3B2F – Safety & Risk II, Michaelmas 2024
Dr Felix Leach
Please submit problem sheet feedback to felix.leach@eng.ox.ac.uk
Recommended reading:
• Boyle, T. “Health and Safety: Risk Management” 3rd edition; Routledge
(2008)
• Bentley, J. “Introduction to Reliability & Quality Engineering” 2nd edition;
Prentice Hall (1999)
• Cameron, I.T. and R. Raman. “Process Systems Risk Management”;
Elsevier (2005)
• Crowl, D.A. and J.F. Louvar. “Chemical Process Safety”; Prentice Hall
(2007)
• Lees, F.P. “Loss Prevention in the Process Industries. Hazard
Identification, Assessment and Control”; Butterworth-Heinemann (1991)
• Tweedale, M. “Managing Risk and Reliability of Process Plants” Gulf
Professional Publishing (2003)
Section A – Reliability and failure
Question 1
A manufacturer sells 36 million hard disk drives of a certain model in one year.
During the first three years of use 350,000 of these fail. Calculate:
a) The failure rate
b) The reliability
c) The unreliability
d) The MTBF
Question 2
a) A component has a failure rate µ(t) (failures per component per hour),
which varies throughout its lifetime. A batch of N components is tested
and n are operating at time τ. Write down an expression for the number
of failures δn occurring during the time interval δτ. Hence show that the
reliability of the component R(t) is given by:
𝑡
𝑅(𝑡) = exp (⁡ − ∫ 𝜇(𝜏)𝑑𝜏)
0
b) A new component has a failure rate described by the Weibull
distribution:
𝛽 𝑡 − 𝑡0 𝛽−1
𝜇(𝑡) = (
)
𝜂
𝜂
where β, η, and t0 are constants.
What is the reliability of the component in terms of β, η, t, and t0?
Section B – Quantitative Methods of Risk Analysis
Question 3
Clean steam for a sterilizer is generated by an electrical immersion heater
inside a pressure vessel, as shown in Figure 1. Normal operating pressure is
maintained by controlling the power supplied to the heater, using a
proportional controller connected to a temperature sensor placed in the
boiling water above the heater. The vessel is protected against overpressurization by:
i.
ii.
a pressure switch, set to a value above the operating pressure, which
opens a circuit breaker on the supply to the heater;
a spring-loaded pressure relief valve. The controller is powered from an
a.c. mains supply to the heater.
Failure of the controller or its power supply does not necessarily disconnect
the power supply to the heater. An internal battery powers the pressure
switch.
Figure 1
a) Draw a fault tree for the hazard of over-pressurization and estimate the
frequency of occurrence from the data supplied (assume the failures are
unrevealed).
b) Evaluate whether it would be better to power the pressure switch from
the mains supply (like the controller) instead of from a battery.
c) Comment on the sufficiency of the protection system.
Data
A: Probability that relief valve fails to open = 10-2 per demand
B: Probability that circuit breaker combined with the pressure switch
fails to open = 10-2 per demand
C: Probability of battery being discharged = 0.2
D: Controller electronic failure frequency = 10-1 per year
E: Temperature sensor failure frequency = 10-2 per year
F: Instrument mains supply failure frequency = 1.2 per year
Question 4
Carry out a FMECA criticality analysis for a torch (flashlight). Include in your
analysis the component under consideration, the failure mode, effect,
detection, provisions, loss frequency, and criticality. Some example values of λp
and α are given. You must estimate other parameters.
Item
Battery
Failure rate (λp)
Proportion of failures in
per million hours
mode (α)
3.3x105
Low power: 0.75
Empty: 0.25
Bulb
50
1
Contacts
15
Poor: 0.5
Intermittent: 0.2
No contact: 0.3
Switch
10
Intermittent: 0.3
Stuck Closed: 0.1
Stuck Open: 0.6
Make a suggestion to improve the reliability of the torch.
Section C – Human factors Quantitative Methods of Risk
Analysis
Question 5
Write a risk assessment for playing a sport of your choice. Make sure that you
include a range of risks (at least 8 separate risks), together with appropriate
risk control measures.
For guidance on putting together a risk assessment, see ‘Five steps to risk
assessment’: http://www.hse.gov.uk/pubns/indg163.pdf
For a blank template see: https://intranet.eng.ox.ac.uk/media/3pmlg1xv/riskassessment-template-rev-4-june-2017.doc
Question 6
Define slips, violations, and mistakes. Distinguish between the three, explain
how they can each be avoided, and state which (in your opinion) is the most
important in creating safe systems with humans in. (approx. 500 wds)
Question 7
The final question is intended to give practice in writing a structured piece of
technical writing and gives an opportunity to put into practice the techniques
presented in the ‘Technical Writing and Communication Skills’ course.
Assess the extent to which cultural issues contributed to the crashes of Korean
Air Lines flight 801 on 6th August 1997 and Flight 8509 on 22nd December 1999.
Highlight the measures taken by Korean Air Lines to address these issues, and
discuss their relevance in other industries
To answer the question above:
i.
Conduct some basic research and take notes – there is a wide range of
web resources on these incidents. See, for example, three pdfs in the
canvas folder.
ii.
Produce an outline plan for the written piece; giving an overall structure
and bullet points for the arguments that you wish to make. Ensure that
the key points of the question are adequately addressed. [This plan
should be handed in to your tutor, in addition to the answer to (iii)
below].
iii.
Using the above plan as a basis, write an answer of approx.1500 words
addressing the question.
ANSWERS
1d 2.70 million hours
3a 2.75×10-3 / year; b 12×10-3 / year