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This study focuses on easing or eliminating potential punch-through in two-layer stiff-over-soft clay deposits by tweaking spudcan shapes. The effectiveness of skirted spudcans with and without holes, relative to a generic spudcan used conventionally, is tested. Three-dimensional large deformation finite element (LDFE) analyses are carried out systematically; encompassing a parametric study on the length of the skirt on the bottom periphery, number of holes through the spudcan base, and the slope of the spudcan base. Numerical results lead to selection of two skirted spudcans with and without holes, and both with a skirt length of ∼0.23 spudcan diameter. The performance of those two spudcans is then tested through a series of centrifuge model tests; varying the strength ratio between the two layers, and the thickness of the stiff layer relative to the spudcan diameter. Overall, the results of eighteen penetration tests suggest that the skirted spudcan without holes is more effective in easing or even eliminating the punch-through that is associated with the penetration of the generic spudcan. For using the skirted spudcan without holes in the field, an improved analytical model is proposed for assessing penetration resistance profile or embedment depth under a given preload. • US$5–50 million/incident costing jack-up rig punch-through failure. • Easing or mitigation of punch-through failure. • Stratified stiff-over-soft clay deposits. • Tweaking spudcan shape. • Combined centrifuge modelling and large deformation finite element analyses. [ABSTRACT FROM AUTHOR]

A cryogenic pipeline is a pipe-in-pipe with larger outer diameter, and is heavier and stiffer, compared to conventional deepwater steel flowlines or export pipelines. Cryogenic pipelines are being considered to be placed on shallow water sandy seabeds for transporting liquefied natural gas to shore. This paper reports a series of centrifuge tests to assess the as-laid embedment of a cryogenic pipeline during the dynamic laying process, and the uplift capacity of a trench-laid pipeline. Two seabed conditions were tested – a pipe laid onto the base of a trench and a flat silica sand deposit. A total of 12 dynamic pipe laying tests were conducted varying vertical force and pipe oscillation amplitude, frequency and number of cycles; encompassing typical laying processes. The embedment depth was found to be less than 0.5 diameters, owing to the resultant effect of lateral ploughing and sand densification. For the trench-laid cases, after the completion of simulating the pipe laying process, the trench was filled with a layer of sand and a layer of rock armour; varying the thickness of the layers. The pipe was then extracted vertically to evaluate the uplift resistance, which was shown to increase with increasing the thickness of the rock layer. • Laying of cryogenic pipelines on sandy seabed. • Surface and trench laying. • Dynamic embedment during laying in sand. • Optimum thickness of rock armour. • Uplift capacity. [ABSTRACT FROM AUTHOR]

Spudcan foundations of various geometries are commonly used in the field for supporting offshore oil and gas drilling (jack-up) rigs. This paper investigates the effect of the spudcan and its base geometry on the penetration and extraction resistance in soft normally and lightly overconsolidated clay. A series of centrifuge tests was undertaken on model spudcans of three different geometries typically used in the field. The spudcans were extracted from penetrations of 2·64∼3 (area equivalent) diameters and after an operation period of ∼19 months. Additional large-deformation finite-element analyses allowed for exploring a range of spudcan base geometries. Corresponding field data were accumulated. The results from this study have confirmed the accuracy of the assumption of a circular conical-based footing and the use of area equivalent diameter for assessing spudcan penetration resistance, as suggested by ISO guidelines 19905-1. Simulation of a flat-based circular plate resulted in 19% higher resistance compared to that of conical-based footings. For spudcans with cutouts and with a peripheral skirt, maximum penetration resistances were, respectively, 14 and 12% lower, and maximum extraction resistances were 20 and 14% lower compared to those for a general spudcan. The corresponding required jack-up leg length was 9 and 7% higher. The effect of peripheral skirt length was shown to be minimal (at least for length relative to spudcan diameter ≤ 0·42). [ABSTRACT FROM AUTHOR]

Challenges associated with dynamically installed anchors include prediction of the anchor embedment depth, which dictates the anchor's holding capacity. This is particularly true for calcareous sediments, as very little performance data exist for this anchor type in these soils. This paper reports results from a series of model tests undertaken to provide insight into the behaviour of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the drop height and consequently the impact velocity, and the consolidation period prior to anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge model test data were used to calibrate: () an analytical dynamic embedment model, based on conventional bearing and frictional resistance factors but with strain-rate-dependent undrained shear strength for the soil; and () an analytical quasi-static vertical pullout capacity model, accounting for reverse end bearing and frictional resistance. A total energy based expression, appropriate for calcareous silts, was proposed for predicting anchor embedment depth for a given anchor geometry, mass and impact velocity. For assessing anchor vertical holding capacity, a piezocone based direct design approach was also proposed, deriving anchor end bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Anchor capacity under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of anchor net resistance, which agreed well with a finite-element based envelope developed for embedded foundations. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework. [ABSTRACT FROM AUTHOR]

Mitigation of spudcan punch-through is one of the principal aspects for inclusion in the ISO standard 19905-1. This paper investigates the potential for using skirted foundations as an alternative to spudcans in an attempt to mitigate punch-through failure of jack-up rigs during installation and preloading in sand-over-clay deposits. A series of centrifuge model tests is undertaken on a spudcan and two skirted foundations, varying the length of the skirt relative to the foundation diameter. The thickness of the sand layer is also varied to cover a range of practical interest. The results from this study confirm that the use of skirted foundations as an alternative to spudcans can eliminate the potential of punch-through failure on sand-over-clay deposits. The degree of post-peak bearing reduction, one of the key measures of the severity of punch-through failure, is shown to be linked directly to the effective sand layer thickness beneath the foundations. Skirted foundations assist in mitigating the severity of punch-through failure by reducing the effective sand layer thickness. [ABSTRACT FROM AUTHOR]

Highly layered soils are prevalent in several areas of current offshore activity and in particular in emerging provinces and fields. However, no investigation has been carried out for spudcan penetration in stratified sediments with more than two layers. This paper reports centrifuge modelling of spudcan foundations penetrating through 3∼6 clay layers with interbedded stronger clay, silica sand and carbonate sand layers. The aim was to provide insight into the behaviour of spudcan foundations in stratified sediments comprising layers of different drainage conditions and mineralogy. The motivation for this paper emanated directly from the future needs identified by the recent revised version of ISO 19905-1. It was found that the downward soil deformation was extended into 3∼4 underlying layers of sand and clay with thickness ratios (/) less than 0·6. The limiting squeezing depth was 0·17∼0·18 when approaching stiff clay-moderate clay layers and 0·28 when approaching dense sand-stiff clay layers. These depths were enhanced (to 0·3∼0·35) by the soil plug trapped beneath the advancing spudcan. The values obtained using the ISO criteria either overestimated or underestimated these depths. Punch-through failure at a shallow penetration depth occurred with a load-spreading gradient of 1 (horizontal):3 (vertical) in silica sand and with a much smaller gradient of 1:7 in carbonate sand. In both sands, the spreading angle reduced with increasing depth of punch-through failure, and diminished quickly immediately after a failure. In contrast to ISO guidelines, the plug base, where the end bearing was mobilised, was located in one of the lower layers moved down with the advancing spudcan. For all cases, a soil plug developed at the base of the advancing spudcan, with the height of the plug accumulating through trapped clay and sand, regardless of their strength, from layers above and below the advancing spudcan. The height of a soil plug was reduced partly by soil backflow and squeezing out from contact with a strong layer. The observed soil failure mechanisms and corresponding penetration resistance profiles highlight uncertainties in ISO 19905-1. Guidelines are provided to improve the suggested punch-through and squeezing methods in ISO 19905-1. Implications for the bottom-up design approach proposed by ISO, and also a top-down design approach used by some practitioners, are discussed. [ABSTRACT FROM AUTHOR]

Earthmoving equipment working on a progressively placed cover layer over mine tailings often experiences catastrophic 'rotational' (or 'punch-through') failure, with potential for loss of the equipment and harm to the operator. Although the performance of foundations on a homogeneous sand or clay slope is routinely calculated and investigated, comprehensive investigation of a thin, stronger slope overlying a softer layer is scarce. This paper reports the results of centrifuge model tests undertaken to provide insight into strip foundation behaviour during penetration, with freedom in horizontal displacement and rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). Variables were the set-back of the edge of the foundation from the crest of the slope, the height of the slope relative to the foundation size, and the normalised strength of the lower clay layer. Soil movement was captured continuously by a digital camera, and subsequently quantified through particle image velocimetry (PIV) analysis. The load-penetration responses were separately recorded. The effect of normalised set-back ratio ( = /), slope height ( = /) and clay strength ( / ) on the evolving soil flow mechanisms and the penetration resistance profile is discussed in the context of the likelihood and severity of failure. Rotational failure, with a peak in penetration resistance followed by some reduction, occurred for all cases investigated except for a higher set-back of = 1·5. The severity of failure was greater the closer the proximity of the footing to the slope crest, and the greater the height of the slope, whereas it reduced as the normalised strength of the lower layer increased. Typical critical failure occurred in clear shear planes pushing a (nominally) rigid block of soil, with the shape of a hemisphere followed by a wedge, towards the slope. [ABSTRACT FROM AUTHOR]

Structural stiffeners placed inside caissons have created significant uncertainty regarding soil flow mechanisms, side friction, and end bearing, and consequently in the prediction of the underpressure required for installation. This paper describes centrifuge modelling of the penetration of stiffened skirts in overconsolidated clay, identifying the soil flow mechanisms around and between stiffeners and the penetration resistance. The effects of stiffener width and spacing as well as soil strength ratio are discussed. The present guidelines for predicting the point of soil backflow in the gap that forms between the stiffeners are shown to be invalid and a new expression, based on a stability factor ( su/ γ′ B, where su and γ′ are the undrained shear strength and effective unit weight of the soil, respectively, and B is the combined width of the skirt and stiffener close to the caisson tip) is proposed. End bearing at the skirt tip and base stiffener are seen to dominate installation resistance. The normalized installation resistance stabilizes at a value of ∼17.5 at deep penetrations and is shown to be independent of whether the cavity between the stiffeners remains open or is backfilled with a soil-water mixture. This factor of 17.5 is shown to be in good agreement with reported field data and centrifuge test data and hence may be used to obtain a first-order estimate of the installation resistance of a caisson with stiffeners. [ABSTRACT FROM AUTHOR]

Installation of independent-legged jack-up rigs in sea-bed sediments where a strong layer overlays weaker soil can lead to catastrophic 'punch-through', with potential leg buckling or toppling of the unit. Although approximate methods of analysis exist for estimating the penetration resistance, these do not account for the distortion of the upper layer as it punches through into the lower layer and there has been only limited visual depiction of the failure mechanism that occurs for the case of strong clay overlying weaker soil. This paper reports results of centrifuge model tests undertaken to provide insight into spudcan foundation behaviour during undrained vertical penetration through a stronger clay layer into weaker material, varying the strength ratio between lower and upper soil layers, the thickness of the upper layer relative to the spudcan diameter and the strength gradient of the lower layer. The model tests included half-spudcan tests against a transparent window, allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry (PIV) analysis, and separate full-spudcan tests to measure the penetration resistance. Four interesting aspects of the soil flow mechanisms were identified: () vertically downwards motion of the soil and consequent deformation of the layer interface; () trapping of the stronger material beneath the spudcan, with this material being carried down into the underlying soft layer; () delayed back-flow of soil around the spudcan into the cavity formed above the spudcan; () eventual localised flow around the embedded spudcan. The effect of normalised layer soil properties and geometry on the soil flow mechanisms and the form of the penetration resistance profile is discussed in the context of the likelihood and severity of punch-through failure. Typical critical failure modes involved punching shear, with clear shear planes in the shape of a truncated cone forming in the upper layer below the spudcan. Potential punch-through, with a peak in penetration resistance followed by some reduction, occurred for almost all cases investigated. The reduction in resistance became more severe as the strength ratio of the underlying layer to the upper layer reduced, and also as the upper layer thickness increased. The failure modes assumed by the currently available recommended practices are not consistent with those observed from the study, suggesting a more rational approach is needed. [ABSTRACT FROM AUTHOR]