SIS Functional Specification and SRS Development
We carry out the IEC 61511 Phase 5 front end work that turns your PHA and LOPA output into a complete, design ready safety requirements specification
SIS Functional Specification
and SRS Development
Safety instrumented system functional specification is the bridge between hazard study output and detailed system design. The work begins after PHA and LOPA have identified each safety instrumented function and allocated a SIL target, and it ends with a fully baselined safety requirements specification that is ready to hand off to the functional safety engineering team for architectural design, FMEDA grade selection and lifecycle verification. The discipline is fundamentally about completeness and traceability. Every function has to trace back to a HAZOP cause and consequence, every safe state has to be physically realisable from the process operating envelope, every demand mode classification has to reflect how the plant actually behaves, and the independence of the basic process control system from the safety instrumented system has to be confirmed before any platform decisions are made. The common front end failures, which include incomplete safe state definitions, missed common cause initiators, a system that shares control system sensors and response time budgets that ignore valve stroke time, show up later as IEC 61511 Phase 6 redesigns or as FSA Stage 2 audit findings, and our team prevents them at the source.
SIS Functional Specification and SRS Development workflow
Catalogue Safety Instrumented Functions from HAZOP / LOPA action register, document demand scenario, initiating event, and process consequence, classify demand mode (low vs continuous / high) per IEC 61511 Cl.3.5.
Define safe state per SIF (de energise / vent / isolate / depressurise), establish process safety time, response time budget, and process operating envelope, align with HAZOP cause consequence pathway.
Receive SIL allocation from LOPA / Risk Graph workflow, convert to SRS outline per IEC 61511 Cl.10, functional, integrity, environmental, operational, and interface requirements per SIF.
Verify SIS independence from BPCS sensors, logic, and final elements per IEC 61511 Cl.11.2.4, cross check SIS credit against LOPA IPL register, flag shared element gaps for redesign.
Draft cause and effect matrix linking SIF initiators to ESD outputs at a functional level (no platform binding), specify voting intent, bypass authorisation, and reset philosophy, hand to detailed design team.
Issue baseline SRS per IEC 61511 Phase 5, compile design hand off package (PHA/LOPA references, SIF inventory, SRS, functional C&E) for downstream detailed engineering / FMEDA / FAT/SAT work.
Every deliverable from basis to handover
Complete SIS Functional Specification and SRS Development scope covering every calculation, drawing, specification, and construction support activity.
Outcomes of SIS Functional Specification and SRS Development
- We make sure the system provides the last automatic layer of protection before a major accident
- We design functions correctly so that hazardous events do not escalate
- We make sure a proof tested system maintains its designed risk reduction over the plant life
- We use fail safe architecture so that a single failure does not lose the safety function
- We deliver IEC 61511 lifecycle compliance for each function
- We produce the OSHA PSM documentation and verification for the system
- We provide the system performance assurance your COMAH and Seveso obligations need
- We meet your insurance underwriter system verification requirements
- We write comprehensive factory and site acceptance test procedures so the system is correctly commissioned
- We design proof test procedures that maintain the SIL without excessive downtime
- We provide a bypass management procedure that prevents extended impairment
- We support safe operating window management through system status monitoring
- We use a specification driven design to prevent over specification and an unnecessary SIL 3 system
- We use thorough acceptance testing to prevent costly startup delays
- We set the correct proof test intervals to minimise maintenance cost
- We document the system basis so you spend less effort negotiating with regulators
Codes & standards we work to
Triggers that signal the need
Where SIS Functional Specification and SRS Development applies
Wellheads, separators, gas compression, FPSO topsides, produced water systems.
Distillation columns, reactors, heat exchangers, storage spheres, LPG handling.
Cryogenic exchangers, liquefaction trains, BOG compressors, storage and sendout.
Reactive systems, batch reactors, solvent handling, runaway reaction scenarios.
Boilers, HRSGs, steam headers, hydrogen systems, ammonia SCR units.
Sterile vessels, CIP/SIP, pressure fermenters, solvent recovery, spray dryers.
Tangible deliverables
- Baseline safety requirements specification per IEC 61511 Clause 10
- Function inventory and boundary register with HAZOP and LOPA traceability
- Safe state and process safety time table per function
- Demand mode classification register
- Control system and safety system independence audit report
- Platform agnostic functional cause and effect matrix
- Design hand off package for the functional safety engineering and detailed engineering team
- Specification MOC and revision control procedure
Ready to start your project?
Speak with our team to scope an engagement tailored to your facility, regulatory context, and lifecycle stage.