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Optimizing the design of unbonded flexible pipelines with more realistic predictions of ph and H2s content in the annulus
TechnipFMC, France.
TechnipFMC, France.
Institut Français du Pétrole, France.
Institut Français du Pétrole, France.
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2017 (English)In: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE, American Society of Mechanical Engineers (ASME) , 2017Conference paper, Published paper (Refereed)
Abstract [en]

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 longterm 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. 

Place, publisher, year, edition, pages
American Society of Mechanical Engineers (ASME) , 2017.
Keywords [en]
Corrosion, Flexible pipe, Flexible pipe annulus, H2S consumption, Supersaturated pH, Arctic engineering, Carbon, Carbon dioxide, Carbon steel, Cost effectiveness, Marine risers, Oil field development, Oil fields, Pipelines, Supersaturation, Commercial projects, Confined configuration, Cost-effective solutions, Flexible pipes, Oil field conditions, Thermodynamic equilibria, Thermodynamic model, Pipe
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-40017DOI: 10.1115/OMAE2017-61129Scopus ID: s2.0-85031912817ISBN: 9780791857694 (print)OAI: oai:DiVA.org:ri-40017DiVA, id: diva2:1361312
Conference
ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering, OMAE 2017, 25 June 2017 through 30 June 2017
Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-15Bibliographically approved

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