Suction caissons for wind turbines

Transcription

1 Frontiers in Offshore Geotechnics: ISFOG 25 Gourvenec & Cassidy (eds) 25 Taylor & Francis Group, London, ISBN X Suction caissons for wind turbines Guy T. Houlsby 1, Lars Bo Ibsen 2 & Byron W. Byrne 1 1 Department of Engineering Science, Oxford University, UK 2 Department of Civil Engineering, Aalborg University, Denmark ABSTRACT: Suction caissons may be used in the future as the foundations for offshore wind turbines. We review recent research on the development of design methods for suction caissons for these applications. We give some attention to installation, but concentrate on design for in-service performance. Whilst much can be learned from previous offshore experience, the wind turbine problem poses a particularly challenging combination of a relatively light structure, with large imposed horizontal forces and overturning moments. Monopod or tripod/tetrapod foundations result in very different loading regimes on the foundations, and we consider both cases. The results of laboratory studies and field trials are reported. We also outline briefly relevant numerical and theoretical work. Extensive references are given to sources of further information. 1 INTRODUCTION The purpose of this paper is to review recent research work on the design of suction caisson foundations for offshore wind turbines. Most of the relevant work has been conducted at, or in co-operation with, the universities of Oxford and Aalborg, so we report here mainly the work of our own research groups. Suction caissons have been extensively used as anchors, principally in clays, and have also been used as foundations for a small number of offshore platforms in the North Sea. They are currently being considered as possible foundations for offshore wind turbines. As discussed by Houlsby and Byrne (2) and by Byrne and Houlsby (23), it is important to realise that the loading regimes on offshore turbines differ in several respects from those on structures usually encountered in the offshore oil and gas industry. Firstly the structures are likely to be founded in much shallower water: 1 m to 2 m is typical of the early developments, although deeper water applications are already being planned. Typically the structures are relatively light, with a mass of say 6t (vertical deadload 6 MN), but in proportion to the vertical load the horizontal loads and overturning moments are large. For instance the horizontal load under extreme conditions may be about 6% of the vertical load. An important consideration is that, unlike the oil and gas industry where large one-off structures dominate, many relatively small and inexpensive foundations are required for a wind farm development, which might involve anything from 3 to 25 turbines. The dominant device used for large scale wind power generation is a horizontal axis, 3-bladed turbine with the blades upwind of the tower, as shown in Figure 1. The details of the generator, rotational speed and blade pitch control vary between designs. Most offshore turbines installed to date generate 2 MW rated power, and typically have a rotor about 8 m in diameter with a hub about 8 m above mean sea level. The size of turbines available is increasing rapidly, and prototypes of 5 MW turbines already exist. These involve a rotor of about 128 m diameter at a hub height of about 1 m. The loads on a typical 3.5 MW turbine are shown in Figure 2, which is intended to give no more than a broad indication of the magnitude of the problem. Note that in conditions as might be encountered in the North Sea, the horizontal load from waves (say 3 MN) is significantly larger than that from the wind (say 1 MN). However, because the latter acts at a much higher point (say 9 m above the foundation) it provides more of the overturning moment than the wave loading, which may only act at say 1 m above the foundation. Using these figures the overturning moment of 12 MNm would divide as 9 MNm due to wind and 3 MNm due to waves. Realistic combinations of loads need to be considered. For instance the maximum thrust on the turbine occurs when it is generating at the maximum allowable wind speed for generation (say 25 m/s). At higher wind speeds the blades will be feathered and provide much less wind resistance. It is thus unlikely that the maximum storm wave loading would occur at the 75

2 1 m 6 MN 9 m 4 MN h 3 m Figure 2. Typical loads on a 3.5 MW offshore wind turbine. Figure 1. Offshore tests in Frederikshavn, Denmark. Front: Vestas V9 3. MW turbine. Back: Nordex 2.3 MW turbine. same time as maximum thrust. Turbine designers must also consider important load cases such as emergency braking. It is important to recognise that the design of a turbine foundation is not usually governed by considerations of ultimate capacity, but is typically dominated by (a) considerations of stiffness of the foundation and (b) performance under fatigue loading. An operational wind turbine is subjected to harmonic excitation from the rotor. The rotor s rotational frequency is the first excitation frequency and is commonly referred to as 1P. The second excitation frequency to consider is the blade passing frequency, often called 3P (for a three-bladed wind turbine) at three times the 1P frequency. Figure 3 shows a representative frequency plot of a selection of measured displacements for the Vestas V9 3. MW wind turbine in operational mode. The foundation is a suction caisson. The measured data, monitoring system and Output-Only Modal Analysis used to establish the frequency plot are described in Ibsen and Liingaard (25). The first mode of the structure is estimated, and corresponds to the frequency observed from idling conditions. The peak to the left of the first natural frequency is the forced vibration from the rotor at 1P. To the right of the first natural frequency is the 3P frequency. It should be noted that the 1P and 3P frequencies in general cover frequency bands and not just two particular values, because the Vestas wind turbine is a variable speed device. To avoid resonances in the structure at the key excitation frequencies (1P, 3P) the structural designer needs to know the stiffness of the foundation with some confidence, this means that problems of deformation and stiffness are as important as capacity. Furthermore, much of the structural design is dictated by considerations of high cycle fatigue (up to about 1 8 cycles), and the foundation too must be designed for these conditions. 2 CASES FOR STUDY The two main problems that need to be studied in design of a suction caisson as a foundation are: installation; in service performance. In this review we shall discuss installation methods briefly, but shall concentrate mainly on design for in service performance. The relevant studies involve techniques as diverse as laboratory model testing, centrifuge model testing, field trials at reduced scale, 76

3 db 1. / Hz 2 1P First mode Frequency Domain Decomposition - Peak Picking Average of the Normalized Singular Values of Spectral Density Matrices of all Data Sets. 3P Frequency Figure 3. Frequency plot of measured displacements for a wind turbine in operational mode. and a full-scale field installation. Complementing these experiments are numerical studies using finite element techniques, and the development of plasticity-based models to represent the foundation behaviour. Suction caissons may be installed in a variety of soils, but we shall consider here two somewhat idealised cases: a caisson installed either in clay, which may be treated as undrained, or in sand. For typical sands the combination of permeability value, size of caisson and loading rates leads to partially drained conditions, although much of the testing we shall report is under fully drained conditions. In this paper we report mainly work on sands. We shall consider two significantly different loading regimes, which depend on the nature of the structure supporting the wind turbine. Most offshore wind turbines to date have been supported on a monopile a single large diameter pile, which in effect is a direct extension of the tubular steel tower which supports the turbine. Some turbines have been supported on circular gravity bases. An obvious alternative is to use a single suction caisson to support the turbine, and we shall call this a monopod foundation, Figure 4(a). The monopod resists the overturning moment (usually the most important loading component) directly by its rotational fixity in the seabed. As turbines become larger, monopod designs may become sufficiently large to be uneconomic, and an alternative is a structure founded on three or four D (a) L Figure 4. Caisson foundations for a wind turbine, (a) monopod, (b) tripod/tetrapod. smaller foundations: a tripod or tetrapod, Figure 4(b). In either of these configurations the overturning moment on the structure is resisted principally by push pull action of opposing vertical loads on the L s (b) D 77

4 upwind and downwind foundations. Alternatives using asymmetric designs of tripod, and those employing jacket type substructures are also under consideration. V Mudline 3 NORMALISATION PROCEDURES A number of studies have been conducted at different scales and it is necessary to compare the results from these various studies. To do this it is appropriate to normalise all the results so that they can be represented in non-dimensional form. This procedure also allows more confident extrapolation to full scale. The geometry of a caisson is shown in Figure 5. The outside radius is R (diameter D o ), skirt length is L and wall thickness t. In practice caissons may also involve stiffeners on the inside of the caisson, these being necessary to prevent buckling instability during suction installation, but we ignore these in a simplified analysis. Geometric similarity is achieved by requiring similar values of L/2R and t/2r. The sign convention for applied loads and displacements is shown in Figure 6. The rotation of the caisson is already dimensionless, and we normalise the displacements simply by dividing by the caisson diameter, to give w/2r and u/2r. In sand it is straightforward to show that, for similar values of dimensionless bearing capacity factor, the loads at failure would be proportional to and to R 3. We therefore normalise vertical and horizontal loads as V/2 R 3 and H/2 R 3, where we have included the factor 2 to give the normalisation factor a simple physical meaning: it is the effective weight of a cylinder of soil of the same diameter of the caisson, and depth equal to the diameter. In a similar way we normalise the overturning moment as M/4 R 4. Use of the above normalisation is appropriate for comparing tests in sands with similar angles of friction and dilation. We recognise that these angles both decrease slightly with pressure and increase rapidly with Relative Density (Bolton, 1986). This means that comparable tests at smaller scales (and therefore lower stress levels) will need to be at lower Relative Densities to be comparable with field tests. In clay the vertical capacity is proportional to a representative undrained shear strength s u and to R 2, so we normalise loads as V/ R 2 s u and H/ R 2 s u, and the moment as M/2 R 3 s u. In order to be comparable, tests at different scales will need the profile of undrained strength with depth to be similar. If the strength profile is fitted by a simple straight-line fit s u s uo z, then this requires similar values of the factor 2R /s uo. Scaling of results using the above methods should give satisfactory results in terms of capacity. For clays it should also lead to satisfactory comparisons in terms h Figure 5. L D i D o Geometry of a caisson foundation. Figure 6. Loading and displacement conventions for a caisson foundation (displacements exaggerated). of stiffness, provided that the clays being compared have similar values of I r G/s u. This condition is usually satisfied if the clays are of similar composition and overconsolidation ratio. For sands, however, an extra consideration needs to be taken into account. The shear modulus of a sand does not increase in proportion to the stress level, but instead can reasonably be expressed by: (1) where g and n are dimensionless constants, and p a is atmospheric pressure (used as a reference pressure). The value of n is typically about.5, so that the stiffness is proportional roughly to the square root of pressure. Comparing rotational stiffnesses on the basis of a plot of M/4 R 4 against effectively makes the assumption that the shear stiffness is proportional to 2R, which may be regarded as a representative stress level. Since in fact the stiffness increases at a lower rate with stress level, this comparison will result t 78

5 in larger scale tests giving lower apparent normalised stiffness. This effect can be reduced by multiplying the scale by the dimensionless factor ( p a /2R ) 1 n, which compensates for the stiffness variation with stress level. Thus we recommend that to compare both stiffness and capacity data for sands one should plot M/4 R 4 against (p a /2R ).5 (assuming n.5) for moment tests, and V/2 R 3 against (w/2r)(p a /2R ).5 for vertical loading tests. A fuller description of these normalisation procedures is given by Kelly et al. (25a). 4 INSTALLATION STUDIES The principal difference between installation of a suction caisson for an offshore wind turbine and for previous applications is that the turbines are likely to be installed in much shallower water. There is a popular misconception that suction caissons can only be installed in deep water, where a very substantial head difference can be established across the lid of the caisson. In shallow water the net suction that can be achieved is indeed much smaller (being limited by the efficiency of the pumps, as the absolute pressure approaches zero), but the suctions that can be achieved are nevertheless sufficient for installation in most circumstances. Only in stiff clays is it likely that some possible caisson designs, which might otherwise be suitable as far as in-service conditions are concerned, could not be installed by suction in shallow water. In Table 1 we list the main instances where caissons have been installed in shallow water, as appropriate to wind turbine installations. The water depths h w are approximate only. In addition to the field tests listed, a large number of small scale model tests of installation have been carried out at Oxford University (on caissons of.1 m to.4 m diameter), the University of Western Australia (UWA), Aalborg and elsewhere. The largest completed installation in shallow water is that of a prototype suction caisson, shown in Figure 7, installed in the offshore research test facility in Frederikshavn, Denmark. The prototype has a diameter of 12 m and a skirt length of 6 m. The operational water depth is 4 m, and as the site is in a basin, no wave or ice loads are applied. As seen in Figure 7 the suction caisson was installed in only 1 m of water in the basin. The steel construction has a mass of approximately 14 t, and the caisson was placed in late October 22. The installation period was about 12 hours, with the soil penetration time being 6 hours. A computer system was used to control the inclination, suction pressure and penetration rate. Det Norske Veritas (DNV) has certified the design of the prototype in Frederikshavn to B level. The Vestas V9 3. MW turbine was erected on the foundation in December 22. The development of the design procedure for the bucket foundation is Table 1. Installations in shallow water. h w D L Site Soil (m) (m) (m) Ref. Wilhelmshaven Sand Installation April 25 Frederikshavn Sand Frederikshavn Sand Sandy Haven Sand Tenby Sand Burry Port Sand Luce Bay Sand Bothkennar Clay Figure 7. Installation of the prototype foundation at the test site in Frederikshavn: (a) during installation, (b) at the end of installation. described in Ibsen and Brincker (24). An even larger installation is currently in progress at Wilhelmshaven, Denmark. There are two main ways of predicting firstly the self-weight penetration of the caisson and secondly 79

6 Suction, s (Pa) Penetration, h (mm) Figure 8. SCIP Results Experimental Result Comparison of SCIP with model test. the suction required to achieve full installation. The first method (Houlsby and Byrne, 25a, b) involves use of adaptations of pile capacity analysis, in which the resistance to penetration is calculated as the sum of an end bearing term on the rim and friction on the inside and outside. In sands the seepage pattern set up by the suction processes alters the effective stress regime in a way that aids installation. The calculation has been implemented in a spreadsheet program SCIP. Figure 8 shows for example a comparison between variation of measured suction in a model test installation with tip penetration of the caisson (Sanham, 23), and the SCIP calculation. The other approach involves use of CPT data to infer directly the resistance R d to penetration of the caisson. The required suction u req to penetrate the caisson to depth d is calculated as: Figure 9. Suction required for installation at Frederikshavn. where k t is an empirical coefficient relating q t to the tip resistance during static penetration of the caisson, r t is the maximum reduction in tip resistance. u crit is the critical suction resulting in the critical hydraulic gradient i crit 1 along the skirt. t is an empirical factor. K out and K in are coefficients relating f s to the unit skin friction on the outside and inside of the skirt. The water flow along the skirt changes the skin friction. For the inside skin friction the coefficient reduces the skin friction when suction is applied, whereas on the outside the skin friction is increased. The coefficients are established as: (2) where G (d) is the self-weight of the caisson at penetration depth d (reduced for buoyancy), and A suc is the area inside the caisson, where the suction is applied. The penetration resistance is calculated from the following expression, which is based on calibration against measured data: (3) where q t is the corrected cone resistance and f s the sleeve friction at depth z. K t is a coefficient relating q t to the unit tip resistance on the rim. This resistance is adjusted for the reduction due to the applied suction by the expression: (4) (5a,b) where out and in are empirical coefficients relating f s to the unit skin friction during static penetration of the caisson. r out and r in are the maximum changes in skirt friction. out and in are empirical factors. The required suction u req to penetrate the prototype in Frederikshavn was predicted using equation (2). The result of the analysis is shown in Figure 9. The lower line represents u req calculated from the CPT tests. The curved line represents the limiting suction u pip which would cause piping to occur. u max is the theoretical maximum net suction, limited by the possibility of cavitation within the caisson, as the absolute pressure approaches zero, so that u max 1 kpa above water level and increases linearly with the water depth, as shown by Figure 9. u max is used to calculate the accessible net suction, which is limited by the efficiency of the pumps, u pump. As is seen, the suction in shallow 8

7 Penetration, h (mm) Volume, (1-3 m 3 ) Cell 1 Cell Total 3 Seepage Volume Volume 35 Volume Displaced 4 Figure 1. The limiting suction u pip has been achieved and soil failure by piping has occurred. water can be limited either by the suction causing piping or by the accessible net suction available from the pumps. The suction u pip causing piping has been studied at the test site in Frederikshavn by installation tests on 2 2m and 4 4m caissons. Figure 1 shows a 4 4m caisson where the limiting suction u pip has been achieved, and soil failure by piping has occurred. The soil outside of the skirt is sucked into the caisson and the penetration of the caisson cannot proceed. If a tripod or tetrapod structure is to be installed, then levelling of the structure can be achieved by separately controlling the suction in each of the caissons. For a monopod structure, however, an alternative strategy has to be adopted. Experience suggests that for installation in either clay or sand, the level of the caisson is rather sensitive to the application of eccentric loads (moments), especially in the early stages of installation. This offers one possibility for controlling the level of the caisson: by use of an eccentric load that can be adjusted in position to keep the caisson level. An alternative strategy, which has proven to be highly successful for installation in sand, is to divide the rim into sections and to control the pressures at the skirt tip in each section individually. By applying pressure over one segment of the caisson rim the upward hydraulic gradient within the caisson can be enhanced locally, thus encouraging additional downward movement for that sector. By controlling the pressures at a number of points the caisson may be maintained level. This method would not be applicable in clays. One possibility, as yet untried at large scale, for controlling level in clays would be to use a segmented caisson in which the suctions in the different segments could be controlled independently. Some preliminary small scale tests suggest that this approach might be successful in sand too (Coldicott, 25). Figure 11 shows the volumes of water pumped from the two halves of a 4 mm diameter caisson split by a diametral vertical wall. About 6% of the Figure 11. Penetration, h (mm) Volumes pumped from 2-cell caisson in sand. Suction, s (Pa) Cell 1 Cell Figure 12. Suctions required for installation of 2-cell caisson in sand. water pumped represents the volume displaced by the descending caisson, whilst about 4% represents seepage beneath the caisson rim. Figure 12 shows that during the installation the suctions developed in the two halves were (as would be expected in a uniform material) almost equal. 5 CAISSON PERFORMANCE: MONOPOD A large number of tests have been devoted to studying the performance of a caisson under moment loading at relatively small vertical loads, as is relevant to the wind turbine design. Some details of the test programmes are given in Table Sand: field tests The largest test involves the instrumented Vestas V9 3. MW prototype turbine at Frederikshavn, Denmark. The caisson is installed in a shallow 4 m depth lagoon next to the sea, and the turbine is fully operational. The only significant difference between this installation and an offshore one is that the structure is not subjected to wave loading. The test program involving the prototype (turbine and caisson) is focusing on long-term deformations, 81

8 Table 2. Moment loading tests. Site Soil D (m) L (m) Ref. Frederikshavn Sand Frederikshavn Sand Sandy Haven Sand Burry Port Sand Luce Bay Sand , , 7 Oxford laboratory Sand , , 42, , , 42, Aalborg laboratory Sand Bothkennar Clay Oxford laboratory Clay UWA centrifuge.2 (1g) Clay Level IV: 89 m Level III: 46 m Level II: 13 m Level I: 6 m soil structure interaction, stiffness and fatigue. The prototype has been equipped with: an online monitoring system that measures the dynamic deformation modes of the foundation and the wind turbine, a monitoring system that measures the long-time deflection and rotation of the caisson, a monitoring system that measures the pore pressure along the inside of the skirt. The online monitoring system that measures the modes of deformation of the foundation and wind turbine involves 15 accelerometers and a real-time dataacquisition system. The accelerometers are placed at three different levels in the turbine tower and at one level in compartments inside the caisson foundation. The positions are shown in Figure 13, and the locations and measuring directions are defined in Figure14. Output-only Modal Analysis has been used to analyze the structural behaviour of the wind turbine during various operational conditions. The modal analysis has shown highly damped mode shapes of the foundation/wind turbine system, which the present aeroelastic codes for wind turbine design cannot model. Further studies are to be carried out with respect to soil-structure interaction. A detailed description of the measuring system and the Output-Only Modal Analysis is given by Ibsen and Liingaard (25). The static moment tests referred to in Table 2 at Sandy Haven and at Burry Port were relatively Figure 13. Sensor positions in tower and foundation. straightforward, with very simple instrumentation, but those at Frederikshavn test site and at Luce Bay were detailed investigations. The large scale tests at Frederikshavn is part of a research and development program concerning caisson foundation for offshore wind turbines. The research program is a co-operation between Aalborg University and MBD offshore power (Ibsen et al., 23). The large scale tests are complemented by laboratory studies. The laboratory and large scale tests are intended to model the prototype in Frederikshavn directly. In order to design a caisson foundation for offshore wind turbines several load combinations have to be investigated. Each load combination is represented by a height of load h above the foundation and a horizontal force H. The moment at the seabed is calculated as M hh. Table 3 shows that the resulting loading height varies from 1 m (for a wave force in shallow water) to 14.4 m (force from normal production of a 3 MW turbine in 2 m of water). Scaling of the tests is achieved by: (6) where D is the diameter of the caisson and index m and p are for model and prototype. The values of the loading height in the test program are shown in Table 3. 82

9 Figure 15. Caisson for large scale test at Frederikshavn. Figure 14. Sensor mountings in the tower and foundation at Frederikshavn. Table 3. Loading heights in the Aalborg test program. Field Prototype LaboratoryModel Model D p 12 m D m.2 m.3 m.4 m 2. m h p [m] h m [m] h m [m] The large scale tests at Frederikshavn employ loading by applying a horizontal load at a fixed height, under constant vertical load. A steel caisson with an outer diameter of 2 m and a skirt length of 2 m has been used. The skirt is made of 12 mm thick steel plate. Figure 15 shows the caisson prior to installation, and Figure 16 the overall test setup. Currently 1 experiments have been conducted, but the testing program is ongoing. Each test has three phases: 1. Installation phase: The caisson is installed by means of suction. CPT tests are performed before and after installation of the caisson. Figure 16. Setup for combined loading of 2 2m caisson at Frederikshavn (Back: prototype 3 MW Vestas wind turbine on the 12 6m caisson). 2. Loading phase: An old tower from a wind turbine is mounted on top of the caisson. The caisson is loaded by pulling the tower horizontally with a wire. The combined loading (H, M) is controlled by changing the height of loading. 3. Dismantling phase: The caisson is removed by applying overpressure inside the bucket. Figure 17 shows the moment rotation curve for a test on the 2 2m caisson at Frederikshavn. The test is performed with h m 17.4 m and a vertical load on the caisson of 37.3 kn. The fluctuations in the curve are caused by wind on the tower. Tests at Luce Bay were designed by Oxford University and conducted by Fugro Ltd.. The moment loading tests were of two types. Firstly small amplitude (but relatively high frequency) loading was applied by a Structural Eccentric Mass Vibrator (SEMV) in which rotating masses are used to apply 83

10 inertial loads at frequencies up to about 12 Hz. Secondly larger amplitude, but lower frequency, cycles were applied using a hydraulic jack. A diagram of the loading rig, which allowed both moment and vertical loading tests, is shown in Figure 18. The SEMV test involve cycles of moment loading at increasing amplitude as the frequency increases. Figure 19 shows the hysteresis loops obtained from a series of these cycles at different amplitudes. As the cycles become larger the stiffness reduces but hysteresis increases. The tests were interpreted (Houlsby et al., 25b) using the theory of Wolf (1994), which takes account of the dynamic effects in the soil, and the equivalent secant shear modulus for each amplitude of cycling determined. Figure 2 shows the moment rotation curves for much larger amplitude cycling applied by the hydraulic jack. Again hysteresis increases and secant stiffness decreases as the amplitude increases. The unusual waisted shape of the hysteresis loops at very large amplitude is due to gapping occurring at the sides of the caisson. The secant stiffnesses deduced from both the SEMV tests and the hydraulic jacking tests are combined in Figure 21, where they are plotted against the amplitude of cyclic rotation. It is clear that the two Moment (knm) 6Hz 7Hz 3 2 8Hz 9Hz 1 1H Figure Rotation (radians) Hysteresis loops from SEMV tests on 3 m caisson. Figure 17. Moment-rotation test on 2 2m caisson. Moment (knm) Rotation of caisson centre (2Rθ ) (m) Figure 2. Hysteresis loops from hydraulic jacking tests on 3 m caisson. Figure 18. Field testing equipment, dimensions in mm. Water level and displacement reference frames not shown. (a) arrangement for jacking tests on 1.5 m and 3. m caissons, (b) alternative arrangement during SEMV tests. Labels indicate (A) A-frame, (B) concrete block, (C) caissons, (H ) hydraulic jacks, (L) load cells, (R) foundations of reaction frame, (V) SEMV, (W) weight providing offset load for SEMV tests. 84

11 groups of tests give a consistent pattern of reduction of shear modulus with strain amplitude, similar to that obtained for instance from laboratory tests. 5.2 Sand: laboratory tests Turning now to model testing, a large number of tests have been carried out both at Aalborg and at Oxford. Almost all the model tests have involved in plane loading (in which the moment is about an axis perpendicular to the horizontal load). However, a test rig capable of applying full 6 degree-of-freedom loading has recently been developed by Byrne and Houlsby (25). The model tests at Aalborg are performed by the test rig shown in Figure 22. The rig consists of a test box and loading frame. The test box consists of a steel frame with an inner width of 1.6 m 1.6 m and an inner total depth of.65 m. The test box is filled with Aalborg University Sand No. After each experiment the sand in the box is prepared in a systematic way to ensure homogeneity within the box, and between the different test boxes. The sand is saturated by the water reservoir shown in Figure 22. Before each experiment CPT-tests are performed to verify the density and strength of the sand. The caisson is then installed and loaded with a constant vertical load. The vertical load is kept constant through the experiment, while the horizontal force is applied to the tower by the loading device mounted on the loading frame, see Figure 21. The tower and the loading device are connected by a wire. The combined loading (H, M) is controlled by the height of loading h. The loading frame allows the possibility of changing h from.1 m to 4. m above the sand surface (Table 3). The horizontal force H is measured by a transducer connected to the wire. The deformation of the foundation and the moment are measured with the measuring cell mounted on the top of the caisson, as shown by Figure 23. Laboratory tests at Oxford University have used a versatile 3 degree-of-freedom loading rig designed by Martin (1994) and adapted by Byrne (2) (see also Martin and Houlsby (2) and Gottardi et al. (1999)). The rig is shown in Figure 24, and is capable of applying a wide range of combinations of vertical, horizontal and moment loading under either displacement or load control. Typical moment loading tests involve applying a fixed vertical load, and then cycling the rotation at increasing amplitude. An example is given in Figure 25. The first interpretation of such tests is to determine the yield surface for a single surface plasticity model (see section 7.2 below, and also Martin and Houlsby Figure 22. The caisson test rig at Aalborg University. G (MPa) Jacking SEMV Hyperbolic curve fit θ (radians) Figure 21. Shear modulus against rotation amplitude. Figure 23. the tower. The measuring cell connecting the caisson and 85

12 Experiment, M/2RH = 1 Fitted Yield Surface Soil Plug Weight Moment Load, M/2R (N) Vertical Load, V (N) Figure 26. V-M plane. Experimentally determined yield surface in Figure 24. Three degree-of-freedom testing rig at Oxford University. Moment Load, M/2R (N) Rotational Displacement, 2Rθ (mm) Figure 25. Moment-rotation test on sand. (21), Houlsby and Cassidy (22), Houlsby (23), Cassidy et al. (24)). An example of the yield points obtained, plotted in the vertical load-moment plane, is given in Figure 26. Of particular importance is the fact that at very low vertical loads there is a significant moment capacity, and that this extends even into the tensile load range. In these drained tests the ultimate load in tension is a significant fraction of the weight of the soil plug inside the caisson. Sections of the yield surface can also be plotted in H-M space as shown in Figure 27, where the data here have been assembled from many tests at different Moment Load, M/2R(N) Incremental Rotation, 2Rdtheta (mm) V = -5 N V = N V = 5 N Horizontal Load, H(N) Incremental Horizontal Displacement, du (mm) Figure 27. Yield surfaces and flow vectors in H-M space. stress levels. The flow vectors are also plotted in this figure, and show that in this plane (unlike the V-M plane) associated flow is a reasonable approximation to the behaviour. Feld (21) has observed similar shapes of a yield surface for a caisson in sand. We now consider the possibility of scaling the results of laboratory tests to the field. The test at Frederikshavn shown in Figure 17 was on a caisson with a ratio L/2R 1, at an M/2RH value of approximately 8.7, and with a value of V/2 R 3 of about.62. Using the data from the Oxford laboratory on.2.2 m caissons this requires a vertical load of about 6N. In fact a test had been carried out with L/2R 1 and V 5 N. According to the scaling relationships discussed in section 3, the moment should be scaled according to R 4 (a factor of 625) and the rotational displacement 2R according to R 3 (a factor of 25). Figures 26 and 27 suggest that for a vertical load of 6 N rather than 5 N a moment capacity say 5% higher might be expected, and that for the higher value of M/2RH a further increase of say 15% is appropriate. We therefore apply a factor of 75 to the moments and 25 to the rotational displacements. The result is shown 86

13 Moment, M (knm) Rotational Displacement, 2Rθ (m) Figure 28. Laboratory moment test scaled to field conditions for comparison with Figure 17. in Figure 28. It can be seen that after scaling the moment at a 2R value of.4 m is about 12 knm, compared to about 28 knm measured in the field. Although there is a factor of about 2 between these values, it must be borne in mind that there are a number of possible causes of difference between the tests (e.g. the sand in the field test may be much denser), and also that a factor of 75 has already been applied: a factor of 2 is relatively small by comparison. 5.3 Clay: field and laboratory tests Less work has been carried out on clay than on sand. The large scale trials at Bothkennar (Houlsby et al. 25b) are complemented by laboratory studies intended to model these trials directly, and therefore add confidence to the scaling of the results to prototype size caissons (Kelly et al., 25a). At Bothkennar, moment loads were applied to a 3m 1.5 m caisson by two means. Small amplitude, but relatively high frequency (1 Hz) loading was applied by means of the SEMV device described above, and larger amplitude cycles, but at much lower frequency, were applied using a hydraulic jack. In both cases the loading was 4 m above the caisson, so that h load /D The most important observation from these tests was the gradual reduction of secant stiffness (and increase in hysteresis) as the amplitude of the load cycles increases. The laboratory tests, specifically modelling the field tests, involved just relatively low frequency loading. After the scaling relationships described in section 3 were applied, there was a satisfactory agreement between laboratory and field data, especially at relatively small amplitudes of movement. As an example, Figure 29(a) shows the results (in dimensionless form) for rotation of the 3. m diameter caisson in the field, and Figure 29(b) the equivalent results, also in dimensionless form, from the small scale model test. The pattern of behaviour is remarkably similar in the two tests. M/[s u (2R) 3 ] M/[s u (2R) 3 ] θ (a) field test θ (b) model test Figure 29. Moment-rotation results presented in nondimensional form for laboratory and field tests. This sort of comparison is vital to establish confidence in the use of model testing to develop design guidelines. 6 CAISSON PERFORMANCE: TETRAPOD OR TRIPOD In the following, in which we consider multiple footing designs to support the wind turbine, we shall refer principally to a tetrapod (four footings) rather than a tripod. As a tripod is perhaps the most obvious multiple footing design to use, and has the obvious advantage of simplicity, our preference for the tetrapod deserves some explanation. As is discussed below, prudent design of a multiple footing structure will avoid tension being applied to any of the foundations (except under the most extreme of circumstances). This in effect dictates the separation of the foundations for a given overturning moment and weight of structure. Approximate calculations indicate that the tetrapod structure is usually a more favourable configuration to avoid tension, as it requires somewhat less material. The differences are not large, and a tripod may be preferred in some circumstances, but we shall refer to a tetrapod, as this will probably be more efficient. The important mechanism is the same in both cases: the overturning moment is resisted by opposing push pull action on the foundations. 87

14 Table 4. Vertical loading tests. Site Soil D (m) L (m) Ref. Luce Bay Sand , ,5 Oxford Sand laboratory , 32, 33, 35 Bothkennar Clay UWA.2 centrifuge Clay (1g).6 Vertical Load, V (N) Normalised Vertical Displacement, w/d Vertical Displacement, w (mm) Figure 3. Vertical load-penetration curves for caissons of different L/D ratios. In Table 4 we list the tests that have been carried out on vertical loading of caissons relevant to the wind turbine problem. In addition to these studies there are a number of other relevant studies which have been directed towards vertical loading of caissons for structures in the oil and gas industry or for use as anchors. 6.1 Sand: field and laboratory tests The simplest tests on vertical loading of caissons in sand, which are relevant both to installation and to subsequent performance, simply involve pushing caissons vertically into sand to determine the vertical load-displacement response. Figure 3 shows the results of a set of such tests on caissons of different L/D ratios, Byrne et al. (23). It is clear from the figure that there is a well-established pattern. While the caisson skirt is penetrating the sand there is relatively low vertical capacity, but as soon as the top plate makes contact with the sand there is a sudden increase in capacity. The envelope of the ultimate capacities of footings of different initial L/D ratios also forms a single consistent line. Of most importance, however, is the performance of the caissons under cyclic vertical loading. Figure 31 shows the results of tests on a 3 mm diameter caisson subjected to rapid cyclic loading. Smallamplitude cycles show a stiff response, with larger cycles showing both more hysteresis and more accumulated displacement per cycle. The most important observation is that as soon as the cycles go into tension, a much softer response is observed, and the hysteresis loops acquire a characteristic banana shape. Clearly the soft response on achieving tension should be avoided in design. Closer examination of the curves reveals that the softening in fact occurs once the drained frictional capacity of the skirts has been exceeded, rather than simply the transition into tension. Paradoxically, although additional accumulated displacement is observed once tension is reached, this accumulated displacement is downwards (not upwards as one might expect because of the tensile loading). Vertical Stress (kpa) Vertical Displacement (mm) Figure 31. Cyclic vertical loading of model caisson. The above observations mean that tension must be avoided in a prudent design of a tripod or tetrapod foundation for a wind turbine. However, in all but the shallowest of water, avoiding this tension means that either the foundation must have a large spacing between the footings, or that ballasting must be used. The latter may in fact be a cost effective measure in deep water. Some designers may wish to reduce conservatism by allowing for the possibility of tension under extreme circumstances. It is therefore useful to examine the ultimate tensile capacity under rapid loading. Figure 32 shows the result of three such tests. The slowest test (at 5 mm/s) is almost drained, and a very low capacity in tension is indicated. The capacity in this case is simply the friction on the skirts. The test at 1 mm/s (but zero ambient water pressure) shows a larger capacity, and it is straightforward to show that this is controlled by cavitation beneath the foundation. This means that at elevated water pressures (as in the third test) the capacity rises approximately in step with to the ambient water pressure, as correspondingly larger pressure changes are required to cause cavitation. This problem is studied in more detail by Houlsby et al. (25a). It is important to note, however, that although ambient water pressure increases the ultimate capacity, it 88

15 Vertical Stress (kpa) Vertical Displacement (mm) Direction of movement 5mm/s, kpa 1mm/s, kpa 1mm/s, 2kPa Figure 32. Tensile capacity of model caisson pulled at different rates and at different ambient pressures. [w/(2r)][p a /(2Rγ )] 1/ Min Max Number of Cycles Figure 34. Accumulated displacement during long term cyclic vertical loading on sand. V/[γ'(2R) 3 ] m Field.15m Suction.2m Pushed.15m Pushed [w/(2r)][p a /(2Rγ')] 1/2 Figure 33. and rates. Hysteresis loops from tests at different scales has negligible influence on the tensile load at which a flexible response begins to occur. Comparison of cyclic loading tests at different scales and at different speeds shows that it is difficult to scale reliably the accumulated displacements, which reduce with larger tests and higher loading rates. However, when the scaling rules described earlier are applied, the shapes of individual hysteresis loops at different scales and at different rates become remarkably similar. Figure 33 shows a comparison, for instance, of loops at three different load amplitudes from four different tests. At each particular load amplitude the loops from the different tests are very similar. The accumulation of displacement after very large numbers of cycles is difficult to predict, and so far few data are available. Rushton (25) has carried out vertical loading tests to about 1 cycles on a model caisson in sand, using a simple loading rig which employs a rotating mass and a series of pulleys to apply a cyclic load. A typical result is shown in Figure 34, on a caisson 2 mm diameter and 1 mm deep, with cycling between N. The caisson is therefore subjected (at the minimum vertical load) to a small tension, but less than the frictional capacity of the skirts. The dimensionless accumulated vertical displacement Vertical Stress, V/A (kpa) Figure 35. Test 1: Post Bearing Capacity Test 2: Pre Bearing Capacity Normalised Displacement, (w + L)/D Tension tests on caisson foundations in clay. is seen in Figure 34 to increase approximately with the logarithm of the number of cycles of loading (after about 1 cycles). Note that even in this case where there is a tensile loading in part of the cycle, the net movement is downwards. The displacement is of course very sensitive also to the amplitude of the cycling. 6.2 Clay: field and laboratory tests Very few vertical loading tests relevant to the wind turbine problem have been completed on caissons in clay, although there have been a number of studies directed towards suction caissons used as tension anchors, e.g. El-Gharbawy (1998), Watson (1999), House (22). At Bothkennar tests were carried out in which inclined (but near vertical) loading was applied to a 1.5 m diameter caisson (Houlsby et al., 25b). Difficulties were encountered with the control of the loads using a hydraulic system, and the resulting load paths are therefore rather complex, leading to difficulties in interpretation. Further work on vertical loading in clay is required before definitive conclusions can be drawn, and in particular the issue of tensile loading in clay needs attention. Some preliminary results (Byrne and Cassidy, 22), shown in Figure 35, show that the tensile response may be sensitive to prior compressive loading. Footings loaded in tension immediately after 89

16 installation showed a stiff tensile response, whilst those loaded after first applying a compressive load to failure showed a more flexible tensile response. 7 NUMERICAL STUDIES 7.1 Finite element studies A number of analyses of suction caissons for offshore wind farms have been carried out as part of commercial investigations for possible projects. A more detailed research project was carried out by Feld (21). Finite element analysis is particularly appropriate for establishing the effects of design parameters on the elastic behaviour of caissons, and has been used by Doherty et al. (24a, b) to determine elastic stiffness coefficients for caisson design which take into account the flexibility of the caisson wall as well as coupling effects between horizontal and moment loading. 7.2 Plasticity models An important tool for the analysis of soil-structure interaction problems, particularly those involving dynamically sensitive structures are force resultant models. In these the behaviour of the foundation is represented purely through the force resultants acting upon it, and the resulting displacements (see Figure 4). Details of stresses and deformations within the soil are ignored. The models are usually framed within the context of work-hardening plasticity theory. Examples include models for foundations on clay (Martin and Houlsby, 21) and on sand (Houlsby and Cassidy, 22). Overviews of the development of these models are given by Houlsby (23) and Cassidy et al. (24). These models have been further developed specifically for the offshore wind turbine application. The developments include: Generalisation to full three-dimensional loading conditions, Inclusion of special features to represent the caisson geometry, Expression of the models within the continuous hyperplasticity framework to allow realistic description of hysteretic response during cyclic loading. A model with all these features is described by Lam and Houlsby (25). The fitting of cyclic data to a continuous hyperplastic model is discussed by Byrne et al. (22a). 8 OTHER CONSIDERATIONS We have concentrated here on the design of caisson foundations as far as capacity and stiffness are concerned for in-service conditions. However, there a number of other issues which need to be addressed in a caisson design, and we mention them here briefly. 8.1 Scour Scour is more important for caissons, since they are relatively shallow, than for piles. The size of caissons, and the fact that part of the caisson inevitably protrudes above mudline level, creates rather aggressive conditions for scour. The fact that the caissons may be installed in mobile shallowwater environments means that proper consideration of this problem is essential, especially in sands. If the scour depth can be determined with sufficient confidence (e.g. from comprehensive model testing) then it may be possible to permit the scour to occur, and simply allow for this in the design by ensuring that the caisson is deep enough. It is more likely, however, that scour protection measures such as rock-dumping will need to be employed. Practical experience suggests that such protection must be placed very soon after caisson installation, as scour can occur very rapidly. In highly mobile environments, significant scour can, for instance, occur due to the currents in a single tide. Model testing indicates, however, that scour protection measures can be effective in preventing further erosion (R. Whitehouse: private communication). For in-service conditions regular monitoring for the possibility of scour would be prudent. 8.2 Liquefaction The transient pore pressures induced in the seabed can induce liquefaction, especially if the seabed is partially saturated due to the presence of gas (as can occur in shallow seabeds, largely due to decay of organic matter). The problem is a complex one, but typically, at one stage in the wave cycle, the pore pressure in the seabed can become equal to the overburden stress, and the effective stress falls to zero. This problem is further complicated by the presence of a structure, which clearly modifies the pore pressure pattern that would occur in the far field. Although some progress has been made, the interactions are complex, and theoretical modelling of the problem is not straightforward. 8.3 Wave-induced forces A quite different problem from liquefaction is also related to the fact that the principal forces on the structure are wave induced. As a wave passes the column of the structure it exerts large horizontal forces (of the order of a few meganewtons for a large wave), which also cause overturning moments. However, at 9