Quan%fying Scour Overview:

Transcription

1 Quan%fying Scour Overview: CFD Sea state is treated as the superposion of two monochromac waves Can spaally map shear stress around the device, but cannot directly quanfy scour Different device geometries can be treated Can idenfy shear stress hotspots due to certain elements of geometry Empirical Formulaons Sea state is treated as a uniform monochromac wave Cannot spaally map shear stress around the device, but can quanfy a general scour depth Simple geometries only Global formulaons are derived from studies on cylindrical piles Local formulaons not yet addressed here Deployment observaons Limited to -3 qualitave assessments by divers over the week deployment Scour noted to beginning shortly afer deployment Greatest scour in the lager days of deployment (Regime 4) Scour was confined to the the device ends and under the caissons at depths of ~ 5 cm Accreon was noted in the mid secon along the edges ~5cm

2 Plan for Quan%fying Scour Aim: Build on Ryan s analysis Correlate CFD with empirical formulaons Check that methods are consistent in esmang shear stress associated with undisturbed flow Global Scour/Shear stresses Empirical formulaons for block geometry CFD for block geometry Relate CFD shear stress output to scour potenal CFD for caisson geometry (accounts for shear stresses that may cause local scour) Local scour formulaons using a characterisc element (like horizontal pipe in Ryan s analysis) Verify results are consistent with deployment observaons determine whether scour was more likely due to local, global, or both scour processes determine appropriate empirical formulaons for general applicaon Apply empirical formulaons to 4 condions Idenfy some representave nominal and extreme 4 condions based on empirical scour results Employ CFD to provide corresponding shear stress esmates Infer scour potenal from shear stress mapping

3 Empirical Formula%ons Text of reference: The Mechanics of Scour in the Marine Environment (Sumer and Fredsoe, 4) Formulaons based on undisturbed flow properes (nearby seabed) esmated using: H rms, T p, u m, d 5, s, ν, h, α Valid for live-bed condions ( > cr ) when undisturbed seabed velocies can cause sediment moon and contribute to the filling of scour holes downstream

4 Empirical Formula%ons (Global Scour) Empirical formulaons were derived from those for cylindrical piles under waves. K factors expand formulaon to account for sediment size, device shape, alignment with respect to waves, finite device height, and sea state exposure me. This mulstep approach is supported in text, p.9 for KC < O() scour is due to leewake vortex shedding with an onset dependent upon KC and device geometry

5 Empirical Es%mates for September 4 APEX deployment site and block geometry Text recommends using u m : H =.4 m T p = 3. s α = 58 o H =.5 m T p =. s α = 93 o But since bimodal wave superposion occasionally results in greater seabed velocies, also considered using u sig :

6 Assuming block geometry Empirical Es%mates for September 4 APEX deployment site Shields Parameters cr S (m) d 5 =. mm D =.5 m APEX = 3 o Scour Depth Equilibrium Scour Time-limited Scour Shields Parameters Day of September 4 s τ (hrs) Scour Equilibrium Time Scale Moving Average (-hour window) Day of September 4

7 Possible Alterna%ve u sig instead of u m Assuming block geometry Shields Parameters cr S (m) d 5 =. mm D =.5 m APEX = 3 o Scour Depth Equilibrium Scour Time-limited Scour Shields Parameters Day of September 4 s τ (hrs) Scour Equilibrium Time Scale Moving Average (-hour window) Day of September 4 *bimodal maximum seabed velocity is at mes closer to u sig (u sig =.5 m/s for Regime 4)

8 CFD for September 7 (Regime 4) APEX deployment site and block geometry Shields Parameters H. =.4 m T p = 3. s α = 58 o H =.5 m T p =. s α = 93 o cr

9 CFD Kinematic Shear Stress (magnitude) (m /s ) Kinematic Shear Stress (magnitude) (m /s ) -3 Shear Stress Averaged over 4 Peak Wave Periods X (m) -3 Maximum Shear Stress Time = 9 s X (m) Empirical Formulaon Shields Parameters Above: H rms =. m T p = 3. s u m =.37 m/s f w =.47 u fm =.8 m/s τ w = u fm =.3x -3 m /s Sh =. KC =.9 < 3, no scour CFD (sampled undisturbed seabed) u m ~.44 m/s (3cm above floor) τ w ~.35 x -3 m /s u fm = sqrt(τ w ) =.9 m/s Regime 4 Sept 7, 4 cr

10 *max shear stress at device seems to occur just before undisturbed maxima when gradients across the domain are high 3 Maximum Shear Stress -3 Regime 4 Regime 4 Sept 7, 4 Maximum Kinematic Shear Stress (m /s ) top % mesh values Time (s) empty domain sampled at center, 5 cm above floor What would be the more appropriate way to define U m for empirical esmates? With respect to H rms (since recommended by text assuming single-mode spectra) With respect to H sig (since bimodal spectra result in occasional higher maxima than would be seen for single-mode)

11 CFD sampling of undisturbed seabed Undisturbed Conditions (Regime 4 maximum) Undisturbed Conditions (Regime 4 maximum) Regime 4 Sept 7, Velocity Magnitude (m/s) u m ~.44 m/s (3cm above floor) Velocity Magnitude (m/s) u m ~.47 m/s (3cm above floor) mag(u) t = 9 s max shear stress at device.36 mag(u) t = s max undisturbed condions 4.e-4 Undisturbed Conditions (Regime 4 maximum) 4.e-4 Undisturbed Conditions (Regime 4 maximum) Kinematic Shear Stress Magnitude (m/s) 4.e-4 3.8e-4 3.6e-4 3.4e-4 3.e-4 3.e-4.8e-4.6e-4 τ w ~.35 x -3 m /s u fm = sqrt(τ w ) =.9 m/s Kinematic Shear Stress Magnitude (m/s) 4.e-4 3.8e-4 3.6e-4 3.4e-4 3.e-4 3.e-4.8e-4.6e-4 τ w ~.4 x -3 m /s u fm = sqrt(τ w ) =. m/s sampling along line.4e-4.4e-4.e-4 mag(wallshearstress) -5 5.e-4 mag(wallshearstress) -5 5

12 CFD Kinematic Shear Stress (magnitude) (m /s ) Undisturbed Flow Maxima t = s X (m) 5 CFD (sampled undisturbed maxima) t = t=9 u m (m/s) ~.47 ~.44 τ w (m /s ) ~.4 x -3 ~.35 x -3 u fm (m/s) ~. ~.9 u fm /u m ~.43 ~.43 Sh ~.3 ~. Empirical Formulaon Shields Parameters Shields ParametersAbove (Sept 7): H rms =. m T p = 3. s u m =.37 m/s f w =.47 u fm =.8 m/s u fm /u m =.46 Regime 4 Sept 7, 4.4. (Cylindrical pile, Ryan): H s =.66 m τ w = u fm =.3x -3 m /s 3 T 5 p = 3. 7 s Sh = *u m =.6 m/s Day of September 4 KC =.9 < 3, no scour *f w =. *u fm =. m/s u fm /u m =.33 τ w = u fm =.4x -3 m /s Sh =.3 KC = 3. < 6, no scour cr s Alternave (Above, but u m = u s ): H s =.66 m T p = 3. s u m =.5 m/s f w =.4 u fm =.4 m/s u fm /u m =.46 τ w = u fm =.65x -3 m /s Sh =.8 KC =.7 < 3, no scour

13 Scour was noted by divers for caisson geometry, especially for Regime 4 condi%ons How do we relate empirical scour esmates to shear stress map? Regime 4 Sept 7, 4 mag(u) mag(wallshearstress) m/s.3 mag(u) mag(wallshearstress) m/s Regime 4 Time: 9. Regime 4 Time: 9. Notable hotspots from Plunging over the caissons Interior caisson corners (interior vortex?)

14 Regime 4 CFD: Average shear stress of caisson geometry versus block geometry Sept 7, 4 Since shear stresses are generally smaller for caisson geometry, could we infer that local scour would be less significant than global scour in this instance? Or perhaps things would look different if we averaged shear stress components instead of overall magnitudes -3 Shear Stress Averaged over 4 Peak Wave Periods. Caisson geometry (local and global shear stress) Block geometry empirical global scour esmates apply -3 Shear Stress Averaged over 4 Peak Wave Periods. Kinematic Shear Stress (magnitude) (m /s ) Kinematic Shear Stress (magnitude) (m /s ) X (m) X (m)