The effects of terrain modelling on risk contours for a Liquid Biogas (LBG) plant

Introduction

VEAS, Norway’s largest wastewater treatment plant, is setting out to produce Liquid Biogas (LBG) for carbon-neutral public transportation in the Oslo area.

Gexcon has assisted VEAS in assessing the risk and establishing a risk analysis for approval by the Norwegian Directorate for Civil Protection (DSB).

VEAS’s c hallenge

VEAS is located in Asker, south of Oslo, and treats wastewater from 600,000 inhabitants in the greater Oslo region. Off-gas from treatment is to be purified and liquified at a new liquid biogas plant on their site. On the surrounding area of VEAS, there is a business park, residential properties and a recreational area with nature trails.

Before installing a plant handling flammable substances, a Qualitative Risk Analysis (QRA) must be undertaken to get approval from from DSB. The QRA establishes risk contours that define an inner, a middle, and an outer risk zone of the plant. The inner zone should be limited to the plant’s area and inside its fences, while the middle and outer zone defines areas around the plant with limitations on activities and habitation.

An early phase risk assessment used simple 2D empirical tools that did not account for the 3D effects of terrain and buildings. This approach resulted in risk zones covering a large circular area around the proposed plant in conflict with existing activities and habitation in the neighbourhood.

Gexcon’s solution

Releases of LBG, which is liquified methane produced from biogas, will result in a liquid pool on the ground and a cold methane gas cloud with high density evaporating from the liquid.

Taking into consideration the hilly terrain surrounding the planned LBG plant and the nature of LBG releases, Gexcon suggested a CFD (Computational Fluid Dynamics) based risk analysis using FLACS-CFD modelling software to simulate the dispersion of LBG releases in a 3D model of the proposed new plant and its surrounding terrain.

Using FLACS-CFD to simulate the behaviour of LBG releases in a realistic terrain-model in various weather conditions, Gexcon proved that the influence of terrain and buildings on the dispersion of flammable gas from an accidental release of LBG was significant. Hills and peaks in the terrain surrounding the plant had considerable shielding and channelling effects on the heavy gas evaporating from a liquid methane pool caused by a spill of LBG.

By combining accidental releases frequencies calculated through the risk analysis with actual weather conditions and CFD simulations, 3D risk contours could be calculated and presented in the 3D geometry model.

Conclusion

As a result, nearby residential areas were found to be unaffected by the new risk zones introduced by the planned LBG plant because the houses are located at higher elevations and are shielded from the new plant by a hill. These findings would not have been possible without the use of FLACS-CFD and a detailed 3D terrain model. The full QRA for the LBG plant is public available from DSB’s website³.