Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Marx Ferdinand Ahlinhan (1) F P
(1) IMPaC Offshore Engineering GmbH, Hamburg, Germany
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Validation of suction can foundations for offshore wind structures
A suction can is a large cylindrical structure, which penetrates into the seabed due to self-weight and a suction pressure. This foundation type is promising for offshore structures due to some differential advantages in comparison with conventional pile foundations: e.g. environmental friendly installation and decommissioning lower noise, shorter installation time and the ability to retract the structure in case of a penetration refusal. This abstract demonstrates that a geotechnical analysis for offshore installations with suction cans in the German North Sea using a FEM is applicable to fulfill the requirements of the German Maritime and Hydrographic Agency (BSH).
In designing the suction can foundation a geotechnical engineer must consider installation processes and in-place performance. The geotechnical analysis of suction can includes the bearing, sliding and overturning capacity. Furthermore, the pore water pressure accumulation and dissipation in surrounding soil due to cyclic loading should be considered. Existing formulae for checking the suction cans would hardly take into account the pore water pressure accumulation and dissipation in surrounding soil due to cyclic loading. Therefore, a FE model has been developed to account for this issue and for scouring, which is applicable for offshore foundations according to BSH requirements.
Main body of abstract
Investigations showed that the soil at installation site consists of silty sand and loose sand in the upper level and dense sand at levels below. It should be noted that pore water pressure would be generated and dissipated in the silty sand due to cyclic loading.
First, the pore water pressure accumulation and dissipation has been modeled by means of the commercial FE method software PLAXIS. Secondly, the shear stress degradation of the soil has been calculated and the degraded interne friction angle has been derived. Finally, a 3D numerical half model has been developed to verify the geotechnical stability of the suction can.
The calculation was executed in steps. Firstly, an initial stress state was generated by defining vertical stresses according to the overburden height. The initial horizontal stresses were determined by considering the earth pressure at rest, using the earth pressure coefficient at rest k0. In the second step, the suction can, the expected degraded soil profile, and the interfaces are activated. In the subsequent calculation steps the loads are applied. Post installation scour has been modeled by excavating the surrounding soil over the allowable scour depth. The evaluated characteristic extreme load is increased linearly at the same rate up to model failure.
The accumulated rotation has been computed and compared to the tolerable rotation for the offshore structure. Also, the load displacement curve of the suction can, which allows to calculate the safety factor of the bearing and to evaluate the performance of the suction can foundation.
A validated guideline for the design analysis of a suction can foundation does still not exist. Therefore, a FE model has been developed taken into account design aspects such as post installation scouring, pore water pressure accumulation and dissipation due the cyclic loading from wave, wind and current.
The analysis shows that the required suctions can stability checks according to applicable German regulations can be carried out and the structural integrity can be proven by means of a FE modelling in connection with a sound engineer judgment.
The risk regarding the design, the approval, fabrication and installation costs of suction can foundations can be well controlled. Therefore, suction cans are a promising, economic and environmental friendly foundation option for the offshore wind industry.