AXIS Explores
Offshore foundations
Overview
Offshore wind turbines can have either fixed or floating foundations.
Here we are focusing on fixed foundations which are grouped into three main types:
- Monopiles
- Jackets
- Other, including gravity based solutions and suction-cassion foundations
Monopiles and jacket foundations may be hammered, drilled and/or grouted into the seabed.
Every foundation is site-specific and must withstand the force of waves and tides, as well as loading from the Wind Turbine Generator (WTG) above. This is an engineering challenge that requires a good understanding of the seabed geology and metocean conditions.
As the offshore wind industry has matured, teething problems with early foundation designs have been largely designed out.
Nevertheless, damage to foundations can still occur during installation, and some common issues are still seen due to design or maintenance shortcomings.
Fixed foundations: (A) Monopile. (B) Jacket. (C) Gravity
Overview
- Monopile - up to 130 feet The preferred foundation for shallower waters, a rolled steel cylinder is drilled or hammered into the seabed. The wind turbine tower is then bolted onto a transition piece which sits on top of the monopile
- Jackets - up to 200 feet Common in the oil and gas industry, jackets are steel structures with three or four legs which are more cost effective than monopiles at greater depths. At the base of the legs are piles (smaller monopiles) inserted into the seabed using the same techniques as a full-size monopile, providing great structural strength
- Gravity Based Solutions (GBS) - up to 100 feet The least common in offshore wind, GBS are widely used in oil and gas. Though very heavy, they can often be towed into place by tugs, saving jack-up or vessel costs. They are typically constructed of concrete and/or steel
Construction phase issues
- During the installation phase, two main issues can be encountered in relation to pile foundation installation: pile refusal and pile run
- Pile refusal pertains to a monopile foundation, or jacket leg pile, encountering significant resistance into the soil, for instance in the presence of rocks or very hard soil layers. In this case, the pile cannot be driven further, and in the worst case can also be damaged. If damage is severe, a whole replacement pile may be required which can result in a large delay in start-up if spares have not been ordered
- Pile run refers to the uncontrolled fall of the pile through soil layers. In the worst case, the pile falls beyond its target penetration depth and cannot be retrieved without risking damage
- The risk of either event occurring can be mitigated by conducting detailed geotechnical surveys of the seabed prior to construction to identify challenging seabed conditions. The correct piling methodology and tools can then be selected based on this detailed geotechnical knowledge of the site, or alternative turbine locations chosen
- Suction-caisson foundations are only suited to certain ground conditions. If these are not met the foundation may become unstable following installation, or in extreme cases may be lost (for example if the seabed is mobile and allows the foundation to drift)
Operational phase issues
Catastrophic failure of the foundation is rare during the operational phase of an offshore wind farm, however:
- Defective welding of monopiles has been found on some projects and required very expensive subsea repairs
- Grouted connections between the foundation and the transition piece have weakened over time on many monopile projects, requiring repairs to avoid movement. Newer designs have overcome this issue
Corrosion (subsea) Anodes are fixed to the foundation to stop the foundation rusting. They require replacement over time.
Corrosion (in the splash zone) Paintwork on the secondary steel work (ladders, railings, fenders) must be maintained to avoid rusting. Poor quality control during manufacturing must be addressed during installation.
Erosion Currents can erode the sea bed around the base of the foundation causing instability. Scour protection is required to avoid this risk.
