Fundamental aspects of helical wire reinforcement for flexible pipe pressure armour
This thesis describes work undertaken to enhance the current understanding of the fundamental mechanics of flexible pipe structures. In particular, this work relates to flexible pipes that are used as flowlines and risers to convey oil and gas in offshore environments. These are subjected to various loadings such as axial, torsional and bending forces, and variations in external and internal pressure. The structure of these pipes is complex, comprising several concentric layers of various materials and includes steel armouring, which also resists the internal pressure due to the fluid. It is the behaviour of the armouring (which in some designs is interlocked) that forms the basis of this thesis. In order to understand the mechanics of this 'pressure armour', the work presented here reduces the layer to its most basic form - that of a helical spring. This rationale has permitted several loading modes to be characterized, including axial loading, internal pressure loading and the combination of axial-internal pressure loading. Analytical solutions were developed for the modes of loading considered, followed by numerical solutions and finally experimental investigation. The results provide a better understanding of armour behaviour and hence can assist in the design of pressure armour. Unexpected, non-conventional responses were seen in helices that were subjected to combined axial and internal pressure loads. Surface contact between adjacent coils in a helix was also studied, given that this is a feature of interlocked pressure armour designs, and when combined with applied loads, this can give rise to failure mechanisms such as fretting fatigue. Attempts were made to understand some of the significant factors in this aspect. Studies were also undertaken to improve the design of pressure armour profiles for use in high pressure environments based on a patented design, the Omega cross-section, which has an interlocking feature.