Accidental loading such as fire, blast, ship impact, and dropped objects constitute the great majority of potential and actual fatalities in both onshore and offshore operations. In order to help prevent human loss and for increased safe design or assessment of an onshore/offshore asset, the risks of these accidental loadings should be quantified, in terms of both probability/frequency and consequence aspects. In this paper we propose a unified risk-based approach for the assessment of all accidental loadings mentioned above.
This approach will address both human and asset risk assessment against accidental loadings. Technical safety and process safety inputs such as hazard curves, frequency exceedance curves, and return periods are where the process all begins. Based on these inputs, an event frequency analysis is carried out to determine how often one should expect the event to happen, i.e., the frequency aspect of the assessment. Depending on the type of the accidental loadings and the subject of the risk assessment (human or asset), the output can include a fragility curve, damage frequency matrix, and individual risk matrix. Additionally, a consequence analysis is carried out to determine the severity levels of the event. In a risk-based consequence analysis, the severity levels are fully determined based on the risk associated with the event. Based on both event frequency and consequence, the risk ranking is identified using a risk matrix. If the risk is not acceptable according to the risk matrix, mitigation/repair/strengthening options are then developed.
As an example, a risk-based structural fire analysis is compared with the conventional approach which is widely used in the industry. In a conventional Passive Fire Protection (PFP) analysis using Ductility Limit Analysis (DLA) according to API RP 2FB, fire loads are deterministically applied to a structure whose response is then analyzed. The initial PFP scheme is developed based on the analysis and then optimized based on input from all disciplines (e.g. technical safety, operations, structures, etc.). This approach is sometimes misinterpreted as a “risk-based” approach; however, it does not take into account the frequency aspect of the risk assessment. In a risk-based PFP analysis using DLA, fire scenarios are developed in a particular target zone based on exceedance curves (i.e., input from technical safety discipline). The ductility level analysis is performed to determine the structural consequence. If personnel safety is of interest, the consequence of the structure is then linked to individual risk to determine fatalities. The amount of PFP to be applied on the structure is fully based on the risk that is produced by the fire scenarios in the target zones or areas.
Followed by the approach, a new perspective on safe design of onshore/offshore structures for accidental loadings is outlined to estimate the associated risk to potential targets such as human personnel as well as asset components. The proposed assessment methodology will contribute towards identifying the mitigation measures and safety-critical procedures and equipment, and toward a safer design.