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Spudcan extraction from deep embedment in soft clayApplied Ocean Research

Spudcan extraction from deep embedment in soft clayApplied Ocean Research

Spudcan extraction from deep embedment in soft clay

Manuscript submitted to Applied Ocean Research

Omid Kohan (corresponding author)

PhD Candidate

Centre for Offshore Foundation Systems and ARC Centre of Excellence for Geotechnical Science and Engineering

University of Western Australia

Perth, WA 6009

Australia

Christophe Gaudin

Professorial Fellow

Centre for Offshore Foundation Systems and ARC Centre of Excellence for Geotechnical Science and Engineering

University of Western Australia

Perth, WA 6009

Australia

Mark J. Cassidy

Winthrop Professor

Centre for Offshore Foundation Systems and ARC Centre of Excellence for Geotechnical Science and Engineering

University of Western Australia

Perth, WA 6009

Australia

Britta Bienen

Associate Professor

Centre for Offshore Foundation Systems and ARC Centre of Excellence for Geotechnical Science and Engineering

University of Western Australia

Perth, WA 6009

Australia

Words: 4632 (excluding abstract and references)

Figures: 16

Tables: 4

Abstract

After drilling is completed, spudcan footings of mobile jack-up rigs are extracted from the seabed before the jack-up is manoeuvred to a new location. In some instances, the extraction may prove to be difficult and time consuming, especially when the spudcans are deeply embedded, because the pull-out capacity of the rig is less than the extraction resistance of the spudcans. In soft soil, the extraction resistance may be significantly augmented by the development of suction at the spudcan invert. To investigate this phenomenon, a deeply embedded 30 mm diameter model spudcan was extracted in a series of physical model experiments conducted at an acceleration of 200g in a geotechnical beam centrifuge. The spudcan, instrumented with two pore pressure transducers, one at the top and one at the bottom face, was extracted from normally consolidated clay and under undrained conditions. Eight tests are reported exhibiting embedments ranging from 1.5 to 3 spudcan diameters and varying operation periods. The excess pore pressure and maximum breakout force measured reveal insights into the magnitude of the suction forces at the spudcan invert, which were observed to increase with the embedment depth. No change in failure mechanism was observed between 1.5 and 3 spudcan diameters depth.

Key words: Spudcan; centrifuge modelling; soft clay; suction; extraction.

Introduction

Self-elevated mobile jack-up units (Figure 1) play an important role in offshore drilling in shallow waters, up to approximately 150 m depth. The inverted conical footings of jack-ups, which are known as spudcans and can be in excess of 20 m in diameter in a modern jack-up (Cassidy et al., 2009), can be penetrated in a wide range of soil conditions. In softer soils, spudcans require large penetration before meeting sufficient bearing capacity to withstand the jack-ups self-weight and the expected operational loads. Penetration of up to two or three spudcan diameters may be necessary before reaching equilibrium during the preloading process (Endley et al., 1981; Menzies and Roper, 2008).

When a jack-up rig is removed from a site and redeployed, its spudcans must be extracted from the seabed. To overcome the soil resistance, the hull is floated, and lowered beyond neutral draft. However, tolerances on the maximum allowable overdraft within the marine operations manual restrict the maximum extraction pull to between 30 and 50% of the maximum compressive load that can be applied during installation (Purwana et al. 2009). In soft soils for deep spudcan penetration (> 1.5 times the spudcan diameter) and long operation periods, the buoyancy of the hull may not be sufficient to extract the spudcan. It is reported that spudcan extraction from penetration depths of one or two spudcan diameters can require one or two weeks, and in some extreme cases, up to ten weeks (InSafe JIP, 2008). The spudcan extraction process, especially from soft clay, may therefore be a time-consuming process. With average jack up day rate in the range US$60,000 to US$160,000 (depending on the water depth), this has significant financial impact.

Figure 2 shows failure mechanisms during initiation of undrained spudcan extraction, as developed by Gaudin et al. (2011) from observations from Particle Image Velocimetry (PIV) analysis of physical tests by Purwana et al. (2006) and numerical analysis by Zhou et al. (2009). In the first stage of the undrained extraction of the spudcan, the main soil resistance is comprised of the weight of the soil above the spudcan, the resistance along a shear plane generated above the spudcan and negative excess pore pressure, namely suction, that is developed at the spudcan base in undrained extraction. In fact, the extraction mechanism is a combination of an uplift mechanism of the soil at the top of the spudcan and reverse end bearing at the spudcan invert due to suction. The contribution of both theses mechanisms is influenced by the duration of the jack-up operation. At the top of the spudcan, Purwana et al. (2009) measured via T-bar tests a reduction of 67% of the shear strength immediately after spudcan installation, followed by an increase of 30% (raising the shear strength to 87% of the undisturbed undrained shear strength) after an operation period of 400 days. Similarly, the gain in shear strength underneath the spudcan after the same operation period time was evaluated as 1.70 times the undisturbed strength by Purwana et al. (2009) from numerical analysis. Both outcomes imply an increase in effective stresses within the soil underneath and at the top of the spudcan, resulting from dissipation of the excess pore pressures generated during the penetration process, albeit at a different rate and magnitude. The phenomena governing the changes in effective stresses in the soil are complex and for the soil at the top, potentially include changes in total stresses due to arching.

In a second stage, the extraction resistance typically reaches a peak followed by a dramatic reduction in resistance. The failure mechanism is then replaced by a localised flow around mechanism, still associated with an uplift mechanism of the soil above the spudcan (Gaudin et al., 2011).

From the observed failure mechanism described, it may be inferred that suction forces contribute significantly to the peak undrained extraction resistance. The importance of base suction generated during spudcan extraction was first revealed by a series of centrifuge tests performed at an acceleration of 100 g and simulating the installation and extraction of spudcans from uniform soft clay with an undrained shear strength in the range of 12-40 kPa (Craig and Chua, 1990). Results indicated that the magnitude of suction was related to the compressive loading history and the associated embedment ratio prior to extraction. However, issues such as the operational period that the jack-up is installed for were not studied by and these form an important component of the testing programme discussed in this paper.

Purwana et al. (2005) experimentally investigated the effect of operation period and operating load magnitude level on spudcan extraction. Results demonstrated that the extraction resistance increases with the operation period. In contrast, the level of jack-up operating load (i.e. the load maintained during the operation period) has an insignificant effect on spudcan extraction in comparison with the time that a jack-up is under operation. It is noteworthy that Purwana et al. (2005) investigated spudcan extraction from embedment up to 1.5 spudcan diameters. To the authors knowledge, the deepest spudcan penetration reported is 78 meters in the Gulf of Mexico, corresponding to an embedment ratio of 5.6 (Menzies and Lopez, 2011), although this is exceptional and penetrations up to a maximum of three spudcan diameters are more common (Menzies and Roper, 2008).

The objective of the present study is to extend the database of Purwana et al. (2005) to embedment up to 3 times the spudcan diameter, to notably investigate if a change of mechanism at deeper embedment may affect the suction generation at the spudcan invert. For this purpose, a series of centrifuge tests were performed, featuring penetration and extraction after varying operating period of a model spudcan penetrated at embedment ratio between 1.5 and 3.

Vertical loads and pore pressures at the top and bottom of the spudcan during the installation, operation period and extraction of the spudcan were monitored, and the results are reported and discussed.

Soil preparation and characterisation

Commercial Kaolin clay with characteristics provided in Table 1 was used to create a soft soil sample in the beam centrifuge at the University of Western Australia (Stewart and Randolph, 1991; Gaudin et al., 2011). The mixture of Kaolin and water at a moisture level of twice the liquid limit formed a de-aired clay slurry, which was then poured into a rectangular strongbox over a 15 mm thick drainage sand layer.

Thereafter, the sample was consolidated under self-weight in the centrifuge at an acceleration of 200 g for a period of approximately five days. Over the consolidation time, settlement of the sample was measured, and at the end, the final height of the soil specimen was approximately 180mm.

A 15 mm diameter miniature piezoball penetrometer (as also used by Mahmoodzadeh et al., 2011) was used to derive the undrained shear strength profile of the sample with a bearing capacity factor of 10.5 (Low et al., 2011; Lee et al., 2012; Lee et al., 2013). The test was performed in flight at a rate of 1 mm/s to ensure that undrained conditions were measured (Chung et al., 2006). The average shear strength gradient was approximately 1.1 kPa/m (Figure 3).

Experimental programme and procedure

A model spudcan with diameter D of 30 mm was fabricated to investigate the extraction of deeply embedded spudcans (Figure 4). The spudcan was manufactured from aluminium alloy 6061-T6 and was connected to a two-dimensional actuator via a load cell. The model spudcan was instrumented with two pore pressure transducers (one at the top face and one at the base) that were installed at approximately half the distance between the centre and the edge of the spudcan. The cross-section of the pore pressure transducers at the top and base of the spudcan is illustrated in Figure 5.

Eight tests were performed at an acceleration of 200 g in a beam geotechnical centrifuge (Randolph et al., 1991). Tests one to four were designed to investigate the effect of the embedment depth on spudcan extraction, whereas tests five to eight were performed to investigate the effect of the duration of operation time on spudcan extraction. In the first four tests, the spudcan installation depth was varied from 1.5 to 3 times the spudcan diameter. In these tests, spudcan extraction occurred after two years operating load (in prototype scale). In the remainder of the tests, the operation period varied from immediate extraction to three years, and the spudcan embedment ratio was 1.5D. Details of the test programme are provided in Table 2.

Spudcan penetration and extraction was undertaken at a penetration rate v of 0.3 mm/s, resulting in a normalised velocity V=vD/cv greater than 30 (assuming a coefficient of consolidation cv of 3.99 m2/y, at a stress level consistent with the spudcan embedment, see Table 2). This ensured that spudcan installation and extraction occurred under undrained conditions (Finnie and Randolph, 1994), mimicking insitu conditions. In the field, successful spudcan extraction may require between 6 hours and 30 hours. Considering spudcan diameters in the range 10 to 20 m and coefficient of consolidation in the range 0.1 to 100 m2/year, normalised extraction velocity insitu are typically greater than 30.

The same test procedure was used for all cases and consisted of three stages. In the first stage, spudcan penetration was performed in-flight in displacement-control mode. The embedment depth ranged from approximately 8.8 m to 18.1 m (prototype scale) corresponding to an embedment ratio of 1.5 to 3, respectively. In the second stage, the jack-up operation period was simulated by holding a constant vertical load of approximately 85% of the maximum installation load for up to three years in prototype scale. For operating period of 2 years and above, pore pressure measurements at the spudcan invert indicated that at least 85% of consolidation was achieved. Finally, in the third stage, spudcan extraction was performed at a constant rate of 0.3 mm/s.

For all stages, the vertical force on the spudcan (corresponding to the penetration resistance, the applied load, and the extraction resistance for the three stages of testing, respectively) and pore pressures at the top and the invert of the spudcan were monitored.

Experimental Results

Installation resistance

The development of penetration resistance Qp, excess pore pressure (with respect to the hydrostatic pressure) at the spudcan invert ui and at the spudcan top ut, are presented in Figure 6, Figure 7, and Figure 8, respectively, for the installation, operation and extraction stages.

Figure 9 presents the normalised net vertical load Qp/(A.su) where Qp is the net penetration resistance measured by the load cell, A the projected area of the spudcan and su the undisturbed shear strength at the spudcan embedment, against the normalised embedment H/D, where H is the penetration depth and D the spudcan diameter. Note that the spudcan embedment is defined at the lowest full diameter of the shoulder of the spudcan. This provides insight into the net bearing capacity factors during penetration.

During installation, excess pore pressures, both at the top and the invert of the spudcan, increase linearly with depth. Tests performed by Purwana et al. (2005) on a larger spudcan, instrumented with both total and pore pressure transducer at the top and invert of the spudcan, demonstrated that excess pore pressures where equal to the change in total pressures during penetration, indicating no change in effective stresses and so a fully undrained process. Based on the same assumption, the penetrating pressure, comprising of the applied pressure qp = Qp/A and the excess pore pressure at the top of the spudcan ut,ins, is compared to the resisting pressure ui,ins corresponding to the excess pore pressure at the spudcan invert in Figure 10. Values at the end of the installation phase presented in Table 3. The agreement is reasonably good throughout the full penetration process, confirming the observations from Purwana et al. (2005), and demonstrating the undrained response of the soil.

This result is however surprising. The phenomena governing the changes in pore pressures at the invert and at the top of the spudcan are complex and involve changes in both effective and total stresses. At the spudcan invert, the soil is essentially sheared so an element of soil underneath the spudcan is expected to experience a reduction in effective stresses, reflecting the remoulding of the soil, as well as an increase in pore pressures. The magnitude of the reduction in effective stresses is difficult to assess and is likely to vary along the spudcan. At the top of the spudcan, the phenomenon is even more complex. Pore pressures at the top of the spudcan are likely induced from the shearing of the soil (which is flowing from underneath the spudcan), but also from a cavity expansion mechanism associated with the cylindrical leg of the spudcan, and a reduction in total stresses due to arching and potential silo effect along the column of soil on the top of the spudcan. Similarly to the invert of the spudcan, changes in effective stresses are expected, although they were not observed by Purwana et al. (2005), and are not suggested by Figure 10.

Indeed, accurate assessment of the contribution of the various components to the penetration resistance is difficult as both the top and invert pore pressure measurements are local measurements extrapolated over the entire surface. Purwana et al. (2005), using a larger model with several pore pressure transducers, showed that the excess pore pressures at the spudcan invert increased towards the centre of the spudcan. In addition, the pore pressures were measured at the soil spudcan interface (rather then in the soil body) and do not necessarily reflect changes within the soil underneath and at the top of the spudcan.

While spudcan penetration is a complex problem, it is noteworthy that it can be elegantly captured by only two parameters, a bearing factor Nc and the undrained shear strength su, as demonstrated in Figure 9. Immediate back-flow on the top of the spudcan was observed visually during testing. This confirms the analysis made by Hossain et al. (2005), indicating that deep failure mechanism, characterised by symmetrical flow-around, occurs at a relatively shallow embedment for soft soils. Indeed, the normalised net vertical load development in Figure 9 exhibits a constant value from an embedment ratio of about 0.7. Bearing factors calculated from the experimental measurements are compared in Figure 9 with large deformation finite element (LDFE) analysis in ideal Tresca soil and Tresca soil modified to account for strain softening and strain rate effects (Hossain and Randolph, 2009). The centrifuge results lean towards the modified numerical solution, i.e. yielding a bearing factor in the range 9-10.4, indicating that undrained conditions are prevalent within the soil and that significant strain softening takes place.

Operating period

Following penetration, 85% of the maximum penetration load (except for Test 2.0D2.0Y in which the holding load was 100% of the installation load due to a temporary technical problem in the centrifuge) was maintained on the spudcan for operating times ranging from 0 to 3 years prototype (see Table 3). This stage resulted in the consolidation of the soil underneath (and to a reduced degree at the top of) the spudcan and additional spudcan settlement as summarised in Table 3. During the operating period, excess pore pressure at the top and bottom of the spudcan dissipated, as shown in Figure 11, which presents the development of the degree of consolidation with the time factor Tv = tcv/D2, where t is the time since the beginning of the operational period and cv has been assumed to be the virgin coefficient of consolidation (estimated as a function of the stress level, see Table 2).

It is noteworthy that degrees of consolidation ranging from 85% to 90% were achieved at the spudcan base at the end of the operation period for all tests, whereas at the top of the spudcan, the degree of consolidation of about 40% to 60% was inferred (Figure 11). The lower degree of consolidation at the top is best explained by a reduction of the coefficient of consolidation by potentially one order of magnitude. Such a large reduction may be explained partially by the lower stress level experienced by the highly remoulded soil at the top of the spudcan, but also by a significantly higher modulus of compressibility. It is however important to recognise, as for the installation process, that the pore pressure measurements are undertaken at one single point and do not necessarily reflect the behaviour of the entire mass of soil at the bottom and at the top of the spudcan.

Spudcan extraction - Increasing embedment depth and constant operating period Tests 1 to 4

As previously reported by Purwana et al. (2005), Bienen et al. (2009), and Kohan et al. (2013), negative excess pore pressures (or suction) generated during extraction reach a peak at the point of maximum extraction resistance, also termed breakout point. In the present case, maximum suction was measured slightly after the breakout point, after displacements ranging from 0.02D to 0.06D. There is no explanation for this behaviour, except potential delay in the pore pressure measurements resulting from poor saturation of the transducer porous stone. Accordingly, the analysis assumes that both peak suction and peak extraction resistance occur simultaneously.

Figure 7 presents the development of suction with spudcan penetration. Peak suction values are reported in Table 4. It is noteworthy that the excess pore pressure at the end of the operation period is relatively similar for tests 1 to 4 (see Table 3). This is expected because they all experienced the same operation period of 2 years. Accordingly, the change in magnitude of suction force during extraction is solely related to the spudcan embedment.

To investigate this point further, the peak suction is plotted against the initial effective stress v0 in Figure 12. It is evident that the magnitude of peak suction developed at the spudcan invert increases linearly with the initial effective stresses. Under undrained extraction, the variation of effective stresses during shearing is identical for all embedment depths and is related to the spacing between the normal consolidation line and the critical state line (or an identical portion if the operation period has not allowed full reconsolidation). Accordingly, the suction generated is the difference between the change in effective stresses and the change in total stresses. This will increase with the increasing change in total stresses as the embedment increases. Therefore, unless there is a change in mechanism (and thus a change in total stress), a linearly increasing relationship between effective stress and excess pore pressure generated, as observed in Figure 12, is expected. The only factor limiting the suction developed is the cavitation pressure. At ambient temperature, water will undergo cavitation at pressure about 80-95 kPa below the atmospheric pressure (Thorn et al., 2004). Considering the range of suction pressure measured (see Table 4), with respect to the hydrostatic pressure (from 88 to 170 kPa), it is evident that cavitation cannot occur in any of the tests. It can therefore be reasonably concluded that the extraction mechanism described for the embedment depth of 1.5D by Gaudin et al. (2011) is also valid for embedments up to 3D.

This is further demonstrated by the value of the ratio of peak suction generated at the breakout point to extraction resistance. For the four tests considered, the ratio varies within a narrow range of 70% to 80%, independent of the spudcan embedment. Gaudin et al. (2011) reported values of about 70% for a spudcan embedded at 1.5 D and with a degree of consolidation of 90% at the end of the operating period, while Purwana et al. (2005) reported value of about 60% for spudcans with long operation periods and an embedment ratio of 1.5D.

Additional insights are provided in Table 5, which compares the variation of load q between the end of the operation period and the peak extraction resistance with the variation of pore pressures at the top and bottom of the spudcan u = ui + ut, both contributing to the extraction resistance. The ratio u/q is lower than 1 for all tests (but test 8), indicating that the change in load is not entirely accounted for by the change in pore pressures. Interestingly, the weight of the soil plug on top of the spudcan varies between 54 and 108 kPa (increasing with depth), assuming a value of equal to 6 kN/m3 and contributes essentially for the difference between q and u (although a significant scatter is acknowledged, that may be explained by (i) the uncertainty of the unit weight of the remoulded soil plug at the top of the spudcan, (ii) the single point measurement of the excess pore pressures and (iii) maybe more importantly, the contribution of the friction along the shearing planes of the soil plug). This observation validates the extraction mechanism at peak extraction presented in Figure 2.

Spudcan extraction - varying operation period at an embedment ratio of 1.5 Tests 4 to 8

Five tests (test numbers four to eight) were performed to investigate the effect of the operation period on the mechanism associated with spudcan extraction. The operation period ranged from less than one day to three years (in prototype dimensions), all for an embedment ratio of 1.5 (see Table 2).

Figure 13 presents the comparisons of the loads developed with displacement during installation, operation, and extraction for different operating periods. The excess pore pressures generated during installation dissipate during the operation period and reach a value close to the hydrostatic pressure for operation time of 2 years and longer (Table 4).

Longer consolidation periods result in higher extraction resistance (Figure 13), which are concomitant with a higher development of suction at the spudcan invert (Figure 14). This is better illustrated in Figure 15, which presents the evolution of peak suction at the spudcan invert and peak extraction resistance with the time factor Tv.

The comparison of the magnitude of the excess pore pressure at the beginning of extraction and at the breakout point ui = ui,op - ui,ex in Table 3 and Table 4 shows that the negative excess pore pressure ui generated during extraction is approximately constant between the tests and falls within a relatively narrow range of 104-107 kPa, with the noticeable exception of the test without an operation period, where the difference is 122 kPa (this point is discussed latter in the paper). This is illustrated in Figure 14, and in Figure 16, which presents the measured pore pressures at the invert and at the top of the spudcan and the end of the operation period, and at peak extraction, as a function of the time factor Tv.

Consequently, the total level of suction generated, which directly governs the magnitude of the extraction resistance depends on the pore pressure at the end of the operating period, This was also observed by Purwana et al. (2005), although a higher magnitude of excess pore pressure between the end of the operation period and the peak extraction was reported (201-230 kPa), but for a different type of clay, with a higher soil strength ratio (0.24 compared with 0.18) and a different initial strength at spudcan embedment (~30 kPa versus ~10 kPa).

When the spudcan is extracted immediately after penetration, the suction developed brings the absolute pore pressure at the spudcan invert to a value close to zero, such that no active suction at the spudcan invert contributes to the extraction resistance. This potentially indicates that a different mechanism takes place compared to the cases where extraction is performed after a period of consolidation. Three other elements confirm that hypothesis:

1 The load extraction curve (Figure 13) exhibits a smooth reduction post peak, while a sharper reduction is observed for the tests with consolidation period. This indicates a change in mechanism post peak for tests with a consolidation period (as discussed in Gaudin et al., 2011), which does not occur for immediate extraction.

2 The excess pore pressure curve at the invert (Figure 14) exhibits no changes post peak, while a sharp reduction in suction is observed post peak for the tests with a consolidation period. This indicates that a partial suction relief mechanism occurs for the test with a consolidation period, which is not observed for immediate extraction. This reinforces the observation from point 1.

3 The ratio of total change in load to the total change in pore pressure at the invert and at the top of the spudcan q/u presented in Table 5 is 1 for immediate extraction, while it ranges from 0.67 to 0.71 for tests with consolidation period.

The combination of these three observations demonstrates that a flow round mechanism takes place for immediate extraction (q/u = 1), while a partial reverse end bearing mechanism takes place for tests with a consolidation period, due to the heterogeneous effective stress field resulting from localised consolidation. This difference in mechanism explains the difference in change in pore pressure at the invert ui = ui,op - ui,ex between immediate and delayed extraction (122 kPa against 104-107 kPa). The zero active suction indicates that the soil flows from the top below the spudcan, without being sucked in, which is consistent with a full flow mechanism. Interestingly, the ratio of extraction to penetration resistance for immediate extraction is 0.64. This is close to the same ratio for T-bar tests in normally consolidated clay ( 0.7), indicating that immediate spudcan extraction resistance can potentially be assessed from in-situ T-bar tests results.

As the operation period increases, the difference in excess pore pressures ui reduces to 104-107 kPa and remains constant regardless of the consolidation time This indicates that the extraction mechanism remains identical for all non-zero consolidation times. Comments made about the ratio u/q for tests with increasing embedment are equally valid for tests with increasing consolidation time. The ratio u/q is in the range 0.66 0.71, decreasing with consolidation time, reflecting an increasing contribution of the soil plug, most likely due to an increase of the friction along the shearing planes. Indeed, the excess pore pressures measured at the top of the spudcan at peak extraction remain relatively constant, around 50 kPa (see Figure 15), with consolidation time, indicating a constant contribution of the weight of the soil plug. This assumes that the pore pressures and total pressures at the top are equal during extraction, as observed by Purwana et al. (2005).

Conclusions

Centrifuge tests have been performed to investigate spudcan extraction resistance in normally consolidated soil as a function of the initial embedment and the operation period. Results demonstrate that the mechanism at the point of maximum extraction resistance involves a reverse end bearing mechanism associated with plug uplift. This mechanism is valid for initial embedment ratio up to 3 times the spudcan diameter and when there is an operational hold of vertical load on the spudcan. For immediate extraction, the mechanism consists of a full flow round, with a ratio of extraction to penetration resistance similar to that measured in a T-bar test.

It was also demonstrated that the contribution of the plug uplift is constant with the operation period. This is in contrast with the peak suction at the spudcan invert, which increases with the operation period, so longer operation periods result in higher extraction resistance. However, the difference in pore pressure between the end of the operation period and the peak suction is approximately constant. Additional work is required to link this constant value with particular mechanisms and soil characteristics (including strength softening and hardening due to consolidation), enabling its assessment for a wide range of spudcan geometry and soil strength.

The above conclusions are restricted to the range of the experimental centrifuge tests, but are believed to provide relevant insights into the extraction mechanisms taking place for a deeply embedded spudcan. Further studies are required to understand whether the extraction mechanism is different for spudcan embedment ratios greater than 3.

Acknowledgments

This work forms part of the activities of the Centre for Offshore Foundation Systems (COFS), currently supported as a node of the Australian Research Council Centre of Excellence for Geotechnical Science and Engineering and as a Centre of Excellence by the Lloyd's Register Foundation. Lloyds Register Foundation invests in science, engineering and technology for public benefit, worldwide. The first author is also supported by the Robert and Maude Gledden Postgraduate Research Scholarships. The third and fourth authors are the recipients of an Australian Research Council (ARC) Laureate Fellowship and Postdoctoral Fellowship (DP110101603) respectively. This support is gratefully acknowledged.

Notations

A: contact area of spudcan

cv: coefficient of consolidation

D: diameter of spudcan

Gs: specific gravity

g: gravity acceleration

H: depth

LL: liquid limit

Nc: undrained bearing capacity factor

Nball:undrained bearing capacity factor of the ball penetrometer

PL: plastic limit

q: Pressure resistance

qe: extraction resistance

qp: penetration resistance

qp-op: operational pressure

Qnet vertical load

Qppenetration load

Qeextraction load

sop: settlement during operation

su: undrained shear strength

Toperoperation period

Tv:time factor

Udegree of consolidation

V: normalised penetration velocity

v: spudcan penetration or extraction velocity

b: breakout depth

ui,exexcess pore pressure at invert of spudcan at the breakout point

ui,insexcess pore pressure at invert of spudcan at the end of the installation

ui,opexcess pore pressure at invert of spudcan at the end of the operational period

ut,exexcess pore pressure at top of spudcan at the breakout point

ut,insexcess pore pressure at top of spudcan at the end of the installation

ut,opexcess pore pressure at top of spudcan at the end of the operational period

uexcess pore pressure

'v: effective vertical stress

': submerged unit weight

':angle of internal friction

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