Pipe-in-Pipe Substructure Modeling in Deepwater Riser Design Analysis

Substructure modeling using pipe-in-pipe (PiP) elements in a finite element program allows representation of dynamic interaction between riser components. This modeling technique is especially useful when it comes to the design of a complex riser system in deepwater applications. In this paper, the ABAQUS finite element program was used to illustrate the substructure models and the results for dynamic analysis of a classic Spar top tension riser (TTR) system in the Gulf of Mexico subjected to a given Hurricane Rita sea state. Nonlinear contacts between the buoyancy can and compliant guides are represented by two different substructure models: compliant guide surface model with friction and frictionless compliant guide spring model. The effects of centerwell hydrodynamic forces were considered. ABAQUS dynamic results were compared between the PiP substructure model and a conventional structure model treating the buoyancy can and the riser inside as a composite beam. The PiP guide friction surface model with centerwell hydrodynamics theoretically is the most representative model for riser analysis. However, the PiP guide spring model is more computationally efficient. It generates comparable guide loads but produces lower riser fatigue damage than the PiP guide friction surface model. The composite beam model leads to guide loads comparable to the PiP model, but cannot be used to determine the spacer loads between the buoyancy can and riser. The composite model also could underestimate riser stresses and riser fatigue damage within the buoyancy can region. The riser guide loads and riser damages from the calculation models without centerwell hydrodynamics are generally higher than those by the same calculation models with such consideration. It was concluded the PiP guide spring model can be used for riser design in lieu of the PiP guide surface model. The additional fatigue damage contribution from axial tension variation due to guide surface friction could be accounted for by adding a damage factor to the total fatigue damage along the riser.