Fatigue Assessment of Free-Span Subsea Pipelines

Abstract The structural integrity of subsea oil and gas pipelines in free span conditions must be assessed considering the interaction with the seabed, the high external pressure and the action of waves and currents. Extreme environmental loads can also result in vortex-induced vibration (VIV) due to the action of waves and currents in the free spans. Fatigue failures can occur in free spans of subsea pipelines, whether or not associated with VIV. The studied scenario is located at a depth between 20 m and 50 m, in the Campos Basin, Brazil. This study is based on the prediction of fatigue damage in free-span subsea pipelines, contributing to the prevention of failures that can result in severe socio-environmental damage. A numerical model was developed, based on the finite element method, to reproduce the interaction with the seabed and the environmental loads of waves and currents on free span steel pipes. The model allows estimating the resulting stress variations for the assessment of fatigue damage according to an appropriate S-N curve and considering the Palmgren-Miner linear damage accumulation rule. Nautical area 74 was considered to estimate long-term wave distribution parameters to achieve sea states with significant wave heights (HS), corresponding to the zero-crossing wave period (TZ), according to the DNVGL-RP-C205 standard. The stiffness of the soil (sand) was considered in a simplified way based on the DNVGL-RP-F114 standard, estimating the dynamic stiffness values in the axial, vertical and lateral directions in the pipe-soil interaction. The non-linear 5th order Stokes wave theory was adopted. The numerical model was used in a parametric study with varying pipe dimensions (ratio between outside diameter to wall-thickness, D/t, and span length), water depth and sea state. The stability of the analyzed cases was evaluated according to the DNV-RP-F109 standard to assess the possibility of VIV occurring due to the action of wave and current loads, characterized by the DNVGL-RP-F105 standard. Based on the stress variations obtained in the numerical simulations and using the S-N curve “D” (seawater with cathodic protection), proposed in the DNV-GL-RP-C203 standard, and the Palmgren-Miner rule for linear accumulation of damage, the fatigue lives of the analyzed cases were estimated.