Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Hélène Robic (1) F P Antoine Neau (1) Maxime Philippe (1) Hakim Mouslim (1)
(1) INNOSEA, Nantes, France
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Maxime Phillipe is R&D manager at INNOSEA, an independent engineering firm specialized in Marine Renewable Energies. He obtained his PhD, working on the numerical simulation of floating wind turbines.
Predin, a preliminary design tool for offshore wind turbine sub-structures
In the early phase of an offshore wind farm development, a preliminary design of sub-structure based on simple data is a great asset for foundations designers.
During this stage, it can be interesting to be able to compare different types of foundation: monopile, jacket structure or gravity base structure and to evaluate the influence of the structure choice on final cost. A first economical evaluation of the wind farm can be determined with a preliminary design. Thus, having a first estimation of main dimensions like diameters and lengths of members, and the total mass is crucial.
In this context, INNOSEA has developed a preliminary design tool for offshore wind sub-structures named PREDIN. INNOSEA’s interest was to be able to quickly estimate the overall dimension of an offshore wind sub-structure during an early preliminary design work. The aim is to be able to quickly evaluate the main dimensions and final mass of structures. At this time, PREDIN concentrates on two types of structures: monopiles and jackets. Design checks and structural integrity verifications are performed following some of the DNV-GL standards.
Main body of abstract
The methodology is based on a simple set of data: soil conditions, water levels, meteocean conditions on site and wind turbine data, and it is based on analytical formulations for the computation of loads and the evaluation of structural integrity. Three main steps are performed in order to determine the foundation dimensions.
The natural frequencies of the whole structure (rotor-nacelle assembly, sub-structure, foundation piles) are determined. They have to avoid 1P-3P turbine excitation frequencies.
The response to extreme environmental loads checks the structure resistance in an extreme environment. Members’ resistance against yielding and buckling are investigated. Foundation stability and piles’ resistance are also checked.
For jacket structure for which fatigue is a design driven, one more step corresponding to fatigue assessment, is added. It aims to estimate the fatigue life of every member and joint. Given that the great number of required data and calculations generates substantial simulation times, some restrictions have to be done.
These steps are crossed one by one. The design is iterative: if one criterion is not respected or too easily respected, a dimension has to be changed and the program comes back to the first step.
Applications of PREDIN on two structures, one monopile and one jacket, are presented in the paper. Also, comparisons of structural assessment results are compared with results provided by commercial finite element software, ANSYS Structural with ANSYS Offshore module.
This study has shown potential capabilities of PREDIN methodology to evaluate a preliminary design of monopiles and jackets in a very short time.
The comparison with ANSYS structural results has shown a good agreement on natural frequencies calculations, structural assessment (utilization ratio inside members in ULS) and fatigue assessment of joints for jackets. The resulting designs for both monopile and jacket are coherent since dimensions (diameters, thicknesses, lengths, masses…) are close to existing structures installed on wind farms.
Further developments will extend the methodology to gravity base foundations. Further verifications and validations of the tool will have to be performed to verify the methodology applicability for different design conditions.