Plate anchors for offshore floating facilities: Soil-anchor-floating system interactions
Dr. Katherine Kwa and Prof. David White, University of Southampton
There are a variety of anchoring and mooring systems that can be embedded in the sea floor to hold offshore floating renewable energy facilities in place. These foundation systems are subjected to a sustained load in still water conditions and also need to withstand large numbers of variable cyclicloads due to wave and wind forces which act on the renewable energy structures. Previous research has shown that in soft soils, these foundations can strengthen as a result of the cyclic loading conditions, and this strengthening can result in some added capacity to the foundation. If these gains in capacity are harnessed, then more efficient and cost-effective anchoring and mooring systems can be designed and used for the foundations of offshore floating facilities. This research project aims to improve the foundation design for embedded plate anchors, an anchoring system that is an efficient solution for the foundations for offshore floating facilities. Plate anchors are currently used in various offshore applications, and can be installed by a range of means. For example, the plate may be connected to a harness and installed by drag embedment or via a suction anchor. Alternatively, helical pile or screw anchors gain their capacity from a plate element located at the base. The project investigates the soil behaviour around these plate anchors, particularly when the plate is subjected to sustained and cyclic loads based on different sea states that fluctuate in severity based on weather cycles and seasonal variations. The behaviour of the soil and its interaction with the plate anchor will also be integrated into existing models, such as WEC-Sim (Wave Energy Converter Simulator), which are currently being used by structural and fluid dynamics engineering researchersto understand the overall behaviour of floating offshore renewable energy structures. Current work over the past three months has focused on using PLAXIS 2D, a finite element software that is designed for modelling deformation and stability in geotechnical engineering, to understand the soil behaviour around an embedded plate anchor. Initially, the plate anchor was loaded monotonically to failure as a benchmarking exercise to check that the ultimate bearing capacity (Vuu) obtained from PLAXIS 2D agreed with the existing, well established, analytical solutions for the ultimate bearing capacities of surface and embedded footings. Once the monotonic solutions were validated, the embedded plate anchor was subjected to a preload, a force consisting of a fraction between 0.1 to 0.65 of the ultimate bearing capacity, before being consolidated and then being pushed to failure. The resulting gains in strength within the soil, and therefore gains in capacity of the plate anchor when pulled to failure, were elaborated by separating the overall failure mechanism around the plate, into the behaviour of the soil above and below the plate. The soil above the plate strengthened as the preload was increased. Conversely, the soil below the plate became weaker as the preload was increased. The net effect represented the global increase in strength within the soil and capacity of the plate anchor. The patterns of changing strength were affected by a gap opening beneath the plate under certain loading conditions and filling with water. The soil constitutive model adopted in the analysis is able to replicate the effect of a zone of freewater forming beneath the plate, which is an improvement over previous modelling of this problem. Future work will extend these numerical analyses to different cyclic loading conditions. Integration of the interaction between the soil and plate anchor into existing fluid dynamic and structural models such as WEC-sim is an ongoing collaborative task. The centrifuge is currently being prepared for testing and once ready, will be used to performing model tests to obtain the experimental data to further validate the finite element results obtained from PLAXIS 2D, and to observe and measure the soil behaviour and capacity of the plate anchor under different cyclic loading conditions.