Measured and calculated spudcan penetration profiles for case histories in sand-over-clay
Introduction
Spudcan-supported jack-up rig installation and preloading in stratified seabed deposits which contain a strong layer of sand over a weak clay layer remains problematic within the offshore industry [[1], [2], [3]]. Punching the sand layer into the weaker clay can cause a rapid penetration, potentially buckling the leg and even toppling the jack-up. Layered soils are prevalent in the Sunda Shelf offshore Malaysia, Australia’s Bass Strait and North-West Shelf, the Gulf of Thailand, the South China Sea, offshore Bombay High, the Persian Gulf, and even the Gulf of Mexico [4,5].
This background has motivated a number of researchers to explore spudcan penetration in two-layer sand-over-clay soils and multilayer soils with interbedded sand layers [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] and this has led to new mechanism-based design approaches for assessing spudcan penetration resistance [21,22,15,[23], [24], [25]]. Synthesis of centrifuge model test data and large deformation finite element (LDFE) analysis results have underpinned these new approaches with measured data from jack-up installations in the field rarely reported in the public domain. This hinders an immediate evaluation of the performance of these methods in estimating the spudcan penetration resistance in offshore sand-over-clay deposits. As such, the recently finalised version of ISO guidelines 19905-1 [26] still recommends the ‘punching shear’ [6] and ‘projected area’ [27] methods (see review in [28]). The approach by Teh et al. [21] was suggested as an alternative method.
The aim of this paper is to present five case histories of jack-up installations in soil profiles with a dominant sand-over-clay strata. Retrospective estimation of the spudcan load-penetration profiles at each site, calculated using the ISO methods, the mechanism-based approach of [[23], [24], [25]] and computed using three-dimensional large deformation finite element (LDFE) analyses, are then compared and the lessons learnt are discussed. Only the main methods (punching shear and projected area) suggested in ISO, which are commonly used by the industries, were considered.
Five case histories were collected from different locations in the Gulf of Mexico, as summarised in Table 1. The field data are reported here for the first time. The soils were two- and four-layer deposits (within the depth of interest) consisting of alternating layers of loose to very dense fine silica sand and soft to stiff clay with an undrained shear strength increasing with depth. The documentation of these case histories includes (a) site-specific soil information, (b) accurate spudcan penetration records (excluding the unload-reload steps), and (c) well-documented spudcan geometries and applied loading. The full details of the soil profiles and spudcan geometries are given in Table 1, Table 2.
At each site, a single boring was drilled and sampled by driving thin-wall 2.25-inch (57.15 mm) diameter Shelby tubes and pushing 3.0-in tubes. The samples were wireline driven with a slide-type (165 pounds or 74.84 kg) hammer on a winch that can free fall 5 ft (1.52 m). For clay, the offshore and onshore laboratory tests carried out on the cored samples included the use of (a) pocket penetrometer (PP), (b) torvane, (c) remote vane, (d) miniature vane, and (e) unconsolidated undrained triaxial (UU) tests. In this study, the undrained shear strength, su, profiles are adequately represented using a trend line that increases linearly with depth, which can be expressed aswhere sum is the undrained shear strength at the sand-clay interface and k is the rate of increase in su with depth z. The selection of the design undrained shear strength su was based on a statistical method [29,4], with the best linear profile obtained aswhere
i = 1,…., n is the total number of data points; the design strength profiles for all cases are noted in Table 1. The clays have a sensitivity, St, between 1.5 and 3.5, which are derived from the remoulded tests on the cored samples.
For the sand layers, the relative densities were calculated based on blow counts from the hammer sample. The blow counts from the offshore wireline thin-wall Shelby tube sampler were converted to an SPT N60 (blows for 300 mm) value by using a ratio of 1:2 (based on local experience). The N60 values were used to calculate the relative densities of the fine sandy soils using the methods proposed by Terzaghi and Peck [27], Skempton [30] and Kulhawy and Mayne [46]. The equations and derived average relative densities are listed in Table 3, showing consistent estimation.
The friction and dilation angles were then determined using the general strength-dilatancy framework established by Bolton [31]where ID is the relative density of the sand; IR is the dilatancy index, p′ is the mean effective stress, and ϕ′ and ψ are the friction and dilation angles of the sand, respectively. The values are summarised in Table 1.
Two different types of spudcan geometry, hexagonal Marathon LeTourneau Design Class 82-SDC (MLT 82-SDC) and dodecagonal Marathon LeTourneau Design Class 116-C (MLT 116-C), were used for the selected case histories. Fig. 1 shows schematic diagrams of the spudcans, and the (plan area) equivalent diameter, plan area, volume, lightship-plus-variable load, maximum preload, and maximum bearing pressure (under the maximum preload) applied during preloading and installation are presented in Table 2. The spudcan (plan area) equivalent diameters, D, are 12.1 and 13.5 m, and the bearing pressures range from 309.5 to 390.0 kPa.
Section snippets
Analysis details
Three-dimensional LDFE analyses were undertaken to simulate the continuous penetration of the spudcans into the seabed surface, providing a complete penetration resistance profile. Spudcan foundation shapes matching the real geometry (including cut-outs, see Fig. 1) were considered. The analyses were performed using the Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit. The spudcan and soils were discretised using Lagrangian and Eulerian meshes,
Methods
In this study, the two main ISO methods, (a) the load spread method [26,27] and (b) the punching shear method [6,26], and one mechanism-based design approach [[23], [24], [25]] were considered. These methods are discussed briefly below with the full details tabulated in Appendix.
Comparison among measured field data, estimation and LDFE results
Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6 plot the reported spudcan penetration data for Sites 1∼5 (Table 1, Table 2), estimations obtained using the design methods, and the results from the 3D LDFE analyses. For the ISO [26] design methods, the estimations from the load spread method with ns values of 3 and 5 were made and are presented together with those from the punching shear method. The ISO guideline recommends the use of the expression for Nc given in Houlsby and Martin [42], so it was
Concluding remarks
Three different methods for estimating the penetration resistance of spudcans in sand overlying clay soils have been compared with high-quality data from field installations. A brief summary was provided about three design methods, comprising the two ISO [26] methods and the mechanism-based design approach of [[23], [24], [25]]. Five case histories were considered, all from different locations in the Gulf of Mexico, where the soils were relatively homogeneous, predominantly sand on top of
Acknowledgements
The research presented herein was undertaken in collaboration with Fugro-McClelland Marine Geosciences, Inc., Houston, Texas; and with support from the Australian Research Council (ARC) through the Linkage Project LP140100066. This work forms part of the activities of the Centre for Offshore Foundation Systems (COFS), currently supported as a node of the Australian Research Council Centre of Excellence for Geotechnical Science and Engineering and as a Centre of Excellence by the Lloyd’s
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