This case study provides information involved in conducting a Layer of Protection Analysis (LOPA) relevant to transferring of gasoline products into a storage tank. LOPA is one technique of risk assessment that can be used in determining if sufficient protection is provided to mitigate against the perceived risk. The study is normally conducted with a team that have various skills, relevant to the process being considered. The team conducting this LOPA involved managers, operators, engineers and health & safety representatives.
Following on from the Buncefield Incident, the MIIB and HSE requested all Storage Terminals to carry out risk assessments on their facilities.
The purpose of the LOPA study was to analyse the specific operations at the terminal and identify if sufficient safe guards were in place.
Normally the requirement for a LOPA study would arise from a HAZOP. In this case it was conducted as part of the requirements arising from the MIIB recommendations. The LOPA study starts with a numerical Target Frequency, that is the frequency at which the event can be tolerated. All Initiating Events which could lead to the event occurring are considered and an initiating event frequency is assigned to them. Once this is completed, the overall frequency of the event occurring is calculated. The team then considers various other aspects such as enabling events, conditional modifiers and the various protection devices that are included within the system to prevent the event from occurring. Once all the aspects have been defined then the mitigated frequency is compared to the Target Frequency. If there is a shortfall then it would be necessary to consider further risk reduction.
In the case of this study, further risk reduction resulted in the installation of a Safety Instrumented System to automatically stop import to the tank on detection of a high level. As well as being used for safety assessments LOPA can be used as a risk assessment tool for environmental and financial consequence studies.