Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Marijn Dekker (1) F P
(1) SPT Offshore, Woerden, The Netherlands
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Mr. Dekker is one of the first graduates of the European Wind Energy Master from TU Delft, DTU, NTNU and Oldenburg University, with a specialization in offshore engineering. He is now working as a geotechnical engineer for SPT Offshore in Woerden, The Netherlands. His focus is on soil-structure interaction for jackets founded on multiple suction bucket foundations.
Non-linear foundation modeling in multi-footing suction bucket design
Suction buckets are a type of offshore foundation that allow for fast and noise-free installation and decommissioning of offshore structures. They have been used for 20 years in the oil and gas industry and are also becoming more common in the offshore wind sector as a promising way to cost reduction, both for foundations of single turbines and substation foundations. The suction buckets are installed by applying a differential pressure between the inner and outer sides of the buckets, which pushes the buckets into the soil.
When a platform is founded on multiple suction buckets, the way the wind and wave loads are transferred through the structure and various buckets into the soil will depend on the stiffness of both the platform and the buckets. The design of the substructure and suction buckets should thus be combined to include the effects of soil-structure interaction. This is currently done by modeling the suction bucket foundations as a set of linear-elastic springs attached to the substructure. These springs are however not able to capture the non-linear behavior that is often characteristic for soils.
Main body of abstract
When the stresses in the soil increase the soil will behave softer, leading to an increase of the displacements of the suction buckets for high loads. The substructure will help decrease large differences between the displacements of the suction buckets, resulting in a more even load distribution over the foundations and thus smaller design loads for the suction buckets. This allows for smaller suction buckets and savings of material and costs.
A method has been developed to model the suction bucket foundations using non-linear springs. The characteristics of the springs are determined using FEM calculations with a nonlinear soil model in the geotechnical software PLAXIS. The springs are then implemented in a structural model of the substructure and the multiple foundations in the structural software SACS.
The resulting loads on and displacements of the suction bucket foundations for a test load case have been compared to results from a complete FE model and a model that uses linear-elastic springs for various soil stiffnesses. The comparison has shown that the developed model gives results that match well with the results from the full FE model.
The linear-elastic model on the other hand gives results that deviate significantly for high loads. As the model with nonlinear springs is able to predict the behavior of the soil and the substructure accurately it can be used to make a more efficient design of suction bucket foundations.