Spudcan Foundations Research Articles

Effects of Prior Footprints on the Bearing Capacity of Spudcan Foundations: A Case Study

A jack-up rig is a self-elevating offshore drilling platform designed for mobile operations in the marine environment. The rig is supported by 3 or 4 legs, with spudcan foundations at the end. These spudcan provide the necessary stability and load-bearing capacity for rig foundation during drilling operations. The mobile rig sometimes revisits sites where prior installation, operation, and extraction activities have resulted in identifiable footprints on the seabed. The penetration of spudcan close to, or partially overlapping, footprints shall be carefully considered because the spudcan-footprint interaction may cause a change in the ultimate vertical bearing capacity (Fv). ISO 19905-1:2016 states that the interaction between a spudcan and a footprint is expected to be minimal when the edge-to-edge distance exceeds one spudcan diameter. This paper presents the effect of existing footprints on the vertical bearing capacity of a spudcan with various spacing variations from 0.5B to 1.75B, where B is the diameter of the spudcan. A 3D finite element method (3D FEM) is employed as the primary analysis tool in analyzing the effects of footprints on layered clay soil. The 3D FEM analysis is conducted because the analytical approach outlined in ISO 19905-1:2016 does not consider the influence of the footprint on the spudcan capacity. The analysis results show that bearing capacity may reduce by more than 50% when the center-to-center distance is less than 1B. Whereas, for distances greater than 1.5B, the influence of the footprint is relatively insignificant, only reducing less than 10% of the ultimate capacity of the spudcan.

Effects of spudcan installation processes on the bearing capacity of foundations under combined loading in soft-over-stiff clay

The bearing capacity of spudcan foundations under vertical, horizontal, and moment loading must be adequately assessed prior to the operation of jack-up platforms. However, studies the effects of combined loading on spudcan foundations in soft-over-stiff clays, considering the effect of installation processes, have been limited. In this study, the two-dimensional remeshing and interpolation with small strain method was used to simulate the large deformations experienced by the soil during the installation of a spudcan, and a three-dimensional model was employed for the small strain analysis regarding the bearing capacity. The undrained shear strength of stiff clay influenced the failure mechanisms of the soil during vertical and moment loading. However, it had minimal impact during horizontal loading. Therefore, in this study, an equation for the bearing capacity factors for vertical and moment loading conditions is proposed. The properties and parameters of the combined bearing capacity failure envelope with respect to different dimensions are analysed and corresponding equations for the failure envelope are proposed, which aid the on-site assessment of jack-up platforms prior to them becoming operational.

Experimental modelling of spudcan penetration in spatially variable soils

ABSTRACT The influence of spatial variability of soils on spudcan penetration responses is found significant through numerical methods. However, due to the difficulty of preparing spatially variable soils, the penetration mechanism of the spudcan through random soils remains physically unclear. Therefore, this study aims to investigate the effects of the spatial variability of soils on the penetration behaviour of spudcan foundations by laboratory tests. A new method to prepare the spatially variable soils was proposed first. Then, three physical soil models with different scales of fluctuation and coefficients of variation were carefully prepared. The spudcan penetration was conducted in these three soil models and the penetration mechanism of the spudcan was investigated. The penetration resistances of the three cases were obtained and compared with that predicted by the methods suggested by ISO and SNAME standards. The results show that the predicted error of the penetration resistance can reach 50% when the soils have a large coefficient of variation and a small horizontal scale of fluctuation. The large difference is the significant spatial variability that has been ignored in the standards. This study paves the way to an accurate prediction of the spudcan penetration analysis in strong variable soil seabed.

Probabilistic prediction of the spudcan extraction from sand-over-clay based on monitored data

Upon completion of the operation on a site, spudcan foundations are pulled out of the seabed before relocating the jack-up platform to another candidate site. It is critical to predict the peak extraction resistance before commencing the jack-up extraction. Though sand-over-clay seabed is commonly found in active marine oil and gas exploration zones, the models for estimating the extraction resistance are very rare. This paper proposes a Bayesian prediction model based on 51 groups of centrifuge tests on foundations extracting from sand overlying clay soils. Model factors are introduced to define the uncertainties in the prediction. Combined with the monitored data during foundation extraction, possible breakout range of resistance and depth is predicted and updated in real time. The most likely breakout resistance and depth in the area are also estimated. The results show that (1) the predicted range of breakout resistance and depth may occur are concentrated around the measured values with the update of the monitored data; (2) the relative error between the most likely breakout depth and the measured value is kept within 10%; and (3) regardless the shape of the foundation, the Bayesian prediction model can effectively predict the extraction behaviour of the foundation.

Effects of the skirt length and embedment depth on the capacity of a skirted spudcan foundation under combined loading

Skirted spudcan foundations can provide a considerable bearing capacity and moment fixity for resisting the complex combined vertical, horizontal, and moment loadings arising from the harsh environment in the offshore oil and gas industry. These potential benefits are conducive to the extension of mobile jack-up drilling rigs into deep water. In this study, a series of probe tests were carried out using finite element analysis to investigate the effects of the skirt length and embedment depth on the bearing capacity of skirted spudcan foundations in soft clay under combined loading. The results indicate that both the skirt length and the embedment depth can significantly affect the behavior of a skirted spudcan and apparently improve the various bearing capacities under combined loading. As the skirt length and embedment depth increase, the failure envelope's size gradually expands and the failure envelope's shape changes accordingly. A spudcan with a skirt length of 0.25D embedded in about 2D of sediments is suggested to obtain an optimal application in practice. A closed-form algebraic expression was derived to interpret and predict the behavior of skirted spudcan foundations, which properly considers the effects of the skirt length and embedment depth.

Centrifuge tests and numerical simulations on vertical bearing capacity of mat foundation on marine clay seabed

Mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed, which avoids the problems of excessive insertion and extraction difficulties of spudcan foundation on soft clay seabed. Evaluating the vertical bearing capacity of the mat foundation on the clay seabed is of great significance for the safe operation of jack-up platform in marine environment. In this research, the vertical bearing characteristics of mat foundation on horizontal and sloping clay seabed were studied through centrifuge tests. Then, numerical simulations were conducted to study the vertical bearing capacity of the full-scale mat foundation on clay seabed, in which the effects of the large mat foundation size, pile leg insertion depth, preloading, consolidation degree of soil, and soil plasticity were analyzed. The results show that the vertical bearing capacity of the mat foundations on the sloping seabed is reduced, and the soil under the mat foundation shows obvious sliding failure. The contribution of the insertion depth of pile legs on the horizontal bearing capacity is more obvious than that of the vertical bearing capacity. The enhancement of the vertical bearing capacity is mainly due to the dissipation of the excess pore pressure and the increase of the mean effective stress of the soil. The effects of preloading, consolidation degree, and soil property on the vertical bearing capacity of the mat foundation are significant.

Numerical Assessment of Jackup Spudcan Foundations by varying Angle of Spud in 3d Fem in Geotechnical Engineering

A jack-up structure is a bottom-mounted floating unit having an adjustable hull and movable legs. Particular jack-up structures are more susceptible to hazards under environmental loads during their operation and life cycle in the ocean. Authors attempted here to evaluate vertical load carrying capacity of soil and predicting its behaviour. Simulation of loaddeformation behavior under axial compression has been carried out for advancing research. The entire study has been carried out using three-dimensional finite element-based software Plaxis 3D A.E. 2017. The geometrical variation of substructure spud can have an inverted spud at the centre bottom from 1200 to 1800 studied for knowing behaviour under axial compression, axial tension and lateral forces. The simulated numerical model is used to develop empirical expressions for axial capacities estimation. The numerical analysis results indicate that spud can have an inverted angle of cone 1300 which is most beneficial under the static combined vertical, moment and horizontal loading (3D loading) in marine clay. The axial load carrying capacities in compression, tension and lateral loading follow the same sequence in ascending order from 1800,1750, 1500, 1300 and highest in 1200. Stiffness and undrained shear strength of soft clay contribute more than the diameter of the spud and embedment depth in compression and tension.

Evaluation of vertical bearing capacity factors for conical footing with varying base roughness using FELA and MARS model

ABSTRACT The conical base of spudcan foundation supporting offshore structures is also called as conical footing contributes to the overall capacity of the foundation. In the study, the vertical bearing capacity factors for the conical footing considering different base roughness (α = 0 and 1), varying friction angles of soil (f = 0–40°), and different apex angles (b = 30–180°) are evaluated by performing two–dimensional lower and upper bound finite element limit analysis (FELA). The base roughness α = 0 represents a smooth base footing and α = 1 represents a perfectly rough base footing and the apex angle b = 180° represents a flat base circular footing. Multivariate adaptive regression splines models are developed, influence of investigated parameters (α, f, and b) on the bearing capacity factors are examined and the empirical equations are proposed for the evaluation of the bearing capacity factors of conical footing.

Evolution of breakout force with operation period for deeply embedded spudcan in clay

The extraction of spudcan foundation in clay can often be difficult and time-consuming considering the large pull-out force required to overcome the breakout resistance. This is particularly an issue for long operation periods, during which the dissipation of excess pore pressure in the soil will increase the base suction and undrained shear strength of soil. This paper investigates the spudcan breakout force in clay deposits through a coupled pore fluid-effective stress large deformation finite element approach. The complete process of spudcan installation, operation and removal is simulated. The numerical approach is verified against existing centrifuge test results. A good agreement can be obtained between the numerical and experimental results in terms of normalised breakout force and operation period. A series of parametric study is then carried out to explore the effects of installation depth, operational load and soil parameters. Based on the numerical results, a unified relationship can be established between the dimensionless forms of breakout force and operation period, for which a simple algebraic expression is proposed for the convenient assessment of breakout force recovery.

Capacity of spudcan foundation on dense sand overlying clay under combined loading

The evaluation of spudcan foundation capacity under combined vertical, horizontal and moment loading is critical for the site-specific assessment of jack-up rigs. However, the investigation on the behavior of a spudcan on the surface of a sand layer overlying clay under combined loading is limited. This paper reports the results from a series of finite element analyses conducted to explore the failure envelope of spudcan on dense sand overlying clay before punch-through, considering the effects of the preload level on spudcan, the thickness ratio and the relative density of sand, and the underlying clay strength through parametric study. The numerical results provide insights that when the sand layer is sufficiently thick, the failure envelope expands with the increase of vertical load level, leading to higher horizontal and moment capacities; however, for the spudcan on a relatively thin sand layer, the horizontal and moment capacity may decline with rising vertical load level, resulting in eccentricity and shrinkage of the failure envelope. New design formulas for the failure envelope are proposed covering a comprehensive range of soil parameters and layer geometries of practical interest, which facilitates their general application in the site-specific assessment of the jack-up platforms.

Determination of elastic stiffness coefficients for spudcan foundations in a spatially varying clayey seabed

Determining the elastic stiffness coefficient of spudcan foundations is a challenging task due to the natural variability of soil properties coupled with the complexity of loading conditions. Uncertainty in the elastic stiffness coefficient is a potential risk factor for the safe and stable operation of offshore jack-up platforms, and there are no guidelines for operators to assess its confidence level. This paper conducted a study on this specific subject using the three-dimensional random finite element method. The influence of soil strength heterogeneity, the coefficient of variation and the scale of fluctuation of the shear modulus, embedment depth and soil backflow on the deformation mechanism and elastic stiffness coefficient of spudcans were systemically discussed. The results suggest that the above parameters, combined with the soil's spatial variability, can alter the deformation mechanism, thereby the elastic stiffness coefficient. If spatial variability is ignored in platform design, the safety probability of spudcans is only 50%. So, a matrix algebraic expression is proposed to accurately predict the elastic stiffness coefficient in spatially variable clay at any given confidence level. The study findings may facilitate incorporating spatial variability into elastic stiffness research and supplement the calculation method in the existing jack-up assessment guidelines.

Penetration Behavior of the Footing of Jack-Up Vessel of OWTs in Thin Stiff over NC Clay

This study investigated the behavior of the spudcan foundation of jack-up vessels of offshore wind turbines during the undrained vertical penetration into thin stiff-over-normally consolidated clay. Large deformation finite element (LDFE) analyses were used to simulate the continuous spudcan penetration into the seabed surface. Detailed parametric analysis was performed to explore a range of normalized soil properties and layer geometry and roughness of the soil–spudcan interface. The results were validated against previously reported data. The LDFE results were consistent with those of centrifuge tests. The evolving soil-failure patterns revealed soil backflow and the trapping of stronger top-layer material beneath the spudcan. The plug shape was influenced by the top layer thickness, the strength gradient of the bottom layer, and the relative strength ratio, which also affected the penetration resistance of soils. In this study, an expression was derived to quantify the plug shape with the aim of providing a theoretical basis for the design of spudcan footings with penetration resistance suitable for thin stiff-over-soft clay.

Numerical pushover analysis of jack-up units in soft clay overlying sand

A site-specific assessment is required to ensure the safe operation of a mobile jack-up unit under a design storm. The evaluation of jack-up structural response strongly depends on an accurate assessment of the interaction between all spudcan foundations and the supporting soils. Macro-element models have been increasingly applied to describe the non-linear spudcan foundation behaviour under combined loading, which can be coupled directly with the numerical analysis of the structure. To address the need to extend the application of the macro-element modelling approach to layered soils, this paper presents example jack-up quasi-static pushover analyses using a newly proposed macro-element model for describing the behaviour of spudcan footings in soft clay overlying sand. When employed within a structural analysis, the macro-element model allows the evolution of the load-displacement response of each individual spudcan as well as the global system response to be examined. The presence of the underlying stronger sand layer is shown to increase the pushover capacity and the system stiffness of the jack-up units. The results emphasize the benefit of using the macro-element foundation model for accurate assessment of the jack-up capacity when subjected to external loading from the ocean's wind, waves and current, which can contribute to the improvement of the site-specific assessment of jack-up units installed in clay overlying sand.

Probabilistic bearing capacity of spudcan foundations under combined loading in spatially variable soils

Spudcan foundations are widely used in the offshore gas and oil industry to support mobile jack-up drilling rigs. During the service period, the spudcan foundations need to resist large vertical-horizontal-moment (VHM) loads that are transmitted from the superstructure. The bearing capacity of foundations in complicated stress conditions is usually based on VHM failure envelopes. These are commonly constructed by using numerical methods where the soil properties are assumed to be deterministic, ignoring the inherent spatial variability. This paper investigates the influence of spatially variable undrained shear strength and embedment depth on the capacity of spudcan foundations under VHM combined loads. The linearly increasing undrained shear strength with depth obeying log-normal distribution was taken into account. Probabilistic failure envelops were constructed to analyse the effect of spatial variability on the bearing capacity of spudcan foundations at various embedment depths when subjected to different combinations of VHM loads. The influence of horizontal scale of fluctuation for a specific embedment depth was also investigated. The results indicated that under combined loading the embedment depth significantly influenced the shape and size of failure envelopes. The probability of failure at each embedment depth based on the obtained probabilistic failure envelopes was intuitionally represented by means of a factor of safety, which can provide specific guidance for foundation reliable designs.

Elastic stiffness coefficients of a skirted spudcan foundation in clay

Skirted spudcan foundations can potentially provide considerable bearing capacity and moment fixity that can withstand the complex combined vertical, horizontal, and moment loadings arising from the harsh environments encountered in the offshore industry. The analysis of a skirted spudcan foundation's stiffness should be given special consideration during the preliminary stage of the design process. This technical note carries out a series of probe tests using finite element analysis to investigate the elastic stiffness coefficients of a skirted spudcan foundation in soft clay. The elastic stiffness coefficients of skirted spudcan footings with different soil shear strengths, embedment depths, and skirt lengths are presented. The results indicate that skirted spudcans can provide higher elastic stiffness coefficients, and the elastic stiffness coefficients are significantly influenced by the soil shear strength, embedment depth, soil backflow, and skirt length. An approximation expression with three special variables is developed for predicting the elastic stiffness coefficients of a foundation, except for the footing on the surface. This may potentially complement the elastic stiffness coefficients in the existing jack-up assessment guidelines, which do not account for skirted spudcan shaped footings embedded at different depths.

Effects of installation on combined bearing capacity of a spudcan foundation in spatially variable clay

Mobile jack-up drilling rigs with spudcan foundations are extensively used in offshore industry due to their flexibility, mobility and cost-effectiveness. A sufficient bearing capacity of the spudcan foundation is needed to resist the combined environmental loading associated with storms, winds, waves and currents. In most previous studies, the combined bearing capacity was defined for the wished-in-place spudcan foundation, ignoring the disturbance to the seabed soils cause by spudcan installation. It is shown in this paper how the installation affects the failure mechanism and bearing capacity of spudcan foundation in spatially variable soil. Random large deformation finite element method is first used to simulate the installation process of a spudcan foundation in order to obtain the disturbed soil strength profiles. Then the profiles are mapped into random small-strain finite element model for calculation of bearing capacity of the spudcan foundation. Results show that a large amount of the softer soils in the surface of seabed flow back to top of spudcan foundation, which causes the bearing capacity to reduce significantly. Moreover, a Monte-Carlo simulation is carried out to define the probabilistic failure envelopes of the spudcan foundation, considering the disturbance of installation to the spatially variable soils.

Spudcan–pile interaction in sand-over-clay: centrifuge modelling

Large soil movements arising from the installation of jack-up spudcan foundations may induce detrimental effects on piles supporting adjacent permanent jacket platforms. Particularly, in a dense sand-over-clay profile, there is a risk of spudcan rapid penetration due to underlying clay bearing capacity failure, known as spudcan punch-through failure, which can cause further damage to piles in the proximity. This study investigates the foundation interaction between the spudcan and adjacent pile in a sand-overlying-clay soil profile by way of centrifuge modelling. Issues investigated include thickness of the upper sand layer with incidence of sand plug beneath the spudcan and spudcan–pile clearance. The experimental findings reveal that spudcan punch-through hazard and the magnitude of induced pile moments amplifies primarily due to the increase in the thickness of the upper sand. In addition, the magnitude of induced pile moment in a sand-over-clay profile is significantly higher than that in a single clay profile because of the post-peak bearing behaviour observed in stratified soil, yielding to a greater risk to pile integrity. Soil pressure profiles on the piles are also back-analysed. Two generalised induced soil pressure profiles are deduced, with the first profile representing the onset of spudcan punch-through and the second profile denoting the situation of post-punch-through failure.

Aspects of soft clay behaviour important for correct prediction of spudcan foundation penetration

An advanced hypoplastic constitutive model for soft clay has been developed with the following features: pre-yield non-linearity, strength anisotropy (different critical state friction angles in compression and in extension), stress- and OCR-dependency of undrained shear strength and soil structure and its degradation during loading. The model features are explored here through simulation of spudcan foundation penetration into a carbonate silty clay that was obtained from the Timor Sea. As different parameters control individual features of the model, model properties that are relevant to spudcan bearing pressure predictions can be separated from the properties that do not affect the results substantially. As expected, it is confirmed that pre-yield non-linearity is among the properties not important for the predictions. On the contrary, strength anisotropy, stress- and OCR-dependency of undrained shear strength and structure degradation are shown to be important aspects to be considered, indicating limitations of the use of simple elasto-plastic models for spudcan bearing pressure predictions. The conclusions can be adopted in the definition of a suitable experimental programme targeting the features of soil behaviour relevant to this geotechnical problem.

A macro-element model for predicting the combined load behaviour of spudcan foundations in clay overlying sand

A macro-element model for predicting the load–displacement behaviour of a spudcan foundation in clay overlying sand when subjected to combined vertical, horizontal and moment loading is introduced. Observations from detailed drum centrifuge tests that measured the effect of the underlying sand layer on the foundation behaviour are combined with finite-element results and theoretical developments to derive the components of the model. The yield surface defined by the centrifuge test results suggests that as the spudcan nears the underlying sand layer, the absolute horizontal capacity remains relatively constant, while the vertical and moment capacities increase at approximately the same normalised rate. The model is demonstrated to accurately predict foundation behaviour by retrospectively simulating the experimental results. This macro-element model has the advantage that it can be integrated into the structural analyses of jack-up platforms required for site-specific assessments.

Characterising the model uncertainty of ISO methods for punch-through capacity prediction

The model uncertainty is explored for two ISO methods to estimate the peak penetration resistance in an offshore spudcan foundation punch-through incident. The model factor, defined as the ratio of measured resistance to calculated resistance, is introduced in the characterisation of the model uncertainty. Model uncertainty of a coupled Eulerian–Lagrangian (CEL) method is first characterised, wherein the model factor of the CEL method (MCEL) is found to be a random variable following a lognormal distribution, with a mean of 1.00 and coefficient of variation of 0.15. Based on results from the CEL method, regression analyses are performed, and conventional methods are modified by multiplying the results of the ISO method by a correction factor f obtained from statistical analysis of the CEL results. The model factors of modified ISO methods (M′) for centrifuge tests are calculated for further verification. Results show that, with the modifications, the systematic part of uncertainty is removed from both ISO methods, and M′ can be directly regarded as a lognormally distributed random variable. Finally, to illustrate the workability of the proposed modified ISO method, a case history is conducted. Results show that the ISO estimated punch-through capacity is improved by that modification.