Onshore Explosion Studies –Benefits of CFD-Modelling

Integral or phenomenological consequence models are extensively used for explosion and dispersion studies at onshore petrochemical facilities. These models will generally ignore the influence of the geometry of the facility on the ventilation and flow patterns, the generation of flammable gas clouds, and any subsequent explosions. Another significant weakness of these models is the inability to handle dense vapour cloud dispersion in low wind conditions. Risk and consequence studies performed according to API-RP 752 or Seveso-II are mostly referred to as worst-case assessments. In reality these represent some kind of a probabilistic assessment as only “maximum credible” release scenarios are considered. Typically non-conservative gas cloud sizes are predicted, deflagration-to-detonation transitions (DDT) potential is ignored, and the ability to predict the effect of mitigation is limited when using such tools. Computational Fluid Dynamics (CFD), on the other hand, can incorporate the 3D geometry of the facility and how it influences dispersion and explosion processes. In the decades after the Piper-Alpha explosion and fire (1988), the need for CFD-based consequence models was recognized for offshore oil and gas facilities, and standards like ISO 13702 (1999) and ISO19901:3 (2010) were introduced. This approach has significantly reduced explosion risk on offshore platforms today. In contrast, authorities seldom require CFD-based consequence studies for onshore plants, but instead accept tools that ignore geometry and important physics, and are unable to provide guidance on optimal mitigation methods. Numerous major explosion accidents on onshore facilities in recent years indicate that the explosion risk has not improved over the past couple decades. There are reasons to believe that the conservative attitude against the use of improved consequence modelling (CFD) is part of the explanation to the poor safety performance in recent years. This article will discuss the situation within industrial process safety and consequence calculations. The potential and benefits of using CFD for explosion studies on onshore facilities will be illustrated with examples.