Investigating into the Temporal Hydrodynamic Forces Exerted on Offshore Piggyback Pipelines due to Steady Currents

For the gas and oil pipelines in marine environment, the forces exerted on these structres are regarded to be of the important parameters in designing them. These pipelines are usually installed with twin arrangements as tandom or side by side. The diameters of the two pipelines may also be identical or different. Neverthless, a number of them may also be configured as piggyback, an special case of side by side arrangements when their diameters are not equal. The parameters around this special arrangement have already been studied both experimentally and numerically by a few number of researchers, namely, Zhao et al. (2007), Zang et al. (2012), Hakimzadeh and Mosahebi Mohammadi (2016), Kardan and Hakimzadeh (2018). However, for this research study, the momentary hydrodynamic forces acting on the piggyback pipelines due to steady currents have been investigated using numerical simulation. All simulations were performed in 3D using ANSYS FLUENT 16 software environment. First, the capability of the software for the current study was investigated. The number of cells and various turbulence models were considered to find the proper mesh size and model, respectively. Then, considering that for flows around a single pipe, the conventional turbulence models may not provide accurate results, therefore, by examining different cell types and turbulence models, an attempt was made to select the appropriate type that can provide more accurate results by spending a reasonable computational cost. Then, it was found that a structured cell formation together with LES turbulence model reproduced the flow patterns around the pipe with a reasonable accuracy when compared with the experimental data. For the numerical simulation results, first, the effects of the inlet flow velocity and main pipe diameter (i.e., the Reynolds number) variations on the coefficients of instantaneous hydrodynamic forces (i.e., drag and lift) exerted on piggyback pipelines were determined. Further, effects of the distance between pipes (G/D), the proximity to the bed (e/D) and the relative diameter (d/D) variations on the drag and lift coefficients were considered through computer simulations.