The Challenge.

The client asked Optimus to assess, by means of non-linear finite element analysis, the structural integrity of a cabin module when subjected to a specific blast loading. The analysis was carried out as a dynamic time dependent analysis using the Abaqus explicit solver. This solver allows high speed events with highly non-linear deformations to be modelled where inertia is taken into account. The design criteria were that the maximum deflections during blast and the residual post blast deformation were sufficient to allow egress of personnel. Critical strain for rupture was 15% in the main structure and 5% plastic strain at the welded areas (see ASCE “Design of Blast-Resistance Building in Petrochemical Facilities”).

Model Build.

Drawings were supplied of the module and the model was built from scratch in the ABAQUS/CAE pre-processor using shell elements. Parts of the cabin, such as openings for air ducts and mesh doors, which were considered to not contribute to the structural response of the module, were omitted from the model. This was done to simplify the modelling, ensuring efficient analysis time. The steel material was modelled including plasticity with isotropic hardening, in order to capture any permanent deformation post-blast. The outer corners of the floor were constrained in all degrees of freedom to represent the fixings at the base. The blast load was applied to all faces simultaneously with the time history shown in the figure below. This pressure time-history was supplied by the client. A mesh density study was undertaken to show that the mesh was fine enough to capture the stresses accurately.

Outcome.

The plastic strain was found to be acceptable in the majority of areas and the permanent deformation was small enough to allow safe egress of personnel. However, the analysis showed that the cabin failed to meet the design criteria, with the welded area between the perimeter beams and the cladding shown to exceed the 5% strain criteria. Three design modifications were generated after discussion with the client, which were implemented in the model and analysed, with one demonstrating an improvement in the strain level at the welds. The figures below show the displacement (left) and the von Mises Stress (right) at 26msecs. The deformation scale factor is set to x5. The Optimus detail design team produced the required documentation of the modifications.