Foundation Technologies for Offshore Shallow Water Renewable Energy Projects

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1 Foundation Technologies for Offshore Shallow Water Renewable Energy Projects Master student: Matthias Laga Supervisor: Prof. Peter Bourne-Webb Abstract: Bucket foundations have been widely used in oil and gas offshore structures. However, this type of foundation is also suitable for offshore wind turbines. This dissertation presents the results of a twodimensional and three-dimensional finite element analyses of bucket foundations embedded in clay in undrained conditions, where frictional contact is considered between bucket skirt and subsoil. Two bucket configurations, solid and shell, are executed and compared with each other. Two soil cases are modelled, with uniform undrained shear strength and with increasing undrained shear strength. Aspect ratios L/D for the foundations are taken 0.5, 1 and 2. The performance of a simplified bucket foundation model under vertical and horizontal load is investigated. The conducted bearing capacities from FEA are compared with calculated analytical approaches as well as previous studies. It is shown that FEA agrees well with the analytical approaches and previous studies. Regarding the ultimate vertical capacity, shell bucket are likely to behave in the same way as solid bucket configurations. Keywords: Bucket foundation, finite element, bearing capacity 1. Introduction In the last decades more and more energy was needed to fulfil the needs of humankind. It has become clear that fossil fuels will run down in some decades and new cost-efficient solutions will have to be found. Where nuclear power seems to be cheap, it has already been proven to be harmful to humans and environment. A wellknown solution for this particular problem is the use of alternative energy methods. After hydropower, which amounts for 64% of total renewable power energy at the end of 2013, wind power follows with 20% [1]. The European Union (EU) already entered a set of binding targets that aims for twenty percent of energy consumption to come from renewable sources at the end of 2020 [2]. Nowadays, wind energy has become a mainstream power resource and will be one of the most rising markets of the future, especially in offshore shallow waters [3]. Because approximately 40 % of the cost of a turbine lies on the foundation support, developers are searching for cost-efficient methods to establish these offshore wind turbines (OWT) by looking at the foundation [4]. One of the most promising foundation solution for shallow water OWT are skirt bucket foundations. This paper aims to fill some gaps regarding the understanding of vertical and horizontal behaviour of skirt bucket foundations with different aspect ratios, in conjugation with other past investigations and analytical approaches. Also a comparison between solid buckets (gravity Base Structure) and shell buckets (skirt foundations) will be executed and the gap-closing between bucket lid and seafloor will be explained. First a brief overview of consisting foundations types for OWT is given

2 2. Shallow foundations for OWT s The first offshore well dates back to the 19 th century, so companies have already gained a lot of experience in this particular field. It seems that foundations for OWT could easily be designed based on the design principles of the gas and oil industry. However, this is only partially true. The dominant loading of an offshore gas- or oil structure is typically from lateral loading from waves and the vertical weight of the platform, whereas for OWT, lateral loads due to wind and waves remain high while the vertical weight is low. Horizontal and moment loads from wave and wind action play a significant part in the analysis of the total OWT system [5, 6]. The main OWT foundations can be categorized in four types, Figure 1. A B C D beneath the footings due to short repeatedly or cyclic wave loading. A solution for this particular problem is the use of passive drainage on the bottom of the foundation. This facilitates dissipation of excess pore water pressures created by the short repeatedly loadings [8]. 2.2 Mono-pile Foundation Due to its relatively simple design and easy installation, the most common support structure for shallow OWT are mono-piles, Figure 1 C. The structure consists of a large diameter cylindrical steel pile driven into the seabed. Depending on the soil next to the foundation, the steel pile is rammed into the sea floor using hammering or vibration to a depth of 35 m to 40m. This is an attractive concept in shallow waters with suitable external conditions, this means the soil must consist of sand or clay. To prevent scour or erosion holes, scour protection on the seabed is needed after installation [9]. 2.3 Multi-pile Foundation d 2.1 Gravity Base Foundation d Figure 1: Shallow foundations for OWT - A) Gravity Base Structure (GBS) B) Skirt Bucket Foundation C) Mono-Pile D) Multi-piles Since 1990, this type of foundations has been the second most common foundation type for OWT in shallow waters, Figure 1 A. Nowadays they consist of hollow reinforced and prestressed concrete structures but also examples constructed by steel and hybrid of concrete and steel exist. After transportation on place, they are filled with ballast to obtain the full design weight. The deepest registered offshore GBS for OWT s has a water depth up to 27 m [7]. A major issue that needs to be considered in this type of foundation is the potential for liquefaction of saturated or partially saturated, loose, sandy soil d The main support structure is a cylindrical steel tube that extends into the tower and the lower part consists of braces and legs, as seen in Figure 1 D. In each corner of the tripod the substructure is fixed to the seabed using piles installed through sleeves. The legs are connected to the main cylindrical tube making the transition with the tower. The multi-pile foundation is also applicable for jacket support structures where piles are driven through and eventually grouted tot the pile guides/sleeves in the corner of the jacket. In contrast to a monopile, this type does not require any special seabed preparations or scour protection. Multipile structures are designed for water depths from 25 m to 50 m [10, 11]. 2.4 Skirt Bucket Foundation Unlike GBS, this foundation type is a lightweight structure equipped with skirts of significant length. Skirt bucket foundations can provide a technical and cost efficient solution for shallow OWT foundations. This foundation type finds its origin in the suction anchors used for floating tension leg platforms in the oil and gas industry. The bucket foundation is a steel, cylinder-shaped upturned structure that is embedded in the - 2 -

3 seabed and is closed at the top. In most cases embedment is achieved by self-weight, by pushing the caisson skirts downwards and by creating negative pressure inside the bucket Components of skirt bucket foundation The upturned structure may also have ring stiffeners and/or longitudinal stiffeners inside the bucket. A simplified model is illustrated in Figure 2 the effective weight of the bucket but can be increased by additional weight on the bucket lid which is recommended to ensure enough penetration depth for the successful application of suction. The installation procedure can be seen in Figure 4. The structure is lowered into the seabed by gravity (1). When the penetration depth of the skirts are sufficient enough, the relative pressure at the lid of the bucket will be lowered by sucking water and air (2). A force will be generated that pushes the structure into the soil (3). No seepage flow is occurring along the skirt-ends (4). When the buckets have reached their final position, the pumps are released from the lid of the bucket (5). Finally, there is a possibility to grout the structure by injecting a thin layer of concrete between the seabed and bucket lid (6) [13]. Figure 3: Skirt bucket foundation configuration Where L is the length of the bucket, h the embedment of the structure and Di and Do inner and outer diameter, respectively. In reality, a small gap will exist between seafloor and the bucket lid. In some cases this will be filled with high pressure grout, which has already proven to increase the moment stiffness of buckets under cyclic loading. Also the vertical settlement of the buckets under cyclic loadings may be reduced by pressure grouting [12]. However, in the simplified FEA model, the bucket gap is assumed to be an empty void of 0.15m. The wall thickness of the shell bucket was taken 0.02m and the thickness of the bucket lid assumed to be 0.03m Installation procedure: Clayey soil Figure 4: Installation of bucket foundation in clay An analytical approach for the installation in clayey soils is characterised by an undrained strength, which is assumed to be constant with depth. Eq. 1 can be given for the calculation of the concentrated force V that is necessary to penetrate the bucket into the soil. It comprises the summation for adhesion on the inside and outside and end bearing on annulus or skirt end [14]. (Eq. 1) The installation procedure of shell buckets mainly consists of two critical phases; the selfweight penetration followed by suction-assisted penetration. The penetration phase depends on - 3 -

4 Where su1 the average undrained shear strength over the embedment of the skirts, su2 the undrained shear strength at the end of the skirts, h the embedment and s the vertical displacement of the bucket. The recommended bearing capacity factor (Nc) is the same as for a strip foundation corresponding to the undrained shear strength of the soil is taken π+2 [15] Ultimate vertical bearing capacity The ultimate vertical bearing capacity on the base (Rb) of skirt bucket foundations in undrained homogeneous soils, where su equals the undrained shear strength, can be approximated by using the conventional method of the bearing capacity calculation from Eq. 2 [16]. V = N c ζ s ζ d A s u (Eq. 2) Where A is the plan area of the bucket foundation, ζs en ζd are factors that include the effect of shape of footing and the effects of embedment of the foundations. The shape factor for a circular footing is suggested 1+0.2D/D and embedment factor for a circular foundation has been suggested 1+0.4tan-1(L/ Do) if L/ Do 1 and (L/ Do) if L/Do < 1. It should be noticed that no adhesion along the sides of the bucket (Rs,o) is included in Eq. 2 so this equation can then be modified by appending an extra term for the skirt adhesion π.l.do.su [17]. Where L is the length of the bucket skirts and Do the outer diameter. This add-in can also be written in function of A.su which gives 4.(L/Do).A.su, Eq. 3 and Figure Limit equilibrium solutions vertical pull out capacity In actual practice, the bucket foundation is placed by the use of self-weight and by the high ratio of internal to external water pressure. Knowing this, the foundation will be placed by the use of active suction. After placement, the foundation will be subjected to external forces. If, for example, an indication of tension movement occurs, the bucket will mobilize significant shortterm pull-out capacity through the development of negative changes of pore water pressure in the soil inside the bucket. This is known as passive suction. However, the applied load can be of long-term condition where the soil will behave in another way. Various possibilities were given, Figure 5 [18]: a) Continuous long-term load with a slow rate and migration of the pore fluid within the surrounding soil. (drained condition); b) pull out load with an increased rate and partially drained; c) short-time load with a rapid rate where no migration of pore fluid within the surrounding soil is possible. (undrained conditions). a) b) c) V = N c ζ s ζ d A s u + 4 ( L D o ) A s u (Eq. 3) Figure 5: uplifting response skirt bucket a) drained condition b) partially drained condition c) undrained condition [18]. Figure 4: Ultimate vertical bearing capacity skirt bucket foundation The focus in this study is the undrained failure of the soil-structure so only c) will be taken into account. The undrained response (clay) of the bucket foundation can be explained by the great amount of capacity that mobilizes after applying a short-time tension load to the structure. Passive suction will arise in the soil at the bottom of the bucket and the surrounding soil will contribute to the total stress, Figure 5.c. In this case, an estimation of the pull out capacity of the shell - 4 -

5 bucket can be given by using the reverse bearing capacity theory in cohesive soils, Eq. 3 [15]. Regarding reverse end bearing, purely practically there is a probability that water will flow inside the bucket. The passive suction will dissipate and resistance will be reduced to the frictional resistance mobilizing along the foundation skirts, a) drained condition. The undrained shear strength is radius of the Mohr circle of total stress with σ1 the major principal stress and σ3 the minor principal stress at failure Eq. 5 and Figure Ultimate lateral capacity The purely lateral resistance of a bucket foundation in undrained homogeneous soil can be estimated by the use of Eq. 4. The central point of the horizontal load must be placed at the optimum level, i.e. above the mudline level. H ult = N p L D o s u (Eq. 4) Where Hult is equal to the ultimate lateral capacity of the skirt bucket foundation, Np is the lateral bearing capacity factor, su is the average undrained shear strength over the skirt length, L and Do the outer diameter of the shell bucket [16]. If rotation of the bucket is allowed, Np will decrease by approximately 40%. 3. Soil modelling For the modelling of the soil, an undrained and cohesive soil was considered. A Tresca Criterion was used for the interpretation of the shear strength of a clay soil in undrained conditions with constant volume shearing [17]. The shear strength under undrained loading only depends on the void ratio or initial water content or initial total confining pressure [17]. In order to implement the Tresca failure criterion in Abaqus, the criterion can be seen as a reduced Mohr-coulomb criterion where the angle of internal friction φ = 0 and the shear strength is described by the undrained shear strength, su, Eq. 5. Figure 6: Tresca Yield criterion 4. Finite element model First, the vertical bearing capacity and gapclosing behaviour of the shell buckets in twodimensional (2D) configuration was examined. Thereafter, solid and shell bucket models were developed for the analysis of the vertical bearing capacity. Finally, three-dimensional (3D) models were implemented to compare the horizontal failure loads from the Finite Element Analysis (FEA) with analytical calculations and previous studies. In this dissertation only short term stability problems of saturated clays is discussed and undrained conditions can reasonably be assumed to apply. The soil is modelled as a linear elastic perfectly plastic material with a Tresca failure criterion [18]. The parameter properties for soil and steel bucket foundation are summarized in Table 1. Table 1: Parameter properties solid and shell bucket τ = c = s u = (σ 1 σ 3 ) 2 (Eq. 5) - 5 -

6 15D+L L For both, shell and solid, bucket configuration, the elastic parameters assigned to the clay are a Poisson s ratio of (a) D/2 Two soil cases were executed: Case 1: the uniform undrained shear strength, su was set as 60 kpa with a Young s Modulus of 40 MPa; Case 2: the soil part was divided in layers in order to define increasing undrained shear strength in the soil. On the surface of the soil, the undrained shear strength was assumed to be 5 kpa. Below this the undrained shear strength increased at a rate of 1.5 kpa/m, Eq. 6. The Young s modulus was assigned a value of 500su in each layer. (b) 15D s u = 5kPa + x 1.5 kpa m (Eq. 6) Where x equals the depth of the soil. The geometry of the 2D models were executed as axi-symmetric. This to reduce arithmetical effort and running time. The discretized soil area is taken 15 times the bucket diameter (D) horizontally and 15D+L vertically. Vertical boundaries at the end of discretized zone are fixed in the horizontal direction and the bottom boundary was taken fixed in all directions. The investigation of the behaviour of this model was executed by using solid and shell bucket aspect ratios, L/D of 0.5, 1 and 2 where D was taken 8m in all configurations. Soil-steel contact was taken rough with no separation allowed and normal hard contact was used to describe the detachment. All FE analyses were executed with Abaqus software where 4-node bilinear axisymmetric quadrilateral elements (CAX4) were used in 2D configurations and eight node linear brick elements (C3D8) in 3D models. Coarse mesh elements are used near the boundaries and fine mesh elements were introduced close to the solid bucket. The same mesh cross-section was used during the 2D and 3D modeling, the only difference is the rotation of 180 degrees around the Y-axis in the latter, Figure 7. Figure 7: 2D - axisymmetric configuration (a) and 3D configuration (b) 5. Finite element results In order to valid the model, the FEA was compared to an analytical evaluation of the vertical installation bearing capacity, Eq 1. The displacement-load analysis was carried out with the shell bucket ratio L/D = 1 Case 1. To ensure the soil next to the bucket skirt was fully mobilized, the arbitrary displacement of the shell bucket was taken m. This resulted in a Vanal.= kn and a numerical vertical bearing load of Vinstal = 26145kN, as seen in Figure 8. It can be said that the difference between the numerical results and analytical calculation is approximately 8% which is acceptable because the bearing capacity factor is an estimated factor

7 Displacement top bucket (m) initially plateaus as the side resistance is fully mobilized and then at a displacement of about 0.15 m the gap closes and a further increase in resistance is mobilized. Examining the loaddisplacement graph for the bucket with L/D = 1 (R1), the leap in the curve flattens off and for L/D = 2 (R2) is absent. Figure 8 : Numerical displacement-load control shell bucket ratio L/D =1 5.1 Vertical bearing capacity shell buckets The total vertical bearing capacity for shell bucket in Case 1 can be calculated with modified Eq. 3. Where V equals the summation of the base resistance (Rb), which includes the compressed soil inside the bucket, and the side resistance (Rs), The vertical bearing capacity for shell buckets in a soil with increasing undrained shear strength with depth Case 2 can also be calculated with Eq.3. Because the foundation footing was modelled at a depth where two layers with different undrained shear strength and young s modulus merge into each other, the average of undrained shear strength in the two layers was taken for the calculation of the ultimate vertical bearing capacity. Results of both, Case 1 and 2 are summarized in Table 2. This can be explained by the length of the bucket skirts; the longer the skirts are, the more friction will occur along the inside of the bucket. Deeper embedment of the skirts induces more vertical capacity attributable to the mobilization of the skirt friction. As seen in Figure 10 the deformed mesh for bucket aspect ratio L/D =2 (R2) shows that the gap configuration will stay more or less in place moving soil and bucket downwards (plugging). It also has to be noticed that this effect depends on the undrained shear strength of the clay soil. In Case 2, shell bucket with ratio L/D = 2 will touch the mudline and thus the bearing capacity will increase subsequently. 0-0,05-0,1-0,15-0,2 Load (kn) Table 2: Analytical vertical bearing capacity for shell bucket Case 1 and 2 Aspect ratio L/D shell bucket Case 1 constant su Case 2 increasing su kn kn kn 4706 kn 9570 kn kn The vertical lines in Figure 9 represent the analytical results of the calculation for vertical bearing capacity. As seen in Figure 9 Case 1, the resistance of the bucket with an aspect ratio L/D = 0.5 (R0.5) -0,25-0,3 R0.5 R1 R2 Theory_R0.5 Theory_R1 Theory_R2 Figure 9: Vertical baring capacity shell buckets - Case 1 Figure 11 shows R1 and R2 - Case 1 at a displacement of -0.10m (dashed line in Figure 9). In these figures the red dots indicate the occurrence of yield within the soil. Regarding these analyses it is possible to understand why the gap for shell buckets with R2 does not close

8 Displacement top bucket (m) Figure 10: Undefromed (transparent) and deformed (green) mesh after displacement of 0.15m - R1 (above) and R2 (under) (not to scale) For shell buckets R1, failure of the soil occurs adjacent to the skirt on the inside and outside of the bucket skirt as well as base failure. The adhesion is not great enough to maintain the gap. After a displacement of 0.18 m the gap will touch the mudline, Case 1, Figure 9. For ratio R2, plasticity in the soil is less at the end of the bucket skirt and no plasticity occurs on the inside of the bucket skirts. This means that the soil inside the bucket is plugged and moves down with the bucket and the gap is maintained. It can be said that the foundation will act as a solid bucket. Comparison between shell and solid bucket will be discussed in Figure 12. aspect ratios L/D = 0.5 and 1. As discussed above, because the soil & bucket move together, the load-displacement responses of the shell bucket and the solid bucket with an aspect ratio L/D = 2 are largely indistinguishable. Here a difference of the vertical bearing capacity between both foundations for a displacement of 0.30 m was only 0.6%. At the same displacement, for ratio L/D = 0.5 and 1, the difference was respectively 7.2% and 5.9%. Same results were obtained with Case 2 where the difference of the vertical bearing capacity between both foundations for a displacement of 0.30 m for the aspect ratios L/D = 0.5, 1 and 2 were 2.2%, 9.0% and 5.6% respectively. 0-0,05-0,1-0,15-0,2 Load (kn) ,25-0,3 2D_shellbucket_R0.5 2D_shellbucket_R2 2D_shellbucket_R1 2D_Solidbucket_R0.5 Figure 12: Comparison vertical bearing capacity solid and shell bucket - Case Reverse bearing capacity shell buckets Figure 11: Plastic zones for aspect ratio L/D = 1 and 2 - Case 1 - displacement of m (not to scale) 5.2 Comparison vertical bearing capacity solid and shell bucket As seen in Figure 12 the load-displacement response of the shell buckets Case 1 are converging to those for the solid buckets for In order to simulate the particular problem of the reverse bearing capacity, shell buckets Case 1 were also modelled without a gap configuration and with a tied constraint between bucket lid and seafloor. When a tension load is applied to the bucket, the soil inside the bucket remains attached to the bucket lid and follows the displacement of the structure. It has to be noticed that this assertion only can be done because a great amount of capacity that will arise after applying tension to the structure, this in undrained short-term conditions. Friction between outer bucket skirt also contributes to the failure tension load

9 Displacement top bucket (m) As seen in Figure 13 the tension R0.5 and tension R1 curves with gap formation have practically the same capacity as the compression case prior to gap closing. This was predictable as the total resistance is obtained by the friction between skirt and soil, and the friction will be the same in tension and compression. For tension R2 the side friction is sufficient enough to allow the formation of reverse bearing capacity effect and the response is indistinguishable from the compression case. The dashed line represent the no gap shell bucket configurations where lid is tied to the soil. It can be seen that failure mode of the shell bucket subjected to an uplifting load under undrained conditions may be considered a reverse bearing capacity mechanism. However, these analyses involved a major simplification of the mechanism, if the suction effect would have been implemented, the model had to be coupled like [15]. 0,2 In order to compare the gained results with previous studies, the normalized vertical bearing capacity Vult / (A.su) was executed with the 3D results. Here Vult is the ultimate vertical load, A the plane area of the shell bucket and su the undrained shear strength. At a normalized displacement ratio of 3.75%, the ultimate vertical bearing load was reached. Further displacements were not possible within the software. The normalized ultimate vertical capacity factors for aspect ratios L/D = 0.5, 1 and 2 are 9.3, 11.4 and Figure 14 compares the normalized ultimate bearing capacity between the FEA, modified Eq. 3, and the results from [19], [20] and [21]. It can be seen that values from the current FEA are slightly lower that previous studies. This can be explained because the ultimate vertical capacity in the FEA was taken at a normalized depth of 3.75%. Previous studies considered the ultimate vertical capacity as a cut-off value where the vertical settlement approaches a normalized depth of 5%. 0,1 0-0,1 Load (kn) ,2-0,3 Tension R0.5 Tension R1 Tension R2 compression R0.5 compression R1 compression R2 No Gap Tension R0.5 No gap tension R1 Figure 13: Reverse bearing capacity shell buckets 5.4 Normalized ultimate vertical bearing capacity (3D) First, a validation of the 3D models was executed and compared to 2D. The loaddisplacement results showed differences between 2D and 3D, for R0.5, R1 and R2 at a penetration depth of 0.3m, of 2.01 %, 1.11% and 0.06% respectively. Figure 14: Comparison normalized ultimate vertical-displacement curve 5.5 ultimate horizontal bearing capacity (3D) As seen in Figure 15, the lateral bearing capacity factor Np for horizontal load (Eq. 4), with and without the allowance of rotation, in soils with uniform undrained shear strength, gained from FEA is acceptable compared with theoretical assumptions.. For aspect ratio L/D = 0.5, 1 and 2 the normalized ultimate horizontal capacity Hult / (A.su) is 4.5, 5.8 and 9.6 respectively. These values are also within the values of previous studies [19], [20] and [21]

10 Figure15: lateral bearing capacity factor Np in function of the aspect ratios L/D 6. Conclusion Based on these series of numerical analyses, the following conclusions can be drawn: The ultimate vertical capacity for FEA for shell buckets agrees well with the modified conventional method (Eq. 3). Also 2D and 3D load-displacement differences are negligible. The longer the skirts are, the more adhesion will develop along the skirt edges which ensures that the soil plug inside the bucket will be subjected to a downwards movement. Adhesion also depends also on su. It was shown that the load-displacement response of the shell buckets are converging to those for the solid buckets for aspect ratios L/D = 0.5 and 1. This response for shell bucket and the solid with R2 is largely indistinguishable. The reverse bearing capacity in undrained conditions (short-term) for shell buckets has been proven. However, these analyses involved a major simplification of the mechanism. The normalized ultimate vertical and horizontal capacity for FEA agrees well with theoretical assumption and previous studies. 7. References [1] REN21. (2014). Renewables 2014 Global status report (1st ed.). Retrieved from REN21 website: /gsr/2014/gsr2014_full%20report_low%20res.pdf [2] Communication from the commission to the European parliament, the council, the European economic and social committee and the committee of the regions Energy A strategy for competitive, sustainable and secure energy. [3] Wind Energy the facts (2011). "Projecting targets for the EU-27 up to 2030." Chapter 2. Website: [4] IRENA, I. (2012). Renewable energy technologies: Cost analysis series.concentrating solar power. [5] ORECCA. (2011). WP3 Technologies stat of the art (Final version). Retrieved from ORECCA website: d=144f87d6-c41a-4a ebdc88f5a5c&groupId=10129 [6] Randolph, M., & Gourvenec, S. (2011). Offshore geotechnical engineering: CRC Press [7] Peire, K., Nonneman, H., & Bosschem, E. (2009). Gravity base foundations for the thornton bank offshore wind farm. Terra et Aqua, 115, [8] Humpheson, C Foundation design of Wandoo B concrete gravity structure. Offshore Site Investigation and Foundation Behaviour, Soc. For Underwater Technology, [9] Den Boon, J., Sutherland, J., Whitehouse, R., Soulsby, R., Stam, C., Verhoeven, K.,... Hald, T. (2004). Scour behaviour and scour protection for monopile foundations of offshore wind turbines. Paper presented at the Proceedings 2004 European Wind Energy Conference, London, UK. European Wind Energy Association. [10] Yanguas Miñambres, Ó. (2012). Assessment of current offshore wind support structures concepts: challenges and technological requirements by [11] De Vries, W., Vemula, N. K., Passon, P., Fischer, T., Kaufer, D., Matha, D.,... Vorpahl, F. (2011). Final report WP 4.2: Support Structure Concepts for Deep Water Sites: Deliverable D (WP4: offshore foundations and support structures): Upwind. [12] Gourvenec, S., & White, D. (Eds.). (2010). Frontiers in Offshore Geotechnics II. CRC Press. Pg. 569 Installation of suction caissons for offshore renewable energy. [13] Romp, R. H. (2013). Installation-effects of suction caissons in non-standard soil conditions (Doctoral dissertation, TU Delft, Delft University of Technology) [14] Houlsby, G. T., & Byrne, B. W. (2004). Calculation procedures for installation of suction caissons. Report No. OUEL2268/04, University of Oxford. [15] Deng, W., & Carter, J. P. (2002). A theoretical study of the vertical uplift capacity of suction caissons. International Journal of Offshore and Polar Engineering, 12(2), [16] Randolph, M., & Gourvenec, S. (2011). Offshore geotechnical engineering. CRC Press. [17] Lambe, T. W., & Whitman, R. V. (2008). Soil mechanics SI version. John Wiley & Sons. [18] Kim, S. R. (2012). Evaluation of vertical and horizontal bearing capacities of bucket foundations in clay. Ocean Engineering, 52, [19] Le Chi Hung, S. R. K. (2012). Evaluation of vertical and horizontal bearing capacities of bucket foundations in clay. Ocean Engineering, 52, [20] Zhan, Y. G., & Liu, F. C. (2010). Numerical analysis of bearing capacity of suction bucket foundation for offshore wind turbines. EJGE, 15, [21] Taiebat, H. A., & Carter, J. P. (2005). A failure surface for caisson foundations in undrained soils. Frontiers in Offshore Geotechnics: ISFOG,