An Irregular Wave Approach to Fatigue Life Capacity for Flexible Risers With Severed or Corroded Tensile Armors

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Abstract A novel simulation-based approach for assessing the remaining fatigue life of flexible risers with damaged or broken tensile armors has been developed that can be used to challenge the need for premature replacement, and to modify risers’ Integrity Management to ensure safe operation. A series of potential corrosion or damage scenarios, such as generalized loss of thickness or number of broken wires, are evaluated using high-fidelity finite element models and computationally efficient irregular wave simulations. Results from the various damage scenarios are used to construct curves showing the timeline of the remaining fatigue life of the flexible riser as a function of the number of broken wires. These timeline curves can then be used to modify riser Integrity Management via scheduling periodic NDT inspections to check the evolution of broken wires and provide advanced warning for approaching end-of-life conditions to operation engineers. This novel approach is demonstrated through a case study involving a 7” ID flexible riser with a known history of annulus breach and subsequent repairs near the splash zone, with the potential for tensile armor corrosion to have occurred. Several hypothetical damage configurations involving combinations of severed (broken) tensile armors are evaluated under 100-year design loading to identify the minimum number of wires lost which increases the stress utilization of both the pressure and tensile armors up to a limit that can be considered a pre-cursor of upcoming rupture or loss of containment. The results from these analyses are used to construct a timeline of the remaining fatigue life of the flexible riser as a function of the number of broken wires. This comprehensive demonstration of accurate capture of flexible pipe damaged wire kinematics by high fidelity finite element models and nonlinear simulations has direct applications to flexible pipe integrity management and remnant life assessments. Given that the nonlinear NDS framework allows highly efficient accelerated computation, it is now feasible to execute real-time irregular wave local fatigue simulations with finite element models that include damaged wire data from physical inspections to more accurately predict remnant life.