ABSTRACT

Unexploded ordnance (UXO) lies on the bottom of many of the world's seas, often near the shore. When installing or operating a submarine pipeline, a safe distance should be maintained from any UXO that is left in place. This paper presents the results of a recent study on the impact of an underwater explosion (UNDEX) on a near-shore submarine pipeline. The response has been assessed for two pipeline configurations: lying on the seabed and suspended in a free span. Moreover, the influence of internal pressure was studied. A purpose-built finite-element model has been used to predict the local response and assess the associated integrity of the pipeline. This coupled structural-acoustic model simulates the evolution of a shock wave generated by an underwater explosion and the subsequent impact on the pipeline structure. It must be safeguarded that the pipeline can remain in service and the ultimate strength is not exceeded. The result is a quantitative estimate of the minimum distance between the explosive charge and the pipeline system that ensures that the system can be installed and operated safely and responsibly.

INTRODUCTION

UXO found in and on the floor of the world's seas stems from military exercise, disposal, accidents or conflicts. The most straightforward way of ensuring safe installation and operation of a pipeline is avoiding such areas; however, this is not always practical and economical. A possible solution is relocating or defusing the explosives; however, this can also be problematic. Alternatively, a more sensible solution is to maintain a safe distance between the pipeline and the recorded position of the UXO. The remaining question is then how far a distance this should be. If too short, the safety of the pipeline cannot be guaranteed; however, when too long, pipeline installation and operation may become difficult or economically unattractive. The target is to strike a fair balance and to obtain a reasonable safe distance.

The answer to how much the minimum safe distance should be depends on many aspects. It depends on, for instance, the type of explosive, its shape, its charge mass, the pipeline's properties, its configuration, its coatings, the seawater characteristics, the water depth, the loads acting on the pipeline, and the seabed properties and morphology.

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