- Simple construction
- Well documented method
- Grouting crumbles over time
- Need for scour protection
- Large hydrodynamic loads
Wind turbine supported by a monopile. Photo: DONG Energy
If one encounters a random offshore wind turbine there is a two-thirds probability that it will be supported by a monopile. The giant steel pipe is by far the most popular support structure in the world. At the end of 2012, 1923 of the world’s 2688 offshore wind turbines used monopiles for support.
The reasons are several:
- Simplicity in design and production – it is a long tube, making both calculations and production manageable;
- the shape allows for effective transportation to site; and,
- the installation technique is well known and widely used by the construction industry.
However, the simple shape also calls for a large diameter of the monopile – ranging from 3.5 to 6.0 meters. As a result, the structure attracts high hydrodynamic loads from the water (UpWind, 2007) – the water pushes and pulls the monopile and this affects the structure much more than for instance a jacket constructed out of smaller tubes.
The monopile typically weighs around 500 tons, making it one of the lighter support structures. On deeper sites like Walney 2, the monopiles weigh up to 810 tons and are up to 69 meters long.
All things considered monopiles are a suited choice for support structure in water depths ranging from 0 to 25 meters (DNV, 2010).
All in one
By definition, the monopile is both a foundation and a support structure in one. The tube rests in the soil – often with the same length below the seabed as above – and goes up all the way above the waterline. On top, a transition piece provides the connection between support structure and the wind turbine tower.
The transition piece is also a tube. It has a slightly larger diameter than the monopile and can thus be mounted over the monopile. On top of the transition piece a flange secures connection with the tower using nuts and bolts. The transition piece typically weighs from 145 tons (Belwind) to 252 tons Baltic 1) and is around 25 meters high.
The transition piece is necessary because of the method used to install the monopile into the sea bed. The structure is hammered into the soil using a hydraulic hammer – the same method used on land to pile foundation for buildings or sheet piling. At sea, the hammer of course is mounted on a barge, as one can see on this video of the process – in this case from the London Array wind farm.
Hammering of monopiles into the seabed with hydraulic hammers is, however, extremely noisy. Rising concerns about the health of fish and sea mammals means that new installation restrictions are going to be enforced in Europe in order to mitigate the noise.
Because of the hydraulic hammer method the monopile cannot have a flange on top for the tower to be mounted on. It would simply be damaged. And more importantly, the method does not guarantee a leveling of the monopile within the normal margins of 0.5 degrees tilt. It is too difficult to keep it perfectly straight when hammering it into the soil.
Transition pieces on the quay at Bladt Industries, Denmark. Notice the working platform – made out of concrete instead of steel.
Photo: Aarsleff Bilfinger Berger Joint Venture
In the main, the transition piece has the function of adding a perfect flange on top, leveling the transition to the tower, and not least providing the whole structure with a boat landing, stairs and a working platform.
At the same time, however, the transition piece represents the main weakness of the monopile concept. The transition piece is connected to the monopile using cement or grout. A solid filling is needed to transfer all loads and forces from the wind turbine tower through the transition piece down to the support structure.
And as the tower rocks and vibrates over the years due to the dynamic loads from wind and waves, the grouting crumbles. In many cases, owners have to refill with new grout.
Solution 1: Conical instead of tubular
Research on grouted connections has been going on in the last years to solve the problem. DNV has performed this Joint Industry Project, which ended January 2011. This has resulted in a conical design concept for the transition piece as seen on the illustrations below.
DNV's report can be downloaded from LORC Knowledge - go to the references at the bottom of this page.
Old design: Grouted connection with gaps (DNV)
New design: Conical connection (DNV)
On new constructions like the Walney 2 offshore wind farm, the grouting problem is also solved this way (OceanWise, March 2011, page 24-25). This conical design should supposedly minimize the risk of grout crumbling.
Solution 2: Drilling instead of hammering
The only way to avoid the use of a transition piece is to drill the monopile into the soil. Though rarely used, because of the high cost associated with it, drilling is the method of choice in cases where hammering the monopile down is not an option due to hard rock in the soil. See for instance the Bockstigen wind farm in Gotland, Sweden.
Another solution that is likely to render the use of the transition piece obsolete is the Concrete Monopile. Still at the prototype stage, the concrete monopile with its core pre-stressed reinforced concrete, is cheaper to produce but at the same time more costly to install because it involves drilling of the sea floor. However, this allows for construction without the transition piece, which in turn may end up being more affordable.