Research report | The Whipping Response of a Submerged Platform Subjected to Near-field, Non-Contact Underwater Explosions
An experimental investigation of an underwater explosion (UNDEX) induced whipping response was conducted on a submerged platform. The platform was subjected to eight near-field, non-contact UNDEX events, using two explosive charge sizes at three longitudinal positions along the hull length, coinciding with predetermined peak and node positions of the natural bending mode responses of the platform. We found that stand-off positions at amidships, coinciding with the peak response of the first bending mode, produced the most severe whipping response. Stand-off positions located at the node of the first bending mode had a greatly reduced whipping response for the same charge size and transverse stand-off distance. Stand-off positions away from amidships demonstrated multiple bending mode responses, which for the larger charge size resulted in the peak response occurring away from the initial stand-off position. These results have an implication on how navy platform assessments are undertaken, and suggests that whipping responses require more detailed consideration to understand a platform's limitations against an UNDEX event.
An underwater explosion poses a significant threat to navy platforms. Understanding the performance of platforms against underwater explosions means designers and operators are aware of the requirements that need to be met for survivability and the limitations of the platform. To explore the responses that a submerged platform may undergo from an underwater explosion, a series of experimental tests were performed on a submerged generic cylindrical structure, approximately 12 m long and 0.4 m in diameter. A total of eight tests were performed using two Pentolite charge masses of 250 g and 43 g at different stand-off distances and locations along the hull length. Each test investigated how the charge size and stand-off location would affect the whipping response of this submerged platform.
We found that stand-off locations at amidships produced the most severe whipping response for both charge sizes, where the whipping response was dominated by the first bending mode of the platform for these events. For events where the charge stand-off location was near the theoretical nodal response position of the first bending mode, the whipping response was substantially reduced to the point that the initial shock response was more severe for both charge sizes. For charge stand-off locations near the theoretical peak response of the second bending mode of the submerged platform, the whipping response consisted of a superimposed response of the first, second, and third bending modes of the submerged platform. While the overall peak response was lower for stand-off locations away from amidships, some locations along the hull experienced similar severity during the whipping response. Notably, the larger 250 g charge at a 2.8 m aft stand-off from amidships had its peak response at the forward end of the platform, i.e. the most severe response occurred at the opposite end of the platform to where the charge was detonated.
These outcomes highlight the need to assess the platform as a whole model in a whipping analysis, to ensure that the global platform response to underwater explosion events is properly characterised. Individual compartment models are insufficient to capture these results. The results of these experiments will be used to validate numerical modelling procedures for underwater explosion induced whipping analysis of submerged platforms.