Piggyback Pipeline Research Articles

Numerical study of scour around a pipeline bundle

A well-established two-dimensional numerical model was employed to study the scour-hole development around a pipeline bundle, which was composed of a main pipeline and a piggyback pipeline. Both the current and wave flow situations were simulated with parameters chosen to represent prototype pipeline scour in adverse weather conditions. Under the same hydraulic and soil conditions, the scour around the main pipeline alone and around an equivalent pipeline was also simulated, and these served as reference cases for studying the scour around pipeline bundles. The focus of this paper is the equilibrium scour depth and the development rate. It was found that the pipeline bundle produced a deeper scour depth with a larger time-scale in comparison with the scour occurring for the single-pipeline case; thus the equivalent-pipeline representation may lead to an unsafe design. The differences between the pipeline bundle and single pipeline scour were larger under the current alone flow condition than those under the wave condition. These differences in the scour development can be explained by examining the flow fields around the pipeline.

Numerical Modeling of Flow and Hydrodynamic Forces around a Piggyback Pipeline near the Seabed

Steady boundary layer flows around a two-cylinder configuration near a plane wall is investigated numerically. The Reynolds-averaged Navier-Stokes equations are solved using an upwind finite-element method with a k-ω turbulence model closure. The numerical model is validated against independent experimental data for flow past a single cylinder near a plane wall. The two cylinders investigated in the present study are structured in the so-called piggyback configuration, in which the small cylinder is placed directly above the large cylinder. The diameter ratio of the small cylinder (d) to the large cylinder (D) is set at a constant value of 0.2. Different values of the gap (e) between the large cylinder and the plane wall and the spacing (G) between the two cylinders are investigated. The effects of the gap ratio e∕D and the spacing ratio G∕D on the flow around and the hydrodynamic forces on the cylinders are investigated. Four vortex shedding modes are found around the two-cylinder system. It is found tha...

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, CD and CL, 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 CD 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 CL< 0, indicating that the transverse force acts toward the seabed (pipeline self-burial). In contrast, CL> 0 at higher (critical) R and this may increase the likelihood of generating free-spans along the pipeline.