action/intent does not take place.• MORE : More action is achieved. This may be one of the

following:– Sequential Repeat - the same actions take place repeatedly.– Parallel Repeat - the same actions take place concurrently.– Extreme intent - some scalar attribute of the action is affected

(e.g. extreme parameter values are used in service invocations).• LESS : Less action is achieved than intended.• AS WELL AS : Parallel action - as well as the intended

or normal action, some unexpected supplementary actions occur or are intended.

• OTHER THAN : The action achieves an incorrect result. Alternatively, the actor may use facilities for purposes other than those intended, i.e. abuse of privilege.

HAZOP: Actors Guidewordsin security (cont.)

• Capability• – NO : Lack of the capability to perform the action.• – MORE : More general capability, allowing more to be

achieved than intended.• – LESS : Less general capability, allowing less to be

achieved than is required.• – PART OF : The actor has only part of, or is missing a

specific part of the capability.• – AS WELL AS : As well as the specific capabilities

required, the actor has other• specific capabilities.

Associations

• Associations of actors with use case model the roles that interact with the system through the functionality modelled in the use case.

• Interactions may represent the exchange of information or invocation of system operations.

• We need a clear idea of which actors should be able to access which use case for what purpose

• Covert channels can be thought of as unintended associations.

HAZOP: Association Guidewords in security

• NO : Association does not/can not take place.• MORE : Superfluous - Interface permits greater functionality to a

particular actor than is required. Association is not constrained as required. Further divisions are:– In-parallel with - More functionality is provided/occurs simultaneously

with the permitted ones.– In-sequence with - More functionality provided/occurs before or after the

permitted ones.• LESS : Interface permits less functionality to a particular actor than

is required. Association is over-constrained.• AS WELL AS : Associations to a particular use case take place with

other actors• as well.• REVERSE : Interaction takes place in the reverse direction.• OTHER THAN : Wrong association is defined. Swapping roles -

swapping of associations between actors or individuals.

Use cases

• Use cases represent tasks that the system must achieve on behalf of the associated actors. Use case documentation includes pre-conditions, post-conditions and sequence of actions.

• We can address the causes and effects of variation by considering the deviations on each step in the use case.

• Consider also deviations from pre- and post conditions

Use Case Elements Guidewords• State (defined in a pre-condition or a post-condition)• NO : The state or condition does not take place or is not

detected.• AS WELL AS : Additional conditions apply. This may

mean that more stringent checks are made, or else a more restrictive state results than is strictly required. Errors of commission might be considered here.

• PART OF : Only a subset of the required conditions apply. This might result from incomplete checks , incomplete implementation, or because the consequences of system behaviour are not fully understood.

• OTHER THAN : An incorrect condition applies. Perhaps the wrong data is used.

Use Case Elements Guidewords• Action• NO : No action takes place.• MORE : More action is achieved. This may be one of the

following:• Repeat - the same actions take place repeatedly.• Superfluous - the system does additional actions to

those intended or required.• LESS : Less is achieved than intended. For example, an

action is incomplete, or an action takes place for a shorter time than required, or an action stopped earlier than expected.

• OTHER THAN : An incorrect action takes place.

Use Case Elements GuidewordsSequence of Actions (scenarios)LESS : Less is achieved than intended. For example, Drop - miss one or more

parts of action. Additionally, a sequence of action takes place for a shorter time than required.

AS WELL AS : The sequence does the intended actions plus others.REVERSE : The sequence of actions take place in reverse order (and other

outof-order concepts).EARLY : The action sequence or its components takes place before it is

expected (timing).LATE : The action sequence or its components takes place after it is expected(timing).BEFORE : The action sequence or its components happens before another

action that is expected to precede it.AFTER : The action sequence or its components happens after another action

that is expected to come after it.OTHER THAN : An incorrect action sequence takes place.

Analysis

• The team systematically investigates each use case element to identify deviation from requirements.

• Each deviant is further investigated for possible causes and effects.

AnalysisThe process has the following steps:1. From a use case, identify and record:– intent/action and capability of actors;– associations between actors and use cases;– components from use case description: pre-condition, main flow of events,alternative flow of events (i.e. normal but perhaps less likely and exceptional/error), post-condition.2. Apply HAZOP guidewords with the appropriate interpretation, to each

elementidentified in step 1, to suggest deviations.3. Evaluate whether the identified deviations violate, or could violate, any

securityproperties. Investigate possible causes of the deviations.4. Identify consequences that may arise from the deviations (extract affectedassets from the identified consequences).5. Categorise the deviations identified, and generalise each group.6. Provide recommendations on the identified problems/threats.