Piggyback Pipeline Research Articles

Experimental Studies on Vortex-Induced Vibration of a Piggyback Pipeline

Offshore pipelines of different diameters are often seen in piggyback arrangements in close proximity. Under the effects of external flows, the pipelines may experience vibration. Reliable prediction of the vibration amplitudes is important for the design and operation of these structures. In the present study, the effect of the position angle (α) and gap ratio (G/D) of a piggyback pipeline on the amplitude of 1DOF vortex-induced vibration (VIV) was investigated experimentally in a wind tunnel. The diameter ratio d/D of the two cylinders was 0.5. Five position angles, namely, α = 0°, 45°, 90°, 135°, and 180°, and six gap ratios at each angle, G/D = 0, 0.1, 0.2, 0.3, 0.4, 0.5, were tested. It was found that both α and G/D affected the amplitude of vibrations significantly. For all gap ratios, the amplitude of vibrations increased from α = 0° to α = 90° and then decreased to a minimum value around α = 135°. The maximum amplitude occurred around α = 90° when G/D = 0, and the minimum occurred around α = 135°, when G/D = 0.2–0.3. At other position angles, the vibration amplitude was less sensitive to G/D, especially when the latter was between 0.1 and 0.4. These results verified those obtained using numerical methods and are invaluable to engineers when designing offshore piggyback pipelines.

Numerical simulation of piggyback pipelaying under current loadings

For piggyback pipelaying operations, current-induced force and its effect on the piggyback pipe have not been thoroughly studied. In the present study, an improved method in hydrodynamic load calculation and structural modelling is proposed to simulate the pipelaying of a piggyback pipeline. In order to obtain the mean drag and lift force coefficients for the piggyback pipeline subjected to different inflow angles, two-dimensional Computational Fluid Dynamics (CFD) simulations are performed by modelling the piggyback pipeline as two cylinders attached to each other without gap. Then, the acquired force coefficients are used to calculate the hydrodynamic loads through a user-defined function in OrcaFlex based on a cross-flow principle approach. The interaction between the pipeline and the piggyback cable is modelled using two types contact elements which are ring penetrator and non-penetrating contact. The present proposed method is compared with other two widely used engineering methods based on (1) the equivalent diameter and (2) two separate cylinders without accounting for hydrodynamic interaction, in terms of the top tension, and the bending moments at Hang-off Clamps (HOC) and sagbend of the pipeline. The comparison shows that the two widely used engineering methods are not always conservative in force and response predictions. Hence, it is important to consider the hydrodynamic and structural interactions between the piggyback cable and the pipeline. With different current directions, the bending moments at the HOC predicted by the present method vary from 40% lower to 100% higher than those predicted by the two widely used engineering methods.

Scour Characteristics and Equilibrium Scour Depth Prediction around a Submarine Piggyback Pipeline

Local scour around a submarine piggyback pipeline in combined waves and current is investigated experimentally. Based on the experimental results, the scour evolution and scour morphology are firstly analyzed. Then, a comparison with the equilibrium scour depth Seq between the present experimental data and predicted results is conducted. After that, the correlation between the dimensionless scour timescale T* and the maximum Shields parameter θcw is investigated, and a formula is obtained to describe the variation trend between T* and θcw for different gap ratios G/D. Furthermore, the parametric study is carried out to study the effects of Reynolds number Red and θcw on Seq, respectively. The results indicate that the Seq below the piggyback pipeline increases when the gap ratio G/D increases from 0 to 0.1, and it gradually decreases when G/D > 0.1. For a given KC, the Seq increases with the increase of the ratio of velocities Ucw. In addition, when Ucw is fixed, a higher KC results in a greater Seq. The T* is closely related to θcw and G/D. The higher Red and θcw both tend to result in the greater scour depth below a piggyback pipeline in combined waves and current.

Study on Hydrodynamic Coefficients of a Submarine Piggyback Pipeline under the Action of Waves and Current

In this study, physical model tests are used to investigate the effects of a varying number of wave and current parameters, the gap ratios between the pipeline and seabed, the spacing ratios between the two pipelines and the diameter ratios on the hydrodynamic coefficients of the large, small pipeline and pipeline system (bundle) in a piggyback configuration under the combined action of waves and current. The results show that, compared with the pure wave field, the existence of the steady current will lead to a decrease in hydrodynamic coefficients. In addition, the results indicate clear differences between the hydrodynamic coefficients of the large pipeline, small pipeline and piggyback pipeline system. The experimental results on hydrodynamic coefficients can be used as an important basis for the safety design of a submarine piggyback pipeline.

Numerical Investigation of Scour Beneath Pipelines Subjected to an Oscillatory Flow Condition

The present study carries out two-dimensional numerical simulations to investigate scour beneath a single pipeline and piggyback pipelines subjected to an oscillatory flow condition at a Keulegan–Carpenter (KC) number of 11 using SedFoam (an open-source, multi-dimensional Eulerian two-phase solver for sediment transport based on OpenFOAM). The turbulence flow is resolved using the two-phase modified k−ω 2006 model. The particle stresses due to the binary collisions and enduring contacts among the sediments are modeled using the rheology model of granular flow. The present numerical model is validated for the scour beneath a single pipeline, and the simulated sediment profiles are compared with published experimental data and numerical simulation results. The scour process beneath three different piggyback pipelines under the same flow condition are also considered, and the scour development and surrounding flow patterns are discussed in detail. Typical steady-streaming structures around the pipeline due to the oscillatory flow condition are captured. The scour depth during the initial development of the scour process for the piggyback pipeline with the small pipeline placed above the large one is the largest among all the investigated configurations. The phase-averaged flow fields show that the flow patterns are influenced by the additional small pipeline.

Numerical investigation of the influence of the small pipeline on local scour morphology around the piggyback pipeline

This paper presents the results from a numerical simulation study to investigate the effect of the position angle (α) of small pipeline on the local scour and the hydrodynamic force around the piggyback pipeline in steady current conditions. Results show that the local scour depth around the piggyback pipeline increases first and then decreases with the increase of α. The scour depth and width reach the maximum values as the small pipe locates at the top of the large pipeline (i.e. α = 90°). The scour around the piggyback pipeline is accelerated when α ranges between 30° and 165°, while for α = 0°–30° and 165°–180°, the local scour around the piggyback pipeline is inhibited. Furthermore, the small pipe placed in front of the large pipe has slightly larger effect on the scour hole morphology than that when it is placed behind the large pipe. The drag force coefficient increases first and reaches the maximum value at α = 75°, and then decreases with the increase of α. Eventually the drag force coefficient approaches roughly a constant. The lift force coefficient is approximately a V-shaped with the variation of α and has the maximum value at α = 90°.

Vortex induced vibrations of short side by side cylinders

Staggered cylinders of different diameters is a common configuration in offshore design, including e.g. clamped risers, piggy-back solutions and piping systems. Current design practice is not clearly formulated in relevant design codes such as DNVGL-RP-F105 and DNVGL-RP-C205. Conservatively, it is often assumed that flow acceleration due to a neighboring cylinder causes up to a doubling of the incoming flow velocity depending on incidence angle and difference in cylinder diameter. Doubling the incoming particle velocity compared to free stream would decrease the required support distance by around 30%. Particularly in conditions with significant diameter disparity, flow acceleration effects due to the larger cylinder of the two can thus cause strict requirements to support distances on the smaller. Designers may want to reduce the number of support points due to cost, maintenance requirements and rigid structure availability, particularly in piping system arrangements. Recent research on piggy-back pipeline vortex induced vibrations have complemented traditional understanding of such configurations, allowing for improved design guidelines. In the present study, a detailed meta study of available laboratory and numerical work is performed, and the resulting findings are translated into simple design guidelines for piggy-back and other staggered arrangements. The present paper provides clear recommendations for improvements to the relevant recommended practices.

Vortex-induced vibrations of piggyback pipelines near the horizontal plane wall in the upper transition regime

Slender subsea structures like pipelines, jumpers and umbilicals when exposed to currents may experience vortex-induced vibrations (VIV), which can shorten their fatigue life and increase the risk of structural failure. In the present study, flow around different configurations of a piggyback pipeline close to a flat seabed has been investigated using the two-dimensional (2D) Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with the k − ω Shear Stress Transport (SST) turbulence model. The Reynolds number (based on the free stream velocity and large cylinder diameter) is equal 3.6 × 106 corresponding to upper-transition regime. The drag forces acting on the cylinders and base pressure coefficient value are well predicted by the present simulations, while the other hydrodynamic quantities (root-mean-square lift coefficient, Strouhal number) are predicted reasonably well as compared to published experimental data. The piggyback pipeline in the present study is modeled as two circular cylinders with a diameter ratio d/D = 0.2 (D denoting diameter of the large cylinder, d is diameter of the small cylinder). These two cylinders are clamped together at a distance G/D = 0.2. The two rigidly coupled cylinders are elastically supported and free to vibrate in two degrees of freedom. The effects on the vibration amplitudes and hydrodynamic forces are analyzed. The flow structures around the cylinders are investigated to explain the variations in observed structural responses. Depending on the angular position (α) of the small cylinder, the lock-in regime is narrower (α = 0°) or significantly wider (α = 180°) when compared to that of a single cylinder.

Study on dynamic slope angle of sandy seabed around the submarine piggyback pipeline in steady flow

Submarine piggyback pipeline is an important tool for oil and gas transport, especially, in the marginal oil field. When the scour happens in the seabed, the pipeline slides easily, which may cause the pipeline failure. The dynamic slope angle is one of the most important characteristics of the scour. In this work, the dynamic slope angles of the sandy seabed under piggyback pipelines in steady flow are investigated experimentally and numerically in detail. The physical experiments are conducted in an annular flume and the numerical simulations are carried out using a two-phase flow model which is resolved by the finite volume method (FVM). In the numerical model, the free surface is tracked by the volume of fluid (VOF), and both bed and suspended loads are considered in the scour module. Depending on the comparison and convergence analysis, the numerical results are in good agreement with the experimental results. The results indicate that the dynamic slope angle is significantly affected by the incoming flow velocity, gap-ratio, spacing-ratio and pipe diameter. With the reduction of gap-ratio and space-ratio, the upstream dynamic slope angle α increases slightly, but the downstream dynamic slope angle β decreases. With the increase of the grain Reynolds number, the angle α increases slightly, but the angle β decreases severely. The conclusions drawn from this paper could provide the reference for the design of submarine piggyback pipeline.

Investigation on scour protection of submarine piggyback pipeline

This paper presents the results of laboratory experiments and numerical simulations to investigate the effect of different piggyback pipeline configuration on the morphology of local seabed scour subject to steady currents. Piggyback pipeline configuration investigated includes the commonly used piggyback pipeline, namely a small pipe attached on the top of large pipe and new form of piggyback pipeline proposed in this study in which a small pipe is attached to the large pipe on the upstream and downstream side, respectively. Pressure gradient, drag coefficient, lift coefficient and scour extent around pipelines are measured and analyzed for a range of pipelines and current conditions. Results show that the vortex strength downstream of the commonly used piggyback pipeline is larger than that for a single as well as the new piggyback pipeline under the same condition. This new type piggyback pipeline can effectively reduce the depth and width of the scour hole. In particular, when the ratio of the small pipe diameter over the large pipe diameter is greater than 0.3, little scour under this new type piggyback pipeline occurs for the test conditions. The bed topography downstream of the pipe has also been altered to favor the backfill.

Investigation on scour scale of piggyback pipeline under wave conditions

Laboratory experiments are presented to investigate the effect of different piggyback pipeline configurations on the morphology of local scour under wave conditions. Scour depth and width around the pipelines under regular and irregular waves are measured and analyzed for a range of pipeline and wave conditions; such as the spacing between two pipes (G), gap between the main pipe and seabed (e), pipe diameter (D), wave height (H) and period (T). Experimental results reveal that both the scour depth and width around piggyback pipeline is much larger than those around single pipe under the same wave conditions. Scour depth increases with the increase of the Keulegan-Carpenter (KC) number and decreases with increase of G and e. When e exceeds 0.5D, scour depth tends to approach 0. When spacing G is greater than 0.4D, the destabilization from small pipe to large one is greatly reduced, resulting in scour depth around piggyback pipeline being close to that around single pipe. Similar to scour depth, scour width broadens with the increase of KC number increasing and decreases with the increase of G. Experiments also show that the effect of e on scour depth is greater than that of G under the same test conditions, while their impact on scour width is opposite. Furthermore, scour width under irregular waves is extended slightly compared with regular wave for otherwise the identical conditions.

Innovations in a submarine piggyback pipeline project in the East China Sea.

Submarine pipelines are widely used to transport offshore oil and gas. Bundling service pipes and cables with these pipelines can cut costs but such ‘piggyback’ installations can lead to problems with torsion and buckling, particularly at crossings. This paper reports on an innovative anti-rotation device that was successfully implemented in a piggyback pipeline project in the East China Sea. A three-layer concrete mattress together with secondary pipe protection devices was developed for crossings, and advanced engineering analysis was conducted to evaluate risks from shallow gas eruptions.

Vortex-induced vibration of a piggyback pipeline half buried in the seabed

Experiments were conducted in a tow tank on the steady current-induced vibration of the smaller flexible cylinder (aspect ratio L∕d = 61 and mass ratio m0 = 1.38) in a piggyback configuration with the main pipe half-buried in the seabed. The effect of the gap-to-diameter ratio (G∕d) based on the smaller cylinder diameter is studied for five different configurations from 1.0 to 4.0 and ∞ (open flow) at different reduced velocities (Ur) from 2 to 10 based on the 1st modal natural frequency. An underwater optical measurement system was used in the current experiment to acquire both cross-flow (CF) and in-line (IL) vibration with a both spatially and temporally dense format. The results show that the existence of the half-buried larger cylinder will increase the mean drag coefficient for the smaller flexible cylinder and induce a positive mean lift coefficient pushing it away from the larger cylinder. For the smallest gap-to-diameter ratio of G∕d = 1.0 when the interaction is the strongest, experiments show that the root mean square (RMS) of the CF vibration will increase due to the appearance of the stronger higher harmonic motion compared to the open flow case. The results of the inversely calculated vortex forces reveal that the 3rd harmonic fluid force in the CF direction is even stronger than the 1st harmonic fluid force for G∕d = 1.0. In addition, it is found that there is a strong correlation between the positive lift coefficient in phase with velocity (Clv) and the counter-clockwise trajectory between the CF and the IL vibrations of the smaller flexible cylinder, even with the existence of the large half-buried main pipeline.

Experimental and Numerical Investigation of Local Scour Around Submarine Piggyback Pipeline Under Steady Current

As a new type of submarine pipeline, the piggyback pipeline has been gradually adopted in engineering practice to enhance the performance and safety of submarine pipelines. However, limited simulation work and few experimental studies have been published on the scour around the piggyback pipeline under steady current. This study numerically and experimentally investigates the local scour of the piggyback pipe under steady current. The influence of prominent factors such as pipe diameter, inflow Reynolds number, and gap between the main and small pipes, on the maximum scour depth have been examined and discussed in detail. Furthermore, one formula to predict the maximum scour depth under the piggyback pipeline has been derived based on the theoretical analysis of scour equilibrium. The feasibility of the proposed formula has been effectively calibrated by both experimental data and numerical results. The findings drawn from this study are instructive in the future design and application of the piggyback pipeline.

Hydrodynamics and vortex shedding characteristics of two tandem cylinders of different diameters in steady flow

Viscous flow past two tandem circular cylinders of different diameters is numerically investigated by using finite volume method. The diameter ratio between the two cylinders ranges from 0.1 to 1.6. The Reynolds number based on the diameter of cylinders is 800 for the large cylinder and 100 for the smallest cylinder. The spacing between two tandem cylinders ranges from 0.1 to 1.6 times the diameter of the large cylinder. Identified from the numerical results for all the cases of tandem configurations, spacing ratio* (S/d) = 1.5 is the critical point. The root mean square lift coefficient on both of the two cylinders abruptly increases at the critical point. The values of the fluctuating forces acting on the large cylinder are generally lower than those on a single cylinder. Three different wake modes are identified based on the vortex shedding characteristics behind two tandem cylinders, the one wake mode, the transitional mode and the reappearance mode. “Attachment” has been found in the transitional mode for the diameter ratio of d/D = 0.75 at the critical point. The numerical results are powerful basis for design of piggyback pipeline.

Steady current induced vibration of near-bed piggyback pipelines: Configuration effects on VIV suppression

A series of experiments on steady current induced vibration of piggyback pipelines close to a plane seabed were conducted with a hydro-elastic facility in a conventional water flume. The effects of the mass-damping parameter, the diameter ratio, the gap-to-diameter ratio, the spacing-to-diameter ratio and the position angle on the VIV response were studied. The VIV suppression for the piggyback pipeline system by the small pipe was investigated based on the analysis of the vibration amplitude and the critical reduced velocity for the onset of VIV. Comparison with the prediction with the modified Griffin plot by Govardhan and Williamson (2006) [10] shows that the peak vibration amplitude of near-wall piggyback pipelines is smaller than that for a wall-free single pipe. The configuration parameters of piggyback pipelines have significant effects on the VIV suppression. For the configuration of the small pipe above the main pipe (θ=90°), the minimum peak amplitude and the maximum critical reduced velocity occur at the spacing-to-diameter ratio of G/D≈0.25, indicating that VIV is suppressed most effectively by the small pipe at this value of G/D. For a constant value of G/D=0.25, both the minimum peak amplitude and the maximum critical reduced velocity occur at the position angle of θ≈120°.

Influence of Proximity of the Seabed on Hydrodynamic Forces on a Submarine Piggyback Pipeline Under Wave Action

Experimental investigations were mainly carried out to clarify the influence of the seabed proximity on hydrodynamic forces on a submarine piggyback pipeline under regular and irregular wave action. Nondimensional force coefficients for drag, inertia and lift on the piggyback pipeline were obtained with an equivalent diameter based on the Morison equation. The effect of the gap ratio e/D between the bottom of the large pipeline and the seabed on force coefficients of the piggyback pipeline was studied. The results indicated that the force coefficients initially decreased and then remained constant when e/D was beyond 0.5. In addition, a two-dimensional hybrid numerical model, FEM-k-ω-VOF, was applied for a numerical analysis. A comparison of numerical and experimental results showed that the calculated values of wave forces agreed well with those of the experiments and that the numerical model can be employed to predict the hydrodynamic forces on the submarine piggyback pipeline.

Influence of the Position Angle of the Small Pipeline on Vortex Shedding Flow Around a Sub-Sea Piggyback Pipeline

The vortex shedding flow past a piggyback pipeline close to a plane boundary is investigated numerically. The piggyback pipeline is comprised of a large pipeline and a small pipeline. The aim of the study is focused on the effects of the position angle of the small pipeline on the vortex shedding and the hydrodynamic forces of the pipeline bundle. The two-dimensional Reynolds-averaged Navier-Stokes equations are solved using the finite element method. The equations are enclosed by the κ–ω turbulent model. The ratio of the small pipeline diameter (d) to the large one (D) is 0.2. The Reynolds number based on the free stream velocity (U 0) and the large pipeline diameter (D) is Re = 2 × 104. Simulations are carried out for the positional angles (α) of the smaller pipeline relative to the larger one ranging from 0 to 180° with an interval of 15°, and the gaps (G) between the large pipeline and the plane boundary ranging from 0.2 to 0.6D. It is found that the critical gap ratio, below which the vortex shedding is suppressed, is a function of the positional angle of the small pipeline. The effects of the position angle of the small pipeline on the hydrodynamic forces on the pipeline system are investigated numerically.

Hydrodynamic Forces on a Large Pipeline and a Small Pipeline in Piggyback Configuration under Wave Action

Experiments on a large pipeline and a small pipeline in a piggyback configuration under regular and irregular waves are carried out. A comparison of the wave forces on the large pipeline and the small pipeline is made. The nondimensional force coefficients for the drag, inertia, and lift are obtained based on the Morison equation. The effects of the Keulegan-Carpenter (KC) number and the spacing ratio between the two pipelines on the force coefficients for the large pipeline, small pipeline, and piggyback pipeline system are investigated. The experimental results indicate that the variation tendencies of the drag coefficients and inertia coefficients with the KC number or the spacing ratio for the large pipeline, small pipeline, and piggyback pipeline system are similar. The variation tendency of the lift coefficient with the spacing ratio for the large pipeline is opposite to that for the small one.

Numerical Modeling of Local Scour below a Piggyback Pipeline in Currents

Local scour below a piggyback pipeline in steady currents is investigated numerically. A piggyback pipeline comprises two pipelines that are arranged in the so-called piggyback configuration with the small pipeline being located directly above the large pipeline. The Reynolds-averaged Navier–Stokes equations and the transport equation for suspended sediment concentration are solved using a finite element method. The bed scour profile is determined through solving sediment mass conservation equation. The numerical model is validated against experimental data available in literature on scour below a single pipeline. Computations are carried out for the diameter ratio [the small pipe diameter (d) to the larger one (D) ] of 0.2 and the gap ( G , between the two pipelines) to the large diameter ratio G∕D ranging from 0.0 to 0.5. It is found that the flow and the scour profiles are influenced significantly by the gap ratio.