What is a CHAZOP?
A Control Hazard and Operability Study (CHAZOP) is a procedure for carrying out the safety and reliability analysis of existing or planned control and computer systems.
In the early 1980's, the trend of replacing manual operations with more advanced automated operations began, and continues to this very day. Localized control panels were moved to centralized control rooms, and the industry saw the evolution and sophistication of programmable electronic systems, including
Distributed Control Systems (DCS)
Hardwired relay systems replaced by programmable logic controllers (PLCs)
Supervisory Control and Data Acquisition (SCADA) systems
This trend has given rise to greater operability, flexibility and capability within facilities, but it is also accompanied by increased complexity and more components that have a certain probability of malfunctioning or failing. Ultimately, in the 2000's, the components of a Control System became potential hazards that needed to be included as part of the process hazard analysis. This is where the CHAZOP was born.
When Do You Conduct a CHAZOP Study?
Typically, CHAZOPs are carried for the following reasons:
To support the traditional PHA process (HAZOPs, LOPAs and SIL studies)
To determine the capability of the equipment
Reason 1 - To Support the traditional PHA Process
To support the PHA process within a HAZOP, it is possible to use CHAZOP techniques to look at global issues such as loss of power, hydraulics or instrument air and determine what are the associated control system failure modes and overall availability. Another approach is to supplement the HAZOP with a CHAZOP taking various approaches, such as:
Analyzing every control loop (this is very intensive, and not always recommended)
Analyzing specific loops that have higher demand or have a higher requirement of availability
Identifying common cause failure modes of control systems and safety systems
Reason 2 - To determine the Capability of Equipment
CHAZOPs can have a high value for an end user, because it allows them to benchmark the design of a control system to evaluate against specific internal criteria requirements or to compare specific equipment from different suppliers. The equipment capability is also important to know for regulatory compliance and to determine failure rates to support LOPA and SIL verification exercises.
Ultimately, identifying what your team is trying to achieve with a CHAZOP is crucial to help you decide what approach is best suited for your needs.
Similar to HAZOPs, CHAZOPs use a framework of guidewords to analyze potential causes that lead to hazardous consequences. Some examples of CHAZOP guidewords are as follows:
Loss: evaluates the impact to the design when a function is lost, including power supply, processor capability, memory, communications channels from/to I/O signals, etc.
Range: evaluates the impact to the design if a signal is either being distorted or is out of range
Mixture: evaluates the impact to the design when there is a failure pattern of inappropriate combinations of I/O channels in relation to the hardware organization of the system
Version: evaluates potential consequences of either changes to the hardware platform or upgrades to the system software on the integrity of the application software
Security: evaluates the potential consequences of unauthorized access to the system
Although not as commonly seen as HAZOPs, CHAZOPs are an important component in the process safety management of facilities. Knowing when and how to conduct these studies will lead to safer and more reliable assets.