One main objective of assessing vortex-induced vibration (VIV) in deep-water (1000 m) riser design analysis is to determine the VIV fatigue damage. Whilst the VIV assessment methodology is well known for steel catenary risers, the unbonded flexible risers require special attention due to their complex composite behaviour. This is especially true when using the superposition modal approach, which is used in Shear7. The VIV response of unbonded flexible risers when modelled by Shear7 is complex and depends on several parameters, mainly: bending stiffness, modal damping and riser configuration.

Flexible risers exhibit a non-linear moment response due to the stick/slip hysteresis phenomena of the metallic wires. For low curvatures, a flexible pipe is characterised by a high bending stiffness. However, when the applied loads exceed a threshold level given by the friction resistance between the layers, slippage between the layers occur and the corresponding bending stiffness is significantly reduced, which leads to higher curvature levels. This behaviour is characterised by the moment-curvature response, which is specific to each cross section and is highly dependent on operating conditions (pressure, temperature), external conditions (temperature and hydrostatic pressure) and riser configuration (tension distribution).

The objective of this paper is to establish a consistent VIV analysis methodology for unbonded flexible risers taking some of the main critical parameters such as bending stiffness, modal damping, riser configuration and environmental loading into account. This methodology has been developed for a large West African offshore project, where different types of flexible risers with different operating conditions have been assessed. The impact of the pre-slip and post-slip bending stiffness on the VIV response has been studied and an energy-based approach has been used to calculate the modal damping associated with the frictional energy loss due to the stick/slip phenomenon. This energy loss is directly dependent on the curvatures extracted from the modes excited by the oceanic currents acting on the risers. Different riser configurations have also been considered to assess the impact of the vessel offset on the VIV curvature response.

Finally, VIV induced fatigue damage of the flexible risers has been determined using both extreme and long term current profiles assuming steady state current in the VIV model. The fatigue damage has been calculated at several critical locations along each flexible riser by converting the curvature histograms obtained from the VIV assessment into stresses using specific S-N data converting stress cycles into aggregated damage benchmarked against wave-induced damage.

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