Pulse pressure testing and analysis of steel plates with openings

Steel plates are widely used in a variety of civil engineering applications for load bearing structural components, due to their favourable strength to weight ratio. Many of these plates have openings that are commonly used for reducing weight, access for utilities or for inspection in shipping and offshore installations. However the influence of these openings to the structural component’s robustness and resilience against blast loading is relatively unknown, with limited research conducted in this subject to date. Due to the high costs associated with offshore facilities they are typically very congested. This coupled with the producing, processing, storing and transporting of hydrocarbon materials means that explosions and subsequent fires are major hazards with severe consequences. In the event of an explosion, the blast load will initially impact the secondary structure (large spanning plated sections) and then transfer through to the primary structure, highlighting their critical consideration in safety assessments. Plated structures are also known to cause confinement, which in turn will results in higher overpressures, making the consequences of an event more severe. The aim of this research was to investigate the combined influence that openings have on the overpressure and the structural response of thin ductile plates subjected to extreme dynamic transverse loads. This was achieved by conducting a set of well-defined experiments investigating the response of 1/8 scale (0.5 m square) mild steel plates with openings subjected to pulse pressure loading. Six central (scaled) openings were considered; circular (50, 75 and 100 mm) and extended circular (50 by 75, 75 by 100 and 100 by 125 mm) representative of typical offshore and shipping applications. Each plate design was assessed with two boundary conditions (restrained and non-restrained) and two nominal loading conditions. The boundary conditions adopted in this study allowed the response to be bounded, and enabled them to be practicably modelled in FEA-analyses and in the simplified analytical approaches. A pulse pressure test facility was used to generate nominal pulse pressure loads (25 and 50 psi) applied over a time (100 to 200+ ms load duration) representative of extreme explosion loading conditions offshore. All plates exhibited a mode I type failure (large inelastic deformation) highlighting the large reserve strength in such members. The work has shown that the inclusion of an opening (<5% of the exposed panel area) does not significantly degrade the structural resistance when damage is restricted to large inelastic deformation. The reduction in stiffness due to the hole is compensated by the reduced area to which the load is applied. The data generated in the laboratory tests was used to develop and validate finite element models. In general, excellent correlation was observed between the experimental failure modes and the permanent displacements, within an average difference of 12% and 15% for the restrained and non-restrained plates respectively. The finite element models also provided a useful insight into the various failure processes and transient behaviour which could not be observed experimentally. A simplified analytical model was developed to predict the response of the plates and was validated against the experimental data. The results for the permanent displacements compared favourably with the restrained plates at the two nominal pressures (6.5% at 25 psi and 7% at 50 psi), but correlated less well with the non- restrained ones (10% at 25 psi and 3% at 50 psi). Correct definition of support conditions along with a detailed description of the development of plasticity, as shown in the finite element models was fundamental in accurately predicting response of the non-restrained plates. The simplified techniques developed are cost effective compared with more sophisticated finite element methods making them suitable for preliminary engineering design studies. Ultimately this study provides evidence to suggest that small (circular or extended circular) openings positioned away from areas of high stress, could be used as a passive system to mitigate the influences of an explosion event offshore. This has many benefits in the form of reducing weight, reducing confinement (thus lowering overpressures) and reducing the loading applied to these members, and subsequently reducing the loading transferred through to the primary structure.