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 best capture the complex kinematics and produce accurate stresses. The local model is typically constructed of few “pitch lengths” of the 3D FEM. Local analysis involves enforcing tension and nodal rotation time-histories on the local model and extracting wire stresses at critical fatigue locations along risers. While local analysis involving a few bending cycles can be executed on modern multi-core computers, static simulations typically require computation times of 24–48 hours for a single cycle and do not capture the effect of dynamics of the local model. With this computational constraint, 1-hr long irregular wave fatigue simulations with 3D FEM local model become computationally infeasible. The nonlinear dynamic substructuring (NDS) approach has been utilized in the past to overcome this computation challenge.
Reduced order models are numerical methods for efficiently solving high fidelity FEM. NDS utilizes reduced-order models and numerical algorithms to significantly decrease the computation time associated with the irregular wave fatigue simulations of the high fidelity flexible FEM. Because NDS is a simulation-based approach, effects such as local model tension stiffening and inertial resistance to the global rotation inputs are fully captured and the impact on wire stresses can be discerned.
A 14” inner diameter (ID) flexible riser with a four-tensile armor layer configuration is modeled and simulated using the NDS approach. The 5m long local model is first driven at different “speeds” of harmonic (regular wave) rotation inputs to illustrate inertial effects. For the faster input, the impact of local model inertia on wire stresses is immediately apparent by the increase in wire stresses and change in the shape of the wire stress hysteresis curve. Next, the local model is simulated to irregular wave inputs. It is again shown that the inclusion of local model inertia increases wire stresses and modifies the shape of the wire stress hysteresis.