Drum Centrifuge Model Tests Comparing the Performance of Spudcans and Caissons in Kaolin Clay

Transcription

1 Drum Centrifuge Model Tests Comparing the Performance of Spudcans and Caissons in Kaolin Clay by M.J. Cassidy and B.W. Byrne Report No. OUEL 48/ University of Oxford Department of Engineering Science Parks Road, Oxford, OX 3PJ, U.K. Tel /833 Fax

2 Cassidy, M.J. and Byrne, B.W. Drum Centrifuge Model Tests Comparing the Performance of Spudcans and Caissons in Kaolin Clay M.J. Cassidy and B.W. Byrne August-September Centre for Offshore Foundation Systems, University of Western Australia Department of Engineering Science, University of Oxford ABSTRACT This report presents the results of tests undertaken to compare the response of different offshore foundations under similar loading conditions. The first foundation examined was a spudcan, typical of the footings used for mobile drilling units (commonly known as jack-up rigs). The second foundation examined was representative of a suction caisson. These might be used to anchor jack-up units on a more permanent basis, as well as, in many other offshore applications. The foundations were tested in a normally consolidated Kaolin clay bed. The tests were performed on the newly established drum centrifuge facility located at the University of Western Australia and were carried out at an acceleration equivalent to that of the Earth s gravity. The main benefit of using a drum centrifuge for these types of tests is the large plan area of uniform testing material available. For each of the two samples tested 3 individual footing test sites were used. The first sample concentrated on vertical loading behaviour, including the effect of cyclic loading at various vertical loading levels. The second investigated the response under combined loading with horizontal and moment loading applied during swipe, radial displacement and constant vertical loading tests. Different moment to horizontal load combinations were investigated through the use of an internal hinge on the loading leg. All of the data recorded in flight has been processed and plotted in a standard format. Further analysis of the results is not presented here, nor the details of the background theory supporting the experiments. These details will be presented in forthcoming publications. However, some introductory notes describing the drum centrifuge, the loading legs, the foundations used and details of the tests performed are included. INTRODUCTION Most jack-up rigs use circular conical footings known as spudcans as their foundations and there is considerable interest within the offshore industry to determine the amount of moment fixity they provide. Usually designers have assumed that there is no moment fixity at the spudcan, however, this is an incorrect assumption and can lead to a misunderstanding about the rigs behaviour during extreme events (see Cassidy et al. () for instance). In most cases jack-ups are used for short term drilling, maintenance or construction operations. They are also being considered for deeper water and harsher environments necessitating a critical assessment of their performance under increased loading. Furthermore, several jack-ups have been considered for permanent deployment at oil and gas fields. This may mean that stricter reliability criteria and harsher environmental conditions might be applied in the design process. A critical component in any assessment is the spudcan fixity, as any moment assumed at the footing, may reduce the moments further up the leg, particularly at the leg-hull connection point. To allow for increased loads and higher reliability, one course of action might be to replace the spudcan footings with suction caisson footings. A suction caisson is essentially a shallow foundation that is skirted around the periphery. The foundation is installed by forcing it into the

3 Cassidy, M.J. and Byrne, B.W. ground by applying a pressure across the top of the footing through withdrawing water from within the caisson compartment (i.e. sucking it into the ground). The skirt of the caisson may lead to a greater moment and horizontal capacity than the spudcan. At present there has been no study devoted to comparing the two footings when subjected to the same loading conditions. This study aims to address this gap in the knowledge base by providing experimental evidence of the different responses. There have been, of course, many numerical studies investigating the effects of skirts on shallow foundations. These experiments will not only indicate areas where these numerical studies agree closely but may also highlight areas where more effort is required in the numerical studies. The experiments have been designed and conducted so that the results can be interpreted within a work hardening plasticity framework. The concept of using plasticity models for foundations was established by Roscoe and Schofield (956), however, it has become very popular recently as they can be implemented into structural analyses programs (see for example, Schotman, 989, Martin, 994, Williams et al., 998, Cassidy et al., ). This is important when carrying out reliability assessments of offshore structures. See Martin (994) or Martin and Houlsby (, ) for further information about the development of plasticity macromodels for footings on clay. The main motivation for this study is to compare the response of shallow foundation options for the long-term deployment of jack-up units. However, there are benefits for other applications. One example is the major drive for the development of offshore wind farms in Europe (see Houlsby and Byrne, ). This will involve putting large numbers of wind turbines, which might be about m high with blades over 7m in diameter, in the offshore environment. Due to the nature of the loading there will be large moment loads but very small vertical loads imposed on the foundation system (a different load condition to the jack-up, which has a substantial vertical self-weight). Obviously for some locations piling will be adequate, however, at other locations shallow foundations may be preferable (see Figure ). One structure under consideration is a monopod, where the moment and vertical loads will be imposed directly onto the foundation. An alternative system might be a tripod structural configuration with three caisson foundations. The moment loads will be transferred into vertical loads applied at opposite foundations. In this case the tensile loading on the upwind foundation may be critical. EXPERIMENTAL SET-UP Drum Centrifuge The testing described here was performed in the drum centrifuge located at the University of Western Australia and shown in Figure. This facility was installed in 998 and has a.m diameter and a maximum acceleration level of 485g. The soil sample is contained in the outer channel, which is 3mm high (vertically) and has a mm radial depth. A central actuator provides both vertical and radial motion (and combined with the hinged leg described below a combination of vertical, horizontal and rotational motion on a footing can be achieved). By using two concentrically driven shafts connected by a Dynaserv motor, relative motion between the outer channel and the central tool table can be achieved and controlled. The system has been designed such that the tool table can be stopped and raised out of the main testing area (known as parked ) while the channel continues to rotate. This allows the instrumented testing tools that are fixed to the actuator to be modified or changed without affecting the acceleration level on the soil. This was done between each test described here, allowing for the spudcans and caissons to be cleaned and interchanged.

4 Cassidy, M.J. and Byrne, B.W. The primary advantage of using a drum centrifuge for the testing of shallow foundation is the large plan area of the test sample. The sample depth was approximately 4mm (a prototype depth of 4m), and with all tests performed at g, this corresponded to a prototype testing area of 3m by 44.5m and depth 4m. Further technical details of the drum centrifuge can be found in Stewart et al. (998). Loading Apparatus The centrifuge is a very useful experimental apparatus as it allows for correct replication of the stress levels associated with large-scale geotechnical problems. However, due to space restrictions and high acceleration levels it is often the case that the complexity of the experimental system is reduced. For foundation problems the true loading is six degree of freedom however it is usually idealised as planar, with a vertical (V), moment (M/R) and horizontal (H) load applied to the footing. In the study described in this report it was possible to apply two different loading regimes where the ratio of moment (M) to horizontal (H) load was varied. This was accomplished by using two different loading legs; a) the standard fixed leg, and, b) one containing a hinge so that the ratio of M to H was comparable with what is applied to spudcan/caisson foundations on jack-up rigs. It was possible to investigate different sections of the yield surface in these moment horizontal loading planes. A loading leg that can be attached to the actuator of the centrifuge has been designed and is shown in Figure 3. It is assembled from individual components allowing for a combination of tests. The central component can either be a hinged or a fully fixed piece, and the footings can also be altered. The footing diameters were kept constant at 6mm (prototype 6m) so that comparisons between footings could be carried out. The spudcan and caisson attachments (skirt length to diameter (L/D) ratio of.5) are shown in Figure 4. By changing the skirt length, caissons with L/D of.33 and. were also used. Photographs of the loading arm and footings are shown in Figure 5. Two load reference points were used when processing the caisson data: one at the base of the caisson skirts and one directly beneath the baseplate. For the spudcan the load reference point is the first point of maximum diameter, assuming a penetrating spudcan. The load reference points have been indicated on the spudcan and caisson depicted in Figure 4. All of the caissons had a specially designed valve attached to the baseplate, allowing for a realistic in-flight installation procedure. The valve was left open until the caisson was installed (i.e. the inside soil plug reached the top of the inside section of the caisson). It was then closed by applying a pressure of 4kPa through an airline to the valve. This valve has been previously used in the beam centrifuge at the University of Western Australia and is described in more detail in Watson (999). As indicated in Figure 3 the loading leg was strain gauged in four locations and consisted of top and bottom axial and top and bottom bending gauges. During the fixed arm test the more accurate top axial gauge was not on the loading leg and the axial readings solely relied on the bottom gauge. The bending gauges could be used to determine a shear force. This was extrapolated to calculate the moment at the footing's load reference point (as shown on Figure 4). A laser was used to measure the distance between the upper piece and the hinged lower piece. The change in this distance could be translated into the angle of rotation of the hinge and hence the rotation of the footing. Appendix A details the calculation procedure. The components of the measured axial and shear forces can then be resolved, based on the rotation, so that the true vertical and horizontal loads acting on the footing can be determined. The calibration factors for the load cells and the laser are given in Table. 3

5 Cassidy, M.J. and Byrne, B.W. Soil Type The key properties of the Kaolin clay used are shown in Table. This clay has been used in many laboratory investigations at UWA and so has been well studied. The sample was prepared in a standard procedure as described: Step Mixing: Kaolin powder was mixed with water in a conventional barrel mixer, deaired under a kpa vacuum, for 4 hours. The slurry had a moisture content of %, approximately twice the liquid limit of the Kaolin. Step Placement in Drum: To create a drain around the base of the sample a sand layer of 5mm thickness was sprayed onto the drum through air. A total of approximately 6kg of the mixed slurry was then slowly pumped into the channel whilst it was spinning at g. This was, however, performed in two stages. The first represented about half of the slurry and was allowed to consolidate overnight at g. The acceleration was lowered back to g to lay the second half of the clay sample. The complete sample was consolidated at g for about two days, until the pore pressures stabilised. The sample was approximately 4mm deep (4m) with a normally consolidated profile. Soil characterisation was performed using a t-bar penetrometer (Watson, 999). One t-bar test was performed next to and on the same day as every footing test (3 t-bars per sample). The t-bar profiles of the two tanks have been included in the figures and are located at the beginning of each section. These profiles have been interpreted in terms of the undrained shear strength of the soil with depth of penetration of the t-bar. RESULTS The results presented in this report represent the processed data. All results are presented in the two sections at the end of the report. The data measured during the testing procedure has been processed to reflect prototype engineering units. The results are presented in sections corresponding to the two tanks and are plotted in terms of the combined loads {V:M/R:H} and displacements {w:rθ:u} imposed on the footings at the load reference point. The sign convention followed is that recommended by Butterfield et al. (997) and shown in Figure 6. The load reference points were shown previously in Figure 4. No thorough interpretation of the data is given here but will be published in forthcoming articles. Two series of tests were performed. The first tank concentrated on vertical loading and the testing program is shown in Table 5. The hinged leg was used for all tests with a combination of spudcans and all three caisson L/D ratios. The data presented are just the vertical loading data, as this is the only set of variables changing. The figures consist of a) time : vertical load, b) time : vertical displacement, and, c) vertical displacement : vertical load. The second tank concentrated on investigating the behaviour and yield surface shape under combined vertical, horizontal and moment loading. Details of Tank are given in Table 6. Both the hinged and fixed loading arms were used, but with only the spudcan and the caisson with L/D =.5. The majority of tests performed were swipe tests. These are tests where the vertical displacement is kept constant whilst a combination of horizontal and/or rotations are applied to the footing. The resulting load path approximately tracks the yield surface at that vertical plastic displacement (Tan, 99; Martin, 994). However, radial displacement (a constant ratio of horizontal and vertical displacement applied) and constant vertical load (vertical load held constant whilst a combination of horizontal and/or rotations are applied) 4

6 Cassidy, M.J. and Byrne, B.W. tests were also performed and are presented here. These test give an indication of the flow rule associated with the expansion of the yield surface. In this series of tests there is more data to present and so the number of figures has increased from that of Tank. In the most part the Figures are self explanatory, however, several of the Figures require a little explanation. During the swipe tests several swipes were performed at different levels of plastic penetration. Usually the footing was swiped in one direction then swiped in the reverse direction and so on. So that the swipe event data are easier to follow when plotting in say {V:H} space the reverse direction swipe was reflected about the V-axis. As horizontal and moment loading should be symmetrical this plotting system is acceptable. Two of the graphs show results normalised by V o. The value used is the maximum vertical compressive load that has been previously applied to the footing. Occasionally a figure has been left empty, as there was no appropriate data to plot. The loading leg was designed with a hinge so that a combination of moment to horizontal load could be applied to the footing. In order to carry out a test that is commensurate with the definition of a swipe test (i.e. with no vertical displacement of the reference point) an assumption is required about the movement of the hinge. In this instance the decision was taken to rotate the hinge in a circular arc about the load reference point for each footing. As the test is carried out there is likely to be small amounts of vertical, rotational and horizontal movements as the soil-structure system is not infinitely stiff. Of these movements the vertical is most critical with regards to the theoretical nature of the test. The question is whether this surface approximates the yield surface. The initial hypothesis of the plasticity theories is that hardening of the yield surface is associated with vertical plastic penetration. In these tests the ratio of the elastic to plastic vertical stiffness is very large (of the order of 4) so actually a small amount of vertical penetration would not significantly affect the shape of the surface that is swiped. The pseudo-swipe tests will therefore track a close approximation to the yield surface. For the caisson the rotation point was taken to be at the centre of the base of the caisson skirts. The data for the caissons have been plotted in two sets of graphs. The first is where the load reference point is assumed to be at the centre of the bottom of the caisson baseplate (i.e. mudline). The second set is where the load reference point is assumed to be at the centre of base of the caisson skirts. If the rotations (for say a swipe) indicated in Table 6 are ±6 the actuator movements in the vertical and radial direction are.97 and.67mm respectively for the spudcan and 5.6 and.799mm respectively for the caisson. The actuator displacements required for other footing rotations are indicated in Tables 3 and 4. The actual vertical and horizontal movements of the footings can be calculated using the procedure set out in Appendix A. This provides a correction on the actuator movements accounting for the rotation of the footing about the hinge. CONCLUSIONS In this report the experimental data obtained from two drum centrifuge tests of both spudcan and caisson footings on Kaolin Clay have been presented. The purpose of the tests is to make a comparative assessment of the two shallow foundation options. The basis for the comparison is the response under loading conditions that might be experienced by jack-up rigs. This response is characterised by a yield surface approach in the three degree of freedom loading space {V:M/R:H}. This report presents the experimental data in its entirety with no thorough interpretation given. However, this will be given in forthcoming publications. 5

7 Cassidy, M.J. and Byrne, B.W. ACKNOWLEDGEMENTS This work was undertaken with support from Woodside Energy Limited (Po No:754) and an IREX grant from the Australian Research Council. The second author also acknowledges the support of the Royal Commission for the Exhibition of 85, the Apgar Prize and Magdalen College, Oxford. These experiments could not have been performed without the support of the drum centrifuge technician Mr Bart Thompson. The helpful advice and comments of Prof. Mark Randolph, Mr George Vlahos and Mr Andrew House were appreciated during the design of the loading leg and the course of the testing. The Centre for Offshore Foundation Systems was established and is funded under the Australian Government s Special Research Centres Program. REFERENCES Butterfield, R., Houlsby, G.T. and Gottardi, G. (997). Standardised sign conventions and notation for generally loaded foundations. Géotechnique 47, N o 4, pp 5-5. Cassidy, M.J., Eatock Taylor, R., Houlsby, G.T. (). Analysis of jack-up units using a Constrained NewWave methodology. Applied Ocean Research 3, pp -34. Houlsby, G.T. and Byrne, B.W. (). Suction caisson foundations for offshore wind turbines and anemometer masts. Journal of Wind Engineering 4, N o 4, pp Martin, C.M. (994). Physical and numerical modelling of offshore foundations under combined loads. DPhil Thesis, University of Oxford. Martin, C.M. and Houlsby, G.T. (). Combined loading of spudcan foundations on clay: laboratory tests. Géotechnique 5, N o 4, pp Martin, C.M. and Houlsby, G.T. (). Combined loading of spudcan foundations on clay: numerical modelling. Géotechnique 5, N o 8, pp Roscoe, K.H. and Schofield, A.N. (956). The stability of short pier foundations in sand. British Welding Journal, August, pp Schotman, G.J.M. (989). The effects of displacements on the stability of jack-up spud-can foundations, Proc. st Offshore Technology Conference, Houston, pp 55-54, OTC 66. Stewart, D.P. (99). Lateral loading of piled bridge abutments due to embankment construction. PhD Thesis, the University of Western Australia. Stewart, D.P., Boyle, R.S. and Randolph, M.F. (998). Experience with a new drum centrifuge. Proc. Int. Conf. Centrifuge 98, Tokyo, Japan,, pp Tan, F.S.C. (99). Centrifuge and numerical modelling of conical footings on sand. PhD Thesis, University of Cambridge. Watson, P.G. (999). Performance of skirted foundations for offshore structures. PhD Thesis, University of Western Australia. Williams M.S., Thompson R.S.G., Houlsby G.T. (998). Non-linear dynamic analysis of offshore jack-up units. Computers and Structures 69, N o, pp

8 Cassidy, M.J. and Byrne, B.W. TABLES Table Calibration factors Load Cell Factor Top Bending Nm/bit Bottom Bending Nm/bit Top Axial.6735 Nm/bit Bottom Axial.6434 N/bit Laser. mm/bit Table Kaolin clay properties (after Stewart, 99) Property Value Liquid Limit, LL 6 % Plastic Limit, PL 7 % Plasticity Index, I P 34 % Specific Gravity, G s.6 Angle of Internal Friction, φ 3 Consolidation Coefficient (mean), c v m /year Submerged unit weight, γ 6.8 kn/m 3 Table 3 - Swipe movements for spudcan (L.R.P. at first maximum radius) q ( ) Radial (mm) Vertical (mm) Table 4 - Swipe movements for caisson (L.R.P. at bottom of skirts) q ( ) Radial (mm) Vertical (mm)

9 Cassidy, M.J. and Byrne, B.W. Table 5 - Vertical loading tests on spudcans and caissons (Tank ) Test Name Footing Comments BBMCT Spudcan Vertical BBMCT Spudcan Vertical BBMCT3 Spudcan N N V m =N (±N) pull-out BBMCT4 Spudcan N N V m =N (±N) pull-out BBMCT5 L/D=.5 Vertical BBMCT6 L/D=.5 Pull-out after installation 5N pull-out BBMCT7 L/D=.5 Installation 5N N 5 V m =N (±N, N, 5N) pull-out BBMCT8 L/D=.5 Installation N N 5 V m =N (±N, N, 5N) pull-out BBMCT9 Spudcan N 5N V m =5 (±N, N, 3N, 5N, 6N) pull-out BBMCT Spudcan Vertical BBMCT L/D=.5 Push in / pull out valve not closed BBMCT L/D=.5 Push in / pull out valve closed BBMCT3 L/D=.5 Push in / pull out valve closed BBMCT4 L/D=.33 Push in / pull out valve closed BBMCT5 L/D=.33 Installation N N V m =N (±N, N, 3N, 5N) N N V m =N (±N, 3N, 5N) pull-out BBMCT6 L/D=.33 Push in / pull out valve closed BBMCT7 L/D=.5 Push in / pull out valve not closed BBMCT8 L/D=.5 Installation N V m =N (±N) pull-out BBMCT9 L/D=.5 Valve not properly closed BBMCT L/D=.5 Installation 5N 5 V m =5N (±N) 9mm pull-out BBMCT L/D=.5 Installation N 5N 5 V m =5N (±N) pull-out BBMCT L/D=.5 Push in / pull out using filters (no valves) BBMCT3 L/D=.5 Push in / pull out no valves or filters BBMCT4 L/D=. Push in / pull out valve not closed BBMCT5 L/D=. Push in / pull out valve closed BBMCT6 L/D=.5 Installation 5N 5 V m =5N (±N) mm pull-out BBMCT7 L/D=.5 Installation 5N N 5 V m =N (±N) 9mm pull-out BBMCT8 L/D=.5 Installation 5N 5 V m =5N (±N except every cycles have one load V min = -5N) 9mm pull-out BBMCT9 Spudcan N 5N 5 V m =5N (±N except every cycles have one load V min = -N) 9mm pull-out BBMCT3 Spudcan N 5N 5 V m =5N (±N except every cycles have one load V min = -3N) 9mm pull-out BBMCT3 Spudcan N N 5 V m =N (±N except every cycles have one load V min = -4N) 5N N 5 V m =N (±N except every cycles have one load V min = -4N) 9mm pull-out BBMCT3 L/D=.5 5N N 5 V m =N (±N except every cycles have one load V min = -N) 5N N 5 V m =N (±N except every cycles have one load V min = -N) 9mm pull-out 8

10 Cassidy, M.J. and Byrne, B.W. Table 6 Combined loading tests on spudcans and caissons (Tank ) Test Name Footing Hinge Test Type Comments BBMCT Test Aborted BBMCT Test Aborted BBMCT3 Test Aborted BBMCT4 Spudcan Hinge Swipe 5N Swipe centre ±5 5N Swipe to outside ± 5 9N Swipe to centre ±5 pull-out BBMCT5 L/D=.5 Hinge Swipe Installation 5N Swipe to centre ±5 (using spudcan LRP) 5N Swipe to outside ±5 N Swipe to centre ±5 pull-out BBMCT6 L/D=.5 Hinge Swipe Installation 5N Swipe to centre ±5 (using caisson LRP) 5N Swipe to outside ±5 N Swipe to centre ±5 pull-out BBMCT7 L/D=.5 Fixed Arm Swipe Installation 5N Swipe to centre ±mm N Swipe to outside ±mm 5N Swipe to centre ±mm N Swipe to outside ±mm pull-out BBMCT8 Spudcan Fixed Arm Swipe 3mm Swipe centre ±mm N Swipe to outside ± mm 5N Swipe to centre ±mm pull-out BBMCT9 Spudcan Fixed Arm Swipe 3mm 5N Swipe to centre ± 8mm 75N 87.5N Swipe to outside ±8mm pull-out BBMCT L/D=.5 Fixed Arm Swipe Installation N 5N Swipe to centre ± 8mm 85N 87.5N Swipe to outside ±8mm N N Swipe to centre ± 8mm pull-out BBMCT L/D=.5 Fixed arm Swipe Installation 6N N Swipe to centre ± 8mm 5N N Swipe to outside ±8mm 6N N Swipe to centre ± 8mm 9N -N Swipe to outside ±8mm pull-out BBMCT Spudcan Fixed Arm Swipe 3mm N Swipe to centre ± 8mm 5N N Swipe to outside ±8mm 6N N Swipe to centre ± 8mm 8N N Swipe to outside ±8mm pull-out BBMCT3 Spudcan Hinge Swipe 3mm N Swipe to centre ±6 5N N Swipe to outside ±6 6N N Swipe to centre ±6 pull-out 9

11 Cassidy, M.J. and Byrne, B.W. Test Name Footing Hinge Test Type Comments BBMCT4 L/D=.5 Hinge Swipe Installation 5N N Swipe to centre ±6 N -5N Swipe to outside ±6 5N -5N Swipe to centre ±6 N -5N Swipe to outside ±6 N pull-out BBMCT5 Spudcan Hinge Swipe 3mm -N Swipe to centre ±6 N -5N Swipe to outside ±6 5N -5N Swipe to centre ±6 N -5N Swipe to outside ±6 pull-out BBMCT6 L/D=.5 Hinge Swipe Installation N -3N Swipe to centre ±6 5N -5N Swipe to outside ±6 5N -5N Swipe to centre ±6 N -5N Swipe to outside ±6 N pull-out BBMCT7 Spudcan Fixed Arm Swipe 3mm -N Swipe to centre ±6 N -5N Swipe to outside ±6 5N -5N Swipe to centre ±6 6N -5N Swipe to outside ±6 pull-out BBMCT8 L/D=.5 Fixed Arm Swipe Installation N -5N Swipe to centre ±6 5N -5N Swipe to centre ±6 5N -5N Swipe to centre ±6 pull-out BBMCT9 Spudcan Fixed Arm Radial displ. test 3mm N concurrently du=mm dw=4mm (du/dw=.5) BBMCT Spudcan Fixed Arm Radial displ. test 3mm (du/dw=.) N conc. du=mm dw=5mm (du/dw=.666) N conc. du=3mm dw=5mm (du/dw=.) pull-out BBMCT L/D=.5 Fixed Arm Radial displ. test Installation (du/dw=.) N conc. du=mm dw=5mm (du/dw=.666) N conc. du=3mm dw=5mm (du/dw=.) pull-out BBMCT Spudcan Fixed Arm Constant V 3mm N hold V=N mm mm Vel=.mm/s 3N hold V=3N mm mm Vel=.mm/s (came back to V=8N) hold V=3N mm mm Vel=.mm/s pull-out

12 Cassidy, M.J. and Byrne, B.W. Test Name Footing Hinge Test Type Comments BBMCT3 Spudcan Hinge Radial displ. test 3mm (du/dw=.) N conc. du=mm dw=5mm (du/dw=.666) N conc. du=3mm dw=5mm (du/dw=.) pull-out BBMCT4 L/D=.5 Hinge Radial displ. test Installation (du/dw=.) N conc. du=mm dw=5mm (du/dw=.666) N conc. du=3mm dw=5mm (du/dw=.) pull-out BBMCT5 Spudcan Hinge Cyclic swipes 3mm V/V =.5 3 cyclic swipes ±.5mm 3N cyclic swipes ±.8mm 3N cyclic swipes ±.mm 3N cyclic swipes ±.4mm 3N cyclic swipes ±4.8mm 3N cyclic swipes ±9.6mm N pull-out BBMCT6 L/D=.5 Hinge Swipes Installation -N Swipe to centre ±6 5N -N Swipe to outside ±4 N pull-out BBMCT7 L/D=.5 Hinge Swipes Installation (not successful i.e. no valve closure) -N Swipe to centre ±6 N -5N Swipe to outside ±6 5N Swipe to centre ±6 N pull-out BBMCT8 L/D=.5 Hinge Swipes Installation -N Swipe to centre ±6 N -5N Swipe to outside ±6 5N Swipe to centre ±6 N pull-out BBMCT9 L/D=.5 Hinge Cyclic swipes 3mm V/V =.5 3 cyclic swipes ±.5mm 3N cyclic swipes ±.8mm 3N cyclic swipes ±.mm 3N cyclic swipes ±.4mm 3N cyclic swipes ±4.8mm 3N cyclic swipes ±9.6mm N pull-out BBMCT3 L/D=.5 Fixed Arm Swipes Installation -N Swipe to centre ±mm 5N Swipe to outside ±mm additional mm pull-out BBMCT3 L/D=.5 Fixed Arm Swipes Installation -N Swipe to centre ±mm N Swipe to outside ±mm 5N Swipe to centre ±mm -5N Swipe to outside ±mm N pull-out

13 Cassidy, M.J. and Byrne, B.W. Test Name Footing Hinge Test Type Comments BBMCT3 Spudcan Fixed Arm Swipes 3mm -3N Swipe to centre ±mm N Swipe to outside ±mm 5N Swipe to centre ±mm -5N Swipe to outside ±mm -5N Swipe to centre ±mm mm pull-out

14 Cassidy, M.J. and Byrne, B.W. Anemometer mast or turbine support structure (a) (b) (c) Water surface Seabed Caissons Caisson Steel pile NOT TO SCALE Figure Structural options for offshore wind applications. Figure UWA Drum Centrifuge. 3

15 Cassidy, M.J. and Byrne, B.W. screws into drum centrifuge tool table laser attached to measure relative displacment of hinged bottom section Allows rotation around the load reference point 83.5mm top axial guage top bending guage bottom axial guage bottom bending guage 5mm 5mm.5mm.5mm Two arm options: hinged or fixed arm Caisson can be replaced with spudcan Figure 3 Design of loading leg used in the experiments. 8 mm thread 8 mm thread "poppet" valve Load Reference Point Spudcan Caisson L/D=.5 Load Reference Points Figure 4 Model spudcan and caisson footings used in experiments. 4

16 Cassidy, M.J. and Byrne, B.W. (a) fixed arm with spudcan (b) hinged arm with caisson (L/D=.5) Figure 5 Photographs of the loading arm. R Reference position w M H Current position u θ V Figure 6 Sign convention adopted (after Butterfield et al., 997). 5

17 Tank Vertical Loading

18 Tank, Shear Strength, s u (kpa) Depth (m) 6 8

19 BBMCT

20 BBMCT

21 BBMCT3

22 BBMCT4

23 BBMCT5

24 BBMCT6

25 BBMCT7

26 BBMCT8

27 BBMCT9

28 Time, t (hours) Time, t (hours) BBMCT

29 BBMCT

30 BBMCT

31 BBMCT3

32 BBMCT4

33 BBMCT5

34 BBMCT6

35 BBMCT7

36 BBMCT8

37 BBMCT9

38 BBMCT

39 BBMCT

40 BBMCT

41 BBMCT3

42 BBMCT4

43 BBMCT5

44 BBMCT6

45 BBMCT7

46 BBMCT8

47 BBMCT9

48 BBMCT3

49 BBMCT3

50 BBMCT3

51 Tank Combined Loading

52 Tank, Shear Strength, s u (kpa) Vertical Penetration, w (m)

53 Rotational Displacement, R q (m) BBMCT4

54 Rotational Displacement, Rq (m) H /V o. M /RV o BBMCT4

55 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT5

56 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Caisson Baseplate BBMCT5

57 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT5

58 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Base of Caisson Skirts BBMCT5

59 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT6

60 H /V o Rotational Displacement, Rq (m) M /RV o LRP at Caisson Baseplate BBMCT6

61 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT6

62 Rotational Displacement, Rq (m) H /V o..5 M /RV o LRP at Base of Caisson Skirts BBMCT6

63 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT7

64 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Caisson Baseplate BBMCT7

65 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT7

66 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Base of Caisson Skirts BBMCT7

67 Rotational Displacement, R q (m) BBMCT8

68 H /V o Rotational Displacement, Rq (m) M /RV o BBMCT8

69 Rotational Displacement, R q (m) BBMCT9

70 Rotational Displacement, Rq (m) H /V o.5..5 M /RV o BBMCT9

71 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT

72 H /V o Rotational Displacement, Rq (m) M /RV o LRP at Caisson Baseplate BBMCT

73 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT

74 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Base of Caisson Skirts BBMCT

75 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT

76 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Caisson Baseplate BBMCT

77 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT

78 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Base of Caisson Skirts BBMCT

79 Rotational Displacement, R q (m) BBMCT

80 Rotational Displacement, Rq (m) H /V o.5 M /RV o BBMCT

81 Rotational Displacement, R q (m) BBMCT3

82 Rotational Displacement, Rq (m) H /V o M /RV o BBMCT3

83 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT4

84 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Caisson Baseplate BBMCT4

85 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT4

86 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Base of Caisson Skirts BBMCT4

87 Rotational Displacement, R q (m) BBMCT5

88 Rotational Displacement, Rq (m) H /V o M /RV o BBMCT5

89 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT6

90 Rotational Displacement, Rq (m) H /V o.4. M /RV o LRP at Caisson Baseplate BBMCT6

91 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT6

92 Rotational Displacement, Rq (m) H /V o.4. M /RV o LRP at Base of Caisson Skirts BBMCT6

93 Rotational Displacement, R q (m) BBMCT7

94 Rotational Displacement, Rq (m) H /V o..5 M /RV o BBMCT7

95 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT8

96 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Caisson Baseplate BBMCT8

97 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT8

98 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Base of Caisson Skirts BBMCT8

99 Rotational Displacement, R q (m) BBMCT9

100 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o BBMCT9

101 Rotational Displacement, R q (m) BBMCT

102 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o BBMCT

103 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT

104 Rotational Displacement, Rq (m) H /V o.6 M /RV o LRP at Caisson Baseplate BBMCT

105 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT

106 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Base of Caisson Skirts BBMCT

107 Rotational Displacement, R q (m) BBMCT

108 Rotational Displacement, Rq (m) H /V o M /RV o BBMCT

109 Rotational Displacement, R q (m) BBMCT3

110 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o BBMCT3

111 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT4

112 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Caisson Baseplate BBMCT4

113 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT4

114 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o LRP at Base of Caisson Skirts BBMCT4

115 Rotational Displacement, R q (m) BBMCT5

116 Rotational Displacement, Rq (m) H /V o.6.4 M /RV o BBMCT5

117 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT6

118 Rotational Displacement, Rq (m) H /V o.4.3 M /RV o LRP at Caisson Baseplate BBMCT6

119 Rotational Displacement, R q (m) LRP at Base of Caisson Skirts BBMCT6

120 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Base of Caisson Skirts BBMCT6

121 Rotational Displacement, R q (m) LRP at Caisson Baseplate BBMCT7

122 Rotational Displacement, Rq (m) H /V o M /RV o LRP at Caisson Baseplate BBMCT7