Suction bucket foundations for offshore wind turbines are steadily gaining prominence and were recently constructed in commercial wind parks. The increasing turbine size and greater water depth cause stringent requirements on the stiffness of the foundation and structure. To meet these criteria, multipod foundations with multiple suction buckets, which are mainly axially loaded, are favourable. The predominant loading direction is compressive, but extreme loads may induce tensile loads on individual foundations. The stiffness and resistance under tensile loads is considerably lower than under compressive loads. Consequently, the tensile bearing behaviour is a relevant subject of current research. The tensile response can be analysed in physical model tests, but scaling of the results is heavily discussed. While model tests conducted at 1g suffer from an insufficient stress state, tests in a geotechnical centrifuge overcome this problem, but are not able to appropriately scale grain size and thin structures (such as the suction bucket’s skirt). This paper presents the approach of testing different model scales at 1g and to simulate these tests in a finite element model. The successful retrospective numerical analyses are taken as basis to extrapolate the scale of the finite element model up to prototype scale. At prototype scale, the tensile response can be quantitatively evaluated. The results point out that few tensile loads may be acceptable. The load magnitude can even exceed the drained resistance, but the load must neither recur shortly thereafter nor have a too long period.
Patrick Gütz and Martin Achmus
Keywords: suction bucket, tensile loading, scaling, model tests, finite element analysis
Publisher: DFI