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This paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of a novel dynamically installed anchor (DIA), termed as fish DIA, during dynamic installation and monotonic pullout in non-homogeneous clay and calcareous silt. The fish DIA has an elliptic-shaped shaft, which reduces hydrodynamic drag resistance. The shaft is shaped to be thicker near the head to lower the mass centroid, and increase its diving potential with a specialised padeye position. A series of large deformation finite element analyses have been carried out considering the relevant range of parameters in terms of soil undrained shear strength; impact velocity, padeye offset ratio and pullout angle. Considering the difference in soil undrained shear strength, and DIA dimensions and mass; overall anchor tip embedment depths of the fish DIA lied in the range of that of the OMNI-Max DIA. The tracked anchor trajectory confirmed that, compared to the OMNI-Max DIA, the fish DIA dove deeper and earlier for a wide range of padeye offset ratio. These are more critical and beneficial for calcareous silt where the achieved embedment depths are generally lower compared to those in clay. [ABSTRACT FROM AUTHOR]

The interaction between a spudcan and an existing footprint is one of the major concerns during jack-up rig installation. The influence of spudcan-footprint interaction has recently been well addressed by a number of researchers. A lack of investigation exists in mitigating spudcan-footprint interaction issues. In the field, stomping and successive repositioning is conventionally used in installing a rig adjacent to an existing footprint. Water jetting and perforation drilling are also sometimes suggested. This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of a spudcan with 4 slots tested through model tests carried out at 1 g on the laboratory floor. The soil conditions tested simulate soft to moderate seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation locations of 0.5D and 1D (D = spudcan diameter) and existing footprint depths of 0.33D and 0.66D were investigated. By comparing with a conventional spudcan, the spudcan with slots reduced the induced maximum moment, horizontal force, and horizontal sliding distance by up to 80%, 40%, and 98% respectively. Critically, no additional operations, such as stomping/repositioning, perforation drilling, water jetting, are required to be performed offshore. [ABSTRACT FROM AUTHOR]

Spudcan punch-through during installation and preloading process is one of the key concerns for the jack-up industry. This incident occurs in layered deposits, with new design approaches for spudcan penetration in sand-over-clay deposits reported recently. This paper reports a novel design approach for spudcan penetration in stiff-over-soft clay deposits. Large-deformation finite element (LDFE) analyses were carried out using the Coupled Eulerian-Lagrangian (CEL) approach. The clay was modelled using the extended elastic - perfectly plastic Tresca soil model allowing strain softening and rate dependency of the undrained shear strength. A detailed parametric study was undertaken, varying the strength ratio between bottom and top soil layers, the thickness of the top layer relative to the spudcan diameter, and degree of nonhomogeneity of the bottom layer. Existing data from centrifuge model tests were first used to validate the LDFE results, and then the measured and computed datasets were used to develop the formulas in the proposed design approach. The approach accounts for the soil plug in the bottom layer, and the corresponding additional resistance. Where there is the potential for punch-through, the approach provides estimations of the depth and bearing capacity at punch-through, the bearing capacity at the stiff-soft layer interface, and the bearing capacity in the bottom layer. Comparison shows that the punch-through method suggested in ISO standard 19905-1 provides a conservative estimate of the bearing capacity at punch-through, with guidelines provided to improve the method. [ABSTRACT FROM AUTHOR]

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]

The interaction between a spudcan and an existing footprint is a major concern during jack-up rig installation. No guidelines were provided in the recently finalised version of ISO guidelines 19905-1 in regards to mitigating spudcan–footprint interaction issues except some proposed considerations due to scarce of detailed investigation. This paper reports a measure for easing spudcan–footprint interaction issues, with the efficiency of perforation drilling tested through model tests carried out at 1 g. The soil conditions tested simulate soft seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation locations of 0.5 D and 1 D ( D =spudcan diameter) and existing footprint depth of 0.33 D were investigated. The removal of soil (through perforation) inside the spudcan perimeter, with an area of 9% perforated, reduced the induced maximum moment, horizontal force, and horizontal sliding distance by up to 80%, 83%, and 76% respectively. [ABSTRACT FROM AUTHOR]

This paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation in non-homogeneous clay. The large deformation finite element (LDFE) analyses were carried out using the coupled Eulerian–Lagrangian approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength using the Herschel–Bulkley model. The results were validated against field data and centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of anchor shaft length and diameter; number, width and length of fins; impact velocity and soil strength. The anchor velocity profile during penetration in clay showed that the dynamic installation process consisted of two stages: (a) in Stage 1, the soil resistance was less than the submerged weight of the anchor and hence the anchor accelerated; (b) in Stage 2, at greater penetration, frictional and end bearing resistance dominated and the anchor decelerated. The corresponding soil failure patterns revealed two interesting aspects including (a) mobilization of an end bearing mechanism at the base of the anchor shaft and fins and (b) formation of a cavity above the shaft of the installing anchor and subsequent soil backflow into the cavity depending on the soil undrained shear strength. To predict the embedment depth in the field, an improved rational analytical embedment model, based on strain rate dependent shearing resistance and fluid mechanics drag resistance, was proposed, with the LDFE data used to calibrate the model. [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]

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]

The spherical ball penetrometer is increasingly being used for profiling the undrained shear strength of soils in centrifuge and offshore site investigations. This paper reports the results from large-deformation finite-element (LDFE) analysis undertaken to provide insight into ball penetrometer behaviour during undrained vertical penetration through uniform and stratified clay deposits. The LDFE analyses simulated continuous penetration of ball penetrometers from the seabed surface. The results were validated against centrifuge test data and plasticity solutions prior to undertaking a detailed parametric study, exploring a range of normalised soil properties and layer thicknesses and roughnesses of the soil/ball interface. The influence of the shaft (or area ratio) was also identified. The evolving soil failure patterns in single- and double-layer soils revealed two interesting aspects: soil backflow above the penetrometer, and trapping of the stronger material beneath the penetrometer. A framework has been proposed to account for these effects, which will allow accurate interpretation of soil undrained shear strength from the ball penetration resistance. [ABSTRACT FROM AUTHOR]

This paper presents results of large deformation finite element (LDFE) analyses that investigate the penetration response of deep-penetrating spudcan foundations in stratified soil comprising a strong clay layer overlying weaker clay. Such strength profiles give rise to potential punch-through failure, with a local maximum penetration resistance followed by some reduction. The paper is a companion paper to one that presents results from centrifuge model tests of the same problem, with the overall aim being to investigate the potential for punch-through failure and its severity, as a basis for improving design methods for evaluating spudcan penetration. The LDFE analyses have simulated continuous penetration of smooth and rough spudcan foundations from the sea-bed surface. A detailed parametric study has been undertaken, exploring the relevant range of layer thickness (relative to the spudcan diameter), strength ratios and spudcan base roughness. The results have been validated against previously published results and the centrifuge test data presented in a companion paper. Excellent agreement was obtained between the results from LDFE analyses and centrifuge tests. As the ratio of the shear strength of the bottom layer to that of the top layer decreased, upwards soil flow around the spudcan shoulder became less pronounced and the failure mechanism was dominated by a steep-sided plug of soil from the top layer being forced deep into the lower layer. The potential for severe punch-through was demonstrated by a significant reduction in the resistance profile as the spudcan approached the interface between the two layers. A preliminary design chart is proposed to estimate the cavity depth above the penetrating spudcan, which was significantly greater than that for a single layer clay. The penetration responses have been presented in terms of profiles of normalised net bearing pressure for a range of layer geometries and soil properties. Potential punch-through occurred for all cases where the strength ratio was less than or equal to 0·6, with the reduction in resistance being greater the lower the strength ratio of the underlying layer to the upper layer, and the thicker the upper layer, but reducing as the strength gradient in the lower layer increased. The current approach suggested in offshore design guidelines is shown to underestimate the penetration resistance significantly, in most cases, and give poor estimates of the likelihood and severity of spudcan punch-through. [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]

Accurate prediction of the final embedment depth and installation-induced excess pore pressure of dynamically installed anchors (DIAs) are prerequisites for the design of subsequent anchor set-up during a waiting period and capacity under operational loadings. This paper presents a dynamic large deformation finite element approach within the effective stress framework and investigates the installation behaviour of DIAs in clay. An elasto-viscoplastic model is adopted to capture strain rate dependency of the undrained shear strength. Development of excess pore pressure for various impact velocities, anchor geometries, viscosity parameters are quantified. An expression is proposed for estimating the final embedment depth of DIAs in clay. • A dynamic LDFE approach within the effective stress framework is developed, incorporating a strain rate-dependent model. • The excess pore pressures caused by rapid penetration of a dynamically installed anchor can be captured. • A simple equation is proposed to predict the final embedment depth of anchor. [ABSTRACT FROM AUTHOR]