Comparison of Software for Analysis of Risers Tied Back With Pull Tubes

Dynamic analysis of steel catenary risers (SCRs) tied back to a Spar through pull tubes has most commonly been performed with the general purpose finite element analysis program ABAQUS, which is generally considered to be most suitable for this type of configuration involving pipe-in-pipe contact between the riser and the pull tube. Version 9.6a and subsequent versions of the riser specific software package OrcaFlex include the capability to handle pipe-in-pipe contact through a line contact model. This model is similar to the contact model in ABAQUS in that it supports relative axial motion and axial friction between the contact lines. A benchmark study is performed to compare riser strength and fatigue performance of the two software packages. The focus of the study is on the pull tube and the riser inside the pull tube, where pipe-in-pipe contact occurs. Bending moment and von Mises stress from the two programs in design storms are found to agree closely. Governing loads on the pull tube guides computed with ABAQUS agree well with the loads computed with OrcaFlex. The two software packages compute similar vessel-motion induced fatigue damage of the bottom of the pull tube (pull tube stress joint). Agreement is not as close for the pull tube above the pull tube stress joint or for the riser pipe. Dynamic run times are comparable between the two programs. Results of the study can assist in the selection of the most suitable software for pull tube riser design and analysis, and in understanding the differences in results from the two software packages.