# Ease Extraction Resistance of Spudcan by Adding an Extra Layer of Sand above Clay

Authors: Keqin Xu; Peng Zhang; Shengfa Yang; Qing Deng
Journal: Mobile Information Systems
 (2022)

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## Abstract

Jack-up rig, supported by a number of movable and independent legs with a footing at the bottom called spudcan, is a type of oil rig platform which is used in self-elevating system to support offshore platforms in shallow to moderate waters. Being different from the normal gravity structures, jack-up rigs can be mobilized to other sites after the exploration of oil and gas reserve is completed by extracting the legs with spudcans. However, extraction of the spudcan can be difficult and time consuming. In this paper, the spudcan penetration and extraction process is simulated using the finite element method. The research concentrates on the nonuniform clay that the shear strength profiles increase with the depth of clay, and ignores the shaft friction. In order to ensure that the effects of sand property on the bearing capacity and extraction resistance, three models for different dilation angles of sand were built. Furthermore, considering the influence of sand thickness, five models with different sand thicknesses were generated, in which the penetration depth is one time the diameter of the spudcan. In addition, the spudcan penetration depths are extended to twice and half of the spudcan diameter to verify the effect of penetration depth. Results obtained from these model analyses reveal that the required extraction force of the spudcan with a thin sand layer above clay is lower than that without the sand layer. Moreover, the results also show that thick sand layers can produce better effect of reducing extraction resistance of spudcan.


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## Body

## 1. Introduction

In shallow to moderate waters, up to around 150 m in water depth, jack-up rigs, also named self-elevating units, are the popular self-elevating system to support offshore platforms, due to their properties of “flexibility, mobility, and cost effectiveness” [1]. Being different from the normal gravity structures, jack-up rigs can be mobilized to other sites after exploration is completed.The working procedure of spudcan can be divided into 3 stages. The first working stage of spudcan is installation. During the installation stage, the jack-up rig will be installed at the working station. Then, all the machines on the jack-up rig can start to operate. It is named the operation stage. Following the operation stage, it is the extraction stage. During this stage, the spudcan will be extracted and the jack-up rig will be removed to a new working station. In order to change the drilling location of jack-up rigs, it requires the removal of the spudcan at each site. Many studies have tried to do the experiments and simulated the installation and extraction procedure of spudcan foundation. During the installation procedure of a spudcan, soil backflow will occur, leading to backfilled soil covering the top of the spudcan [2]. For the extraction procedure, after the compression of load, it releases an upward displacement which provides an extraction resistance. Moreover, a relative mass of clay is stuck to the top of the spudcan after the extraction procedures completed [3]. For extraction of the spudcan, the process can be difficult and time consuming, which regularly requires one or two weeks, and even up to ten weeks in some cases. With the decreased operation time at each site, the rate of rigs moving per year increased significantly during the last 10 years [4]. Different from the installation process of jack-up rigs, there are no standard procedures for the extraction of spudcan. Based on previous experience, each operator seems to have their own operative skills for pulling out the legs.As mentioned by Purwana et al., based on the assumption of ignoring the friction on the side wall of the spudcan, the total spudcan extraction resistance can be divided into three parts: resistance of soil above spudcan (Qtop), resistance of soil below the spudcan (Qbottom), and the effective submerged weight of the leg and the spudcan (We). The equation could be shown as(1)Qtotal=Qtop+Qbottom+We,Qtop=0.25πB2Nc,topcu,topFg,top,Qbottom=0.25πB2Nc,basecu,baseFg,base−γ′sideDbSb,where B is the spudcan diameter; Nc,top is the breakout factor; cu,top is the average shear strength of backfilled soil; Fg,top is the shear strength change of the soil above spudcan; cu,base is the spudcan base average shear strength; Fg,bas is the shear strength change of the soil below spudcan; γ′side is the average unit weight; Sb is the empirical coefficient; Db is the depth of spudcan, as shown in Figure 1.Figure 1
The components of the Spudcan extraction force (Purwana et al.).The resistance of soil below the spudcan, or often called “base suction”, is a suction force “developed at the interface between the spudcan base and the underlying soil,” which increases with the spudcan operation time. Unlike the base suction, the growth of the resistance of soil above the spudcan is slight with the spudcan operation period. Therefore, for jack-up rigs with a long operation period, base suction would be the main contribution of the total extraction resistance required to pull out the spudcan footings.Generally, the uplift force is comprised by hull buoyancy. An overdraft, larger than neutral draft by several feet, can provide greater buoyancy for the hull, which can ease the extraction resistance effectively. But it is unsure if maximum possible buoyancy force can be imposed. So, there is a risk of inundating hull. Most jack-up rigs cannot apply enough tensile force to pull out the spudcan during the extraction procedure [5]. In order to ease the extraction resistance, a water jetting system is applied to the structure of the spudcan. For the water jetting system, it is usually placed at the top and bottom of the spudcan. With high water pressure of up to 1000 psi, the water jetting system is an effective system to destroy the adhesion on the surface of the spudcan and the accumulated sediments above. However, knowledge is limited for the actual behavior of the water jetting system and how much it can contribute to the pulling out force. In addition, the filling ratio of the water jetting system has a significant influence to the efficiency of jetting. Potential contribution of the sea state to the pulling out force and the extraction resistance of the spudcan should also be taken into consideration. The cyclic loading, supplied by the prolonged action of small waves, combined with the potential consolidation of soil can change the share strength of the soil to a certain extent. The geometry of spudcan also influences on the penetration and extraction resistance. In the paper of ‘Effect of Spudan Geometry on Penetration and Extraction Resistance in Clay’ [6], it explains the relationship between spudcan geometry and penetration and extraction resistance. Three centrifuge models (normal spudcan, cutouts spudcan, and spudcan with peripheral shirt) and large deformation finite element methods have been used to investigate the results. Compared with the normal spudcan, the penetration resistance of cutouts spudcan is 14% lower and the extraction resistance is 20% lower, with 9% higher leg lengths. And for the spudcan with peripheral skirt, the penetration resistance is 12% lower and the extraction resistance is 14% lower. But leg lengths are 7% higher than the normal spudcan leg lengths.


## 2. Experimental Methodology

### 2.1. Numerical Modeling

The coupled Eulerian–Lagrangian (CEL) method is applied in this study. In Tho et al.’s papers [7], the CEL approach was first adopted to research the performance of spudcan and compare the results of penetration resistances and soil flow mechanisms with the results of centrifuge tests [8]. The bearing capacity and extraction resistance can be investigated through the CEL method, and the reaction force of the spudcan can be reflected by applying the displacement on the spudcan. The penetration and extraction processes of a spudcan with a diameter of 14 m have been simulated. The first part for the whole model is spudcan which has been set as a rigid part and the material is steel with Young’s modulus equal to 2.1×108kPa, and Poisson’s ratio equal to 0.3. The density of the steel was set to 0 and the buoyancy force of the spudcan was considered. And there is a reference point on the top jump of the spudcan which is used to control the speed of penetration and extraction processes of the spudcan. For the second part, it is a deformable soil part with three different layers (void, sand, and clay). A void layer is placed at the top of the soil part, which thickness is indicated as a ratio of the spudcan diameter 0.2D. Then, it is a sand layer with the friction angle equal to 31° and the unit weight equal to 10kN/m3. Below that, it is a layer of clay and the unit weight of clay equal to 7kN/m3. The CEL simulation is well performed for generating a quasistatic response with a penetration and extraction rate of 0.2m/s. Mesh size would influence the accuracy of the estimation of the penetration and extraction resistance. One typical element size (1/30D) was used for the focus zone, as shown in Figure 2.Figure 2
Model of a spudcan above soil.This study concentrates on an infinite area of the sand layer. Dilation angle as one of the parameters controls the plastic volumetric strain which is developed during plastic shearing. As for sand, the dilation angle depends on the internal friction angle [9]. Moreover, different sand thicknesses were considered into this project to get the influence to extraction resistance of thin and thick sand layers. In addition, considering different penetration depth, the results of the effects to extraction resistance can be obtained. The details of parametric plan are listed in Table 1.Table 1
Parametric plan.
ItemsVariateCommentsDilation angles0°,2°,&5°Sand thickness (0.25D) and penetration depth (1D)Sand thickness0.1D, 0.25D 0.35D, 0.5D, and 0.75DDilation angle (2°) and penetration depth (1D)Penetration depth1D and 2DDilation angle (2°) and sand thickness (0.5D)Note. D is the diameter of the spudcan.


### 2.2. Model Validation

In this paper, the results of the numerical models are compared with other published results to validate the models. The uniform clay, yield stress equaling 5 kPa, and nonuniform clay, the yield stress (Su) equal to the yield stress on the top surface (Sum) plus the increments of yield stress with depth (kz), are simulated and investigated. A graph of Nc value and d/D can be obtained, which is quite similar with the results of Hossain, and the trend of two graphs is similar [10]. With the variation of Sum and k, the bearing capacity factor (Nc) will reach a constant value about 10.8, as shown in Figures 3 and 4.Figure 3
d/D plotted against Nc values.Figure 4
Nc Value results from centrifuge tests (Hossain 2008).According to Hu et al., the bearing capacity of the clay can be theoretically expressed as the following equation:(2)qclay=NcSuo+Vfγc′A.This equation is constructed by two terms. The first term is the bearing capacity for a weightless soil, whereNc is the bearing capacity factor and Suo is the soil strength at the point of the widest cross sectional area. And the second term is the buoyancy force of the spudcan, where A is the area of the spudcan foundation; γc′ is the effective unit weight of the soil; Vf is the volume of the embedded foundation. For the penetration on sand overlying clay, the bearing capacity factor (Nc) is related to the trapped sand plug height (Hplug). Teh et al. announced that the trapped sand plug height (Hplug) is approximately equal to the height of the sand layer (Hs). Furthermore, there is an experimental formula performed by Lee et al. to calculate the bearing capacity factor (Nc) of a sand layer overlying clay [11].(3)Nc=14HsD+9.5,where D is the diameter of spudcan. The above formula is suitable for the ratio of sand layer over the diameter of spudcan (Hs/D) between 0.21 and 1.12. Comparing the theoretical and numerical results, the modeling results of bearing capacity are close to the theoretical results, as shown in Figure 5.Figure 5
Comparison between theoretical and numerical results of bearing capacity against sand thickness.


## 2.1. Numerical Modeling

The coupled Eulerian–Lagrangian (CEL) method is applied in this study. In Tho et al.’s papers [7], the CEL approach was first adopted to research the performance of spudcan and compare the results of penetration resistances and soil flow mechanisms with the results of centrifuge tests [8]. The bearing capacity and extraction resistance can be investigated through the CEL method, and the reaction force of the spudcan can be reflected by applying the displacement on the spudcan. The penetration and extraction processes of a spudcan with a diameter of 14 m have been simulated. The first part for the whole model is spudcan which has been set as a rigid part and the material is steel with Young’s modulus equal to 2.1×108kPa, and Poisson’s ratio equal to 0.3. The density of the steel was set to 0 and the buoyancy force of the spudcan was considered. And there is a reference point on the top jump of the spudcan which is used to control the speed of penetration and extraction processes of the spudcan. For the second part, it is a deformable soil part with three different layers (void, sand, and clay). A void layer is placed at the top of the soil part, which thickness is indicated as a ratio of the spudcan diameter 0.2D. Then, it is a sand layer with the friction angle equal to 31° and the unit weight equal to 10kN/m3. Below that, it is a layer of clay and the unit weight of clay equal to 7kN/m3. The CEL simulation is well performed for generating a quasistatic response with a penetration and extraction rate of 0.2m/s. Mesh size would influence the accuracy of the estimation of the penetration and extraction resistance. One typical element size (1/30D) was used for the focus zone, as shown in Figure 2.Figure 2
Model of a spudcan above soil.This study concentrates on an infinite area of the sand layer. Dilation angle as one of the parameters controls the plastic volumetric strain which is developed during plastic shearing. As for sand, the dilation angle depends on the internal friction angle [9]. Moreover, different sand thicknesses were considered into this project to get the influence to extraction resistance of thin and thick sand layers. In addition, considering different penetration depth, the results of the effects to extraction resistance can be obtained. The details of parametric plan are listed in Table 1.Table 1
Parametric plan.
ItemsVariateCommentsDilation angles0°,2°,&5°Sand thickness (0.25D) and penetration depth (1D)Sand thickness0.1D, 0.25D 0.35D, 0.5D, and 0.75DDilation angle (2°) and penetration depth (1D)Penetration depth1D and 2DDilation angle (2°) and sand thickness (0.5D)Note. D is the diameter of the spudcan.


## 2.2. Model Validation

In this paper, the results of the numerical models are compared with other published results to validate the models. The uniform clay, yield stress equaling 5 kPa, and nonuniform clay, the yield stress (Su) equal to the yield stress on the top surface (Sum) plus the increments of yield stress with depth (kz), are simulated and investigated. A graph of Nc value and d/D can be obtained, which is quite similar with the results of Hossain, and the trend of two graphs is similar [10]. With the variation of Sum and k, the bearing capacity factor (Nc) will reach a constant value about 10.8, as shown in Figures 3 and 4.Figure 3
d/D plotted against Nc values.Figure 4
Nc Value results from centrifuge tests (Hossain 2008).According to Hu et al., the bearing capacity of the clay can be theoretically expressed as the following equation:(2)qclay=NcSuo+Vfγc′A.This equation is constructed by two terms. The first term is the bearing capacity for a weightless soil, whereNc is the bearing capacity factor and Suo is the soil strength at the point of the widest cross sectional area. And the second term is the buoyancy force of the spudcan, where A is the area of the spudcan foundation; γc′ is the effective unit weight of the soil; Vf is the volume of the embedded foundation. For the penetration on sand overlying clay, the bearing capacity factor (Nc) is related to the trapped sand plug height (Hplug). Teh et al. announced that the trapped sand plug height (Hplug) is approximately equal to the height of the sand layer (Hs). Furthermore, there is an experimental formula performed by Lee et al. to calculate the bearing capacity factor (Nc) of a sand layer overlying clay [11].(3)Nc=14HsD+9.5,where D is the diameter of spudcan. The above formula is suitable for the ratio of sand layer over the diameter of spudcan (Hs/D) between 0.21 and 1.12. Comparing the theoretical and numerical results, the modeling results of bearing capacity are close to the theoretical results, as shown in Figure 5.Figure 5
Comparison between theoretical and numerical results of bearing capacity against sand thickness.


## 3. Results

A reduction ratio of extraction resistance is applied to express the proportion that the easing extraction resistance taking up on the bearing capacity. The greater the reduction ratio is, the better reduction effects that the sand layer provides.(4)Reduction Ratio=1−extraction resistancebearing capacity.


### 3.1. Different Sand Dilation Angles

The different dilation angles of sand, with constant sand thickness (0.25D) and penetration depth (1D) which are indicated with the diameter of the spudcan (14 m), were considered. The bearing capacity can be calculated by using the reaction force of the reference point to divide the cross section area of the foundation. Figure6 shows the displacement and pressure curve of the spudcan for different dilation angles. When the sand dilation angle equals to 0°, the bearing capacity (247.2 kPa) and the extraction resistance (116.07 kPa) can be obtained, with the reduction ratio 53.05%. With the sand dilation angle equaling to 5°, the bearing capacity (267.06 kPa) and the extraction resistance (112.82 kPa) can be obtained, with the reduction ratio 57.55%, as shown in Figure 7.Figure 6
Displacement and pressure curves for different dilation angles.Figure 7
Reduction ratio with different sand dilation angles.


### 3.2. Different Sand Thicknesses

For different thicknesses of sand with constant dilation angle (2°) and penetration depth (1D) indicated with the diameter of the spudcan (14 m), the reaction force of the reference point can be obtained. Figure 8 indicates the results of bearing capacity and extraction resistance for different thicknesses of sand. The extraction resistance decreases dramatically, while the bearing capacity increasing with the thickness of sand. The reduction ratio increases with the sand thickness from 42.7% with sand thickness equaling to 1D to 69.26% with 0.75D sand thickness, as shown in Figure 9.Figure 8
Combined displacement and pressure curves for different sand thicknesses.Figure 9
Reduction ratio with different sand thicknesses.


### 3.3. Different Penetration Depth

The models are simulated, with the different penetration depth of spudcan but the constant sand dilation angle (2°) and sand thickness (0.5D). The results of the extraction resistance can be analyzed. When the penetration depth equals to 1D, where D is the diameter of the spudcan, the extraction resistance is 144.38 kPa with 356.37 kPa bearing capacity and the reduction ratio is 59.49%. While the penetration depth increases to 2D, the extraction resistance raises slightly to 270.61 kPa and the reduction ratio is 58.30%. Displacement and pressure curves for different penetration depth are shown in Figure 10; reduction ratio with different penetration depth is shown in Figure 11.Figure 10
Displacement and pressure curves for different penetration depth.Figure 11
Reduction ratio with different penetration depth.


## 3.1. Different Sand Dilation Angles

The different dilation angles of sand, with constant sand thickness (0.25D) and penetration depth (1D) which are indicated with the diameter of the spudcan (14 m), were considered. The bearing capacity can be calculated by using the reaction force of the reference point to divide the cross section area of the foundation. Figure6 shows the displacement and pressure curve of the spudcan for different dilation angles. When the sand dilation angle equals to 0°, the bearing capacity (247.2 kPa) and the extraction resistance (116.07 kPa) can be obtained, with the reduction ratio 53.05%. With the sand dilation angle equaling to 5°, the bearing capacity (267.06 kPa) and the extraction resistance (112.82 kPa) can be obtained, with the reduction ratio 57.55%, as shown in Figure 7.Figure 6
Displacement and pressure curves for different dilation angles.Figure 7
Reduction ratio with different sand dilation angles.


## 3.2. Different Sand Thicknesses

For different thicknesses of sand with constant dilation angle (2°) and penetration depth (1D) indicated with the diameter of the spudcan (14 m), the reaction force of the reference point can be obtained. Figure 8 indicates the results of bearing capacity and extraction resistance for different thicknesses of sand. The extraction resistance decreases dramatically, while the bearing capacity increasing with the thickness of sand. The reduction ratio increases with the sand thickness from 42.7% with sand thickness equaling to 1D to 69.26% with 0.75D sand thickness, as shown in Figure 9.Figure 8
Combined displacement and pressure curves for different sand thicknesses.Figure 9
Reduction ratio with different sand thicknesses.


## 3.3. Different Penetration Depth

The models are simulated, with the different penetration depth of spudcan but the constant sand dilation angle (2°) and sand thickness (0.5D). The results of the extraction resistance can be analyzed. When the penetration depth equals to 1D, where D is the diameter of the spudcan, the extraction resistance is 144.38 kPa with 356.37 kPa bearing capacity and the reduction ratio is 59.49%. While the penetration depth increases to 2D, the extraction resistance raises slightly to 270.61 kPa and the reduction ratio is 58.30%. Displacement and pressure curves for different penetration depth are shown in Figure 10; reduction ratio with different penetration depth is shown in Figure 11.Figure 10
Displacement and pressure curves for different penetration depth.Figure 11
Reduction ratio with different penetration depth.


## 4. Discussion

The soil deformation can explain the reason of why adding an extra sand layer above clay can ease the extraction resistance of spudcan. A sand plug under the spudcan and two sand columns above the spudcan are then formed. Because sand has lesser suction force compared to clay, it reduces the force required to pull out the spudcan foundation [12–14]. For different sand dilation angles, the extraction resistance is much less than the bearing capacity, which means adding an extra sand layer above clay can effectively ease the extraction resistance of spudcan, and the greater dilation angle gives the better reduction effects. Even though a thick sand layer can produce greater extraction resistance, the reduction ratio raises with the increasing of the sand thickness, which means thick sand layer has better reduction effects. It is clear to see that the penetration depth does have effects on the bearing capacity and extraction resistance [15, 16]. The spudcan penetrates deeper; the greater bearing capacity and extraction resistance can be obtained. However, the reduction ratios of the extraction resistance for different penetration depth are similar, so the reduction effects would not be influenced by the penetration depth of the spudcan.


## 5. Conclusions

Adding an infinite sand layer above the clay, penetrating and extracting procedure of the spudcan with a diameter of 14 m was studied by using numerical models. The bearing capacity and extraction resistance were investigated through the CEL method, and the reaction force of the spudcan was reflected by applying the displacement on the reference point of the spudcan. A three-dimensional configuration was used. The effects of different sand dilation angles, sand thickness, and spudcan penetration depth were investigated. A reduction ratio was applied to parameterize the effects of reducing the extraction resistance by adding an extra sand layer. The major findings of the study are as follows:(1)
Sand with different dilation angles was considered. The greater dilation angle can give better reduction effects.(2)
For different thicknesses of sand, the reduction ratio increases with the increasing of sand thickness, which means the thicker sand layer gives better results of easing extraction resistance.(3)
The bearing capacity and extraction resistance increase with a deeper penetration of the spudcan, but the reduction effects of the sand layer do not have much difference for the different penetration depth.



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*Source: 9614870-2022-07-15.xml*