Findings From Dissection and Testing of Used Flexible Risers

During Statoil’s recent riser replacement project more than 30 used risers have been dissected onshore [1]. The main objective has been to reveal the root cause of carcass axial tearing failures. However, this also gave the opportunity to investigate details of other carcass damages, armor corrosion, annulus environment, external sheath breaches and polymer ageing as described in [5], [6] and [7]. This paper discusses some of the new knowledge obtained from the dissections.New tools and methods have been developed for carcass pitch measurements in order to determine axial movements, maximum load level seen by the carcass and determine the utilization against tearing.Offshore free-volume annulus testing and gas sampling is compared to liquid sampling and actual liquid found when dissecting, with significant deviations observed. Expected annulus environment is assessed in light of actual observed corrosion, with generally less corrosion than expected, however at some few selected areas significant corrosion attacks are found. Residual stresses and condition of polymer layers is quantified, with generally more degeneration and larger changes than anticipated before riser recovery.Copyright © 2014 by ASME