Underwater mines and explosives, left in ports and harbours after World War II, can still pose a threat to subsea pipelines. In case of an accidental explosion, or even during controlled detonation, such explosives can cause significant damage to subsea pipelines. To assess the safety of pipelines exposed to an underwater explosion, finite element analyses are performed to predict the transient response of the pipeline to an acoustic pressure shock wave. This type of problem is characterized by a strong coupling between the structural response of the pipe and the acoustic pressure on the wetted interface between the pipe surface and the surrounding seawater. The spherical pressure wave induced by an underwater explosion is characterized by a very steep wave front, where the maximum pressure is attained over an extremely short rise time. The pressure then drops off exponentially over a significantly longer period of time. As a result, the structural behaviour is a combination of a long time response, dominated by an added mass effect (low frequency), a short time response, governed by radiation damping (high frequency), and an intermediate time-frequency response, where both added mass and radiation damping effects are present. In this paper, a finite element model is presented to simulate the transient response of a subsea pipeline subjected to an underwater explosion. The close coupling between acoustic pressure and structural response gives rise to numerical challenges like the accurate formulation and representation of the shock wave, the mesh requirements for the acoustic domain, and the position of the surface based absorbing radiation boundaries. An explicit dynamic solver is used to tackle these challenges, and to predict the behaviour of subsea pipelines exposed to an underwater explosion. The numerical results are compared to published experimental data, and can be used to assess the safety of submerged pipelines in the vicinity of explosives. |
