Industrialized offshore foundations: ‘The Tetra Concept’

By Eize de Vries

Five years after former Siemens Wind Power (now SGRE) chief technical officer Henrik Stiesdal retired, his new business Stiesdal Offshore Technologies aims to deploy a novel industrial floater later this year. We spoke with the Danish wind pioneer on the background of especially his modular industrialized Tetra floater concept and how this could decisively lower the cost of floating wind.

The shared principle behind the Tetra Concept (Tetra floaters and fixed-bottom TetraBase) is a triangular steel structure composed of pre-manufactured industrialized modules. The use of standardized components allows advanced simplified manufacturing and welding automation. The Tetra system is fully modular with all components and subassemblies factory pre-produced and pre-painted, whereby many process steps are either fully identical or largely comparable for different floating and bottom-fixed applications.

Stiesdal’s current Tetra project setup comes available in four different functional configurations for water depths between 40 – 1000m+. It all commenced in 2015 with an open source project aiming to bring the industrialized approach to the market. Stiesdal: “We had valuable dialogue with parts of the offshore wind community, and in particular DNV GL generously provided very competent technical support free of charge. However, after having by the end of 2015 completed the initial design, we discovered that while universities and suppliers found the open source approach very intriguing, project developers showed no interest in an open source set-up. It was then decided to establish a commercial business and create original in-house IP.”  

This resulted in a ‘basis’ TetraSpar configuration incorporating a unique radical innovation, a retractable keel that can be pulled up during quay-based turbine mounting and towing operations, and lowered in operating mode. The solution offers the smooth stable operating dynamics of a spar floater, but with very shallow draft in port and during towing instead of typically 80-100m+ water depth requirements for conventional spar-type. TetraSpar is suited for 100-1000m+ water depths. Stiesdal later introduced a second TLP-variant focuses at sites with ‘narrow footprint’ like sea areas where floating wind competes with fishing, and it comes with a comparable 100-1000m+ water depth range. The third and final addition is semi-sub configuration suited for a 40-1000m+ depths range.

Stiesdal: “Ready modules for TetraSpar are transported by road to an onshore quayside with sufficient heavy load handling capacity, where all floater parts are assembled without requiring additional cutting or welding. Our goal is that in a serial version the entire assembly process will take only a few days.”

In a next step the turbine is mounted using a land-based mobile crane, and the ready unit is then in floating mode towed to a wind farm site without requiring installation vessels.

Stiesdal: “This first, open-source project stage was a paper-exercise only, successfully concluded by an initial technology validation. The second, commercial stage has involved the full-scale floater design, and numerous wave tank tests. We have tested a 1:80 model of the concept design at the DHI wave tank in Hørsholm (DK), a much more detailed 1:43 model of the near-complete design using the comprehensive wind-wave test facilities at the University of Maine (US), and a 1:35 model of the final design at the FORCE wave tank in Lyngby (DK). A key element of the tests has been to validate our simulation tools, and in addition we have tested a wide range of installation and operation situations, including extreme waves with 50-year and 1000-year recurrence levels.”

After leaving SWP, Stiesdal had no intent to reengage in something new that would compete with his former wind industry colleagues, and opted instead for two largely unexplored wind industry areas. The Tetra Project offered fresh opportunities for reutilizing his comprehensive experience in industrializing wind turbines. A second GridScale project involves a novel low-cost ‘hot rock’ energy storage solution aimed at enabling ‘an unlimited share of renewables on grid.’ He considers both technologies essential building blocks in the ongoing energy transition process with a main role for renewables, but GridScale will not be discussed further in this article.

Stiesdal’s selection of floating offshore wind as a “largely unexplored area” did not reflect on any perception of lack of skills or competences by incumbent players. Stiesdal emphasizes his deep respect for the early movers, not least Equinor (Norway), Principle Power (US) and Ideol (France). Rather, it was driven by a desire to see floating offshore wind develop much faster than has been the case for conventional bottom-fixed offshore wind power.

“Important continous success factors are modularization and standardization”

The main reason for the dramatic cost reductions in offshore wind since 2015 has been that the supply chain has finally been industrialized. Stiesdal’s observation on the emerging floating offshore wind sector was straightforward – “Current floater designs are typically not industrialized – we need to do something!”

Stiesdal: “These structures tend to be quite heavy set against their matching 2 – 9MW+ class turbines, and also tend to use ‘traditional’ construction methods from shipbuilding and oil & gas. In addition, fabrication is typically conducted at ports of floater launch, with built times measured in months, and manufacturing/assembly processes, whether in concrete or steel, are generally labour intensive.”

Based on his decade-long experience with industrializing wind turbines, Stiesdal further considered especially at the huge success of tubular steel towers. He explained that over 20,000 of such towers in many different sizes are manufactured annually across the globe and with highly industrialized processes. These tubular steel towers are further characterized by low cost per kg, advanced (welding) automation, and end products with high-quality surface protection coating.

“Important continuous success factors are physical separation of fabrication and installation locations, modularization and standardization, and no significant IP issues. The latter factor is crucial because it promotes cost-effective manufacture through open competition”, he said.

Another observation Stiesdal made is that often things cannot be made as big as we would prefer. Like for instance how to get increasingly larger structures like wind turbines and their towers to launch ports. The answer is in pieces! This means in the wind turbine example splitting the entire unit into nacelle, (gearbox-) generator, blades, and tower segments. It may well be that turbine owners would prefer a tower made in one piece, but this requires facilities close to wind farm construction sites. Splitting the tower in sections is a logical compromise, and the extra flanges and bolt-joints are a small penalty relative to the cost savings of the modular approach. Comparable modularity principles are applied with Tetra encompassing solutions considering all aspects from manufacture, transport logistics, to assembly. Equally important for the Tetra concept is that manufacturability has clear preference over innovation and achieving ideal technical solutions.

Stiesdal: “It meant for instance that our initial pride of the innovative aspects of the TetraSpar retractable keel concept gradually was replaced by a realization that we could use the manufacturing concept for other, less innovative configurations also. For example, we could make more conventional TLP and semi-sub configurations with the same manufacturing benefits.”

He added that some of these observations and practical lessons learned were put in practice already for the TetraSpar pilot project. Examples are that no part should be longer than a state-of-the-art rotor blade, no part should have larger diameter than the largest part of the tower, or heavier than the heaviest tower part. 

Developing TetraSpar unavoidably led to constraints too, but the choice for a fully modular industrialized structure worked out well in the end said Stiesdal: “The power of industrialization through industrializing supply chains is simply overwhelming and reflected by huge cost reductions and added benefits. This is clearly illustrated by the cost reductions achieved during the last century, and by the ongoing solar PV price drop from 100US$ per watt in 1978 to less than 1US$ per Watt today.”

Stiesdal predicts a big future for floating wind in many geographical regions, with key enabling market conditions major population centres and (near-shore) water depths above 40 – 50m like for instance California and Japan. He quoted the floating wind capacity potential in California at more than 500 GW (source NREL). This translates potentially in 2,000,000GWh gross annual energy production potential assuming 40% capacity factor, which equals 10x the current total power generation (GWh) in this populous wealthy US state. The floating wind capacity in densely populated Japan is according the Japanese Wind Power Association 300GW.

“Floating wind has a big future in many geographical regions, like for instance California and Japan”

The TetraSpar Demo unit is financed by Dutch/UK oil & gas giant Shell through its New Energies division, and by German energy company Innogy. The two giants have 66% and 33% shares, with Stiesdal Offshore Technologies taking the remaining 1%. Danish steel construction specialist Welcon currently builds the components at their tower facilities in Give. These ready components will then be transported to the Danish port Grenaa for final assembly and mounting a 3.6MW Siemens Gamesa SG 3.6-130 DD direct drive turbine with 130-metre rotor.

The following step planned for late summer is towing the floating unit to a demonstration location in the Norwegian northern North Sea 10km off the Stavanger coast, and mooring it with three anchor lines. Stiesdal concludes: The test site has 200-metre water depth and belongs the Norway’s Marine Energy Test Centre (Metcentre). Actual testing and validation will commence later this autumn.

“Our next goal is entering the floating wind market commercially. We hope to reach a 50 – 100 US$/MWh LCOE level once we are in volume production. This would be comparable to bottom-fixed offshore wind in many parts of the world.”

The Tetra concept can also be applied to bottom-fixed foundations for 10-60m water depths. The basic principles are the same as for the floating variants – factory-made modules assembled at quayside.

TetraBase installation process is straightforward. The foundation is set on seabed in port for turbine mounting with the aid of temporary ballast tanks. These are de-ballasted for towing to site. After arrival, these tanks are ballasted again for lowering the foundation to the seabed. Finally, the foundation is water-ballasted and the temporary tanks floated off.

The main potential for bottom-fixed foundations is today in Northern Europe and along China’s long coastline, with a major expansion potential at the US East Coast expected in the years to come. But Stiesdal does not have immediately the European market dominated by monopiles in mind for TetraBase ‘because it is very hard to compete with something as simple and efficient as monopiles.’ He elaborated further: “We might have better chances in the US, which has no ready supply chain for monopiles, with additional restrictions regarding installation noise, and other bottlenecks imposed by the Jones Act. Fresh opportunities could on the other hand arise for next generation 15MW class offshore turbines, which would require 2500-3000-tonne range monopiles and these are perhaps not easy to manufacture and install. On the other hand, the monopile foundations have over and over again proven more stretchable than expected. It may well be that in the long run we are going to see bottom-fixed monopiles out to a certain depth, and then from there onwards floating foundations, with nothing in between!”