Centrifuge model study on spudcan footprint interaction 3

Chapter – Literature Review Chapter Literature Review 2.1 Introduction Owing to complexity of the problem and relatively few publications in the public domain, the issue of spudcan-footprint interaction is still not wellunderstood. This chapter reviews the literatures on spudcan-footprint interaction including the existing guideline (SNAME, 2002a, b), conceptual soil-structure interactions, experimental investigations, numerical simulations and industry adopted precautious/mitigation measures. Summary of the literature review on spudcan-footprint interaction (S-F-I) is tabulated in Table 2.1. As the interaction may be significantly affected by the footprint features which are directly related to the spudcan foundation behaviour of the first installation and also the soil conditions for both intact ground and after the footprint is formed, the later part of this chapter reviews the soil failure mechanisms of spudcan penetration and extraction. Penetrometers, particularly T-bar and ball, which are used to measure the soil shear strength profile, will also be reviewed. 2.2 Guideline - SNAME 2002 SNAME (2002a) considered the footprint problem in two operational sequences as follows: 18 Chapter – Literature Review i) Use an identical jackup design (same footing geometries and leg spacing) and locate it in exactly the same position as the previous rig. ii) However, it is unlikely that two jack-up units have an identical design as the structures of most units are often custom-made and the deployment of units is subject to availability. In this situation, SNAME (2002a) suggests to carefully position the jackup on a new heading, or/and with one footing located over a footprint and the others in virgin soil, to alleviate the potential for spudcan sliding. In the commentaries (SNAME, 2002b), a general guideline on the acceptable minimum distance of one spudcan diameter (measured from the edge of the bearing area to the edge of footprint) is recommended. However, footprint features due to various factors (such as soil type, penetration depth and time) are not clearly defined in the guideline. When and where the spudcan-footprint interaction becomes problematic to new rig installation still remain unidentified. If it is not possible to avoid spudcan-footprint interaction, the guideline suggests infilling the footprints with imported materials.The material selection should recognize the potential for material removal by scour, and differences in material stiffness. No further detail is given in the guideline. The suitability and effectiveness of infilling to ease the footprint problem in various soil conditions are still questionable. 19 Chapter – Literature Review 2.3 Spudcan-footprint interaction 2.3.1 A single leg When a spudcan penetrates into the seabed, there are two force components that are equal in magnitude but acting in opposite direction, so that the whole system is in equilibrium. These two force components are the soil bearing resistance and the preload from the structure. Hence, the interaction between a new spudcan installation and old footprint is not merely dependent on soil conditions but also the manner the structure interacts with it. If the jackup leg is fully restrained where no lateral movement is permitted, the spudcan would tend to penetrate vertically and large lateral loads will develop. On the other hand, if the jackup leg is fully flexible where no lateral movement is resisted, the spudcan will tend to slide into the adjacent footprint (Stewart & Finnie, 2001). The leg penetration responses with fully restrained and fully flexible at the leg are illustrated schematically in Fig. 2.1. In reality, the jackup leg-hull connections are neither fully rigid nor fully flexible and that makes the problem complex. The interrelation between structural response and geotechnical response with these two conditions can be further illustrated by examining the predicted horizontal load-displacement response as shown in Fig. 2.2 (Stewart & Finnie, 2001). The intersection point of the geotechnical and the purely elastic structural response predicts the potential horizontal load that develops in the jackup leg and the corresponding horizontal displacement at the spudcan. This implies that the load imposed on the spudcan is a function of horizontal displacement and vice-versa. Dean & Serra (2004) examined this problem differently. Conceptually, they considered the forces needed to install a spudcan vertically over a 20 Chapter – Literature Review boundary between a region of stronger soil (less disturbed soil) and a region of weaker soil (disturbed soil), see Fig. 2.3. The force needed to push aside the stronger soil is larger than what is needed for the weaker soil. Depending on various factors, a net reaction force inclined to vertical will then be obtained. This force has to be provided by the jackup structure for the spudcan to move vertically or else it will move in an inclined direction and slip towards the weaker soil. They termed the phenomenon as a ‘weak-seeking’ response. On the other hand, if an open cylinder is pushed vertically, a larger reaction force from the stronger soil, producing a resisting moment that must be provided by the jack-up. Else, the footing would rotate towards the stronger soil. They termed this as ‘strong-seeking’ behaviour. The probable spudcan-footprint interaction described above is solely purely based on postulations without being experimentally proven. An important aspect that was not considered in the postulations is that the soil failure mechanism which may alter the foundation behaviour. Both studies assumed the uneven soil bearing resistance dominates the interaction, which may not be the case for footprint problem. As far as the author is concerned, there is very little study on the soil failure mechanism reported in the public domain. 2.3.2 System behaviour Most of the footprint studies were conducted based on a single leg. In reality, a rig typically consists of 3-leg. The legs pass through openings in a barge hull where its deck serves as the platform for drilling equipment and other machinery. The basic function of leg-hull connection is to allow forces to transit between the legs and the hull. Each connection commonly consists of a 21 Chapter – Literature Review pair of upper and lower guides and a jacking system and/or fixation system (Fig. 1.5). Hence, the spudcan-footprint interaction is not only governed by how a single leg interacts with the footprint, but also how the whole rig response to the induced forces and/or displacements. The interaction involving the entire rig is termed as system behaviour. When the system behaviour is accounted for, the footprint does not just interact with a single jack-up leg but instead with the whole jack-up. The structural stiffness is not only dependent on the leg above the spudcan and its connection to the hull, but also the stiffness of the hull, the hull connections to the other legs and their stiffness, lastly foundation stiffness of the other spudcans (Jardine et al., 2001). Randolph et al. (2005) postulated that if the movement of the jack-up structure as a system is discounted, the spudcan loads and displacement measured are potentially significantly lower than those if the system behaviour was accounted for. They recommended full three-legged jack-up experimental studies. However, to-date, no simulation either numerically or experimentally on the system behaviour is reported in the public domain. However, owing to the complexity of the problem due to many parameters are involved, it becomes extremely difficult to study the influence of each parameter accurately if a full jack-up is modelled. 2.3.3 Experimental modelling 2.3.3.1 Effect of offset distance Stewart & Finnie (2001) conducted a series of centrifuge model tests to examine the effect of offset distance of spudcan installations adjacent to footprints. The offset distance is the distance from the spudcan centre to the footprint centre. The tests were performed using the 1.2 m diameter drum 22 Chapter – Literature Review centrifuge at University of Western Australia. Over-consolidated kaolin clay with an undrained shear strength, su of 12 kPa at the mudline to 53 kPa at a depth of 20 m was used. The tests were conducted at 200g with a 60 mm diameter model spudcan (12 m in prototype). The jack-up leg was rigidly fixed to the vertical actuator. A footprint was created by performing spudcan penetration and extraction. The spudcan was then re-penetrated at offset distance 0, 0.25, 0.5, 0.75, 1.0, 1.5 and 2.0 times footing diameter, D. They found that the vertical load was reduced to between 40% and 70% of the load obtained during initial penetration for various offset distances (Fig. 2.4). An important finding was that considerable large horizontal forces developed during re-penetration at the offset distance from 0.5D to 1.0D and a peak of 1.3 MN horizontal force developed at the offset distance of 0.75D (Figs. 2.5(a) & (b)). Cassidy et al. (2009) reported a similar qualitative trend of the behaviour due to the offset distance for spudcan re-penetration. The model spudcan used followed the design of jack-up rig Mod V with a prototype diameter of 18.2 m. The tests were conducted in 250g in clay of lightly overconsolidated profile. The undrained shear strength profile measured using the T-bar was approximated to be 7.5 kPa at the mudline with kPa/m increase with depth. Fig. 2.6 shows the induced maximum horizontal H and moment M loads on the spudcan. They found that the effect of footprint is minimal for offset distance of greater than one and a half spudcan diameters. Among four offset distances tested (D/4, D/2, D, 3D/.2), the induced H and M at an offset distance of D/2 was found to be the greatest. 23 Chapter – Literature Review The above studies had identified a range of critical distance (where relatively high horizontal force and moment are likely to occur) that should be avoided to minimize the risk of sliding into footpritnt. As the same spudcan was used to create the footprint and to perform the spudcan re-penetration, the effect of varying spudcan sizes was not addressed in both studies. 2.3.3.2 Effect of leg stiffness It has been suggested that the leg flexural rigidity would influence the interaction between a spudcan and a footprint (Stewart and Finnie, 2001; Foo et al. 2003; Dean and Serra, 2004). Fig. 2.7 shows a comparison of the equivalent stiffness EI/L3 of four different jack-up rigs over the leg length L by Foo et al. (2003). They pointed out that the equivalent stiffness difference becomes less distinct when a jack-up operates in deeper water with longer leg length. It is apparent that there is no typical leg stiffness and the equivalent leg stiffness is dependent on the length. Foo et al. (2003) postulated that the infinitely stiff leg would take the most bending moment as compared to one that is fully flexible. Cassidy et al. (2009) reported a set of drum centrifuge test results investigating the effect of the leg stiffness on spudcan-footprint interaction. The leg stiffness of two jack-ups, the 116C and the Mod V, was modelled in the centrifuge. Two sets of legs were manufactured for each jack-up type, as shown in Fig. 2.8. The model legs used have prototype stiffnesses ranging from 1.69 × 1011 Nm2 to 2.75 × 1012 Nm2. They found no distinguishable difference in the induced horizontal force, H during spudcan-footprint interaction for the different leg stiffnesses with a similar experimental set-up (rigid connection), see Fig. 2.9. They attributed this surprising result to the 24 Chapter – Literature Review experimental set-up. With a fixed connection between the top of the loading leg and the centrifuge actuator, the displacement and rotational movements were restrained and subsequently reduced the contribution of the leg stiffness to the overall system feasibility. However, this finding may only be applicable to the range of leg stiffnesses and forces that were tested. Beyond these stiffness and loading ranges, the penetration response may be different. 2.3.3.3 Effect of preload Three test results of different preloads for the initial penetration were reported by Cassidy et al. (2009), see Fig. 2.10. The preloads level were 40 MN, 60 MN and 80 MN, corresponding to preload pressure of 154 kPa, 231 kPa and 308 kPa respectively for the Mod V spudcan (prototype footing area of 259.7 m2). The peak horizontal force for reinstallation at offset distance 0.5D from footprint created by preload of 60 MN and 80 MN were very similar, but was significantly less for the 40 MN preload. They accounted this as a reflection of the change in footprint shape for different preloads as a deeper crater was created in a higher preload case. On the other hand, higher moment was induced on the spudcan for the higher preload case. As the undisturbed soil had a linearly increase shear strength profile, the variation in strength of the soil beneath the footprint and the differential in bearing pressures also increased with depth. This may attribute to higher moment was induced in the higher preload case. 2.3.3.4 Effect of seabed irregularities Teh et al. (2006) performed a series of 1g tests to investigate the effects of sloping seabed (30o inclined to the horizontal) and footprint on loads 25 Chapter – Literature Review developed in jack-up leg on sand and clay. Two types of foundation namely spudcan and skirted foundation were used. Similar to Stewart & Finnie (2001), the rig flexibility was not accounted for in the model. They observed a much higher horizontal force for the installation on the footprint than on the slope. In the study on the spudcan and skirted foundation behaviour interaction with footprints in clay, they found that the maximum moment that developed on both spudcan and skirted foundation was comparable, but the moment profile varied with penetration depth (Fig. 2.11). The horizontal loads on these two different foundations acted in the opposite direction. This implies that the overall moment developed at the lower guide of a jack-up might be different. The probable mechanism that caused this behaviour was not discussed in the paper. 2.3.4 Numerical modelling 2.3.4.1 Limit equilibrium method Stewart & Finnie (2001) modelled the spudcan-footprint interaction using a simple analytical technique: slope stability coupled with bearing capacity solutions. Footprint was modelled as a steep-sided crater with uniform shear strength. Differential earth pressures due to non-uniformly embedment were estimated using the equation presented by Rowe & Davis (1982). Loads developed at the spudcan due to the collapse of crater wall were estimated assuming limiting equilibrium on a simplified prescribed failure plane extending from the crater toe up to the spudcan edge. They claimed that the results are loosely consistent with what were measured in the centrifuge tests. The authors assumed the induced loads on the spudcan were due to the collapse of the crater wall. This may not be the case as depending on the soil 26 Chapter – Literature Review types, the footprint crater may more likely be a sloping ground rather than a steep-sided wall. In addition, the authors ignored the effect of the nonuniformity of the shear strength profile of a footprint. Hence, the validity of this simulation approach is in question. 2.3.4.2 Finite element simulation Jardine et al. (2001) conducted numerical study on jackup penetration into infilled footprint craters on layered cohesive material. Two infill types namely medium dense sand and dense gravel were used. An idealized structural model for plane strain model was adopted where the spudcan was considered as an infinitely long prism, shown in Fig. 2.12. The footprint was modelled as vertical sides with firm base (Fig. 2.13(a)). This is not a realistic feature as sloping upper sides are more common as observed in the field. Separate finite element runs with a mesh representing a different stage of penetration were performed (Fig. 2.13(b)). The initial shear strength was downgraded by 7% and used as su for the modeled footprint. Three sets of leg stiffness were also studied. They found that the vertical capacity was significantly reduced by 20%, and the full capacity could only be achieved with sustaining relatively large lateral leg movement, leg forces and leg bending moment. The result also revealed that the developed leg forces and bending moment were a function of leg stiffness where the leg forces increase systematically with leg stiffness. They pointed out the difference in stiffness between the infilled material and the subsoil would also greatly affect the effectiveness of this mitigation method. Jardine et al. (2002) presented an updated version of the analysis in which the finite element mesh employed was further refined. With the improved analysis, it was found that the composite foundation capacity 27 Table 2.1 Summary of the literature review on spudcan-footprint interaction (S-F-I) Area Guideline Literature SNAME (2002a & b) Description General recommendations on rig positioning and mitigation method Concept Stewart and Finnie (2001) Postulation S-F-I based on the structural rigidity Dean and Serra (2004) Postulation S-F-I based on the forces required to install footings Experimental study Numerical simulation Mitigation/preventive measures Jardine et al. (2001) & Randolph et al. (2005) System behaviour Stewart and Finnie (2001) Effect of offset distance Cassidy et al. (2009) Effect of offset distance, leg stiffness, preload Teh et al. (2008) Seabed irregularities; skirted footing and spudcan Jardine et al. (2001 and 2002) FE modelling on infilling crater with imported granular material on clayey soil Grammatikopoulou et al. (2007) Jardine et al. (2001) and MSL (2004) FE modelling on infilling craters with granular material and combined crater infilling with capping gravel loading platforms Brief discussions on stomping Foo et al. (2003) RPD monitoring Osborne et al. (2006) Brief introductions on the industry adopted methods including reaming, leg ‘ploughing’, seabed excavation, swiss-cheessing etc. Chapter Literature Review Fig. 2.1 Predicted movement of spudcan penetration with two extreme jackup leg connections (after Stewart & Finnie, 2001) Fig. 2.2 Horizontal load-displacement response (after Stewart & Finnie, 2001) 39 Chapter Literature Review (a) Spudcan (b) Open cylinder Fig. 2.3 Aspects of resisting forces from soil for footings moving vertically with strong/weak lateral soil variability (after Dean & Serra, 2004) Fig. 2.4 Measured vertical load at various offset distances. Offsets are expressed as multiple of spudcan diameter (after Stewart & Finnie, 2001) 40 Chapter Literature Review (a) (b) Fig. 2.5 a) Measured horizontal load at various offset distances; b) Summary of peak horizontal forces (after Stewart & Finnie, 2001) 41 Chapter Literature Review Fig. 2.6 Maximum horizontal and moment load on Mod V spudcan during reinstallation (after Cassidy et al., 2009) Fig. 2.7 Equivalent jack-up stiffness over leg length (after Foo et al., 2003) 42 Chapter Literature Review Fig. 2.8 Four model loading legs (after Cassidy et al., 2009) Fig. 2.9 Measured horizontal loads at spudcan for four tests of varying leg stiffness (after Cassidy et al., 2009) 43 Chapter Literature Review Fig. 2.10 Maximum H and M on Mod V spudcan for different preloads (after Cassidy et al., 2009) Fig. 2.11 a) V, b) H and c) M/2R profiles of spudcan and skirted foundation in clay (Teh et al., 2006) 44 Chapter Literature Review Fig. 2.12 Idealised structural model for plain strain analysis (after Jardine et. al., 2001) (a) (b) Fig. 2.13 a) Soil layering; & b) finite element mesh (after Jardine et. al., 2001) 45 Chapter Literature Review (a) (b) Fig. 2.14 Examples of geometry for a) infilled crater and b) infilled crater combined with gravel loading platform (after Grammatikopoulou et al., 2007) 46 Chapter Literature Review Fig. 2.15 Process of stomping (after Jardine et. al. 2001) Fig. 2.16 Schematic view of RPD effect (after MSL, 2004) 47 Chapter Literature Review (a) (b) (c) Fig. 2.17 Soil flow mechanisms from top view of centrifuge test; a) Surface foundation; b) soil back flow; c) deep foundation (after Hossain et. al. 2004) (a) (b) (c) Fig. 2.18 Soil failure mechanisms from FE analysis; a) soil heave; b) soil back flow and c) localized flow (after Hossain et. al. 2004) 48 Chapter Literature Review Fig. 2.19 Schematic diagram showing spudcan extraction mechanism in soft clay (after Purwana, 2007) 49 Chapter Literature Review Fig. 2.20 Strength measurements within and near to a footprint (after Siciliano et al., 1990) Fig. 2.21 Strength measurements within and near to a footprint (after Stewart, 2005) 50 Chapter Literature Review Fig. 2.22 Alternative full-flow penetrometers with cone (after Randolph et al., 2005) Fig. 2.23 Bearing capacity factor for T-bar with interface roughness (after Randolph & Houlsby, 1984) 51 Chapter Literature Review 0.2 0.4 Interface friction angle,  Fig. 2.24 Bearing capacity factors for ball and T-bar with interface roughness (after Randolph et al., 2000) (a) (b) Fig. 2.25 Penetration responses from centrifuge tests at 100g: a) normally consolidated kaolin; b) heavily overconsolidated kaolin (after Watson, 1999) 52 Chapter Literature Review Fig. 2.26 Penetration resistance profile of constant rate (1 mm/s) for various shaped model penetrometers (after Chung et al., 2006) Fig. 2.27 Penetration resistance profile of constant rate (1 mm/s) for model Tbars with various aspect ratios (after Chung et al., 2006) 53 . literatures on spudcan-footprint interaction including the existing guideline (SNAME, 2002a, b), conceptual soil-structure interactions, experimental investigations, numerical simulations and industry. soil conditions are still questionable. Chapter 2 – Literature Review 20 2 .3 Spudcan-footprint interaction 2 .3. 1 A single leg When a spudcan penetrates into the seabed, there are two force components that. observed a much higher horizontal force for the installation on the footprint than on the slope. In the study on the spudcan and skirted foundation behaviour interaction with footprints in clay,