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1. Introduction1.1 Jack-up and spudcan backgroundJack-up units are commonly used for oil or gas exploita-tion in water depths up to 120m (Figure 1). They consist of a buoyant triangular hull connected to three independent truss-work legs, with a conical shape foundation (known as a spudcan) at the base. During installation, the legs are low-ered into the seabed independently and usually one after each other. The loading process includes stages of preload-ing where additional load (in comparison to the in-service load) is applied on the spudcan. Once the drilling or work-over is complete, the jack-up unit is removed, leaving on the seabed footprints which may be up to 10m deep and 20m wide in soft clay1 .

Reinstallation of jack-up units nearby pre-existing foot-prints is one of the challenges currently faced by the jack-up industry (Figure 2). During the installation process, vertical load is applied directly though the centre of the spudcan. In the case of installation near a pre-existing footprint, where the soil surface is uneven, an eccentric and/or inclined reac-tion from the soil will be applied to the spudcan. This will tend to cause tilting of the spudcan that is resisted by the development of bending moment in the leg, leading poten-tially to overloading of the legs. Although this problem has been clearly identied2, 3, there are still no guidelines to as-sist operators in a safe reinstallation, aside from the recom-mendation to monitor leg loads via rack phase dierence (RPD) during installation.

Previous investigations carried out by Clunie-Ross4 and Stewart and Finnie5 showed that the critical oset ratio, , when the maximum leg bending moment is developed is 0.75 (measured as the centre-to-centre oset distance be-tween the two installations, a, divided by the spudcan di-ameter, D see Figure 2). On the other hand, the highest horizontal load was recorded at a normalised oset distance of about 1.25. A more recent study6 aimed to quantify the

SPUDCAN REINSTALLATION NEAR EXISTING FOOTPRINTS

Christophe Gaudin, Mark J Cassidy and Tim DonovanCentre for Oshore Foundation Systems, The University of Western Australia, Australia

AbstractThe requirement to re-install spudcan foundations close to existing footprints has a signicant and often detrimental effect both structurally on the jack-up legs load and stability and, geotechnically, on the bearing capacity of the spudcan. The inuence of an existing footprint on the bearing capacity and potential horizontal displacement of the spudcan have therefore been investigated experimentally using a geotechnical centrifuge. The experimental arrangements feature a fully instrumented jack-up leg, measuring axial forces and bending moments, coupled to a sliding device that allows free lateral displacement of the spudcan (advancing previous experimental and nite element studies that have prevented movement of the jack-up leg). This paper presents experimental results obtained by varying the offset distance between the spudcan and the footprint for normally consolidated clay. Implications for the reinstallation of jack-ups under these conditions are discussed.

Figure 1: Three-legged jack-up

bending moment developed in the leg during reinstallation using centrifuge testing. The parameters investigated were the stiness of the leg, the spudcan geometry and oset ratio, and the pre-load level. The results showed similar trends with a critical oset ratio around 0.5, independent of the exural stiness of the leg. More surprising was the lack of inuence of the exural stiness of the leg on the maximum bending moment developed. One reason advanced by the authors to explain this result was the xed connection between the leg

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and the experimental apparatus, which prevented any lateral or rotational movement. Lateral displacements were evalu-ated using simple rst-order bending theory. However, such evaluation may be misleading due to the xities of the appa-ratus which inuences the development of bending moment and may not be relevant to prototype conditions.

The present study aimed to extend this series of results by performing centrifuge tests allowing for free lateral move-ment of the leg during the reinstallation. Although dier-ent to the prototype condition, this case, like the previously discussed study, presented extreme assumptions which are believed to be bound to the real prototype.The investigation was limited to a circular spudcan and a leg stiness corre-sponding to a specic jack-up unit. During the test, vertical load, bending moment at the top of the model leg and ver-tical and horizontal spudcan displacements were recorded. The oset ratio, , was the main parameter investigated. Finally, the performance of a technical solution used to limit the lateral displacement of the spudcan was assessed.

1.2 Physical modelling backgroundSelf-weight forces are the dominant load in geotechnical engineering, and the conning pressures generated by them govern the behaviour of the soil mass (i.e. the behaviour of the soil is stress dependent). Therefore, to respect the simili-tude principles (i.e. to observe the same behaviour) between a full scale structure, (referred to as the prototype case) and a 1/nth reduced-scale model, it is necessary to replicate the gravity-induced stresses by testing the model in a gravi-tational eld n times larger than that of the prototype. A centrifuge is the most convenient way to apply a high accel-eration eld to a model. Applying dimensional analysis to this particular case7, 8, 9 allows for the scaling of the dimen-sions of the reduced scale model and the determination of their values in terms of the prototype dimensions. Table 1 presents the scale factors used for centrifuge modelling.

Centrifuge modelling is now common practice and has been used with success for many years to study oshore geotech-nical structures, including piled structures, gravity-based structures, suction caissons, shallow foundations, plate an-chors and embedded anchors and spudcan foundations10, 11, 12. The University of Western Australia (UWA) hosts a drum centrifuge, which was used for the present study.

Figure 2: Spudcan reinstallation scenario

2. Centrifuge Testing2.1 Experimental setupAll tests were carried out at n = 250g in the drum cen-trifuge at UWA (Figure 3), as this oered the possibility of conducting multiple spudcan penetration tests with one sample. The ring channel of this machine has an outer di-ameter of 1.2m, an inner diameter of 0.8m and a channel height (sample width) of 0.3m. When testing at 250g, this size provides a full-scale testing area of 360m by 75m (with a depth of 35m). An actuator is mounted on an independ-ently rotating central tool table to which the spudcan leg is connected prior to each penetration test, thereby avoiding the need to halt the centrifuge in between tests when the spudcan must be cleaned. A complete technical description of this facility is presented by Stewart et al.13

The model spudcan is based on the prototype Mod V A Class Spudcan. It is a 60mm-diameter (15m prototype) cir-cular spudcan made from aluminium, with a conventional conical shape with a spigot at its centre (Figure 4). No sur-face treatment was applied to the spudcan, and the interface can be considered as smooth. The spudcan was rigidly con-nected to a model leg made from aluminium, 11.4mm in diameter and 190mm long (47.5m prototype).

The Mod V A Class jack-up is operational in water depths exceeding 100m. However, due to space restrictions be-tween the central tool table and the channel of the drum centrifuge, it was not possible to model the full leg length. With the model leg length restricted, a reduced scale factor = 0.5 was therefore applied on the prototype leg length before applying the centrifuge scale factor. Dimensional analysis shows that in such a case, the scale factor on the stiness of the leg should be n4 3(instead of n4) in order to respect the similitude between the model and the prototype leg. Hence, although the leg is 190mm, at 250g it replicates a 95m-long prototype leg. Though the jack-up may some-times operate with leg lengths exceeding 95m, the length used is considered to represent average operational condi-tions. The model and prototype dimensions of the leg and the spudcan are summarised in Table 2.

The model leg was instrumented by two sets of bending strain gauges and one set of axial strain gauges to record the

Dimensions Scale Factors

Density 1

Length 1/n

Displacement 1/n

Strain 1

Stress 1

Bending Stiness n4

Acceleration n

Force n2

Time (dynamic eects) 1/n

Time (consolidation eect) 1/n2

Table 1: Scale factors in centrifuge modelling

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vertical load applied and the bending moment generated during testing.

The key feature of the device was the sliding displacement mechanism allowing free lateral displacement of the leg dur-ing the reinstallation (Figure 5a). It consists of an immov-able rectangular brushed aluminium bracket housing two highly polished non-friction aluminium shafts installed par-allel to each other. This conguration was used to ensure the leg connection underwent no rotation during the testing process. The leg itself was attached to an aluminium block into which the parallel shafts were threaded through. This restricted the block so that it would slide freely only on the horizontal plane. A linear displacement transducer (LDT) was connected to the sliding block to record the lateral dis-placement. The sliding block could also be locked into place by four circular discs positioned on either side, hence preventing any displacement during the initial penetration.

The sliding system featured connections at its bottom and

Figure 3: UWA drum centrifuge

Figure 4: Model spudcan

Prototype Model

MOD V A Class Leg

Length, L 95m* 190mm

Inertia, I 13.8m4 1253mm4*

Stiness, EI 2.75 x 1015Nm2 88107Nm2

MOD V A Class Spudcan

Diameter 15m 60mm

* This was achieved in the model by using an allow tube 11.4mm in diameter and 1.5mm wall thickness

top face for connection to the spudcan and the actuator (Figure 5b). Once setup, the actuator was able to drive the spudcan into the channel (vertical to the sample) and at dierent positions within the channel.

2.2 Soil sample preparation and characterisationNormally consolidated kaolin clay was used as the soil model (see the properties tabulated in Table 3). The clay was mixed under vacuum at a water content of 120% (twice the liquid limit) and poured into the drum channel. The consolidation process was achieved in the centrifuge under self-weight at an acceleration of 250g. Pore pressures at three dierent depths were measured to monitor the con-solidation process.

T-bar penetrometer tests14 were performed before each spud-can penetration test at a penetration rate of 1mm/s. This speed ensured undrained behaviour, and the tests were used to assess the undrained shear strength. The T-bar is an in situ testing tool consisting of a cylindrical bar (5mm by 20mm) attached perpendicularly to the end of a shaft. Strain gauges located at the connection recorded the bearing resistance, qT-bar, continuously during penetration of the cylindrical bar. Using a bearing capacity factor, NT-bar, derived from plasticity theory of 10.515, the following expression was used to calculate the undrained shear strength, su,

(1)

Figure 6 presents the undrained shear strength proles ob-tained over the duration of the spudcan tests. They showed a good consistency with minimal change in the shear strength prole over the duration of the tests. The undrained shear

Table 2: Model and prototype dimensions of the leg and spudcan

Liquid Limit, LL 61%

Plastic Limit, PL 27%

Specic Gravity, Gs 2.60

Angle of Internal Friction, 23Voids Ratio at p = 1kPa on critical state line, ecs 2.140

Slope of normal consolidation line, 0.205Slope of swelling line, 0.044Ratio of pressures on normal compression and critical state lines

2.48

Coecient of consolidation, cv (at over-consolidation ratio = 1 and v = 20kPa)

2m2/yr

Figure 5: (a) The sliding mechanism and (b) the spudcan and sliding mechanism connected to the actuator

Table 3: Properties of UWA Kaolin

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strength, su (in kPa), may be approximate by the following expression

su = z (2)

where z is the prototype depth expressed in metres.

2.3 Experimental procedure and programmeA total of eight tests were performed by penetrating the spud-can into the clay at a constant rate of 0.1mm/s, ensuring un-drained behaviour of the clay16 . The process was as follows:

1. The rst penetration was carried out, with the sliding sys-tem locked in order to prevent any lateral displacement, up to a penetration depth of 90mm (22.5m prototype), corresponding to an embedment of 1.5 diameters.

2. The spudcan was immediately extracted at 0.1mm/s and subsequently cleaned. Note that in the time period nec-essary to stop the tool table, clean the spudcan and set it up in position for reinstallation (corresponding to about 1.2 year prototype), the clay had undertaken some re-consolidation at the location of the rst penetration.

3. The spudcan was reinstalled at 0.1mm/s, up to the same embedment ratio, nearby the initial footprint at an oset ratio, (dened as the centre-to-centre distance divided by the diameter of the spudcan), of 0.25, 0.50 (twice), 0.75, 1.00 and 1.25 (Figure 7). For these six tests, the sliding system was unlocked, allowing for free lateral dis-placement. Two tests were repeated with the sliding sys-

Figure 6: Undrained shear strength proles in the sample from T-bar penetrometer tests

tem locked at an oset ratio of 0.5 and 1.0 for comparison with data obtained previously by Stewart and Finnie5.

During each test, the vertical load, bending moment and vertical and horizontal displacements were recorded at a sampling rate of 10Hz.

3. Experimental ResultsThe adopted sign convention is presented in Figure 8. Vertical displacement is positive downwards. Positive lateral displacement is dened as towards the centre of the pre-existing footprint. All the gures are plotted in prototype scale units, unless mentioned otherwise.

3.1 Penetration resistance during reinstallationFigure 9 presents the penetration resistance for all tests performed with the sliding device unlocked, in compari-son to the initial penetration. The displacement origin was the same for all tests and corresponded to the touchdown of the spigot during the initial penetration. The depression created by the footprint was about 2 spudcan diameters in width (30m prototype) and 3 to 4m deep, meaning that all reinstallation tests were preformed with diering portions of the spudcan initially in contact with the footprint. This was reected in the penetration curves of the reinstallation tests which exhibited little to no penetration resistance at the very beginning of the penetration, over a depth increas-ing with the reduction of the oset ratio. Once the spudcan was in full contact with the soil, the penetration resistance increased linearly with depth (as expected in normally con-solidated clay). Also expected was the immediate clay back-ow observed behind the spudcan penetration in all cases. In addition, the following points are noted:

The bearing capacity factor deduced from the penetra-tion resistance during the initial penetration was about 11.3. This result is consistent with previous results re-ported by Randolph et al.17

As the oset ratio decreased, the penetration resistance during reinstallation reduced, illustrating (a) the remould-ing of clay and resulting loss of strength during the initial penetration and (b) that the spudcan is more inuenced by this remoulding as the reinstallation is closer to the initial penetration. The maximum reduction was observed for an oset ratio of 0.25 and corresponded to a reduction of shear strength of about 33%. This is signicantly lower than that observed by Stewart and Finnie5 (about 60%), but this dierence may be explained by the reconsolidation un-dergone by the clay during the cleaning and repositioning

Figure 7: Spudcan reinstallation process

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Figure 8: Sign convention for V, H and M loads

ratio. This was surprising, as the lateral displacement during the unlocked reinstallation (discussed in the fol-lowing section) demonstrated the displacement of the spudcan towards more remoulded clay, which might have been expected to result in a lower penetration re-sistance compared to the locked case.

3.2 Lateral displacements during reinstallationThe lateral displacement proles during the reinstallation for all unlocked tests are presented in Figure 10. Interestingly, they all exhibited the same pattern, with a rapid increase in lateral displacement starting at a depth increasing with the reduction of the oset ratio. Beyond a certain depth, the lateral displacement tended to reach a stationary value. This behaviour is more pronounced for a high oset ratio, no-tably for = 1.25, where the limit is reached at a shallower depth and where the lateral displacement actually reduces with further penetration.

These patterns are easily explained by the geometry of the footprint and its interaction with the spudcan. As the o-set ratio decreases, the spudcan has less distance to travel before reaching an equilibrium position, which is close to the centre of the previous footprint. As the oset ratio in-creases, the spudcan has a higher distance to travel and it moves laterally until either the embedment is high enough to generate enough lateral resistance or the spudcan is no longer aected by the footprint. Consequently, the highest lateral displacement is observed for a critical oset ratio of = 1, where the spudcan is close enough to be inuenced by the previous footprint and far enough to travel a signi-cant distance before reaching an equilibrium. This critical oset, leading to the maximum leg displacement, is in con-trast with results obtained by Foo et al.3, Clunie-Ross4 and Stewart and Finnie5. They observed that for locked rein-stallation the critical oset, which led to the maximum leg bending moment, was smaller, ranging from 0.5 to 0.75. The evolution of the lateral displacement expressed as a ra-tio of the spudcan diameter with the initial oset ratio is presented in Figure 11. The maximum lateral displacement is observed for an oset ratio of = 1, and is ~5.3m, i.e. nearly 35% of the spudcan diameter.

Figure 9: Penetration resistance during reinstallation with depth for the ve dierent osets

Figure 10: Lateral spudcan displace-ment comparisons during reinstalla-

tion

of the spudcan before the reinstallation in these tests. Although the previous tests were also conducted in the same drum centrifuge, the spudcan re-installation took place immediately after the initial footprint was produced.

Comparison between unlocked and locked reinstalla-tion (not presented here) did not show any signicant dierence in penetration resistance for a given oset

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In addition, the following points may be noted:

For the lowest oset ratio, = 0.25, the lateral displace-ments are initiated after a penetration of 14m. This re-sult seems to show that the lateral displacement is not only due to the geometry of the footprint, but also to the dierent soil shear strength between the side close to the footprint and the one far from the footprint, resulting in an asymmetric bearing pressure applied at the invert of the spudcan.

During the extraction, all tests exhibited a specic pat-tern of movement, as presented in Figure 12 (only the result for the oset ratio of 1 is presented for clarity). The lateral displacements tend to decrease with reduc-ing embedment until it increases again at an embedment ratio of 0.3. This behaviour is consistent for all tests. This phenomenon is not well understood yet and will require more investigation. It may result from the specic stress distribution around the spudcan due to the successive installation and reinstallation. Note that in all cases, the spudcan came back to the original position once fully extracted due to the radial centrifuge force acting on the apparatus. However, this force cannot be the explanation of the behaviour described. This force was sucient to move the spudcan once it did not experienced any lateral resistance, but was insignicant in comparison to the lat-eral resistance acting on the spudcan once penetrated. This is proven by the nearly nil bending moment in the leg during the locked penetration (see Figure 13).

3.3 Bending moment proles during reinstallationFigure 13 presents the bending moment proles for an oset ratio of 1 for the locked and unlocked cases, in addition to the prole during initial penetration. During the initial penetration, the bending moment at the platform level (i.e. the head of the leg) uctuated around zero. This is likely to be due to small heterogeneities in the soil resulting in an uneven penetration resistance along the spudcan and, more impor-tantly, by the small play in the sliding system which does not provide perfect xity. This defect of the device, unfortunately inevitable, is highlighted in the bending moment proles

Figure 11: Maximum lateral spudcan displacement during reinstallation

during reinstallation, which exhibit some erratic variations.

The mechanisms that cause the development of bending mo-ment along the leg of the spudcan depend on the geometry of the footprint and the heterogeneity in the strength of the material from the initial penetration. Two mechanisms are likely to occur, resulting in opposite or additional bending moments in the leg according to the xity at the top of the leg. The rst one corresponds to the tilting of the spudcan as it rests on an uneven surface and only a section of the whole spudcan invert is in contact with the soil at the early pen-etration stage. This mechanism generates a positive bending moment (according to the convention presented in Figure 8) through an eccentric vertical load that increases with penetra-tion, as more soil comes in contact with the spudcan, until half of the spudcan is touching the soil.

Once the spudcan becomes fully embedded, a second mech-anism takes place. The asymmetry in soil shear strength on each side of the spudcan generates an asymmetry in the bearing pressure at the invert of the spudcan, resulting in a further positive bending moment in the leg, also due to an

Figure 12: Typical lateral spudcan displacement curve during reinstallation with oset, = 1 (prototype scale)

Figure 13: Bending moment comparison between the locked and unlocked reinstallation with oset, = 1 (prototype scale)

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eccentric vertical load. However, when the leg is free to move horizontally, this asymmetry generates a horizontal force at the spudcan level, resulting in a horizontal displacement to-wards the weaker material in the centre of the footprint, and a corresponding lateral soil resistance acting on the leg. Both forces (for which resultant is nil) apply at dierent location in the structure and create a negative bending moment. The magnitude of the latter depends on the penetration depth and the magnitude in lateral displacement.

These mechanisms are clearly observable in Figure 13. Both locked and unlocked proles feature a rapid increase in bending moment in the rst few metres of penetration due to the eccentric load on the spudcan. Once full embedment is achieved, the proles diverge with a continuous increase of bending moment in the locked reinstallation, as already observed by Steart and Finnie5, and a decrease and reversal of bending moment with depth for the unlocked case, which reaches its maximum negative value at a penetration of about 12m before increasing again. It should be noted that the subsequent decrease and increase of bending mo-ment for the unlocked reinstallation coincides exactly with the beginning and the end of the lateral displacement, re-spectively, validating the assumptions regarding the mecha-nisms presented previously. Another important observation is the reduction in bending moment magnitude by a factor of 2 when the leg is free to move horizontally.

Figure 14 presents the bending moment proles of all tests per-formed with the sliding device unlocked, in comparison to the one from the initial penetration. The following is observed:

During the rst few metres of penetration, a positive bending moment is generated, as described earlier. The amplitude of this bending moment and the depth of its maximum value depend on the oset ratio, although the variation is relatively small (between 4.9 and 5.2MNm over a depth varying from 3 to 4m).

Beyond an embedment of 4m, there is a clear dierence in behaviour between spudcans reinstalled at an oset ratio, , lower or equal to 0.75 and those reinstalled at

an oset ratio, , higher or equal to 1. In the rst case, the bending moment values reduce to reach eventually a maximum negative value between 12.5 and 17.5m depth. In the second case, the bending moment values decrease before increasing again beyond 12.5m depth to reach a maximum value at the maximum embedment depth. It is uncertain at that stage how these mechanisms combine at deep embedment and if other mechanisms are taking place as the spudcan penetrates deeper. Further investiga-tions are required to understand these patterns.

From Figure 14, it is concluded that the maximum bending moment is generated for an oset ratio of = 1.25 at the maximum embedment depth. This result is in contrast to that obtained by Stewart and Finnie5, who observed that for a locked reinstallation, the maximum bending moment was generated for a penetration equal to half the spudcan diameter and for an oset ratio of = 1.25.

This dierence highlights the dominant role played by the horizontal leg restraint in the development of bending mo-ment in the leg. However, it is not possible to conclude what the exact inuence the oset ratio has on the ampli-tude of bending moment in the leg for cases of intermediate restraint, between the xed and free-sliding cases considered here. A more detailed analysis in term of failure mechanism during penetration and interaction between the remoulded zones generated by the initial penetration and the zone af-fected by the reinstallation would be required. This is how-ever beyond the scope of this paper.

3.4 Technical solution to limit lateral displacementsIn order to investigate methods of limiting the lateral leg displacement during reinstallation, a small thin-walled skirt was added to the base of the spudcan. This skirt was 10mm high, 40mm in diameter and 1mm thick (respectively 2.5m, 10m and 0.25m in eld scale units). It aimed to in-crease the lateral resistance by adding the bearing capacity of the skirt to the frictional resistance of the bottom face of the spudcan. One test was performed with this device for an oset ratio of 1, where the maximum lateral displace-

Figure 14: Bending moment at the platform level during rein-stallation at ve dierent osets with unlocked reinstallation

free horizontal movement cases (prototype scale)

Figure 15: Lateral spudcan displacement comparison between the normal spudcan and the skirted spudcan, with oset,

= 1 (prototype scale)

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ment was observed. The results obtained were encouraging. The lateral displacement was reduced by about 27% (from 5.2 to 4.95m) as shown in Figure 15. Most importantly, this reduction of lateral displacement did not result in an increase of the bending moment in the leg, as demonstrated in Figure 16.

4. ConclusionsCentrifuge tests were performed to assess the behaviour of a spudcan foundation reinstalled nearby a previous foot-print. These tests featured a new experimental device where free lateral displacement of the spudcan was permitted and were performed in addition to a previous test campaign in which the lateral displacement was constrained. The main conclusions are

Allowing free lateral spudcan displacement leads to a sig-nicant reduction in the bending moment generated in the spudcan leg during reinstallation.

The critical oset ratio dened in this case as the po-sition of maximum lateral displacement is found to be 1. The maximum lateral displacement is reached at an embedment of 1.33 diameter of the spudcan and is ~35% of the spudcan diameter.

The lateral displacement may be signicantly reduced without increasing the bending moment in the leg by equipping the spudcan with a circular skirt.

These results are a rst step towards the understanding of the reinstallation behaviour of the spudcan next to an exist-ing footprint. A more complex analysis is now required to account for the full interaction between the spudcans, legs and hull, and to integrate a structural analysis in addition to the geotechnical analysis.

AcknowledgmentsThe authors acknowledge the contribution of Mr Bart Thompson, drum centrifuge operator, who assisted with the centrifuge experiments, and Messrs Tuarn Brown and Phil Hortin, who assisted with the development of

the experimental apparatus. Helpful discussions with Dr Dave White are gratefully acknowledged. The Centre for Oshore Foundation Systems was established under the Australian Research Councils Research Centres Programme and is supported by the state government of Western Australia through the Centres of Excellence in Science and Innovation Program. The research discussed in this paper was conduced when the third author was an enrolled stu-dent at the University of Western Australia.

References1. Osborne JJ, Pelley D, Nelson C and Hunt R. (2006).

Unpredicted jack-up foundation performance. Proceedings of Jack-Up Asia Conference and Exhibition, PetroMin, Singapore, 78 December.

2. Dean ETR and Serra H. (2004). Concepts for mitigation of spud-can-footprint interaction in normally consolidated clay. Proc. 14th Int. Symp. of Oshore and Polar Engng, Toulon, France.

3. Foo KS, Quah MCK, Wildberger P and Vazquez JH. (2003). Spudcan footing interaction and rack phase dierence. Proceedings of the 9th Int. Conf. Jack-Up Platform Design, Construction and Operation, City University, London, UK.

4. Clunie-Ross B. (1999). Reinstallation of spudcan footings in clay. Honours Thesis, the University of Western Australia, Perth.

5. Stewart DP and Finnie IMS. (2001). Spudcan-footprint interac-tion during jack-up workovers. Proc. 11th Int. Symp. of Oshore and Polar Engng, Stavanger, Norway.

6. Cassidy MJ, Quah M and Foo KS. (2006). Experimental inves-tigation of the reinstallation of mobile jack-up platforms close to existing footprints. Available as report C:2248, the University of Western Australia.

7. Buckingham E. (1914). On physically similar systems: illustrat-ing the use of dimensional analysis. Physical Review 4, 345379.

8. Buttereld R. (1999). Dimensional analysis for geotechnical en-gineers. Gotechnique 49(3), 357366.

9. Taylor RN. (1995). Geotechnical Centrifuge Technology. London: Chapman and Hall.

10. Mur JD. (1996). The geotechnical centrifuge in oshore engi-neering. Proc. Oshore Tech. Conf., Houston, USA.

11. Garnier J. (2004). Centrifuge modelling in oshore geotechnical engineering. Proc. 14th Int. Symp. of Oshore and Polar Engng, Toulon, France, 2328 May.

12. Gaudin C, OLoughlin CD and Randolph MF. (2006). New in-sights from model tests of foundation and anchoring systems in oshore geomechanics. Proc. 6th Int. Conf. on Physical Modelling in Geotech., ICPMG06, 46 August, Hong-Kong, Vol. 1, 4762.

13. Stewart DP, Boyle RS and Randoph MF. (1998). Experience with a new drum centrifuge. Proc. Int. Conf. on Centrifuge Modelling, Centrifuge 98. Rotterdam: Balkema, 3540.

14. Stewart D and Randolph MF. (1991). A new site investigation tool for centrifuge. Proc. Int. Conf. on Centrifuge Modelling, Centrifuge 91, Boulder, USA, 531538.

15. Einav I and Randolph MF. (2003). Characterisation of soft soil for deepwater developments. Theoretical analysis of T-bar resist-ance. Report Geo 03302, the Centre for Oshore Foundations Systems, The University of Western Australia, 28p.

16. Finnie IMS. (1993). Performance of shallow foundations in cal-careous soils. PhD Thesis, University of Western Australia.

17. Randolph MF, Jamiolkowski MB and Zdravkovic L. (2004). Load carrying capacity of foundations. In Jardine RJ, Potts DM and Higgins KG (eds.), Advances in Geotechnical Engineering: The Skempton Conference, Vol. 1. London: Thomas Telford, 207240.

Figure 16: Bending moment comparison between the nor-mal spudcan and the skirted spudcan, with oset, = 1

(prototype scale)

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