The preferred foundation for offshore wind turbine generators (WTG) is a monopile (MP) foundation with a transition piece (TP). In this combination, the TP fulfills numerous functions, including:
The most common connection in European offshore wind farm foundations between MP and TP is a grouted connection with an ultra-high strength grout material.
Although grouted connections have been proved a successful technique, the industry has started using alternative connections methods, such as flanged connections, in which the MP and TP are bolted together - comparable to the flanged connection within the tower. The boat landing and the corrosion protection (anodes or ICCP) are directly connected to the TP in a typical grouted connection monopile foundation.
For monopile foundations with bolted connections, the boat landing and corrosion protection are either partially connected to an outer skirt or connected directly onto the monopile. The offshore industry is currently considering two options: one, back-filling the skirt with a special grout, or two, leaving the skirt unfilled.
The skirt is also becoming shorter in recent projects, and does not extend below water level, which means that boat landings and corrosion protections are being considered as part of the monopile (separate from offshore operation during installation).
The trend in the offshore foundations seems to go towards flanged connections between the MP and TP since installations can be faster and more cost-efficient. However, major risks are associated with this procedure, such as corrosion and pre-tension aspects, leading to an unexpected increase in maintenance costs over the lifetime of the WTG.
On the other hand, MPs with grouted connections have a proven track record, especially after the implementation of the new design guidelines. They overcome all problems associated with flanged connections, including corrosion, secondary steel and maintenance costs.
Load transfer from the WTG to the foundation with the classic MP foundation is achieved by the grouted connection. Since the foundations are not only exposed to axial loads, but predominantly to bending and torsion moments, MP foundations with a grouted connection are typically designed with conical cross sections or cylindrical cross sections in combination with shear keys. Each layout has to overcome the earlier slipping problems with pure cylindrical MP/TP connections. The use of shear keys considerably increases the load-bearing capacity as the load transfer is the result of the struts effect between opposing shear keys.
Also with conical-shaped grouted connections, the bending moments and axial loads are effectively distributed and transferred to the foundation. Even minor angles of 1.5° considerably increase the load capacity of these foundation types.
Grouts and grouted connections are well covered by offshore standards. The guideline DNV-OS-C502 and the Z.i.E. (single case of approval) process in the German exclusive economic zone (AWZ) define requirements that have positively contributed to the quality of the grouted connections. Improved installation methods together with the possibility of applying the grout in shorter weather windows have strongly improved the durability and longevity of MP foundations with grouted connections.
Flanged connections are a well-known technique in steel construction, like onshore or offshore wind turbine towers. These connections, however, come with many hurdles and challenges when used near seawater conditions, especially if the MP-sided flange is impact driven during installation.
The bolted connections need special attention because the highly corrosive environment of the seawater causes steel corrosion of the normally galvanised components. Damages to the MP flange from impact driving need to be retrofitted, while imperfections in verticality need to be corrected using foundation-specific solutions, such as adapter rings. Due to the magnitude of current turbine generation, and corresponding loads acting on flanged connections, typically large bolts (M64-M72) are used at the monopile to transition piece connection.
With a view on the future of the industry and the current development trends, it is safe to assume that the WTGs are getting bigger with larger loads, which would require bigger and more bolts in the flanged connection. Therefore, the flanged connections are likely to be used with 8-10 MW WTG generations if further bolt sizes will not be standardised.
In order to avoid corrosion, additional measures need to be taken. Such measures include installing seals within the skirt to protect the connection from water, or filling the skirt with a grout material. However, these protection measures have to be verified concerning their long-term durability. In this context, the constructive design and execution details of each are essential for the durability. Based on the number of projects with grouted connections in the splash zone, one can assume that the failure rates of a grout-material filled skirt should be less than the relatively recent versions of groutless protection concepts.
MP foundations with grouted connections, with their pros and cons, have proved successful with the advantages clearly outweighing the shortcomings. Flanged connections, however, have only been recently used, and need special attention and careful design to guarantee a durable and low-maintenance foundation. With such flanged connections, volume stable grouts with validated quality should be used to guarantee the high durability and longevity of monopile foundations.