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Scour at wind turbine tripod foundation under steady flow
Ocean Engineering 141 (2017) 277–282
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Scour at wind turbine tripod foundation under steady flow Chunguang Yuan a b c
a,b,⁎
c
MARK
c
, Bruce W. Melville , Keith N. Adams
Tianjin Research Institute for Water Transport Engineering, M.O.T., Tianjin 300000, China Hohai University, College of Harbour, Coastal and Offshore Engineering, Nanjing 210098, China The University of Auckland, Department of Civil and Environmental Engineering, Auckland, New Zealand
A R T I C L E I N F O
A BS T RAC T
Keywords: Tripod Scour depth Steady flow Clear-water scour Wind turbine foundations
Results of a physical modelling study of clear-water scour at a tripod obstacle in uniform bed sediment under steady flow conditions are presented. 4 flow depths, 2 flow velocities and 3 angles of attack were considered. Maximum scour depth increased with increase in flow depth or velocity. For deep flows and high flow rate, the maximum value of the ratio of the equilibrium scour depth to the diameter of the tripod’s leg was 3.5, which is significantly greater than values of 1.3 or 2.5 suggested by guidelines for offshore wind turbine monopiles design. This ratio may be even larger because of the increased shear stress when waves are superimposed on current. All maximum equilibrium scour depths occurred at the upstream surface of a tripod leg despite the centre pile having a larger diameter. The alignment angle had a small effect on scour depth. But it is notable that there is an increase in scour development depth at 60° alignment angle. Equations are suggested for estimating the clear-water scour depth at wind turbine tripod foundations.
1. Introduction As an important source of clean, renewable energy, the offshore wind industry is currently experiencing worldwide development. Five main types of foundation structures are common, the choice depending on water depth and flow conditions: monopiles, gravity-based, tripods, jackets and floating. Monopiles and gravity-based foundations are preferred for shallow waters ( < 25 m). For larger depths, XXL monopiles (diameter 8–10 m), tripods, jackets, and floating platforms may be reasonable choices. It is well known that local scour around the foundations of a hydraulic structure can lead to damage or even to overall failure. Several design guidelines for offshore wind turbines provide details for the design of the entire wind turbine. However, many of them (e.g., BSH, 2007, and DIN, 2009) do not provide any explicit design guidance on scour depth estimation for different types of wind turbine foundations. Available guidance on maximum scour depth at a monopile foundation ranges from 1. 3D (DNV, 2013) to 2. 5D (GL, 2005). In fluvial environments, the scour mechanisms for a circular cylinder in clear-water and live-bed are well known from the results of many experiments (e.g., Briaud et al., 1999; Chiew, 1984; Dey et al., 1995; Ettema, 1980; Melville and Coleman, 2000; Jones and Sheppard, 2000; Sumer and Fredsøe, 2002). But new problems arise for offshore wind turbine foundations because they often have complex structural shapes. Whitehouse et al. (2008) and Harris et al. (2010) carried out
⁎
Corresponding author. E-mail address: [email protected] (C. Yuan).
http://dx.doi.org/10.1016/j.oceaneng.2017.06.038 Received 12 October 2016; Received in revised form 28 April 2017; Accepted 17 June 2017 0029-8018/ © 2017 Elsevier Ltd. All rights reserved.
experiments on wind turbine foundation scour in a coastal environment, but for monopiles. The results of circular cylinder foundation research cannot be directly applied to more complex foundations because foundation shape itself has an effect on scour depth. To the present, apart from work by Stahlmann and Schlurmann (2010) and Stahlmann (2013, 2014), there has been no systematic physical experiments on scour around tripod foundations in a steady current. This study investigated the effect of flow depth, flow velocity and tripod alignment on the maximum scour depth. The combination effect of current-wave on scour depth will be discussed in the future. 2. Experimental setup and data Experiments were carried out in the Hydraulic Engineering Laboratory of the Department of Civil and Environmental Engineering, University of Auckland, using a 2.4 m wide, 0.3 m deep and 16.5 m long flume. Uniform sand with median size d50 = 0. 85mm , specific gravity of 2.65 and geometric standard deviation σg = 1. 3 was used as the bed sediment material for all scour experiments. All sediment was placed in a 2.8 m long 0.45 m deep recess which was located 7 m downstream of the inlet tank. The tripod model, based on tripod foundations of prototype wind turbines recently established in the East China Sea, was manufactured with a scale of 1:60. A circular cylinder with the same diameter and length as the tripod leg was also considered in some experiments. The
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Fig. 1. Model details: (a) top view, (b) front view, (c)–(e) the three orientations investigated, (f)–(g) lateral scour extent and red dots showing the locations of the scour depth measurement points. And red dots showing the locations of the scour depth measurement points, α is the angle between Leg 1 and the flow direction.
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Because only one kind of uniform sand (d50 = 0.85mm ) was used in this study, Eq. (1) can be simplified to
Table 1 Experimental conditions and results of scour experiments. No.
t (h)
V/Vc
y (m)
α (°)
ds (mm)
ds /D
E /R
POMSD
Code
ds / D = dsmax / D∙KI Ky Kα = CKI Ky Kα
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25C 26C 27C 28C
30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9
0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10
60 60 60 60 60 60 60 60 30 30 30 30 30 30 30 30 0 0 0 0 0 0 0 0 – – – –
172 140 127 80 40 26 17 13 158 140 138 91 56 26 17 16 149 130 130 88 40 30 18 14 75 92 93 82
3.2 2.50 2.35 1.48 0.74 0.48 0.31 0.24 2.93 2.59 2.56 1.59 1.04 0.48 0.31 0.30 2.75 2.41 2.40 1.63 0.74 0.56 0.33 0.26 1.39 1.70 1.72 1.52
2.07 1.94 1.78 1.46 1.19 1.39 1.01 1.00 2.05 2.01 1.99 1.71 1.19 1.14 1.14 1.12 1.83 1.75 1.64 1.52 1.43 1.23 1.19 1.15 0.69 0.71 0.69 0.68
2 2 1 or 1 or 1 or 1 or 1 or 1 or 3 3 3 3 3 3 3 3 2 or 2 or 2 or 2 or 2 or 2 or 2 or 2 or – – – –
60_HF_25 60_HF_20 60_HF_15 60_HF_10 60_LF_25 60_LF_20 60_LF_15 60_LF_10 30_HF_25 30_HF_20 30_HF_15 30_HF_10 30_LF_25 30_LF_20 30_LF_15 30_LF_10 0_HF_25 0_HF_20 0_HF_15 0_HF_10 0_LF_25 0_LF_20 0_LF_15 0_LF_10 HF_25 HF_20 HF_15 HF_10
where ds max is the maximum equilibrium scour depth that could reasonably be expected for this experimental setup, as defined later in this paper.
3 3 3 3 3 3
3 3 3 3 3 3 3 3
(2)
3.1. Equilibrium scour position From Table 1, for all experiments, the maximum scour depths always occurred at the upstream surface of a leg, despite the centre pile having the largest diameter of all the elements in the structure. Because the centre pile’s base is level with the unscoured surface (just below the underside of the horizontal beams) most of the downflow passes downstream through the newly generated scour hole beneath the pile. As a result, less energy is imparted to the horseshoe vortex system and the scour hole is shallower. With α = 0°, because of the bilateral symmetry of the tripod foundation along the flow direction, the maximum scour depth always occurred at a downstream leg (Leg 2 or 3). With α = 30°, the lateral distance between the centre pile and Legs 1 and 2 was a minimum, and the obstruction effect of the pile group in this region was significantly greater than that at Leg 3, and flow was redirected towards Leg 3. Therefore, with α = 30°, the maximum scour depth always occurred at Leg 3. With α = 60°, the situation was more complicated. For low flow intensity (0. 5Vc ), the maximum scour depth always occurred at an upstream leg (Legs 1 or 3). For high flow intensity, the position of the maximum scour depth depended on the flow depth: for low flow depth the maximum scour depth occurred at an upstream leg (Legs 1 or 3); for high flow depths, maximum scour depth occurred at a downstream leg (Leg 2). With α = 60°, as the flow depth increased, submergence of the legs and angled struts increased, and a larger amount of near-surface flow was forced to change its direction into downflow, resulting in deepening of the scour hole at the centre pile and at the downstream leg. Because the maximum scour depth always occurred at a leg, it is reasonable and convenient to normalize the scour depth (ds ) with the tripod leg diameter (D = 54mm ).
Note: t is the scour time, V is the mean flow velocity, Vc is the threshold velocity of bed sediment material motion, y is the flow depth, ds is the maximum scour depth which includes both local scour and global scour, E is the lateral extent of global scour at 0.9Vc for tripod model and the lateral extent of local scour for the circular cylinder only which was measured from the centre of the centre pile or the circular cylinder to the edge of the scour hole, R (in all cases) equals 262 mm, the distance between the centre of the centre pile and the outer edges of the legs (Fig. 1), and POMSD indicates the leg at which the maximum scour depth occurred. The suffixal ‘C’ in a test number indicates that the experimental model was only a circular cylinder with the same diameter as the tripod leg.
geometry of the model and the locations of the scour depth measurement points are shown in Fig. 1. All legs go all the way to the recess bottom. All experimental conditions and results of the tests are shown in Table 1 below. Because of the complex structural shape, all scour depths at red-dots positions were measured manually with the help of a ruler and scale marks painted on the model surface after experiments were finished. The red-dots were positioned at 0°, ± 90° and 180° from flow direction for the legs and the centre pile and their connection points with beams. And the scour depth of the beam was measured in the mid-point. Because of the typical dimensions of laboratory flumes as well as the lower limit of cohesionless sediment size, it is difficult to satisfy the similitude requirements relating scour in the flume to scour in reality. Experiments carried out in this study were for determining the possibility of scour at tripod foundations in non-ripple-forming bed material and relationships between scour depth and the main variables influencing it.
3.2. Temporal evolution of the scour depth According to Melville and Chiew (2000), for clear-water scour at bridge piers, the time to equilibrium scour depth is proportional to both mean flow velocity and depth. Therefore, experiments with the largest flow depth (0.25 m), the highest flow intensity (0.9Vc ), and α = 60° were selected to determine the time for development of equilibrium maximum scour depth, ds , for the tripod model. Other scour experiments with lower flow intensity would be expected to reach their equilibrium scour depth sooner. In Fig. 2, the maximum scour depths (ds ) normalized by the diameter of the leg (D ) are plotted against time for each component of the tripod. Unlike the field data measured at Alpha Ventus (Stahlmann, 2013), where the deepest position was found very close
3. Analysis of the experimental results Following Melville and Sutherland (1988), an expression for the equilibrium scour depth ds for a tripod in a steady current can be written as
ds / D = KI Ky Kd Kσ Kα
(1)
where ds is the maximum scour depth, D is the diameter of tripod’s legs, and the Ks are expressions describing effects of different variables: KI for flow intensity, Ky for flow depth, Kd for sediment size, Kσ for sediment gradation and Kα for orientation.
Fig. 2. Time history of scour depth for downstream leg, upstream legs, centre pile and beams (alignment angle 60°, D = 54 mm).
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to the centre pile, the maximum scour depth in the steady current experiments in this study always occurred at a leg for both 0.5Vc and 0.9 Vc . It may be that the occurrence of waves causes the maximum scour depth to be near the centre pile. For 0. 9Vc and alignment angle 60°, the maximum scour depth position transferred from an upstream leg to a downstream leg at about 12 h after the experiment started. For a two-pile in tandem arrangement, Hannah (1978) found that scour at the front pile was always greater than at the rear one. But, for alignment angle α = 60° in this study, the opposite occurred at the centre pile and downstream leg. In fact scour depth at the centre pile was always less than at legs for all experiments in this study, because of the centre pile’s non-inserted root. In a study of three-pile group scour in current and in waves (Sumer and Fredsøe, 1998), scour depth at the downstream leg was always less than at the upstream legs, which was also contrary to the results from this study. When flow intensity is weak, because most flow could pass through the structure, the horseshoe vortex is relatively small and weak and the scour below the centre pile is shallow. Some of the sand eroded from the upstream area is deposited in the vicinity of the downstream leg. The shielding effect of the centre pile also protects the downstream leg from scouring. Therefore, the maximum scour depth is located at the same places as for the 2-pile group at the same alignment angle (i.e., the scour depth is larger at the front pile than at the rear one). When the power of flow increases, the sediment deposited around the rear leg can be flushed away completely. Because the bottom of the centre pile is at the sediment surface, when the scour hole is deep enough, the centre pile and the downstream leg appear to act as an integral unit in the streamline direction. The equivalent diameter of the combination is much greater than that of an upstream leg. In addition, the upstream legs would be submerged at higher water level, which mitigates the scour depth development. For high flow rate (0.9Vc ) in this study, scour holes of the individual components merged together, influenced by the slope of the deeper scour holes at the legs and the scour depth under the beams (which are larger than centre pile). However, for weak flow (0.5Vc ), because the scour holes were separated from each other, there was little scour under the beams, and sometimes several small aggradation mounds were observed. The time for equilibrium scour holes to form in every part of the tripod model was about 30 h. It should be noted that the maximum scour depth measured in this study was about 3. 2D , where D is the diameter of the leg. This is significantly larger than the 1. 3D or 2. 5D recommended in guidelines for monopiles. This amplification is probably due to the pile group effect of the centre pile, the angled struts and the beams near the bed. A substantial amount of water is diverted to the gap between the underside of the beams and the bed, leading to very large velocity in this area and presumably resulting in large shear stress and increased scour depth. The ratio may be even larger in wave-current conditions, because the bed material is more easily entrained by the increased bed shear stress and is transported away from the tripod foundation by the current.
Fig. 3. Flow depth effect on normalized scour depth (the scour depth of circularcylinder-only is normalized by the maximum scour depth in this study namely 3.2D).
In this study, D / d50 is high (for the tripod leg D / d50 = 64 , for the centre pile Dm / d50 = 84 , and if the pile group effect of the tripod is taken into considerationD / d50 would be even larger), and y / D is greater than 1 in all experiments, so results of previous research for bridge piers suggest that, in this study, flow depth should not affect scour depth. However, a significant increase of scour depth occurred when y / D increased from 1.85 to 4.63, this indicates that there is a clear difference between tripod scour and single pile scour. The scour depth for circular cylinder only is similar to the case of alignment angle 60° under low flow depth. Because the circular-cylinder-only model was submerged when flow depths reached 0.2 m and 0.25 m, its scour depth first increased then decreased with flow depth increase (cf. Dey et al., 2008). It is evident that the pile group effect of the tripod foundation may play an important role in how scour depth changes with flow depth. The flow depth factor, Ky , in Eq. (2) is defined as Ky = ds ( y / D )/ ds ( y / D = 4.63) . Fig. 3 shows the equilibrium scour depths for V / Vc = 0.9; the scour depths are normalized by the maximum equilibrium scour depth for this study. It is expected that scour depths would be slightly larger for V / Vc = 1.0 (Chee, 1982), so an appropriate expression for Ky should bracket the normalized scour depth values in Fig. 3,and have a value of 1 at y / D = 5.0 , and a value of 0 at y / D = 0 . For design purposes, the expression recommended for Ky is (Fig. 3),
Ky = 1 − exp (−0. 8y / D )( y / D<5. 0)
(3)
3.4. Approach flow velocity effects The flow continuously supplies energy to horseshoe vortices near the bed and to lee-wake vortices, and the bed shear stress around the structure may far exceed the undisturbed mean bed shear stress. For a single pile, local scour is initiated when the approaching mean flow velocity V exceeds about 0.5 times the threshold velocity Vc of the sediment. For 0. 51, livebed local scour occurs, with sediment transport occurring over the entire bed and the scour hole is continually supplied with sediment by the approach flow. With increasing flow velocity, the scour depth reaches two local maxima: the first at V / Vc = 1, and the second at the range of velocities producing a transition flat-bed; sediment transport into the scour hole decreases the scour hole depth between the two peak values. (Chee, 1982). In this study, as the flow velocity increased, the influence of water depth on the increase in equilibrium scour became larger. Two flow intensities (V / Vc = 0.5 and V / Vc = 0.9 ) were used in the experiments. The scour depths for V / Vc = 0.9 were larger than for V / Vc = 0.5, and the increases in scour depths (for a 0.15 m increase in flow depth) were also larger for V / Vc = 0.9 . The flow intensity factor, KI , in Eq. (2) is defined as KI = ds (V )/ ds (V = Vc ). Fig. 4 shows the relationship between scour depth ds and flow intensity V / Vc for flow depth y = 0.25m (i.e. for
3.3. Influence of flow depth on equilibrium scour depth of tripod For bridge piers, Chiew (1984), Ettema (1980), Melville and Coleman (2000), Jones and Sheppard (2000), and others found that scour depth increases with flow depth, up to a limiting value of the flow depth ratio y / D , beyond which there is no influence of flow depth. In addition, Ettema (1980) concluded that the influence of flow depth is affected by the relative sizes of the pier and sediment, D / d50 . He found that for high values of D / d50 , the scour depth was almost independent of flow depth wheny / D > 1, while for low values of D / d50 it was still dependent on flow depth for values of y / D as high as 6. Jones and Sheppard (2000) proposed that the maximum relative clear water scour occurred when D / d50 is about 46, and that, for constant values of y / D and flow intensity, V/ Vc , scour depth was less for D / d50 values larger or smaller than 46. 280
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Fig. 4. Effect of flow intensity on equilibrium scour, for 0.25 m flow depth.
Ky = 1). From the best fit line through the data points, dsmax , the maximum equilibrium scour depth that could reasonably be expected for this experimental setup (i.e. ds for V / Vc = 1), is 188 mm. The value of C in Eq. (2) becomes 3.5. The normalized scour depths ds / dsmax are plotted against V / Vc in Fig. 5. Because only two flow intensities were considered in this study, a linear relationship was assumed between the normalized scour depth ds / dsmax and relative velocity V / Vc , which is in agreement with the present knowledge of clear-water scour (Chiew, 1984; Ettema, 1980; Melville and Coleman, 2000). The best fit line gives the equation for KI as KI = 1. 52V / Vc−0. 52(0. 35
Fig. 6. Effect of alignment on normalized scour depth, for V /Vc = 0.9 .
mainly waves acting on a physical model of a tripod foundation, the results of small-scale tests indicated that an alignment of α = 30°produced the smallest scour depth. And for large-scale experiments, Stahlmann found that an alignment of α = 60° produced a much greater equilibrium scour depth than 0°. In this study, alignments of α = 0° usually produced a lower value of scour depth than other alignment angles. However, in a marine environment, the flood and ebb tides have opposite flow directions. If the tripod is aligned with α = 0° for minimum scour during the flood tide, it will experience the maximum scour during the ebb tide, for which the effective alignment will have changed to α = 60°. More attention needs to be paid to cases of high flow intensity and flow depth at 60° alignment angle.
(5)
This equation suggests that the flow intensity for the initiation of scour at a tripod is for a V / Vc value of approximately 0.35, which is similar to the initiation flow intensity proposed by Melville and Coleman (2000). Because only two flow intensities were considered in this study, Eq. (5) should be applied with discretion. 3.5. Alignment effects The scour depth is related to the strength of the downflow at the upstream surfaces of the elements of the tripod, the former depending on the amount of flow blockage caused by the tripod at different alignment angles. For α as defined in Fig. 1, the blockage effect (and hence the strength of the downflow and horseshoe vortex) increases as α increases. Fig. 6 shows, for V / Vc = 0.9 , the relationship between the normalized equilibrium scour depth and alignment angle, α . It can be seen, from Fig. 6 and Table 1, that α has only a small influence on equilibrium scour depth. In addition, the trend of the relationship between scour depth and α is different for different flow depths. Therefore, it is conservatively recommended that a value of 1.0 be adopted for Kα . However, other factors have to be considered when deciding on an appropriate orientation. In a study by Stahlmann (2013) involving
3.6. Lateral extent of the scour At high flow intensity (0. 9Vc ), all the individual scour holes at the foundation components join up in the form of a scour hole. Also, the lateral extent of scour increases linearly with increase in the maximum scour depth ds (Fig. 7). The best fit line gives the equation for E as
⎛d ⎞ E = 1. 5 ⎜ s ⎟ +1 ⎝R⎠ R
(6)
where R is defined as the distance between the centre of the centre pile and the leg surface (Fig. 1).
Fig. 5. Effect of flow intensity on normalized scour depth, for 0.25 m flow depth.
Fig. 7. Lateral extent of scour.
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supported by the Cultivation of Jiangsu Province Graduate Innovation Project (grant numbers KYZZ_0151) and Natural Science & Technology Pillar Program (grant number 2012BAB03B01).
4. Conclusions The following conclusions can be drawn from this study:
References
(1) Although the centre pile has a larger diameter than the tripod legs, the maximum scour depths of all tripod scour experiments were found at a leg for the clear-water, current-only scour cases. The specific locations of the equilibrium scour depth were influenced by the alignment angles of the tripod, flow intensity, and flow depth. The occurrence of waves could result in the maximum scour depth being located near to the centre pile. (2) The ratio of the maximum scour depth to the diameter of the tripod leg, dsmax / D , reached 3.2 in this study for V / Vc = 0.9 , and analysis suggests that this ratio would be 3.5 for V / Vc = 1.0 . Because of the extra turbulence and pile group effect of the complex structural shapes, this ratio is significantly larger than that in available guidance for monopile foundations, which ranges from 1. 3D (DNV 2014) to 2. 5D (GL, 2005). (3) Similarly to the scour around a single pile, the equilibrium scour depth of a tripod increased with flow depth and flow intensity (at the approach section). The relationship between the equilibrium scour depth and the tripod alignment is imprecise. (4) For high flow depth and intensity, the scour depth tended to increase with the alignment angle. Alignment angle α = 60° should be avoided to the high flow velocity and strong waves as possible. (5) For estimating scour depth ds / D where D is the pile diameter of a tripod leg, it is suggested Eq. (2) (with C = 3.5) can be combined with Eq. (3) for KI , Eq. (5) for Ky and Kα =1 to estimate the clearwater equilibrium scour depth. And the lateral extent of the scour hole can be computed by Eq. (6).
Briaud, J.-L., Ting, F.C.K., Chen, H.C., Gudavalli, R., Perugu, S., Wei, G., 1999. SRICOS: prediction of scour rate in cohesive soils at bridge piers. J. Geotech. Geoenvironmential Eng., ASCE 125 (4), 237–246. BSH, 2007. Hydrographie: Standard – Konstruktive Ausführung von OffshoreWindenergieanlagen. No. 7005. Bundesamt fur Seechiffahrt und Hydrographie, Hamburg. Chee, R.K.W., 1982. Live-bed Scour at Bridge Piers (Report No.290, (Report No.290, Ph.D. Thesis. University of Auckland, Auckland. Chiew, Y.M., 1984. Local Scour at Bridge Piers Report No. 355, Report No. 355, Ph.D. Thesis. University of Auckland, Auckland, Report No. 355, Report No. 355, Ph.D. Thesis〈http://hdl.handle.net/2292/2520〉. Dey, S., Bose, S.K., Sastry, G.L.N., 1995. Clear water scour at circular piers: a model. J. Hydraul. Eng. 131 (12), 1126–1135. Dey, S., Raikar, R.V., Roy, A., 2008. Scour at submerged cylindrical obstacles under steady flow. J. Hydraul. Eng. 134 (1), 105–109. DIN, 2009. DIN EN 61400-3. Windenergieanlagen – Teil 3: AuslegungAnforderungen für Windenergieanlagen auf offener See (IEC 61400-3:2009). Deutsches Istitutfür Normung e.V. DNV, 2013. Design of Offshore Wind Turbine Structures. Offshore Standard DNV-OSJ101, Det Norske Veritas AS, Høvik. Ettema, R., 1980. Scour at Bridge Piers (Report No. 215. (Report No. 215. Ph.D. Thesis. University of Auckland, Auckland. GL, 2005. Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd Wind Energie GmbH, Hamburg. Hannah, C.R., 1978. Scour at Pile Groups No. 78–3. University of Canterbury N.Z., 92. Harris, J.M., Whitehouse, R.J.S., Benson, T., 2010. The time evolution of scour around offshore structures. Marit. Eng. 163 (1), 3–17. Jones, J.S., Sheppard, D.M., 2000. Scour at wide bridge piers. In: Hotchkiss, R.H., Michael Glade, M. (Eds), Building Partnerships: Joint Conference on Water Resources Engineering and Water Resources Planning and Management, ASCE, Minneapolis, pp. 1–10. Melville, B.W., Chiew, Y.M., 2000. Time scale for local scour at bridge piers. J. Hydraul. Eng. 126 (10), 59–65. Melville, B.W., Coleman, S.E., 2000. Bridge Scour. Water Resources Publications, LLC, Highlands Ranch, CO. Melville, B.W., Sutherland, A.J., 1988. Design method for local scour at bridge piers. J. Hydraul. Eng. 114 (10), 1210–1226. Stahlmann, A., 2013. Experimental and Numerical Model of Scour at Offshore Wind Turbines. Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering. Leibniz Universität Hannover, Hannover, Germany. Stahlmann, A., 2014. Experimental and numerical model of scour at offshore wind turbines. J. Ocean Wind Energy 1 (2), 82–89. Stahlmann, A., Schlurmann, T., 2010. Physical Modeling of Scour around Tripod Foundation Structures for Offshore Wind Energy Converters. Proceedings of the International Conference on Coastal Engineering, NO. 32, Shanghai, China, 1-12. Sumer, B.M., Fredsøe, J., 1998. Wave scour around group of vertical piles. J. Waterw., Port., Coast. Ocean Eng. 124 (5), 248–256. Sumer, B.M., Fredsøe, J., 2002. The mechanics of scour in the marine environment. World Scientific, Singapore. Whitehouse, R., Harris, J. Sutherland, J., Rees, J., 2008. An assessment of field data for scour at offshore wind turbine foundations. 4th International Conference on Scour and Erosion, Tokyo, pp. 1–8.
All the conclusions above are derived from tripod scour experiments; this has two consequences. First, for the same bed material, the higher Froude number resulting from scaling effects in the model can lead to a scour depth larger than that for a prototype tripod foundation with the same V/Vc; this is an inherent in laboratory flume experiments. Second, for finer non-cohesive bed material, because of the lower incipient velocity, the scour depth could be even larger. Therefore, the upper-bound scour depth estimate from the suggested equations can be regarded as a possible and conservative method of scour depth prediction for coarse bed material, but the estimate is more likely to be achieved for finer non-cohesive sands. Acknowledgements This study was carried out at the University of Auckland and
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Short communication
Scour at wind turbine tripod foundation under steady flow Chunguang Yuan a b c
a,b,⁎
c
MARK
c
, Bruce W. Melville , Keith N. Adams
Tianjin Research Institute for Water Transport Engineering, M.O.T., Tianjin 300000, China Hohai University, College of Harbour, Coastal and Offshore Engineering, Nanjing 210098, China The University of Auckland, Department of Civil and Environmental Engineering, Auckland, New Zealand
A R T I C L E I N F O
A BS T RAC T
Keywords: Tripod Scour depth Steady flow Clear-water scour Wind turbine foundations
Results of a physical modelling study of clear-water scour at a tripod obstacle in uniform bed sediment under steady flow conditions are presented. 4 flow depths, 2 flow velocities and 3 angles of attack were considered. Maximum scour depth increased with increase in flow depth or velocity. For deep flows and high flow rate, the maximum value of the ratio of the equilibrium scour depth to the diameter of the tripod’s leg was 3.5, which is significantly greater than values of 1.3 or 2.5 suggested by guidelines for offshore wind turbine monopiles design. This ratio may be even larger because of the increased shear stress when waves are superimposed on current. All maximum equilibrium scour depths occurred at the upstream surface of a tripod leg despite the centre pile having a larger diameter. The alignment angle had a small effect on scour depth. But it is notable that there is an increase in scour development depth at 60° alignment angle. Equations are suggested for estimating the clear-water scour depth at wind turbine tripod foundations.
1. Introduction As an important source of clean, renewable energy, the offshore wind industry is currently experiencing worldwide development. Five main types of foundation structures are common, the choice depending on water depth and flow conditions: monopiles, gravity-based, tripods, jackets and floating. Monopiles and gravity-based foundations are preferred for shallow waters ( < 25 m). For larger depths, XXL monopiles (diameter 8–10 m), tripods, jackets, and floating platforms may be reasonable choices. It is well known that local scour around the foundations of a hydraulic structure can lead to damage or even to overall failure. Several design guidelines for offshore wind turbines provide details for the design of the entire wind turbine. However, many of them (e.g., BSH, 2007, and DIN, 2009) do not provide any explicit design guidance on scour depth estimation for different types of wind turbine foundations. Available guidance on maximum scour depth at a monopile foundation ranges from 1. 3D (DNV, 2013) to 2. 5D (GL, 2005). In fluvial environments, the scour mechanisms for a circular cylinder in clear-water and live-bed are well known from the results of many experiments (e.g., Briaud et al., 1999; Chiew, 1984; Dey et al., 1995; Ettema, 1980; Melville and Coleman, 2000; Jones and Sheppard, 2000; Sumer and Fredsøe, 2002). But new problems arise for offshore wind turbine foundations because they often have complex structural shapes. Whitehouse et al. (2008) and Harris et al. (2010) carried out
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Corresponding author. E-mail address: [email protected] (C. Yuan).
http://dx.doi.org/10.1016/j.oceaneng.2017.06.038 Received 12 October 2016; Received in revised form 28 April 2017; Accepted 17 June 2017 0029-8018/ © 2017 Elsevier Ltd. All rights reserved.
experiments on wind turbine foundation scour in a coastal environment, but for monopiles. The results of circular cylinder foundation research cannot be directly applied to more complex foundations because foundation shape itself has an effect on scour depth. To the present, apart from work by Stahlmann and Schlurmann (2010) and Stahlmann (2013, 2014), there has been no systematic physical experiments on scour around tripod foundations in a steady current. This study investigated the effect of flow depth, flow velocity and tripod alignment on the maximum scour depth. The combination effect of current-wave on scour depth will be discussed in the future. 2. Experimental setup and data Experiments were carried out in the Hydraulic Engineering Laboratory of the Department of Civil and Environmental Engineering, University of Auckland, using a 2.4 m wide, 0.3 m deep and 16.5 m long flume. Uniform sand with median size d50 = 0. 85mm , specific gravity of 2.65 and geometric standard deviation σg = 1. 3 was used as the bed sediment material for all scour experiments. All sediment was placed in a 2.8 m long 0.45 m deep recess which was located 7 m downstream of the inlet tank. The tripod model, based on tripod foundations of prototype wind turbines recently established in the East China Sea, was manufactured with a scale of 1:60. A circular cylinder with the same diameter and length as the tripod leg was also considered in some experiments. The
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Fig. 1. Model details: (a) top view, (b) front view, (c)–(e) the three orientations investigated, (f)–(g) lateral scour extent and red dots showing the locations of the scour depth measurement points. And red dots showing the locations of the scour depth measurement points, α is the angle between Leg 1 and the flow direction.
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Because only one kind of uniform sand (d50 = 0.85mm ) was used in this study, Eq. (1) can be simplified to
Table 1 Experimental conditions and results of scour experiments. No.
t (h)
V/Vc
y (m)
α (°)
ds (mm)
ds /D
E /R
POMSD
Code
ds / D = dsmax / D∙KI Ky Kα = CKI Ky Kα
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25C 26C 27C 28C
30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9 0.5 0.5 0.5 0.5 0.9 0.9 0.9 0.9
0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10 0.25 0.20 0.15 0.10
60 60 60 60 60 60 60 60 30 30 30 30 30 30 30 30 0 0 0 0 0 0 0 0 – – – –
172 140 127 80 40 26 17 13 158 140 138 91 56 26 17 16 149 130 130 88 40 30 18 14 75 92 93 82
3.2 2.50 2.35 1.48 0.74 0.48 0.31 0.24 2.93 2.59 2.56 1.59 1.04 0.48 0.31 0.30 2.75 2.41 2.40 1.63 0.74 0.56 0.33 0.26 1.39 1.70 1.72 1.52
2.07 1.94 1.78 1.46 1.19 1.39 1.01 1.00 2.05 2.01 1.99 1.71 1.19 1.14 1.14 1.12 1.83 1.75 1.64 1.52 1.43 1.23 1.19 1.15 0.69 0.71 0.69 0.68
2 2 1 or 1 or 1 or 1 or 1 or 1 or 3 3 3 3 3 3 3 3 2 or 2 or 2 or 2 or 2 or 2 or 2 or 2 or – – – –
60_HF_25 60_HF_20 60_HF_15 60_HF_10 60_LF_25 60_LF_20 60_LF_15 60_LF_10 30_HF_25 30_HF_20 30_HF_15 30_HF_10 30_LF_25 30_LF_20 30_LF_15 30_LF_10 0_HF_25 0_HF_20 0_HF_15 0_HF_10 0_LF_25 0_LF_20 0_LF_15 0_LF_10 HF_25 HF_20 HF_15 HF_10
where ds max is the maximum equilibrium scour depth that could reasonably be expected for this experimental setup, as defined later in this paper.
3 3 3 3 3 3
3 3 3 3 3 3 3 3
(2)
3.1. Equilibrium scour position From Table 1, for all experiments, the maximum scour depths always occurred at the upstream surface of a leg, despite the centre pile having the largest diameter of all the elements in the structure. Because the centre pile’s base is level with the unscoured surface (just below the underside of the horizontal beams) most of the downflow passes downstream through the newly generated scour hole beneath the pile. As a result, less energy is imparted to the horseshoe vortex system and the scour hole is shallower. With α = 0°, because of the bilateral symmetry of the tripod foundation along the flow direction, the maximum scour depth always occurred at a downstream leg (Leg 2 or 3). With α = 30°, the lateral distance between the centre pile and Legs 1 and 2 was a minimum, and the obstruction effect of the pile group in this region was significantly greater than that at Leg 3, and flow was redirected towards Leg 3. Therefore, with α = 30°, the maximum scour depth always occurred at Leg 3. With α = 60°, the situation was more complicated. For low flow intensity (0. 5Vc ), the maximum scour depth always occurred at an upstream leg (Legs 1 or 3). For high flow intensity, the position of the maximum scour depth depended on the flow depth: for low flow depth the maximum scour depth occurred at an upstream leg (Legs 1 or 3); for high flow depths, maximum scour depth occurred at a downstream leg (Leg 2). With α = 60°, as the flow depth increased, submergence of the legs and angled struts increased, and a larger amount of near-surface flow was forced to change its direction into downflow, resulting in deepening of the scour hole at the centre pile and at the downstream leg. Because the maximum scour depth always occurred at a leg, it is reasonable and convenient to normalize the scour depth (ds ) with the tripod leg diameter (D = 54mm ).
Note: t is the scour time, V is the mean flow velocity, Vc is the threshold velocity of bed sediment material motion, y is the flow depth, ds is the maximum scour depth which includes both local scour and global scour, E is the lateral extent of global scour at 0.9Vc for tripod model and the lateral extent of local scour for the circular cylinder only which was measured from the centre of the centre pile or the circular cylinder to the edge of the scour hole, R (in all cases) equals 262 mm, the distance between the centre of the centre pile and the outer edges of the legs (Fig. 1), and POMSD indicates the leg at which the maximum scour depth occurred. The suffixal ‘C’ in a test number indicates that the experimental model was only a circular cylinder with the same diameter as the tripod leg.
geometry of the model and the locations of the scour depth measurement points are shown in Fig. 1. All legs go all the way to the recess bottom. All experimental conditions and results of the tests are shown in Table 1 below. Because of the complex structural shape, all scour depths at red-dots positions were measured manually with the help of a ruler and scale marks painted on the model surface after experiments were finished. The red-dots were positioned at 0°, ± 90° and 180° from flow direction for the legs and the centre pile and their connection points with beams. And the scour depth of the beam was measured in the mid-point. Because of the typical dimensions of laboratory flumes as well as the lower limit of cohesionless sediment size, it is difficult to satisfy the similitude requirements relating scour in the flume to scour in reality. Experiments carried out in this study were for determining the possibility of scour at tripod foundations in non-ripple-forming bed material and relationships between scour depth and the main variables influencing it.
3.2. Temporal evolution of the scour depth According to Melville and Chiew (2000), for clear-water scour at bridge piers, the time to equilibrium scour depth is proportional to both mean flow velocity and depth. Therefore, experiments with the largest flow depth (0.25 m), the highest flow intensity (0.9Vc ), and α = 60° were selected to determine the time for development of equilibrium maximum scour depth, ds , for the tripod model. Other scour experiments with lower flow intensity would be expected to reach their equilibrium scour depth sooner. In Fig. 2, the maximum scour depths (ds ) normalized by the diameter of the leg (D ) are plotted against time for each component of the tripod. Unlike the field data measured at Alpha Ventus (Stahlmann, 2013), where the deepest position was found very close
3. Analysis of the experimental results Following Melville and Sutherland (1988), an expression for the equilibrium scour depth ds for a tripod in a steady current can be written as
ds / D = KI Ky Kd Kσ Kα
(1)
where ds is the maximum scour depth, D is the diameter of tripod’s legs, and the Ks are expressions describing effects of different variables: KI for flow intensity, Ky for flow depth, Kd for sediment size, Kσ for sediment gradation and Kα for orientation.
Fig. 2. Time history of scour depth for downstream leg, upstream legs, centre pile and beams (alignment angle 60°, D = 54 mm).
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to the centre pile, the maximum scour depth in the steady current experiments in this study always occurred at a leg for both 0.5Vc and 0.9 Vc . It may be that the occurrence of waves causes the maximum scour depth to be near the centre pile. For 0. 9Vc and alignment angle 60°, the maximum scour depth position transferred from an upstream leg to a downstream leg at about 12 h after the experiment started. For a two-pile in tandem arrangement, Hannah (1978) found that scour at the front pile was always greater than at the rear one. But, for alignment angle α = 60° in this study, the opposite occurred at the centre pile and downstream leg. In fact scour depth at the centre pile was always less than at legs for all experiments in this study, because of the centre pile’s non-inserted root. In a study of three-pile group scour in current and in waves (Sumer and Fredsøe, 1998), scour depth at the downstream leg was always less than at the upstream legs, which was also contrary to the results from this study. When flow intensity is weak, because most flow could pass through the structure, the horseshoe vortex is relatively small and weak and the scour below the centre pile is shallow. Some of the sand eroded from the upstream area is deposited in the vicinity of the downstream leg. The shielding effect of the centre pile also protects the downstream leg from scouring. Therefore, the maximum scour depth is located at the same places as for the 2-pile group at the same alignment angle (i.e., the scour depth is larger at the front pile than at the rear one). When the power of flow increases, the sediment deposited around the rear leg can be flushed away completely. Because the bottom of the centre pile is at the sediment surface, when the scour hole is deep enough, the centre pile and the downstream leg appear to act as an integral unit in the streamline direction. The equivalent diameter of the combination is much greater than that of an upstream leg. In addition, the upstream legs would be submerged at higher water level, which mitigates the scour depth development. For high flow rate (0.9Vc ) in this study, scour holes of the individual components merged together, influenced by the slope of the deeper scour holes at the legs and the scour depth under the beams (which are larger than centre pile). However, for weak flow (0.5Vc ), because the scour holes were separated from each other, there was little scour under the beams, and sometimes several small aggradation mounds were observed. The time for equilibrium scour holes to form in every part of the tripod model was about 30 h. It should be noted that the maximum scour depth measured in this study was about 3. 2D , where D is the diameter of the leg. This is significantly larger than the 1. 3D or 2. 5D recommended in guidelines for monopiles. This amplification is probably due to the pile group effect of the centre pile, the angled struts and the beams near the bed. A substantial amount of water is diverted to the gap between the underside of the beams and the bed, leading to very large velocity in this area and presumably resulting in large shear stress and increased scour depth. The ratio may be even larger in wave-current conditions, because the bed material is more easily entrained by the increased bed shear stress and is transported away from the tripod foundation by the current.
Fig. 3. Flow depth effect on normalized scour depth (the scour depth of circularcylinder-only is normalized by the maximum scour depth in this study namely 3.2D).
In this study, D / d50 is high (for the tripod leg D / d50 = 64 , for the centre pile Dm / d50 = 84 , and if the pile group effect of the tripod is taken into considerationD / d50 would be even larger), and y / D is greater than 1 in all experiments, so results of previous research for bridge piers suggest that, in this study, flow depth should not affect scour depth. However, a significant increase of scour depth occurred when y / D increased from 1.85 to 4.63, this indicates that there is a clear difference between tripod scour and single pile scour. The scour depth for circular cylinder only is similar to the case of alignment angle 60° under low flow depth. Because the circular-cylinder-only model was submerged when flow depths reached 0.2 m and 0.25 m, its scour depth first increased then decreased with flow depth increase (cf. Dey et al., 2008). It is evident that the pile group effect of the tripod foundation may play an important role in how scour depth changes with flow depth. The flow depth factor, Ky , in Eq. (2) is defined as Ky = ds ( y / D )/ ds ( y / D = 4.63) . Fig. 3 shows the equilibrium scour depths for V / Vc = 0.9; the scour depths are normalized by the maximum equilibrium scour depth for this study. It is expected that scour depths would be slightly larger for V / Vc = 1.0 (Chee, 1982), so an appropriate expression for Ky should bracket the normalized scour depth values in Fig. 3,and have a value of 1 at y / D = 5.0 , and a value of 0 at y / D = 0 . For design purposes, the expression recommended for Ky is (Fig. 3),
Ky = 1 − exp (−0. 8y / D )( y / D<5. 0)
(3)
3.4. Approach flow velocity effects The flow continuously supplies energy to horseshoe vortices near the bed and to lee-wake vortices, and the bed shear stress around the structure may far exceed the undisturbed mean bed shear stress. For a single pile, local scour is initiated when the approaching mean flow velocity V exceeds about 0.5 times the threshold velocity Vc of the sediment. For 0. 5
3.3. Influence of flow depth on equilibrium scour depth of tripod For bridge piers, Chiew (1984), Ettema (1980), Melville and Coleman (2000), Jones and Sheppard (2000), and others found that scour depth increases with flow depth, up to a limiting value of the flow depth ratio y / D , beyond which there is no influence of flow depth. In addition, Ettema (1980) concluded that the influence of flow depth is affected by the relative sizes of the pier and sediment, D / d50 . He found that for high values of D / d50 , the scour depth was almost independent of flow depth wheny / D > 1, while for low values of D / d50 it was still dependent on flow depth for values of y / D as high as 6. Jones and Sheppard (2000) proposed that the maximum relative clear water scour occurred when D / d50 is about 46, and that, for constant values of y / D and flow intensity, V/ Vc , scour depth was less for D / d50 values larger or smaller than 46. 280
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Fig. 4. Effect of flow intensity on equilibrium scour, for 0.25 m flow depth.
Ky = 1). From the best fit line through the data points, dsmax , the maximum equilibrium scour depth that could reasonably be expected for this experimental setup (i.e. ds for V / Vc = 1), is 188 mm. The value of C in Eq. (2) becomes 3.5. The normalized scour depths ds / dsmax are plotted against V / Vc in Fig. 5. Because only two flow intensities were considered in this study, a linear relationship was assumed between the normalized scour depth ds / dsmax and relative velocity V / Vc , which is in agreement with the present knowledge of clear-water scour (Chiew, 1984; Ettema, 1980; Melville and Coleman, 2000). The best fit line gives the equation for KI as KI = 1. 52V / Vc−0. 52(0. 35
Fig. 6. Effect of alignment on normalized scour depth, for V /Vc = 0.9 .
mainly waves acting on a physical model of a tripod foundation, the results of small-scale tests indicated that an alignment of α = 30°produced the smallest scour depth. And for large-scale experiments, Stahlmann found that an alignment of α = 60° produced a much greater equilibrium scour depth than 0°. In this study, alignments of α = 0° usually produced a lower value of scour depth than other alignment angles. However, in a marine environment, the flood and ebb tides have opposite flow directions. If the tripod is aligned with α = 0° for minimum scour during the flood tide, it will experience the maximum scour during the ebb tide, for which the effective alignment will have changed to α = 60°. More attention needs to be paid to cases of high flow intensity and flow depth at 60° alignment angle.
(5)
This equation suggests that the flow intensity for the initiation of scour at a tripod is for a V / Vc value of approximately 0.35, which is similar to the initiation flow intensity proposed by Melville and Coleman (2000). Because only two flow intensities were considered in this study, Eq. (5) should be applied with discretion. 3.5. Alignment effects The scour depth is related to the strength of the downflow at the upstream surfaces of the elements of the tripod, the former depending on the amount of flow blockage caused by the tripod at different alignment angles. For α as defined in Fig. 1, the blockage effect (and hence the strength of the downflow and horseshoe vortex) increases as α increases. Fig. 6 shows, for V / Vc = 0.9 , the relationship between the normalized equilibrium scour depth and alignment angle, α . It can be seen, from Fig. 6 and Table 1, that α has only a small influence on equilibrium scour depth. In addition, the trend of the relationship between scour depth and α is different for different flow depths. Therefore, it is conservatively recommended that a value of 1.0 be adopted for Kα . However, other factors have to be considered when deciding on an appropriate orientation. In a study by Stahlmann (2013) involving
3.6. Lateral extent of the scour At high flow intensity (0. 9Vc ), all the individual scour holes at the foundation components join up in the form of a scour hole. Also, the lateral extent of scour increases linearly with increase in the maximum scour depth ds (Fig. 7). The best fit line gives the equation for E as
⎛d ⎞ E = 1. 5 ⎜ s ⎟ +1 ⎝R⎠ R
(6)
where R is defined as the distance between the centre of the centre pile and the leg surface (Fig. 1).
Fig. 5. Effect of flow intensity on normalized scour depth, for 0.25 m flow depth.
Fig. 7. Lateral extent of scour.
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supported by the Cultivation of Jiangsu Province Graduate Innovation Project (grant numbers KYZZ_0151) and Natural Science & Technology Pillar Program (grant number 2012BAB03B01).
4. Conclusions The following conclusions can be drawn from this study:
References
(1) Although the centre pile has a larger diameter than the tripod legs, the maximum scour depths of all tripod scour experiments were found at a leg for the clear-water, current-only scour cases. The specific locations of the equilibrium scour depth were influenced by the alignment angles of the tripod, flow intensity, and flow depth. The occurrence of waves could result in the maximum scour depth being located near to the centre pile. (2) The ratio of the maximum scour depth to the diameter of the tripod leg, dsmax / D , reached 3.2 in this study for V / Vc = 0.9 , and analysis suggests that this ratio would be 3.5 for V / Vc = 1.0 . Because of the extra turbulence and pile group effect of the complex structural shapes, this ratio is significantly larger than that in available guidance for monopile foundations, which ranges from 1. 3D (DNV 2014) to 2. 5D (GL, 2005). (3) Similarly to the scour around a single pile, the equilibrium scour depth of a tripod increased with flow depth and flow intensity (at the approach section). The relationship between the equilibrium scour depth and the tripod alignment is imprecise. (4) For high flow depth and intensity, the scour depth tended to increase with the alignment angle. Alignment angle α = 60° should be avoided to the high flow velocity and strong waves as possible. (5) For estimating scour depth ds / D where D is the pile diameter of a tripod leg, it is suggested Eq. (2) (with C = 3.5) can be combined with Eq. (3) for KI , Eq. (5) for Ky and Kα =1 to estimate the clearwater equilibrium scour depth. And the lateral extent of the scour hole can be computed by Eq. (6).
Briaud, J.-L., Ting, F.C.K., Chen, H.C., Gudavalli, R., Perugu, S., Wei, G., 1999. SRICOS: prediction of scour rate in cohesive soils at bridge piers. J. Geotech. Geoenvironmential Eng., ASCE 125 (4), 237–246. BSH, 2007. Hydrographie: Standard – Konstruktive Ausführung von OffshoreWindenergieanlagen. No. 7005. Bundesamt fur Seechiffahrt und Hydrographie, Hamburg. Chee, R.K.W., 1982. Live-bed Scour at Bridge Piers (Report No.290, (Report No.290, Ph.D. Thesis. University of Auckland, Auckland. Chiew, Y.M., 1984. Local Scour at Bridge Piers Report No. 355, Report No. 355, Ph.D. Thesis. University of Auckland, Auckland, Report No. 355, Report No. 355, Ph.D. Thesis〈http://hdl.handle.net/2292/2520〉. Dey, S., Bose, S.K., Sastry, G.L.N., 1995. Clear water scour at circular piers: a model. J. Hydraul. Eng. 131 (12), 1126–1135. Dey, S., Raikar, R.V., Roy, A., 2008. Scour at submerged cylindrical obstacles under steady flow. J. Hydraul. Eng. 134 (1), 105–109. DIN, 2009. DIN EN 61400-3. Windenergieanlagen – Teil 3: AuslegungAnforderungen für Windenergieanlagen auf offener See (IEC 61400-3:2009). Deutsches Istitutfür Normung e.V. DNV, 2013. Design of Offshore Wind Turbine Structures. Offshore Standard DNV-OSJ101, Det Norske Veritas AS, Høvik. Ettema, R., 1980. Scour at Bridge Piers (Report No. 215. (Report No. 215. Ph.D. Thesis. University of Auckland, Auckland. GL, 2005. Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd Wind Energie GmbH, Hamburg. Hannah, C.R., 1978. Scour at Pile Groups No. 78–3. University of Canterbury N.Z., 92. Harris, J.M., Whitehouse, R.J.S., Benson, T., 2010. The time evolution of scour around offshore structures. Marit. Eng. 163 (1), 3–17. Jones, J.S., Sheppard, D.M., 2000. Scour at wide bridge piers. In: Hotchkiss, R.H., Michael Glade, M. (Eds), Building Partnerships: Joint Conference on Water Resources Engineering and Water Resources Planning and Management, ASCE, Minneapolis, pp. 1–10. Melville, B.W., Chiew, Y.M., 2000. Time scale for local scour at bridge piers. J. Hydraul. Eng. 126 (10), 59–65. Melville, B.W., Coleman, S.E., 2000. Bridge Scour. Water Resources Publications, LLC, Highlands Ranch, CO. Melville, B.W., Sutherland, A.J., 1988. Design method for local scour at bridge piers. J. Hydraul. Eng. 114 (10), 1210–1226. Stahlmann, A., 2013. Experimental and Numerical Model of Scour at Offshore Wind Turbines. Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering. Leibniz Universität Hannover, Hannover, Germany. Stahlmann, A., 2014. Experimental and numerical model of scour at offshore wind turbines. J. Ocean Wind Energy 1 (2), 82–89. Stahlmann, A., Schlurmann, T., 2010. Physical Modeling of Scour around Tripod Foundation Structures for Offshore Wind Energy Converters. Proceedings of the International Conference on Coastal Engineering, NO. 32, Shanghai, China, 1-12. Sumer, B.M., Fredsøe, J., 1998. Wave scour around group of vertical piles. J. Waterw., Port., Coast. Ocean Eng. 124 (5), 248–256. Sumer, B.M., Fredsøe, J., 2002. The mechanics of scour in the marine environment. World Scientific, Singapore. Whitehouse, R., Harris, J. Sutherland, J., Rees, J., 2008. An assessment of field data for scour at offshore wind turbine foundations. 4th International Conference on Scour and Erosion, Tokyo, pp. 1–8.
All the conclusions above are derived from tripod scour experiments; this has two consequences. First, for the same bed material, the higher Froude number resulting from scaling effects in the model can lead to a scour depth larger than that for a prototype tripod foundation with the same V/Vc; this is an inherent in laboratory flume experiments. Second, for finer non-cohesive bed material, because of the lower incipient velocity, the scour depth could be even larger. Therefore, the upper-bound scour depth estimate from the suggested equations can be regarded as a possible and conservative method of scour depth prediction for coarse bed material, but the estimate is more likely to be achieved for finer non-cohesive sands. Acknowledgements This study was carried out at the University of Auckland and
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