Dropped objects encompass a wide variety of accidents from objects dropped during lifting and drilling operations. Dropped objects develop a significant amount of kinetic energy that may impair the performance of structures and equipment impacted. Proper study of these events requires specialized techniques to address the dropped object trajectory and subsequent likelihood of striking another structure or equipment as well as predicting the consequences of such subsequent impacts.
Dropped objects generally have relatively high velocities (the most critical cases) and the influence of inertia forces on the response of the impacted structure is large. The response is very local and the damage in the impact area includes yielding of materials, local buckling and in some cases local failure/penetration. In the interest of safety and economy it is essential that the analysis is accurate and takes full account of the energy absorption capacity of the actual structure. The analyses must therefore be both non-linear and dynamic, and the models should represent the local structure very accurately. The analytical evaluation of dropped object event establishes basis for an optimum strengthening and/or consequence evaluation for use in the risk analysis.
Typical scope of work for dropped object consequence assessment includes:
- Based on probabilistic or deterministic methods define impact energy. Establish geometry, orientation and strength characteristics of the dropped object and impacted structures. Establish acceptance criteria for the impact event.
- Develop a nonlinear model for both structures involved.
- Perform nonlinear dynamic strength analysis (using LS-DYNA) to assess impact force and energy absorption in the structure and the falling object.
Deliverables – Technical Report documenting:
- Impact force and absorbed energy in the structure (and in dropped object if modelled) as a function of deflection of the impact area.
- Maximum displacements and stresses in the structure round the impact area.
- Permanent displacements and stresses in the structure around the impact area.
- Plastic strains or crack propagation in the damaged structure