Blast & Impact Loading & Response of Structures
Underwater munitions in shallow waters represent a significant hazard to waterfront infrastructure such as bridges, piers, wharves, docks, etc. Currently, when unexploded ordnance (UXO) is found at shorelines, particularly in proximity to personnel and infrastructure and it cannot be safely moved or rendered safe, a blow-in-place (BIP) will be employed to destroy the UXO. However, current BIP techniques require the closure of significant portions of waterways and often pose a substantial risk to waterfront infrastructure within the closed areas. Therefore, advance technologies are needed to cost-effectively and safely dispose of UXO in the underwater environment. Recently, Naval Facilities Engineering and Expeditionary Warfare Center (NAVFAC EXWC) performed a research project of “Modeling a Robust Caisson Structure to Resist Effects from Blow-In-Place of Underwater Unexploded Ordnance” to mitigate these hazardous effects. This research project was sponsored by the Strategic Environmental Research and Development Program (SERDP), a joint Research and Development (R&D) program by the US Department of Defense (DoD), the Environmental Protection Agency (EPA) and the Department of Energy (DOE). The objective of this project was to develop a robust caisson structure that effectively mitigates blast effects generated by underwater explosions (UNDEX) during BIP detonation operations, in particular munitions that cannot be moved due to explosive safety concerns with the hazardous condition of the items. Specific objectives are to: (1) demonstrate a reduction in UNDEX blast pressures and acoustics with the use of the shielding concept over a baseline case; (2) determine a performance requirement that optimizes mitigation efficacy to waterfront infrastructure as well as safety and health of human and marine life; and (3) quantify a relation between charge size and shielding performance.
This paper presents a computer modeling technique to develop concepts for the robust caisson structure using the Dynamic System Mechanics Advanced Simulation (DYSMAS) software, a fully-coupled and extensively validated hydro-code, which is owned and developed by the cooperation between the U.S. and German governments. Four types of the caisson structure with nine alternative configurations were tested for the efficacy. The three candidates identified for further investigation included a double steel hull filled with air, an air pressurized tank with a blast-proof bottom system, and a tube extending from the seabed to the top of the shallow waters where the water in the tube is pumped out. The significant reductions in terms of blast peak pressure, impulse intensity, and energy flux density from the baseline scenario were demonstrated by the DYSMAS simulation results. The development of the performance requirements for the optimal design and construction of the robust caisson structures is underway, along with quantifying the relation between the charge size and these performance requirements.
Finally, this paper proposes a practical approach for developing a risk mitigation strategy for waterfront infrastructure subjected to UNDEX. The roles of multiple parties involved, such as federal, state, regional governments and local communities in addressing structural safety concerns, along with the environmental, economic, and social aspects of risk management of the waterfront infrastructure, are also explicitly considered.