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

Abstract Equinor1 has more than 200 dynamic unbonded flexible risers in operation, and many of them are approaching their original design life. Risers that are in good technical condition and have low risk of failure are candidates for lifetime extension studies. The flexible pipe suppliers’ SN-curves for the individual armor 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. Company has developed a corrosion fatigue test methodology together with an independent research institute, which simulates steady state annulus environment and wire surface condition of a long-operated flexible pipe. The methodology enables Company to generate SN-curves for specific armor 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 shakedown, pre-fatigue treatment, film consolidation and corrosion fatigue testing. An in-house permeation program determines the annulus gas composition, and thermodynamic simulation results define the water chemistry. The new test methodology and a field case application is presented in the paper.