Life Extension of Unbonded Flexible Risers and Flowlines in Subsea Applications: Experience from North Sea Assets

Abstract

An increasing number of offshore assets in the North Sea are now operating beyond, or close to, their original design life. Industry’s best practices for integrity management recommend conducting life extension assessments to guide decisions on extending the operational life of subsea pipeline systems beyond their intended service period. Reliable life extension assessments are becoming increasingly critical as operators aim to manage ageing infrastructure in a safe, reliable, and cost-effective manner while maintaining regulatory compliance.

Assessing the feasibility of life extension for flexible pipes presents unique challenges due to their complex construction, use of multiple grades of metallic and polymeric materials, wide range of potential failure modes, and the limited availability of proven, practical inspection and monitoring techniques.

This paper presents key insights and lessons learned from life extension assessments of approximately 500 dynamic and static flexible pipes across 10 offshore assets in the UK and Norwegian sectors of the North Sea over the last decade. These pipes were originally designed for 20-25 years of operation, with proposed life extensions ranging from 20 to 45 years.

The paper focuses on sharing key findings and engineering insights, specifically:

• A practical approach to the life extension assessment process.
• Common design gaps identified during the gap analysis phase, often stemming from changes in design codes and standards since the original pipe design.
• Common uncertainties and vulnerabilities revealed during current condition evaluation and risk assessment phases, frequently driven by significant changes in operating conditions or limitations in inspection and maintenance programs.

This paper also discusses examples of uncertainties and risks that cannot be mitigated through follow-up analyses or inspections, as well as technological advancements that have enhanced the reliability of life extension assessments. The paper concludes with key takeaways for operators, emphasizing the importance of proactive integrity management, early identification of potential issues, and the critical role of data availability in determining life extension feasibility.

Authors

Damir Tadjiev
Principal Engineer at 4Subsea

Andreas Gjendal
Senior Advisor at 4Subsea