The safety of personnel working within onshore and offshore oil and gas processing facilities with respect to vapor cloud explosion (VCE) scenarios revolves around the risk associated with the event. Conducting a risk assessment is quite typical within the offshore and onshore loss prevention communities. The consequence of a VCE event is a very complex phenomenon however. It varies based on several blast load parameters. With improved understanding of blast loading parameters such as blast overpressure, impulse, time lag, net blast loading, etc. structural designers have gained increased confidence in their design and have reduced conservatism in their blast designs. However, there is a significant gap in the risk assessment of onshore and offshore facilities between the process safety and the structural design groups. Using structural insight in combination with rigorous process safety risk evaluation techniques we can reduce the gap between the risk and consequence of a blast design, providing the stakeholders with a more accurate understanding of the risk and consequence of a blast event.
This paper will attempt to address the impact of uncertainty in pressure exceedance curves and;
- how the confidence level impacts the design blast loads;
- how the slope of the exceedance curves impacts the blast design loads;
- two different blast events can result in similar design blast loads for a given risk;
- how a small change is frequency can lead to a significant change in design blast loads;
- how the negative phase of a blast scenario could lead to failure of some specific type of structures;
- how the response of long wall or piping structure is impacted by time-lag of the design blast load;
- how the design blast pressure and design blast impulse impacts a structure;
- how non-structural members, typically not considered in blast design, impact the blast design;
- how simultaneous application of blast load on different parts of structure, taking into account time-lag, affects the blast design.
Currently for a blast event, both blast pressure and blast impulse are defined as independent variables whereas in reality they are coupled. The correlation between the impulse and pressure will also be discussed in the paper. Defining appropriate pressure time-history is critical for structural design. Rise time, positive-phase and negative-phase duration are unknowns in a blast design. How an appropriate time-history is established will also be addressed.
Prior knowledge of implications of a blast event for an associated risk is helpful during structural blast design. Early involvement of structural engineers within the process safety procedure is a recommended solution to such questions. Enhanced loss prevention of offshore and onshore structures can only be established by always keeping the consequence for a blast event for a given risk in mind.