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Concrete Gravity Base Substructure

Introduction and objective

Currently most offshore wind turbine foundations are made of steel and depend on the use of piling and the installation with heavy lift equipment. The objective of this project is to explore an alternative concrete gravity base substructure (GBS) that does not require piling (no restriction for installation as a result of noise) and is self-floating (obviating the need for heavy lift vessels and equipment), to be able to introduce a competitive alternative foundation onto the market.

During the course of the FLOW project the design, fabrication, installation and the business case have been explored. A Basic and Detail Design has been made to demonstrate technical feasibility, to base a fabrication and installation method upon and to determine the cost price of such a design, as one of the parameters for the business case.

Constructability as well as “install-ability” have influenced the design to a large degree, focussing on the design and materialisation of the shaft. A steel (not concrete) shaft has shown to be the optimal solution because it shows a more efficient use of material (better resistance to bending moments, smaller dimensions, lower loads), high floating stability during transport and installation with improved workability and shorter construction time. These factors have resulted in a compact and cost effective design. To be able to fully consider this, many aspects influencing the design had to be balanced. A feasible geotechnical design providing sufficient bearing capacity for the load distribution to the seabed, floating stability and draught during various stages of transport and sufficient strength of the construction to withstand environmental (wave) loading as well as the various load cases from the WTG.

Much effort has been put into the geotechnical design, understanding the effect of cyclic loading on the seabed in relation to the effects of pore pressure generation and dissipation. The connection between the steel shaft and concrete caisson has undergone many design steps to reach a feasible solution for transferring and withstanding particularly the fatigue loading during the lifetime of the foundation. Detailed Finite Element modelling with various software packages and third party review has taken place.
A Detail Design is currently in progress for a demonstrator project where an integrated load analysis for a new generation 8MW wind turbine has been completed. The design has been declared certifiable by DNV GL.

Model testing
BAM have performed model tests for the floating stability and seakeeping behaviour of the GBS which have led to modifications in the design improving the workability and safety.
Further model testing was undertaken by Deltares, assessing the wave loading and wave run up on the structure as well as the required scour protection. Completing the physical model testing has resulted a better understanding of environmental loads and in a major downgrade of the necessary layer thickness and rock size allowing a substantial decrease in cost.

Fabrication, construction and installation study
The study into fabrication of the GBS has resulted in solutions for optimised construction utilising pre-fabrication and solving the logistics and load-out of the GBS while using regular materials and equipment readily available. Particular effort went into optimising the construction site logistics; the handling of large volumes of aggregate (concrete, steel rebar), prefabrication of different components, and also handling the large weights and sizes of the concrete components as well as the steel shaft. Together with Mammoet a solution was engineered for the installation of the shaft as well as the load out of the GBS from land to water using Self Propelled Modular Transporters (SPMTs) and a submersible barge.

Fabrication will take place in a yard where a production rate of 80 units per year can be achieved, aligned with the number of units that can be installed in one season.
For the installation study BAM (by design) and Van Oord (by working method) have worked close together to optimise the available weather window and improve workability of the GBS. Methods have been developed for preparing the seabed and the installation of a gravelbed and scour protection and for the immersion of the GBS by waterballasting and finally the sandballasting of the GBS.

Cost Reduction
Pricing comparison with the jacket has shown that the GBS can be competitive and will have lower maintenance costs. Partner RWE completed a business case analysis for the GBS showing it to be cost competitive for a windfarm with 182 foundations at 48.7m average waterdepth The FLOW Offshore Wind Cost Model hase been used, showing a 4.8% potential cost saving compared to the generic jacket. In the comparison with monopiles in shallower water and with fewer numbers of foundation the GBS does not show to be competitive. But for specific conditions, particularly when placed on a hard soil that does not allow for the piling that a jacket requires and utilising the new generation 8MW wind turbines the benefits for the GBS will further increase.

Demonstrator project
With the concept for the GBS BAM and Van Oord have participated in multiple pre-qualifications, design competitions and tenders allowing further development of the concept. The technical evaluation of the GBS has reduced the risks that were associated to the concept to the extent that his has now been selected for a demonstrator project in the UK.



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