Penetration response of spudcans in layered sands

The behaviour of spudcan foundations during the installation and preloading in two-layer sand sediments was investigated through large deformation finite element (LDFE) analyses. The LDFE analyses were carried out using the coupled Eulerian-Lagrangian approach, modifying Mohr-Coulomb soil model to capture hardening and subsequent softening effects of sand. Parametric analyses were undertaken varying the top layer thickness, relative density of sand and spudcan diameter. Both loose to medium dense-over-dense and dense-over-loose to medium dense sand deposits were explored. The results showed that, for the investigated relatively thin top layer thickness of ≤ 5 m, spudcan behaviour was dictated by the bottom sand layer with a minimal influence of the top layer. For assessing the penetration resistance profile in two-layer sands, the performance of the ISO, SNAME, InSafeJIP, and other existing theoretical design methods were evaluated.     

A finite element approach for predicting the full resistance profile of a spudcan deeply penetrating in dense sand overlying clay

This paper presents a finite element approach to calculate the full resistance profile of a spudcan deeply penetrating in dense sand overlying clay, in which a potential for an installing spudcan to experience a sudden uncontrolled punch-through failure exists. A modified Mohr-Coulomb model characterized by incorporating a four-phase variation of the mobilized strength and dilation parameters with an equivalent accumulated plastic strain is developed and tested for the overlying dense silica sand. An extended Tresca model is used for the strain softening of the underlying clay. A series of large deformation finite-element (LDFE) analyses are carried out, varying the strength and dilation parameters as well as the spudcan geometries. A fairly good performance of the present approach is verified by validating against groups of centrifuge tests data, allowing the numerical study to be extended parametrically. The four-phase variation of the mobilized strength and dilation parameters involved in the progressive failure of the upper dense sand is parametrically studied and extended to cover the range of sand relative densities that are of practical interest. Additionally, comparisons with the typical existing LDFE analyses using both simple and sophisticated constitutive models are carried out. It shows that the present approach performs fairly well to calculate the full resistance profile of a spudcan deeply penetration in both thin and thick dense sand overlying clay, especially the peak and post-peak resistance, within around 5% of the corresponding centrifuge tests results.     

Recent contributions of geotechnical centrifuge modelling to the understanding of jack-up spudcan behaviour

The paper presents an overview of the recent contributions of centrifuge modelling to the understanding of soil-structure interaction and the development of design and predictive methods in the field of mobile jack-up drilling rig foundations. Both advantages and limitations of the centrifuge methods are detailed and key examples are presented. The benefits provided by centrifuge modelling to the development of analysis methods that are now being used within the jack-up industry are highlighted. To conclude, industry trends and research opportunities are discussed, as is the possible role of the geotechnical centrifuge in finding solutions to these new needs.     

Post-installation pore-pressure changes around spudcan and long-term spudcan behaviour in soft clay

This paper presents a dual-stage Eulerian–Lagrangian analysis for modelling the entire process of spudcan installation in soft clay, followed by consolidation and working load operation. The analysis consists of three components, namely undrained effective stress Eulerian analysis of spudcan installation, mesh-to-mesh variable mapping and coupled-flow Lagrangian analysis for the post-installation spudcan working behaviour. The results show good agreement with centrifuge model data but also highlight the importance of replicating the hysteretic behaviour of the soil. The findings also show that while a wished-in-place approach was able to model the long-term bearing response of the spudcan, rotational stiffness was over-estimated. This is due to the fact that, while the wished-in-place analysis was able to model the hardening of the soil ahead of the spudcan, it was unable to model the softening of back-flowed soil behind spudcan. The latter influences the spudcan fixity significantly, but not bearing response. Although the analyses were conducted using ABAQUS, they can, in principle, be conducted using other codes.     

Experimental investigation of the system behaviour of a model three-legged jack-up on clay

Results from combined loading experiments dedicated to evaluating the capacity and stiffness of a single footing have significantly improved the understanding of the response of circular shallow foundations. However, due to experimental practicalities, little corroborative evidence is available to confirm predicted behaviour when multiple footings act within a structural system. This paper addresses this concern by presenting the results of a series of experimental tests on a three-legged model jack-up unit founded on soft, heavily overconsolidated clay. The model adopted geometric features representative of a large prototype rig by appropriately scaling leg height, leg separation and flexural stiffness, as well as the ‘spudcan’ footing diameter and profile. Pushover events were considered by subjecting the model jack-up to monotonic horizontal loading at hull level. By measuring loads and displacements at the hull and at each spudcan, the system behaviour of the jack-up could be analysed. Nine separate tests were performed, revealing how load orientation, leg length and preload ratio changed the system capacity and affected the load–displacement paths of the jack-up and spudcan system. The test results are compared with those derived from single spudcan experiments, and are interpreted within the combined load yield surface approach gaining acceptance within the offshore industry.     

Experimental investigation of jetting effect on spudcan extraction from soft clay

The jack-up unit may suffer difficult extraction from soft clay attributed to the large embedment and suction. To ease spudcan extraction, the jetting technique is extensively adopted. The jetting effect on spudcan extraction is investigated with a series of model tests. Firstly, the effectiveness of top jetting is investigated, and it is found that the top jetting is not effective in reducing extraction resistance. Secondly, the efficiency of different jetting procedures are studied. It is revealed that jetting prior to extraction reduces the suction by increasing the excess pore pressure at spudcan base. And the jetting after extraction begins reduces the suction by filling the gap formed under the spudcan with jetting water and eliminating cavitation. Finally, tests with different jetting times, jetting rates, jetting pressures, and jetting nozzle locations and numbers are conducted. It is found that the effect of jetting time converges as it increases. The flow rate of jetting prior to extraction has little effect on jetting efficiency, whereas the flow rate of jetting after the extraction begins has significant influences on the jetting efficiency. Jetting pressure also has great effect on jetting efficiency, and converges as it increases. The closer the nozzles are located to the spudcan edge, the sooner the post-peak extraction resistance decreases. And the jetting efficiency increases with the nozzle number. The findings from the experimental studies can serve as a reference for future studies on the operation and optimization design of a jack-up jetting system.