Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 318

Monopile foundation (Fig. 7.13, centre). This foundations type consists of a single
foundation pile (monopole), which quasi extends the wind energy converter tower
up to the seabed. Depending on the plant size and support structure design its di-
ameter varies between 3 and 4.5 m and its mass between 100 to 400 t. Installation
on the seabed is performed by pile-driving, vibrations, and drilling, whereby the
penetration depth varies between 18 and 25 m. The actual tower of the wind en-
ergy converter and the foundation pile are subsequently connected by a joining
element to compensate for a possibly oblique position of the foundation pile.
From a current viewpoint, the maximum water depth, up to which the installation
of monopile foundation is sensible, is reported to be 25 m. Monopile foundation
technology seams comparatively economic for water depths up to 20 m and fa-
vourable ocean floor conditions and has, for instance, been applied for the off-
shore wind parks of Bockstigen and Utgrunden (both located in Sweden).
Tripod foundation (Fig. 7.13, right). Tripod foundations consist of a central col-
umn element connected to the tower of the wind energy converter and a three-
legged three-dimensional steel truss, which transmits the forces and the torque ef-
fective onto the construction corners. The diameter of the foundation piles an-
chored into the seabed by pile-driving, drilling and vibrations, amounts to about
0.9 m. Depending on the respective soil conditions, the desired seabed penetration
depth varies between 10 and 20 m. Tripod foundations are most suitable for water
depths exceeding 20 m, as landing water vehicles may collide with the steel truss
submerged into the water at water depths below 7 m. To date, the described tripod
foundations have not yet been applied for offshore wind energy converters. How-
ever, their application at a water depth of about 30 m is planned for the first Ger-
man offshore wind park "Borkum West", currently being planned for the North
Sea. The mass of such a foundation designed for the planned multi-megawatt con-
verters amounts to approximately 800 t.
Fig. 7.13 Floor-mounted support structure of offshore wind energy converters (gravity
foundation (left), monopile (centre), tripod (right); see /7-7/, /7-8/)