Tensile-loaded suction bucket foundations for offshore structures in sand

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Autoren

  • Patrick Sven Gütz

Organisationseinheiten

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Details

OriginalspracheEnglisch
QualifikationDoktor der Ingenieurwissenschaften
Gradverleihende Hochschule
Betreut von
  • Martin Achmus, Betreuer*in
Datum der Verleihung des Grades22 Sept. 2020
ErscheinungsortHannover
PublikationsstatusVeröffentlicht - 2020

Abstract

Der aktuell große Bedarf an regenerativen Energien wird sich in Zukunft erwartungsgemäß weiter steigern. Offshore Windenergieanlagen (OWEAs) produzieren bereits jetzt einen großen Anteil der benötigten regenerativen Energie und es wird von einem fortschreitenden Ausbau ausgegangen. An die Gründungen von OWEAs werden hohe Anforderungen gestellt, um einen sicheren und zuverlässigen Betrieb bei gleichzeitig geringen Kosten gewährleisten zu können. Die Gründung einer OWEA auf mehrere Suction Buckets stellt hierbei ein vielversprechendes Konzept dar, dessen Umsetzbarkeit bereits in Form von Prototypen in mehreren kommerziellen Windparks nachgewiesen wurde. Die OWEA erfährt hauptsächlich horizontale Belastungen, welche durch die Unterstruktur in axiale Belastungen auf die gegenüberliegenden Gründungen übertragen werden. Das Eigengewicht einer OWEA bedingt eine mittlere Drucklast, sodass diese zusätzlichen Belastungen eine Variation der axialen Last hervorrufen. Sofern die Gründungen nicht besonders weit voneinander entfernt angeordnet sind oder die OWEA ein übermäßiges Eigengewicht aufweist, werden zeitweilig Zugkräfte auf die Gründungen einwirken. Das Tragverhalten von Suction Buckets unter Zugbelastung ist bislang nicht vollständig bekannt, weshalb die ökonomischen und ökologischen Vorteile dieser Gründung eingeschränkt sind. In der Vergangenheit befassten sich unterschiedliche Studien mit dem axialen Tragverhalten von Suction Buckets, jedoch konnte das Verhalten unter Zugbelastung bislang nicht ganzheitlich bewertet werden. Unter sehr langsamen Belastungsraten und in durchlässigem Boden verhält sich das Suction Bucket dräniert, sodass dessen Widerstand aus der Mantelreibung resultiert. Hingegen bestehen Unsicherheiten hinsichtlich des Verhaltens unter zyklischer Belastung und höheren Belastungsraten, bei denen die Entwicklung eines Unterdrucks hinzukommt, der als zusätzlicher Widerstand unter dem Deckel des Suction Buckets der einwirkenden Kraft entgegenwirkt. Weder die Größe der daraus resultierenden Unterdruckkraft noch die Verschiebung des Suction Buckets zur Mobilisierung des Unterdrucks können bislang zuverlässig bestimmt werden. Darüber hinaus können zyklisch wiederkehrende Zugkräfte zu einer Akkumulation der Verschiebungen führen, welche ebenfalls noch nicht prognostiziert werden können. Aus diesen Gründen wird das Auftreten von Zugkräften im Rahmen einer konservativen Bemessung von OWEAs konstruktiv verhindert. In dieser Dissertation werden die Ergebnisse von umfassenden Untersuchungen vorgestellt, welche auf die Ermittlung des Zugtragverhaltens im partiell drainierten Zustand und die Bewertung der Anwendbarkeit zum Auftreten von Zuglasten ausgerichtet waren. Hierzu wurde der aktuelle Stand der Wissenschaft ausführlich dargestellt und daraus die zielgerichtete Analyse mit Hilfe von physikalischen Modellversuchen und numerischen Simulationen abgeleitet und durchgeführt. Schließlich wurden die gewonnenen Erkenntnisse in die Verbesserung eines bestehenden analytischen Modells übertragen. Die physikalischen Modellversuche ergaben höhere Zugwiderstände bei geringerer Drainage, jedoch erforderte deren Mobilisierung größere Verschiebungen. Kraftgesteuerte Versuche ermöglichten die Ermittlung der Verschiebungsakkumulation. In Abhängigkeit der hierbei aufgebrachten Belastungen widerstand das Suction Bucket einer gewissen Anzahl von Zyklen bis schließlich größere Verschiebungen hervorgerufen wurden. Bei Belastungen, die die dränierte Tragfähigkeit um ein Vielfaches überschritten, trat dies bereits nach wenigen Zyklen auf, jedoch wäre das Eintreten einzelner Lastereignisse unter Berücksichtigung tolerierbarer Verschiebungen zulässig. Aus den Beobachtungen der Modellversuche wurden die essentiellen Merkmale abgeleitet, welche in dem numerischen Modell implementiert wurden. Außerdem lieferten die gemessenen Ergebnisse die Basis für die Validierung des numerischen Modells, dessen Ergebnisse mit denen der Modellversuche grundlegend übereinstimmten. Eine weitreichende Parameterstudie wurde mit Hilfe von numerischen Simulationen im Prototypmaßstab durchgeführt, um die Bandbreite der in den Modellversuchen untersuchten Randbedingungen zu erweitern. Hierin wurden die Einflüsse der Drainagebedingung und der Abmessung des Suction Buckets auf dessen Steifigkeit und Zugwiderstand ausgewertet. Darüber hinaus wurden die Verschiebungen und Verschiebungsraten in Folge unterschiedlicher statischer und zyklischer Belastungen in Abhängigkeit der Größe des Suction Buckets untersucht. Abschließend wurde ein Ansatz formuliert und erfolgreich demonstriert, der die Abschätzung der zyklischen Verschiebungsakkumulation durch eine äquivalente Last approximiert. Die gewonnen Erkenntnisse aus den Modellversuchen und Simulationen wurden in der Weiterentwicklung und Kalibrierung eines analytischen Modells berücksichtigt, welches trotz gewisser Einschränkungen adäquate Abschätzungen liefert. Die Ergebnisse dieser Dissertation zeigen auf, dass die Einwirkung einiger Zuglasten zulässig sein könnte, sofern gewährleistet ist, dass die Belastung gewisse Größen nicht überschreitet und solche Lasten nicht wiederholt in zu kurzem Abstand auftreten, um eine zwischenzeitliche Dissipation des Porendrucks zu ermöglichen. Ein anwendungsorientierter Ansatz wurde vorgeschlagen, der die zyklische Verschiebungsakkumulation durch eine vereinfachte Belastung abschätzt. Außerdem wurde die Anwendbarkeit eines analytischen Modells aufgezeigt. Es sei darauf hingewiesen, dass diese Untersuchungen ausschließlich Zugkräfte berücksichtigten, diese jedoch üblicherweise mit Drucklasten interagieren, welche abwärtsgerichtete Verschiebungen und unterschiedliche Steifigkeiten hervorrufen.

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Zitieren

Tensile-loaded suction bucket foundations for offshore structures in sand. / Gütz, Patrick Sven.
Hannover, 2020. 273 S.

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Gütz, PS 2020, 'Tensile-loaded suction bucket foundations for offshore structures in sand', Doktor der Ingenieurwissenschaften, Gottfried Wilhelm Leibniz Universität Hannover, Hannover. https://doi.org/10.15488/10152
Gütz, P. S. (2020). Tensile-loaded suction bucket foundations for offshore structures in sand. [Dissertation, Gottfried Wilhelm Leibniz Universität Hannover]. https://doi.org/10.15488/10152
Gütz PS. Tensile-loaded suction bucket foundations for offshore structures in sand. Hannover, 2020. 273 S. doi: 10.15488/10152
Gütz, Patrick Sven. / Tensile-loaded suction bucket foundations for offshore structures in sand. Hannover, 2020. 273 S.
Download
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title = "Tensile-loaded suction bucket foundations for offshore structures in sand",
abstract = "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{\textquoteright}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.",
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Download

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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.

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DO - 10.15488/10152

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CY - Hannover

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