A Control Systems HAZOP (CHAZOP) is a systematic and structured approach to identifying potential hazards and operability issues associated with a control system. The conventional Hazard and Operability (HAZOP) analysis assesses the significant safety risks associated with a process or function that is integral to the operation of a plant, facility, machinery, or any other element of a workplace.

Most CHAZOPs look at two parts of the control system. Initially, it is imperative to verify whether the control loops, which are the collection of instruments responsible for executing the managed functions, are suitable for the intended process. Second, they check to see if the control system is properly connected to the main process and makes sure it doesn’t pose a risk to the workers.

A CHAZOP measures the safety risks that come with a certain control system, and as a result, it measures how well that control system works.

The CHAZOP Study objective includes identifying the following:

  • Potentially critical failure cases
  • A set of suggestions for design modifications or additional investigation
  • A means of determining Single critical points of failure
  • To Minimize downtime and Production Outrage

The Exact difference between HAZOP and CHAZOP is the focus. A HAZOP looks at the entire facility, the mechanical equipment and the process, where a CHAZOP looks at the control system itself. A CHAZOP goes into the control system and fails individual components. The Control Systems can have many single point failures which can have severe consequences.

Control System Faults can send the wrong signals or information to operator which can affect decision making factor of Operator or even can remove the plant operation out of the hands of operator. To accommodate this safety issue CHAZOP study is performed.

The full CHAZOP involves a detailed analysis of each part of Programmable Electronic System: The computer system / environment, Input /Output signals and control schemes, safety shutdown system etc.

The CHAZOP methodology involves the following steps:

  • The complete system to be studied is divided into various study nodes.
  • When evaluating each node, it is necessary to take into account a range of parameters, guidewords, and potential deviations.
  • For each deviation, realistic causes are identified (if any cause is not credible, it is ignored).
  • Consequences are identified for each credible hazard, assuming the absence of any safety measures.
  • The identification of existing protections for each consequence involves an evaluation of current practices, procedures, engineering and safety systems to determine their effectiveness in preventing the cause or reducing the impact of potential consequences.
  • Where the CHAZOP Team judges that the engineering and safety systems are unlikely to cope with the consequences of all deviations, necessary actions are recommended.
  • Where recommendations are given risk ranking shall be performed in accordance with the provided guidelines.

In CHAZOP method, the guidewords were systematically applied to a segment of Control Systems in order to promote discussion on possible deviations from the design intention. CHAZOP study is based on the documents such as Instrument & Control System Architecture diagrams, Functional Design Specifications etc., which shall be made available to CHAZOP team.

For detailed CHAZOP preparation following documents are needed

  1. System Architecture Diagram / Control
  2. System Block Diagram
  3. P&ID / PFD / Process Flow Block Diagram
  4. Control and Shutdown Philosophy
  5. Instrumentation Design Basis
  6. Shutdown Hierarchy Diagram
  7. Alarm and Trip Schedule
  8. Control Room / Local Room Layout
  9. Main UPS Single Line Diagram
  10. IO List (Preferably the full dump of the Database – SPI)
  11. Detailed System Architecture / Network Diagram
  12. Functional Design Specification – Hardware
  13. Functional Design Specification – Software
  14. Typical Loop Diagram for All System
  15. Powering Scheme diagram
  16. Earthing Layout / Grounding Layouts
  17. IO Channel Assignment List or Nest Loading
  18. Typical or Prototype – GA drawing for Cabinets (System / Marshalling / PDB / Server / Network cabinets)

CHAZOP proceedings shall be recorded in the CHAZOP worksheets. Information recorded in columns on the work sheets is as follows: Guideword, Deviation, Causes, Consequences, Safeguards, Risk Ranking, Recommendations and Responsibility

CHAZOP recording is done using a Software Tool ‘PHA-Pro. The use of software enables On-line recording of the team discussion. The discussion from the study is recorded in log sheets.

The recommendations are evaluated for implementation based on their criticality. Risk ranking is used as a means of prioritizing recommendations based on severity and probability of occurrences of undesirable consequences. The consequences arising out of various deviations are analyzed and the severity and likelihood of such occurrences are assessed during CHAZOP study.

The Risks are ranked based on severity and likelihood of each event. The Recommendations are categorized based on the available Risk Ranking matrix document.

CHAZOP methodology For identifying hazards on each nodes

Safety Risk Tolerability Criteria

Risk CategoryControl Regime
Unacceptable  RegionThe activity shall not be carried out as the Risk is intolerable / unacceptable.
ALARP RegionThe Risk is tolerable.
Acceptable RegionThe Risk is acceptable.

A Sample Guideline for assessing Likelihood of Occurrence is described below

Guideline for Assessing “Likelihood of Occurrence”

Frequency  Estimation
Frequency CategoryQualitative Interpretation GuidanceQualitative Interpretation Guidance (yr-1)
AVery Unlikely Has happened a few times, or not at all in Oil & Gas industryLess than E-05
BUnlikely Have been as an isolated occurrence or has happened several times in Oil & Gas industryBetween E-05 and E-04
CPossible Has not been happened at the Asset or has happened a few times.Between E-04 and E-03
DLikely Has happened once before, or several times.Between E-03 and E-02
EFrequent Has happened several times, or many times.Greater than E- 01

A Sample Guidelines for assessing Hazard Severity is described below

Guidelines for Assessing “Hazard Severity”

Identifying the risk matrix and risk tolerability of people, Assets and reputation

The CHAZOP meeting discussions are recorded in a Sample CHAZOP worksheet as shown below