title: Experimental and Numerical Study on Propagation Buckling in Pipe-in-Pipe Systems
Creators: name: Alrsai, Mahmoud S
Publisher: name: Griffith University
Year: 2020
Subjects: subject: pipe-in-pipe systems
subject: buckle propagation
subject: collapse mechanisms

description: Subsea pipe-in-pipe (PIP) systems are being used extensively in the design of high-pressure and high-temperature (HP/HT) flowlines because of their outstanding thermal insulation performance. A typical PIP system consists of concentric inner and outer pipes, bulkheads, and centralizers. The inner pipe (flowline) conveys production fluids, and the outer pipe (carrier pipe) protects the system from high external pressure (water depths up to 3000 m) and mechanical damage. These two pipes are isolated by centralizers at joints and connected through bulkheads at both ends of the pipeline. The annulus (the space between the tubes) is either empty or filled with non-structural insulation material such as foam or water. The external pressure in the vicinity of local dents or ovality in the outer pipe-wall can cause a local collapse, which may catastrophically propagate along with the PIP system, known as propagation buckling. The lowest pressure required to sustain such a buckle propagation is known as the propagation pressure Pp, which is only 15–30% of the initiation pressure PI of the intact pipe. The propagation buckling (or buckle propagation) has been extensively investigated in single pipelines using analytical, experimental and numerical methods. Unlike single pipelines, buckle propagation and collapse mechanisms in PIPs have only been marginally addressed. Therefore, this research program aims to investigate propagation buckling of subsea pipe-in-pipe (PIP) systems under hydrostatic pressure; experimentally and numerically. By using experimental results, a finite element (FE) model is validated and is used to conduct a comprehensive parametric study to evaluate the effect of geometric and material parameters affecting the propagation buckling pressure of PIP system. The experimental study performed herein showed that the buckling mechanisms and collapse pressure of the inner pipe of a PIP system are different from those of a single pipeline. Therefore, a chapter of the thesis is dedicated to the investigation of the collapse mechanisms of the inner pipe of a PIP system. Moreover, a parametric study is conducted and the collapse mechanisms of PIPs with various combinations of outer and inner pipes in a broad practical range of diameter-to-thickness ratios (D/t) are evaluated. The thesis is concluded by proposing and investigating the appropriateness of a novel carbon fibre buckle arrestor. The thesis makes the following contributions to the knowledge and practice in the area of offshore pipelines. A more accurate simplified analytical expression is proposed to predict the propagation buckling pressure of PIP system. The buckle propagation modes of PIPs are identified and discussed. Subsequently, empirical expressions for each mode are proposed to predict the propagation buckling pressure of PIP systems. Moreover, a parametric study is conducted on the collapse mechanisms of the inner pipe of a PIP system. Comprehensive empirical expressions are proposed for the collapse pressure of the inner pipe, and the upper and lower pressure capacity bounds are quantified. Design charts are proposed to estimate the collapse pressure of the inner pipe and its corresponding collapse modes. In the last part of this research, CFRP buckle arrestors are proposed for subsea pipelines and are shown to achieve higher arresting efficiency compared with the conventional buckle arrestors.