Lifetime Extension of Flexible Risers Based On Field Experiences
Authors: Muren Jan , Eide Jørgen , Engelbreth Knut Inge , Christensen Henning , Nilsen-Aas Christoffer 
Published: April 2016

There are more than 3500 dynamic unbonded flexible risers in operation worldwide. In addition, there is a considerable number and kilometres of flexible flowlines and jumpers on the seabed. The average riser age is more than 10 years and a number of risers are soon reaching their original design service life of 15-25 years. There is a need to learn more about the time driven degradation processes in flexible risers in order to ensure continued safe operation, and to meet the increasing demand for service life extension.

By investigating a large range of damaged and intact unbonded flexible risers from West Africa, UK, The Netherlands and Norway, 4Subsea is continuously improving the understanding on how flexible risers and flowlines in operation degrade over time. As the variability of the degradation and failure development is significant, a high number of observations is needed to establish consistent trends and basis for reliable analysis and assessments. Trends of emerging challenges are observed through these investigations, however most of all it is recognized that the complexity of the annulus environment, corrosion processes and polymer degradation is high and deserves continuous attention.

4Subsea has investigated some 60 000 m of flexible risers, flowlines, jumpers, cables and umbilicals from 13 fields and 6 operators. Examination of used flexible pipes by 4Subsea and others have to a large degree validated the design assumptions and conclusions, however there are exceptions. Variability in degradation mechanisms and their development is found to be significant. Some weaknesses are identified and improvements are implemented in the recent updates of API specifications and recommended practices.

The referenced investigations are performed for several operators, and it is seen that sharing information will give the operators considerable benefits. Cooperation initiatives overcoming confidentiality issues are progressing and will give improvements to reliability and safety for flexible pipes. Combined, significant improvements in basis for safety-, reliability- and life time analysis for the operated assets are experienced.

In broad terms the experiences show that polymer ageing issues, and in particular challenges related to high temperature operation, need to be prioritized when investigating the possibility for life extension. Current methods for degradation modelling are uncertain. Tensile armour wire fatigue analysis needs to take into account dynamics from calibrated models, response measurements and advanced stress predictions as well as correct annulus environment to be representative for the real exploitable life of the structures. Pitting corrosion and loss of material due to corrosion are key factors to take into account in fatigue modelling. Serious corrosion issues are correlated with oxygen access as typically experienced in connection with large external sheath damages near the water surface.

A successful life extension for a particular flexible pipe requires a thorough process starting with an assessment of current status of the flexible pipe with basis in original design data, operational data records, inspection, test and monitoring data. When the assessment of the current status concludes that the complete system is well suited for further safe operation, a prognostic assessment is performed based on best-available information on future service conditions. As the lifetime of a flexible riser or pipeline may be several decades, the premise for assessing safe operation may have to be re-established, incorporating new methods, knowledge and differing industry experiences.

Experiences from investigations of more than 75 used risers and umbilicals provide a unique basis for focusing on life time assessment issues, as well as target areas of inspection, monitoring and research. This also involves various compensating measures such as external sheath repair, improvement of annulus vent flow, online condition monitoring or special inspection campaigns.