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Optimizing the Design of Unbonded Flexible Pipelines With More Realistic Predictions of pH and H2S Content in the Annulus

[+] Author Affiliations
Li Ke, Carol Taravel-Condat

TechnipFMC, Le Trait, France

Jean Kittel, Rémy Mingant

Institut Français du Pétrole Energies Nouvelles, Solaize, France

Claude Duret-Thual, Virginie Querez

Institut de la Corrosion, Saint-Etienne, France

Paper No. OMAE2017-61129, pp. V05AT04A005; 10 pages
  • ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 5A: Pipelines, Risers, and Subsea Systems
  • Trondheim, Norway, June 25–30, 2017
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-5769-4
  • Copyright © 2017 by ASME


Due to its high metallic confinement, the annulus of unbonded flexible pipelines is a specific and mild corrosive medium for carbon steel armour wires. This environment presents high supersaturation levels of dissolved iron, leading to pH values far above thermodynamic equilibrium. Furthermore, the permeation of acidic gases (such as CO2 and H2S) through the polymer pressure sheath occurs at very low flow rates. Since the annulus is supersaturated with dissolved iron, part of the H2S is consumed as it slowly arrives into the annulus. Therefore, the annular medium contains low levels of gas far below those predicted by standard thermodynamic models, and less H2S is available to trigger sour cracking.

The recent development of harsher oil field conditions (higher water depths, increased CO2 content, presence of H2S...) induced the need to refine the design of flexible pipes to propose more cost effective solutions. As pH and H2S content are key parameters for the selection of steel grades, taking into account the supersaturation and the H2S consumption in the annulus allows major optimization of flexible pipes by using for instance steel grades with higher strength. Therefore, extensive experimental work was conducted over the past years to better characterize the annulus and predict more realistic pH and H2S levels. In this paper, the following developments are presented:

– A kinetic corrosion model named FlexCor was derived from numerous corrosion tests done at various CO2 pressures in confined configuration, with in-situ pH monitoring. These tests were performed over long durations (3 months) in order to capture the effective long term supersaturated pH. The kinetic model is able to simulate the transient and stationary phases of the supersaturated pH evolution up to 45 bara of CO2, providing a good fit with the experimental data. The tests also demonstrated that the annulus environment remains supersaturated even at high CO2 partial pressures.

– A methodology taking into account the H2S consumption was developed based on extensive long-term small scale and full scale testing (> 2 years), where low flow rates of H2S were imposed. The experimental results show that H2S consumption is far from negligible, even when the annulus is not fully flooded. This H2S consumption methodology was certified by an Independent Verification Agency and is now being applied on commercial projects.

Copyright © 2017 by ASME
Topics: Design , Pipelines , Annulus



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