TetraSpar

A floating wind turbine prototype, the TetraSpar Demonstration Project, is now operational at the Marine Energy Test Centre (METcentre) 10 km off the coast near Stavanger, Norway. The TetraSpar is holding a 3.6 MW Siemens Gamesa Renewable Energy wind turbine at an ocean depth of 200 meters. The TetraSpar Demonstrator is developed by Stiesdal Offshore Technologies and CREADIS provided engineering solutions for the foundation. The TetraSpar Demonstrator is owned by Stiesdal, Shell, RWE and TEPCO.

The pyramid-shaped floating structures are made from pre-made tubular steel components manufactured offsite. The tetrahedral foundation parts are connected by “pins,” and no welding is needed at the harbour.

“You can use relatively inexpensive cranes at the harbour, where you fully assemble the structure and mount the turbine and keel, then tow it to the installation site. It’s an unprecedented operation,” says Simon Christensen, Senior Engineer at CREADIS. “Nothing like this has been done before. Being able to put the foundation together simply is very much what has driven the design.” He adds that easy assembly and scalability were a large part of the development work with several partners, such as Scandinavian crane specialists BMS.

One-tenth the time to produce
According to Stiesdal Offshore Technologies, the TetraSpar is expected to offer important competitive advantages with its potential for lean manufacturing, lean assembly and installation processes and low material costs. As the “world’s first fully industrialized offshore wind foundation,” Stiesdal estimates that it will reduce manufacturing hours by 85-90%.

CREADIS provided engineering services to the product development, structural calculations and engineering of the structure. More specifically, the CREADIS work comprised elements of the primary structure design – including connection elements – and the secondary structure design, including the external work platform, boat landing, cable I-tube, and cable pull-in equipment. Work also included solutions and procedures for assembly, load-out, towing and ballasting, as well as engineering support for transport, lifting and installation equipment according to current regulatory standards.

Next step: monitoring data
The next step is monitoring the data from the installation site.

“The TetraSpar is packed with sensors, and the turbine itself is also producing a lot of data,” says Rene Holmgaard, Specialist Mechanical Engineering. “We are processing this data and producing packages for the investors so they can look at the dynamic behaviour of the foundation and power production and so on.”

Offshore wind foundations have until now been mostly limited to structures mounted on the seabed. The TetraSpar floating foundation relies on buoyancy and stability from a heavy keel that hangs 30 meters below the structure for a relatively deep-water concept. “It is basically irrelevant how deep the water is,” says Rene Holmgaard. “But of course you need to be connected with your mooring system and the grid cable.”

The floating foundations will therefore make it possible to place turbines at great depths that have otherwise been technologically and economically impractical.

“Offshore wind is growing, and eventually it will move into deeper waters where fixed foundations aren’t possible, but the wind is better. That’s where the floating foundation comes in,” says Victor Digiovine, Business Development Manager at CREADIS.