Some uncertainties are still present though, as the barite is a soft material and it might partly penetrate through the top sandy soil. However, the presence of the voids is confirmed in different cases by the needed volume of barite, injected to fill these gaps.
AXISYMMETRIC CONDITION PLAXIS 2D ANCHOR FULL
In other situations, the underside of the spudcan might have already some remaining soil from previous installations, which can block the spudcan to reach full penetrations. While the spudcan penetrates, it might happen that the soil volume displaced from the spudcan, possibly moves up filling the gaps, before the achievement of full skirt penetration. The presence of the voids is however difficult to be visually confirmed, as the spudcan is confined by the outer skirts. However, in other spudcans it is not possible to apply suction. In some jack-up rigs, the spudcans have the facilities to apply suction within the created voids, which helps in increasing the skirt penetration. This is associated with a gap between the seabed and the underside of the spudcan equipped with outer and inner skirts, as seen in Figure 1 a). In specific sites, due to the special geometry of the skirted spudcans and due to reduced preload capacity of different rigs, combined sometimes with a dense sandy seabed, penetrations less than full base contact can occur during preloading or rig installation. Other aspects, such as applying suctions in order to increase the skirt penetration and (V-H-M) capacity are also investigated in. The studies have been based on real industry projects / case histories and supported by feedback from the jack-up installations and operations. On this aspect, previous research studies have been carried out by, , using conventional, 2D and 3D FE analyses for skirted footings under combined loads, considering multi-layered soil profiles (sand overlying clay with varying strength, sand overlying soft clay followed by sand and stiff clay) and full base skirted spudcan penetrations. For conditions when combined Vertical (V), Horizontal (H), and Moment (M) loadingĢ apply, the conventional approach has shown to be less reliable. However, very limited information is provided with regard to the design methods for predicting their capacity under combined loading. The possibly gained additional contact area is shown in red Figure 1 Skirted spudcan penetrating to depth less than full base contact The benefits of skirted spudcans are mentioned in the current industry guidelines such as:,. a) Skirted spudcan penetrated to depth corresponding to less than full base contact during preloading (V) b) Approximate additional penetrations of skirted spudcan under (V-H-M) loads, exceeding the yield capacity derived based on the preload. Some of the benefits of the skirted footing include: improvement of the foundation capacity by increasing the effective depth of the footing improvement of the horizontal capacity by reducing the risk for sliding improvement of the foundation fixity and provision of protection from scour when that is expected to be a major issue. KEY WORDS: skirted spudcans, (V-H-M) capacity, fixity analyses, limit equilibrium, finite element INTRODUCTION Jack-up rigs equipped with skirted spudcans are commonly used in the offshore industry. Important conclusions are derived and recommendations given with regard to identifying the governing failure mechanisms and predicting the behavior and capacity in the elevated conditions of the skirted footings suffering limited penetrations during preloading. Due to the gap presence, large deformation, updated mesh (UM) analyses are carried out in order to account for increased (V-H- M) bearing capacity developed due to additional penetrations and gained contact area of the underside of the spudcan and the seabed soil. Finite element (FE) analyses are carried out by modelling the spudcan in 2D and in 3D (allowing for accurate modelling of the inner skirts). In this paper, for a jack-up rig installed in the North Sea, experiencing penetrations less than full base contact, the assessment of footing-soil system stability and stiffness under storm load conditions is carried out. In such a case, there are limitations in the existing guidelines with regard to the calculations of (V-H-M) capacity based on the conventional methods. Sundararajan Geo, Copenhagen, Denmark *corresponding author: Lindita Kellezi: ABSTRACT When a jack-up rig, equipped with skirted spudcans, is installed on competent seabed soil, limited penetrations (less than full base contact) can be experienced. 1 SKIRTED SPUDCAN- SOIL INTERACTION UNDER COMBINED LOADING - GAP CONDITIONS AFTER PRELOADING L.