Fuhrman, David R.

Larsen, Bjarke Eltard

Sumer, B. Mutlu

Scour beneath submarine pipelines has been the subject of much past research see eg. Sumer and Fredsøe (2002).To date most research, both numerical and experimental, has focused on scour induced by either pure waves or currents, while comparatively few studies have involved combined wave-plus-current environments. The present study, which is published in Larsen et al. (2016) focuses on the numerical simulation of wave-plus-current induced scour beneath submarine pipelines, based on a model solving Reynolds-averaged Navier-Stokes (RANS) equations, fully coupled with turbulence closure, bed and suspended load sediment transport descriptions, and a seabed morphological model. The model was utilized in simulating breaker bar development by Jacobsen et al. (2014) and has been used in simulating wave induced scour beneath pipelines by Fuhrman et al. (2014) .The model is utilized for the numerical study of combined wave-plus-current scour processes beneath pipelines. The results of 77 simulated wave-plus-current scour cases will be presented and analysed. The cases considered will consist of waves characterized by 10 different Keulegan-Carpenter numbers, KC=UmTw/D and up to eight different values of m=Uc/(Uc+Um) which defines the relative strength of the current i.e. m=0 corresponds to pure-wave conditions, with m=1 corresponding to pure-current conditions. The resulting equilibrium scour depths are shown to be in accordance with existing experimentally-based expressions. In Figure 1 the time scale of the scour process is compared to the existing empirical formula from Fredsøe et al. (1992) for the time scale governing both the wave and current-induced scour, T*=1/50 θ -5/3. In Figure 1 θcw can be interpreted as the maximum Shields parameter of the combined wave-current flow. As can be seen the modelled time scales generally agree well for the pure wave and pure current cases whereas the time scales for combined waves and current are generally larger. The time scales are systematically reinvestigated and a new general expression for the time scale is proposed.This expression is fully consistent with existing experimentally based relations at both pure-current and pure-wave limits, and is appropriate for engineering use. In Figure 2 the model results are compared to the new general expression which is of the form T*=Г(m) θcw-5/3, where Г(m) is a relatively simple close form expression. The tight clustering, compared with Figure 1 around the full line in Figure 2, demonstrates that the generalized expression effectively unites the time scales for pure-current, pure-wave, as well as the combined wave-plus-current flows.<br/

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MutluPublication date:2016Document VersionPeer reviewed versionLink back to DTU OrbitCitation (APA):Eltard-Larsen, B., Fuhrman, D. R., & Sumer, B. M. (2016). Simulation of Wave-Plus-Current Induced ScourBeneath Submarine Pipelines. Abstract from 35th International Conference on Coastal Engineering (ICCE'16),Antalya, Turkey.   SIMULATION OF WAVE-PLUS-CURRENT INDUCED SCOUR BENEATH SUBMARINE PIPELINES   Bjarke Eltard Larsen, Technical University of Denmark, bjelt@mek.dtu.dk David R. Fuhrman,Technical University of Denmark, drf@mek.dtu.dk B. Mutlu Sumer, Technical University of Denmark, bms@mek.dtu.dk  Scour beneath submarine pipelines has been the subject of much past research see eg. Sumer and Fredsøe (2002).To date most research, both numerical and experimental, has focused on scour induced by either pure waves or currents, while comparatively few studies have involved combined wave-plus-current environments. The present study, which is published in Larsen et al. (2016) focuses on the numerical simulation of wave-plus-current induced scour beneath submarine pipelines, based on a model solving Reynolds-averaged Navier-Stokes (RANS) equations, fully coupled with turbulence closure, bed and suspended load sediment transport descriptions, and a seabed morphological model. The model was utilized in simulating breaker bar development by Jacobsen et al. (2014) and has been used in simulating wave induced scour beneath pipelines by Fuhrman et al. (2014) .  The model is utilized for the numerical study of combined wave-plus-current scour processes beneath pipelines. The results of 77 simulated wave-plus-current scour cases will be presented and analysed.  The cases considered will consist of waves characterized by 10 different Keulegan-Carpenter numbers, KC=UmTw/D  and up to eight different values of m=Uc/(Uc+Um) which defines the relative strength of the current i.e. m=0 corresponds to pure-wave conditions, with m=1 corresponding to pure-current conditions. The resulting equilibrium scour depths are shown to be in accordance with existing experimentally-based expressions. In Figure 1 the time scale of the scour process is compared to the existing empirical formula from Fredsøe et al. (1992) for the time scale governing both the wave and current-induced scour, T*=1/50 θ -5/3. In Figure 1 θcw can be interpreted as the maximum Shields parameter of the combined wave-current flow.  As can be seen the modelled time scales generally agree well for the pure wave and pure current cases whereas the time scales for combined waves and current are generally larger. The time scales are systematically reinvestigated and a new general expression for the time scale is proposed. Figure 1 – Time scale of the scour development. This expression is fully consistent with existing experimentally based relations at both pure-current and pure-wave limits, and is appropriate for engineering use. In Figure 2 the model results are compared to the new general expression which is of the form T*=Г(m) θcw-5/3, where Г(m)  is a relatively simple close form expression. The tight clustering, compared with Figure 1 around the full line in Figure 2, demonstrates that the generalized expression effectively unites the time scales for pure-current, pure-wave, as well as the combined wave-plus-current flows.   Figure 2 – Time scale of the scour development.  AKNOWLEDGEMENTS The authors acknowledge support from the European Union project ASTARTE--Assessment, Strategy And Risk Reduction for Tsunamis in Europe, Grant no.~603839 (FP7-ENV-2013.6.4-3).The third author additionally acknowledges Innovative Multi-purpose Offshore Platforms: Planning, Design and Operation (MERMAID), 2012–2016, Grant Agreement No. 288710 of European Commission, 7th Framework Programme for Research.  REFERENCES Fredsøe, J., Sumer, B. M., Arnskov, M. M., (1992). Time scale for wave/current scour below pipelines. Int. J. Offshore Polar Eng. 2, 13–17. Fuhrman, D. R., Baykal, C., Sumer, B. M., Jacobsen, N. G., Fredsøe, J., (2014). Numerical simulation of wave-induced scour and backfilling processes beneath submarine pipelines. Coast. Eng. 94, 10–22. Jacobsen, N. G., Fredsøe, J., Jensen, J. H., (2014). Formation and development of a breaker bar under regular waves. Part 1: Model description and hydrodynamics. Coast. Eng. 88, 182–193 Larsen, B. E., Fuhrman, D.R., Sumer, B.M, (2016). Simulation of Wave-Plus-Current Scour beneath Submarine Pipelines. J. Waterway, Port, Coastal, Ocean Eng.- ASCE, article no. 04016003 Sumer, B. M., Fredsøe, J., (2002). The Mechanics of Scour in the Marine Environment. World Scientific, Singapore 

Simulation of Wave-Plus-Current Induced Scour Beneath Submarine Pipelines

Bjarke Eltard Larsen

David R. Fuhrman

B. Mutlu Sumer

Crossref

Journal of Waterway Port Coastal and Ocean Engineering

Simulation of Wave-Plus-Current Scour beneath Submarine Pipelines

A fully coupled hydrodynamic and morphologic numerical model was utilized for the simulation of wave-plus-current scour beneath submarine pipelines. The model was based on incompressible Reynolds-averaged Navier–Stokes equations, coupled with k-ω turbulence closure, with additional bed and suspended load descriptions forming the basis for seabed morphology. The model was successfully validated against experimental measurements involving scour development and eventual equilibrium in pure-current flows over a range of Shields parameters characteristic of both clear-water and live-bed regimes. This validation complements previously demonstrated accuracy for the same model in simulating pipeline scour processes in pure-wave environments. The model was subsequently utilized to simulate combined wave-plus-current scour over a wide range of combined Keulegan–Carpenter numbers and relative current strengths. The resulting equilibrium scour depths and trends were shown to be in accordance with existing experimentally based expressions from the literature. The variety of scour profile types emerging under various flow conditions is detailed and reconciled with experimental observations. The resulting matrix of scour depth time series was systematically analyzed, resulting in a new generalized expression for the scour time scale in combined wave-plus-current flow environments. This expression is fully consistent with existing experimentally based relations at both pure-current and pure-wave limits and is appropriate for engineering use

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Simulation of Wave-Plus-Current Scour beneath Submarine Pipelines

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Online Research Database In Technology