PetroStreet

Companies develop & maintain their risk matrix having their own listed risk with values /references.
However, what are the unified best practices to apply the risk matrix when assessing a risk, and when applying controls to bring down the risk to ALARP?

I think this is what all involved in a risk assessment may think at some point in time when generic practices are applied.
In real, applying a risk matrix effectively requires more than just picking a color on a grid; it requires a disciplined approach to ensure that "High" risk means the same thing to every engineer in the room.
Lately, we had a workshop, a refresher you can call, and following are some of the key discussions, we had:

1. Initial Assessment
Before looking at the matrix, you must define the scenario clearly to avoid "risk creep" or over-conservative estimates.
Define the "Credible" Worst Case: Do not assess the "absolute" worst case (e.g., a meteor hitting the plant) unless it is a credible threat. Focus on the most severe outcome that is actually possible given the equipment and process.
Assess Unmitigated Risk First: Determine the risk level as if no active or administrative controls are functioning. This establishes the "Raw Risk" and justifies why certain safety systems (like SIS or relief valves) are critical.
Consistency in Consequence Categories: Use a multi-disciplinary approach. A risk might be "Low" for Safety (no injuries) but "High" for Environment (large spill) or Reputation. The highest individual category determines the final risk rank.
Independence of Likelihood and Consequence: A common error is lowering the consequence because the likelihood is low. These must be treated as independent variables.

2. Applying the Hierarchy of Controls
Once a risk is identified, the goal is to move it "down and to the left" on the matrix. Best practices dictate following the Hierarchy of Controls in strict order:
a. Elimination (meaning) Physically remove the hazard (e.g., using a non-toxic chemical). (effectiveness) Highest
b. Substitution (meaning) Replace the hazard with something less risky (e.g., lower pressure). (effectiveness) High
c. Engineering (meaning) Isolate people from the hazard (e.g., blast walls, automated trips). (effectiveness) Moderate
d. Administrative (meaning) Change the way people work (e.g., training, permits, SOPs). (effectiveness) Low
e. PPE (meaning) Protect the worker with equipment (e.g., respirators, fire suits). (effectiveness) Lowest

3. Reaching ALARP (As Low As Reasonably Practicable)ALARP is the point where the cost (time, money, effort) of further reduction is grossly disproportionate to the benefit gained.
The Three Zones
Unacceptable Zone (Red): Risk must be reduced regardless of cost. Operation is usually prohibited.
ALARP Zone (Yellow/Amber): Risk is tolerable only if it can be demonstrated that further reduction is not reasonably practicable.
Broadly Acceptable Zone (Green): Risk is low enough that no further action is required, though continuous monitoring is expected.

The "Good Practice" Test
To prove a risk is ALARP, you must demonstrate that you have met or exceeded industry codes and standards (e.g., ASME, API, ISO). If a standard says a specific safety valve is required, you cannot claim ALARP without it just because it's expensive.

Cost-Benefit Analysis (CBA)In complex cases, a formal CBA is used. If the cost of an additional safety layer is $10$ times higher than the "value" of the risk reduction it provides, it may be deemed not "reasonably practicable."

4. Common Pitfalls to Avoid
Risk Normalization: Just because a high-risk activity hasn't caused an accident in 10 years doesn't mean the likelihood is "Rare." Use data/frequencies, not just "gut feeling."
Double Counting Controls: Don't credit the same safety system twice (e.g., counting a pressure transmitter as both an alarm and a trip if they use the same sensor).
Ignoring Cumulative Risk: Assessing ten "Medium" risks individually might seem fine, but if they all occur in the same area, the aggregate risk to the facility might be "High."

I hope these notes would guide you well.
Share with colleagues when you start a risk assessment next time.

Apart from all the good practices, refreshers, and practical trainings on applying the risk matrix while performing the risk assessments, there still are some challenges, I would say, like:
1. Getting competent team on board
2. Achieving a minimum experience level (team, as a whole)
3. Facilitation role
4. Re-assessing frequency
5. Involvement of Licensor, OEM, Consultant etc.

Though point 1 and 2 are connected but I understand that an individual competence, and overall team experience are still two different variables to be judged separately.
I do not believe in a risk assessment which is not facilitated either by Process, Process Safety, or by HSE. People working with the hazards directly may have normalized risk for themselves. That happens in many routine cases!
We perform risk assessment for a particular scenario today, and then there are changes tomorrow, who is gonna re-assess those changes?
We talk all good between ourselves but when an involvement of licensor, OEM or a consultant is felt, do we actually get them onboard during the risk assessment workshops?

Some very good points have been shared by opo21.
Practically, risk assessment is one of the best practice to be implemented whenever there is a condition to the operating assets. I have seen cases where meaningful mitigations in place has saved the operating facility both from a costly shutdown and at the same time any damaging event.
Focus should be to have new trainees trained on this very important process and at the same time, the experienced personnel should spent more time in actually simulating the event in front of them to have visualtion to an extent that risk matrix should be applied properly. Missing information or information not enough would never lead to a meaningful risk assessment.

So happy to have this thread, and the responses.
An area which requires an all time attention for the kind of facilities we have been working in.