Joints in offshore structures

“Everything is getting bigger and heavier,” says Haico van der Heijden of TNO. “This is particularly relevant to joints. Take a simple bolted joint. It’s been around for about 200 years, so hardly worth making a fuss about one may think. But once you start scaling it up, you discover that you’re approaching the limits of what has been tested and conceived in the past. A bolt with a diameter of 36 mm is already a big chap. But a wind turbine is secured with an M72 twice that size.” 

Suitable for scaling up 

Because certain assumptions are no longer correct, there’s a chance that joints, such as from steel to steel or steel to composite, will break. “That’s why we’re researching which joints are best suited to being scaled up,” continues van der Heijden. “To start with, we analyse in the laboratory environment which parameters do or do not influence things. We do this in close cooperation with industry in order to find out what they’re up against and what else they experience.” 

Optical and acoustic sensors 

Another special area of TNO expertise is the development of sensors that monitor the status of joints, such as an optical sensor integrated into the blade of a wind turbine. “Sensors currently last two to three years, but we’re committed to optical, acoustic and other sensors with a lifespan of 20 years. This is especially desirable with the advent of digital twinning, in which we can also monitor structures and joints virtually.” 

Mimicking reality  

“Even an M72 can be pulled or vibrated apart,” explains Van der Heijden. “It’s a matter of force. But instead of deliberately destroying something, we mimic reality to gain insights into the behaviour of a joint. Sometimes, we do this with sensors or joints that we’ve developed ourselves; other times, we test and quantify a company’s application. In this way, every piece of research is a new and interesting challenge.”