Centrifuge model study on spudcan footprint interaction 5

Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction Chapter Footprint Characteristics and Their Influence on Spudcan-footprint Interaction An extensive series of centrifuge model tests was carried out to investigate the spudcan-footprint interaction problem in clays of various strengths. The first part of this chapter focuses on identifying the footprint characteristics, whereas the second part is to investigate the effects of these footprint characteristics on the new spudcan installation. The spudcan-footprint interaction is evaluated in terms of horizontal force and moment profiles acting on the spudcan during installation. Half spudcan tests were also conducted to observe the soil failure mechanism during the initial penetration, extraction and the spudcan repenetration at 0.5 times spudcan diameter offset from the initial site. The results will be presented in this chapter. 4.1 Footprint definition As discussed in Section 1.3, spudcan footprints are referred to ground conditions that experience:  Changes in physical profile of the seabed (existence of depression) and  Changes in soil properties (shear strength of soils beneath a footprint is highly non-uniform). 99 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction The above footprint characteristics can significantly influence the manner in which a spudcan interacts with it. The footprint characteristics are dependent on factors such as spudcan shape and size, soil type and strength, previous spudcan penetration, and the elapsed time after previous spudcan extraction. Spudcan-footprint interaction can occur in a wide range of soil conditions (Osborne et al., 2006). In this chapter, the footprint characteristics and the mechanism of interaction between spudcan and existing footprint in clay with different shear strength profiles were investigated using centrifuge modelling technique. Details of the centrifuge test results and practical implications of the findings are reported in this paper. 4.2 Test programme 4.2.1 Formation of a spudcan footprint All the centrifuge model tests were carried out at 100g in the National University of Singapore geotechnical centrifuge. The tests were conducted using Malaysia kaolin clay with the physical properties as shown in Table 3.5. Details of the experimental model set-up and soil sample preparation can be found in Chapter 3. As the present study aims to investigate the soil responses in the spudcan-footprint interaction in detail, only a single leg with fully rigid connection was modelled. A 100 mm diameter model spudcan was used. The spudcan has an 11o base angle and an 80o conical tip. A schematic of the spudcan and the model leg (Leg 1) with instrumentation details are given in Section 3.3.1. The leg consists of a thin walled circular hollow section with a prototype length of 28 m and a flexural stiffness of 1.18×1012 Nm2 at 100g. This flexural stiffness value is similar to Jack-up 116-C (Global Maritime, 2003). The model leg is rigidly connected to the spudcan and the other end is 100 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction rigidly connected to the loading actuator. The leg was instrumented with levels of axial strain gauges and levels of bending gauges. During a test, the loads on the spudcan were measured from the strain gauges at second interval. To form a footprint, the spudcan was first penetrated into the model ground at the desired location at a penetration rate of mm/s. Based on the velocity group parameter proposed by Finnie (1993), undrained condition for the penetration process was preserved at this penetration rate. The spudcan continued penetrating until the desired preload pressure was achieved. The required preload level in the field is dependent on several factors such as number of footings, size of footing, environmental loadings and soil bearing resistance. For earlier rigs, the average bearing pressures on soil beneath the spudcan lie between ranges of 200 to 350 kPa (Le Tirant, 1979). For a modern jack-up, maximum leg loads can be higher than 140 MN that produces average vertical bearing pressures in excess of 400 kPa for a fully embedded spudcan (Randolph et al., 2005). Some rigs have considerably higher preload pressures ranging from 575 to 960 kPa (Poulos, 1988). In general, there has been an increase in the maximum vertical installation bearing stresses from a range of 200 to 400 kPa, to around 400 to greater than 600 kPa for the popular rig classes (Osborne et al., 2006). The maximum preload pressure used was 460 kPa in all the tests. This is deemed to fall within the normal preload pressure. The spudcan was then withdrawn immediately at a rate of mm/s and a footprint was created. This is to simulate the scenario where a rig is engaged for a very short operational period. Depending on the type of work engaged, the operational period may range from a week to up to year. Time is an 101 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction important variable in this problem. This is particularly the case for footprints formed in fine grained soils, where a re-consolidation process can occur. The effect of time on spudcan-footprint interaction will be studied in Chapter 5. 4.2.2 Evaluation of spudcan-footprint interaction As the leg-hull connection was modelled as fully rigid, where no lateral displacement and rotational movement are allowed, the spudcan-footprint interaction is evaluated in term of three major ‘resultant’ load components (vertical force, V, horizontal force, H and moment, M) acting at the spudcan load reference point (L.R.P.). These forces are computed based on the strain gauge measurements on the leg using the formulae shown in Appendix A. 4.2.3 Evaluation of soil condition – Ball penetrometer test For the tests presented in this chapter, a miniature ball penetrometer consisting of an 11.9 mm diameter ball connected to a narrow shaft was used to evaluate the undrained shear strength profile of the clay prior to spudcan penetration and after the formation of the footprint. After several trials, the ball penetrometer was found to provide comparable but more stable readings than the commonly used T-bar for the measurements of shear strength profile of a footprint. This is because the soil profile within a footprint is highly variable. This results in bending of the T-bar owing to asymmetrical resistance, and this affects its measurements. In this situation, the axisymmetric ball penetrometer has an advantage over the T-bar as it is less susceptible to bending. The ball was installed at a penetration rate of mm/s. Such a rate is sufficiently fast to maintain the penetration process under undrained condition based on the velocity group parameter proposed by Finnie (1993).The 102 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction undrained shear strength profile of the soil can be obtained by measuring the penetration resistance of the ball while penetrating into the clay (Randolph et al., 2000, Chung et al., 2006). Following Watson et al. (1998), the bearing capacity factor for the ball penetrometer, Nball, is taken as 10.5. 4.2.4 Experiment procedure After the clay sample achieved at least 90% degree of consolidation, ball penetrometer test was conducted in-flight at a position far from the intended footprint location to measure the original undisturbed shear strength profile of the clay. The movable platform was shifted to different locations such that ball penetrometer tests could be conducted at various offset distances from the centre of the footprint, see Fig. 4.1. Upon completion of shear strength measurements, spudcan re-penetration was then immediately performed at a rate of mm/s at the desired location. Throughout the spudcan installation, extraction and re-installation, all the strain gauges were monitored at second intervals. 4.3 Experimental results and discussions 4.3.1 Shear strength profiles and spudcan penetration depths Four centrifuge model tests, namely tests CS_1 to CS_4, were conducted in clay with various shear strength profiles. In all tests, the spudcan used has a prototype diameter D of 10 m. The ball penetrometer test results obtained prior to spudcan installation reveal that the undrained shear strength, su, increases almost linearly with depth for the four clay samples (Fig. 4.2). A summary of the su profile presented in the form of (sum+kz) where sum is the shear strength at mudline in kPa, k is the gradient of shear strength profile in kPa/m and z is 103 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction the depth from the mudline in m, is given in Table 4.1. Among the four clay samples, test CS_1 has the strongest shear strength profile representing firm clay while test CS_4 has the weakest soil strength profile representing soft clay. Unless otherwise stated, all the test results presented hereinafter are in prototype units, which are derived using appropriate scaling laws (Taylor, 1995) from the model units. In the present study, a systematic sign convention as recommended by Butterfield et al. (1997) is adopted where downward vertical force, V, is positive, horizontal force towards footprint, H, is positive and clock-wise moment, M, is positive, see Fig. 4.3 for these forces relative to the footprint position. All the load components (V, H and M) and penetration depth presented in this report are those acting at the spudcan load reference point shown in Fig. 4.3. The preload pressure, q for spudcan initial and repenetration can be obtained by the following relationship: Preload pressure, q = V/A (4.1) where V is the measured vertical load and A is the largest bearing area of the spudcan, which is 78.5 m2. On the other hand, though M and H at the reference point cannot be measured directly from the experiment but they are evaluated through extrapolation of the bending moments measured by bending gauges instrumented at the spudcan legs using the equations derived in Appendix A. For all cases, the spudcan penetration was terminated when the vertical soil bearing resistance reached 460 kPa. For the same spudcan, the final spudcan penetration depth is dependent on the soil strength. The preload pressure-displacement curves during spudcan penetration and extraction for 104 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction tests CS_1 to CS_4 are shown in Fig. 4.4. It should be noted that a penetration of m refers to the widest spudcan section is right at the mudline. The final penetration depth of the initial spudcan installation, is about m, m, m and 13 m for tests CS_1, CS_2, CS_3 and CS_4, respectively. 4.3.2 Characteristics and physical profile of footprint Spudcan extraction was performed immediately (within seconds in model time) after spudcan installation. A miniature camera was mounted on the container to capture the movement of the surface soil during spudcan installation and re-installation. Unfortunately, the footprint characteristics for Test CS_3 could not be captured due to problem with the camera. Fig. 4.5a reveals that the footprint formed in test CS_1 having an initial penetration depth of m is about one spudcan diameter wide with an almost vertical cylindrical circumference and a relatively less disturbed base. A number of tension cracks were observed around the circumference of the footprint. Fig. 4.5b shows that the footprint from test CS_2 (do = m) consists of a bowlshaped depression with its deepest point of about 2.3 m at the centre. The size of the circular depression of heavily disturbed soil is about 1.6 spudcan diameters. For the footprint formed in test CS_4 (Fig. 4.5c) with = 13 m, its diameter is about spudcan diameters and its deepest point at the centre is almost m deep. Soil heave along the footprint periphery is clearly visible. Within the depression, the soil is heavily remoulded and the highly irregular and ‘lumpy-like’ soil surface indicates that the soil is rather soft. 105 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction 4.3.3 Soil failure mechanism during spudcan penetration and extraction The observations in Section 4.3.2 showed that the seabed profile of a footprint varies with the penetration depth under the same preload pressure. To investigate how the spudcan penetration and extraction alter the footprint profile, two half-spudcan tests, namely tests CS_1A and CS_2A were conducted to observe the soil movement on soil having similar shear strength profile as tests CS_1 and CS_2. Unlike tests CS_1 and CS_2 which employed a reduced-scale full spudcan, only one half of the full spudcan was employed in tests CS_1A and CS_2A with the face of the half-cut spudcan placed behind a transparent Perspex window of a rectangular model container. The clay sample was textured with black flocks and beads and a digital camera was placed in front of the Perspex window to capture high resolution photographs throughout the tests. The photographs were analysed using Particle Image Velocimetry technique employing GeoPIV8 software (White & Take, 2002; White et al. 2003). The detail of the half-spudcan test set-up was presented in Chapter 3. The soil movement patterns at various selected stages of penetration and extraction shown are in the forms of velocity fields and the corresponded normalized velocity contours. The velocity field represents the increment of resultant displacements taken from a pair of sequential images at sec interval. The normalized velocity contours indicate the ratio of the soil displacements to the spudcan movement, which was mainly in the vertical motion. The major displacement field is defined as soil displacements equal to or greater than 10% (or ratio 0.1 as shown in velocity contour plots) with respect to the 106 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction spudcan movement. The results presented have a physical dimension in a prototype unit of meter. 4.3.3.1 Spudcan penetration in undisturbed ground – test CS_2A The soil movement during the spudcan penetration at various penetration depths are presented in a pair of velocity vector and velocity contour plots and shown in Figs. 4.6 and 4.7. Only the results obtained from test CS_2A is reported in this section. The surface soil movements of the entire penetration process were captured in the full spudcan test (test CS_2) and are presented in Figs. 4.10a – d. When the largest surface of the spudcan completely touched the seabed, the soil was pushed downward and outward. The extent of the major soil displacement was about 0.8D (vertical) from the spudcan load reference level and and 1D (lateral) from the spudcan centre. As the spudcan continues penetrating to about m, some minor crack lines began to appear on the soil surface (see Fig. 4.10b). When the spudcan continued the penetration below m, soil beneath the spudcan was continuously being pushed outward and downward with substantial volume of soil being brought down together with the advancing spudcan. The major vertical soil displacement extended to about 1D while the radial lateral extent was still confined within 0.8 – 1D. A stable open cavity on top of the spudcan was formed without significant soil backflow over the spudcan penetration depth. At this stage, more crack lines were developed on the surface and the crack width enlarges with the penetrating spudcan (see fig. 4.10c & d). As in an undrained condition, no change in volumetric strain, the soil surrounded the spudcan’s circumference experienced heaving to accommodate the displaced soil mass of the equal 107 Chapter – Footprint Characteristics and Their Influence on Spudcan-footprint Interaction volume of the cavity and the spudcan. The heaving motion induced tensile force on the soil and resulted in the development of tension cracks. Until penetration depth of m as the desired preload pressure of 460 kPa was achieved, a cavity on top of the spudcan remained stable with substantial cracks formed (see Figs. 4.9 and 4.10d). 4.3.3.2 Spudcan extraction – test CS_2A At the initial stage of extraction, soil beneath the spudcan experiences substantial rebound and moved upwards with the extracting spudcan (see Fig. 4.11). The tension cracks width decreased with the uplifting movement of the spudcan, as observed in Fig. 4.15a. Substantial upward movement of the underlying soil continued until the onset of spudcan breakout (Fig. 4.12) at depth of 2.1 m or a spudcan uplift movement of 2.9 m. At this stage, the crack lines were closed up as observed in Fig. 4.15b. After spudcan breakout, uplifting of the underlying soil was less dominant with more soil from the sides moving inward. After another small uplift displacement of 0.l m, the separation of soil from the spudcan base was observed, see Fig. 4.13. From this stage onwards, the soil movement only involved soil flow from the sides to beneath the base of spudcan. Fig. 4.15c shows a circular crack of about 1.6D formed at the soil surface when the spudcan is at 1.9 m. Shortly after the spudcan was fully extracted from the model ground (see Fig. 4.14 and 4.15d), an abrupt circular slide was observed to occur where the soils at the sides slid towards the depression with the creation of a 2.3 m deep bowl-shaped depression. After spudcan extraction, soil upheaval was clearly observed surrounded the footprint circumference, as shown in Fig. 4.14. 108 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector c) Velocity contour Fig. 4.13 Soil movement velocity soon after spudcan breakout [depth = 1.9 m] 132 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction Fig. 4.14 Side elevation of footprint after spudcan extraction for test CS_2A a) m (at initial stage of up-lifting) b) 2.1 m (at breakout) c) 1.9 m (collapsed of surrounding soil) d) m Fig. 4.15 Top view of soil surface failure mechanism during spudcan extraction at various stages; a) m; b) 2.1m; c) 1.9 m; & f) m 133 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction (a) Initial stage of extraction [depth = m] (b) Spudcan breakout [depth = 1.5 m] 134 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction (c) Side elevation of footprint after spudcan extraction Fig. 4.16 Soil movement vectors during and after spudcan extraction from test CS_1A su (kPa) 20 40 60 su (kPa) 80 100 20 40 60 su (kPa) 80 100 Undisturbed su Footprint (a) 0.56D Depth (m) Depth (m) Depth (m) 40 60 80 100 (b) 0.92D 20 (c) 1.28D Fig. 4.17 Undrained Shear strength su profile at various distances from footprint centre for test CS_1 (D = spudcan diameter) 135 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction su (kPa) 20 40 su (kPa) 60 80 100 20 40 60 80 100 80 100 Undisturbed su Depth (m) Depth (m) Footprint 10 10 12 12 14 14 (a) 0.2D (b) 0.56D su (kPa) 20 40 60 su (kPa) 80 100 Depth (m) Depth (m) 10 12 12 (c) 0.92D 40 60 10 14 20 14 (d) 1.28D Fig. 4.18 Undrained shear strength su profile at various distances from footprint centre for test CS_2 (D = spudcan diameter) 136 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction 20 su (kPa) 40 60 80 20 su (kPa) 40 60 80 20 su (kPa) 40 60 80 Footprint Depth (m) Depth (m) 10 Depth (m) Undisturbed su 10 10 12 12 12 14 14 14 16 16 16 18 18 (a) 0.2D 18 (b) 0.56D (c) 0.92D Fig. 4.19 Undrained shear strength su profile at various distances from footprint centre for test CS_4 (D = spudcan diameter) Radial Distance/Spudcan Diameter -1.00 -0.75 -0.50 -0.25 0.50 0.75 1.00 1.25 Crater Depth (m) 0.25 0.5 1.0 1.5 2.0 2.5 3.0 -12.5 -10.0 -7.5 -5.0 -2.5 2.5 5.0 7.5 10.0 12.5 Radial Distance (m) (a) Undisturbed (b) Beneath footprint Fig. 4.20 su profile (in kPa) from test CS_1 137 3.5 Depth/Initial Penetration Depth -1.25 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction Radial Distance/Spudcan Diameter -1.25 -1.00 -0.75 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 Crater Su= 0.4 0.8 Depth (m) 1.0 1.2 1.6 10 2.0 12 2.4 -12.5 -7.5 -10.0 -5.0 -2.5 2.5 5.0 7.5 10.0 12.5 Radial Distance (m) (a) Undisturbed (b) Beneath footprint Fig. 4.21 su profile (in kPa) from test CS_2 Radial Distance/Spudcan Diameter -0.75 -0.50 0.25 0.50 0.75 1.00 Depth (m) -0.25 0.15 su = 0.30 0.45 0.6 0.75 10 0.9 12 Depth/Initial Penetration Depth -1.00 1.0 1.05 14 1.2 16 -10.0 -7.5 -5.0 -2.5 2.5 5.0 7.5 10.0 Radial Distance (m) (a) Undisturbed (b) Beneath footprint Fig. 4.22 su profile (in kPa) from test CS_4 138 Depth/Initial Penetration Depth Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction Preload Pressure, q (kPa) -2 0.2 0.4 0.6 0.8 1.2 1.4 Initial penetration Re-penetration 12 16 20 16 20 16 20 16 20 Depth (m) M (MNm) -2 Depth (m) Depth (m) H (MN) -2 100 200 300 400 500 600 (a) Test CS_1 Preload Pressure, q (kPa) -2 H (MN) -2 100 200 300 400 500 600 0.2 0.4 0.6 0.8 1.2 1.4 Depth (m) Initial penetration Re-penetration M (MNm) -2 Depth (m) Depth (m) 12 (b) Test CS_2 Preload Pressure, q (kPa) -2 0.2 0.4 0.6 0.8 1.2 1.4 Depth (m) Depth (m) 10 10 H (MN) -2 100 200 300 400 500 600 0.2 0.4 0.6 0.8 1.2 1.4 Depth (m) 10 10 10 12 12 12 14 14 Initial penetration Re-penetration 12 14 M (MNm) -2 12 (c) Test CS_3 Preload Pressure, q (kPa) -2 Initial penetration Re-penetration M (MNm) -2 10 Depth (m) H (MN) -2 100 200 300 400 500 600 Depth (m) Depth (m) (d) Test CS_4 Fig. 4.23 Measured profiles of preload pressure q; horizontal force H; and moment M for spudcan installation and re-installation 139 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction Idealised new seabed profile after footprint formed C footprint Mudline M H (a) Mudline C footprint M' H' (b) Fig. 4.24 Illustration of spudcan-footprint interaction for shallow penetration in firm clay 140 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction su (kPa) 20 40 60 80 -2 100 Preload Pressure, q (kPa) 100 200 300 400 500 600 Depth (m) Depth (m) CS_1 - Footprint CS_1B - Pre-formed crater CS_1 - footprint CS_1B - Pre-formed crater 10 CS_1B - Pre-formed crater a) Undisturbed su -2 0.4 0.8 1.2 30 40 b) V M (MNm) 1.6 10 15 20 25 Depth (m) Depth (m) V (MN) -2 H (MN) 20 c) H d) M Fig. 4.25 Comparison on VHM between footprint and pre-formed crater 141 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector b) Velocity contour c) Lateral contour d) Vertical Contour Fig. 4.26 Development of soil movements during spudcan re-penetration at 0.5D from footprint centre at m 142 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector b) Velocity contour c) Lateral contour d) Vertical Contour Fig. 4.27 Development of soil movements during spudcan re-penetration at 0.5D from footprint centre at m 143 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector c) Lateral contour b) Velocity contour d) Vertical Contour Fig. 4.28 Development of soil movements during spudcan re-penetration at 0.5D from footprint centre at 2.6 m (depth of Hmax) 144 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector c) Lateral contour b) Velocity contour d) Vertical Contour Fig. 4.29 Development of soil movements during spudcan re-penetration at 0.5D from footprint centre at m (initial penetration depth) 145 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction a) Velocity vector c) Lateral contour b) Velocity contour d) Vertical Contour Fig. 4.30 Development of soil movements during spudcan re-penetration at 0.5D from footprint centre at m 146 Chapter – Footprint Characteristics and their Influence on Spudcan-footprint Interaction C footprint Model Ground 0.5D Depth (m) Crater su = -1.25 -1.00 -0.75 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 Radial Distance/Spudcan Diameter (a) Footprint condition prior to spudcan re-penetration C footprint Mbearing Hbearing Model Ground  e Rs (b) Shallow penetration – soil bearing failure predominates C footprint Model Ground Mslide Mbearing Hbearing Hslide Sliding soil mass Rs1 Sliding failure plane e1 (c) Deep penetration - Sliding failure predominates Fig. 4.31 Simplified diagram of probable soil failure mechanisms during penetration at 0.5D from footprint centre 147 . occur. The effect of time on spudcan-footprint interaction will be studied in Chapter 5. 4.2.2 Evaluation of spudcan-footprint interaction As the leg-hull connection was modelled as fully rigid,. Influence on Spudcan-footprint Interaction 99 C C h h a a p p t t e e r r 4 4 Footprint Characteristics and Their Influence on Spudcan-footprint Interaction An extensive series of centrifuge model. soil shear strength profile on the spudcan re- penetration. 4.3 .5 Spudcan-footprint interaction To investigate the most critical spudcan-footprint interaction situation, the same spudcan was used