Guideline for Lẹg Jetting System

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Gaudin, C. et al. (2011). Geotechnique 61, No. 12, 1043–1054 [http://dx.doi.org/10.1680/geot.8.P.152]

1043

Investigation of the potential of bottom water jetting to ease spudcanextraction in soft clay

C. GAUDIN�, B. BIENEN� and M. J. CASSIDY�

Retrieval of jack-up rigs at the end of their operations iscommon practice in the offshore industry, notably tomove the rigs to another drilling location. In some casesthe process is difficult, time consuming and costly, be-cause the high extraction resistance on the jack-up’sspudcan footings can exceed the capacity of the rig topull. This is particularly the case in soft clay, wheresignificant suction is developed at the spudcan invert.The main option available to operators to ease thespudcan extraction resistance is to use a jetting system atthe spudcan invert to attempt to break the suction gener-ated. However, there is a general consensus within theoffshore industry that jetting systems, under their cur-rent configurations, have a limited efficiency. Centrifugeexperiments have been performed at the University ofWestern Australia in order to understand the mechan-isms taking place during jetting extraction, and to pro-vide recommendations to optimise the jettingperformance. A reduced-scale spudcan model simulatinga 17.11 m diameter prototype spudcan with jets has beentested at 200 times the acceleration of Earth’s gravity(known as 200g). It was extracted from penetrations ofup to 1.5 diameters in normally consolidated clay atvariable extraction rates and variable jetting flow rates.Measurements of the generated suction and the totalextraction resistance after a preloading period have pro-vided insight into the extraction mechanisms with jetting.The study has demonstrated that jetting can lead tosignificant reduction in extraction resistance, providedthat the extraction rate is fast enough to ensure un-drained extraction, and that a sufficiently high flow rateis applied with respect to the extraction rate.

KEYWORDS: centrifuge modelling; clays; consolidation;offshore engineering; suction; water flow

La desinstallation des plateformes mobiles est une pra-tique courante dans l’industrie offshore, notamment afinde deplacer une plateforme d’un point de forage a unautre. Cette operation peut parfois s’averer technique-ment difficile and economiquement desavantageuse. C’estnotamment le cas pour les sols argileux ou la resistance al’extraction des fondations est superieure aux capacitesde traction de la plateforme, a cause des phenomenes desuccion generer a la base des fondations. Une des tech-niques couramment mise en place par les operateurspour faciliter l’extraction consiste a injecter de l’eau ahaute pression a la base des fondations, afin de limiter ledeveloppement des phenomenes de succion et ainsi re-duire la resistance developpee. Un consensus existe cepen-dant parmi les operateurs pour admettre l’efficacite tresaleatoire de cette technique. Des essais en centrifugeusegeotechnique ont ete realises a l’Universite d’AustralieOccidentale afin de comprendre les mecanismes develop-pement de succion a la base des fondations de plateformemobile et la performance des techniques d’injectiond’eau. Les essais ont ete realises sous une acceleration de200g, sur un modele reduit de fondation de forme con-ique simulant une fondation de 17,11 m de diametre.Apres installation du modele dans une argile normale-ment consolidee a une profondeur de equivalent a 1,5diametre, la fondation a ete retiree du sol, a differentesvitesses d’extraction et a differents debits et pressiond’injection d’eau. La mesure de la resistance a l’extrac-tion et du developpement de pressions interstitielles a labase de la fondation, demontre l’efficacite des techniquesd’injection d’eau si et seulement si, l’extraction est con-duite suffisamment rapidement pour generer un compor-tement non draine du sol et si le debit d’injection estsuffisamment eleve.

INTRODUCTIONMost of the world’s offshore drilling in shallow water (up to120 m) is performed from self-elevated mobile units, whichare generally referred to as ‘jack-up’ rigs (Fig. 1). The unitsconsist of a buoyant triangular platform resting on threeindependent truss-work legs, each with a rack and pinionsystem used to jack the legs up and down through the deck.The foundations of independent-leg jack-up platforms ap-proximate large inverted cones, commonly know as ‘spud-cans’. In modern jack-ups the spudcan diameter can be inexcess of 20 m (Fig. 1).

When a jack-up rig is to be relocated, its spudcans arerequired to be extracted from the seabed. The extraction iscompleted by lowering the rig’s hull into the sea in order to

use its buoyancy to pull the spudcans, hopefully overcomingthe soil resistance. However, depending on the level of embed-ment and the nature of the soil, the buoyancy of the hull maynot generate sufficient pullout force to extract the foundation(which typically ranges from 20% to 50% of the maximumpenetration force). Extraction from penetration depths of oneor two spudcan diameters is reported to regularly take one ortwo weeks, and in some cases up to 10 weeks have beenrequired to free the legs (Purwana et al., 2009).

To overcome this issue, most modern mobile drilling rigs areequipped with a water jetting system (with flow rate typicallyaround 60 gal/min or 4 l/s) integrated into the spudcans toassist in the extraction of the foundations. In sandy and siltymaterial, the water jetting system aims to fluidise the soilsurrounding the spudcan, resulting in a significant decrease ofthe effective stresses, and hence a significant reduction of theextraction resistance (Lin, 1987). In clay material, where sig-nificant suction may be developed at the spudcan invert, thejetting aims to break the suction and hence reduce the extrac-tion resistance to the sole component of the bearing capacityprovided by the soil resting on top of the spudcan.

Manuscript received 10 December 2008; revised manuscript accepted15 October 2010. Published online ahead of print 28 July 2011.Discussion on this paper closes on 1 May 2012, for further details seep. ii.� Centre for Offshore Foundation Systems, University of WesternAustralia, Perth, Australia.


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Very few references exist in the literature to assess thereduction in soil resistance generated by water jetting. Lin(1987) developed and described a jetting system for ageneric conical footing. Reduced-scale model tests wereperformed at 1g on a 609 mm diameter spudcan embeddedin a pit filled with saturated beach sand. Different nozzleconfigurations were investigated, varying the inclination ofthe jetting (from 458 to 908 with respect to the spudcansurface) and the location of the nozzles (at the top and theinvert of the spudcan). The jetting was delivered at apressure of 0.4 MPa and at a flow rate of up to 4.1 l/s.Results demonstrated the potential efficiency of the jettingsystem, with a reduction of the pullout resistance by up to80%. The reduction increased with the flow rate and theinclination of the jetting. Visual observation during the testsconfirmed the fluidisation of the sand around the spudcan,with several sand boilings occurring during the jetting. Sub-sequent analytical work indicated a reduction in pulloutresistance of about 76%, in good agreement with the 80%observed during the experiments (Lin, 1995).

To the authors’ knowledge, no such experiments havebeen performed in soft clay. In addition, based on feedbackreceived from drilling contractors, leg extraction can proveto be time consuming, even when combined with waterjetting at the top and bottom of spudcans. Leg extractionattempts for moderate leg penetration depths up to two timesspudcan diameter often take 1–2 weeks, and in some ex-treme cases up to 10 weeks were required to free the legs. Itis also found that extraction problems generally occur whendeep leg penetration is encountered, or the unit has been onlocation for an extended operation period prior to extraction.

Despite the fact that water jetting systems are commonlyfitted to jack-up rigs, very little is known about theirmechanisms or effectiveness, particularly for their applica-tions in clayey material. In order to better understand waterjetting problems, the Centre for Offshore Foundation Sys-tems (COFS), in collaboration with Keppel Offshore Tech-nology Development (OTD), carried out a series ofcentrifuge tests to investigate the mechanisms and perform-ance of spudcan jetting extraction in normally consolidatedclay (Gaudin et al., 2006). The experimental study focusedon measurements of the total extraction force and thesuction developed at the spudcan invert for variable extrac-tion and variable jetting flow rates. Different jetting config-

urations, varying nozzle locations and diameters were alsoinvestigated. The present study aims to shed light on theefficiency of water jetting issues and the mechanisms in-volved, and to serve as a starting point for further in-depthinvestigations into potential improvements to existing waterjetting system.

A second series of tests focused on using load controlduring extraction. These more closely mimicked in situextraction procedure. These results and recommendations foroffshore practice are presented in Bienen et al. (2009).

EXPERIMENTAL SET-UP AND PROCEDUREFacility and test set-up

The experiments were performed in the beam geotechnicalcentrifuge at the University of Western Australia (Randolphet al., 1991). The centrifuge has a swinging platform radiusof 1.8 m with a nominal working radius of 1.55 m, andis rated at 40 g-tonnes (this equates to a maximumpayload of 200 kg at the acceleration of 200g used inthese experiments). The platform seats standard rectangular‘strongboxes’, which have internal dimensions of650 mm 3 390 mm 3 325 mm (L 3W 3 H), representing aprototype test bed of up to 80 m wide by 130 m long by60 m deep at 200g.

Water jetting was applied using a syringe pump housedwithin the centrifuge platform (House, 2002). It consists of apiston moving inside a cylinder, which is connected by ahose to an inlet in the strongbox. The cylinder has an innerdiameter of 50 mm and a stroke of 190 mm. This providesan internal volume of 373 cm3. The drive rate of the motorshaft ranges from 0.001 to 3 mm/s, corresponding to fluidrates of 0.02 and 5.9 cm3/s respectively (in model dimen-sions).

A two-dimensional actuator mounted on top of the strong-box was used to penetrate and extract the spudcan in theunderlying soil.

Model and instrumentationThe centrifuge tests were performed at an acceleration of

200g. The shape and size of a 17.11 m diameter spudcanused offshore was modelled, with 1:200 model dimensions

Over160 m

Over20 m

All dimensionsapproximate

~20 m

~4 m

Fig. 1. Typical jack-up and spudcan foundations (after Reardon, 1986)

1044 GAUDIN, BIENEN AND CASSIDY


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provided in Fig. 2. The spudcan model is made fromaluminium and is 85.56 mm in diameter. It features

(a) a connection to a cylindrical hollow leg to which a loadcell is attached to measure the penetration and extractionload

(b) two pore pressure transducers, located 20 and 32 mmfrom the centre, to measure the pore pressure at thespudcan invert

(c) three sets of 12 jetting nozzles, each arranged onconcentric circles. These sets can be switched on andoff, allowing three different jetting configurations to bemodelled.

Details of the model geometry are presented in the mechani-cal sketch of Fig. 2, and a photo of the model is presentedin Fig. 3.

The prototype jetting nozzles for the spudcan tested are38 mm in diameter, which translates to 0.19 mm in a1/200th reduced scale model. However, it is not technicallypossible to manufacture nozzles with such a small diameterin the spudcan model, and consequently, nozzles 2 mm(400 mm prototype) and 0.5 mm (100 mm prototype) indiameter were used in the model. The diameter of thenozzles influences the jetting pressure applied for a givenflow rate (with a lower diameter resulting in a higherpressure). As demonstrated later in the paper, the jettingflow rate is dominant over the jetting pressure in thephenomenon of jetted extraction. Consequently, in order tomodel the prototype conditions correctly, the scaling factoron the jetting flow rate has to be determined accounting forthe incorrect scaling of the nozzle diameter.

Scaling considerations for jetting flow rateThe water jetting of spudcans during extraction is a

complex phenomenon, which depends on numerous vari-ables. An analogy can, however, be made with erosion ofsand by water jets (which has been studied to model scourin the vicinity of submerged foundations; Rajaratnam &Mazurek, 2003), to help identify the key variables involvedin the mechanism. It is reasonable to assume that the proper-ties of the soil and of the jetted fluid, and the dimensionsand kinematics of the spudcan, are involved in the mechan-ism of jetted extraction. Consequently, the variables identi-fied are

(a) for the spudcan, the extraction velocity v, the extractionforce Qt, the diameter of the spudcan D, the diameter ofthe jetting nozzle dn, the density rf and pressure pf of thejetted fluid, the flow rate Q, and the depth of the spudcan z

(b) for the soil, the strength of the soil su, the density of the

soil r, and the confining pressure around the spudcan,defined by the total mean stress �mean

(c) for both the soil and the spudcan, the gravity g.

This results in a total of 11 variables, which have only threedimensions: mass M, length L and time T. According toBuckingham’s (1914) theorem, there must be 12 � 3 ¼ 9dimensionless groups (or � groups) governing the system.The purpose of this analysis is not to determine all thesegroups (that would be beyond the scope of this paper), but to

OuterPPT

Top

14·00

510

4·31 10

5%

22·8

6

85·56Invert

6·01

Jettingnozzles

Jettingnozzles

InnerPPT

OuterPPT

Fig. 2. Spudcan model (dimensions in mm)

(a)

(b)

Spudcan legOutletguard

Jetting nozzle

Pore pressuretransducers

Fig. 3. (a) Details of jetting nozzles and outlet guards; (b) jettingin action

BOTTOM WATER JETTING TO EASE SPUDCAN EXTRACTION IN SOFT CLAY 1045


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identify the ones providing information about the scaling ofthe flow rate Q, in regard to the extraction of the spudcan.

Two such groups can be identified. They are �mean/rv2,and Qv=gd3

n, which characterises the potential of the jettedfluid to fill up the gap left behind the spudcan as it isextracted.

From the former it can be concluded that the scalingfactor on the extraction velocity is 1, which means that theundrained conditions achieved during the experiment aremodelling undrained conditions in the prototype (as thenormalised velocity vD/cv is n times higher in the proto-type). From the latter it is possible to determine the scalingfactor on the flow rate. Considering that the diameter ofthe model nozzle scaled as dn-m ¼ Ædn-p/n, where n is theacceleration level and Æ is the reduction factor betweenthe correctly scaled and the actual model nozzle diameters,the flow rate is expressed as

Qp ¼ n2

Æ3Qm (1)

where Qp is the prototype flow rate and Qm is the modelflow rate. The scaling factor on the flow rate is therefore n2/Æ3, and is used to determine the range of flow rate usedduring the experiments.

Although correct scaling to prototype conditions has beenfollowed in the experimental programme, all of the results inthis paper are presented in model units. Further details ofthe prototype conditions are provided in Bienen et al.(2009).

Jetting configurationsIn a first stage of the study, jetting nozzles of 2 mm

diameter were used. M2 screws were inserted in the nozzlesnot in use to block the flow, allowing the three jettingconfigurations to be investigated: an inner jetting configura-tion with only the nozzles in the inner circle open, anintermediate jetting configuration, with only the nozzles inthe middle circle open, and an outer jetting configuration,with only the nozzles in the outer circle open, as shown inFig. 2. On each nozzle, an outlet guard of half a pipe 4 mmin diameter and 4 mm long was fixed to redirect the jettingflow along the bottom face of the spudcan (Fig. 3(b)).Although not properly scaled, this device is believed to havethe same effect as the one used in the field.

In a second stage of the study, only the inner jettingconfiguration was investigated, and the nozzle diameter wasreduced to 0.5 mm. The use of two different diameters givesan insight into the influence of the nozzle diameter on thejetting efficiency, and hence allows for extrapolation of thecentrifuge tests results to prototype conditions.

No attempt was made to model the jack-up leg or the legbracing of the prototype spudcan. Instead, a cylindricalhollow leg 14 mm in external diameter and 10 mm ininternal diameter was used (Fig. 3(a)).

Soil preparation and characterisationCommercially available kaolin clay was used to reconsti-

tute the soil samples, with characteristics given in Table 1.The kaolin was mixed to a slurry with a water content of120% (twice the liquid limit). During mechanical mixing,the slurry was de-aired with a vacuum pump to ensure fullsaturation. The slurry was poured into a rectangular strong-box over a 15 mm deep sand drain. The consolidationprocess was achieved in the centrifuge under self-weight atan acceleration of 200g for a period of about 4 days. Theconsolidation process was monitored by settlement measure-ments of the sample, and periodic penetrometer tests.

Three samples were reconstituted in total. The total heightin sample 1 was about 220 mm, but the total height ofsamples 2 and 3 was limited to 160 mm owing to temporarytechnical constraints in the centrifuge. This resulted in dif-ferent spudcan penetrations between the samples.

A T-bar, 5 mm in diameter and 20 mm long, was used toderive continuous undrained shear strength profiles of thesample (Stewart & Randolph, 1991). A bearing capacityfactor of NT-bar of 10.5 was used, with typical profilespresented in Fig. 4 for the three samples tested. The threesamples show good consistency, with a shear strength ratiosu=� 9v of 0.195 (assuming ª9 ¼ 6 kN/m3). This is typical ofnormally consolidated kaolin in a centrifuge. T-bar testsperformed at different locations within the sample alsodemonstrated each of the samples to be homogeneous (re-sults not presented here).

Experimental programme and procedureAll tests were performed at 200g. A total of 13 tests are

presented in this paper. Three tests without jetting wereperformed in samples 1 and 2 to provide base referenceextraction resistance. Two were performed under undrainedconditions, with a normalised velocity V ¼ vD/cv

y exceeding

Table 1. Kaolin clay characteristics (after Stewart, 1992)

Property Value

Liquid limit, LL: % 61Plastic limit, PL: % 27Specific gravity, Gs 2.60Angle of internal friction, �9: degrees 238Void ratio at p9 ¼ 1 kPa on critical-state line, ecs 2.140Slope of normal consolidation line, º 0.205Slope of swelling line, k 0.044Spacing ratio, r 2.14Coefficient of consolidation, cv

(at OCR ¼ 1 and � 9v ¼ 130 kPa)1.33 3 10�7 m2/s

0

20

40

60

80

100

120

140

0 10 20 30 40 50

Undrained shear strength, : kPasu

Mod

el d

epth

,: m

mz m

Sample 1Sample 2Sample 3

su v/ 0·195σ� �

Fig. 4. Shear strength profiles in the three samples

† The diameter D was calculated as the equivalent diameter of a disc(noting the non-circular shape of the spudcan in plan view, Fig. 2)with an area equal to the projected area of the spudcan. Thecoefficient of consolidation cv at a stress level relevant to the spudcanembedment is provided in Table 1. v is the velocity of the spudcanpenetration/extraction.



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30 (Finnie & Randolph, 1994) and one under partiallydrained conditions, with a normalised velocity less than 1.Ten tests with jetting were performed, investigating the threedifferent jetting configurations and variable filling ratios f,defined as the ratio of the volume of the water jetted to thevolume of the void created by the extraction of the spudcan.This is a theoretical void, calculated as the area of thespudcan multiplied by the spudcan displacement. The testingprogramme is summarised in Table 2.

The experimental procedure followed three phases.

(a) In all tests, spudcan penetration was achieved at anundrained rate of 0.1 mm/s and to a depth z of 125 mmin sample 1 and 90 mm in samples 2 and 3 (correspond-ing to embedment ratios z/D of 1.46 and 1.05 respec-tively).

(b) An operational load, of about 90% of the load achievedafter full penetration, was applied for 1 h (4.56 yearsprototype). Although typical jack-up operations areusually less than three months, the load was maintainedfor a longer time in the centrifuge so that nearly fullconsolidation could be achieved, hence simplifying thesubsequent analysis and investigating jetting efficiency ina ‘worst-case scenario’. Measurements of the porepressure at the spudcan invert indicated that indeed about85% of consolidation was completed.

(c) The extraction was then performed under displacementcontrol, at rates ranging from less than 0.001 to0.1 mm/s,{ spanning from undrained to partially drainedbehaviour. In jetting tests, the flow was appliedsimultaneously with the extraction.

During the entire experiment, the data measured includedthe extraction/penetration load, the displacement of the spud-can, the pore pressure at two locations on the spudcan invert(Fig. 2), the displacement of the syringe pump piston (andhence the jetting rate), and the jetting pressure inside thejetting chamber.

EXPERIMENTAL RESULTSAll results are presented in model dimensions, unless

otherwise stated.

Non-jetted extractionFigures 5 and 6 present the net penetration and extraction

resistances (exclusive of the self-weight of the spudcan) andthe excess pore pressure and suction generated at thespudcan invert for tests S1-DEnJ and S1-UEnJ respectively.These were both performed without jetting, and are thereforeused as the base reference cases.

Spudcan penetration/installation. Both spudcans were jackedto a depth of about 125 mm, corresponding to an embedmentratio of 1.46. The maximum penetration resistance reachedabout 1.9 kN (or 400 kPa, which is typical of modern-dayjack-up; Osborne et al., 2006), resulting in a bearing capacityfactor Nc of about 10.5, similar to values observed for deepmechanism spudcan installation (Randolph et al., 2004).Immediate back-flow on the top of the spudcan was observedvisually during testing, and it is expected that the deep failuremechanism, characterised by a symmetrical flow-aroundmechanism, occurred at an embedment ratio of about 0.7,as described by Hossain et al. (2005).

Table 2. Testing programme

Sample Test name Jettingconfiguration

Nozzlediameter: mm

Extraction rate,v: mm/s

Normalisedextraction rate, V

Jetting flowrate, j: mm3/s

Filling ratio,f

1 S1-UEnJ No jetting N/A 0.1 60.8 N/A N/AS1-DEnJ No jetting N/A 0.001 0.61 N/A N/AS1-UEJ1-1 Inner 2 0.1 60.8 392 0.769S1-UEJ2-1 Intermediate 2 0.08 48.6 314 0.769S1-UEJ3-1 Outer 2 0.08 48.6 334 0.817

2 S2-UEnJ No jetting N/A 0.1 60.8 N/A N/AS2-UEJ1-3 Inner 0.5 0.085 51.8 22 0.051S2-UEJ1-4 Inner 0.5 0.076 46.3 89 0.229S2-UEJ1-5 Inner 0.5 0.057 34.4 223 0.772

3 S3-UEJ1-6 Inner 0.5 0.084 51.3 149 0.346S3-UEJ1-7 Inner 0.5 0.080 48.8 237 0.577S3-UEJ1-8 Inner 0.5 0.096 58.2 285 0.583S3-UEJ1-9 Inner 0.5 0.099 60 209 0.416

Test nomenclature: U, undrained; D, partially drained; nJ, no jetting; J, jetting; E, extraction.

‡ Note that the displacement rates used during the centrifuge tests were not intended to model the prototype displacement rates, but the drainageconditions that may be encountered in the field.

0

20

40

60

80

100

120

140

�3000 �2000 �1000 0 1000 2000 3000Net load, : NQt

Mod

el d

epth

,: m

mz m

Extraction at 0·001 mm/s

Extraction at0·1 mm/s

Extraction at0·009 mm/s

Weight of clayaccumulated on top

of spudcan

Operation load

Spudcanbreakaway

Installation

ExtractionS1-DEnJPartiallydrained

S1-UEnJUndrained

Fig. 5. Penetration and extraction resistance for non-jetted tests



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In both tests S1-UEnJ and S1-DEnJ, excess pore pressuresincrease linearly with depth, and no significant differencebetween the measurements of the two transducers is ob-served. Integrating the pore pressure measurements over thearea of the spudcan results in a total bearing resistance of2.2 kN. Evidently, the soil back-flow contributes to an in-crease of the penetration load by 0.3 kN. Preliminary assess-ment of the weight of the soil at the top of the spudcanyields a value of about 0.5 kN (accounting for the fact thatback-flow does not fill the entire cavity left behind), butmeasurements reported by Purwana et al. (2005) in similarconditions showed that the excess pore pressure increasestowards the centre of the spudcan, and consequently thecontribution of the excess pore pressure in the present caseis likely to be underestimated, owing to the location of thetransducer.

Nevertheless, the assessment of the respective contributionof the soil at the top of the spudcan and the spudcan invertindicates that shearing at the spudcan invert is under verylow effective stresses (no friction) and hence undrainedconditions. As already observed by Purwana et al. (2005),back-flow is accompanied by limited excess pore pressure atthe top of the spudcan, likely to be the remains of the excesspore pressure generated at the spudcan invert as the materialflows from the bottom to the top of the spudcan.

Operational period. At the end of the penetration, anoperational load was maintained constant at a value of about90% of the maximum penetration resistance for 4.56 years(prototype), leading to a degree of consolidation of about85% for both tests. This consolidation resulted in additionalembedment of the spudcan of about 2 mm (0.025D).Although this process is a simplification of the loadingconditions of a spudcan in the field, it is intended to replicatethe gain in strength of the soil due to consolidation.

Undrained spudcan extraction. In test S1-UEnJ, the extrac-tion was performed at a constant rate of 0.1 mm/s(corresponding to a normalised velocity of 60.8, and ensuringfully undrained conditions). Results show a maximumextraction resistance of about 2.1 kN after 5.6 mm of

extraction (6.5% of the spudcan diameter). This is about8% higher than the penetration resistance due to the increasein effective stresses and resulting gain in strength of the soilresulting from the consolidation. The increase in extractionresistance in comparison with the penetration resistance issimilar to that observed by Purwana et al. (2005) for similarconditions. However, it appears to be limited when consid-ering the expected increase of undrained shear strength of thesoil due to consolidation. For similar levels of operation loadand consolidation, Gaudin et al. (2007) reported, for skirtedfoundations, an increase of penetration resistance of up to70%. Although a different response would be expectedbetween pushing through consolidated material and pullingaway from consolidated material, if a symmetrical flow-around took place in both extraction and penetration (as for aT-bar test), it would be expected that the consolidated soilwould account for a higher contribution in the extractionresistance. It is therefore likely that the large difference inextraction and penetration resistance after consolidation isdue to a change in failure mechanism.

Insights into the extraction mechanism are provided byexamination of Figs 5 and 6. During extraction, the suctiondeveloped a peak at �282 kPa before dropping significantlyto an average value of �20 kPa after 8 mm (0.1D). (Notethat, in contrast to the penetration, the inner and outer porepressure transducers (PPTs) show different values of nega-tive pore pressure, with more suction developed towards thecentre of the spudcan, which is consistent with observationsmade by Purwana et al., 2005.) A similar drop is observedin the load–displacement curve in Fig. 5. In parallel, themaximum suction generated accounts for about 70% of themaximum extraction resistance, highlighting the contributionof the soil on the top of the spudcan, in contrast to thepenetration process, and hence the difference in failuremechanism. As already observed by Purwana et al. (2005),although no suction was developed at the top of the spudcanduring penetration (the penetration resistance being entirelyprovided by the excess pore pressure at the spudcan invert),both negative pore pressures at the invert and positive porepressures at the top contribute to the extraction resistance.

During penetration, the failure mechanism is a flow-around mechanism (beyond an embedment ratio of 0.7), asdescribed by Hossain et al. (2005). During extraction, differ-ent mechanisms take place. In the first stage of extraction,the mechanism appears to be a combination of reverse endbearing at the spudcan invert (resulting in a high level ofsuction) and an uplift mechanism of the soil above thespudcan. This has been identified by both particle imagevelocimetry (PIV) analysis (Purwana et al., 2006) andnumerical analysis (Zhou et al., 2009) of spudcan extractionin normally consolidated clay. This is probably explained bythe fact that a symmetrical flow-round mechanism with shearplanes going through the overconsolidated soil at the spud-can invert would require more work (due to the strengthincrease) than a reverse end bearing mechanism accompa-nied by an uplift mechanism on the top of the spudcan.

Once the work required to lift the soil below the spudcanexceeds that required for a deep flow mechanism, themechanism changes to a localised flow around the edge ofthe spudcan, resulting in a sudden drop in the suction devel-oped. This localised flow-around mechanism is still accom-panied by an uplift mechanism of the soil on top of thespudcan. This mechanism has also been identified by Zhouet al. (2009), although it is not observed by Purwana et al.(2006), most likely because insufficient sealing at the spud-can/window interface resulted in a breakaway mechanism.After about 25 mm of extraction (0.31D), the extractionresistance decreases regularly with depth, while the suctionpressure remains relatively constant. This decrease is a result

0

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140

�400 �300 �200 �100 0 100 200 300 400 500

Excess/negative pore pressure, : kPapM

odel

dep

th,

: mm

z m

Inner PPTOuter PPT

S1-UEnJUndrained

S1-DEnJPartially drained

Fig. 6. Excess pore pressure and suction developed at spudcaninvert during penetration and extraction of non-jetted tests



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of a reduction in the weight of soil above as the spudcanmoves towards the surface.

Note that results of test S2-UEnJ (not presented here)showed the same behaviour despite a lower final embedment(90 mm instead of 120 mm; z/D of 1.05 instead of 1.46),indicating that the same mechanisms apply.

Partially drained spudcan extraction. In test S1-DEnJ,extraction was initially performed at 0.001 mm/s (normalisedvelocity vD/cv of 0.6), before being accelerated once themaximum suction and extraction resistance were overcome.Such a normalised velocity results in a partially drainedextraction, as demonstrated by the low level of suctiondeveloped at the spudcan invert, in contrast to the undrainedextraction (Fig. 6). Although the suction developed in theearly stage of the extraction reached �65 kPa (24% of theundrained extraction), the maximum extraction resistance isstill as high as 1.6 kN (75% of the undrained extraction, withsuction contributing 20% to the total extraction resistance).Evidently, some of the excess pore pressure at the top of thespudcan generated during the undrained extraction dissipatesduring the partially drained extraction, resulting in anincrease in the effective stresses at the top of the spudcan,and hence a higher contribution of the material above thespudcan to the extraction resistance. This has particularrelevance to prototype conditions when spudcans cannot beextracted, and sustained tensile leg loads possibly consolidatethe soil above the spudcan, further exacerbating efforts toextract the spudcans (as will be discussed further in thispaper). The limited suction developed at the spudcan invertand the softer post-peak decrease in extraction resistanceseem to indicate either the absence of the reverse end bearingmechanism or a faster and smoother transition between thisand a localised flow-around mechanism. These assumptionsneed to be validated, however, through a PIV analysis ofpartially drained spudcan extraction.

The difference in extraction resistance between the un-drained and partially drained cases is further highlighted bythe higher post-peak extraction resistance (at shallowerdepth) and the increased weight of material at the top of thespudcan, demonstrated both by the load reading once thespudcan is fully extracted and by visual observations aftertesting, which showed a larger amount of soil resting on topof the spudcan. This results from a wider wedge of soilinvolved in the uplift failure mechanism during the partiallydrained extraction. As the soil on top of the spudcanconsolidates, the failure plane tends to form at a wider anglethrough weaker soil. Das & Singh (1994) have reportedsimilar differences in uplift mechanism of shallow anchorsbetween soft and stiff clay, and have identified similarmechanisms.

Potential of jetting extraction. These results demonstrate thatthe suction at the spudcan invert contributes significantly tothe undrained extraction resistance. The transition from anundrained to a partially drained extraction results in areduction of the suction developed, as time is given for thepressure to dissipate. However, this is compensated for by anincrease of the contribution to the extraction resistance of thematerial on top of the spudcan. This increased contributioncomes from (a) an increase of the friction along the failureplane due to the increase in effective stresses, and (b) theassociated change in the shape of the uplift failuremechanism (from a vertical column to a wedge) resultingin a larger weight of soil. (Fig. 7 summarises the differentmechanisms believed to take place during undrained deeppenetration and undrained as well as partially drained

extraction.) These mechanisms are indeed simplified, and adetailed PIV analysis of both undrained and partially drainedextractions would provide a more thorough understanding ofthe mechanisms.

It is therefore inferred that the minimum possible extrac-tion resistance corresponds to a vented extraction, where nosuction is developed at the spudcan invert, and the contribu-tion of the soil above the spudcan is minimised withundrained behaviour and vertical failure planes. As thesuction measured in test S1-UEnJ contributes 70% of thetotal extraction resistance, the optimum benefit that can beexpected from jetting is a reduction of about 70% of theextraction resistance (if no suction is generated).

Undrained jetted extractionsThe tests labelled UEJ were all performed under displace-

ment control, at similar undrained extraction rates and con-stant jetting flow rates (see Table 2). This allows directcomparison with the non-jetted tests. Fig. 8 presents typicalextraction resistances for two jetted tests and their equivalentnon-jetted tests, and Fig. 9 shows the suction developed atthe spudcan invert (for clarity, only the inner measurementsare shown). The two jetting tests presented featured thelowest and one of the highest filling ratios investigated forthe inner jetting configuration (0.051 and 0.769 for tests S2-UEJ1-3 and S1-UEJ1-1 respectively).

High filling ratio. It is evident from Fig. 8 that a high fillingratio results in a significant reduction of extraction resistance,whereas a very low filling ratio has negligible effect. Toallow comparison between tests an extraction resistance ratiois defined as the ratio of the maximum jetted extractionresistance Qtj to the maximum non-jetted extraction resis-tance Qtu. For test S1-UEJ1-1 of Fig. 8 the ratio is 0.26 (or inother words a reduction in extraction resistance of 74%). Thepeak extraction resistance occurred after a displacement of7 mm (0.086D). This is a softer response than observedduring the non-jetted test, and may indicate that a differentmechanism takes place. Indeed, examination of the porepressures developed at the spudcan invert in Fig. 9 demon-strates that, after an immediate development of suction (dueto some delay between the beginning of the extraction andthe application of jetting}), the jetting creates a gap at thespudcan invert, resulting in a sharp reduction in suction. Inthis particular case, the jetting flow rate and correspondingfilling ratio actually results in positive pore pressures appliedat the spudcan invert, hence contributing actively to thereduction in extraction resistance. As the extraction pro-gresses, the mechanism reaches an equilibrium, where theexcess pore pressure at the spudcan invert decreases slowlywhile the extraction load decreases more sharply. At about60 mm depth (0.74D), where the level of excess pore pressureis approximately the same as the effective vertical stress inthe soil, some piping occurred, resulting in boiling at the soilsurface (observed visually during the test) and in a reductionin the excess pore pressure at the spudcan invert and hencean increase in the extraction resistance. Finally, at 30 mmdepth (0.37D), the spudcan broke away, leading to theexpected result of no excess pore pressure at the spudcaninvert, and an increase in the extraction load (as the positivepore pressure was no longer contributing to the extraction) to

§ Although extraction and jetting were initiated simultaneously, asmall delay of about 0.5 s was observed between the displacement ofthe piston and the application of the jetting at the spudcan invert.



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a value equal to the weight of the soil accumulated at the topof the spudcan.

Low filling ratio. Results of test S2-UEJ1-3, performed at theminimum jetting flow rate of 22 mm3/s and a filling ratio of0.051, show a different pattern. During the first stage ofextraction, the reduction in extraction resistance (Fig. 8)appears to be insignificant. This is because the high level ofsuction developed at the spudcan invert is of similarmagnitude to that developed during the non-jetted test, asshown in Fig. 9. The maximum extraction resistance andmaximum suction occur at similar spudcan displacements, aswas the case in the non-jetted test, and it is reasonable toassume that at that stage the small amount of jetting does notaffect the mechanisms taking place. Figs 8 and 9 highlightsome difference in the post-peak behaviour, once the

localised flow mechanism replaces the reverse end bearingmechanism, with limited excess pore pressures developed atthe spudcan invert.

Effect of filling ratio on jetting performance. The totality ofthe experimental results is summarised in Table 3, whichpresents for each test

(a) the non-dimensional extraction velocity(b) the filling ratio(c) the extraction resistance ratio (Qtj/Qtu)(d ) the reduction of suction at the spudcan invert,}

characterised by the ratio of the maximum (jetted)

Deep penetration

Qt

Qt

Qt

Qt

Undrained extractionEarly stage

Undrained extractionPost peak

Partially drained extractionHypothesised mechanism

Fig. 7. Simplified failure mechanisms during deep undrained penetration and undrained and partially undrained extraction

¶ Note that a negative number indicates the development of excesspore pressure at the spudcan invert.



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suction at the spudcan invert, psj, to the maximum non-jetted suction, psu

(e) the ratio of the suction at the spudcan invert, ps, to thetotal extraction resistance qt (defined as Qt/A).

In order to quantify the performance of the jetting, the firsttwo ratios are plotted against the filling ratio in Figs 10 and11 respectively.

It is evident from Fig. 10 that an increase in the filling

0

20

40

60

80

100

120

140

�2500 �2000 �1500 �1000 �500 0

Net load, : NQt

Mod

el d

epth

,: m

mz m

Spudcan breakaway

Piping

Extraction stoppedbriefly owing totechnical difficulties

S1-UEnJ

S2-UEnJ

S1-UEJ1-1

S2-UEJ1 3-

Fig. 8. Comparison of extraction resistance developed betweennon-jetted and jetted tests

0

20

40

60

80

100

120

140

�400 �300 �200 �100 0 100 200 300

Excess/negative pore pressure, : kPap

Mod

el d

epth

,: m

mz m

Spudcan breakaway

Equilibirumreached

Piping

Extractionstoppedbriefly owing totechnical difficulties

S2-UEnJ

S1-UEJ1 1-

S2-UEJ1 3-

Fig. 9. Comparison of development of pore pressure at spudcaninvert between non-jetted and jetted tests

Table 3. Summary of testing results

Sample Test name Normalised extractionrate, V

Filling ratio,f: %

Extraction resistanceratio, Qt/Qtu

Base suction ratio,psj/psu

Contribution of base suction tototal extraction resistance, ps/qt

1 S1-UEnJ 60.8 N/A 1 1 0.67S1-DEnJ 0.61 N/A 1 1 0.22S1-UEJ1-1 60.8 0.769 0.26 �0.22 �0.14S1-UEJ2-1 48.6 0.769 0.40 �0.14 �0.10S1-UEJ3-1 48.6 0.817 0.37 �0.17 �0.12

2 S2-UEnJ 60.8 N/A 1 1 0.85S2-UEJ1-3 51.8 0.051 0.99 0.95 0.84S2-UEJ1-4 46.3 0.229 0.71 0.57 0.41S2-UEJ1-5 34.4 0.772 0.36 �0.05 �0.06

3 S3-UEJ1-6 51.3 0.346 0.62 0.25 0.22S3-UEJ1–7 48.8 0.577 0.60 0.34 0.28S3-UEJ1-8 58.2 0.583 0.54 0.30 0.24S3-UEJ1-9 60 0.416 0.68 0.44 0.43

0

0·20

0·40

0·60

0·80

1·00

1·20

0 0·1 0·2 0·3 0·4 0·5 0·6 0·7 0·8 0·9 1·0

Filling ratio, f

Ext

ract

ion

resi

stan

ce r

atio

,/

QQ

tjtu

Sample 1Sample 2Sample 3

Fig. 10. Jetting extraction performance in terms of extractionresistance reduction

�0·4

�0·2

0

0·2

0·4

0·6

0·8

1·0

1·2

0 0·1 0·2 0·3 0·4 0·5 0·6 0·7 0·8 0·9 1·0Filling ratio, f

Suc

tion

ratio

,/

pp

sjsu

Sample 1

Sample 2

Sample 3

Best fit

Fig. 11. Jetting extraction performance in terms of suction atspudcan invert reduction



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ratio reduces the extraction resistance, with a linear fitreasonably representing the data. This is consistent with Fig.11, which shows a linear decrease of the suction developedat the spudcan invert with the filling ratio. From a best-fitline through the experimental data, a filling ratio of 0.7results in no suction developed at the spudcan invert (i.e. noexcess pore pressure).

It appears, therefore, that the optimum jetting performancedoes not correspond to a purely vented extraction (fillingratio of 1), but to a mechanism where localised flow-aroundat the spudcan edge still takes place, in addition to the upliftmechanism of the soil above the spudcan. This mechanismis similar to the one developed during post-peak non-jettedextraction, but with a gap at the spudcan invert, whichcovers about 0.7 times of the total spudcan–soil contactarea. This mechanism is in equilibrium as long as the jettingflow creates and maintains this gap. For higher filling ratiosthe same mechanism takes place, but with generation ofexcess pore pressure at the spudcan invert to a maximumlevel corresponding to the effective vertical stress in the soilat which point hydraulic fracturing occurs. For lower fillingratios, where limited suction still develops, it can be as-sumed that the localised flow extends further towards thecentre of the spudcan, similarly to the post-peak non-jettedundrained extraction mechanism.

Bearing capacity factors during jetting extraction. Tovalidate this assumption, bearing capacity factors werecalculated and compared with upper-bound calculations.

Bearing capacity factors, Nc, were back-calculated for allof the jetting tests, as the net extraction resistance Qt dividedby the area of the spudcan A and by the shear strength ofthe soil su at that embedment depth (and accounting for theincrease in shear strength resulting from the consolidationunder operating load). These are shown in Fig. 12. Adecrease in bearing capacity factor is observed from thetypical undrained extraction value of 10.8 to an averagevalue of 5 beyond a filling ratio of 0.6.

In parallel, an upper-bound calculation based on bearingcapacity solutions from Kusakabe et al. (1986) was per-formed, assuming a failure mechanism involving localisedflow at the spudcan edges, in combination with an upliftmechanism of the soil on top of the spudcan. The extent ofthe localised flow is such that the area of the spudcan invertnot involved in the process is 0.7 of the total area of thespudcan. This relates to the experimental observation thatthe suction is negated at the spudcan invert at a filling ratioof 0.7. The mechanism assumed in the upper-bound calcula-tion is presented in Fig. 13, and yields a bearing capacity

factor of 4.7 for the conditions of the experiments. This isin good agreement with the experimental results, and con-firms the assumptions made for the mechanism developedduring jetted undrained extraction.

Conceptual framework describing jetted extraction. In orderto describe in a simple manner the effect of the jetting on theextraction resistance, the experimental results have beengathered into a conceptual framework, which is presented inFig. 14, which presents the ratio between the direct loadapplied on the spudcan, Qt, and the ultimate undrainedunjetted extraction resistance Qult as a function of the fillingratio f. Qult can be estimated from bearing capacity theoryusing an Nc factor that depends on the length of theoperational period and the load magnitude applied. Fromunjetted extraction tests presented here, Fig. 12 yields a valueof Nc � 10.8.

A filling ratio of zero represents no jetting, and theresistance to overcome corresponds to the undrained extrac-tion resistance (Qt/Qult ¼ 1). For a filling ratio of 0.7 noexcess pore pressures were measured, and the breakawayextraction resistance factor of 4.7 can be applied, whichresults in a ratio Qt/Qult of 0.43. The line of successfulundrained jetted extraction is then defined as the line passingthrough these two points. The experimental results represent-ing successful jetted undrained extraction distribute evenlyalong this line, which validates the conceptual framework.Fig. 14 also presents the path followed during one of thejetted tests, which is performed under displacement control.The measured extraction load increased in the tests (whilethe filling ratio remained constant) until the peak extractionload Qt was measured. For all the other tests only the peakextraction load ratio is shown.

The line of successful undrained jetted extraction definesthe required extraction load as a function of the unjettedundrained resistance. It is relevant only if the rate ofextraction represents undrained conditions. As discussedabove, the benefit of lowering the negative pore pressures atthe spudcan invert is negated in a slower, partially drainedextraction owing to (partial) consolidation of the soil abovethe spudcan. This results in both an increase in shearstrength and a change in failure mechanism (see Fig. 7).Consequently, the extraction resistance increases.

The relevance and efficiency of this conceptual frameworkare discussed further in Bienen et al. (2009). Its applicationto offshore prototype conditions is also covered.

Influence of jetting pressure, jetting flow rate and jettingconfiguration

The influence of the jetting pressure on the jetting per-formance can be investigated by comparing tests S1-UEJ1-1

0

2·0

4·0

6·0

8·0

10·0

12·0

14·0

0 0·1 0·2 0·3 0·4 0·5 0·6 0·7 0·8 0·9 1·0 1·1Filling ratio, f

Bea

ring

capa

city

fact

or, N

c

Sample 1

Sample 2

Sample 3

Fig. 12. Net bearing capacity factors for all jetting tests

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16 18 20Radius, : mr

Dep

th,

: mz

Spudcan Failure mechanism

Fig. 13. Upper-bound failure mechanism for a filling ratio of 0.7



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and S2-UEJ1-5, which feature the same jetting configura-tions (nozzles in the inner circle) and similar filling ratios(0.769 and 0.772 respectively), but different nozzle dia-meters and hence different jetting pressures. Although thejetting pressure is not measured, from Bernoulli’s equation areduction of the nozzle diameter by a factor of 4 (asbetween sample 1 and subsequent samples) will result in anincrease of the jetting pressure by 4. The difference inextraction resistance reduction and suction reduction appearsto be slightly lower for the higher jetting pressure (test S2-UEJ1-5). This may be explained by hydraulic fracturingpotentially occurring under high jetting pressure. This wouldreduce the jetting performance, as the volume under thespudcan is no longer filled.

In contrast, reduction of the flow rate by one order ofmagnitude, from test S2-UEJ1-5 to test S2-UEJ1-3, featuringthe same nozzle diameter and the same jetting configura-tions, results in a significant reduction of the jetting effi-ciency, with the ratio of jetted extraction resistance to non-jetted extraction resistance increasing from 0.36 to 0.99. Itthen appears clear that the jetting flow rate is a dominantparameter over the jetting pressure, although it is likely thata minimum pressure is required at the beginning of theprocess to break the suction. Further tests performed toinvestigate the influence of jetting pressure reported byGaudin et al. (2006) actually demonstrated that under highjetting pressure, once the excess pore pressure at the spud-can invert exceeds the effective vertical stress in the soil,hydraulic fracturing and subsequent piping occur, negatingthe benefit of jetting in the reduction of extraction resis-tance.

Examination of the results of tests S1-UEJ1-1, S1-UEJ2-1and S1-UEJ3-1 in Table 3, featuring the same jetting flowrate and the same nozzle diameter, give insight into theinfluence of the jetting configuration on the jetting perform-ance. The inner jetting configuration (subsequently kept fortests in samples 2 and 3) seems to provide the best perform-ance, with an extraction resistance reduction ratio of 0.26. Itmay be assumed that in such a configuration the jetting ismore effective, as the jetted water moves from the centre ofthe spudcan to the edge, hence involving a larger part of thespudcan invert in the phenomenon, but surprisingly, the outer

configuration provides better results than the intermediateconfiguration. It should also be noted that for all threejetting configurations the inner and outer pore pressuretransducers measured the same level of pore pressure, indi-cating that the jetting was spread evenly at the spudcaninvert, regardless of the position of the nozzles. Thus resultsappear inconclusive in regard to the jetting configuration,and further work is necessary, both on the position of thenozzles and on the shape and orientation of the outlet guardsto maximise the performance of the jetting system for anoptimum filling ratio.

CONCLUSIONSThis paper reports a programme of centrifuge experiments

initiated to understand the causes of the limited efficiencyobserved in situ of water jetting methods used to ease theextraction of jack-up spudcans. Tests were performed on a1:200 scale spudcan model, and investigated parametersincluding the jetting flow rate, jetting pressure and extractionrate.

Extraction tests using water jetting at the spudcan invertindicated that jetting is efficient only if the spudcan exhibitsan extraction rate that results in undrained behaviour andhence a lesser contribution of the material at the top of thespudcan to the total extraction resistance. During partiallydrained extraction, the reduction of suction at the spudcaninvert due to jetting is balanced by an increase in effectivestresses at the top of the spudcan due to consolidation. Ifundrained conditions are achieved, sufficient jetting pressureis required to be applied at the beginning of the process tobreak the suction (although the jetting pressure has to bemaintained below the effective stresses of the soil to ensurethat no hydraulic fracturing occurs). Thereafter, a jettingflow rate corresponding to a filling ratio of 0.7 is required tonegate the development of suction at the spudcan invert, andhence to reduce its contribution to the extraction resistanceto a minimum.

In partially drained conditions jetting becomes inefficient,even at high filling ratios, owing to the increasing extractionresistance resulting from the consolidation of the soil on topof the spudcan.

Undrained unjetted extraction resistance

Negative pore pressuresdeveloped at spudcan invert

Line of successful undrained jetting extraction

Area of unsuccessful jettingExtraction (below line)

Breakaway extraction resistance

Path of displacementcontrol centrifuge tests

Positive pore pressuresdeveloped at

spudcan invertE

xtra

ctio

n re

sist

ance

ra

tio,

/Q

Qdi

rect

ult

1·00

0·90

0·80

0·70

0·60

0·50

0·40

0·30

0·20

0·10

00 0·1 0·2 0·3 0·4 0·5 0·6 0·7 0·8 0·9 1·0

Filling ratio, f

Fig. 14. Conceptual framework describing jetted extraction



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These conclusions are limited to the experiments under-taken, but are believed to provide relevant insight into themechanisms taking place in situ. Further studies are neces-sary, notably focusing on the size, orientation and locationof the jetting nozzles, and modelling more accurately theextraction process in the field, which is performed underload control rather than displacement control.

ACKNOWLEDGEMENTSThis project has been partially funded by Keppel Offshore

Technology Development. Their contribution and permissionto report these results are gratefully acknowledged, alongwith the fruitful discussions with Dr Okky A. Purwana. Theauthors acknowledge the contribution of Don Herley, beamcentrifuge operator, who assisted with the centrifuge experi-ments, and Tuarn Brown, who assisted in the developmentof the experimental device. The authors would like to thankProfessor Mark Randolph for his assistance on the bearingcapacity calculation, and Professor Martin Fahey for hisfruitful ideas and discussions during the COFS workshop.

NOTATIONA contact area of spudcancv coefficient of consolidationD equivalent diameter of spudcan (m for model, p for

prototype)ecs void ratio at p9 ¼ 1 kPa on critical-state line

f filling ratioGs specific gravityg gravity accelerationH height of the strongboxL length of strongbox

LL liquid limitNc bearing capacity factor

NT-bar bearing factor of T-barPL plastic limit

p pressurep9 mean stressps suction at spudcan invertpsj suction at spudcan invert for jetted testspsu suction at spudcan invert for non-jetted testsQ jetting flow rate (m for model, p for prototype)Qt total extraction resistanceQtj total extraction resistance for jetted testsQtu total extraction resistance for non-jetted testsQult ultimate extraction resistance for non-jetted tests

qt total extraction resistancer spacing ratio

su undrained shear strengthV normalised penetration velocityv spudcan penetration or extraction velocity

W width of strongboxz depth

zm model depthÆ reduction factor on diameter of jetting nozzleª9 effective unit weightk slope of swelling lineº slope of normal consolidation liner density of the soil

� 9v effective vertical stress�9 angle of internal friction

REFERENCESBienen, B., Gaudin, C. & Cassidy, M. J. (2009). The influence of

pull-out load on the efficiency of jetting during spudcan extrac-

tion. Appl. Ocean Res. 31, No. 3, 201–211.Buckingham, E. (1914). On physically similar systems: illustrations

of the use of dimensional analysis. Phys. Rev. 4, 345–376.Das, B. M. & Singh, G. (1994). Uplift capacity of plate anchors in

clay. Proc. 4th Int. Conf. on Offshore and Polar Engineering,Osaka, 436–442.

Finnie, I. M. S. & Randolph, M. F. (1994). Punch-through andliquefaction induced failure of shallow foundations on calcar-eous sediments. Proc. 7th Int. Conf. on Behaviour of OffshoreStructures, Boston, 217–230.

Gaudin, C., Brown, T. A., Hodder, M. & Cassidy, M. J. (2006).Centrifuge experiments investigating the use of jetting in spud-can extraction, GEO 06384. Perth: University of WesternAustralia.

Gaudin, C., Landon, M. M. & Cassidy, M. J. (2007). Feasibilityand performance of a hybrid foundation system—Stage 2. Reportto Keppel Offshore and Marine Technology Centre Pte Ltd,GEO 07427. Perth: University of Western Australia.

House, A. R. (2002). Suction caisson foundations for buoyantoffshore foundations. PhD thesis, University of Western Austra-lia.

Hossain, S. M., Hu, Y., Randolph, M. F. & White, D. J. (2005).Limiting cavity depth for spudcan foundations penetrating clay.Geotechnique 55, No. 9, 679–690, doi: 10.1680/geot.2005.55.9.679.

Kusakabe, O., Susuki, H. & Nakase, H. (1986). An upper boundcalculation on bearing capacity of a circular footing on a non-homogeneous clay. Soils Found. 26, No. 3, 143–148.

Lin, S. S. (1987). A universal footing with jetting. Proceedings ofthe offshore technology conference, Houston, TX, 299–306.

Lin, S. S (1995). Effect of jetting on footing penetration and pullout. Proc. 15th Int. Conf. on Offshore and Polar Engineering,The Hague, Netherlands, 426–429.

Osborne, J. J., Nelson, C. & Hunt, R. (2006). Unpredicted jack-upfoundation performance. Proc. 1st Jack-up Asia Conference andExhibition, Singapore.

Purwana, O. A., Leung, C. F., Chow, Y. K. & Foo, K. S. (2005).Influence of base suction on extraction of jack-up spudcans.Geotechnique 55, No. 10, 741–753, doi: 10.1680/geot.2005.55.10.741.

Purwana, O. A., Leung, C. F., Chow, Y. K. & Foo, K. S. (2006).Breakout failure mechanism of jackup spudcan extraction. Proc.6th Int. Conf. on Physical Modelling in Geotechnics, HongKong, 667–672.

Purwana, O. A., Quah, M., Foo, K. S., Nowak, S. & Handidjaja, P.(2009). Leg extraction/pullout resistance: theoretical and practi-cal perspectives. Proc. 12th Int. Conf. on The Jack-Up Platform:Design, Design, Construction and Operation, London.

Rajaratnam, N. & Mazurek, K. A. (2003). Erosion of sand bycircular impinging water jets with small tailwater. ASCE J.Hydraulic Engng 129, No. 3, 225–229.

Randolph, M. F. (2004). Characterisation of soft sediments foroffshore applications. Keynote lecture. Proc. 2nd Int. Conf. onSite Characterisation, Porto 1, 209–231.

Randolph, M. F., Jewell, R. J., Stone, K. J. L. & Brown, T. A.(1991). Establishing a new centrifuge facility. Proc. Int. Conf.on Centrifuge Modelling, Centrifuge 91, Boulder, CO, 3–9

Randolph, M. F., Jamiolkowski, M. B. & Zdravkovic, L. (2004).Load carrying capacity of foundations. In Advances in geo-technical engineering (eds R. J. Jardine, D. M. Potts and K. G.Higgins), Vol. 1, pp. 207–240. London: Thomas Telford.

Reardon, M. J. (1986). Review of the geotechnical aspectcs of jack-up unit operations. Ground Engng 19, No. 7, 21–26.

Stewart, D. P. (1992). Lateral loading of piled bridge abutments dueto embankment construction. PhD thesis, University of WesternAustralia.

Stewart, D. P. & Randolph, M. F. (1991). T-bar penetration testingin soft clay. J. Geotech. Engng ASCE 120, No. 12, 2230–2236.

Zhou, X. X., Chow, Y. K. & Lueng, C. F. (2009). Numericalmodelling of extraction of spudcans. Geotechnique 59, No. 1,29–39, doi: 10.1680/geot.2007.00008.


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