REPORT DECISION. The scour protection shall be designed to avoid progressive collapse of scour protection edges.

Transcription

1 MEMORANDUM TO Stakeholders of the offshore grid DATE September 11, 2017 REFERENCE NLO-TTB FROM Net op Zee SUBJECT Scour protection Hollandse Kust (zuid) Alpha and Beta REPORT DECISION Deltares has conducted a scour assessment for the Hollandse Kust (zuid) offshore high voltage stations Alpha and Beta at the request of TenneT. Based on this assessment and the Borssele physical model tests it has been concluded that scour protection under and around the Hollandse Kust (zuid) platform jackets is required. In order to provide a stable underground for the subsea cables in the vicinity of the platform, a pre-installed scour protection will be placed under and around the platform jacket as part of the scope of the platform contractor. This scour protection will meet the following requirements: Design life: Elevation: Radial extent: Edge stability: The scour protection will be dynamically stable and will be designed to survive for 30 years without maintenance. The maximum extreme event to be accounted for is the 1/100 yrs wave in combination with the 1/10 yrs tidal current. Along the first 5 meter of all the cable routes, measured from the J-tube bell mouths, the top of the scour protection shall be at 2,5m (+/- 0,5m) below the centreline of the J-tube bell mouths. The scour protection will extend 15m around the jacket base outlines, plus the slopes needed to ensure edge stability. The scour protection shall be designed to avoid progressive collapse of scour protection edges. The design will also fulfil scour protection requirements in the way of the legs of jack-ups servicing the platform during testing and commissioning and during future maintenance operations. OWF is responsible for the design and installation of the cable over the platform scour protection, the cable protection system, fixation of the cables on the scour protection (if needed) and the protection of the cable on and near the scour protection edge. The attached report by Deltares shows the assessment and scour protection design.

2 Scour development and conceptual scour protection layout at HKZ Alpha and Beta Greta van Velzen Niek Bruinsma Deltares, 2017, B

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4 Title Scour development and conceptual scour protection layout at HKZ Alpha and Beta Client Tennet TSO B.V. Project Reference HYE-0002 Pages 14 Keywords Offshore wind; flow amplification; CFD; HKZ Alpha; HKZ Beta; offshore substations. Summary TenneT is responsible for the development of the two Offshore High Voltage Stations (OHVS) of Hollandse Kust Zuid Offshore Wind Farm (HKZ OWF): HKZ Alpha and Beta. Deltares was approached to assist them in several scour-related consultancy tasks. One of which was related to the assessment of the scour development and conceptual scour protection layout at HKZ Alpha and Beta. A desk study has been carried out to assess the scour development and conceptual scour protection layout at HKZ Alpha and Beta. Because of the similarity between the design of the HKZ Alpha and Beta jacket with the Borssele Alpha and Beta jackets, the starting point of the desk study was the outcome of the physical model test programme on the scour development and scour protection layout at Borssele Alpha and Beta. By comparing the hydrodynamic conditions at both sites, as well as the effects of (small) differences in the design of the jackets, required alterations in the expected scour development and conceptual scour protection layout could be determined. Both the undisturbed hydrodynamic conditions as well as the local amplification due to the HKZ Alpha or Beta platforms are not expected to differ much from the situation at the Borssele Alpha and Beta platforms. Consequently, the best-estimate for the scour development and the conceptual scour protection at HKZ Alpha and Beta is similar to the outcome of the physical model tests performed for Borssele Alpha and Beta. References Request for proposal: Proposal: Deltares, HYE-0001-Scour protection design OHVS HKZ OWF.pdf. Dated 27 January State final Scour development and conceptual scour protection layout at HKZ Alpha and Beta

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6 HYE-0002, 1 June 2017, final Contents 1 Introduction Objectives Methodology Structure of this report 1 2 System understanding The HKZ Alpha and Beta OHVS Met-ocean conditions 4 3 Comparison hydrodynamic conditions HKZ and Borssele OWF Summary of physical model test Borssele Undisturbed hydrodynamic conditions Numerical flow amplification 8 4 Scour development and conceptual scour protection at HKZ Alpha and Beta 11 5 Conclusions 13 6 References 14 Scour development and conceptual scour protection layout at HKZ Alpha and Beta i

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8 HYE-0002, 1 June 2017, final 1 Introduction TenneT is responsible for the development of the two Offshore High Voltage Stations (OHVS) of Hollandse Kust Zuid Offshore Wind Farm (HKZ OWF): HKZ Alpha and Beta. On 30 November 2016, Deltares was contacted by TenneT with a request for proposal regarding several scour-related consultancy tasks. A quotation was provided by Deltares on 27 January 2017 (Deltares, 2017) for two separate work packages. Deltares was given a notice to proceed according to TenneT s Purchase Order from 18 November 2015 (change order number 2 dated 8 December 2016), which was signed 13 March 2017 (PO T274969). The present document contains the report on the quotation s work package one. This work package entails the assessment of the scour development and conceptual scour protection layout at HKZ Alpha and Beta. 1.1 Objectives The objective of this study is twofold: 1. to provide an indicative estimate of the scour that could occur over the lifetime of the jacket in case of an unprotected jacket. 2. to develop a conceptual loose rock scour protection layout. 1.2 Methodology In the summer of 2016 a physical model test campaign was executed by Deltares in which the scour development and required scour protection around another OHVS was simulated, namely OHVS Borssele Alpha and Beta (Deltares, 2016). Because of the similarity between the design of the Borssele OHVS and HKZ OHVS, the results of the physical model test campaign are used as a starting point for this desk study. By comparing the hydrodynamic conditions at both sites, as well as the effects of (small) differences in the design of the jackets, alterations in the expected scour development and conceptual scour protection layout are suggested. The assessment therefore consists of the following steps: 1. The undisturbed hydrodynamic conditions at HKZ are compared with the undisturbed conditions at Borssele. 2. The numerical modelling results of flow amplification around the HKZ and Borssele jackets are compared. 3. Based on the abovementioned comparisons and the scour development and conceptual scour protection layout simulated for Borssele Alpha and Beta, the scour development and conceptual layout for HKZ Alpha and Beta are developed. 1.3 Structure of this report Chapter 2 presents a description of the HKZ Alpha and Beta platform. In Chapter 3 the comparison between the hydrodynamic loading at Borssele Alpha and Beta and HKZ Alpha and Beta (point 1 and 2 of the abovementioned approach) is discussed. The resulting expected scour development and conceptual scour protection layout for HKZ Alpha and Beta is presented in Chapter 4 (point 3 of the abovementioned approach), followed by the conclusions presented in Chapter 5. Scour development and conceptual scour protection layout at HKZ Alpha and Beta 1 of 14

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10 Length HYE-0002, 1 June 2017, final 2 System understanding 2.1 The HKZ Alpha and Beta OHVS The Dutch Ministry of Economic Affairs presented a road map outlining how the Government plans to achieve its offshore wind goals in accordance with the timeline agreed upon in the Energy Agreement. The road map set out a schedule of tenders offering 700 MW of development each year in the period In the context of this road map, a new tender will open this year for wind farm zone Hollandse Kust Zuid (HKZ); a wind farm with an area of 356 km², divided into 4 sites that can each accommodate 350 MW. TenneT will build grid connections and construct two offshore substations in the wind farm zone. Last year, TenneT has developed a jacket foundation design for the Borssele offshore substations, see Figure 2.1. In view of cost reduction, it is the intention to apply a similar jacket design in the HKZ OWF for the two offshore substations HKZ Alpha and Beta. Both the HKZ Alpha and Beta structures are placed under a 25 degree angle with respect to the true North. MSL N Width Figure 2.1 Borssele Alpha jacket design (provided by TenneT). Left: three-dimensional overview. Right: plan view. The dimensions of the structures are optimised for local hydrodynamic conditions. The footprint of the Borssele Alpha and HKZ Alpha and Beta jackets are presented in Table 2.1. This table shows that the expected footprints of both the HKZ Alpha and HKZ Beta platform are smaller than the footprint of the Borssele Alpha platform. This difference is related to the differences in water depth at the three locations. Apart from the footprint, the exact dimensions of the HKZ Alpha and HKZ Beta platform are unknown yet. In consultation with Tennet, it is assumed that all dimensions scale down from the Borssele Alpha dimensions with a similar ratio as the footprint. Moreover, the numerical simulation on the flow amplification (Deltares, 2017b) showed that the flow amplification around the HKZ Beta platform is expected to be slightly larger than the flow amplification around the HKZ Alpha Scour development and conceptual scour protection layout at HKZ Alpha and Beta 3 of 14

11 HYE-0002, 1 June 2017, final platform. Regarding the flow amplification, the dimensions of HKZ Beta are therefore assumed. Table 2.1 Footprint of the Borssele Alpha and HKZ Alpha and Beta jacket structure, where the length and width are indicated in Figure 2.1 and the scale is the size of the footprint relative to the footprint of Borssele Alpha. Parameter Symbol Borssele Alpha HKZ Alpha HKZ Beta Length L [m] Width W [m] Scale S [%] Met-ocean conditions The met-ocean conditions for the HKZ site are obtained from the met-ocean report (DHI, 2017). The site is characterised by a moderate tidal current, with a dominant direction in the ENE WSW axis. Furthermore, it is noted that the flood currents, going towards Northeast are usually stronger than ebb currents. Waves are predominantly coming from the northerly and north-north-westerly direction. This is in line with the direction of the strongest winds. More than 50% of the time, the waves come from between N and NW with more extreme waves coming from NNW. An overview of the met-ocean conditions is presented in Table 2.3 (HKZ Alpha) and Table 2.4 (HKZ Beta). Please note that the RP100yr storm event is based on one of the Ultimate Limit State (ULS) conditions presented in the guidelines by DNV GL on load combinations (DNVGL-OS-C101), see Table 2.2. This ULS condition is referred to the RP100yr condition in the remaining part of this document. Table 2.2 Return periods of environmental load incorporated in one of the ULS conditions mentioned in DNVLGL- OS-C101. Limit state Waves Current Water level ULS For the tidal conditions, the water depth is based on a water level equal to MSL. For the storm conditions the corresponding minimum still water level is incorporated in the water depth. Table 2.3 Summary of the met-ocean conditions at HKZ Alpha (DHI, 2017) Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Mean tidal conditions Storm conditions: RP100yr Table 2.4 Summary of the met-ocean conditions at HKZ Beta (DHI, 2017) Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Mean tidal conditions Storm conditions: RP100yr of 14 Scour development and conceptual scour protection layout at HKZ Alpha and Beta

12 HYE-0002, 1 June 2017, final 3 Comparison hydrodynamic conditions HKZ and Borssele OWF 3.1 Summary of physical model test Borssele Prior to comparing the conditions at the Borssele site to the HKZ site, a short summary of the physical model test campaign for Borssele OHVS is presented. Hereby we go into the considered met-ocean conditions, conceptual scour protection layout, the observed scour development and the observed scour protection deformation. Met-ocean conditions Prior to the execution of the physical model test campaign it was concluded that the conditions at Borssele Alpha and Beta were very similar. Moreover, it was assessed that the conditions at Borssele Alpha were normative. Based on this assessment the physical model test campaign included loading cases related to the Borssele Alpha location. These are presented in Table 3.1. Please note that the load combinations are defined in a similar manner as described in Section 2.2. Table 3.1 Load combinations considered in the Borssele OHVS physical model test campaign (field scale). Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Tide_mean RP100yr Conceptual scour protection layout The test performed in the Borssele OHVS physical model test campaign were aimed at simulating the external stability of the armour grading; a 3-9 high density (HD) rock grading. Other failure mechanisms, such as winnowing and failure due to edge scour, were not considered. Based on the selected armour grading, three different scour protection concepts were considered prior to the execution of the tests: (1) a single layer system, with post-installed jacket; (2) a two-layer system, with post-installed jacket and (3) a two-layer system in which the armour is installed after the installation of the jacket. All are visualised in Figure 3.1. It was assessed that concept 3 would be most unfavourable for the armour layer stability. A schematisation of this concept (without including the filter layer) was therefore tested, see Figure Figure 3.1 Schematisation of scour protection concepts. Scour development and conceptual scour protection layout at HKZ Alpha and Beta 5 of 14

13 HYE-0002, 1 June 2017, final Figure 3.2 Tested scour protection layout. Scour development observed The scour development was simulated in tidal conditions, as such conditions general result in the deepest scour holes at the foundation piles. After a mean tidal condition two storm conditions (RP10yr and RP100yr) were run consecutively. Please note that in this desk study we only focus on the design condition: RP100yr storm event. The test clearly showed that the tidal conditions are governing for the development of scour holes around the foundation piles (local scour) and for the extent of scour hole around the whole jacket (global scour). In mean tidal conditions equilibrium depths of approximately 7m are found around the foundation piles (including local and global scour). In more severe tidal conditions the scour holes are expected to deepen. In storm conditions or less severe tidal 6 of 14 Scour development and conceptual scour protection layout at HKZ Alpha and Beta

14 HYE-0002, 1 June 2017, final conditions these scour holes around the foundation piles are expected to experience backfilling. Scour protection deformation A RP10yr storm and RP100yr storm were run consecutively during the physical model test programme. After the 100yr storm limited movement of the rock was observed; Some rock moved on top of the mud mats. This occasional movement did not result in significant deformation during the RP100yr storm. Based on the outcome of the tests it was assessed that the 3-9 HD grading would be very stable, either when applied as a pre- or post-installed layer. When applied as a single layer, the thickness of the layer should be sufficient to prevent winnowing. When applied in a twolayer system the thickness of the armour layer should ensure a good coverage of the filter layer; a minimum value of 0.5m was advised. Based on the expected (expert judgement) edge scour hole development of 3m, a similar extent as incorporated in the conceptual layout (12m, see Figure 3.2) was advised for a single layer system. For a two layer system the extent of the armour grading might be optimised, but the extent of the filter layer should then be sufficient to prevent undermining of the armour layer due to edge scour. 3.2 Undisturbed hydrodynamic conditions As mentioned in the methodology, see Section 1.2, the first step in the comparison between the Borssele OHVS and HKZ OHVS, includes the comparison between the undisturbed hydrodynamic conditions. In order to evaluate the combined effect of the differences of all hydraulic parameters, we consider the relative mobility. The relative mobility is defined as: MOB, MOB FIELD MOB (3.1) MODEL c where θ is the Shields parameter [-], θ c is the critical Shields parameter [-] and MOB is the ratio between them [-]. A relative mobility value larger than 1 indicates that a particle is likely to be entrained or unstable, while with a relative mobility value smaller than 1 little to no movement is expected. Please note, that this is related to the undisturbed relative mobility (without the present of a structure). In the vicinity of a structure the load can be increased by the structure and particles can become unstable at lower undisturbed relative mobility values. Table 3.2 presents an overview of the undisturbed relative mobility values for the tested conditions for Borssele OHVS and the design conditions for HKZ Alpha and HKZ Beta. Table 3.2 Overview of hydraulic parameters for Borssele OHVS and HKZ Alpha and HKZ Beta. Load case Sand* MOB [-] 3-9 HD** Borssele OHVS; tested conditions HKZ Alpha HKZ Beta * the mobility of the seabed is determined based on tidal conditions ** the mobility of the 3-9 HD grading is determined based on a RP100yr storm event. Scour development and conceptual scour protection layout at HKZ Alpha and Beta 7 of 14

15 HYE-0002, 1 June 2017, final From Table 3.2 we can conclude that both the sand and the 3-9 HD rock grading are expected to be slightly more mobile at HKZ Alpha and Beta than simulated in the tests for Borssele OHVS. However, the differences are so small that they fall within the accuracy with which a desk study can be performed. 3.3 Numerical flow amplification From the previous section we know that the undisturbed (without a structure) movement of both the sand and a 3-9 HD grading at the location of HKZ Alpha and Beta will be comparable to the movement observed in the tests of Borssele OHVS. However the slight changes in jacket design (linearly scaled down with the footprint), might result in a different amplification of the flow around the HKZ Alpha and Beta than occurred at the Borssele OHVS. As already mentioned in Section 2.1, based on the numerical flow amplification simulations it is expected that the amplification around HKZ Beta is slightly larger than the amplification around HKZ Alpha. Regarding the flow amplification, the dimensions of HKZ Beta are therefore assumed. Below we therefore compare the simulation of the flow around Borssele Alpha (normative for Borssele jackets) with the simulation of the flow around HKZ Beta (normative for HKZ jackets). Based on expert judgment, the difference in the flow amplification is expected to be limited, because of the great similarity between the design of the Borssele and HKZ jacket. To substantiate this, the numerical simulations of the flow patterns around both platforms are compared. We consider the flow patterns in a current-only situation, both expressed as the mean flow velocity and maximum flow velocity. Moreover, we consider two different approach angles (0 and 45 ). For an elaborate description of the numerical simulation we refer to (Deltares, 2017b). Table 3.3 presents an overview of these simulated flow patterns for both Borssele Alpha and HKZ Beta. In the top two figures presented in Table 3.3 the flow patterns are very comparable. Slightly larger flow amplification values are found at the foundation piles of the Borssele Alpha platform. In the middle two figures there is clearly a different interaction between the front and back piles of Borssele Alpha than at HKZ Beta, which is expected to be related to the distance between the piles. Also for this situation the amplification found at Borssele Alpha is larger than at HKZ Beta. In the bottom figures the differences between the flow patterns is again very small. These three examples of the flow patterns show that indeed the difference in expected amplification around the Borssele Alpha and HKZ Beta platform is very limited. Within the accuracy with which a desk study can be performed, we can conclude that the differences are negligibly small. 8 of 14 Scour development and conceptual scour protection layout at HKZ Alpha and Beta

16 HYE-0002, 1 June 2017, final Table 3.3 Comparison of simulated flow patterns around Borssele Alpha and HKZ Beta. Flow condition Borssele Alpha HKZ Beta Orientation: 0 Flow velocity: mean Orientation: 0 Flow velocity: max Orientation: 45 Flow velocity: max Scour development and conceptual scour protection layout at HKZ Alpha and Beta 9 of 14

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18 HYE-0002, 1 June 2017, final 4 Scour development and conceptual scour protection at HKZ Alpha and Beta As concluded in the previous chapter, both the undisturbed hydrodynamic conditions as well as the local amplification due to the HKZ Alpha and Beta platforms is not expected to differ much from the situation at the Borssele Alpha and Beta platforms. Consequently, the best-estimate for the scour development and the conceptual scour protection at HKZ Alpha and Beta is similar to the outcome of the physical model tests performed for Borssele Alpha and Beta. To be complete, a short summary of these expectations is described below. The reader is also referred to Section 3.1 and Deltares (2016). Scour development For the Borssele Alpha and Beta platforms for the mean tidal conditions an equilibrium scour depth of approximately 7m was found around the foundations. Due to the high similarity in both the undisturbed and local flow amplification (see Section 3.2 and 3.3) an approximately equal scour depth is expected for HKZ Alpha and Beta. In more severe tidal conditions the scour holes are expected to deepen. In storm conditions or less severe tidal conditions these scour holes around the foundation piles are expected to experience backfilling. Conceptual scour protection layout An armour layer consisting of a 3-9 HD grading is expected to show limited deformation under the design storm condition (RP100yr). The edge scour hole development for HKZ is expected to be similar to Borssele, which is estimated (expert judgement) to be approximately 3m (see Section 3.1). When applied as a single layer system, an extent of 12m (measured from the contour of the mud mats) is advised. The thickness of such a layer should be sufficient to prevent winnowing. Based on expert judgement a minimum required thickness of 1-1.5m is expected. Please note that both extent and thickness should be further verified. When applied in a two-layer system, the extent of the armour layer can probably be optimized/decreased, provided that a sufficiently stable filter grading is chosen which is applied with a sufficiently large extent. Moreover, a minimum required thickness of 0.5m is expected. A filter layer of 1-3 or 1-3 HD is likely to fulfil the requirements for sand-tightness (prevent winnowing), while being sufficiently stable. For the extent of the filter layer of a twolayer system it is advised to apply 10m additional extent, measured from the contours of the armour layer. This stability and extent of the filter layer should be verified. When the armour layer is post-installed gaps between the armour layer and the mud mats should be prevented, to prevent washing out of sand or filter material close to the mud mats. Scour development and conceptual scour protection layout at HKZ Alpha and Beta 11 of 14

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20 HYE-0002, 1 June 2017, final 5 Conclusions Both the undisturbed hydrodynamic conditions as well as the local amplification due to the HKZ Alpha or Beta platforms is not expected to differ much from the situation at the Borssele Alpha and Beta platforms. Consequently, the best-estimate for the scour development and the conceptual scour protection at HKZ Alpha and Beta is similar to the outcome of the physical model tests performed for Borssele Alpha and Beta. An armour layer consisting of a 3-9 HD grading is expected to show limited deformation under the design storm condition (RP100yr). When applied as a single layer system, an extent of 12m (measured from the contour of the mud mats) is advised. Based on expert judgement a minimum required thickness of 1-1.5m is expected to be sufficient to prevent winnowing. When applied in a two-layer system, both the extent and thickness of the 3-9 HD grading can probably be decreased. In such a two-layer system a 1-3 or 1-3 HD filter layer should be included to prevent winnowing and undermining due to edge scour. An additional extent of this filter layer of 10m (measured from the contours of the armour layer) is advised. Please note this is a conceptual scour protection and that both extent and thickness of the armour layer as well as the stability and extent of the filter layer should be further verified. Scour development and conceptual scour protection layout at HKZ Alpha and Beta 13 of 14

21 HYE-0002, 1 June 2017, final 6 References Deltares. (2017). Quotation: Scour protection design - OHVS HKZ OWF. Ref: HYE DHI. (2017). Wind Farm Zone Hollandse Kust (zuid) & Hollandse Kust (noord) - Metocean Study. 14 of 14 Scour development and conceptual scour protection layout at HKZ Alpha and Beta

22 Borssele OHVS Scour and scour protection Physical modelling test programme Final report October HYE-0011

23 Borssele OHVS - Scour and scour protection Physical modelling test programme Greta van Velzen Hendrik Jan Riezebos Niek Bruinsma Deltares, 2016, B

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25 Deltares Title Borssele OHVS - Scour and scour protection Client Project Tennet TSO B.V Attn. Harry van der Heijden Classification confidential until further notice Reference Pages HYE Keywords Borssele Alpha, Borssele Beta, OHVS, local scour, global scour, cables Summary The project on the scour development around the Offshore High Voltage Stations (OHVS) at the Borssele Wind Farm sites, Borssele Alpha and Borssele Beta, consists of several phases. The first phase included a desk study on the extreme hydrodynamic conditions at Borssele Alpha and Beta (Oeltares, 2016a) and a desk study on the expected scour development at both platforms (Oeltares, 2016b). In the second phase of the project a physical modelling test programme is executed to simulate the scour development around Borssele Alpha and Borssele Beta. The results of these tests indicate that the scour around the platform will reach a significant depth; the maximum allowable scour depth at the platform's foundation piles and at the cable touch down points is expected to be exceeded. Consequently, TenneT decided to extend the scope of the physical model test programme by adding a test series focussing on the stability of a conceptual scour protection layout. In these tests the external stability of a 3-9" HO grading (either preinstalled or post-installed or part of a two-layer or single-layer scour protection system) is simulated. This test series showed that the 3-9" HO grading is very stable during a RP100yr storm and that optimization of this grading might even be considered. This document contains the documentation of the physical model test programme on the scour development and the stability of the conceptual scour protection layout around Borssele Alpha and Beta. References Invitation to tender, discussed during meeting on 8 March Deltares' initial proposal ( HYE-0002), submitted 24 March Deltares' updated proposal ( HYE-0003), submitted 15 Apri Letter of award for phase 1 and 2a (T260812), dated 14 April2016. Letter of award for phase 2b (T261511), dated 4 May Letter of award for scale model fabrication (T262511), dated 20 June Deltares' proposal additional work ( HYE-0009-v2), submitted 3 August Letter of award for additional work (T263797), dated 5 August Version Date Author Initials Review 1.0 Se t Greta van Velzen Hans de Vroe Hendrik Jan Riezebos Niek Bruinsma 2.0 Oct 2016 Greta van Velzen Hans de Vroe Klaas Jan Bos Hendrik Jan Riezebos Niek Bruinsma State final Borssele OHVS - Scour and scour protection

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27 Contents 1 Introduction Background Objective Methodology Content of the report 3 2 Boundary conditions Bathymetry, morphology and soil conditions Soil conditions Morphology Metocean conditions Water levels Current velocities Wave conditions Platform design 10 3 Physical model setup Introduction Background/Terminology Scaling methodology Scaling laws Model scale factor Application of the scaling criteria in the present study Test facility Scale models Measurements and documentation Hydrodynamic measurements Bathymetrical measurements Schematisation of field conditions Soil conditions Hydrodynamic conditions Scour protection Scour protection layout Tested stone grading Test programme 29 4 Test results Introduction Hydrodynamic conditions Scour development Mean tidal conditions Storm conditions Behaviour of the scour protection Deformation of the scour protection in a RP10yr storm Deformation of the scour protection in a RP100yr storm 43 Borssele OHVS - Scour and scour protection i

28 5 Analysis Scour development around foundations piles Dealing with the scour development around the platform scour protection 45 6 Conclusions and recommendations Conclusions Recommendations 47 References 49 Appendices A Scale model design A-1 B Grading curves B-1 C Processed hydrodynamic conditions C-1 D Photographs of the physical model tests D-1 E Underwater camera images E-1 F Results of the stereo-photography technique F-1 G Interface analysis G-1 ii Borssele OHVS - Scour and scour protection

29 1 Introduction 1.1 Background TenneT is responsible for the development of the two 700MW AC offshore substation jacket structures which will be placed inside the Borssele Wind Farm Zone; Borssele Alpha and Borssele Beta (the two black points in Figure 1.1). The electricity cables of the wind turbines in the Borssele Wind Farm Zone (BWFZ) will be connected to the grid via these platforms. Borssele Alpha will service the turbines in BWFZ Sites I & II and Borssele Beta will service the turbines in BWFZ Sites III, IV & V. Figure 1.1 Layout of the Borssele Wind farm (source: RVO (2015), Base Map Borssele Wind Farm Zone (WFS I, II, III, IV & V) ( ), obtained from In view of cost reduction, TenneT has developed a design for the jacket foundation of the offshore substation. This design is further presented in Section 2.3. The intention is to apply the same jacket design for offshore substations of two other wind farms along the Dutch coastline; Noord-Hollande kust (NHK, 1 substation) and Zuid-Hollandse kust (ZHK, 2 substations). The jacket is designed for a scour development of 5.6m below the initial seabed (TenneT, 2016). The sum of the local scour, the global scour and the sea bed level lowering Borssele OHVS - Scour and scour protection 1 of 49

30 due to large scale morphodynamics features such as sand waves should therefore not exceed this 5.6m. TenneT has requested Deltares to address the scour development around Borssele Alpha and Beta in a physical model test programme. Figure 1.2 presents the test layout in Deltares Atlantic Basin. The results of these tests indicate that the scour around the platform will reach a significant depth; the maximum allowable scour depth at the platform s foundation piles and at the cable touch down points is expected to be exceeded. Consequently, TenneT decided to extend the scope of the physical model test programme by adding a test series focussing on the stability of a scour protection layout. Figure 1.2 Two scale models of Borssele Alpha and Beta placed in Deltares Atlantic Basin. 1.2 Objective The objectives of the study are: Simulate the total scour development around the two offshore substations in the Borssele wind farms in normative conditions. Develop a conceptual scour protection layout. Validate the external stability of the conceptual scour protection layout. 1.3 Methodology There are no theoretical guidelines available for an accurate assessment of the scour development at the Borssele OHVS jackets due to their unique structure shape. Guidelines for scour at circular piles can be used for a first rough estimate. However, they cannot be used for an accurate estimate, because they do not incorporate the influence of the combined geometry (pile clusters, array of J-tubes etc.). Therefore, theoretical guidelines alone will not provide a sufficient basis for a detailed estimate of the scour depth for the Borssele OHVS jackets. Our methodology is therefore based on performing physical model tests. In the physical model tests scour around the platforms (both local and global) is simulated directly. Please note that the morphodynamic sea bed level changes are not incorporated in this test programme. 2 of 49 Borssele OHVS - Scour and scour protection

31 Within the physical model test programme we have installed two scale models of the Borssele Alpha jacket in the Atlantic Basin of Deltares. By doing so, two different orientations of the platform, with respect to the current and wave direction, were tested simultaneously. After installation of the scale models the scour development was simulated under different kinds of hydrodynamic conditions which are representative for the project area. The conditions include typical tidal currents (current-only) and extreme storms (combined-waves-and-current). The physical model test programme consisted of two test series. In the first the scour development around the platforms was simulated. In the second a conceptual scour protection layout was applied around both platforms and its external stability was tested. 1.4 Content of the report The boundary conditions, which represent the starting conditions for the physical modelling test programme, are presented in Chapter 2. Chapter 3 presents the physical model setup, including the translation of hydrodynamic conditions from field scale to model scale. In Chapter 4 the test results are presented and in Chapter 5 an interpretation of these test results is discussed. In the final chapter of this report the conclusions and recommendation are presented. Borssele OHVS - Scour and scour protection 3 of 49

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33 2 Boundary conditions 2.1 Bathymetry, morphology and soil conditions Soil conditions An extensive set of geotechnical surveys has been performed for the Borssele Wind Farm sites. At the location of both Borssele Alpha and Borssele Beta a borehole and CPT-survey have been executed, which makes the derivation of the required soil conditions for the present study relatively straightforward. Table 2.1 presents the coordinates of both platforms and the nearest borehole survey. Easting [m] Northing [m] Borssele Alpha BH-WFS2-1A Borssele Beta BH-SubStatB Table 2.1 Coordinates of Borssele Alpha and Beta 1 and the nearest borehole survey (Fugro, 2015 and Fugro, 2016) (ETRS 1989, UTM Zone 31N). The particle size distribution of the top layers is presented in Figure 2.1 and Figure 2.2 for the location of respectively Borssele Alpha and Borssele Beta. At the location of Borssele Alpha gravel is present in the top soil layer. Underneath this top layer (approximately 1-8m) the soil is characterized by medium sand with a median grain size of approximately 290µm. At the location of Borssele Beta we can observe a relatively uniform soil layer up to approximately 8m deep, which is characterized is medium sand with a median grain diameter of approximately 280µm. The gravel fraction present in the top layer at the location of Borssele Alpha might slow down the scour development and even decrease the equilibrium scour depth around the jacket platform. To what extent the scour rate and/or scour depth is decreased will depend on the stability of the gravel fraction and the amount of material available. However, from this survey we cannot determine whether this gravel fraction is present at the whole footprint of the jacket. In the present study we therefore dismiss the presence of this gravel fraction and derive a characteristic medium grain size based on the sand fraction present in the top layers. Please note that this is a conservative approach for the simulation of the scour development. The presence of a gravel fraction the aspect might require more detailed consideration with respect to the installation procedure. For both the location of Borssele Alpha and Borssele Beta we select a characteristic medium grain diameter of 280µm, which is a lowerbound value of the medium grain size found within the upper 8m. Please note that at a larger depth the grain sizes are found to be smaller. However, prior to the execution of the physical model tests a scour depth of more than 8m was not expected. Moreover, scour depths exceeding 5.6m are already not accepted, let alone scour depths exceeding 8m. 1 Based on E Field_Lay_Out_Borssele_Alpha.pdf and E Field_Lay_Out_Borssele_Beta.pdf Borssele OHVS - Scour and scour protection 5 of 49

34 The d 85 /d 15 value found for the upper layer at the location of Borssele Alpha and Borssele Beta is approximately Figure 2.1 Particle size distribution for the upper layers of BH-WFS2-1A (location of Borssele Alpha) (Fugro, 2015). Figure 2.2 Particle size distribution for the upper layers of BH-SubStatB-1 (location of Borssele Beta) (Fugro, 2016). 6 of 49 Borssele OHVS - Scour and scour protection

35 2.1.2 Morphology In preparation of the development of the Borssele Wind Farm Zone several studies of the area were performed. One of these studies involved the morphology and morphodynamics of the seabed in the BWFZ (Deltares, 2015). Besides the qualitative description of the morphology in the BWFZ the study also provided predictions of future seabed level variations. These predictions are summarised in Table 2.2 for the location of Borssele Alpha and Borssele Beta. Location Depth 2015 RSBL MSBL Max. lowering Max. rise Alpha -30.7m LAT -31.4m LAT -28.8m LAT -0.7m +1.9m Beta -32.1m LAT -32.8m LAT -29.9m LAT -0.7m +2.2m Table 2.2 Seabed level predictions for the two Borssele OHVS locations. Depth 2015 is the water depth in 2015 according to the morphology study; RSBL is the Reference Seabed Level, the lowest seabed level predicted till 2046; MSBL is the Maximum Seabed Level, the highest seabed level predicted till Figure 2.3 Maximum predicted seabed level rise around Borssele Alpha (left) and Borssele Beta (right), including an overlay of the platform and cables. Figure 2.4 Maximum predicted seabed level lowering around Borssele Alpha (left) and Borssele Beta (right), including an overlay of the platform and cables. Borssele OHVS - Scour and scour protection 7 of 49

36 Figure 2.3 and Figure 2.4 furthermore show that both platforms are located rather favourable with respect to the morphodynamics (i.e. only limited potential seabed level lowering is predicted) even though the area surrounding the platform shows much higher morphodynamic activity. Only a small potential lowering of the seabed is predicted at the locations of the two OHVS jackets. In the morphology study a larger potential for seabed level rise is caused by migrating sand waves. Therefore it will occur slowly over the course of many years. Note that due to morphodynamic changes (sea bed level rise or lowering) the overall scour pattern (both local and global) can slightly change. This is related to the main scour-inducing structural features having an altered position relative to the surrounding seabed, affecting the vortex patterns. Regardless of these potential changes in scour patterns, the fixation level of the foundation piles is expected to lower, should the predicted seabed lowering occur. If the seabed rises the fixation level of the foundation piles might rise accordingly. Please note that the morphodynamic development is not part of the present scope. It is however mentioned here to provide a complete overview of the boundary conditions at the Borssele site. 2.2 Metocean conditions The basis for the metocean conditions is the metocean study performed by Deltares (2015). Specifically for the location of the substations, an additional assessment was performed prior to the physical model test programme to derive the extreme hydrodynamic conditions for the platforms (Deltares, 2016a). In this section we present a summary of these conditions Water levels An overview of the relevant tidal water level variations on the location of Borssele Alpha and Borssele Beta is given in Table 2.3. The tidal range between Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT) is 3.65m for Borssele Alpha and 3.55m for Borssele Beta. Tide level Borssele Alpha Borssele Beta Rel. to MSL (m) Rel. to LAT (m) Rel. to MSL (m) Rel. to LAT (m) Highest Astronomical Tide (HAT) Mean Water Level (MSL) Lowest Astronomical Level (LAT) Table 2.3 Average tide levels at Borssele Alpha and Borssele Beta, rounded off to the closest 0.05m. Besides the tide, storm surges and set downs were determined. The extreme high and low water levels are provided in Table 2.4 and Table 2.5 for return periods of 1, 10 and 100 years for respectively Borssele Alpha and Borssele Beta. Please note that for the extreme high water levels, no potential sea level rise is included. For this project scope not taking into account sea level rise is the normative situation. Return period Extreme high water level [m MSL] Extreme low water level [m MSL] 1 year HWL LWL year HWL LWL year HWL LWL Table 2.4 Extreme high and low water levels (unassociated) relative to MSL for Borssele Alpha (Deltares, 2016a). 8 of 49 Borssele OHVS - Scour and scour protection

37 Return period Extreme high water level [m MSL] Extreme low water level [m MSL] 1 year HWL LWL year HWL LWL year HWL LWL Table 2.5 Extreme high and low water levels (unassociated) relative to MSL for Borssele Beta (Deltares, 2016a). Please note that the values presented in Table 2.4 and Table 2.5 are best-estimated values Current velocities The mean tidal current velocity is in the order of 0.70m/s and the 90%-upper limit is in the order of 0.85m/s for both Borssele Alpha and Borssele Beta (Deltares, 2016a). The tidal current has a dominant NNE-SSW axis (Deltares, 2015). Table 2.6 presents the extreme depth-averaged current velocities for return periods of 1, 10 and 100 years for both Borssele Alpha and Borssele Beta. Return period Current speed (m/s) Borssele Alpha Borssele Beta 1 year year year Table 2.6 Current speed return value estimates (unassociated) for substation Borssele Alpha and Beta. Please note that the values presented in Table 2.6 are best-estimated values Wave conditions Extreme wave conditions for all output locations were computed by means of an extreme value analysis and wave modelling in the Borssele metocean study (Deltares, 2015) and are available as model output. SWAN computations and the determination of extreme wave conditions are further explained in the Borssele metocean study (Deltares, 2015). The resulting extreme wave conditions consist of omni-directional 1-, 2-, 5-, 10-, 50- and 100-yr return values of the significant wave height and the associated peak wave period. The significant wave height and associated peak wave period at the location of Borssele Alpha and Borssele Beta were extracted from the in-house SWAN model results (Deltares, 2016a). Table 2.7 and Table 2.8 present the best-estimate values of these wave heights and wave periods. Return period Minimum still water level Borssele Alpha Maximum still water level H s (m) T p (s) H s (m) T p (s) 1 year year year Table 2.7 Extreme omni-directional wave condition (H s and T p) for substation Borssele Alpha. Values are rounded off to the nearest 0.05cm and to the nearest 0.1s. Borssele OHVS - Scour and scour protection 9 of 49

38 Return period Minimum still water level Borssele Beta Maximum still water level H s (m) T p (s) H s (m) T p (s) 1 year year year Table 2.8 Extreme omni-directional wave condition (H s and T p) for substation Borssele Beta. Values are rounded off to the nearest 0.05cm and to the nearest 0.1s. The largest waves come predominantly from the NNW direction, followed by the NW- WSW direction, see Figure 2.5. This figure presents the annual wave rose at the selected location for Site II. This annual wave rose does not differ much from those at the other sites and are therefore considered representative for the Borssele Alpha and Borssele Beta locations. Figure 2.5 Annual wave rose at the selected location for Site II. 2.3 Platform design In the present phase a detailed design of the Borssele Alpha platform is available. For the Borssele Beta platform such a detailed design is not yet available. However, the platform design for Borssele Beta is not expected to differ much from the design of Borssele Alpha (based on conversation; 31 March 2016; 14:17, sent by Evert Mom). Minor differences might be incorporated to cope with the slightly larger water depth at the location of Borssele Beta. It was expected that these minor differences between Borssele Alpha and Beta would not significantly influence the scour development. In consultation with TenneT it was therefore decided to base the scale model on the detailed design of Borssele Alpha. The design of Borssele Alpha consists of a four-legged jacket structure with main dimensions of 29m by 35m, see Figure 2.6. Each jacket leg is connected to two pile sleeves and the mudmat through stiffner plates and ring stiffners. At each corner two foundation piles will support the jacket. The diameter of these foundation piles is 1830mm (excluding marine growth). In the metocean report (Deltares, 2015) a value for the marine growth of 50mm is given, which is slightly smaller than the generic value presented in the DNVGL guidelines (100mm). In the physical model test programme an average marine growth is accounted for and we considered a pile diameter of 1980mm (1830mm+2 75mm). The orientation of the Borssele Alpha platform is 25 with respect to the true north. For the Borssele Beta platform a similar orientation is presently foreseen by TenneT. 10 of 49 Borssele OHVS - Scour and scour protection

39 MSL N 35m 29m Figure 2.6 Design of the Borssele Alpha jacket. Left: three-dimensional overview. Right: plan view. Borssele OHVS - Scour and scour protection 11 of 49

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41 3 Physical model setup 3.1 Introduction This chapter discusses the setup of the physical model tests. As a starting point of this chapter background of scour around jackets is presented in Section 3.2. The scaling methodology used to translate the conditions in the field to model scale is introduced in Section 3.3. In this section the scale factor is also presented. The test facility used for the physical model tests is discussed in Section 3.4. Section 3.5 subsequently presents the scale models. The instrumentation that will be used for measuring both hydrodynamics and the bathymetry is introduced in Section 3.6. The schematisation of the test conditions is discussed in Section 3.7 and remarks regarding the scour protection design are presented in 3.8. Tthe test programme is summarised in Section Background/Terminology When looking at the scour patterns around jacket structures a distinction can be made between local scour around the foundation piles and global scour around the whole jacket structure. Local scour (erosion of soil around a structure) occurs as a result of a local disturbance of the hydrodynamics by a structure. When dealing with foundation piles this disturbance of the hydrodynamics is characterized by several hydrodynamic features: a downflow at the upstream side of the pile, which rolls up and is transported along the pile, resulting in a horseshoe vortex at the base of the pile. At the downstream side of the pile vortex shedding is also a distinct phenomenon. Figure 3.1 illustrates these flow features. This disturbance of the flow around the pile amplifies the bed shear stress and the sediment transport around the structure thereby creating a gradient in sediment transport, leading to erosion of the seabed. The scour hole that develops is deepest directly at the structure and extends generally 3-5 times the pile diameter around the foundation pile. The formation of the local scour hole is very dependent on the development of the abovementioned flow features. In relatively shallow water depth, extreme storm events generally result in larger flow velocities at the bed than a tidal condition. However, in these storm events the flow features that result in the local scour holes around the foundation piles (see Figure 3.1) are continuously broken off due to the reversal of the wave-induced flow. The formation of local scour holes is therefore generally dominated by tidal conditions. Figure 3.1 Characteristics features of the flow pattern around a pile (Roulund, 2005). Borssele OHVS - Scour and scour protection 13 of 49

42 Apart from the flow patterns and resulting local scour around single structural elements, the complete set of structural elements of a jacket also results in scour; global scour. This global scour is the result of the turbulence introduced by the combination of the complete set of structural elements of a jacket. In turn this turbulence amplifies the bed shear stress and sediment transport in the surrounding area, typically leading to a shallow wide depression; global scour. This type of scour typically occurs within and around the footprint of jacket foundations. Global scour typically takes years to develop. Please note, that the distinction between global and local scour is generally not very strict, because the two scour patterns will naturally coincide. Another distinction that can be made is related to the upstream sediment supply. Scour is defined as clear water scour if scour occurs only locally near the structure (no sediment input from upstream), whereas live-bed scour refers to the situation where the upstream bed is also mobile (and sediment input occurs from upstream). When a scour protection is applied around a structure, a different scour mechanism is important to consider, namely edge scour. With the area near the structure protected, scour is shifted towards the edge of the protection. These edge scour holes are the result of the disturbance of the flow due to the structure (similar to local and global scour) and the influence of the scour protection itself. Generally, edge scour holes are smaller than local scour holes and take a longer time (in the order of 5 to 10 years) to fully develop. 3.3 Scaling methodology Scaling laws The most important scaling criteria for laboratory tests on the subject of deformation of a loose rock scour protection are: Froude similarity (hydrodynamics, structure dimensions) Mobility similarity (seabed material and rock protection) Reynolds similarity (waves and current) Froude similarity For the water motion in free surface waves, gravitational and inertial forces are the dominant forces. Consequently, the ratio of these forces should be equal in model and nature, so as to preserve dynamic similarity. This is achieved by reproducing the Froude number Fr, 2 2 ( Fr u gl, where u is current velocity or wave orbital velocity, g gravitational acceleration, and L a length scale) on a scale of n Fr = 1. This is called the Froude scaling law. From the Froude law the following scaling relationships, expressed in terms of the model scale factor n scale, are derived: length-related parameters [m] : n = n (e.g. wave height, footing diameter, water depth) 0.5 time-related parameters [s] : n = n (e.g. wave period, sto T scale L 0.5 velocity-related parameters [m/s] : n = n (e.g. current velocity, wave orbital velocity) V scale scale rm duration) where an arbitrary scale factor n scale is defined as the ratio of the prototype value to the model value. Froude scaling can usually be fulfilled for most parameters, such as wave height and structural dimensions. Seabed sediment often cannot be scaled down sufficiently. As soon as the diameter of the sediment becomes smaller than about 60μm, the sediment is 14 of 49 Borssele OHVS - Scour and scour protection

43 characterized as silt, which has different (cohesive) erosion behaviour than sand. As a consequence, Froude scaling of the grain diameter in general would only allow for very large model scales (such as 1:2). Mobility similarity To allow for much smaller model scales (such as 1:20 to 1:60), Froude similarity is no longer satisfied for the particle size. Instead, the scaling methodology is aimed at reproducing a similar relative mobility in the model as in the field. The relative mobility is defined as: MOB, MOB FIELD MOB (3.1) MODEL c where θ is the Shields parameter, θ c is the critical Shields parameter and MOB is the ratio between them. The mobility similarity criterion is used to correctly scale the sediment, ensuring similar mobility in the field and at model scale. A relative mobility value larger than 1 indicates that a particle is likely to be entrained or unstable, while with a relative mobility value smaller than 1 little to no movement is expected. Please note, that this is related to the undisturbed relative mobility (without the present of a structure). In the vicinity of a structure the load can be increased by the structure and particles can become unstable at lower (undisturbed) relative mobility values. Reynolds similarity Besides Froude similarity, the flow in the model has to be sufficiently turbulent. To address this issue the Reynolds similarity can be used. Several definitions of the Reynolds numbers exist, but they are all based on the ratio between the inertia forces (ρ w u 2 L 2 ) and the viscous forces (on a certain length scale; µul), in which ρ w is the fluid density, u is the flow velocity, L is a certain length scale and μ is the dynamic viscosity. For open-channel flow the Reynolds numbers for currents and waves respectively become: Re c u h u & w h Rew v v c w w (3.2) in which u c is the current velocity, u w is the wave orbital velocity, h w is the water depth and ν is the kinematic viscosity (ν = μ/ρ w ). These numbers determine the level of turbulence and the size of the eddies that will develop in the flow. Reynolds numbers are also defined for the flow around a structure, such as jacket legs or foundation piles, with an outer diameter D. In this case, the formulae become: Re ud u & w D Re v v c c; D w; D (3.3) Sumer and Fredsøe (2006) present several vortex regimes for cylindrical piles. Since the kinematic viscosity of water (ν), which is mainly dependent on the water density and water temperature, will not vary much between model and field situation, and the current velocity and the pile diameter will scale down with respectively the square root of the scale factor (n scale 0.5 ) and the scale factor (n scale ), the Reynolds-number will always be smaller in a laboratory model test than in the field. This scale effect is inherent to model testing. To keep the effect as small as possible, it is considered to be sufficient to assure that the flow does not Borssele OHVS - Scour and scour protection 15 of 49

44 become laminar, but remains in the same (turbulent) regime as in the field situation. The solution is to choose the largest model scale possible Model scale factor The model scale factor is governed by the hydrodynamic conditions to be modelled, the basin dimensions and measurement requirements (e.g. using a video camera system inside a model). Within these boundaries, the scale factor is preferably chosen as small as possible to minimize scale effects. Based on the basin dimensions (e.g. the wave makers capacity to generate the design wave height and the maximum discharge), the scale factor for this test programme in the Atlantic Basin is chosen to be 45. The test facility is further discussed in the next section. With this scale factor the hydrodynamic conditions can be scaled with the Froude scaling law (with some minor adjustments to ensure a sufficiently high relative mobility of the sediment). Moreover, this scale factor allows for the placement of cameras inside the foundation piles. See section for this measurement technique Application of the scaling criteria in the present study For water motion in free surface waves, gravitational and inertial forces are the dominant factors. Consequently, Froude scaling was applied for all length scales (structure dimensions, scour protection layout dimensions) and the hydrodynamics (water depth, wave height, wave period). This required that the length dimensions are divided by the scale factor and the time dimensions by the square root of the scale factor. The sediment will not be scaled based on Froude, because that would lead to a grain diameter smaller than 60μm, which has different (cohesive) erosion behaviour. Instead the smallest commercially available grain diameter is applied, that still has non-cohesive properties and can still be compacted (d μm). As mentioned above, the development of the local scour holes will be dominated by the tidal conditions. In these conditions the relatively coarse sand in the scale model would result in an underestimation of the local scour depth when the hydrodynamic conditions are scaled with the Froude scaling law. In the current-only tests, the current velocity was therefore increased to achieve a similar sediment relative mobility in the model as in the field (and compensate for the sediment being too coarse in the model). In the storm tests, this was not necessary, because the sediment was already highly mobile. Please note that this approach is common practice and has proven to be suitable in many earlier projects. The influence of fluid viscosity will be considered based on the Reynolds number. The Reynolds number describes the level of turbulence and size of eddies that will develop for a certain length scale (e.g. water depth, pile diameter or rock size). In general, because the kinematic viscosity of water cannot be scaled down, in laboratory tests the Reynolds number will always be smaller than in the field. Viscous effects are negligible for Reynolds numbers related to the water depth larger than 10,000 (Whitehouse, 1998), which is the case for the proposed tests. The Reynolds number related to the structural dimensions is also sufficiently large to limit viscous effects. 3.4 Test facility The tests will be performed in Deltares Atlantic Basin. This basin has a length of 75m, a width of 8.7m and a maximum water depth of around 1m; see Figure 3.2 and Figure 3.3. The basin is equipped with a paddle type wave generator consisting of 20 individually controlled 16 of 49 Borssele OHVS - Scour and scour protection

45 paddles to generate waves. Each paddle is equipped with Active Reflection Compensation technology to take out reflected wave energy. The wave generator can generate all of the common wave spectra (e.g. JONSWAP, regular waves or a user-defined spectrum). The current in the Atlantic Basin can be generated in two directions (either following the waves or in the opposing direction). This means that a tidal current can be generated (with or without waves). For offshore projects, the use of a tidal current is often preferred in order to avoid over-conservative results, which would occur when only a unidirectional current is used. The maximum pump capacity for generating the current is 3m 3 /s. A test section of 130m 2 (15mx8.7m) is located in the middle of the basin, which is filled with a 0.5m thick layer of compacted non-cohesive sand (d 50 = 170μm). Figure 3.2 Layout of Atlantic Basin with sandy test section in the middle of the basin. In the Atlantic Basin accurate wave-current-interaction can be simulated, because the flow will enter from below the basin floor in front of the wave generator. The wave generator and the current inflow both have a fixed setup and the return flow circulates through the basement below ground floor level. The Atlantic Basin is a state-of-the-art facility (operational since 2009) and has acquired an extensive track record since then. Figure 3.3 Atlantic Basin, looking from the wave spending beach towards the test section in the middle of the basin and the wave paddles at the end of the basin. The large width of the Altantic Basin allows two models to be placed next to each other. By doing so two orientations of the platform can be tested simultaneously. Based on the annual wave and current rose and the orientation of the Borssele Alpha and Borssele Beta platform Borssele OHVS - Scour and scour protection 17 of 49

46 (Figure 3.4; Deltares 2015), the two orientations of the platform as presented in Figure 3.5 are chosen. N Figure 3.4 Annual wave rose (left); Platform orientation (middle) and annual current rose (right). Platform 1 Platform 2 Figure 3.5 Test layout in the Atlantic Basin. The orientation of Platform 1 is 90 with respect to the north of the platform (and 65 with respect to the true north). This orientation corresponds with the main tidal axis, see Figure 3.4. Moreover, it is also representative for waves coming from the SSW and NNE direction. The orientation of Platform 2 is 45 with respect to the north of the platform (and 20 with respect to the true north). This orientation is expected to result in the most severe scour patterns and it corresponds to waves coming from the W-WSW direction and to waves coming from the dominant NNW-N direction (in a mirrored way). Moreover, the orientation of Platform 2 also corresponds to a WSW-ENE tidal axis, which is also observed in the annual current rose due to the slightly rotating tide. Because the chosen platform orientations are 18 of 49 Borssele OHVS - Scour and scour protection

47 representative for several different incoming storm and tidal directions, referring to sides or piles of a platform will be based on the placement in the model, not in the field. In these types of (three-dimensional) studies the basin should be sufficiently wide to limit the effect of the basin walls (i.e. flow should not be forced around the structure due to the narrow width of the basin). As a rule of thumb the obstruction of the structure(s) should not be larger than 1/5 of the basin width. If the jackets in this study were complete solid they would have a combined obstruction width of approximately 1/3.9 of the basin width. However, the jackets are relatively open and water can flow through the jackets. When assuming the jackets to be 25% open and 75% solid, the restriction of the basin width is already complied with. For these type of structure such a percentage of transparency seems really conservative, and the effect of the basin width is therefore expected to be negligible. 3.5 Scale models The two scale models of the Borssele Alpha and Beta platforms were designed based on the three-dimensional model issued by TenneT on the 6 th of April 2016, as well as the prototype drawings dated on the 3th of March The scale models include the main trusswork of the foundation (see Appendix A). There are some small differences between the model and the real structure, which are related to practical considerations (e.g. to use available pipe diameters and facilitate welding) and upward for conservatism. The drawings of the scale model were sent to TenneT for a review on the 19 th of May Figure 3.6 The construction of (left) and the finalized (right) scale models of the Borssele Alpha and Borssele Beta platform. Figure 3.6 presents the scale models of the jackets of the Borssele Alpha and Beta platform. Please note that the top side is not incorporated in this scale model, to allow for a better visual inspection during the test and a better performance of the stereophotography Borssele OHVS - Scour and scour protection 19 of 49

48 technique, see Section The height of the jackets is such that the structures were emerged during the highest waves. All welds or other possible openings in the scale models were sealed off prior to the tests. By doing so, water could not travel through the structure, which might influence the observed scour patterns. If for example, the J-tubes were not sealed off water could enter at the top during an overtopping wave, which would results in increased flow velocities at the bellmouth. At each corner one of the foundation piles is constructed in stainless steel. The other foundation pile is made of Plexiglas to allow for visual inspection with a camera inside the foundation pile, see Section At this Plexiglas foundation pile a LED-ring is incorporated within the mudmat, to provide sufficient light even with small scour depths. 3.6 Measurements and documentation Hydrodynamic measurements Figure 3.7 presents the locations of the wave height meters (WHM) and electro-magnetic current velocity meters (EMS). Flow velocities are measured with four electro-magnetic current velocity meters (type EMS-30, developed in-house by Deltares and sold on the market as well). During the tidal conditions the direction of the flow is consecutively from left to right and from right to left. To measure the undisturbed approach flow velocity for both directions, two EMS sensors were placed at the right side and two EMS sensors were placed at the left side of the model. The instruments measure at 40% of the water depth, where the current velocity is close to the depth-averaged value (assuming a logarithmic velocity profile). The water level fluctuations are measured by means of six resistance-type wave gauges (type WHM-50). Three WHM s are located 2.5m upstream of the models, measuring the undisturbed incoming wave height. The other three WHM s are located throughout the basin. The first is positioned close the wave maker, in order to measure the incoming wave signal. The second is located half way between the wave maker and the test section, giving insight into the propagation (and decay) of wave energy along the length of the basin. The last wave gauge is located in the middle of the basin, 2.5m downstream of the scale models. Figure 3.7 Location of the wave height meters (WHM, indicated in blue) and electro-magnetic current velocity meters (EMS, indicated in red) in the basin. WHM3-WHM6 and EMS1-EMS4 are connected to a beam spanning the basin. WHM1 and WHM2 are installed on a slender tripod and are located upstream of the test section. 20 of 49 Borssele OHVS - Scour and scour protection

49 In addition to the current velocity meters and the wave gauges, two water level gauges and three flow discharge meters are used. The water level gauges are located inside the basin wall, and measure the water level at both ends of the basin. The flow discharge meters measure the discharge of the three pumps of the Atlantic Basin Bathymetrical measurements Before and after each test Before and after each subtest photographs are taken to document the tests and the deformation patterns associated with each layout and condition. Apart from regular photographs, digital stereo-photography is applied to record the scour patterns around the model (Raaijmakers et al., 2012). The setup of this technique consists of two digital cameras with a fixed distance between each other, a laptop and a set of markers. The applied cameras are two synchronised 5 Megapixel cameras (type Flea-2 by Point Grey). The marker set consists of one base plate marker, which defines the origin of the coordinate system and several regular markers. After placing the markers around the model, photographs are taken from different angles around the platforms. The 3D-environment (consisting of all camera and marker positions) is reconstructed from the photographs based on an iterative computational process. Finally, the data are corrected for lens distortion and difference in focal length and position and translated into xyz-coordinates by means of dense stereo-matching. This technique provides high-resolution data on the vertical and lateral extent of the deformation and magnitude of the scour depth around the structure. Figure 3.8 illustrates the execution of the stereo-photography technique and Figure 3.9 presents an example of the 3D-vizualisation of the scour development around offshore platforms. The accuracy of the stereo-photography measurement is 1mm on model scale (Raaijmakers et al., 2012). When considering the difference between two measurements in the present test set up (scale factor of 45) the accuracy on field scale will be smaller than 0.1m. Figure 3.8 Photograph of stereo-photography measurements performed. Borssele OHVS - Scour and scour protection 21 of 49

50 In addition to the stereophotography measurements we have applied a ruler on each steel foundation pile. With this ruler the scour depth at each steel foundation pile can be determined. The accuracy with which the scour depth can be determined based on the ruler is estimated to be smaller than 0.5m Figure 3.9 3D-visualization of scour development around by means of the stereo-photography technique. During the tests During the tests, the scour development is monitored with four underwater cameras. Figure 3.10 presents an example of the images captured with these underwater camera images. These cameras are placed in such a way that they do not disturb the flow and scour patterns around the platforms. Figure 3.10 Example of an underwater camera (top left) and snapshots taken with this camera during a test for the Dolwin3 project (bottom right).the severe lighted area is related to the LED lights incorporated within the mudmats, to allow a good view from the internal cameras. 22 of 49 Borssele OHVS - Scour and scour protection

51 Figure 3.11: Left image: Scour hole that developed around a transparent monopile equipped with a downward looking camera with fisheye lens. Right image: 360 view from the camera inside the monopile. The red line shows the interface between the sediment and the pile, recognised by means of image recognition tools. The yellow line shows the initial interface before the tests. Deltares currently investigates the feasibility of this measurement technique using smaller cameras inside the jacket foundation piles. In addition to these underwater camera images, internal cameras are fitted inside four of the eight foundation piles of each platform (one at each corner). With these cameras the scour development can be measured as a function of time. The left image of Figure 3.11 presents the Perspex foundation piles equipped with a camera. In the right image of this figure the view captured with the camera is presented. The yellow line indicates the initial bed level adjacent to the pile and the red line indicates the bed level at the end of the test. This identification of the contour line, combined with a calibration of the images, provides quantitative information of the scour development in time. Figure 3.12 LED lights are incorporated in the design of the mudmats to allow for a good view with the internal cameras. Even though this internal camera technique has been used many times in (relatively large) monopiles, the application of this technique within the relatively slender foundation piles of this jacket platform is new. Apart from fitting a camera, lens and wiring all within the small and watertight foundation piles, additional lighting is required close to the foundation pile to ensure Borssele OHVS - Scour and scour protection 23 of 49

52 a good view of the scour development. Namely, the jacket structure itself and especially the mudmats and neighbouring foundation pile will block the ambient light. This additional lighting is done by means of a custom made LED-ring, which is incorporated within the mudmat, see Figure Please note that this application of the internal camera technique is newly developed and that in the present scope of this project the information gained from this technique is not essential for meeting the objective, but it will help us better understand the scour development. 3.7 Schematisation of field conditions Soil conditions The Atlantic Basin is equipped with non-cohesive sediment. Appendix B.1 presents gradings curves of several samples taken from the Altantic Basin. This appendix shows that the medium grain size is approximately 170µm. A correction on the Froude scaling of the hydrodynamic conditions is applied to achieve a similar mobility of the sediment in the basin as the expected mobility of the sediment in the field Hydrodynamic conditions Introduction In relatively shallow water depth, extreme storm events generally result in larger flow velocities at the bed than a tidal condition. However, in these storm events the flow patterns that result in the local scour holes around the foundation piles (see Figure 3.1) are continuously broken off due to the reversal of the flow. The formation of local scour holes is therefore generally dominated by tidal conditions. Storm conditions are however also important to consider because these conditions can be important for the development of a global scour hole. Load combinations Based on the above considerations, each test series consisted of a selection of tidal and storm conditions. For the tidal conditions we selected an average tidal flow velocity and a 90% upper limit value. The selected storm conditions include an RP10yr storm event and a RP100yr storm event. The RP100yr storm event is based on one of the Ultimate Limit State (ULS) conditions presented in the guidelines by DNV GL on load combinations (DNVLGL-OS- C101), see Table 3.1. This ULS condition is referred to the RP100yr condition in the remaining part of this document. Limit state Waves Current Water level ULS Table 3.1 Return periods of environmental load incorporated in one of the ULS conditions mentioned in DNVLGL- OS-C101. Please note that the load combination given by the DNVGL guideline, is realtively conservative. Namely, it assumes a design storm event (wave height, wave period and current velocity) with a maximum setdown of the water level, while generally such a storm event is accompanied with a setup of the water level. By evaluating two tidal and two storm conditions (instead of only testing the maximum tidal velocity and design storm) we might be able to derive a trend between scour and the hydraulic load and develop a better understanding of the scour depth. 24 of 49 Borssele OHVS - Scour and scour protection

53 Table 3.2 and Table 3.3 present the environmental conditions of the considered load combinations for respectively Borssele Alpha and Borssele Beta. Please note that the presented values are on field scale. For both tidal conditions the water depth is based on a water level equal to MSL. For the storm condition the corresponding minimum still water level is incorporated in the water depth. Regarding the wave characteristics, we selected the values related to the maximum still water level. These values might be considered as slightly conservative, as we included a relatively low water level. However, the differences in wave characteristics at minimum and maximum still water level are relatively small (see Table 2.7 and Table 2.8) and all within the uncertainty range. Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Tide_mean Tide_max RP10yr RP100yr Table 3.2 Load combination; Borssele Alpha (field scale). Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Tide_mean Tide_max RP10yr RP100yr Table 3.3 Load combination; Borssele Beta (field scale). Because the conditions for Borssele Alpha and Beta are relatively similar, there is no need to simulate the scour development for both locations separately. Instead we simulate the scour development at the normative location. To determine which location is indeed normative we compute the undisturbed relative mobility (MOB) and the Keulegan-Carpenter (KC) number. As mentioned in Section 3.3, the relative mobility can be interpreted as a measure for the stability of a particle and is given as the ratio between the Shields parameter and the critical Shields parameter. A larger undisturbed mobility value therefore indicates more movement of the sediment. The KC-number is a measure for wave-structure interaction, which is typically used to describe the vortex regime (and wave load on the seabed). It includes the effect of water depth, wave height, wave period and structural dimensions: KC ut b p (3.1) D p In which: u b the bed orbital velocity [m/s] KC the Keulegan-Carpenter number [-] T p the peak wave period [s] D p the pile diameter [m] In general a larger KC-number indicates more severe vortex shedding from the jacket legs, bracings etc. and therefore a larger local amplification of the load. Borssele OHVS - Scour and scour protection 25 of 49

54 Load combination MOB [-] KC[-] Alpha Beta Alpha Beta Tide mean Tide max RP10yr RP100yr Table 3.4 Computed undisturbed mobility (MOB) and KC-number for selected load combinations. The undisturbed mobility values and the KC-number for the selected load combinations are presented in Table 3.4. This table shows that generally the mobility values and KC-numbers at Borssele Alpha are slightly larger than at Borssele Beta. However, the differences are very small. The load combinations at Borssele Alpha are therefore considered normative and representative for both Borssele Alpha and Beta. Scaling of load combinations The resulting model scale values of the normative load combinations are presented in Table 3.5. Load case Environmental conditions H m0 [m] T p [s] U c [m/s] Water depth [m] Tide_mean Tide_max RP10yr RP100yr Table 3.5 Normative load combinations (model scale). Because the mobility during storm conditions is relatively high, the scaling of these hydrodynamic conditions is based on Froude scaling: the wave height and water depth are divided by the scale factor and the wave period and flow velocity are divided by the square root of the scale factor. For the tidal conditions corrections are applied to achieve a sufficiently large mobility at model scale (similar to the mobility at field scale). This was done by increasing the flow velocity and decreasing the water depth. Test duration During the tidal test we ran an ebb and flood current alternatively until an equilibrium situation is reached, which is verified by means of the underwater camera images as well as the internal camera images. Each flow condition will be continued for 50 minutes (approximately 6 hours on field scale). For the storm conditions an extreme sea state duration of 6 hours (at field scale) is applied. The resulting test duration is approximately 1 hour at model scale (3220s). 3.8 Scour protection In this section the characteristics of the conceptual scour protection layout that is tested in the second test series are presented. We have made a distinction between the conceptual scour protection layout (Section 3.8.1) and the tested stone grading (Section 3.8.2) Scour protection layout To prevent scour around the foundation piles and to stabilize the seabed near the J-tubes, a conceptual scour protection was developed. Based on experience with similar structures and a theoretical understanding of relevant processes we selected a 3-9 HD (high density) 26 of 49 Borssele OHVS - Scour and scour protection

55 grading as armour grading of the conceptual scour protection layout, which was agreed upon by TenneT. The specifications of this 3-9 HD grading, both on field and model scale are presented the next section. A 3-9 HD armour grading can be incorporated in (1) a pre-installed single-graded system, (2) a pre-installed two-graded system or (3) a pre-installed filter + (partly) post-installed armour system. These different scour protection concepts are illustrated in Figure Which protection concept is selected is not known a-priori and will depend on many considerations, such as installation procedure, pile driveability and economic aspects. The final design of the scour protection, including the choice for a specific scour protection concept, will therefore be made by the contractor. In a two-graded system the armour layer should ensure external stability of the scour protection; the extent and thickness of the armour material should be chosen such that the stability of the underlying filter layer is maintained in a design storm event. In turn the filter layer should be sufficiently flexible (to cope with the development of edge scour holes and global seabed changes) and sand-tight (to prevent winnowing; the washing out of sediment through the scour protection). When applied as a single-graded system the armour layer should not only ensure external stability, but also flexibility and sand-tightness. Consequently, the armour layer generally has a larger thickness and extent in a single-graded system than in a two-graded system, see Figure Figure 3.13 Schematisation of scour protection concepts. The aim of the physical model tests is to simulate the external stability of a 3-9 armour grading, independent of which of the three protection concepts is chosen by contractors. Of the three protection concepts the external stability of the (partly) post-installed armour grading (concept 3) is expected to be least favourable. Consequently, the tested armour layer, which is presented in Figure 3.14, is similar to concept 3 (Figure 3.13). Please note that the thickness and extent of the tested armour layer will depend on the expected deformation of the layer itself. As this was not known a-priori, both thickness and extent were chosen relatively large to make sure that the deformation would remain within the rock layer. Both are based on expert judgement. In addition to the external stability of the scour protection, which is tested in this test programme, other failure mechanisms such as edge failure and winnowing (washing out of sediment through the protection) should be considered in a detailed design of the scour protection. For example, a filter layer consisting of smaller stones (e.g. 1-3 ) might be applied to prevent edge failure and winnowing. Borssele OHVS - Scour and scour protection 27 of 49

56 Figure 3.14 Tested scour protection layout Tested stone grading As mentioned in the previous section the tested scour protection layout included a 3-9 HD grading. The characteristic stone sizes of this grading on field scale are presented in Table 3.6. The characteristics of the 3-9 HD grading are based on characteristics mentioned in the Rock Manual (CIRIA, 2007). Table 3.6 also present the specific density of the stones. Please note that NoRock mentions a density of 3200kg/m 3. Based on previous projects and based on the (model scale) gradings from NoRock that we have available, our experience is however that a density of 3050kg/m 3 is more accurate. We therefore accounted for a density of 3050kg/m 3 for both the field scale and model scale HD gradings. 28 of 49 Borssele OHVS - Scour and scour protection

57 Parameter D 15 [m] D 50 [m] D 85 [m] ρ s [kg/m 3 ] 3-9 HD Table 3.6 Characteristics of a 3-9 HD grading. As mentioned in Section 3.3, the scaling of the rocks (and the sand) is based on a similarity in relative mobility between field and model scale. Apart from the similarity in relative mobility, also the width of the grading is important to consider when scaling down a stone grading. The width of the grading is important for the armouring effect and therefore for the stability of the layer as a whole. Consequently, the d 50 value of the model scale grading is based on a similarity in relative mobility and the d 15 and d 85 values are both based on a similarity in relative mobility but also on a similarity in width of the gradation. Moreover, in the calculation of the relative mobility several roughness formulations can be used. In this scaling procedure either the Soulsby formulation (relatively small gradings) or the Dixen formulation (relatively large gradings) is used. Because the 3-9 grading is a relatively small armour grading, the presented target ranges of this grading also include this difference in roughness formulation. Table 3.7 and Table 3.8 present respectively the D 15, D 50 and D 85 values based on similarity in relative mobility and similarity in width of the gradation. Table 3.9 presents the resulting target ranges for the model scale grading. D 15 [mm] D 50 [mm] D 85 [mm] Stone grading min max min mean max min max 3-9 HD Table 3.7 Scaled stone sizes, based on similarity in relative mobility. D 15 [mm] D 50 [mm] D 85 [mm] Stone grading min max min mean max min max 3-9 HD Table 3.8 Scaled stone sizes, based on similarity in width of the gradation. D 15 [mm] D 50 [mm] D 85 [mm] Stone grading min max min mean max min max 3-9 HD Table 3.9 Target sizes at model scale. The sieve curves of the model scale grading is presented and compared with the target size ranges in Appendix B.2. Please note that the model scale gradings have been provided by NoRock and are fabricated in a similar manner as the field scale grading. Therefore, note only the size but also the angularity and shape of the model scale stones is representative for field scale stones. 3.9 Test programme The test programme, including the hydrodynamic conditions, is summarised in Table The first test series (test series A) was focussed on the scour development around the Borssele Alpha and Beta platforms. As already mentioned in Section 3.7.2, local scour holes are expected to develop deepest under current-only conditions. We therefore start this test series with a mean tide. After that, two increasingly severe storm conditions are simulated. These storm conditions are included because these can be of importance for the development of the global scour hole. Borssele OHVS - Scour and scour protection 29 of 49

58 Please note that a maximum tidal condition is not included in this test programme, while it was considered in the test conditions. The maximum tidal condition was dismissed because the scour development during the mean tidal conditions already exceeded the acceptable limit of 5.6m. Test ID Load case Hydrodynamics at field scale Hydrodynamics at model scale Scale Hm0 Tp Uc Hw Hm0 Tp Uc Hw Duration factor [m] [s] [m/s] [m] [m] [s] [m/s] [m] [s] Test series A: Scour development A01 Tide mean x*3000 A02 RP10yr A03 RP100yr Test series B: Scour protection stability B01 RP10yr B02 RP100yr Table 3.10 Test programme Borssele OHVS. The second test series (test series B) were aimed at simulating the external stability of the conceptual scour protection layout. Because the stability of a scour protection is governed by storm events, this test series includes storm conditions. By simulating two increasingly severe storm conditions, instead of only the design condition, a better understanding is obtained about the stability of the stone grading; e.g. resilience, maintenance requirements etc. 30 of 49 Borssele OHVS - Scour and scour protection

59 4 Test results 4.1 Introduction The physical model tests were performed in the Atlantic Basin of Deltares in Delft from the 1 st of August 2016 until the 10 th of August The tests were witnessed by representatives of TenneT, Ramboll, DONG Energy and Rijkswaterstaat. The test programme consisted of two test series and a total of 5 tests. Within each test series the tests were run consecutively and one test was performed per day. The tests were executed in the afternoon, so that the basin could be drained very slowly during the night and bathymetry measurement could be performed in the morning of the following day. Appendix D presents a selection of photos during the preparation and execution phase of the test programme. An overview of the test conditions is given in Table All test results presented in this chapter are field scale values for easier interpretation. Test description Target conditions (field scale) Test date Test ID Test condition Hm0 Tp Uc Hw [m] [s] [m/s] [m] Test series A: Scour development A01a mean tidal current, ebb A01b mean tidal current, flood Aug A01c mean tidal current, ebb A01d mean tidal current, flood A01e mean tidal current, ebb A01f mean tidal current, flood Aug A02 RP10yr storm Aug A03 RP100yr storm Test series B: Scour protection stability 09-Aug B01 RP10yr storm Aug B02 RP100yr storm Table 4.1 Overview of the (target) test conditions (field scale). 4.2 Hydrodynamic conditions Table 4.2 presents a comparison between the target conditions (Table 4.1) and the tested conditions. Please note, that while Table 4.2 only presents the differences between the target and tested conditions, Appendix C present an extensive overview of the tested conditions. Table 4.2 shows that in all subtests the tested water depth is similar to the target condition. The table furthermore shows that the tested depth-average current velocity, the wave height and the wave peak period only show minor deviations (generally in the order of 3%) with the target conditions. The tested conditions are therefore considered representative. Borssele OHVS - Scour and scour protection 31 of 49

60 Test description Tested conditions (field scale) Test date Test ID Test condition Hm0 Tp Uc Hw [m] [s] [m/s] [m] Test series A: Scour development A01a mean tidal current, ebb % 100% A01b mean tidal current, flood % 100% 01-Aug A01c mean tidal current, ebb % 100% A01d mean tidal current, flood % 100% A01e mean tidal current, ebb % 100% A01f mean tidal current, flood % 100% 02-Aug A02 RP10yr storm 101% 103% 94% 100% 03-Aug A03 RP100yr storm 101% 102% 96% 100% Test series B: Scour protection stability 09-Aug B01 RP10yr storm 101% 103% 98% 100% 10-Aug B02 RP100yr storm 101% 102% 99% 100% Table 4.2 Difference between target and tested conditions. In order to evaluate the combined effect of the deviations of the hydraulic conditions, we consider two non-dimensional parameters; the relative mobility (MOB) and the Keulegan- Carpenter (KC) number. As mentioned in Section 3.3, the relative mobility can be interpreted as a measure for the stability of a particle and is given as the ratio between the Shields parameter and the critical Shields parameter. The KC-number is a measure for wavestructure interaction, which is typically used to describe the vortex regime (and wave load on the seabed). It includes the effect of water depth, wave height, wave period and structural dimensions: KC ut b p (4.1) D p In which: u b the bed orbital velocity [m/s] KC the Keulegan-Carpenter number [-] T p the peak wave period [s] D p the pile diameter [m] Table 4.3 presents these two hydraulic parameters based on the target conditions. The differences between the target values for these hydraulic parameters and the tested values are presented in Table 4.4. Both these tables present the relative mobility value of the sand for each test and the KC-number for the storm tests. For the second test series (scour protection tests) these tables also show the relative mobility of the applied stone grading; 3-9 HD. Please note that the calculation of the relative mobility value includes the determination of the wave friction factor. This wave friction factor can be calculated with the formulae by Soulsby (1997) or by Dixen (2008). The Soulsby formulation was developed for sediments (small gravel and sand range), whereas the Dixen approach, in addition to being more recent, has been explicitly extended to the armour rock sizes. As the scaled 3-9 HD grading is a relatively small armour grading, both formulae are applied and presented in Table 4.3 and Table of 49 Borssele OHVS - Scour and scour protection

61 Test description Target hydraulic parameters MOB [-] KC-number Test date Test ID Test condition 3-9" HD [-] Sand Soulsby Dixen Test series A: Scour development A01a mean tidal current, ebb A01b mean tidal current, flood Aug A01c mean tidal current, ebb A01d mean tidal current, flood A01e mean tidal current, ebb A01f mean tidal current, flood Aug A02 RP10yr storm Aug A03 RP100yr storm Test series B: Scour protection stability 09-Aug B01 RP10yr storm Aug B02 RP100yr storm Table 4.3 Overview of the (target) hydraulic parameters (field scale). Test description Tested hydraulic parameters MOB [-] KC-number Test date Test ID Test condition 3-9" HD [-] Sand Soulsby Dixen Test series A: Scour development A01a mean tidal current, ebb - 96% - - A01b mean tidal current, flood - 102% Aug A01c mean tidal current, ebb - 97% - - A01d mean tidal current, flood - 101% - - A01e mean tidal current, ebb - 96% - - A01f mean tidal current, flood - 101% Aug A02 RP10yr storm 107% 100% Aug A03 RP100yr storm 104% 100% - - Test series B: Scour protection stability 09-Aug B01 RP10yr storm 107% 100% 104% 106% 10-Aug B02 RP100yr storm 104% 100% 102% 105% Table 4.4 Difference between the target and tested hydraulic parameters. Table 4.4 shows that the differences between the target and tested (undisturbed) relative mobility of the sand and the applied stone gradings are very small for all tests. Moreover, in most cases the tested relative mobility is slightly larger than the target values and is therefore slightly on the conservative side. When we consider the hydraulic wave load (expressed as the KC-number), Table 4.4 also shows values that are slightly on the conservative side. Borssele OHVS - Scour and scour protection 33 of 49

62 4.3 Scour development As mentioned in Section 3.4 the chosen orientations of the model scale platforms represent several incoming storm and tidal conditions. In this chapter the specific piles of each platform are therefore referred to based on their placement in the model, not in the field. Figure 4.1 presents an overview of reference names given to each corner of the platforms. Please note that in this figure the blue dots indicate the foundation piles that were made of Perspex (equipped with the internal cameras) and that the red dots indicate the foundation piles that are part of the steel structure (equipped with a ruler). W NW SW Platform 1 N Platform 2 NE SE E S Figure 4.1 Reference of the corners of each platform. Below we present the observed scour development in mean tidal conditions (Section 4.3.1) and storm conditions (Section 4.3.2). In the discussion of the results we frequently refer to the photos taken during the tests, the results of the underwater cameras, stereo-photography technique and the internal cameras, which are presented in respectively Appendix D, E, F and G. Appendix D presents a large selection of photos taking during the preparation and execution phase of the project. A description and the numbering of the other appendices is presented in below. Numbering of Appendix E; underwater camera images E.x.y.z 1; NW-side of Platform 1 2; NE-side of Platform 1 3; SW-side of Platform 2 4; SE-side of Platform 2 1; Pictures after every reversal of the current of the tidal condition. 2; Pictures after the tidal and storm conditions. A; Test series A B; Test series B 34 of 49 Borssele OHVS - Scour and scour protection

63 Numbering of Appendix F; results of the stereo-photography technique F.w.x.y.z 1; Platform 1 2; Platform 2 A; Test series A B; Test series B 1; Before the tests, initial seabed 2; After tidal conditions (for test series A) 2; After applied the scour protection (for test series B) 3; After the RP10 year storm 4; After the RP100 year storm 1; Colour image - plot 2; Bathymetry plot 3; Bathymtery plot (Zoom) Numbering of Appendix G; internal camera images G.x.y.z 1; NW-side of Platform 1 2; NE-side of Platform 1 3; SE-side of Platform 1 4; SW-side of Platform 1 5; N-side of Platform 2 6; E-side of Platform 2 7; S-side of Platform 2 8; W-side of Platform 2 1; Scour development during the tidal condition. 2; Scour development during the tidal and storm conditions. A; Test series A B; Test series B Mean tidal conditions Platform 1 As mentioned in Section 3.4 Platform 1 has an orientation which is most representative for tidal conditions. Therefore the scour development around Platform 1 is most important to consider in this section. Figure 4.2 presents the bathymetry around Platform 1 after mean tidal conditions. The blue arrow indicates the flow direction; an alternating ebb and flood current. The colour bar is chosen such that blue colours indicate deposition; green-yellow colours indicate total scour Borssele OHVS - Scour and scour protection 35 of 49

64 depths smaller than 5.6m and orange-red colours indicate total scour depths larger than 5.6m. Please note that this colour bar provides a very straightforward interpretation with respect to the exceedance of 5.6m scour depth. However, the extent of scour patterns might require more detailed interpretation, because the abrupt change in yellow and orange colour might provide a slightly distorted view. Figure 4.2 shows a symmetrical scour pattern, with total scour holes around the foundation piles all exceeding 5.6m. Figure 4.2 furthermore shows that the area around the foundation piles in which the scour depth exceeds 5.6m is limited to approximately 1 D pile around the foundation pile. When comparing the scour depth around the two foundation piles at each corner, we see that the foundation piles that are most protruding the flow around the sides of the platform experience the largest scour development. Figure 4.2 Bathymetry around Platform 1 after simulation of mean tidal conditions. 36 of 49 Borssele OHVS - Scour and scour protection

65 Apart from the stereo-photography measurements, the scour development is recorded with rulers on the steel foundations piles and internal cameras within the transparent foundation piles. Figure 4.3 presents a photo of the NW-corner of Platform 1 after the simulation of mean tidal conditions. The black dot on the steel foundation pile indicates a scour depth of 4m 2 and each black or white band indicates an additional depth of 0.5m. With this ruler this picture therefore presents a scour depth of 7m around the steel foundation pile at the NW-corner of Platform 1. Appendix G.A.1.1 shows that at this corner the scour depth around the transparent foundation piles is slightly smaller after the simulation of the tidal conditions, namely 6.2m. View from camera Platform 1 Figure 4.3 NW-corner of Platform 1, after mean tidal conditions. Table 4.5 presents an overview of the simulated scour depths around the steel foundation piles (measured with a ruler) and the transparent foundation piles (measured with the internal cameras). Corner of Platform 1 Pile Scour depth [m] NE Transparent 6.8 Steel 6.5 SE Transparent 7.0 Steel 7.0 SW Transparent 7.0 Steel 6.5 NW Transparent 6.2 Steel 7.0 Table 4.5 Overview of the measured scour depth after the simulation of tidal conditions (test A01) around the steel foundation piles (ruler) and the transparent foundation piles (internal cameras) of Platform 1. 2 Only at the SE-pile of Platform 1 the black dot indicates a 5m deep scour hole. Borssele OHVS - Scour and scour protection 37 of 49

66 When comparing the measurements performed with the ruler and the internal cameras (Table 4.5) with the bathymetry obtained with the stereo-photography technique (Figure 4.2), the scour depth around the foundation piles are very much alike. In this comparison the reader is referred to Figure 4.1, which distinguishes the transparent and steel foundation piles. Figure 4.2 furthermore shows that the overall depression, the global scour hole, is approximately 3.5m deep. Directly underneath the northern and southern brace the global scour hole is clearly less deep. However, approximately 10m outward from these braces the scour depth is 4-5.5m. In this north and south side of the platform the global scour holes is not only deepest, but also the extent of the global scour is largest (approximately 20-30m). This relatively large depth and extent of the global scour hole at the north and south side is related to the contraction of the flow around the whole structure and the turbulence introduced by the upstream corner (combination of foundation piles, mudmat, stiffners etc.). View from camera Platform 1 Figure 4.4 Overall depression, global scour hole, at Platform 1 after simulation of mean tidal conditions. Underneath the western and eastern braces the global scour hole is approximately 4-4.5m deep. However, at these sides the extent of the global scour hole is much smaller than at the north and south side, namely 10-20m. Some deposition is even visible at the western and eastern side of the platform. The overall lowering under the platform is also clearly visible in Figure 4.4. Figure 4.5 presents the scour development in time for the NE- and SE-corners of Platform 1, as recorded with the internal cameras. At each time step and in discrete angular bins the scour depth around the whole circumference of the pile is determined. The figure shows the 50% value in black and the 10%-90% range in a blue (ebb) or red (flood) band. A larger band width therefore indicates a larger variation of scour depth around the circumference of the pile. Figure 4.5 shows that during the first ebb flow scour holes developed relatively fast at both eastern corners. For the western corners, which were not facing the flow, the rapid scour development is almost absent in this stage, see Appendices G.A.1.3 and G.A.1.4 and Appendices E.A.1.1 and E.A.1.2. Once the flow direction is reversed, rapid scour development also occurred around to piles at the western corners. The scour holes around the piles at the eastern corners even experienced some backfilling after reversal of the flow. This backfilling process is most pronounced at the NE-corner (top image of Figure 4.5). 38 of 49 Borssele OHVS - Scour and scour protection

67 Figure 4.5 Scour development in time around foundation pile at the NE-corner (top) and SE-corner (bottom) of Platform 1 measured with the internal cameras. The specific foundation pile is indicated with a blue circle. While at the NE-corner (top image of Figure 4.5) the largest variation of scour depth around the foundation pile is observed during ebb (blue), the SE-corner (bottom image of Figure 4.5) shows the largest variation during flood (red). This difference is related to the specific foundation pile around which the time development was recorded with the internal camera (these transparent piles are indicated with a blue circle in Figure 4.5). During flood the transparent pile at the SE-corner was at the downstream side of the steel foundation pile. Consequently, the variation of the depth of the scour hole around the transparent foundation was relatively large during flood. For the transparent foundation pile at the NE-corner this situation occurred during ebb tide. For all four corners, Figure 4.5 and Appendices G.A.1.1- G.A.1.4 show that the scour process is very close to an equilibrium situation. Platform 2 For completeness the scour development in mean tidal conditions around Platform 2 is also presented here. However, this orientation of Platform 2 is not considered representative for Borssele Alpha and Beta. Borssele OHVS - Scour and scour protection 39 of 49

68 Table 4.6, together with Appendices F.A and G.A.2.5-G.A.2.8 show that the local scour holes at the N-corner and S-corner of Platform 2 developed largest and deepest, up to approximately 9m. Again the measurement with all three measurements techniques (stereophotography, internal cameras and ruler) show a very good agreement. Please note that the recording with the internal camera is out of bound for the S-corner, see Appendix G.A.2.7. At the W-corner and E-corner the local scour holes reached a depth of 7-8m. At this platform the depth of the global scour hole is approximately 3.5m with deeper areas close to the braces and downstream of the foundation piles. The extent of the global scour hole is approximately 20-30m. The scour development around the foundation piles of Platform 2 is also very close to an equilibrium value. Corner of Platform 2 Pile Scour depth [m] N Transparent 8.7 Steel 9.0 E Transparent 7.4 Steel 6.5 S Transparent 9.1* Steel 9.0 W Transparent 8.0 Steel 8.0 Table 4.6 Overview of the measured scour depth after the simulation of tidal conditions (test A01) around the steel foundation piles (ruler) and the transparent foundation piles (internal cameras) of Platform 2. *out of bound Storm conditions In the discussion of the effect of storm on the scour development, we first present the scour patterns around Platform 2. This platform has the most representative orientation with respect to the RP10yr and RP100yr storm event. Platform 2 Figure 4.7 and Figure 4.6 clearly show that both storm conditions resulted in backfilling of the local scour holes. This effect is also clearly visible in Appendices E.A.2.3 and E.A.2.4. Figure 4.6 Scour development in time around foundation pile at the N-corner of Platform 2 measured with the internal cameras. The specific foundation pile is indicated with a red circle. Scour development during tide is indicated in blue, during the RP10yr in red and during RP100yr in green. 40 of 49 Borssele OHVS - Scour and scour protection

69 Again a very good agreement is found between the bathymetry obtained from the stereo-photography measurements and the measurements performed with the ruler and the internal cameras, see Figure 4.7 and Table 4.7. Again we refer to Figure 4.1 for the distinction between the steel foundation piles (measurements with ruler) and the transparent foundation piles (measurements with internal cameras). Figure 4.7 and Table 4.7 show that the scour holes around the foundation piles at the W-corner are backfilled most in the RP10yr and RP100yr storm; approximately 2-2.5m. At the downstream side (E-corner) the sediment remains in suspension due to the turbulence introduced by the structure and the scour holes are backfilled less. Figure 4.7 shows that in general the global scour hole decreases slightly in extent. The distinct lobes that were presence in the global scour pattern after the simulation of tidal conditions are backfilled and the global scour hole smoothens out and gets a more oval shape. After the tidal condition an area which experienced minor erosion was present within the platform. After the simulation of the storm conditions the global scour hole also smoothens out within the platform, resulting in a slight deepening within the platform, especially during the RP100yr storm. Figure 4.6 furthermore shows that an equilibrium situation is not reached during either storm condition; if continued for a larger period or followed by a larger amount of storms the local scour depth is expected to decrease further. Figure 4.7 Bathymetry around Platform 2 after simulation of mean tidal conditions (top), RP10yr storm (bottom left) and RP100yr storm (bottom right). Borssele OHVS - Scour and scour protection 41 of 49

70 Corner of Platform 2 N E S W Pile Scour depth [m] A01 A02 A03 Transparent Steel Transparent Steel Transparent 9.1* Steel Transparent Steel Table 4.7 Overview of the measured scour depth after the simulation of tidal and storm conditions around the steel foundation piles (ruler) and the transparent foundation piles (internal cameras) of Platform 2. *out of bound Platform 1 For Platform 1 a similar scour development under the RP10yr and RP100yr storm is observed as for Platform 2. Again both storms resulted in backfilling of the local scour holes, see Table 4.8, Appendices E.A.2.1-E.A.2.2. and Appendices G.A.2.1-G.A.2.4. These appendices furthermore show that also around Platform 1 the scour process has not yet reached an equilibrium situation during the storm tests. Appendices F.A F.A show that around this platform the global scour hole decreased slightly in extent, similar to the scour pattern around Platform 2 The depth of the global scour hole around this platform however did not change much. Corner of Platform 1 NE SE SW NW Pile Scour depth [m] A01 A02 A03 Transparent Steel Transparent Steel Transparent Steel Transparent Steel Table 4.8 Overview of the measured scour depth after the simulation of tidal and storm conditions around the steel foundation piles (ruler) and the transparent foundation piles (internal cameras) of Platform Behaviour of the scour protection Around both platforms the scour protection is applied according to the conceptual design presented in Section 3.8. Prior to installation the larger stones of the 3-9 HD grading are painted for easier recognition. Placement is performed accurately by using moulds, see Figure 4.8. This figure furthermore presents the final layout as applied around Platform 2. This image is the colour-image obtained with the stereo-photography technique. To check the installed thickness of the 3-9 HD layer, stereo-measurements were performed before and after installing the scour protection layout. Appendices F.B F.B and F.B F.B present these measurements and show that the installed thickness is indeed 1m. 42 of 49 Borssele OHVS - Scour and scour protection

71 Figure 4.8 Installation of the 3-9 HD stone grading (left) and the final layout (right) Deformation of the scour protection in a RP10yr storm During the RP10yr storm no significant movement of rocks was observed at either platform. The movies obtained from the underwater camera images (and provided through an ftp-site) show the stability of the 3-9 HD grading during the RP10yr storm nicely. Moreover, the underwater images presented in Appendices E.B.2.1-E.B.2.4 also show that the scour protection after the RP10yr storm is similar to the initial scour protection, with the exception of some sand ripples that ran up the edges of the protection Deformation of the scour protection in a RP100yr storm During the RP100yr storm the movement of rocks was still very limited; some rocks moved on top of the mud mats. Once these rocks are located on the mudmat the rocks tend to move back and forth over the smooth surface of the mudmat, while the rocks in the scour protection are relatively stable. Again the movies obtained from the underwater camera images show this movement nicely. The movement of stones on the mudmats is most clearly visible in Appendix E.B.2.1 and E.B.2.4. Appendices F.B F.B and F.B F.B show that this occasional movement did not result in significant deformation of the 3-9 HD grading during the RP100yr storm. Figure 4.9 presents the limited movement that was observed during the RP10yr and RP100yr storm condition at Platform 1. Please note that the sand that is visible in the bottom left picture of Figure 4.9 is related to sand ripples migrating over the scour protection and not to exposure of the seabed underneath the scour protection. Borssele OHVS - Scour and scour protection 43 of 49

72 Figure 4.9 Top right: situation before the tests at Platform 1. Bottom left: cumulative deformation after the RP10yr and RP100yr storm at Platform 1. Top left: view from the underwater camera is indicated with a black arrow and the storm direction is indicated with a blue arrow. 44 of 49 Borssele OHVS - Scour and scour protection

73 5 Analysis 5.1 Scour development around foundations piles The test clearly showed that the tidal conditions are governing for the development of scour holes around the foundation piles and for the extent of the global scour. In this condition the total scour holes around all foundation piles of both Borssele Alpha and Beta are expected to exceed the previously accounted for value of 5.6m. In mean tidal conditions equilibrium depths of approximately 7m are found (including local and global scour). In more severe tidal conditions the scour holes are expected to deepen. In storm conditions or less severe tidal conditions these scour holes around the foundation piles will experience backfilling. Most backfilling is expected at the foundation piles facing the waves and current (the north and west corner of the Borssele Alpha and Borssele Beta platform in the field scale situation). Please note that due to morphodynamic changes of the seabed the bed level at the foundation piles can experience an additional lowering. Part of the abovementioned total scour depth at the foundation piles can be attributed to the global scour hole. This global scour hole is expected to have an extent of approximately 20m and a depth of 3.5m. This global scour hole can be slightly deeper along the western and eastern sides (field scale situation) of the platform due to the contraction of the flow during tidal conditions and the flow patterns around the corners (combination of foundation piles, pile sleeves, mud mat, stiffners etc.) of the platform. In reflection of the schematisation of the soil conditions it is noted that this schematisation was based on a medium grain size representative for the top 8m. This scour depth was not exceeded at the platform which was most representative for the tidal conditions. The chosen schematisation of the soil conditions is therefore considered accurate. The fact that average tidal conditions are simulated, together with the accuracy of the different measurement techniques and the fact that the different measurements all present similar results, give high confidence in the expectation that scour depths around the foundation piles can develop deeper than 5.6m. 5.2 Dealing with the scour development around the platform scour protection Rock grading: 3-9 HD armour layer An armour layer consisting of 3-9 HD is expected to be very stable when applied as a preinstalled or post-installed layer. Please note that a good coverage at the mudmats is required, which will be more difficult for a post-installed layer. Further optimization of this layer might even be considered. Such optimization can include applying a smaller or less dense grading or a reduction of the layer thickness. Because the 3-9 HD grading was very stable during the RP100yr, stability limits are not found and can therefore not be correlated with hydrodynamic parameters (MOB and KCnumber). Additional verification is therefore recommended when a smaller or less dense rock grading is considered. If applied as a single grading, the thickness of a 3-9 HD grading should be chosen such that winnowing is prevented; i.e. the scour protection should be sand tight. This requirement Borssele OHVS - Scour and scour protection 45 of 49

74 supersedes the observation that the layer thickness can be decreased when considering the external stability. When a filter layer is applied which ensures the sand tightness of the scour protection, then the layer thickness of a 3-9 HD armour layer can be decreased to a value that ensures sufficient coverage of the filter layer (>0.5m). Protection layout When applied in a two-layer system, the extent of the armour layer should be chosen such that it covers the filter layer in the area where the filter layer is not expected to be stable (close to the structure). Moreover, if the armour grading is highly mobile, an increase in the extent can be considered to prevent loss of armour material. Because such high mobility was not observed for a 3-9 HD grading, the extent of this grading (when applied as an armour layer in a two-layer system) should be chosen such that it covers the filter layer sufficiently. Compared to the tested layout, that included an extent of 12m, the extent of the 3-9 HD armour layer applied in a two-layer system can be optimized/decreased. If a more mobile armour grading is considered such optimization of the armour layer extent should be considered more carefully. In a two-layered system the filter layer should be designed such that disintegration of the armour layer due to progressive undermining by edge scour is prevented. In other words, the filter layer should stabilize the slopes of the scour holes that develop at the edges of the scour protection. Generally, edge scour holes develop less deep when the scour protection has a relatively small thickness and a large extent. A frequently used filter layer is a 1-3 or 1-3 high density (HD) grading with a minimum thickness of 0.5m. Such a filter layer might also be considered for a two-layer system for Borssele Alpha and Beta. It s stability should however be verified. When applied as a single-layer system the armour layer itself should stabilize the slopes of the edge scour holes. Because edge scour holes in the order of 3m might be expected around the tested scour protection layout, an optimization of the extent of a single layer protection is not considered feasible. Maybe an increase of the extent might even be required. Another important aspect to consider with respect to the development of edge scour holes is the effect that it can have on cables; i.e. free spans. To prevent such undesirable effects the scour protection can be extended along the cables routes. In a two layer system, the extent of the filter layer might be increased locally and in a single-layer system an additional smaller grading can be applied. Please note that edge scour can never be completely prevented, as it is a scour process related to the transition between the rough scour protection and the smooth sea bed. For the Borssele Alpha and Borssele Beta jacket a 1-3 high density (HD) or 2-8 grading with a minimum thickness of respectively 0.5 and 1m and an additional extent of approximately 20m might be considered. Please note that in this test programme only the external stability of a 3-9 HD grading is considered. All presented values and possible optimization steps regarding the extent and thickness of the 3-9 HD grading and any specification of other gradings should be considered and verified in the development of a final design. 46 of 49 Borssele OHVS - Scour and scour protection

75 6 Conclusions and recommendations A laboratory test programme was performed in the Atlantic Basin at Deltares to quantify the scour development around the Borssele Alpha and Borssele Beta platform and to test the external stability of a conceptual scour protection layout. The hydrodynamic conditions included tidal currents and extreme storms (RP10yr and RP100yr) with combined current and waves. 6.1 Conclusions Scour development around Borssele Alpha and Beta The tests clearly showed that the tidal conditions are governing for the development of scour holes around the foundation piles. In this condition the total scour holes around all foundation piles of both Borssele Alpha and Beta are expected to exceed the previously accounted for value of 5.6m. The simulated total scour depth at the foundation piles in mean tidal conditions is 6-7m, depending on which pile is considered. Backfilling of the local scour holes occurred during the simulated storm conditions. As part of this total scour depth, the global scour hole reached a depth of approximately 3.5m, with locally deeper areas in the order of 4.5m and an extent of approximately 20m. External stability of a 3-9 HD grading During the RP10yr the movement of stones of the 3-9 HD grading was very limited. In the RP100yr storm the movement of stones increased, but this did not result in significant deformation of the scour protection. A 3-9 HD grading can therefore be considered as a suitable armour grading, both in a preinstalled or post-installed and a single- or two-layer scour protection system. 6.2 Recommendations The recommendations include the following: Because the scour development is expected to exceed the value presently accounted for in the design, it is recommended to consider a scour protection and/or adjustment of the design. In the detailed design of the scour protection other failure mechanisms of a scour protection, such as edge failure and winnowing, should be considered. Optimisation of the conceptual scour protection layout may be considered. The available options for optimising the layout are dependent on the type of scour protection that is selected (i.e. single graded vs. two-layer system) and the installation method (pre-installation vs. post-installation). Around the scour protection layout scour holes will develop: edge scour. Because these edge scour holes might have an undesirable effect on the cables (e.g. free spans), it is important to consider the development of edge scour holes in the design Borssele OHVS - Scour and scour protection 47 of 49

76 of the scour protection layout. In this report a first indication on how to deal with cables and edge scour holes is presented; a local extension of the scour protection layout. For the development of the final design such an extension of the scour protection layout should be verified. 48 of 49 Borssele OHVS - Scour and scour protection

77 References Deltares (2015). Metocean study for the Borssele Wind Farm. Ref: HYE-0010; final report, dated January Deltares (2016a). Extreme hydrodynamic conditions for Borssele Offshore High Voltage Stations Alpha and Beta. Ref: HYE-0004; final report, dated May Deltares (2016b). Scour assessment for Borssele Offshore High Voltage Stations Alpha and Beta. Ref: HYE-0005; final report, dated June Fugro (2015) Geotechnical Report/Investigation Data Geotechinical Borehole Locations Borssele Wind Farm Site II. Ref: N6016/03 (4). Final report, dated July Fugro (2016) Geotechnical Report/Investigation Data Geotechinical Borehole Locations Borssele Wind Farm Site III. Ref: N6083/01 (4). Final report, dated February Raaijmakers, T.C., Liefhebber, F., Hofland., B., Meys, P. (2012), Mapping of 3D-bathymetries and structures using stereophotography through an air-water-interface. Proceedings Coastlab12, 2012, Ghent, Belgium. Sumer, B.M., Fredsøe, J. (1997). Hydrodynamics around cylindrical structures. World Scientific Publishing, London. TenneT (2016). Standard Offshore Substation, Scour Assessment Around Jacket Foundations. Document No.: ONL-TTB TenneT (2015). Standard Offshore Substation, Offshore Grid NL Jacket Structure Isometric View. Document No.: ONL-TTB Whitehouse (1998). Scour at marine structures. Thomas Telford Limited, London. Borssele OHVS - Scour and scour protection 49 of 49

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