Hydrodynamic Forces on Piggyback Pipeline Configurations

The external hydrodynamic loading exerted by steady incident flows on submarine pipelines is investigated. Of particular interest are the hydrodynamic interactions between pipeline and seabed, and between two pipelines of different diameters. The latter gives rise to the “piggyback” configuration, where one cylinder (the piggyback) is situated either on top of, or slightly above, a larger diameter cylinder (the main pipeline). Drag and lift coefficients, C_D and C_L, are obtained experimentally for Reynolds numbers R between 9 × 10⁴ and 3 × 10⁵, corresponding to currents of 0.5–1 m/s (1–2 knots) and main pipelines of diameter 400–600 mm (16–24 in.). For piggyback configurations typical of those used by the offshore industry, the proximity of the two pipelines to each other can increase the values of C_D for single cylinders by 50%–100%. The direction of the transverse lift force is also of practical importance, as it determines whether the pipelines have a tendency to self-bury or free-span. This appears to depend on R and the gap ratio e/D between the seabed and (main) pipeline. The characteristics of both a single pipeline and a piggyback configuration near the seabed are discussed. For the piggyback configuration at subcritical R, it is found that C_L< 0, indicating that the transverse force acts toward the seabed (pipeline self-burial). In contrast, C_L> 0 at higher (critical) R and this may increase the likelihood of generating free-spans along the pipeline.