With basis in field experiences from several carcass axial failures in HT/HP unbonded flexible risers a comprehensive program was initiated to determine the root cause and develop a theoretical framework for prediction of the loads and mechanisms leading to damage initiation and development. The physical understanding, theory, tools and methods were developed in a close cooperation between the field operator Statoil, 4Subsea and DNV GL.
Following the discovery of carcass tearing as a new failure mode in May 2011 substantial work has been put into small- and mid-scale testing together with the establishment of various analysis models to describe the carcass tearing phenomenon. This development and test work has been presented at OMAE-2013/14. Since 2011 substantial work has been put into small- and mid-scale testing together with various analysis models to describe the carcass pull-out phenomenon.
As a final full-scale validation, an onshore fully instrumented test was conducted to assess the accuracy of the carcass tearing models and evaluate variability and uncertainties related to use of the developed tools and methods for risk assessment of similar HT/HP pipes still in operation. The flexible riser used for the test was a full length 830m long pipe recovered after 15 years’ successful operation at a North Sea Field. The pipe integrity was not jeopardized and the pipe was fully functional at the time of recovery.
The pipe was recovered due to unfavorable risk assessment and plans for full-scale testing. The full-scale test gathers as many aspects as possible into a single test to verify the analytical solutions proposed during this work. The test pipe was used with the original end-fittings with the full pipe length intact. The end sections of length were unreeled, secured, protected and supported.
As preparation to the test, finite element analyses of the flexible riser were performed to predict the carcass axial load in the end-fitting. The analysis also served as valuable input to the test procedure and instrumentation plan. Along with analyses, a set of novel measurement and instrument solutions were developed. This includes, live video monitoring of carcass movements and strain measurements at carcass, with temperature and pressure well outside standard solutions. Bore temperature and pressure varied from 0–90 °C and 0–500 barg, respectively.
Several temperature and pressure cycles were done. Comparison between detailed finite element analysis and full-scale results shows excellent correlation. The riser and end-fittings have been dissected after testing. Key findings from dissection match the predicted behavior well.