Local buckling response of subsea flexible pipe

Ebrahimi, Alireza (2016) Local buckling response of subsea flexible pipe. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (7MB)

Abstract

Applications of flexible pipe have been growing because of its characteristic features (i.e. low bending stiffness and high axial strength) which are because of various composite and steel layers have been used in the structure of this pipe. These characteristics make flexible pipeline capable to transfer oil and gas from wellhead to the fixed and floating platforms, or to inject water into the wells. There are a number of technical and economic advantages for the use of flexible pipe with respect to conventional rigid line pipe. Rapid installation, typically 5 to 10 km per day, and special polymer material (i.e. elimination of needs for cathodic protection) suggest it may be used as a suitable option for installation in harsh environment fields. Furthermore, the pipe exhibits advantageous mechanical performance characteristics with respect to strength, collapse resistance, thermal expansion and vibration response, and fatigue and abrasion resistance. Flexible pipe comprises of carcass and pressure armours which are interlocked layer wrapped with angle close to 90 degree and stand toward radial pressures; extruded polymer layers which prevents leakage of fluids to the other layers; high strength tape which are considered to prevent radial expansion of tensile armours; tensile armours which are rectangular cross section helical wires with pitch angle close to 35 degrees made by high strength steel to stand for axial and bending and torsional loads. For deepwater flexible pipe systems, in response to local damage and loss of constraint, the tensile armour wires may exhibit two forms of local instability that includes radial buckling (i.e., birdcaging) and lateral buckling. These two failure modes may occur during installation or operational conditions due to pure axial compression and bending curvature. Due to the complex mechanics for integrating the mechanical response of each layer and the corresponding interactions between adjacent layers, there are few analytical and numerical modelling studies addressing the mechanical performance of composite flexible pipe. These investigations are constrained by the underlying idealizations and assumptions used, and the available hardware and software technology. As the technology development and fabrication of flexible pipe is company-specific proprietary, intellectual property, there are few experimental studies available in the public domain. To improve knowledge, and potentially advance current engineering design and practice, it is important to develop a thorough understanding of the pipe mechanical response, strength performance limits and deformation mechanisms. The main goals and major contributions in this thesis are the development and advancement of three-dimensional finite element modelling procedures investigating the local radial and lateral buckling of the tensile armour wires in flexible pipe. This investigation has provided new knowledge and insight, which is either incremental or unique, on these local instability mechanisms for tensile armour wires. The importance of using an implicit solver rather than the traditional use of an explicit solver has also been established. The simplifying assumptions of existing finite element and analytical models mostly have been improved and built sufficient reliability to be used for the different industrial practices. The significance of pipe model characteristics (e.g., element type, topology, segment length), interlayer contact formulations, boundary conditions (i.e., natural, essential), interface friction, hydrostatic loads, damage condition, and curvature on the local instability mechanisms have been examined which is another unique step for consolidating the design standards.

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