Predicting jack-up spudcan installation in sand overlying stiff clay

Abstract

There has been a constant trend towards larger mobile jack-ups capable of operating for extended periods in deeper water and in harsher environmental conditions. This is increasing both the size of their spudcan footings and the operational bearing pressures. Though some new analytical methods to predict the load-penetration profile have been proposed, and shown to fit centrifuge experiments well, these methods were calibrated mainly using experimental and numerical data for sand overlying soft clay. This paper reports six centrifuge tests simulating spudcan installation in sand overlying a stiff clay. These are complemented with large-deformation finite element analyses, simulating the continuous penetration of the spudcan in sand over stiff clay. Modified Mohr-Coulomb and Tresca models were used to describe the sand and clay behaviour, accounting for the effects of strain softening on the response of the soil. These results provide confidence that a recently published analytical method to predict the peak capacity can indeed be extended to high bearing resistances and stronger underlying clays. Bearing capacity factors used to predict capacity in the underlying clay have, however, been updated to reflect the new database of results. A new formulae that explicitly accounts for increasing strength with depth profiles is provided.

Measured and calculated spudcan penetration profiles for case histories in sand-over-clay

“…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 2012methods and the mechanism-based design approach of Hu et al (2015aHu et al ( , 2017Hu et al ( , 2018. 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 normally to lightly overconsolidated clay with the strength increasing essentially linearly with depth.…”

“…In this study, the two main ISO methods, (a) the load spread method (Terzaghi and Peck 1948;ISO, 2012) and (b) the punching shear method (Hanna and Meyerhof 1980;ISO, 2012), and one mechanism-based design approach (Hu et al 2015a(Hu et al , 2017(Hu et al , 2018 were considered.…”

“…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 (Hanna and Meyerhof 1980;Craig and Chua 1990;Teh et al 2008Teh et al , 2010Qiu and Henke 2011;Qiu and Grabe 2012;Tho et al 2012;Hossain et al 2012;Lee et al 2013aLee et al , 2013bHossain, 2014;Hu et al 2014aHu et al , 2016Ullah et al 2017a, b) and this has led to new mechanism-based design approaches for assessing spudcan penetration resistance (Teh et al 2009;Lee et al 2009Lee et al , 2013bHu et al 2015aHu et al , 2017Hu et al , 2018. 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.…”

“…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 loadpenetration profiles at each site, calculated using the ISO methods, the mechanism-based approach of Hu et al (2015aHu et al ( , 2017Hu et al ( , 2018 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.…”

“…It is suggested that the value of this coefficient be obtained from a chart improved from Hanna and Meyerhof (1980). Hu et al (2015aHu et al ( , 2017Hu et al ( , 2018) proposed a 'mechanism-based' design approach based on the soil flow mechanisms observed in model tests and LDFE analyses. The Hu et al approach simplified the spudcan penetration profile as a combination of the peak resistance and postpeak resistance in the sand layer, q peak and q post-peak , and the resistance in the clay layer, q clay .…”