Free hanging deep water flexible risers are associated with high static top tension. In addition comes significant dynamic tension due to the tangential drag forces mobilised along the riser due to floater motions and wave loads. At the upper end fitting where conditions of direct metallic contact between tensile armours might occur, this may result in the fretting fatigue failure mode. This mechanism may be additionally triggered by the increase in longitudinal stress resulting from local bending at the end fitting fixation.
At the touch down point, the pipe cross-section is exposed to high external pressures resulting in true wall compression possibly initiating local buckling modes in the tensile armour wires. The buckling process may be initiated by cyclic bending effects leading to a gradual reduction of each tendon’s capacity resulting in excessive transverse displacements and cross-section failure.
The present paper presents a finite element formulation and analytical models addressing both of the above topics. A case study is further carried out to document the performance of the FE model and to investigate effects related to the transverse bending stress at the end fitting and under which conditions one single armour tendon will fail in different buckling modes.