Fatigue of armour wires is in many cases a limiting factor for the design life of flexible risers. Until a few years ago, fatigue design was based on SN data obtained by component testing in air, with the implicit assumption that the environment in a pipe annulus is benign with regard to fatigue of armour wires. Service experience has shown that a pipe annulus may contain species that are aggressive with respect to steel, and could affect fatigue strength significantly. In a consistent design methodology these effects should be taken into account. MARINTEK in co-operation with SINTEF Materials and Chemistry is running a Joint Industry Project (JIP) with the aim of developing a basis for fatigue design of armour wire in which the effects due to the chemical environment in a pipe annulus are accounted for. The project started in 2001 and is now into a Phase III to be completed in 2008. A Phase IV is proposed, to be carried out in 2009–2011. Testing is carried out on tensile armour wire in air, and in aqueous environments and with H2S and/or CO2 at various partial pressures. SN curves have been obtained for more than 50 different combinations of material grade, environmental composition and loading parameters. In this paper the methodology of the testing is presented, with some general results. The following aspects of corrosion fatigue are discussed: - Procedures for fatigue testing of armour wire in corrosive environments, test protocol. - The scope for establishing common fatigue design criteria for armour wire, based on strength classes. - The effect of loading frequency in corrosion fatigue, assessment of fatigue strength criteria for long lives. - The fatigue limit in corrosion fatigue.

The number of flexible pipes in operation is increasing due to new field developments and the desire to extend the lifetime of existing fields. There have been many challenges with respect to long term operation of such pipes, in many cases with the need to replace or repair before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing. The objective of the present paper is to describe the steps required in order to be able to extend the lifetime of a flexible riser for the case of corrosive annulus environments. A specific riser configuration is considered, and the particular case of outer sheath damage is applied for the purpose of illustration.

The integrity of BP's West of Shetland (WoS) flexible riser systems has been actively managed using a risk-based approach since they were installed in the late 90s. A failure mode that was identified as part of this process was the fatigue of armour wires in those risers with flooded annuli. Annulus flooding WoS was caused by external sheath breaches that occurred during installation and to mitigate the risks of both corrosion and corrosion fatigue, BP sealed the breach locations, displaced the sea water in the annuli with a corrosion inhibitor, and implemented a programme of regular fatigue life reassessments for the affected risers.In 2008, informed by the integrity management process BP put in place, the decision was taken to replace one of the WoS high pressure gas risers which had a flooded annulus. To complete the integrity management cycle, the decommissioned riser was dissected and the various metallic and polymer layers inspected. In addition, armour wire samples were taken from the fatigue critical region of the riser, and fatigue tested to establish S-N curves in an environment representative of the annulus. This paper describes the dissection process and the general condition of the pipe following decommissioning. It shows that generally the condition of both the polymer and metallic layers was good and the introduction of the corrosion inhibitor into the annulus of the pipe had a positive effect. The fatigue testing programme is also presented, and the S-N curves generated using the wires from the decommissioned riser are compared against curves for as-manufactured wire. This indicates a significant level of conservatism in current fatigue life prediction methods.The inventory of flexible risers in the major offshore basins that have flooded annuli is large and the fatigue performance of armour wires in this situation can be a key integrity management risk. Currently assessment of this risk is based on calculation and small scale testing alone, rather than actual operational experience. This paper seeks to address that gap in industry knowledge and to start building a database of actual fatigue performance.

This paper presents detailed learnings into the dissection of a flexible riser section, which had been operating for seven (7) years and subjected to high rates of fatigue damage, after operational field changes. Detailed global and local fatigue analyses were performed to assess the response of the riser system taking into account significant changes between the design and the operational history. These environmental and operational changes were shown to have a negative impact on the fatigue performance of the risers, in particular the top section located inside the bend stiffner. Fatigue analyses are performed using field measured data such as vessel headings, internal pressures and environmental conditions, aiming to replicate the riser response history and calculate fatigue usage to date. Actual field data are shown to be more onerous than design data and therefore a detrimental fatigue response is expected. Results from detailed fatigue analysis shows that flexible risers fatigue usage were above the maximum allowable of 0.1, as outlined in API [1,2]. The most critical location was the pressure armour wires of the top section, located inside the bend stiffener. A mitigation plan was proposed, including an immediate reduction of internal pressure to reduce the risk of riser failure, and the removal of the fatigued riser top section. After removal of the fatigue critical section, the riser was re-terminated and its fatigue performance reinstated. The pipe section removed from operation was carefully dissected. Following the dissection, each riser layer was investigated to determine possible signs of degradation. Additionally, fatigue testing of both pressure and tensile armour wires were performed to determine potential fatigue degradation and confirm analyses findings. Dissection and fatigue tests have demonstrated that the pipe condition was better than expected, highlighting conservatisms in design and analyses methodologies. In-service inspection of flexible riser internal layers is highly complex, with no detailed insight of all the respective layers. Dissection of a flexible riser, in service for seven (7) years and exposed to high rates of fatigue damage, provides valuable information about the state of the different layers comprising the cross section. This is particularly useful since analytical work have shown the riser to have exceeded the fatigue limit of 0.1 as defined per API. The good state of the flexible riser, on the contrary to the prediction, highlights a good level of conservatism in flexible riser design and methodology.

There have been many challenges with respect to long term operation of flexible pipes, in many cases with the need to replace or repair them before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing. In the present paper, some typical issues that arise during the process of integrity assessment and qualification for lifetime extension are outlined. More detailed considerations are illustrated in relation to a particular case study. This involves elaboration of the different steps which are required in order to be able to extend the lifetime of a flexible riser for the case of corrosive annulus environments. The particular case of an outer sheath damage is applied for the purpose of illustration.