Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
William Courtney (1) F P Mathias Stolpe (1) Thomas Buhl (1) Robert Bitsche (1) Nicolai Hallum (2) Søren Nielsen (2)
(1) Technical University of Denmark, Roskilde, Denmark (2) Universal Foundation A/S, Aalborg, Denmark
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
William Courtney is a recent graduate of the MSc. in Wind Energy program at the Technical University of Denmark (DTU). He has been working for the last half year at DTU and is currently employed as a Development Engineer. His research areas include structural optimization of suction bucket and jacket foundations as well as floating wind turbines.
Optimal design of stiffeners for bucket foundations
The Bucket Foundation - developed by Universal Foundation A/S - is a novel foundation concept that can potentially reduce the cost of energy for offshore wind farms in both shallow and deep waters. The project “Cost-Effective mass production of Universal Foundations for large offshore wind parks” aims to move the Bucket Foundation from a research, development, and demonstration phase into commercialization and industrialization within 2016-2017. As part of this project, the objective of this work is to further reduce the cost of the bucket foundation by reducing the mass of the stiffeners through structural optimization while maintaining manufacturability.
In order to obtain the optimal design both shape and topology optimization problems are formulated and solved using the commercial optimization software Tosca Structure coupled with the commercial finite element software Abaqus. The solutions to these optimization problems are then manually interpreted as a new design concept.
Main body of abstract
The foundation is loaded with gravity loads as well as extreme static horizontal forces and moments at the top of the shaft. Zero displacement/rotation boundary conditions are imposed at the bottom of the foundation and therefore soil interactions are assumed to be negligible in the optimization problem.
The shape optimization problem is formulated with the objective function minimize the maximum von Mises stress in the stiffener, subject to various volume constraints, with the design variables being the position of the nodes near the stiffener’s edge.
The topology optimization problem is formulated with the objective function minimize the structure’s compliance, subject to various volume constraints, with the design variables being the density of the shell elements representing the design domain. Results from this problem are then manually interpreted as a new concept taking buckling, manufacturability, and mass production into consideration. Finally, sizing of the new concept ensures von Mises stress levels do not exceed that of the initial concept.
Shape and topology optimization problems are formulated and solved for a specific foundation size. Results from shape optimization show that the maximum von Mises stress in the stiffeners can be reduced by 38% without adding mass to the design. A potential reduction in mass can then be achieved through sizing optimization of the new shape.
Results from topology optimization have led to a new design concept which reduces the total mass by 25%. A mass reduction of this magnitude likely justifies the slightly increased manufacturing complexity of the new design.
This work has resulted in a new design process for suction bucket foundations based upon existing commercial software as well as a new design with a mass reduction of 25%. These mass reductions will contribute to a lower cost of offshore wind energy as the suction bucket foundation begins entering the market.
This work explores the potential for utilizing commercial structural optimization software in the offshore wind industry. While focus is on the bucket foundation, the methods and results can serve as inspiration for other structural optimization problems in the industry.