Buckle propagation in pipe-in-pipe systems.: Part I. Experiments

Pipe-in-pipe systems are extensively used in offshore pipeline applications in which thermal insulation of the pipeline is necessary. Typically, the space between the two pipes is either empty or contains insulation material which provide minimal mechanical support to the system. In deepwater applications, the carrier pipe must be designed to resist collapse due to the ambient external pressure while the design of the inner pipe is usually governed by the pressure of the hydrocarbons it carries. The integrity of the two-pipe system in the event of accidental collapse of the carrier pipe is an issue of concern.In Part I of this two-part report, the results of an extensive experimental study of the problem are presented. The experiments were carried out on two-inch diameter carrier tubes with D/t values of 24.1, 21.1 and 16.7 and inner tubes of several diameters and wall thicknesses. In most cases local collapse of the outer tube led to simultaneous collapse of the inner one. Subsequently, the collapse propagated simultaneously collapsing both pipes. The propagation pressure of the two-pipe system (P_P2) has been quantified parametrically. An interesting second mode of collapse propagation was also discovered in which the carrier pipe collapses leaving the inner one intact. Propagation of collapse affecting both pipes is still possible but usually at a higher pressure level. The pressure at which this switch takes place has been found to closely correspond to the propagation pressure of a carrier pipe with a solid rod insert (P_PS) of the same diameter as the inner pipe. The parametric dependence of P_PS has also been established experimentally. Part I finishes with a discussion of how these new critical pressures of pipe-in-pipe systems should influence the design of such pipelines.