Vattenfall reduces 30% installation costs for Denmark’s largest offshore wind farm thanks to new single-layer approach

One of the most prominent risks to offshore foundations is scouring due to wave and current action, which may significantly affect the total load on wind turbines. Today, scour protection for wind turbine foundations makes up a considerable part of the installation cost. Vattenfall appointed DHI and LIC Engineering to design and optimise the scour protection system for Danish Kriegers Flak (DKF), Denmark's largest offshore wind farm to date. The decision to use the innovative scour protection solution has saved the developer 30% in installation costs.

Challenge

The presence of a monopile in a marine environment changes the flow pattern in its immediate area, resulting in increased local sediment transport. This causes scouring of the seabed around the monopile—a serious risk that may compromise the stability of the wind turbine foundation. In addition, the cables on the seabed may risk exposure due to the eroded seabed around the monopile. The foundation must therefore include relatively costly scour protection.

Vattenfall needed a solution to reduce investment costs for the DKF wind farm located in the Danish part of the Baltic Sea. When operating at its full capacity, the wind farm will produce 604 MW of wind power distributed over 72 wind turbines. DKF is Denmark’s largest offshore wind farm to date and will increase Danish wind production by approximately 16%.

Solution

To maintain the seabed around the monopile foundations at a fixed level, a scour protection system consisting of rock is applied in the design. Scour protection systems are traditionally applied in two consecutive stages, and this includes installing:

• a filter layer around the base of the monopiles consisting of finer material; and

• an armour layer on top of the filter material

The monopiles may be installed prior to both layers, or in between the two; in either case, the total installation comprises three steps. However, significant cost reduction could be achieved if just one layer of graded scour protection needed to be installed—as proven possible in this innovative solution.

Single-layer scour protection significantly reduces installation costs

Modern design principles based on recent research and development has now made it possible to install a single layer of graded scour protection over the traditional two-layer solution. The wide-graded rock material encompasses both the filter and armour effect, and can be installed at the site in one step. Considering the installation of the stone material and the monopiles, the total installation procedure has been reduced from three to two steps, resulting in significant cost savings.

Results

• 30% cost savings thanks to optimised scour protection design
• Installation steps reduced from three to two
• Chance to apply new R&D in the planning and design phase