TWD has designed a hammock to support monopiles during transport. This innovative seafastening is suitable for monopiles with a diameter between 7 and 11 metres. In addition, it divides the forces on the tube more evenly. This allows contractors to safely transport various wind turbine monopiles with a thinner and therefore more vulnerable wall thickness. An interview with Thijs Roethof of TWD.
For what problem have you found a solution?
“The diameter of a monopile varies with the power of the wind turbine and the water depth. It is clear that diameters increase and that wall thicknesses become thinner in relation to the diameter. This makes monopiles vulnerable, creating a problem for the contractors who transport the monopiles. Just as you can easily compress a soda can, a point load that seafastening-pads exert on a monopile is also a risk of damage.
TWD has designed solid steel seafastening systems for over 40 wind farms. Each seafastening is tailormade for every individual diameter, a ‘one-off’. As a result, a contractor must reinvest in the design and manufacturing of a seafastening for each new wind farm. Even within one wind farm there is a trend towards different wind turbines. Moreover, much time is spent to fit and remove these seafastenings before and after the installation campaign. (De-)mobilisation time that could be spent more efficient. The hammock we designed takes away these concerns.”
What’s the core of your solution?
“The construction consists of a steel frame with straps or belts of strong synthetic fibres (Dyneema). These are connected crosswise to transfer the forces in the longitudinal direction to the supporting frame. This geometry prevents the monopile from swinging back and forth. When the monopile is hoisted into the seafastening, the weight causes the frame to lock the monopile and closes, holding the pipe securely. Only a few contact points between the frame and the monopile are necessary to prevent movement in the longitudinal direction. They are designed to minimize the load on the monopile. For each monopile two hammocks are needed.”
What is so pioneering to your solution?
“The special thing about the hammock is that it looks so logical and obvious. Yet, no one has ever thought of this geometry with flexible slings. At the same time the concept is realistic and close to market readiness. The slings are already being used for hoisting the monopiles and according to the manufacturer it is no problem to use them for the continuous fixing of monopiles. The Hammock supports a large part of the pipe, distributing the forces evenly. That’s exactly why a conventional hammock is so comfortable. And just as both a child and an adult can lie relaxed in the same hammock, the Hammock seafastening can handle a monopile diameter ranging from 7 to 11 meters. This flexibility is unique.”
What are the benefits of your system?
“The flexibility and even distribution of forces offers great commercial benefits to installation and transport contractors. No heavy saddle construction is required. On a conventional installation vessel, it is possible to place multiple frames side by side and to mount a second set of hammocks above it. Four monopiles in total. The hammock is also scalable and therefore futureproof: the concept does not change with increasing pile diameters.
The advantage for contractors is that they can save considerably on investment costs and time if they want to build several wind farms using the same vessel. The initial investment costs are comparable. Moreover, they run less risk of damage during transport of the larger and more vulnerable monopiles.”
How far are you now?
“The basic design is ready. All vessel-independent calculations have been carried out. Because each transport vessel is different, we will be doing detailed design in close cooperation with the vessel owner, to get a tailed solution.. Especially installation contractors show serious interest. We are about to make a detail design for existing customers who must consider whether they will go for a conventional, proven seafastening or an innovative, futureproof hammock.”
What are your challenges?
“We are well-known in the wind industry but convincing a launching customer of the benefits of a new technology is a challenging and long process. They look at each other to take the first step. Specifically, questions about the longitudinal movement of the monopiles, keeps them busy. There is no need to worry. Just as with fixed seafastenings, the monopile’s own weight causes enough friction, preventing them to move. In fact, the geometry of the hammock, with its contact pads, increases the friction forces even more, allowing for larger longitudinal accelerations than conventional seafastenings.”
What are your next steps?
“If a contractor would place an order today, we estimate that within six months – three months of detailed design, three months of manufacturing by a third party – the first hammock can be ready. During the detailed design we will undoubtedly come across technical issues, but they are not insurmountable. We have tackled all the showstoppers.”
What is the added value of Offshore Wind Innovators?
“TWD was one of the finalists of the Offshore Wind Innovation Award 2018. The presentation during the Q-meeting in December and the video we made of our concept, gave us a lot of exposure and requests for more information. We notice that our target group is enthusiastic about our concept and we get a lot of positive feedback. All this exposure helps to strengthen our image as an experienced and innovative designer of seafastenings.”