Suction bucket foundation Feasibility and pre-design for the 6 MW DOWEC

Transcription

1 Suction bucket foundation Feasibility and pre-design for the MW DOWEC Dutch Offshore Wind Energy Converter project DOWEC-FW2-MZ-02-0/03-P DOWEC 0 rev. 3 Name: Signature: Date: Written by: M.B. Zaaijer, TUD X 0 June 2002 version Date No of pages 0 /04/02 New document 07/05/02 Comments to ver. 0; Recalculation of installability 2 0/0/02 Comments to ver. 3 20/08/03 Converted to PDF

2 DOWEC-FW2-MZ-02-0/03-P Preface Results are reported for a contribution to the single objective concept alternatives in WP 2 Task of the project DOWEC commissioned by EET. The following persons have contributed to this work: TU Delft Michiel Zaaijer (editor) ECN Sergio Herman NEG-Micon Peter Fish Dick Veldkamp Ballast Nedam Ton Topper The report has been published by TUDelft, DUWIND, under number Delft, June 2002 Page 2 of 7

3 DOWEC-FW2-MZ-02-0/03-P Contents INTRODUCTION MODELLING APPROACH BEARING CAPACITY INSTALLATION BUCKET MASS FEASIBILITY OF BASE CASE LOADING CONDITIONS BEARING CAPACITY INSTALLABILITY FEASIBLE SUCTION BUCKETS INDICATION OF COSTS... 4 FEASIBILITY OF OTHER SCENARIOS DESCRIPTION OF OTHER SCENARIOS FEASIBLE BUCKET DIMENSIONS FOR OTHER SCENARIOS CONCLUSIONS AND CONTINUATION...4 REFERENCES...5 APPENDIX A... Page 3 of 7

4 DOWEC-FW2-MZ-02-0/03-P Introduction Suction buckets are tubular steel foundations that are installed by sealing the top and applying suction inside the bucket. The hydrostatic pressure difference and the deadweight cause the bucket to penetrate the soil. This benign installation procedure allows the buckets to be connected to the rest of the structure before installation, enabling a reduction in steps of the installation procedure. The system has been tried in practice in the Norwegian oil and gas fields in the North Sea [2] and in Angola [0]. Because of the large hydrostatic force that is required for installation, suction buckets commonly have a much lower aspect ratio than driven piles. A diameter to length ratio of 0 is a practical maximum, which depends on water depth and soil properties [3]. For tripod support structures for offshore wind turbines a much lower diameter to length ratio may be more appropriate, as suggested by the studies of Rambøll [9], [5]. Due to the low aspect ratio suction buckets are less suited to comply with the large moments that occur for a monopile concept, although it is known that monobuckets are being tested in Denmark []. Therefore, this study is limited to the combination with a space frame structure, in which axial foundation loads dominate. Furthermore, the low aspect ratio would make suction buckets susceptible to scour and, therefore, scour protection has been assumed. After installation, the suction pump is removed and the bucket top is either sealed or left open. Open buckets can be installed with a reusable cap, thus saving cap costs. The behaviour of these two types of foundations under dynamic loading is very different. The latter resembles that of driven piles, taking account of the dissimilar dimensions. For the first type axial loading causes a pressure/suction built-up below the cap. If the cap touches the soil, the bucket will behave as a traditional skirted footing or plugged pile under compression. Under tension, the suction below the cap will increase the resistance, which would otherwise only consist of deadweight and skin friction. During longer tensile loading the suction area will be drained and the extra resistance will be lost. This time-variance of the resistance, due to drainage, is the cause of the more difficult dynamic behaviour of the suction bucket. Under long term (quasi-) static loading both suction bucket types behave similar, since that corresponds to drained conditions [9]. The objective of this work is twofold: General assessment of feasibility of suction buckets Indication of suction bucket dimensions and costs Both objectives relate to the application of suction buckets as a foundation for a tripod support structure for the MW DOWEC. The pre-design of a tripod for the MW DOWEC will be used to obtain reference foundation loads for the feasibility study and the pre-design of the suction bucket []. It is appreciated that the tripod is not designed for suction buckets. However, a generic bearing and installation assessment is used to obtain insight in the sensitivity of the suction bucket design to modifications of the support structure. In particular, a wider tripod base might be preferred, due to the reduction in axial foundation loads. The feasibility study needs to answer the question whether a suction bucket with sufficient bearing capacity will be installable in shallow water. To avoid the analysis of the complicated dynamic behaviour of the suction bucket, the current work focuses on (quasi-) static behaviour. In case a range of suction bucket dimensions is feasible from a geotechnical perspective, a particular design will be selected and used to get a cost estimate. A more detailed design is outside the scope of this work. An indication of further work for detailed assessment of feasibility and costs will be given at the end of this report. Page 4 of 7

5 DOWEC-FW2-MZ-02-0/03-P 2 Modelling approach 2. Bearing Capacity To determine the static axial capacity of the suction bucket the following contributions are considered: Skin friction on the outside of the bucket End resistance of the bucket tip Skin friction on the inside of the bucket, or end resistance of the plug The skin friction and the end resistance are calculated according to [] and some intermediate results are given in Appendix A as pressures or forces per length of circumference of the bucket. It is not likely that a plug will occur for these shallow and wide buckets. Thus, the last contribution will most likely be the skin friction on the inside of the bucket. However, for a capped suction bucket the end resistance of the plug is included under compressive loading. It has been assumed that ultimate skin friction and end resistance are mobilized simultaneously. The ultimate axial capacity, Q, must satisfy the condition: ( ) P Φ Q ± F D F B, () in which P = vertical force exerted on bucket by tripod, Φ = pile resistance factor (0.7), F D = deadweight of bucket and possible plug, = buoyancy of bucket and possible plug. F B The sign before the deadweight and buoyancy term indicates its contributing or counteracting effect during tensile or compressive loading. The right-hand term of Equation gives the ultimate axial loading of the tripod on the foundation and is plotted in Figure and Figure 2 for a range of suction bucket dimensions with a wall thickness of 20 mm. Soil conditions are taken for site of [7] from [8], assuming maximum sand wave height bucket without cap bucket with cap Figure Maximum allowed static compression load in 0 N Page 5 of 7

6 DOWEC-FW2-MZ-02-0/03-P The marked reduction of maximum allowed compression loading for buckets with a penetration depth between and 20 m is caused by the clay layer, which gives far less end resistance than sandy soils. (The influence of the clay layer on the end resistance in the sand just above it is not included in the model.) For a given extreme compression loading of the tripod on the foundation Figure can be used to establish bucket dimensions that provide sufficient bearing capacity Figure 2 Maximum allowed static tension load in 0 N (bucket with and without cap) Because the skin friction dominates the bearing capacity for the bucket without cap the maximum allowed compression and tension loads are very similar in that case. 2.2 Installation The driving force during installation of the suction bucket is the hydrostatic pressure difference over the cap and the deadweight of the suction bucket. Additional deadweight can be used, e.g. through ballasting or pre-assembling part of the support structure with the suction buckets. It has been assumed that the pressure inside the suction bucket can be reduced to zero. In shallow water of several tens of meters depth this would require a pumping capacity of a few bar. Thus, the potential pressure difference over the cap increases linearly with water depth. Risk of critical suction, at which point liquefaction occurs and soil is pumped up, is not considered. However, this may limit installability and must be considered in a later phase [5]. The assessment of installation is based on the procedure outlined for skirted foundations in [4]. A highest expected skin friction and end resistance are used to determine the resistance during installation. In the absence of cone penetration data the skin friction and end resistance according to [] are used as most probable values and multiplied with suggested ratios between highest expected and most probable. For skin friction in sands the highest expected value is obtained by using the passive pressure coefficient, K p, rather than the coefficient of lateral earth pressure. The highest expected skin friction in clay and end resistance in both sand and clay are between.5 and 2 times the most probable value. However, the highest expected skin friction in sand can exceed the most probable value much more and is approximately 7 times higher at the considered site. Increase of outer friction and reduction of inner friction and tip resistance due to pore pressures during suction are not considered here ([5]). A first assessment of the effect of this omission is given in Appendix B, which can be compared with the results for the base case presented in Section 3.4. Instabilities during installation due to inhomogeneous resistance around the bucket circumference are not considered. When the suction buckets are connected rigidly to the tripod frame during installation stability can be provided through the distribution of suction over the three buckets. Page of 7

7 DOWEC-FW2-MZ-02-0/03-P The deadweight of the suction bucket is included as an inherent driving force during installation. Figure 3 plots the additional deadweight that would be required to install suction buckets of various sizes with a wall thickness of 20 mm. Soil conditions are taken for site of [7] from [8], assuming maximum sand wave height. A water depth of m is used to determine the hydrostatic installation pressure under normal sea conditions Figure 3 Required deadweight to install suction bucket in 0 N (Required deadweight > 0 0 displayed in yellow) 2.3 Bucket mass As a first indication of material use for suction buckets the bucket mass for various sizes with a wall thickness of 20 mm is given in Figure 4. For the cap an effective wall thickness of 40 mm is assumed bucket without cap bucket with cap Figure 4 Suction bucket mass in 0 3 kg, without mass of cap. Page 7 of 7

8 DOWEC-FW2-MZ-02-0/03-P 3 Feasibility of base case 3. Loading Conditions The foundation loads are obtained from the pre-design of a tripod with a 20 m base radius for the MW DOWEC []. The loads are taken from the engineering model that is used in the pre-design phase and are merely an indication of the actual loads. As explained in the introduction, the bearing capacity of a suction bucket is different for static and dynamic loading. Typical time-scales to distinguish between static and dynamic differ very much for different bucket dimensions and different soil types [9]. In this assessment all wave loading and dynamic amplification of wind and wave loading is considered as dynamic loading. The (quasi-) static solution of wind loading during extreme and operating conditions is considered as static loading. The following extreme loads (without safety factor) were observed, all occurring during operating conditions: Compression load (static) (N).29 0 Compression load (static + dynamic) (N) Tension load (static) (N).3 0 Tension load (static + dynamic) (N) Bearing capacity To determine the dimensions of suction buckets that have sufficient bearing capacity, the axial foundation loads are compared with the maximum allowed loads according to Section 2.. A safety factor of.35 is applied to all loads of Section 3.. A distinction is made between buckets with a (sealed) cap and without a cap, as outlined below: Bearing capacity for suction bucket without cap These suction buckets don t experience the increased tension resistance due to suction below the cap. Thus, no distinction is made between static and dynamic loading and the extreme static + dynamic loading may not exceed the maximum allowed compression and tension loading of Figure and Figure 2, respectively. Bearing capacity for suction bucket with cap For compression loading no distinction is made between static and dynamic loading. The extreme static + dynamic compression loading may not exceed the maximum allowed loading of Figure. For tension loading the suction below the cap increases the resistance, for a certain period. Since both the exact dynamic behaviour of the loading and the tension bearing capacity for dynamic loading require a rigorous analysis it is assumed here that the suction effect is sufficient to withstand the entire dynamic part of the tension load. This is a quite strong assumption and must be verified in a later stage. In this feasibility study the extreme static tension load may not exceed the maximum allowed loading of Figure 2. Figure 5 and Figure plot the suction bucket dimensions that have sufficient bearing capacity according to the described approach. For buckets without cap compression loading is dominating the feasible bucket dimensions. Apparently the additional end resistance of the bucket tip is small compared to the additional compression loading, due to the structure weight. On the other hand, for buckets with cap, tension loading is generally dominating. In this case the end resistance of the soil plug outweighs the structure weight, except in the clay layer. Obviously, bucket dimensions with cap can be smaller than without cap. Page 8 of 7

9 DOWEC-FW2-MZ-02-0/03-P tension compression Figure 5 Bearing capacity of bucket without cap; dark (brown) area is OK tension compression Figure Bearing capacity of suction bucket with cap; dark (brown) area is OK 3.3 Installability The required deadweight for installation was plotted in Figure 3. For the base case it is assumed that the buckets and tripod are pre-assembled and thus the deadweight per bucket is the weight of the tripod divided by 3. The tripod mass is kg, up to the tower flange 9 m above MSL. No additional installation ballasting is assumed. Suction buckets that can be installed with this approach are plotted in Figure 7. Evidently, buckets with a large diameter can be installed to greater penetration depths, due to the larger hydrostatic force on the cap. Page 9 of 7

10 DOWEC-FW2-MZ-02-0/03-P Figure 7 Installability with deadweight of tripod in m water depth; dark (brown) area is OK 3.4 Feasible suction buckets Combining the buckets with sufficient bearing capacity of Figure 5 and Figure with the installability of Figure 7, results in feasible suction bucket dimensions for the tripod support structure. These are plotted in Figure 8, for both capped and open buckets. Diam e ter bucket without cap bucket with cap Figure 8 Geotechnically feasible suction buckets; dark (brown) area is OK Within the shown dimensions, up to 5 m diameter and m penetration depth, there is no feasible suction bucket without cap. There is a small area of possible bucket dimensions when the cap is remained and sealed after installation. A first assessment of the omission of drainage currents on the feasibility of buckets is given in Appendix B. Page 0 of 7

11 DOWEC-FW2-MZ-02-0/03-P 3.5 Indication of costs The lightest feasible bucket with a diameter of 8 m and a penetration of 8 m is kg (per bucket), without the mass of the cap. Obviously, the mass of the bucket depends strongly on the assumed wall thickness, which is not properly designed. The installability of this bucket is uncertain, due to the negligence of the negative effect of critical suction and the positive effect of reduced tip resistance and inner friction during installation. When the limit of critical suction causes smaller penetration depths, larger diameters are required. Therefore, it is more conservative to perform an initial estimation of costs with a bucket with a 4 m diameter and 7 m penetration, which has some margin on installability. With a wall thickness of 20 mm this bucket has a mass of kg, without cap. Assuming an average cap wall thickness of 40 mm the cap mass is also kg. Thus, with considerable uncertainty, the total mass of the three suction buckets is in the order of kg. The production process of the suction buckets will involve much welding and other operations, particularly due to the large diameter, the probable strengthening elements of the cap and the connection to the tripod braces. Therefore, a price of 3,- per kg is assumed for material and fabrication. For comparison, prices for monopile and tripod used in the DOWEC concept study are 2.25 and 3.5 /kg, respectively, meaning intermediate fabrication costs are assumed for the buckets. This results in an indication of the procurement costs of the suction buckets of 0.9 M per turbine and 72 M for a farm of 80 turbines. Using the same assumptions for the cap and the price per kg, the lightest bucket with a diameter and penetration of 8 m would have a mass of kg, cost 0.4 M per turbine and cost 32 M for a farm of 80 turbines. Page of 7

12 DOWEC-FW2-MZ-02-0/03-P 4 Feasibility of other scenarios 4. Description of other scenarios To get an impression of the sensitivity of bucket dimensions to several parameters 4 alternatives to the base case are formulated:. No sand wave 2. Installation with tower and nacelle pre-assembled 3. Reduction of loads by 50% 4. Extra ballast Scenario Previous studies stated an economic preference of suction buckets for softer soils, particularly clay [2], [9]. To assess this for the DOWEC support structure the sand wave is omitted from the soil data, shifting the clay layer up to the top 5 m. The loads and installation procedure remain the same as the base case. This is not a realistic scenario for site, since removal of the sand wave would actually lead to a larger water depth, with a different foundation design and loading. Scenario 2 In this scenario the deadweight for installation is increased. The mass of the tower and nacelle is kg, additional to the tripod mass. Scenario 3 The tripod has been optimised for a foundation of driven piles. The lightest bucket of the base case is still much heavier than the driven piles used in the original tripod design. It is expected that the optimum base radius of a tripod with suction buckets is wider, reducing the foundation loads and thus the bucket dimensions. Scenario 4 Since tension loads govern feasible dimensions of buckets with cap, there is reserve capacity against compression loading. By ballasting the suction bucket some of this spare capacity can be used to reduce the net tension load on the bucket. A ballast material with a submerged unit weight of N/m 3 and a height of m over the entire bucket surface has been used. 4.2 Feasible bucket dimensions for other scenarios The results of the feasible bucket dimensions for the 4 alternative scenarios are given in Figure 9. As for the base case, none of the alternative scenarios was feasible without cap, within the analysed range of dimensions. The following can be observed for the buckets with cap: Scenario Due to the larger skin friction of clay the boundary of bucket dimensions with sufficient bearing capacity is shifted to smaller penetration depths. The limit of installable buckets is also shifted to smaller penetration depth, but to a less extend. This is due to the fact that the highest expected skin friction of clay is not as much above the most probable value as for sand. The lightest feasible bucket is kg (per bucket), without the mass of the cap Scenario 2 The additional deadweight doesn t contribute significantly to the installability of the buckets. However, the additional deadweight might be required when critical suction would reduce the limit of installability to smaller penetration depths. The lightest feasible bucket is kg (per bucket), without the mass of the cap. Scenario 3 The increase of possible bucket dimensions with sufficient bearing capacity is significant. The lightest feasible bucket is 0 3 kg (per bucket), without the mass of the cap. Page 2 of 7

13 DOWEC-FW2-MZ-02-0/03-P Scenario 4 As in scenario 2 there is only a small increase in installability, due to the additional deadweight of the ballast material. However, the shift of the boundary of bucket dimensions with sufficient bearing capacity to smaller penetration depths is significant. The lightest feasible bucket is kg (per bucket), without the mass of the cap. In this case the weight of the ballast material equals 2 0 N.. No sand wave 2. Tower and nacelle pre-assembled 3. Reduction of loads by 50% 4. Extra ballast Figure 9 Geotechnically feasible suction buckets for alternative scenarios (with cap); dark (brown) area is OK Page 3 of 7

14 DOWEC-FW2-MZ-02-0/03-P 5 Conclusions and continuation Suction buckets for a tripod support structure for the MW DOWEC appear to be feasible with realistic dimensions, but will have a much larger mass than driven piles. However, this conclusion is based on two strong assumptions: the installation is not subject to critical suction and the suction effect is sufficient to withstand the entire dynamic part of tension loading. Within a range up to 5 m diameter and m penetration depth only suction buckets with a closed cap appear to be feasible. Therefore, suction bucket designs for the MW DOWEC tripod will need to address the complicated dynamic behaviour of capped buckets. To obtain some margin for installability a suction bucket with a diameter of 4 meter and a penetration of 7 m was selected to get an indication of procurement costs. With considerable uncertainty, an estimate is given of 72 M for a farm of 80 turbines. The most optimistic estimate, based on a suction bucket of 8 m diameter and 8 m penetration, is 32 M for 80 turbines. A comparison of these costs with the baseline requires cost estimates of the tripod, scour protection and the installation of a tripod with suction buckets and is outside the scope of this report. Several alternative scenarios were compared with a base case to get an idea of the sensitivity of the bucket design. Increase of deadweight to facilitate installation increases the area of feasible bucket dimensions only marginally. The experience that suction buckets perform better in clay than in sand is confirmed. Reduction of tension loading by increase of the tripod base or application of permanent ballast material results in significantly smaller bucket dimensions. The small difference between the required penetration depth and maximum installable penetration depth of the feasible buckets indicates that nearly the entire potential hydrostatic pressure is required for installation. In turn, this indicates that critical suction must be analysed carefully in a further stage. Some of the alternative scenarios give a larger margin, particularly when additional (permanent) ballast is applied. The following activities are foreseen for detailed bucket design, when it is decided to continue with this concept (despite the estimated economic disadvantage): - Determination of (model for) bearing under dynamic loading - Determination of (model for) dynamic stiffness - Adaptation of dynamic model for simulation program (Flex 5) or vice versa - Generation of useful statistics of dynamic foundation loads - Assessment of critical suction - Assessment of the effect of pore pressures on skin friction and end resistance during installation - Assessment of equipment and procedures for installation - Assessment of scour and scour protection - Design of cap, including assessment of maximum installation and service loads - Design of wall thickness, including buckling analysis - Design of connection with tripod - Optimisation of tripod base width and bucket size Page 4 of 7

15 DOWEC-FW2-MZ-02-0/03-P References [] API, RP 2A-LRFD: API Recommended Practices for Planning, Designing and Constructing Fixed Offshore Platforms Load and Resistance Factor Design, First Edition, July, 993. [2] Birck, C.; Gormsen, C., Recent Developments in Offshore Foundation Design, In: Proceedings of the European Wind Energy Conference, Nice, March 999. [3] Broek, W. van den, Van Oord ACZ, Personal Communication, November 999. [4] DNV, Foundations - Classification Notes, No. 30.4, Høvik, Norway, Februari 992. [5] Feld, T., Design Procedures for Bucket Foundations - a New Innovative Foundation Concept Applied to Offshore Wind Turbines, In: Proceedings of Offshore Wind Energy Special Topic Conference (CD-rom), Brussel, Belgium, December 200. [] Fish, P., NEG Micon UK, Personal communication, May 2002 [7] Goezinne, F., Terms of Reference DOWEC, 7-FG-R0300 V, Neg Micon Holland bv,. Bunnik, September 200. [8] Kooistra, A., DOWEC Soil data, \00C0003.AKO, Ballast Nedam Engineering bv, Amstelveen, June 200. [9] Rasmussen, J. Lorin, Feld, T., Sørensen, P. Hald, Bucket Foundation for Offshore Wind Farms - Comparison of Simplified Model and FE-Calculations, In: Proceedings of the European Seminar on Offshore Windenergy in Mediterranean and Other European Seas, Siracusa, Italy, April [0] [] Zaaijer, M.B., Tripod Report, (Forthcoming). Page 5 of 7

16 DOWEC-FW2-MZ-02-0/03-P Appendix A Table Input data and intermediate results of the geotechnical analysis Depth Type Gamma' Cu phi delta K Kp Local friction Most probable (used for capacity calculations) Depth End integrated resist. friction Highest expected (used for installability calculations) Local Depth End friction integrated resist. friction m sand/clay N/m^3 N/m^2 degrees degrees - - N/m^2 N/m N/m^2 N/m^2 N/m N/m^ sand sand sand sand sand sand sand sand sand sand sand sand sand sand sand clay clay clay clay clay sand sand sand Page of 7

17 DOWEC-FW2-MZ-02-0/03-P Appendix B During installation the pressure difference between the inside and outside of the bucket causes a water current through the soil, going down in the soil outside the bucket and up in the soil inside the bucket. Due to the related changes in effective pressures in the soil the skin friction on the outside increases and the tip resistance and skin friction on the inside decrease. A thorough estimate of these changes is outside the scope of this report, but a very optimistic situation is represented by the assumption the skin friction on the outside is only increased by 0%, while the tip resistance and skin friction on the inside is reduced to zero. The region of feasible bucket dimensions for this situation is shown in Figure 0. As expected, the region is increased in favour of larger possible bucket penetrations, due to the reduced resistance during installation. However, the region is hardly extended in the direction of lighter buckets, because that is limited by the required minimum bearing capacity. Thus, the main effect of water currents during installation will be some extra safety margin for installability. bucket with cap Figure 0 Optimistic estimate of feasible suction buckets; dark (brown) area is OK Page 7 of 7