An innovative construction process

T he DemoSATH prototype is an offshore wind platform designed by SAITEC as a catamaran. The platform, with a length of 70m and a beam of 30m and a height of 24m, is divided into 5 main elements: 2 floats (6), the SPM (Single Mooring Point)(1), transition piece (PT)(2), and the stabilising plate (5).  Of the 5 main elements, 4 have been prefabricated outside the assembly slab (2 floats, SPM and PT).

Building DemoSATH

Building the floats

First, the segments that form the floats have been built. They are two cylindrical floats, with an elliptical section, with a vertical diameter of 13.70m and a horizontal diameter of 10.70m, and a length of 40m.

Each float is made up of 3 segments (2 conical and 1 cylindrical). The segments were built with the longitudinal axis vertical, each in 4 concreting phases. Each phase consisted basically of:

  1. Placement of metal bulkhead on falsework.
  2. Reinforcement of the perimeter beam of the bulkhead and keels (upper and lower).
  3. Exterior formwork
  4. Reinforcement between bulkheads from the inside of the voussoirs
  5. Interior formwork
  6. Concrete pouring
  7. Formwork removal

Did you know that…?

Each float is made up of 7 metal bulkheads of 22tn each. These metal diaphragms have a double function: to increase the resistance of the float and to compartmentalise the float as if it were a ship, so that a series of watertight spaces are generated, which in the event of collapse or flooding of one or more of them, would not compromise the buoyancy of the platform. Inside, in each compartment there is a bilge system.

On the other hand, it has a stern/prow levelling system incorporated, by means of the transfer of fresh water between the watertight spaces delimited in the cones, by means of a pumping installation placed in the lower part of the floats.

A unique manoeuvre for the placement of the segments.

Each segment is approximately 13m high and weighs approximately 430 tonnes for the cylindrical segments and 400 tonnes for the conical segments. Once the 6 segments that make up the two floats had been manufactured, the footing of the assembly slab was built, where the segments were placed on concrete supports at a height of 2m.

Two lattice cranes were used in the operations of lifting, turning, moving and placing the segments, a main 800 ton crane and a secondary 450 ton crane. The secondary crane assisted the main crane in the turning of the pieces by changing the longitudinal axis from vertical to horizontal. The 800 ton crane was used to transport the segments to their final position, and with the use of Ferrovial’s viaduct shoring system, errors of less than one centimetre were achieved in their placement.

The necessary tools for the manoeuvres were defined, calculated and certified. 29 ton metal pieces were fitted to the segments by means of bars and tensioned tendons.

Did you know that?

The construction process defined by SAITEC included one voussoir per bulkhead, which meant having to have 7 joints in the floats. These joints created several problems.  From a constructive point of view, their execution was very complicated, with the assembly of a scaffolding inside the float for each joint that covered and gave access to the entire perimeter, with complicated reinforcement and the use of special formwork that allowed a perfect seal for the injection of high-strength self-compacting mortar. In addition, a key factor: watertightness. The structure is designed to be at sea and therefore, any joint is a weak point, so it was decided to go from 7 to 2 joints. Each float consists of a conical bow voussoir, a central cylindrical voussoir and a conical stern voussoir, and only 2 joints have been made to join these three elements into one.

The stabiliser plate, the key piece to prepare the catamaran for launching

The stabiliser plate was built on a falsework at a height of 2m above the level of the dock, this will allow the SPMT platforms to be introduced through the lower part of the platform. Once the joints between the segments and the stabiliser plate had been made, they were post-tensioned. This allowed us to continue placing the rest of the prefabricated elements and to execute the remaining insitu joining elements.

Single Point Mooring: the part to anchor the wind turbine in order to orient it to the wind and optimise its performance.

The SPM was built on shore and then hoisted into its final position. This part houses a macro-bearing to which the 6 anchors that position the prototype are attached and which not only allows the wind turbine to be oriented towards the wind, but also the platform itself, in order to optimise its performance.

Transition piece to join the mill to the platform

For the construction of the transition piece, one of the footings used for the construction of the segments was used, which was placed on the floats in its definitive position with the use of an 1100 tonne lattice crane. Weighing 345 tonnes, it is a mixed piece of concrete and metal. It is the element that joins the wind mill to the concrete platform.

Finally, with the platform finished and post-tensioned, the provisional supports were removed to introduce the SPMTs trolleys underneath the prototype. With the platform raised 30cms above the trolleys, the permanent supports were removed, freeing the space to carry out the necessary movements to raise the DemoSath to the semi-submersible pontoon placed on the dike’s slope. After 12 hours of ballasting the pontoon, the concrete platform floated.

Did you know?

Another uniqueness of this project is that the wind turbine will be placed in water depths of more than 80m. This technology is cost-effective for water depths greater than 50m. In waters with depths of less than 50m there are the conventional foundations that we know today.