Risk Analysis

Risk Analysis

       Everyone in the engineering profession is familiar with Murphy's Law, "If anything can go wrong, it will.". The extended version which states, "If a series of events can go wrong, it will do so in the worst possible sequence". 

Risk analysis is a sort of Murphy's Law review in which events are analyzed to see the destructive nature that they might produce. Risk analysis is a term that is applied to a number of analytic techniques used to evaluate the level of hazardous occurrences. Technically, risk analysis is a tool by which the probability and consequences of accidental events are evaluated for hazard implications. These techniques can be either qualitative or quantitative.

Risk analysis can be broken down into four main steps:
(1) Identify accident occurrences.
(2) Estimate the frequency of the occurrences.
(3) Determine the consequences of each occurrence.
(4) Develop risk estimates associated with the frequency and consequence.

Risk Identification and Evaluation

  The basic methodology adopted for the formal risk evaluation in the petroleum and related industries, both for existing facilities and new projects, normally contain the following steps:

1. Definition of the Facility - A general description of the facility is identified. Input and outputs to the facility are noted, production, manning, basic process control system (BPCS), ESD, fire protection philosophy, assumptions, hazardous material compositions, etc.
2. Identification of Hazards - A listing of the processes and storage of combustible materials and the process chemistry that can precipitate an incident.
3. Development of Accidental Events - Identified scenarios that can cause an accident to occur.
4. Frequency Analysis - An examination of the probabilities or possibilities of and accident to occur.
5. Consequence Modeling - A description of the possible incidents that can occur.
6. Impact Assessment - The development of the severity of the incident in terms of injuries, damage, environmental impact, business interruption and public reaction.
7. Summation of Risk - The combination of severity and probability estimates an incident to occur.
8. Effect of Safety Measures An evaluation of the mitigation effects of layers of protective systems of different integrity, on the effects or prevention of an incident.
9. Review Against Risk Acceptance Criteria - The comparison of an incident risk which is supplemented by the selected safety measures to achieve the requirements for company safety levels.

During the process hazards identification and definition phase of a project design, a basic process control system (BPCS) strategy is normally developed in conjunction with heat and material balances for the process.

Both qualitative and quantitative evaluation techniques may be used to consider the risk associated with a facility. The level and magnitude of these reviews should be commensurate with the risk that the facility represents.

Qualitative Assessment

Qualitative assessment are studies base on the generic experience of personnel and do not involve mathematical estimations. Overall these reviews are essentially checklist reviews in which questions or process parameters are used to prompt discussions of the process design and operations and possible accident scenarios.

 Some of the examples are HAZOP, What if reviews, Preliminary hazard analysis (PHA), etc. 

Quantitative Assessment

Quantitative reviews are mathematical estimations that rely upon historical evidence or estimates of failures to predict the occurrence of an event. These reviews are sometimes referred to as a Quantitative Risk Assessment (QRA).

Some of the examples are Event tree analysis, Fault tree analysis, Failure mode effect analysis.

There are other studies according to hazards present at the time of process like leak estimation, combustible vapor dispersion, fire water reliability, etc.,

Note: In glossary some terms are given for readers reference.


Glossary

Checklist or Worksheet - A standardized listing which identifies common protection features required for typical facilities is compared against the facility design and operation. Risks are expressed by the omission of safety systems or system features.

Preliminary Hazard Analysis (PHA) - Each hazard is identified with potiential causes and effects. Recommendations or known protective measures are listed.

What-If Reviews - A safety study which by which “What-If’ investigative questions (brainstorming approach) are asked by an experienced team of a hydrocarbon system or components under examination. Risks are normally expressed in a qualitative numerical series (e.g., 1 to 5).

HAZOP - A formal systematic critical safety study where deviations of design intent of each component are formulated and analyzed from a standardized list. Risks are typically expressed in a qualitative numerical series (e.g., 1 to 5) relative to one another. 

Relative Ranking Techniques (DOW and MOND Hazard Indices) - This method assigns relative penalties and awards points for hazards and protection measures respectively in a checklist accounting form. The penalties and award points are combined into an index which is an indication of the relative ranking of the plant risk.

Event Trees (ET) - A mathematical logic model that mathematically and graphically portrays the combination of events and circumstances in an accident sequence, expressed in an annual estimation.

Fault Trees (FT) - A mathematical logic model that mathematically and graphically portrays the combination of failures that can lead to a specific main failure or accident of interest, expressed in an annual estimation.

Failure Modes and Effects Analysis (FMEA) - A systematic, tabular method of evaluating the causes and effects of known types of component failures, expressed in an annual estimation.

Leak Estimation - A mathematical model of the probability and amount of potential hydrocarbon releases that may occur from selected processes or locations.

Depressurization and Blowdown Capabilities - A mathematical calculation of the system sizing and amount of time needed to obtain gas depressurization or liquid blowdown according to the company’s philosophy of plant protection and industry standards (i.e., API RP 521).

Combustible Vapor Dispersion (CVD) - A mathematical estimation of the probability, location, and distance a release of combustible vapors will exist until dilution will naturally reduce the concentration to below the LEL or no longer considered ignitable (typically defined as 50% of the LEL).

Explosion Overpressure - A mathematical estimation of the amount of explosive overpressure that may be expected from an incident. It is portrayed as overpressure radii from the point of initiation until the overpressure magnitudes are of no concern, i.e., less than 0.02 bar (3.0 psio). Evaluations perforned for enclosed areas will also estimate the amount of overpressure venting capability available.

Survivability of Safety Systems - An estimation of the ability for safety systems to maintain integrity from the effects of explosions and fires. (Safety systems may include ESD (Electrostatic discharge), depressurization, fire protection - active and passive, communication, emergency power, evacuation mechanism, etc.).

Firewater Reliability - A mathematical model of the ability of the firewater system to provide firewater upon demand as required by the design of the system without a component failure, e.g., a Mean Time Between Failure (MTBF) analysis.

Fire and Smoke Models - A mathematical estimation model depicting the duration and extent of heat, flame and smoke that may be generated from the ignition of a hydrocarbon release. The results of these estimates are compared against protection mechanisms (e.g., firewater, fireproofing, etc.) afforded to the subject area to determine adequacy.

Emergency Evacuation Modeling - A study of the mechanisms, locations and time estimates to complete an effective removal of all personnel from an immediately endangered location or facility.

Fatality Accident Rates (FAR) or Potential Loss of Life (PLL) - A mathematical estimation of the level of fatalities that may occur at a location or facility due to the nature of work being performed and protection measures provided, may be calculated at an annual rate or for the life of the project.

Human Reliability Analysis (HRA) or Human Error Analysis - A reliability analysis that estimates the potential for human errors to occur due to the work environment, human-machine interfaces, and required operational tasks.