Planning lifecycle activities for safety instrumented systems

Modern industrial safety instrumented systems are often required to be designed in accordance with IEC 61508 or IEC 61511. These standards about functional safety take a lifecycle view on the safety instrumented system. Most people associate this with SIL – or safety integrity levels, which is an important concept in these standards. Many newcomers to functional safety focus only on quantitative measures of reliability and do not engage with the lifecycle process. This leads to poorer designs than necessary, and compliance with requirements from these standards is not possible without taking the whole lifecycle into account.

A good way to look at a safety instrumented system, is to define phases of the lifecycle, and then assign activities for managing the safety instrumented system throughout these phases. Based on IEC 61511 we can define these phases as:

  • Design
  • Construction
  • Commissioning
  • Operation and maintenance
  • Decomissioning

In other words – we need to manage the safety instrumented system from conception to grave – in line with asset management thinking in general. For each these phases there will typically be various activities related to the safety instrumented system that we will need to focus on. For example, in the design phase we need to focus on identifying the necessary risk reduction, performing risk analysis and determining necessary SILs for the different safety instrumented functions making up the system. A key document emerging from this phase is the “Safety Requirement Specification”. Typically, in the same phase one would start to map out vendors and put out requests for offers on equipment to buy. A guideline for vendors on what type of documentation they should provide would also be good to prepare in this early phase. The Norwegian oil and gas association has made a very nice guideline (Guideline No. 070) for application of functional safety in the oil industry; this guideline contains a very good description of what type of documentation would need to be collected. This is a good starting point.

Also part of design, and typically lasting into the construction phase as well, we would find activities such as compliance assessment (it is necessary to check whether the requirements in the SRS are actually fulfilled, based on documentation form eqipment vendors and system integrators). In addition, at this point it is necessary to complete a Functional Safey Assessment (FSA), a third-party review in the form of an audit to check that the work has been done the way the standards require us to.

Part of the plan should be on how to commission the safety instrumented system. When are the different functions tested, what type of verifications are we doing on the programming of actions based on inputs? Who is responsibel for this? All of this should be planned out from the start.

Further, when the system is taken into operation, the complete asset (including the SIS) is delivered to the company that is going to operate it. The owner is then responsible for maintenance of the system, for proof testing and ensuring that all barrier elements necessary for the system to work are in place. These type of activities should be planned as well.

Finally, the end-of-life for the asset should be managed. How to actually manage that should be part of the plan – taking the system out of service as a whole or only in parts shoudl still be done while maintaining the right level of safety for people, environment and other assets that may be harmed if an accident should occur.

Finally, there are a number of aspects that should be included in a plan for managing functional safety, that span over all these lifecycle phases. These are things like competence management of people involved in working with the SIS in the different lifecycles, how to deal with changes of the system or the environment the system is operating in, who is responsible for what and how to communicate across company interfaces – this list is not exhaustive. Consult the standards for looking at the details.

If all organizations involved in functional safety design would plan out their acitivites in a good way fewer changes would occur towards the end of large engineering projects, better quality would be obtained at a lower cost. And this, is a low-hanging fruit that we all should grab.

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