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Numéro : 2794 - Year : 2023

Strength of ships to underwater explosions: nonlinear whipping analysis

Guillaume BARRAS, DGA Techniques navales – Groupe Coques et Structures – Toulon, France

Matthias HIVERT, Natacha LE COQ, Tanguy QUESNEL, PRINCIPIA – Groupe Naval et Défense – Nantes, France

As part of military surface ship projects, the hull girder mechanical strength capacities for beam deformation mode is assessed in longitudinal bending either for wave loading or underwater explosion loads (beam whipping due to the pulsation of explosion bubble).

For relatively small solicitations, the ship response remains in linear elastic material and structural behaviour and certified analysis methods can be applied. On one hand, the bending resistance of the hull girder in static is assessed and both hogging and sagging ultimate strength moments are determined using the well-known “independent components method”. This method was developed in RULTIM tool by PRINCIPIA and is applied at DGA Naval Systems. On the other hand, sea keeping calculation and overall ship response in case of underwater explosion allow determining the maximum expected solicitations. The maximum solicitation in dynamic is then confronted to the ultimate resistance in static for safety analysis.

However, when considering severe loads or accidental scenarios, this method is too much conservative and therefore not valid. In such cases, non-linear behaviour and transient dynamics cannot be neglected. It is the case for example when considering ship vulnerability domain or laying down ammunition performance (mines and torpedo) designed to destroy hulls.

The aim of this study is to develop a new efficient tool capable of evaluating the strength capacity of hull girder when submitting the surface ship to underwater explosion effects. Compared to usual whipping computational methods, limited to hull girder linear response, this method is extended to severe aggressions induced by underwater remotely operated weapons. To that extent, nonlinear behaviour and plastic collapse phenomena under explosion loads are taken into account in hull girder model. It considers primary shock for short time response, which tends to emphasize the long time response (pulsating bubble). PHEDRE, which stands for “dynamic damageable hydro elastic beam regarding explosion”, was developed for that need.

First, this study focuses on the hull girder nonlinear behaviour model under a cyclic and dynamic solicitation. Both hogging and sagging motions are considered. Indeed, underwater explosion and pulsating bubble leads to vertical bending loads switching between positive and negative values.

Second, this work aims to quantify primary shock impact on hull girder long time response. In fact, loads induced by underwater shock and ship interaction tends to reduce ship’s bottom strength ability (with plastic bulge or failure) and thus hull girder on damaged sections. Therefore, the potential local/global coupling is analysed towards short times effects (i.e. bulge under primary shock) that may emphasize the long-term global response (i.e. nonlinear hull girder whipping).