With much larger turbines in development, the offshore wind supply chain needs to respond to the demands they will place on foundations and develop new ones that are cost-effective to manufacture and install. Solutions are at hand
In the long-term, floating foundations could dominate the market where water depths preclude the use of bottom-fixed foundations. However, in shallower water new, more capable bottom-fixed foundations will be required that can be installed quickly and easily with reduced environmental impact and manufactured cost-effectively in large numbers. Potential options include new, larger diameter monopiles, jackets and gravity-base foundations that can be floated into position and submerged.
One example of the shape of things to come can be found off the northeast coast of the UK at the newly-opened Blyth offshore demonstrator windfarm. Here EDF Group installed five turbines with a total generation capacity of 41.5 MW.
The test site was the first use of ‘float and submerge’ technology – in the form of gravity-based foundations – that many in the industry believe has significant potential to simplify installing foundations and turbines and reduce costs.
Installation using the new float and submerge technique was one of a number of EU-funded Demo Wind projects designed to test innovative technology. The aim of the project was to validate the feasibility of float and submerge at all stages of the process, from manufacture and quayside construction to installation and operation.
The gravity-base foundations used in the project were manufactured at Shephard Offshore in the UK at a drydock and floated out in July 2017. They are now in operation with MHI Vestas Offshore Wind’s V164 turbines.
The aim of the project was to move the gravity-based foundation and the float and submerge concept from technology readiness level 6 to 7, and to verify the manufacturing and installation methodology to optimise plans for future gravity-base foundations while comparing actual costs and performance with alternatives. The project partners believe the potential benefits include reduced installation costs, employing standard tugs and self-buoyancy rather than specialised vessels, and lower costs during the operational phase due to reduced inspection and maintenance. Another important benefit is that the foundations can be used at sites where piling is not technically feasible.
New foundations which are evidently quick – and therefore less expensive – to install and more environmentally friendly than those driven into the seabed have been given the ‘thumbs up’ by developer Vattenfall following their use at the European Offshore Wind Deployment Centre (EOWDC).
Using a totally new type of foundation at the EOWDC was always going to be potentially challenging but as the project progressed, installation of the suction bucket jacket foundations turned into one of the project’s biggest successes.
The novel foundation type supports a three-legged jacket which in turn supports a turbine. Unlike a conventional foundation, such as a monopile which is driven into the seabed using a pile driver, the suction bucket jacket can be installed without a pile hammer and without the underwater noise that pile driving causes potentially affecting marine life.
The foundations are lowered into place from an installation vessel. Once in place on the seabed air and water are pumped out of the suction buckets, anchoring them in the seabed. Unlike pile driving, the process is virtually noiseless. It has the added advantage that, when the project is decommissioned, the installation process can be reversed, removing the entire foundation.
Vattenfall’s project director, Adam Ezzamel told OWJ, “There is no doubt about how well the suction bucket jacket foundations worked. It was a massive success for the project. There was undoubtedly a learning curve involved for the installation contractor, but I am in no doubt that they are quicker and easier to install than many other foundations.”
The EOWDC project may have been the first use of suction bucket jacket foundations, but their use is now spreading – April 2018 saw GeoSea’s vessel Innovation loaded with the first suction bucket jacket foundations for Orsted’s 450 MW Borkum Riffgrund 2 offshore windfarm in the North Sea.
As the size of turbines grows and as water depths increase, so jacket foundations have also come into focus, as engineers at Atkins, part of the SNC-Lavalin Group noted in a presentation at Global Offshore Wind 2018 in Manchester in June.
They have been examining the potential benefits of steel jacket structures founded on driven piles. They found that this kind of structure is well-suited for use in the offshore wind industry, especially as turbines get larger and water depths increase. However, although this has been used in the offshore oil and gas industry, in the offshore wind market their potential application differs in a number of important respects.
Atkins chief geotechnical engineer Sebastien Manceau and senior geotechnical engineer Robert McLean explained that, in the offshore wind application, the loading regime for jacket structures on driven piles would differ significantly from that in offshore oil and gas. In the renewables application the focus would be on the structure’s natural frequency and on fatigue-related issues. A production line approach for offshore wind – in which large numbers of identical units would be produced – would also be quite different from offshore oil and gas.
They noted that the selection of a reliable design method to assess pile axial capacity would be critical and, if done right, could lead to significant cost reductions. Another important area requiring further study is potential degradation of pile shaft capacity under cyclical loading. This would require careful assessment, they said.
As in the offshore oil and gas industry, said the Atkins engineers, pre-piling would be advantageous and enable standardisation of the jacket design through a ‘clustering’ approach which would also lead to fabrication and installation efficiencies. Clustering could be useful in other ways – clustering using variations of pile stick up could be used to even out differences in water depth across a site.
Arup associate Steven Downie told the conference that as the industry looks to 12-15 MW turbines (compared with the largest units currently, which are 8-9 MW units), it would need to consider the effect of non-linear wave loading effects on larger monopile foundations, in particular a phenomenon known as ‘ringing’ that has a bearing on the ultimate design strength required in a foundation.
He explained that extreme wave loads on larger diameter monopile foundations – which might be more frequent in windfarms built farther offshore in deeper water – can significantly increase dynamic excitation of the structure due to ringing. He said although ringing is recognised as a potential effect in design specifications for monopiles for offshore wind such as DNV GL-ST-0126, conventional design techniques do not capture ringing response, the level of which has generally been acceptable in smaller diameter monopiles.
However, with the trend towards larger turbines and larger diameter monopile foundations, experience from the offshore oil and gas sector suggests that ringing could become an issue. “Most existing windfarms have avoided the need to carry out detailed ringing assessments and the dominance of fatigue life in foundation design has also played a role in the limited attention ringing has seen to date,” he said.
“In future there will need to be more attention given to non-linear wave load effects that have the potential to become a governing factor in the design of monopile foundations.” He noted that the process adopted by the offshore oil and gas industry to address this issue relies on time-consuming and expensive analysis that is not practical for offshore wind due to the joint probability of wind and wave action. “An alternative method, with common acceptance by the offshore wind industry is needed,” he concluded.