Approximately ten years ago the choice of offshore wind turbine foundations was rather simple: at up to ten meters’ water depth, it would be concrete gravity-based foundations; at 10 – 25 meters, monopiles were the choice; and above 20 – 25 meters it was either jacket or tripod structures — although no one exploited sites at those depths.
A decade later, while these choices still dominate the market, a larger variety of foundation types are available. There are large gravity-based foundations, tripods, twisted jackets, concrete monopiles, suction tripiles and others. So how do you make the right choice for a project?
Wind resources, shipping lanes and impact on nature reserves are the main factors behind site selection — alongside proximity to harbours and good grid connections. Unsurprisingly, foundation selection in the early stages of a project is at best an educated guess. Costly soil investigations are normally scheduled rather late in the development process, leaving little time to look at innovative options.
Once data on soil properties become available, the first theoretical calculations can be made. Combining turbine loads with soil and metocean data will give an idea on foundation type and size. Time and cost constraints make developers unlikely to look closely at non-proven options; if technically feasible, monopiles tend to be favoured for their simple structure and rapid, proven installation method.
Cost jigsaw puzzle
Based on foundation size, the next step is to estimate engineering, procurement, construction and installation costs. This is rather like completing a jigsaw puzzle. The use of several fabrication yards, transport routes, harbours and vessels will be estimated, alongside a range of foundation structures. Using common sense and experience — both relatively rare in an emerging industry — a decision will be made on foundation type, with design, costs and scheduling all being considered. An additional percentage will be factored in for risk.
This process has a big downside. In the current market there is a strong demand for cost reduction — for today’s foundations and for those destined for future offshore projects, which will be further offshore, in deeper waters and with larger turbines. New concepts need to be developed, tested and applied. But time constraints, combined with the legitimate requirements of financers and insurers for robust, proven solutions, are showstoppers for the development of new foundations.
The development of turbines typically goes from theory to prototype to zero-series and certification, culminating in market entry. This route cannot be duplicated for foundations because of factors including:
Insufficient R&D budgets in construction industry
Split responsibility for engineering, fabrication, transport, installation and maintenance
Limited scope for protecting new technical solutions as they are being developed
Uniqueness of each foundation to suit specific turbine and site
Need for large investment even for a first prototype.
There are a number of research and test initiatives in the market, including the Carbon Trust’s Offshore Wind Accelerator, the Innogy 1 test site, Aberdeen Bay and the Frederikshavn test site. These sites are often used to test newly developed turbines too. A turbine prototype is unlikely to be installed on a concept foundation structure. The investment needed to test foundations is in the order of tens of millions of euros — too much for single suppliers and owners.
Confidence in foundation concepts must be underpinned by investment in reliable designs, working methods, certifications, onshore testing and cost forecasts. Cost reductions of €100,000-300,000/MW can be achieved — €4-12 billion by 2020 for the whole sector, based on an estimated 40GW of offshore development. However, this can only be achieved if the offshore wind industry starts acting as an industry. As always, costs must be incurred before profits can be realised.
Edwin van de Brug is commercial manager of foundations supplier Ballast Nedam Offshore