Composite joints for offshore jackets

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TKI Wind op Zee
Afbeelding boom

Start-up Tree Composites develops a composite joint for offshore structures such as jackets, as an alternative to traditional welded joints. This disruptive innovation ensures a superior load transfer that drastically decreases the amount of steel typically used in structures. Moreover, the TC-joint is stronger, corrosion-resistant, and speeds up the assembly of multi-membered structures. Once getting past the hurdle of installing a full-scale offshore demonstrator in combination with a wind turbine, Tree Composites can focus on upscaling, says director Maxim Segeren. 

What is the problem you have found a solution for? 

“Multi-membered structures such as jackets have one of the best steel/stiffness ratios and therefore seem to be the most optimal structure for offshore wind turbines. However, monopiles are currently dominating the offshore wind foundation market even though a monopile is much heavier than a jacket. Furthermore, studies show that monopiles will only become heavier in the future. The relatively low market share of jackets is due to the lower manufacturing speed/capacity and higher cost compared to the monopile. This has to do with the challenge of welded TKY-joints. An offshore jacket in 40-to-50-meter water depth consists of 28 or more joints that require manual welding. In these joints, the stress concentration reduces the fatigue resistance of the structure with factors up to 1000. To ensure a lifetime of 20 to 25 years this results in an increased wall thickness of the tubes and overspending of steel up to 300%.  

What is the core of your solution? 

“As an alternative to welding, we designed a joint made of glass-fibre reinforced composite material. Our TC-joint is formed by connecting steel tubular using a composite wrap. The load is transferred through the wrap which has been applied in layers where it is necessary. With this approach, we utilize a large surface for the load transfer with which we can reduce the stress concentration and ensure a long fatigue life. Additionally, the composite wrap protects the steel against corrosion. With our TC-joint, we use the stiffness of steel and the fatigue resistance of the composite.” 

What is so innovative about your solution? 

“The TC-joint is the first non-welded steel joint in offshore structures where composite functions as the primary load transfer. The joint is inspired by a 150-year-old tree that grew around steel tubes to support and extend the lifetime of its branches. The tree grew around the tubes and added material where required. Our joints are based on the same principle, hence the name. This disruptive innovation is invented by Marko Pavlovic, assistant professor at TU Delft. Composite material is already used in combination with steel in other applications such as aircraft wings and turbine blades, but not as a primary load transfer in offshore structures.’’ 

What are the benefits of the TC-joint? 

“Reduction in time and costs, a longer lifetime, fewer headaches, less steel, less weight, and lower CO₂-emissions. Composites ensure a better load transfer, lowering the stress concentration at the joints. A 100-year lifetime will be viable and affordable. As our joints do not require additional corrosion protection, we save time and important activity during the construction phase. In addition, the construction is lighter and stronger and has a suitable and safe failure mechanism: the connection will inform you when it's failing years in advance. Finally, we double the production capacity, expected from 1 to 2 per week, comparable to the lead time of a monopile. We have also calculated that a TC-joint will reduce 30% to 60% of steel consumption in an offshore jacket structure. Because steel production costs a lot of energy, CO₂-emissions decrease accordingly. All this reduces costs and benefits the environment.” 

How far are you now? 

How far are you now? 

“We started our research four years ago. One year ago, we founded Tree Composites. We have performed 90 tests. and have now obtained a statement of the feasibility of DNV. In May we started the research project WrapNode-l, in which large parties such as HSM Offshore, Smulders, Siemens Gamesa, Shell, AOC, BuFA, Salzgitter Renewables, and Enersea participate, in addition to TU Delft. This project falls under the GROW initiative and receives support from the Renewable Energy Scheme. The aim of this project is to test the scale effects and other influences. So far, everything has gone beyond expectations: the steel always yields or breaks before the composite joint fails. Next to the R&D project, we are busy getting a pilot project off the ground.” 

What are your biggest challenges? 

“The TC-joint is completely new. There are no guidelines on how to design and produce it. Together with TU Delft, we developed models that predict how the joint will behave in saltwater or in the sun. We believe that it is a matter of time to quantify the influence of all factors. The real challenge is the acceptance of the market. For pilot projects, the risks considered by market parties can be manageable. For full implementation in a commercial project these risks change. It will be assessed by various parties on the risks. That is why we work in line and in consultation with DNV as a certification institute and we have the entire value chain involved in the project consortium. In this way, we include the industry in our progress, validation and implementation.”   

A model of the jacket with TC-joints in green.

What are your next steps? 

“The WrapNode-1 project will take another two years. We are now at TRL 5-6 and want to progress to TRL level 6-7. To this end, we are setting up pilot projects were a complete structure will be built and tested, besides full-scale tests with 2-meter diameter tubes and branches with diameters of 800 to 1200 millimetres. We deem it feasible to have a jacket structure with TC-joints incorporated in a wind turbine, in operation within three to four years. That’s the biggest hurdle. Ultimately, we trust the TC-joints will become the new standard for joints for multi-membered structures.” 

What is the added value of TKI wind op zee for you? 

“Through TKI Wind op Zee we can get access to a large network of potential partners. They are very valuable for collaborations between innovative start-ups and the industry. This link between knowledge institutes, government, and industry is good for the Dutch offshore wind industry. TKI Wind op Zee is a central point where everything comes together. They also put offshore testing sites on the agenda which are necessary to demonstrate innovations and accelerate market acceptance.” 

This article was written in cooperation with Offshore Wind Innovators.

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