Abstract
Jack-up installation is typically discontinuous with seawater ballast being taken on and shed again periodically to elevate the hull out of the ocean as the spudcan footings penetrate the soil. Any pauses in the footing penetration provide the opportunity for consolidation to occur in sufficiently permeable cohesive soils. The undrained shear strength underneath the spudcan is increased during consolidation, which enhances the penetration resistance within a limited extent immediately after the consolidation. This phenomenon has been explored in a limited number of centrifuge tests, but cannot be reproduced using conventional finite-element methods. An effective stress large-deformation finite-element approach based on periodic mesh regeneration is employed to capture the entire process of ‘penetration–consolidation–penetration’, with the modified Cam-clay model being incorporated to represent the behaviour of normally consolidated kaolin clays. The numerical model established is verified by comparison with two series of centrifuge tests before exploring the influence of key parameters, including consolidation depth, consolidation duration, coefficient of consolidations, anisotropy of permeability and loads held during consolidation. The normalised consolidation duration is identified as the dominant factor that affects the post-consolidation peak in penetration resistance. A simple method is proposed to estimate the post-consolidation penetration resistance.
Highlights
Prior to the installation of a mobile offshore jack-up platform at a site, the load–penetration curve is required to be predicted
Bearing capacity theory is used at a series of embedment depths, with the soil response assumed to be drained in sand and undrained in clay (ISO, 2012)
Jack-up installation typically is a discontinuous process, with any pauses providing the opportunity for consolidation to occur in sufficiently permeable cohesive soils, which results in a zone of increased strength of the soil and an enhancement in the penetration resistance
Summary
Jack-up installation is typically discontinuous with seawater ballast being taken on and shed again periodically to elevate the hull out of the ocean as the spudcan footings penetrate the soil. Any pauses in the footing penetration provide the opportunity for consolidation to occur in sufficiently permeable cohesive soils. The undrained shear strength underneath the spudcan is increased during consolidation, which enhances the penetration resistance within a limited extent immediately after the consolidation. This phenomenon has been explored in a limited number of centrifuge tests, but cannot be reproduced using conventional finite-element methods.
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