Safety and Security: Two Sides of the Same Coin

A Weakness in Security Creates Increased Risk, Which in Turn Creates a Decrease in Safety, so Safety and Security Are Directly Proportional, but Are Both Inversely Proportional to Risk

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The remainder of this article will present an approach to merge the front-end of the safety and security life cycles to demonstrate the possibility and the benefits of taking an integrated approach to safety and security, especially when designing a new or retrofitting an existing system. While the authors believe it is also possible to merge subsequent phases of the safety and security life cycles, it is beyond the scope of this article to cover the latter phases. Additionally, we feel the greatest similarities are in the front end of the processes, and integrating the processes up-front will provide the greatest benefit throughout the process.

Let's start by taking a look at the life-cycle models for safety system engineering and control system security. The safety life-cycle model from IEC 61511 (also ANSI/ISA S84) has three main phases; Analysis, Realization and Operation. The security level life-cycle model from ANSI/ISA S99.00.01-2007 also has three main phases; Assess, Develop, and Implement and Maintain.

The Safety Analysis and the Security Assess phases have the most similarity by far because, in both cases, the purpose of this phase is to determine the amount of risk present and decide if it is within tolerable limits for the facility. Determining the amount of risk involves identifying the consequences (what could happen and how bad would it be?) and the likelihood of it occurring (how it could happen and how likely it is to happen?).

A typical first step in this process is the hazard and operability analysis or HAZOP. A HAZOP, the most widely used method of hazard analysis in the process industries, is a methodology for identifying and dealing with potential problems in processes, particularly those which would create a hazardous situation or a severe impairment of the process. A HAZOP team, consisting of specialists in the design, operation and maintenance of the process, analyzes the process and determines possible deviations, feasible causes and likely consequences. It is important that the industrial automation and control system (IACS) be listed as a cause if failure of the IACS or unauthorized access could initiate a deviation.

Figure 1 shows the piping and instrumentation diagram (P&ID) for a simple chemical reactor. A portion of an example HAZOP for the process is shown in Figure 2. The text in red highlights the IACS-related causes.

Unfortunately, other than identifying the IACS as a potential cause, a HAZOP doesn't study the details of IACS deviations, which is an important step, especially given the size and complexity of modern control systems. An increasingly popular solution to this problem is a special version of a HAZOP called a control hazards and operability analysis (CHAZOP), which takes the next step in understanding the details of IACS hazards. Another technique is a failure modes and effect analysis (FMEA). Both techniques identify causes and consequences of control system failures. The CHAZOP technique extends the concept of deviations and guidewords from HAZOP, extending the list of guidewords for IACS specific types of deviations. The FMEA process takes a more hardware-centric approach by systematically studying the failure modes of each component and the effects on the system. Either technique is acceptable. However, regardless of the technique selected, it is important to include security deviations or failure modes in the analysis.

Figure 3 shows a portion of an example CHAZOP that looks deeper into possible causes of the IACS deviations. In this example, the red text highlights the security-related deviations.

Upon completion of the HAZOP and CHAZOP or FMEA, we should have identified all of the causes of IACS failure, including security failures, and the consequences of those failures. However, we still have not determined the likelihood of these events occurring, which is a necessary step in quantifying the risk, as risk is the product of likelihood and consequence.

Estimating likelihood, particularly for security, can be a difficult task because it can be very difficult to estimate the skill and determination of an attacker. We can simplify the task by filtering the list to include only those with intolerable consequences (e.g., have the potential to cause injury, death, significant downtime and environmental or major equipment damage). There are a variety of techniques available, including risk matrices and risk graphs. However, in safety, one of the most popular techniques for estimating likelihood is layer-of-protection analysis (LOPA). Part of the reason for its popularity is that it gives the user credit for employing a layer- of-protection strategy to mitigating risk. The security field uses the term defense-in-depth to describe a very similar concept. Yet, even though the terminology is different, the LOPA technique can definitely apply to security threats.

Having completed the LOPA or other method of estimating the likelihood, the final step in the front-end of the combined safety-and-security life cycle is to compare the results with facility or corporate tolerable risk guidelines and to document the results. Whenever the estimated risk exceeds the tolerable risk, there is work to be done. That work is covered in the subsequent phases of the combined life cycle and will be discussed in the subsequent articles. However the following case study from a major U.S. refinery illustrates the analysis phase and subsequent develop/implement phase.

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