With the increasing size of turbines and the greater depth in which they are placed, the monopile foundations of offshore wind turbines are increasing in size, weight and cost. Alternatives to the supporting structures, such as jackets, are lighter but are still more expensive due to their complex welded joints. A significant drawback of complex welds is the low fatigue resistance of the joints. When such fatigue resistance can be increased, the tube wall thicknesses of the legs (chords) and brace members can be reduced, ultimately leading to lower weight and steel costs.
The possibility of using thin-walled tubes without affecting the fatigue strength of the structure is to use composite compounds. Compared to complex welds, the composite joint transfers the load through a composite wrap and not through the small area of the weld. In addition, prefabricated composite joints also offer the possibility of significantly increasing the production speed of the casing constructions. Using wrapped composite joints may reduce pipe thickness by 80%, translating into 40-60% less steel usage. This will provide an estimated cost reduction for offshore wind developers of 25% to 50% and a carbon footprint reduction of 30% to 70% compared to jackets with complex welded joints.
In collaboration with Tree Composites, the TU Delft faculty of Civil Engineering has been developing this composite joint technology since 2017. Since the beginning of 2021, the technology has been further developed in the GROW WrapNode programme, comprising the projects WrapNode-I, WrapNode-II and WrapUp. The programme aims to demonstrate that the wrapped composite joint is a technically superior and cost-effective solution to enable future commercial implementation of the wrapped joint.
WrapNode-II builds on WrapNode-I which forms the scientific and experimental basis and will demonstrate that composite joints can be used to assemble reliable jackets. In addition, we will validate the methods to predict static and fatigue performance. Therefore, we will build a functional jacket structure using composite joints. During this activity, we focus on production methodology, tolerances achieved, quality assessment, repair and replacement methods, and structural capacity validation. In addition, we will expand and validate our prediction tool to reach technology readiness level 6. After the project, the composite joint technology will be ready for use in an offshore demonstrator follow-up project.
In this project, we aim to develop wrapped composite joints that have static and fatigue resistance in a realistic offshore environment that exceeds the resistance of the steel casing pipes to which they are connected, i.e. S-N curve class B1 according to DNV. We will demonstrate that composite joints can be used to assemble a jacket structure. We then design and manufacture full-scale wrapped joints that must exceed the design static loads in a plain shell and have a fatigue resistance higher than the circumferential weld, i.e. S-N curve class C1 according to DNV. The joints are intended to connect prefabricated pipes with a large diameter of 700 mm or more.
The project will deliver the following results:
When successful, the consortium will subsequently initiate the WrapUp project where we will demonstrate a full-scale jacket with wrapped composite joints in an offshore environment.
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