Dynamic Collapse of Underwater Metallic Structures – Recent Investigations: Contributions after the 2011 Murray Lecture

Abstract

The Murray Lecture that was delivered in 2011 included a historical perspective on salient features of research conducted and published by Arun Shukla. A review of experimental work since the Murray Lecture is presented in this paper on the topic of imploding underwater structures. Specifically, the dynamic underwater collapse of metallic structures was studied under free-field and confined environments using novel applications of the Digital Image Correlation (DIC) technique. During these implosion experiments, the implodable volumes became unstable under hydrostatic and combined hydrostatic-explosive conditions. Moreover, there are two types of confinements explored in this paper. A semi-confinement, where a large tubular structure is open to a reservoir at one end and closed at the other end, which leads to water hammer waves on the closed end after implosions. Also, a full-confinement, where a large tubular structure is closed at both ends, which leads to limited hydrostatic potential energy during the implosion. For accurate displacement and velocity measurements of the collapsing structures, the 3D DIC technique is calibrated for underwater measurements in a small-scale setup for each experimental configuration. High-speed cameras are then used to record the dynamic implosion event while dynamic pressure transducers measure the emitted pressure pulses. The results of these experimental series show that the 3D DIC technique can be successfully used for displacement measurements of submerged objects by extracting intrinsic and extrinsic parameters using a submerged calibration grid. The implodable volume in an open-ended semi-confining structure displays a reduced collapse velocity with respect to a free-field configuration. This is primarily caused by a near-field pressure drop from the superposition of low-pressure implosion waves inside confining structure. Semi-confined implosion also exhibits high-pressure hammer pulses at the closed end. Implosions in fully confined vessels show significant reduction/delays in the implosion process due to limited energy present in the vessel. Lastly, during fully confined explosive experiments, the shock wave impact leads to structural vibrations in the implodable volume. The amplitude of these vibrations increases with higher hydrostatic pressures until the implodable volume shows an instability.