divided into two compartments by a watertight deck, and the lower compartment is used to fit a water ballast tank.
Innovatively, a footplate, whose diameter is 18 m and thickness is 10mm, is added to the base of each column to increase the system damping, which is advantageous in terms of sustaining dynamic loads. The cost of the platform was projected to be 7.83 million U.S. dollars in the year of 2002. Furthermore, a bracing system is designed to connect the columns and to support the wind turbine [13].
Under the inspiration of the Dutch tri-floater, Lefebvre and Collu [31] proposed a modified tri-floater in the year of 2012. In the proposal, they focus on the detailed design on the support structure, which is essentially a semi-submersible foundation. In detail, each column of the foundation is divided into three compartments using two horizon bulkheads. T-section stiffeners, Hsection stiffeners and radius ring stiffeners are employed to increase integrity and global/local stiffness of the foundation. Detailed information is available in their manuscript.
The design of the tri-floater is enlightening in many perspec-
tives. Specifically,
1. Lower keel depth could make it possible to assemble thefoundation on a dock in a large harbor.
2. The catenary anchoring cables employed to moor the foundation onto the seabed could lower the cost of the foundation.
(2) WindFloat project
As another example of the three-legged floating foundation, the WindFloat designed by Roddier et al. [17] has been tested in the offshore area of Portugal in 2011 [37].