Computationally Efficient Simulation of Flexible Risers via Nonlinear Dynamic Substructures: Numerical and Experimental Validation

摘要

Accurate time-consistent computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy requires capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. It is generally accepted that high fidelity 3D Finite Element Models (FEMs) can capture this complex kinematics and produce accurate stresses. However, dynamic simulation of the flexible’s detailed FEM for spans longer than a few meters requires prohibitively long computation times when utilizing commercial FE solvers. This has constrained the detailed FEM to a local model for static post-processing of maximum tension and curvature from regular wave global analyses. If more accurate representations of the dynamic environments (and associated fatigue life estimates) are desired, irregular wave tension and curvature time-histories are utilized as inputs to stress transfer functions generated from simplified FEM representations. Nonlinear Dynamic Substructuring (NDS) is designed for efficient computation of large-order nonlinear dynamic systems composed of FEMs. NDS is an extension of the framework of dynamic substructuring capturing all its computational efficiencies while operating in fully nonlinear mode. With NDS, the flexible’s detailed FEM is no longer relegated to short spans and static analysis. Instead, the detailed FEM can be synthesized to longer span local models and utilized in nonlinear dynamic simulations with long duration irregular wave tension/curvature inputs. Alternatively, the detailed FEM can be synthesized to global configurations where the simulations can capture the flexible’s complex kinematics at global geometries, boundary conditions, and dynamic environments. In either local or global NDS application, irregular wave armor wire stress time-histories are directly computed from the simulations and serve as an accurate basis for an enhanced fatigue assessment capturing a large array of stress ranges and associated cycles. This leads to high resolution fatigue spectra for reliable and accurate life estimates. This more accurate life estimate can play an important role in reducing over-conservatisms due to simplified models and idealized environments and allow for more accurate life extension assessments and more robust flexible riser integrity management programs.The validation of the NDS approach involved both numerical benchmarking and experimental validations. Numerical benchmarks of NDS simulations were executed against commercial FEA solvers on a pitch length 3D FEM, approximately 4 million degrees of freedom (DoFs) in size, for axial, torsion, internal pressure, external pressure, and bending cases. Experimental validation of NDS simulations involved comparisons of wire strains against an 8-pitch (4.65m) experiment involving multiple cycles of axial, torsional, and bending inputs. The 3D FEM simulated for the above experimental validation cases is approximately 32 million DoFs, making executions in commercial FEA infeasible whereas the NDS simulations executed in minutes. It is shown that NDS results are in very good agreement for both numerical and experimental cases.