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 OFFSHORE FOUNDATIONS

Overview (contd.)

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 crane costs. They are typically constructed of concrete and/or steel.

Fixed foundations: (A) Monopile. (B) Jacket. (C) Gravity.

OFFSHORE FOUNDATIONS

Construction phase issues

During the installation phase it is easy to damage the foundation if it is not handled correctly by the installation vessel.

If rocks are hit during pile driving, this can also damage the monopile itself, or the piles located at the bottom of the jacket foundation.

If damage is severe, a whole replacement unit may be required which can result in a large delay in start up if spares have not been ordered.

Detailed geotechnical surveys of the seabed prior to construction help mitigate this risk by identifying rocks and other obstacles.

Suction-cassion 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).

OFFSHORE FOUNDATIONS

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.

OFFSHORE FOUNDATIONS

Operational phase issues (contd.)

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.

Scour pic: https://www.jaegergroup.com/en/products/
green-products/marine-technology/scour-protection/