Geotechnical and Structural Clustering of Pre-Piled Offshore Wind Turbine Jackets
Alex Hall, Sebastien Manceau, Trevor Hodgson, Saif Mohammad
Atkins Ltd, UK
Offshore windfarm sites cover vast areas and comprise a large number of widely spaced wind turbine locations, leading to potentially significant variations of water depths and ground conditions (and possibly environmental loads) between individual locations. It is impractical to consider a unique design for every turbine location across a large site. Instead a clustering approach is used, whereby sets of locations are grouped together and a common substructure design is developed for each "cluster".
The clustering strategy, where decisions are made on how many clusters are required and how they are defined, is one of the key design decision affecting all parties involved in the development. It has to balance many different and sometimes conflicting factors. The benefits of increasing the number of clusters include narrower ranges of data and lower load variations per cluster, resulting in reduced substructure and foundation weights and installation efficiencies. The benefits of decreasing the number of clusters include reduced design complexity and programme, a reduced number of coupled analyses and increased fabrication standardisation.
Although clustering is appropriate for all fixed substructure types, in this presentation we will discuss the different factors which are considered when defining the clustering strategy for pre-piled jacket structures, based on recent project experience.
Because offshore wind turbine jackets structures are dynamically sensitive, a reliable assessment of the soil-structure interaction is essential. An integrated geotechnical and structural approach is considered key to ensuring that the clustering strategy enables efficient design.
The presentation will focus on the different factors which need to be considered when defining the clustering strategy for pre-piled jacket structures. The different factors that will be discussed in the presentation include:
There is considered to be no single clustering strategy that is suitable for all wind farms. Specific strategies must be created to suit the water depth range, soils, fabrication and installation contractors.
To illustrate this, examples of different clustering strategies will be compared, quantifying the relative benefits of each. The clustering strategies developed for two offshore wind farm projects will be used to illustrate the decision processes involved and the impact of the clustering strategy on the overall project.
The decision on the number of clusters for a windfarm is a key project decision that has a significant impact on the project programme and CAPEX and affects all stakeholders in the development. Optimising the clustering strategy is an important part of the design process. It is considered essential that this optimisation is undertaken as an integrated geotechnical and structural approach and should consider all of the potentially contradicting requirements of the project.
The key learning will be an appreciation of the numerous factors that influence the clustering strategy with a particular focus on ground conditions and water depths variability and the impact of the chosen clustering strategy on design for pre-piled offshore wind turbine jackets. The optimisation of the clustering strategy is dependent on close collaboration between structural and geotechnical engineers. The intention is that geotechnical engineers will learn some of the key factors influencing the jacket design resulting from the variation in foundation stiffness and that structural engineers appreciate the need to adequately consider ground conditions variability.