Session: 02-11-01 Fatigue and Fracture Reliability

Submission Number: 156959

Fatigue Assessment of Free-Span Subsea Pipelines 

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 socioenvironmental 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. The stiffness of the soil (sand) was reproduced by 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 diameter to 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 an S-N curve and the Palmgren-Miner rule for linear accumulation of damage, the fatigue lives of the analyzed cases were estimated.

Presenting Author: Bianca Pinheiro COPPE/UFRJ

Presenting Author Biography: MSc in Ocean Engineering at COPPE/UFRJ, Brazil
DSc in Mechanical Engineering at the University of Lille, France