New guidance that aims to combat problems of grout failure on swathes of wind farm foundations has been published this week.
NCE exclusively revealed the problem last April, which had hit wind farms across Europe, including UK fleets at Gunfleet Sands off the east coast, and Burbo Bank in Liverpool Bay at the mouth of the River Mersey.
Grout designed to act as an adhesive between a pile foundation and the transition section of the turbine tower base was failing, causing slippage of the tower. But the revised design guidance now recommends a conical connection between the monopile and transition section below the wind turbine tower to prevent slippage (see diagram).
“It’s addressing the problem of slippage on wind turbines,” said wind farm developer RES Offshore lead structural engineer Danny Bonnet.
The new guidance comes in time for much of the Round 2 offshore wind construction with the Lincs WInd Farm, off the English east coast, among the first to adopt the conical connection.
“It’s addressing the problem of slippage on wind turbines”
Danny Bonnet, RES Offshore
Up to 7.2GW of capacity is due to installed during Round 2. While other major projects set to take advantage of the development include Gwynt Y Môr, Humber Gateway and London Array.
The amended guidance – DNV-OS-J101 Design of Offshore Wind Turbine Structures - is the result of a joint industry project led by Norwegian risk and certification firm Det Norske Veritas (DNV).
Original designs used a vertical connection. During installation, a transition piece is jacked up before grouting is carried out. After curing the jacks are removed leaving a gap of 20mm to 30mm between the monopile and the temporary supports.
Wind turbine settlement has caused this gap to close, leaving the transition piece sitting on the temporary supports – a function they were not designed for. Engineers had become concerned the result meant untended forces could lead to fatigue cracking.
As a result, DNV does not recommend this connection, and urges designers to use a conical design. This is where the monopile and transition piece is fabricated with a small cone angle in the grouted connection.
If settlement does occur in this design, then the shape and friction should provide enough resistance to prevent fatigue cracking.
“This limits the stress in the concrete,” said DNV Wind Energy Certification head of department Claus Christensen.
“We have performed full scale testing in up to 1m diameter piles but it is relevant up to 5m,” he added.
However, engineers have warned that costs are set to rise with this new guidance. “It’s a more complicated design so fabrication costs will go up,” said Bonnat.
If developers choose not to use the conical connection, then the guidance suggest installing spring supports, which can also be used to retrofit existing structures.
Bonnat says a 1m spring support should sit on top of the monopile. This allows the vertical load from the transition piece to be transferred to the top of the pile.
RenewableUK’s head of offshore reneweables Peter Madigan added that this is new technology and firms are constantly updating their technology.