Tripod Support Structures
Article published on Wednesday, 05 June 2013
The tripod structure is considered to be a relatively lightweight
three-legged steel jacket compared to a standard lattice structure. Under
the steel central column, which is below the turbine, there is a steel
frame which transfers the forces from the tower into the three steel piles.
Piles are installed at each leg position to anchor the tripod to the seabed.
The three piles are driven 10-20m into the seabed. The tripod can also
be installed using suction buckets.
The tripod foundation has good stability and overall stiffness.
However it's not suitable at water depths less than 6-7m, as this causes
problems to the vessels approaching the foundation as sufficient draught
is need to clear the steel frame.
The foundation is anchored into the seabed using a relatively
small steel pile (typically 0.9m diameter) on each corner.
Proponents state the advantages of the tripod are the suitability
for greater water depths, and a minimum of preparations required at the
site before installation (assuming absence of large boulders etc.). Erosion
is not normally a problem associated with this type of foundation. They
go on to cite the foundations potential to save costs compared to a more
complex jacket design. The effect of scour can be less significant when
compared to the monopile support structure.
Others argue that tripods are not suited for locations
with uneven sea beds with large boulders, and that the complex main joint
has a potentially greater risk of fatigue from the large impact of wind
The tripod support structure is pre-assembled in an onshore
construction yard. The entire structure is placed on a suitable vessel
such as a barge and transported to the offshore location where it is slowly
lowered onto the seabed ensuring that the structure is entirely level.
Mud mats may be used at the three corners of the tripod ensure that the
structure settles onto the seabed in a stable manner, while providing support
until the foundation piles are in place. The foundation pin piles are each
driven through pile sleeves at the three corners at the bottom of the structure
using a submersible hammer. When the piles are at the required depth, a
connection between the top of the pile and the pile sleeve is made by filling
the annulus with grout or by means of a swaged connection.
Scour protection can be reduced if the foundation piles
of a tripod support structure are loaded mainly in the axial direction.
No separate transition piece is required, as the requirements for pile
driving do not apply to the tripod and the appurtenances can be connected
directly to the tripod support structure.
The tripod foundation draws on the experiences gained in
the oil and gas industry where light weight three-legged steel jackets
have been used for marginal offshore fields
Until now, the tripod foundation has been only used at
Phase 1 (under construction), Global
Tech I (under construction) but will also be
used at MEG
Offshore I and Borkum
Phase 2 offshore wind farms.
Standard: ‘Design of Offshore Wind Turbine Structures, DNV-OSJ101’, Det
Norske Veritas (2010) Project
Types - E.ON Windpower