Multi-Scale, Stochastic Fatigue and Fretting in Flexible Marine Risers

Multi-scale, stochastic fatigue and fretting in flexible marine risers

Duration: Sept 2014 to Sept 2018

People:

• Researcher: Patrick Ashton (PhD student)
• Supervisors: Prof. Sean. Leen, Dr. Annette Harte
• Academic Collaborator(s): Prof. Fionn Dunne, Imperial College London
• Technical Team: Mr Bonaventure Kennedy, Mr Pat Kelly
• Industrial partners: Wood group Kenny, Bakaert
• Funding source: Irish Research Council

Summary:

Fatigue is a gradual damage mechanism due to repeated cyclic loading of a structure. Fatigue is the key life limiting factor for marine risers. Flexible marine risers serve the purpose of transporting hydrocarbons from a subsea well to a floating production vessel. Riser systems  are subject to complex dynamic loading due to stochastic wave, wind, and current conditions. As a result, it is extremely challenging to accurately predict the service life of marine risers. Fretting fatigue and wear of these components is particularly difficult to predict. Fretting is surface damage caused by small amplitude motion between two contact surfaces. Flexible risers are susceptible to fretting fatigue due to large hydrostatic and hydrocarbon pressures and marine environmental conditions, leading to the potential for premature failure. The pressure armour layers are especially vulnerable to damage from fretting.

The proposed research will develop a combined experimental and computational methodology to measure and predict fatigue crack initiation, wear and surface damage in the steel alloy used for pressure armour layers of flexible marine risers, as provided by industrial collaborators. The research will investigate the effect of riser material microstructure on fatigue life through crystal plasticity models and micromechanics. Experimental testing will be employed to (i) study the crack nucleation and short crack growth behaviour of the material for identification of modelling parameters and (ii) quantify the surface damage due to wear. The prediction methodology will incorporate the effect of key material microstructure parameters, such as grain size and distribution statistics and the statistics of rough surface contacts (asperities).

Publications:

‘Cyclic crystal plasticity modelling of fatigue with application to fretting’, Proceedings of the 42nd Leeds-Lyon symposium on Tribology, Lyon 2015.

Images

The schematic diagram of research methodology