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  Title Flap pressure and arrest length considerations in propagating shear failure
  Author(s) Dr Brian N Leis and Dr Andrew Cosham  
  Abstract PROPAGATING SHEAR FAILURE refers to the rapid axial growth of a split along a hydrocarbon transmission pipeline transporting a gas or a liquid with a high vapour pressure. Growth of the split continues if its speed exceeds that of the decompression front; otherwise arrest occurs, with the balance between these speeds dependent on the fluid’s properties, the linepipe’s size, the flow and fracture properties, and the backfill conditions.

Interest in propagating fractures was motivated by in-service failures in the 1950s and 60s. Linepipe steels of the 1960s and before offered little resistance to propagating failure which, when it occurred, ran in a brittle mode. While steels were soon developed that overcame such brittle fracture, it became clear by way of full-scale experiments simulating service conditions that a propagating failure could run in a shear mode if the steel was not sufficiently tough. Technology soon emerged to quantify the steel properties required to arrest such failures. Perhaps motivated by the observation that the early failures involved brittle fracture, initially such methods were based on fracture concepts, with alternative plastic-collapse-based concepts emerging as the limitations of fracture-based methods became evident as the toughness required for arrest increased. What is clear – regardless of which concept is proven relevant – is that the failure process is dynamic, making inertia associated with the ‘flaps’ that form in the wake of the split a consideration along with related issues such as the length required to affect arrest.

This paper reviews the basis for an effect of flaps weighed in the balance of phenomenology and analyses, working from Smith and Shoemaker’s 1974 paper that first asserted the potential role of flaps, on through analytical and numerical work. Due to the complex non- linear interaction that develops between the soil, the pipe flow and failure resistance, and the fluid, recourse is also made to discriminating experiments – with a focus on work involving gaseous detonation. Pressure decay and its role in regard to arrest length is then addressed in light of current models used to quantify arrest. Thereafter, the results are integrated with a view to a more general formulation for arrest.

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