Modeling a Robust Caisson Structure to Resist Effects from Blow-In-Place of Underwater Unexploded Ordnance
Mr. Josephy Trotsky | NAVFAC EXWC
Underwater munitions in shallow waters represent a significant threat to human health due to incidental contact from recreational users and in some circumstances commercial dredging operations. Technologies are needed to cost-effectively and safely dispose of these munitions. Current blow-in-place (BIP) techniques result in the closure of significant portions of waterways and adversely impact the marine life.
This project consisted of a modeling and simulation effort to investigate the reduction of blast pressure waves from installing a robust caisson structure around an underwater BIP detonation. This work is responsive to MRSON-16-01 under topic: “Cost-Effective Recovery and Disposal”. A blast shield (e.g. a robust caisson structure) will allow for a BIP of underwater munitions that cannot be moved due to explosive safety concerns and will significantly reduce any impact to marine life or nearby structures.
The objective of this project was to develop a design for a robust caisson structure that effectively mitigates blast effects generated by underwater explosions (UNDEX), in particular munitions that cannot be moved due to explosive safety concerns. Specific objectives were 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; and (3) quantify a relation between charge size and shielding performance.
The Naval Facilities Engineering Command Engineering and Expeditionary Warfare Center (NAVFAC EXWC) performed computer modeling and simulation to develop and validate various concepts for a robust caisson structure using Dynamic System Mechanics Advanced Simulation (DYSMAS) software, a fully-coupled and extensively validated hydro-code developed in cooperation between the U.S. and Germany governments (Naval Surface Warfare Center, Indian Head Technical Report 3062, 2010). The DYSMAS modeling identified the most effective conceptual designs for mitigating blast effects of UNDEX generated from BIP operations. Four types of blast mitigation conceptual designs with various configurations were modeled. 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.
DYSMAS simulations showed that increasing the amount of air around the BIP could reduce the blast effects of the UNDEX. The proposed air tank (Type IV) mitigation was considered the best candidate for further development. This innovative design splits the total blast wave energy into multiple smaller amount of shock waves that arrive at the target locations at the different times, resulting in much lower blast peak pressures and impulses. In addition, the air tank resists the shock waves directly coming from the bottom of the tank so that the material strength of the tank (e.g. steel or concrete) can be effectively utilized to ensure the safety of the air tank itself during the BIP operations.
The tube structure (Type III) was also shown to work very well in shallow waters, particularly in the beaches or the surrounding areas of bridge piers, dry docks or other infrastructure facilities. The double hull with air (Type II) was the modification of traditional air bubble curtain mitigation that could survive only up to 5.0 lb. Trinitrotoluene (TNT), according to the literature reviews. The DYSMAS simulations confirmed that Type II was completely destroyed at 20 lb. TNT. This was because the double hull with air (Type II) had to resist the lateral blast pressure impacts. In general, the lateral resistance capacity of a double hull structure is much less than that in the axial direction (see Type IV).
The benefits of using a robust caisson structure to mitigate the blast effects from BIP operations are: significantly reducing the amount of real estate/waterways requiring closure during a BIP operation, a vast reduction of the blast loading on nearby infrastructure, and a significant decrease in the lethal blast dosing inflicted on nearby marine life.