TY - BOOK

T1 - Tensile-loaded suction bucket foundations for offshore structures in sand

AU - Gütz, Patrick Sven

N1 - Doctoral thesis

PY - 2020

Y1 - 2020

N2 - Currently, a great demand for renewable energy exists and OWT supply a relevant share, which is expected to further increase in the future. The foundations for these structures must fulfill strict requirements to ensure a safe and reliable operation of the OWT while minimising the costs. Suction buckets for multipod foundations are considered as a promising alternative and demonstrated their applicability in commercial projects. The multipod substructure transfers the horizontal loads acting on the OWT to axial loads on the opposing foundations. Owing to the self weight of the OWT, the suction bucket’s mean load is generally compressive and superimposed by a variation of the axial load. Except for wide spacing of the foundations or large self weight of the OWT, tensile loads arise. Due to incomplete knowledge regarding the suction bucket's tensile bearing behaviour, the particular economic and ecological advantages are still limited. Although numerous studies investigated the suction bucket's response to axial loads, the tensile bearing behaviour was not yet elucidated holistically. If the suction bucket is subjected to very low tensile load rates and the soil's hydraulic conductivity is sufficiently high, the tensile resistance constitutes from the drained skin friction inside and outside the suction bucket's skirt. Major uncertainties exist in terms of the response to cyclic loads and the assessment of loading under constrained drainage, where the presence of negative differential pressure beneath the suction bucket's lid contributes to the tensile resistance. It is neither possible to quantify the magnitude of the invoked suction force nor is there any reliable method to predict the required displacement to generate this resistance. Moreover, the displacement accumulation induced by recurring tensile loads cannot be determined with confidence. As a consequence, a conservative design of OWT by means of prohibiting the occurrence of tensile loads is often recommended. This thesis presents results of comprehensive analyses with specific focus on gaining knowledge concerning the suction bucket's tensile bearing behaviour under partial drainage and assessing the applicability of tensile loads. Based on a holistic literature review, the response to a wide range of load types was examined by physical model tests and numerical simulations. Finally, these observations were used to improve an existing analytical model. The physical model tests revealed a significant increase of the tensile resistance with decreasing drainage, but its mobilisation required a larger displacement. Force-controlled tests enabled the determination of the displacement accumulation. Depending on the load magnitude, a significant number of cyclic loads were withstood until relevant displacement took place. Cyclic loads substantially exceeding the drained resistance caused severe displacements after few cycles, but the occurrence of only a limited number of cycles might be acceptable if certain displacement would be tolerable. The results of the physical model tests provided an essential database for the development and validation of a FE model. In general, the numerical simulations agreed with the observations of the model tests and extended the range of investigated specifications within the scope of a broad parametric study at prototype scale. The stiffness as well as the interaction of the resistances were evaluated depending on the drainage condition and the size of the suction bucket. Moreover, the induced displacement rates due to monotonic and cyclic loading were examined for various load configurations and suction bucket geometries. Finally, a concept for defining a cyclic equivalent load was proposed and demonstrated successfully. The conclusions from the physical model tests and numerical simulations were utilised for developing and calibrating an existing analytical model. Although the analytical model exhibited certain limitations, the attained results were reasonable. The findings of this thesis indicate that the occurrence of few tensile load cycles might be acceptable depending on their magnitudes and provided that these loads do not recur in short-term to allow for dissipation. A practical approach to estimate the cyclic displacement accumulation by a simplified load was proposed. Additionally, the applicability of an analytical model was demonstrated. It is remarked that the investigated loads were only tensile, although there is usually an interaction with compressive loads of different magnitudes, which would induce certain downward displacement and a variation of stiffnesses.

AB - Currently, a great demand for renewable energy exists and OWT supply a relevant share, which is expected to further increase in the future. The foundations for these structures must fulfill strict requirements to ensure a safe and reliable operation of the OWT while minimising the costs. Suction buckets for multipod foundations are considered as a promising alternative and demonstrated their applicability in commercial projects. The multipod substructure transfers the horizontal loads acting on the OWT to axial loads on the opposing foundations. Owing to the self weight of the OWT, the suction bucket’s mean load is generally compressive and superimposed by a variation of the axial load. Except for wide spacing of the foundations or large self weight of the OWT, tensile loads arise. Due to incomplete knowledge regarding the suction bucket's tensile bearing behaviour, the particular economic and ecological advantages are still limited. Although numerous studies investigated the suction bucket's response to axial loads, the tensile bearing behaviour was not yet elucidated holistically. If the suction bucket is subjected to very low tensile load rates and the soil's hydraulic conductivity is sufficiently high, the tensile resistance constitutes from the drained skin friction inside and outside the suction bucket's skirt. Major uncertainties exist in terms of the response to cyclic loads and the assessment of loading under constrained drainage, where the presence of negative differential pressure beneath the suction bucket's lid contributes to the tensile resistance. It is neither possible to quantify the magnitude of the invoked suction force nor is there any reliable method to predict the required displacement to generate this resistance. Moreover, the displacement accumulation induced by recurring tensile loads cannot be determined with confidence. As a consequence, a conservative design of OWT by means of prohibiting the occurrence of tensile loads is often recommended. This thesis presents results of comprehensive analyses with specific focus on gaining knowledge concerning the suction bucket's tensile bearing behaviour under partial drainage and assessing the applicability of tensile loads. Based on a holistic literature review, the response to a wide range of load types was examined by physical model tests and numerical simulations. Finally, these observations were used to improve an existing analytical model. The physical model tests revealed a significant increase of the tensile resistance with decreasing drainage, but its mobilisation required a larger displacement. Force-controlled tests enabled the determination of the displacement accumulation. Depending on the load magnitude, a significant number of cyclic loads were withstood until relevant displacement took place. Cyclic loads substantially exceeding the drained resistance caused severe displacements after few cycles, but the occurrence of only a limited number of cycles might be acceptable if certain displacement would be tolerable. The results of the physical model tests provided an essential database for the development and validation of a FE model. In general, the numerical simulations agreed with the observations of the model tests and extended the range of investigated specifications within the scope of a broad parametric study at prototype scale. The stiffness as well as the interaction of the resistances were evaluated depending on the drainage condition and the size of the suction bucket. Moreover, the induced displacement rates due to monotonic and cyclic loading were examined for various load configurations and suction bucket geometries. Finally, a concept for defining a cyclic equivalent load was proposed and demonstrated successfully. The conclusions from the physical model tests and numerical simulations were utilised for developing and calibrating an existing analytical model. Although the analytical model exhibited certain limitations, the attained results were reasonable. The findings of this thesis indicate that the occurrence of few tensile load cycles might be acceptable depending on their magnitudes and provided that these loads do not recur in short-term to allow for dissipation. A practical approach to estimate the cyclic displacement accumulation by a simplified load was proposed. Additionally, the applicability of an analytical model was demonstrated. It is remarked that the investigated loads were only tensile, although there is usually an interaction with compressive loads of different magnitudes, which would induce certain downward displacement and a variation of stiffnesses.

U2 - 10.15488/10152

DO - 10.15488/10152

M3 - Doctoral thesis

CY - Hannover

ER -