Session: 04-01-02 Flexible Pipes and Umbilicals II

Paper Number: 101167

101167 - An Irregular Wave Approach to Fatigue Life Capacity for Flexible Risers With Severed or Corroded Tensile Armors 

A novel simulation-based approach for assessing the remaining fatigue life of flexible risers with 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. The high-fidelity finite element models do not use simplifications like stress concentration factors in the vicinity of broken wires, enabling extraction of stress time series at several points across the cross-section of each intact wire based on the altered dynamic response. 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. This baseline damage configuration, and a few preceding configurations, are then assessed under irregular wave dynamic simulations to determine their remaining fatigue life, after rainflow counting 1-hour long stress time series extracted at several points across each intact wire. 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. An approximation is also introduced to highlight the potential application for generalized loss of thickness.

Presenting Author: Krassimir Doynov ExxonMobil

Presenting Author Biography: Dr. Doynov is ExxonMobil’s Senior Principal Engineer Offshore Pipelines & Risers, with 25-year experience in the Oil and Gas Industry. His responsibilities include providing technology solutions to new Project developments, technical expertise to Operation Units, and coordinating R&D activities in Pipelines & Risers. He has been chairing the API & ISO flexible pipe standardization for 18 years.

Authors:

Krassimir Doynov ExxonMobil
Nathan Cooke Intecsea
Dharma Pasala Intecsea
Arya Majed Intecsea