Suction Caisson

This paper presents the results of three-dimensional finite element analyses of the suction bucket foundation used for offshore wind turbines. The behavior of the bucket and the response of soil supporting the bucket in dense and medium dense sandy soils subjected to static horizontal load are investigated. Field tests results and a centrifuge model test are used to validate the numerical model. Dimensionless horizontal load-displacement and overturning moment-rotation relationships are derived utilizing the Power law and Buckingham's theorem. The results show good agreement between the numerical analysis results and the straight lines obtained from the Power law until a specific value of horizontal load and overturning moment. Regarding stress behavior of soil supporting the bucket, due to soil densification and bucket movement, maximum stresses are seen near the bucket tip at the right inside of the bucket. The major part of the applied load is transferred by the bucket skirt. Numerical analysis modeling results show that the bucket rotation and displacement are highly dependent on the bucket geometry and soil properties in addition to loading conditions. Normalized equations and figures for the ultimate horizontal load and overturning-moment capacities are presented and can be used for the preliminary design of the bucket foundations in sandy soils.

Suction caissons are widely used in mooring systems for deep water oil and gas development projects. The response of a caisson in sand is different from its response in clay under pullout force. In this study, three-dimensional finite element (FE) analyses are conducted to calculate the pullout capacity of a suction caisson subjected to various oblique loadings. The FE modeling is performed using Abaqus FE software. In the mooring systems, the caisson could have a significant movement and rotation before reaching to the maximum pullout force. Therefore, the Arbitrary Lagrangian Eulerian (ALE) method available in the Abaqus/Explicit is used in the present FE analysis to avoid numerical issues due to excessive mesh distortion at large displacements that typically encountered in the FE formulations in the Lagrangian framework. The sand around the caisson is modeled using the Mohr-Coulomb model. The effects of key variables, such as loading angle, mooring position and aspect ratio, on pullout capacity and rotation of the caisson are presented. The comparison between FE and centrifuge test results is also shown.

Three-dimensional finite element (FE) analyses are conducted to calculate the pullout capacity of a suction caisson subjected to oblique loadings. Two sets of FE analyses are performed using Abaqus FE software. In the first set, the sand around the caisson is modeled using the built-in Mohr–Coulomb (MC) model available in Abaqus where constant values of the angle of internal friction and dilation are defined. The effects of key variables, such as loading angle, mooring position, and aspect ratio, on pullout capacity and rotation of the caisson are examined. A comparison between FE and centrifuge test results is also shown. The second set of analyses are performed using a modified Mohr–Coulomb (MMC) model where the prepeak hardening, postpeak softening, and effects of density and confining pressure on stress-strain behavior of dense sand are implemented via a user subroutine by varying and as a function of plastic shear strain and confining pressure. By comparing the failure surface development in the soil with increase in loading for two different models (MC and MMC), it is shown that the mobilized and vary along the failure planes if the MMC model is used, although the capacity of the caisson could be obtained even if appropriate values of constant and are used in the MC model.