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Fatigue and Fretting of Pressure Armour Materials in Flexible Marine Risers
Duration: Sept 2013 to Sept 2017
- Researcher: Sinéad O’Halloran (PhD student)
- Supervisors: Prof. Sean. Leen, Dr. Annette Harte
- Technical Team: Mr Bonaventure Kennedy, Mr Pat Kelly
- Industrial partners: Wood Group Kenny
- Funding source: IRC Enterprise Partnership Scheme (Wood Group Kenny) and NUI Travelling Scholarship
Flexible marine risers are a key component in the delivery of offshore hydrocarbons from the seabed to seawater level, typically to a floating structure, such as a production platform or vessel. Flexible risers rely on a complex, composite cross-sectional architecture of helically-wound, interlocking steel wires and polymer layers to give a unique combination of high bending flexibility, axial and torsional stiffness and internal pressure resistance, as well as internal and external corrosion resistance. The effect of fretting wear and fretting fatigue is a potential problem that is difficult to analyse and solve, so it is not presently considered during design of flexible risers. One of the layers, which can be susceptible to fatigue failure is the pressure armour (see figure). Fretting has a large potential to nucleate fatigue cracks in the pressure armour layer. For the inter-locking steel wires, micro-articulation of nub and groove mechanical contacts plays a key role in achieving this complex combination of exceptional mechanical and structural properties.
Fretting action along the contact surfaces is produced by shearing loads due to bending of the riser. This gives rise to a micro-scale frictional contact phenomenon leading to micro-scale surface damage, typically, a combination of wear and fatigue micro-crack nucleation, ultimately leading to loss of function and fatigue cracking and failure. The predominance of wear or fatigue crack nucleation is dependent on a large number of mechanical and physical variables, so that the development of a combined modelling and experimental capability is critical for a scientifically-based service life prediction (design) methodology.
The key design challenge for pressure armour layers in flexible risers is the determination of the optimum nub-groove contact geometry for resistance to fretting crack nucleation. However, this requires the development of a combined experimental-computational methodology to establish the relevant material tribological characteristics (viz. coefficient of friction and wear coefficient evolutions and dependencies on stroke-load combinations), fatigue characteristics (low- and high-cycle fatigue), as well as an identification of the relevant local loading conditions (contact pressure or load, tangential stroke or slip, substrate stresses) for representative global (riser) loads, viz. bending moment, torque, axial load and external and internal pressure conditions.
Figure. Schematics of (a) a riser system, (b) riser cross section, and (c) loading conditions on the pressure armour layer
- O'Halloran, S.M., Leen, S.B.; Harte, A.M., "Contact Mechanics for Flexible Marine Risers", CERAI Conference 2014, Belfast, Ireland (paper presentation)
- Pierce Malone Scholarships NUI (2014)
- O’Halloran, S.M., Connaire, A.D., Harte, A.M., Leen, S.B., “Wear of the pressure armour wire of flexible marine risers”, EMMC14 – European Mechanics of Materials Conference presentation, Gothenburg August 27-29, 2014
- O’Halloran, S.M., Connaire, A.D., Harte, A.M., Leen, S.B., “A framework for fretting design of flexible marine risers”, 18th Sir Bernard Crossland Symposium and Postgraduate Research Workshop 2015 - 2nd place paper presentation
O’Halloran, S.M., Connaire, A.D., Harte, A.M., Leen, S.B., "Modelling of fretting in the pressure armour layer of flexible marine risers", Tribology International, 42th Leeds-Lyon Symposium on Tribology, September 7-9, 2015, Lyon, France (Paper in review)