Module 5 - SIL Determination

Question 1

What are the three main SIL determination techniques?

Answer 1

Risk graph, LOPA, Fault tree

Question 2

How do you operate a risk graph?

Answer 2

1. Interpret consequence
2. Interpret probability of persons present along consequence branch
3. Interpret probability of avoidance on same branch
4. Interpret probability of hazardous event
5. Intersection of 1,2,3 and W is the SIL

Question 3

What are three advantages of risk graphs?

Answer 3

1. It is semi quantitative and thus easier to use
2. It can be applied as a team exercise to avoid individual bias
3. It can be applied very quickly as a screening tool

Question 4

What is the main source of input to a LOPA?

Answer 4

HAZOP report

Question 5

What is the main source of input to a LOPA?

Answer 5

HAZOP report

Question 6

What are some restrictions on protection layers when conducting a LOPA?

Answer 6

Protection layers must be:

1. Independent
2. Specific
3. Dependable (systematic and random faults)
4. Auditable

Question 7

Does the order of assessment in a LOPA matter?

Answer 7

Yes. You must assess protection layers in the order they would be triggered

Question 8

What are the advantages of a LOPA?

Answer 8

1. They can be used both semi-quantitatively and quantitatively
2. They can account for risk mitigating factors

Question 9

Provide three points of comparison for a risk graph and a LOPA

Answer 9

1. A LOPA can be used quantitatively while a risk graph cannot
2. A risk graph can be applied quickly whilst a LOPA is slow relatively
3. A LOPA can account for risk mitigating factors while a risk graph cannot
4. A risk graph is easier to perform as a team exercise (more visual)
5. A risk graph is coarser than a LOPA

Question 10

What are the advantages of a fault tree?

Answer 10

1. Can represent complex dependencies where protection layers are not independent
2. Can model very large/complex systems
3. Highly quantitative (probabilities of unsafe events, top contributors to unreliability, Markov modelling for time dependent events)

Question 11

What are the disadvantages of a fault tree?

Answer 11

1. Time consuming
2. Requires specialist skills
3. Not a team exercise
4. Good results require good data

Question 12

What is the best approach to allocating protection layers?

Answer 12

A combined approach:

1. screen SIFs using simple tool such as risk graph
2. For SIL2 or greater use a quantitative tool