A Study Of Hydrodynamic Characteristics On Large And Small Pipeline In Submarine Piggyback Configuration

Model tests and numerical simulations were performed to study the hydrodynamic characteristics on the large and the small pipeline in submarine piggyback configuration, in order to improve the understanding of this new submarine piggyback configuration and provide valuable scientific results and theoretical foundation for engineerers and researchers to predict the stability and safety of submarine piggyback configuration.Model tests were designed to measure both the in-line force and the lift force on the large and the small pipeline simultaneously. Hydrodynamic forces on a single large pipeline, a piggyback pipeline system, the large and the small pipeline under regular wave, irregular wave and wave-current coexisting action were investigated. The ratio of loads on the large or the small pipeline to the total loads on the piggyback pipeline system was discussed. The effects of KC number, the gap ratio e/D between the large pipeline and the seabed, the spacing ratio G/D between two pipelines, the diameter ratio d/D and the current ratio Uc/uwm on the hydrodynamic coefficients of the large, the small pipeline and the piggyback pipeline sysem were studied. The experimental results indicate that inline forces on the large and the small pipeline are fluctuating simultaneously, and the inline force peaks of the large and the small pipeline are about74%and26%of those for the piggyback pipeline system respectively. There is a good relationship between force coefficients of the large, the small pipeline and the piggyback pipeline sysem and the influencing factors described above. And the varation tendency of force coefficients with influencing factors under wave-current coexisting action is similar to that under pure wave action.In the numerical study, a three-step finite element method was employed to solve the N-S equation with k-ω turbulent model, and CLEAR-VOF method was used to simulate free surface problems. A numerical wave flume model was established to simulate the hydrodynamic characteristics of submarine piggyback pipeline under wave action. A comparison of numerical and experimental results indicates that the numerical results are in good agreement with the experimental ones. Thus, the numerical model established in this paper can be employed to predict the hydrodynamic forces on the submarine piggyback pipeline under wave action. Based on the established numerical wave flume model, the hydrodynamic characteristics of the large and the small pipeline in piggyback configuration under regular and irregular wave action were investigated. The relationship between the oscillating varation of loads on the large and the small pipeline and the surface pressure distribution and the motion of the vorticity around the pipeline was discussed. The effect of the spacing ratio GID on the vortex shedding characteristic of the large and the small pipeline was analyzed. The numerical results indicate that peaks of loads on the large and the small pipeline are induced by shedding or return of the vortices. And two different vortex shedding patterns under regular wave action are found regarding GID. One vortex shedding pattern denoted "anti-phase-synchronized" pattern is recognized when G/D is smaller, and the other pattern denoted "in-phase-synchronized" pattern is recognized when G/D is larger.