Since Equinor (at that time Statoil) in 2009 installed the world’s first floating MW-scale wind turbine, a lot has happened in the sector. Something that at the time was seen by many as a daunting technology-leap with questionable economic feasibility, has now moved into forming its own separate commercial sector within the offshore wind industry.
Coming from an installed capacity of approx. 75 MW as of 2020 and moving towards a planned pipeline measured in GWs by the middle of the decade, it is clear that this sector is in rapid exponential growth.
This impressive market trajectory is only made possible through a strong technological and economical confidence from the respective developers.
For the curious spectator, the past decade has brought many different foundations designs and interesting concepts to the table. And while all these the concepts in the market might look wildly different, they all obtain their stability from one or more of the following:
To better be able to understand the basic differences between these design principles, the following will try to exemplify each by introducing the three archetypes in floating wind; the ballast stabilized Spar, the buoyancy stabilized Semisubmersible, and the mooring line stabilized tension-leg platform (TLP). While all three archetypes originate from the oil and gas sector, a good understanding of their intrinsic pros and cons will enable the reader to better assess their application and viability in the floating wind energy market.
The spar concept might be the most intuitive and simple of the three. It relies on having a heavy keel, ensuring that the total centre of mass lies below the centre of buoyancy. The distance between the aforementioned determine the static stiffness of the system to counteract e.g. the thrust forces on the wind turbine rotor plane. The often very large total mass of such a spar system is of course from a material usage standpoint not economically attractive, but it does imply that the system will inherently have good dynamic properties since the high system inertia results in natural periods well outside the ones of the wave spectrum. This in turn leads to low accelerations in the nacelle. For station keeping purposes spars often employ a simple three-legged catenary mooring spread where heavy steel chains on the seabed provides a restoring force when the system is laterally displaced.
One of the more significant drawbacks of the spar design is the relatively large draft required for the keel, often in excess of 80 m. In order to be able to assemble the substructure and turbine prior to arriving at site, the foundation needs to be upended in a sheltered area. This severely limits the applicability of this technology on a global scale where such deep ports and associated towing routes often simply do not exist.
The most notable example of a spar in the market is undoubtedly the Hywind foundation by Equinor.
The semi-sub concept instead relies on buoyancy to obtain its static stability, or to be more specific, it relies on the change to the centre of buoyancy to provide stability. Having a number of columns piercing the waterplane which provides a restoring moment when the foundation is exposed to an external force trying to push any of these columns deeper into the water. In this case the centre of buoyancy will move towards the corner pushed into the water, while the centre of mass stays fixed relative to the structure.
A big upside of the semi-sub is that the design often allows for relatively shallow drafts.
This enables access to most standard ports. Hence the expensive offshore lifting operations can be negated and instead handled quay-side by less expensive land-based cranes.
Having these relatively large structures in the waterplane means that the structure is inherently exposed to the severe wave loading in the splash zone. The large hydrostatic stiffness obtained from the large waterplane area also presents another challenge unique to this concept; the heave, pitch, and roll natural periods can come close to the wave excitation periods resulting in undesired dynamic loading. This effect is often counteracted by employing heave plates at the bottom of the columns. These plates ensure more added hydrodynamic mass and hence makes the system response slower, as well as providing some desirable damping properties.
As was the case for the spar concept, a semi-submersible foundation will often utilize a catenary mooring spread. However, this type of mooring is often not attractive in more shallow water cases where large footprints might be necessary to obtain the desired lateral restoring stiffness. To circumvent this effect, a lot of effort is currently going into developing shallow water mooring based on taut inclined compliant mooring lines of synthetic fibres.
Examples of Semi-submersibles in the market include the WindFloat foundation by Principle Power, and while often categorized as a barge, the Damping Pool concept by IDEOL. The semisub and the barge both rely on their waterplane area for stability, and thus, for this purpose can be classified as one. Other notable semi-subs include the V-shaped Shimpuu floater by the Fukushima FORWARD consortium, the Sea Reed by Naval Energies, and the coming OO-Star by Olav Olsen.
THE TENSION-LEG PLATFORM
As mentioned, the TLP obtains its stability directly from the mooring system. The buoyancy of the foundation is exerting a vertical force upwards which is counteracted by the downwards pull provided by the structural stiffness of the mooring line tethers. This results in a very stable system with good static and dynamic properties. As the TLP is not relying on the shape and size of its structural parts for stability, it can often be made as a notably lighter structure compared to the spar and semi-sub, implying reduced material and manufacturing costs.
This slenderer structure also has the benefit of being less susceptible for the larger wave loading associated with e.g. the semi-sub.
That said, a significant part of this cost-saving is channelled directly into the mooring and anchors. Since the TLP is not inherently stable, a failure in either mooring tendons or anchors will result in a system collapse. To be able to resist the large vertical loads in the anchors, simple drag embedded anchors are off the table. Instead, more expensive options such as driven piles or suction anchors must be utilized. This increased dependency to the geotechnical properties at the site is also a limiting factor for the TLP. All in all, this makes the cost of the mooring and anchoring system substantially larger than for the spar and semi-sub concepts. The inherently unstable TLP will also need some sort of auxiliary structure or supporting vessel in order to keep it stable while transporting it to site, again siphoning some of the structural cost savings into added installation cost
Of course, there is no rule without exceptions. The three conventional archetypes all come with their own respective set of upsides and downsides. Thus, some hybrids concepts have also been presented to the market.
These concepts are aiming at leveraging the upsides of multiple archetypes. The two most prominent technologies in the hybrid category is the TetraSpar by Stiesdal Offshore Technologies and the SBM Offshore Wind Floater.
The TetraSpar design aims at combining the hydrodynamic properties of the spar, with the low structural mass of the TLP, while still maintaining the low installation draft of the semi-sub.
This is done by having a suspended keel that can be incrementally lowered and ballasted during the towing and installation of the foundation, effectively transitioning from a semi-sub to a spar. Again, shifting cost and complexity around between design and installation. This transitional approach is also seen in the SBM Offshore TLP design. To negate the unstable towing operation of a true TLP, SBM has instead opted for adjusting the design to act as a semi-sub during towing, and only transition into a tension-leg stabilized system when arriving at site. This is done via a ratcheting functionality in the mooring fairleads. Again, taking CAPEX savings from structural cost and using part of that on moorings and offshore operations.
In Figure 1, some of the mentioned concepts’ stability origin has been plotted. The illustration highlights the fact that from a stability standpoint a lot of the systems are closely related. The two transitional hybrids are also depicted.
COMPARING THE ARCHETYPES
In the previous sections the source of stability for each archetype is presented. The stability of a floating foundation is of course the main design criteria since it ensures the safe operation and reliable energy production of the wind turbine atop. Therefore, many concepts are often compared on their performance in the installed state in which they will operate for multiple decades. While this is a very important data point, it is not sufficient to assess the total cost outlook when choosing between floating wind turbine foundation designs. Besides the already mentioned aspects of material usage, mooring systems, and power cables, other important process- and operational focused parameters that merits a cost/complexity assessment include:
Figure 2 shows an indicative high-level comparison of the three foundation archetypes where a few select areas of competitional difference are included. Since these are based on the pure archetypes, several technology developers in the market will have their own ways of mitigating their inherent downsides while enhancing the upsides.
In summary, the understanding of floating foundations is two-fold. The initial step is to recognize the choice of stability-measure at the installed state including the intrinsic pros and cons of said choice. The second, and probably more interesting step, is to be able to map these design choices in relation to all the other technologic and economic interfaces in a successful complete floating wind project; from engineering, procurement, construction, and installation through operation and eventual decommissioning. The earlier a project is able to complete this mapping, the better is the probability of obtaining a global optimum on the realized levelized cost of energy.
Morten Thøtt Andersen
Chief Specialist – Floating Technologies
Download the Whitepaper on the link below:
BPP – Floating Wind 101 – Foundation Archetypes
If you are interested in understanding the more detailed concepts behind it, please feel free to contact Morten Thøtt Andersen, Chief Specialist, Ph.D. – Floating Technologies, via email (firstname.lastname@example.org) or phone (+45 61 68 61 88)