Appunti completi di Safety Engineering and Management

SAFETY ENGINEERING

• 1. Human & Organizational Factors

  • Environmental effects
  • Failure likelihood
  • Human errors
  • Job design
  • Interaction
  • Organizational structure

• 2. Occupational Health & Safety

  • Health check
  • OSHA Guidelines
  • Risk of dangerous accidents investigation
  • RST
  • SOSH Procedures Assessment

• 3. Human & Organizational Factors (HEP)

  • HT
  • IPER Process Factors
  • Performance shaping factors
  • Prevents HPCP
  • Quality check
  • Redundancy
  • Risk Evaluation

• 4. Threat Assessment & Emergency Management

  • 4 steps method
  • Crisis intervention
  • Prevention methods
  • Risk communication

• 6. Maintainable Operation

  • Components inventory
  • Monitoring
  • Process information

Safety Engineering

Introduction

A group of interacting, interrelated & interdependent elements formed or regarded as forming a cohesive unity.

• A subsystem represents an element of a system that may constitute a system in itself.
• A composite of people, processes and equipment that are integrated to perform a specific operational task or function within a specific environment.

Cyber-physical system - A mechanism that is controlled or monitored by computer-based algorithms, tightly integrated with the internet and its users.

Safety - A degree of confidence that in given circumstances we are free from undergoing or causing harm or loss.

System safety - Characteristics of a system that allows the system to function under predetermined conditions with an acceptable minimum of accidental loss.

System Safety Engineering

It is the combination of management and system engineering practices applied to the evaluation and reduction of safety risk in a system and its operation.

• Identify the hazards in a system.
• Determine the underlying causes of hazards.
• Develop plans & implement options to either eliminate the hazards or mitigate their consequences.
• Verify that the controls are adequate and in place.
• Maintain the system after it has been released and modify it later as needed.

Process

• Define the objectives (How safe is safe enough?).
• System description
• Hazard identification
• Hazard analysis
• Periodic system review

Risk Management

• Risk evaluation
• Hazard controls
• Verification of controls
• Risk acceptance

Hazard - Everything that may have an impact on the safety level of the system.

That can cause harm/loss when interacting with the system.

FMEA ANALYSIS

FAILURE MODE EFFECT ANALYSIS (FMEA)

FMEA is an analytical tool that identifies all the ways a particular item can fail, and what its effects could be on the system at different levels of abstraction.

• Assembles how a failure mode can and cause a failure
• Causes (failure prevention): preventive controls
• Effects?

Reliability engineering tool: analytical barrier (Not a priority setting tool)

Failure mode has to exist for a limited role to be present

• Face for machine analysis of domains

We assume a certain failure mode and we try to analyze what the expected effects on the components of a system.

• Processes: Product/Process FMEA - addresses potential failure of a system
• Process FMEA - addresses potential failure modes occurring during the regular execution of business process

System description

• Highest Level
• Transducer
• Physical
• Lowest Level
• Subsystem

FMEA Worksheet

Identification of failure modes within system.

• Contaminants
• Leaking dust elements - piping joints
• Alternative models or options

Severity Classification

• I - Catastrophic: A failure mode that causes death or complete mission loss
• II - Critical: FM that may cause severe injury or major system degradation, damage, or reduction in mission performance
• III - Marginal: FM that may cause minor injury or degradation in system or mission performance
• IV - Minor: FM that does not cause injury or system degradation but may result in system failure/unresolved maintenance

FMEA & CRITICALITY ANALYSIS (FMECA)

Criticality (quantitative) - measurement of the severity of a FM

Analysis is the accumulation of a probabilistic assessment of the occurrence of the FM and the determination of the impact of the FM on the reliability targets of the systems.

RPN = Risk Priority Number!

Deterministic compilation

FMECA PROCEDURE

• FMHOE and causes
• Mission Phase/Operat. Mode
• Severity class
• Failure Probab., mode sources
• Failure Mode Mechanism/Ratio (B)
• Failure Effect Ratio (α)
• Failure Rate (λ)
• Operating Time (T)
• FM Criticality (Cr)
• Item Corrective Remarks

Common Cause Analysis

• One of the most important task in the complexity of HCS is the identification of possible environmental/external causes of faults common within the system.
• Common causes (CC) have the effect of modifying the base failure rate of single states.

1. Indirect external causes of faults have the disturbing probability of turning BE that were modeled as completely random into dependent BE that occur due to an external cause.

• Vibrational problems
• Temperature
• Corrosion
• Human error

One method for cause analysis is:

• MC₃ = ABC
• MC₂ = ABCE
• MC₃ = ABEFG

Beta factor: The fraction of failures corresponding to the estimate of the additional failure rate due to a common cause

λ = λ + λ_CC

λ_CCF = β * λ_C

Event Tree Analysis (ETA)

• If an inductive process whereby an undesirable event (initiating event) is produced, causing an adverse event, a chain of the operation of a system occurs.
• To analyze the degree loading of the safety barrier to mitigate consequences/remaining living wallings.

You can calculate the P of each branch with FT

Bow-Tie Model

Initiating event → Hazardious event ← Mitigation (Pre-active)

Trigger

Analysis of Complex System

• Accident sequences
• Organizational → Influence the environment

RIF

• External
• Internal

Task & Tools

• Structure
• Human
• Error

Phases

1. Familiarization
   • Information gathering
   • Plant visit
   • Review of procedures/information from system analysis
2. Qualitative Assessment
   • Determine performance requirements
   • Evaluate performance criteria
   • Determine performance objectives
   • Determine potential & human errors
   • Assess human performance
3. Quantitative Assessment
   • Preliminary probability of human error
   • Identify factors/methodology questions human performance
   • Evaluate effects of factors/methodology
   • Account for probabilities of factors giving errors
   • Estimate factors contributing to IR of original failure
4. Incorporation
   • Perform sensitivity analysis
   • Incorporate results to system analysis

Therp Tree Diagram

Basic model used by THERP is the "tree diagram"

Each node is a binary decision point where correct performance (branches take either) and error (opposite route) are the two possible values.

Each node note, the sum of the departing branches probabilities = 1

Definitions of HEP

• Nominal HEP = P of a given human error upon the rejection of an available option for basis not yet been considered
• Basic HEP = P of a HE without considering the conditional influence of other tasks
• Conditional HEP = Evaluation of HEP to account for influence of other tasks/factors
• Joint HEP = P of HE on tasks that must be performed correctly to achieve system end effect

Data Analysis

Maximum-likelihood estimation methodExponential functionE(Xset) = e^(2mu+sigma)Xset = ERx/E^2Therp estimation of HEP (xh)

Stress Levels

• Low = processes unchanged/confirmed during emergency ~ multiplier 1
• Moderate = processes during emergency diff training emphasis ~ multiplier 2
• High = emergency during poor diagnosis mod incorrect conditions ~ multiplier of 5