Lifetime Extension of Unbonded Flexible Risers Using a New Test Philosophy for Corrosion Fatigue

Paper ID: C2025-00064 ABSTRACT: Equinor has over 200 dynamic unbonded flexible risers in operation approaching their original design life. Risers that are in good technical condition and have low risk of failure, are suitable candidates for lifetime extension studies. The flexible pipe suppliers’ SN-curves for the individual armour wire qualities used in fatigue life design are typically chosen based on conservative predictions of annulus environment and dynamic loads. Comparing operational history and the original design premises may reveal conservative assumptions in design which represents a potential to utilize the remaining capacity. Equinor has developed a corrosion fatigue test methodology, together with a research organization, which simulates steady state annulus environment and wire surface condition of a long-operated flexible pipe. The methodology enables Equinor to generate SN-curves for specific armour wire materials and corrosive environments characteristic for any riser cross section and service governing the annulus environment. The method consists of different steps, including sample shake-down, pre-corrosion, solution and gas replacement, film consolidation, and fatigue testing. The in-house permeation program determines the annulus gas composition and thermodynamic simulation results define the water chemistry. Equinor will present a description of the new test philosophy and examples of field case applications.