The master thesis describes, compares current methods in the literature, and proposes new methods for determination of safety/environmental integrity level of safety instrumented systems (SISs). These systems are used widely in many industry sectors to detect the onset of hazardous events and mitigate the consequences to humans, the environment and material assets. The main objective of this thesis has been to investigate the risk based approaches for determination of safety /environmental integrity level of SISs. The focus of the thesis is the risk graph and layer of protection analysis approach for subsea applications where the failure of such systems could lead to significant environmental consequences. The thesis builds on concepts, methods and definitions adopted in two main standards for SIS applications: IEC 61508 and IEC 61511. The proposals of new methods are inspired by these two standards and other relevant literature found during the master thesis project. The main contributions of this thesis are:1. Discussion on current environmental risk acceptance criteria used on Norwegian Continental Shelf and proposal of new environmental risk acceptance criteria based on release volume for subsea SISs applications where the consequences of hazardous events include environmental damages.2. A modified risk graph approach suited for SIL/EIL determinations for subsea SISs. This approach is demonstrated and tested in a case study.3. Detailed discussion on the effect of common cause failures between the designated SIS and the existing protection layers during SIL/EIL determination. A framework for determining SIL/EIL considering such CCFs is developed. This framework includes CCFs quantification in two phases: SIL determination phase and SIL realization phase. A checklist is developed for CCFs quantification in the early phase.