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Defining requirements and optimising safety - part two

Hydrocarbon Engineering,

This is part two, read part one here.

Industry experience

In spite of the introduction of the new safety requirements step in Edition 2 of IEC 61508, ABB’s experience is that many SRS documents are still not comprehensively detailed, leaving safety system suppliers second-guessing about many of the specific criteria that process operators and contractors are looking for.

Not only that, but many organisations lack dedicated and competent functional safety resources. Compared to 25 years ago, when in-house engineering teams were the norm, dedicated functional safety (FS) specialists are now very hard to find, having either retired or moved on. Despite this, the expectations from both industry and regulators alike when it comes to functional safety have never been higher, due in no small part to a crop of recent high profile industry incidents and related safety system failures. Asset owners are therefore under pressure to ensure the highest levels of plant safety are delivered in accordance with their duty of care and are managed and demonstrable to internal and external stakeholders alike.

An important point to remember at the outset of a project is that the contractual necessities for safety system requirements should not be attempted to be resolved too early in the project lifecycle/FEED study. Furthermore, any detailed commercial and technical discussions with the supply chain should start only after the development of the detailed SRS and as based on the information provided from the earlier output of the hazard and risk assessment processes.

At a time when project owners and their main contractors may not have dedicated functional safety expertise in-house, the best advice is to use the available expertise in the supply chain. Responsible suppliers of SIS will generally be a good place to start when trying to bring this information together. The SRS development methodology may form part of a wider suite of functional safety management (FSM) system procedures that can be utilised as part of the broader safety lifecycle management requirements for compliance to the safety standards e.g. an accredited certified FSM system assessed by a recognised third-party accredited certification organisation such as TÜV.

Responsible suppliers such as ABB will typically support end users and contractors to develop the SRS by providing a structured SRS document skeleton that can be used to identify any gaps in the existing information and assumptions. In this way, the SIS supplier can test key assumptions and spot whether there is an opportunity to safely reduce complexity in design and installation and the expected maintenance regimes whilst optimising the overall cost of safety.

For the commercial and responsible supply chain teams involved during a typical tender and bid process, a detailed SRS issued as part of the commercial negotiations will enable greater transparency against requirements and provide a vehicle to test key assumptions. As well as helping to identify areas for reducing cost, complexity in design and installation and expected maintenance regimes, a detailed SRS will also help to:

  • Provide clarification and reduce ambiguity to technical, management and integrity requirements.
  • Provide commercial assurance that the SRS meets the intended risk reduction to be afforded by the SIS.
  • Establish the basis for traceability and audit trail throughout later lifecycle phases.
  • This will provide the level of safety integrity and traceability required to ensure the correct functional design is taken forward through the safety lifecycle management process.


To conclude, a well-specified safety requirements specification will substantially reduce the risk of under, or over specification, affecting both safety risk reduction requirements and capital to be deployed. This means that the system requirements specification meets the desired scope, optimised cost of solution, performance and maintenance criteria, size and complexity of the application.

Written by John Walkington and Stuart Nunns, ABB, UK, and edited for web by Cecilia Rehn

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