Multi-Strip Numerical Analysis for Flexible Riser Response

A multi-strip numerical method, combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a finite-element structural dynamics response, has been developed to analyze the flow-structure interaction of long, flexible risers. This solution methodology combines a number of individual hydrodynamic simulations corresponding to individual axial strips along the riser section with a full 3D structural analysis to predict overall VIV loads and displacements. The hydrodynamic loading for each riser strip is derived from a 2D finite-volume discretization of the governing RANS equations which is applicable to both single and multiple riser configurations. The entire flow-structure solution procedure is carried out in the time domain via a loose coupling strategy, such that the hydrodynamic loads from each riser strip are summed to obtain the overall loading along the span of each riser. This loading is then used to integrate forward a single time-step in the riser equations of motion to obtain an updated riser displacement profile. Closure of the coupled flow-structure method is achieved by updating the riser displacements for each of the corresponding hydrodynamic strips in the next time-step integration. The developed multi-strip method is applied to a single bare riser subjected to both uniform and shear current profiles. The flow conditions and riser configuration were chosen to match the Marintek rotating rig experiments, and comparisons between experimental and numerical results are presented for several flow configurations and axial tensions. In addition, a parametric study is presented using 16, 32, and 64 hydrodynamic strips for a given flow configuration to ascertain the sensitivity of the results to the number of strips chosen.