Improving the ductile behaviour of offshore topside structures under extreme loads

This study investigates the behaviour of a typical offshore topside structure subjected to blast loading caused by hydrocarbon explosions. The pressure load caused by the explosion is idealised as a linearised triangular impulse which is simultaneously applied on two deck levels modelled using the finite element program, ABAQUS Explicit. Two cases were developed: the original case representing a typical offshore fabricated deck flooring with sniped bottom flanges at connections for stringer beams and the strengthened case which was a modified form of the original case but with rolled steel angles welded at sniped bottom flanges of stringer beams and plate girders. Results from the analysis show that load directions and structural configuration play a prominent role in the response of the topside. The strengthening indicates the beneficial redistribution of local yielding at the sniped connections of the beams to mid-spans which consequently improve the performance level of the structural system. Haunch details are found to cause local failures on plate girders but the failure mechanism is useful for developing a technique to avert further damage. Finally, generalised ductility ratios derived from normalisation of responses at end-spans and mid-spans of the structural components are compared with commonly used design values to justify the damage levels and performance under this extreme loading condition.