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Development of Modular Steel Jacket for Offshore Windfarms

Contract RFSR-CT-2015-00024

The majority of the future offshore wind farms will be bigger in terms of capacity, going into deeper water and moving further away from the shore. The potential for wind farms at deeper water is huge provided that costs can be reduced to a competitive level. Considering that much of the investment for offshore wind (around 20% of the capital costs) is paid for substructure, cost savings in the area of support structure fabrication may therefore yield sizeable reductions in overall cost of energy for offshore wind.

The aim of this research activity is to reduce costs by incrementing prefabrication of offshore wind substructures, by the exploitation of Modular Jacket concept, composed by prefabricated standard components, scalable in a significant range of environmental conditions. This leads to increased availability of jacket components, ready for massive jacket fabrication and installation.

A jacket substructure is under consideration since it is suitable for transitional water depth (30-80m) and up scaling turbine size. A jacket is a truss like tower composed by circular hollow sections welded in X, DK or DKT configuration. It is a structure with low mass/stiffenes ratio where the cost of the structure is driven by the large number of complex welded joints to be manufactured. For such a structure, subjected to combined wind and wave loads, fatigue is the design driving limit state.

The Modular Jacket is designed and verified through a wide range of environmental conditions, from the shallow North Sea to the deeper Mediterranean Sea, supporting wind turbine of size 8MW and 10MW. In this new concept, the structural components of the offshore jacket are designed as prefabricated sub-assemblies. Emphasis is given on developing solutions for performing the maximum number of welds in the factory, under controlled conditions. This ensures automation of fabrication, and consequently reduced oscillation in final behavior, potentially leading to higher reliability and lower safety factors to be applied. Fabrication trials and testing of Standard Components are performed and the impact of the new execution concept on costs and quality level is analyzed.

A series of data are made available for the 6 different windfarm scenarios analysed, in particular:

- Jackets geometry

- Environmental load data

- Design charts for foundations dimensioning

- Prefabricated component data (geometry and welding procedures)

- Stress histograms sustained the prefabricated component

- Structural fatigue performances experimentally validated

- Cost analysis

Courtesy Ramboll ®