Typical workover operations involve the switching of subsea well barriers from permanent equipment to temporarily installed barrier systems. During workover activities additional operational hazards are introduced such as drive/drift off and compensator lockup which can have severe consequences including injury to personnel. To account for these operational hazards a workover system will have additional barriers. These barriers consist of instrumented and non-instrumented protective layers. The instrumented protective layers consist of safety instrumented systems (SIS).
The safety instrumented systems fitted to a workover system can vary. Most workover systems consist of three common safety instrumented functions (SIF); Process Shutdown (PSD), Emergency Shutdown (ESD) and Emergency Quick Disconnect (EQD). The non-instrumented protective layers are made up from mechanical barrier elements for example the weak links.
DNV GL on behalf of Statoil performed an analysis to better understand workover hazards and the barriers required to mitigate these. The aim of the work was to assess the effectiveness and suitability of the current safety barriers and to identify what additional functional requirements might be needed. This paper details the results of that work. A generic workover system formed the basis for the study. Hazards were defined through a hazard identification (HAZID) workshop and the effectiveness of the measures was evaluated through a Level of Protection Analysis (LOPA).
This paper documents the results of a study to establish whether the current barriers used during workover operations adequately mitigate the risks. Despite having dissimilar operational modes, it is often expected that the workover system barriers shall be suitable for all operational modes. The results of this study show that some of the barriers in workover operations are not as efficient as perceived, in particular the riser weak link designs and manually initiated EQD functions.