Combined Effects of Waves and Currents on Offshore Pipeline Bundles Available to Purchase

It is common practice to accompany offshore pipelines by smaller diameter service lines or umbilicals to create a bundle. This gives rise to the so-called piggyback configuration. The flow behavior around the bundle is not well-known, leading to concerns on the stability of the configuration. This paper investigates the influence of the piggyback on the hydrodynamic loadings on the bundle in wave plus current condition using Computational Fluid Dynamics (CFD). Key parameters of the configuration that were investigated were the orientation of the smaller pipe with respect to the main pipeline and the flow conditions (different Keulegan-Carpenter numbers). The gap between the seabed and the main pipe was set to zero for all cases investigated. It was found that the hydrodynamic characteristics of the main pipe were significantly influenced by the presence of the piggyback. The numerical results also showed that the orientation of the piggyback plays an essential role in determining the drag, lift and inertia coefficients for the bundle. This phenomenon is explained by examining the vortex flow patterns around the cylinders. It is shown that the established industry practice of assuming the hydrodynamic characteristics of the bundle to be the same as an equivalent diameter cylinder may underestimate the forces on the bundle, and lead to a non-conservative design.