Experimental and numerical investigations into the underwater explosion induced whipping response of submerged platforms

This thesis reports the findings of an experimental and numerical investigation into the whipping response of a submerged platform, due to loading from near-field, non-contact underwater explosion (UNDEX) events. The experimental investigation examined the whipping response of a generic submerged platform subjected to UNDEX. Eight scenarios were investigated, using different explosive charge sizes, longitudinal, and transverse stand-off locations, to explore the effects of these variables on the platform's whipping response, and to provide validation data for the numerical investigation. It was found that UNDEX stand-off locations near the anti-node of the first bending mode shape induced the most severe whipping responses, while stand-offs near the node of the first bending mode shape had greatly reduced whipping responses. A numerical model was developed and validated against the experimental results, using incident pressure and strain measurements. Three numerical studies were conducted with the validated numerical model, investigating the effects of additional stand-off distances at the previously explored longitudinal stand-off locations, the whipping response from an intermediate charge size, and the effects of the UNDEX bubble loading on the whipping response by using a shock-only loading model. From these studies, the following novel contributions were made: The stand-off distance was found to only affect the whipping severity while the charge size and stand-off location determined the modal contributions of the whipping response. A comparison of the peak whipping response to the bubble pressure impulse suggests that three distinct forms of whipping could be induced, and these can be determined by the proximity of the UNDEX stand-off location to the nodes and anti-nodes of the platform's first bending mode shape. These were classified as critical, resilient, and general whipping responses. The critical whipping response occurred when the UNDEX stand-off location was near the anti-node of the first bending mode shape. This was characterised by a dominant response of the first bending mode and minimal contribution from other bending modes in the overall response. The severity of this response increased nonlinearly as the bubble impulse increased and when the bubble pulsation frequency was similar to the first bending mode frequency. This is the most severe form of UNDEX induced whipping. The resilient whipping response occurred when the UNDEX stand-off location was near the node of the first bending mode shape. This was characterised by minimal contribution of the first bending mode compared to others in the overall response. From these scenarios, the bubble loading had minimal effect on the whipping response. This was confirmed from good correlation in comparisons of the strain responses from experimental measurements and the results from the numerical shock-only loading study. This is the least severe form of UNDEX induced whipping and the most desirable outcome from an unavoidable UNDEX threat. The general whipping response occurred when the UNDEX stand-off location was between the nodes and anti-nodes of the first bending mode shape. This was characterised by similar contributions of multiple bending mode responses in the overall response. The severity from this response increased linearly for increases in the bubble impulse. It is suggested this is the most likely form of UNDEX induced whipping to occur, based on the large number of variables involved with UNDEX loading and platform response scenarios. Current analysis methodologies are able to identify the resilient whipping response but some methods that focus on the shock response of a platform may not identify the critical and general whipping responses. This was demonstrated by comparison of the experimental measurements to a numerical shock-only loading models for the most severe UNDEX scenarios, where shock-only loading and response assumptions under-predicted the whipping response by 39 % and 54 % for the critical and general whipping responses respectively. The relative increase in the severity of the whipping response from a reduction in the stand-off distance was consistent for all similar forms of whipping. This suggests that the whipping severity from many different stand-off distances can be approximated through analysis of only a few scenarios at different stand-off locations on a specific naval platform. It is suggested that this knowledge may allow a rapid assessment tool to be developed that could determine a platform's survivability from a wide variety of UNDEX scenarios. The trends from each form of whipping severity compared to bubble pressure impulse were examined with limited extrapolation. This suggested that four whipping response analysis regimes exist: far-field elastic, near-field and non-contact elastic, non-contact plastic, and contact damage. Exact limits of these regimes were not identified, and all are promising areas for further investigation.