An experimental study on the key fretting variables for flexible marine risers

- Experimental testing on representative pressure armour steel for identification of friction and wear parameters.
- Effect of grease-lubrication, contact conformity and loading conditions on fretting variables investigated.
- Low rates of wear and coefficients of friction occur under conditions where grease is retained in the contact.
- Grease retention occurs at larger contact displacements, low contact conformity and low applied load.

This paper presents an experimental investigation into the effects of contact conformity, contact pressure and displacement amplitude on gross-slip fretting behaviour for grease-lubricated cylinder-on-flat contacts in the context of flexible marine riser pressure armour wire, and compares behaviour with that observed in unlubricated conditions. Characterisation of friction and wear is critical to fretting fatigue life prediction in flexible risers since friction directly controls trailing-edge fretting stresses and hence fatigue crack initiation, on the one hand, and on the other hand, directly affects wear via relative tangential slip (displacement). Wear can have a beneficial or detrimental effect on fatigue crack initiation and propagation, depending on relative slip and slip regime. For the grease-lubricated conditions, the behaviour is determined by whether grease can be retained in the contact (as opposed to being extruded out). Retention (or replenishment) of grease in the contact results in low rates of wear and low coefficients of friction; these conditions are favoured by fretting displacements above a critical value, by low contact conformity, and by low applied loads.