With the increasing size of turbines and the deeper location of wind farms, the monopile foundations of offshore wind turbines increase in size, weight, and costs. Jackets can be attractive alternative support structures because they are lighter than monopiles. However, the complex welds result in higher costs and longer production route for jackets than for monopiles.
The complex welds in the joint region significantly reduce the fatigue resistance of structural joints of circular hollow sections. If the fatigue resistance for these joints can be increased, the tube wall thicknesses of the legs (chords) and brace members can be significantly reduced resulting in lighter structures and less material use. Wrapped composite joints offer such improvement. Compared to complex welds, the composite joint transfers the load through a dedicated composite wrap and not through the small area of the weld.
Composite joints can replace all the complex welding in jacket structures allowing for an increase in fatigue resistance. By applying composite joints, the amount of steel in a jacket is reduced with 40% to 60% compared to a welded jacket. In addition, the production time of the jacket is considerably shortened due to the possibility of prefabrication. Compared to monopiles, jackets manufactured with wrapped composite joints offer a potential cost reduction of 25% to 50% for the supporting structures. By applying the innovative joint, the carbon footprint of the turbine foundations will be reduced by 30% to 70%.
To realize future commercial implementation and to demonstrate that the wrapped composite joint is a technically superior and cost-effective solution the development is divided into three phases. In this project (WrapNode-I), we carry out full-scale joint tests and execute a performance validation.
Within the WrapNode-I project, we will test full-scale wrapped composite joints by applying multi-axial loads and investigate the influence of offshore environment. We will validate the technological and economic feasibility of offshore jackets on critical points. We will perform the following activities:
The WrapNode-I project will provide a solid basis for the implementation of the wrapped composite joint in jackets for offshore wind turbines. The project will provide a design of a reference jacket structure that relies on a composite wrap instead of welds to transfer loads. It will also provide test results for full-scale tests, load performance and durability in the offshore environment. In addition, a method becomes available to predict static and fatigue performance. The project will also provide insight into the cost, associated CO2 emissions and production rate of jackets with wrapped composite joints at full-scale.
In the case of positive project results on the full-scale joint tests, multi-axial loading test and environmental influences, we will initiate a follow-up project (WrapNode-II). In this project we will design, assemble and test a scale jacket onshore. In conclusion, we carry out a full-scale offshore demonstrator with the wrapped composite joints. In parallel, we will continue to validate the prediction methods, design procedures and manufacturing procedures for the wrapped joints which are required to enter the market.