Modeling of Nonlinear Structural Response from Explosion Loads on Offshore Structures

This thesis has investigated the structural response of offshore structures subjected to accidental explosion loads. This has been done by introducing two simple models that are representative of relevant load conditions and supports on an offshore module. These two models are a simply supported plate subjected to uniaxial compressive forces and a simply supported beam subjected to an evenly distributed impulse load. Both models has been made in the commercial Finite Element software ANSYS.

The plate model has been researched in regards to the local buckling response arising from the compressive forces, while the beam has been analyzed for the development of local strain and the combined dynamic response of plasticity, lateral buckling and strain rate effects.

It has been found that for local strain development in I-profile beams, there are differences between models modeled with shell elements and models modeled with solid elements. This difference was found to be roughly 10 %. A method for conversion introduced in a former paper on the subject, to establish convergence in a shell element model and plastic strain comparable to solid element models, has been tested and found to be incapable of achieving reliable results.

Transient analyses of the I-profile subjected to a linearized triangular impulse load were carried out with a pressure level that induced plasticity, where the response was for different combinations of elasticity, plasticity and imperfections. It was found that including plasticity in the response increase the midspan response magnitude significantly while adding imperfections, which enables lateral buckling to occur, to the elastic model increased the response by similar amounts. Combining plasticity and lateral buckling gave a response many times larger than the linear elastic response, highlighting the differences these phenomena cause.

These analyses were also implemented in the explicit solver, ANSYS LS-DYNA, which is a subset of ANSYS, and it was found that it could achieve comparable results to the implicit solver while drastically reducing computation time.
Using the ANSYS LS-DYNA module, analyses including the Cowper-Symonds strain rate model were made to measure the influence of strain rate effects on the maximum response. It was found that because the strain rate effects effectively increase the yield stress, the effects of plasticity were greatly reduced and the response of full nonlinear model was almost halved. A modified plasticity model was introduced, were strain rate effects were included by an increase of the initial yield stress based on hand calculations, and the response of this model was found to be comparable to the full strain rate model as long as the load levels remained within reasonable limits.

A number of methods for modeling post buckling behavior were reviewed, and it was found that though some had practical limitations, like being only valid for quadratic plates, they all showed good agreement with results generated by the plate model setup in ANSYS. Additionally methods capable of simulating dynamic buckling as well were also reviewed and matched well with ANSYS, though one showed divergence in results at loads above 2 times the critical load. Common for all the methods, both static and dynamic, was that none demonstrated the capability to model plasticity, although some were capable of sufficiently predict the post critical bearing capacity. 

Modeling of Nonlinear Structural Response from Explosion Loads on Offshore Structures
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