Behaviour of Suction Caisson subjected to Cyclic Loading in Tension

Abstract

As countries around the world are in a race for clean energy, various renewable energy sources are harvested to meet the energy needs. In Western Europe, wind energy is the primary source of renewable energy. To meet the energy demand, higher capacity wind turbines are being installed in deeper waters. In deep waters, monopile foundations are ineffective due to its inability to withstand large overturning moments. Thus, multi-caisson foundations are preferred in deeper waters. Typically jacket or tripod foundations are preferred as multi-caisson foundation and these foundations are supported by either piles or suction caissons. Suction caissons are preferred due to the ease of installation and removal.Based on the literature, it was understood that various kinds of research are being undertaken throughout the world to better understand the performance of suction caisson foundations. Two main components of suction caisson are researched widely, first being the installation behaviour, and second being the behaviour of caisson during repeated cyclic loading. This thesis seeks to gain further insight on the installation behaviour of the suction caisson. For this purpose, a suction installation apparatus was designed to be used in centrifuge. Various installation characteristics such as flow rate, soil profile has been studied in this thesis and detailed analysis is provided. Extraction tests of caisson were also done at a slow rate to determine the drained tensile capacity, which was used for determining tensile amplitudes in cyclic loading tests.Tripod structures withstand load by ’push-pull’ system, where the windward caisson is pulled out of the soil due to the cyclic environmental loads. This movement is compensated by the push behaviour of the other two caissons. Thus, the tensile capacity ofcaissons plays an important role in the design of caissons in a tripod structure. Windward caisson of a tripod foundation was modelled in this thesis to study the behaviour of the caisson under cyclic loading in tension. Cyclic loading tests were done at 100g using the geo-centrifuge at Delft University of Technology. A series of tests was done for various cyclic loading characteristics such as the average load, frequency and soil profile. Settlement and stiffness response of the suction caisson were obtained for each scenario. A brief discussion of the results is made in this thesis report. Very high settlement was observed when one-way compressive load was applied on the caisson installed in layered sand. A test was also done to study the response of caisson to storm loading where very high tensile amplitudes were used. The results obtained from these tests show the complexity of the response under tensile loads. When the tensile amplitude was more than 50% of the tensile capacity of the caisson, significant uplift of the caisson was witnessed along with a significant reduction in the stiffness of the soil.Finally, Finite Element Modelling (FEM) was done using the geotechnical finite element software - Plaxis 2D to compare the cyclic loading results obtained from centrifuge modelling with FEM. It was observed that the initial settlement behaviour was captured well in plaxis. But after few cycles, no more accumulation of strain was witnessed in the centrifuge tests. However, plaxis failed to capture this behaviour and thus higher settlements were observed in FEM analysis.This thesis can be used as a basis for the design of tripod foundations for OWT using suction caissons. The results obtained from this thesis reinforces previous studies and adds confidence in understanding the tensile behaviour of suction caissons.