One of the main features of the technology being developed in ELICAN project is that the foundation is built in a dry-dock, to provide a high performance in the production of units and making this process independent from the weather.
The main feature of a dry-dock is that there is no need to use a crane to load out the vessel or structure to be built in it. Through the use of water pumps and valves, it is possible to ground a boat or structure without the risks of lifting them.
Since for the Arinaga project, only one unit is to be built, the dry-dock has not been permanent, it was made of earth. This way is cheaper to build, but it requires keeping on a pumping system to lower the water level.
The construction included the excavation of the hole, installation of pumping systems and construction of a concrete surface to facilitate a flat and clean surface to work.
The first part of the construction, the excavation of the hole, ended in March 2016. It required placing the pumping system to extract the water from the soil. Once the necessary depth was reached, the work surface was built, pouring gravel and poor concrete providing a robust and clean surface.
The construction of the foundation began immediately after finishing the construction and conditioning of the dry dock. It started by placing the reinforcing steel and pouring the concrete for the bottom slab.
Once the curing of this element ended, the next step was to place the formwork and rebars for the exterior walls and bulkheads.
By the end of April 2016, the entire exterior wall was finished, and works on the bulkheads were started, finishing in May 2016. Next stage was the installation of the columns and the ballasting system: tube, valves, pumps, pressure sensors… The final step was to install the pre-slabs of concrete, in order to close the foundation.
The first stage of the base construction was finalized on July 2016, moment when the base was capable of floating by itself.
When the foundation was built, the pumping system of the dry dock was turned off, letting the water in the dock. This operation was the first test of the water tightness of the concrete and the operation of the pumps and the valves. All the elements performed correctly, so it was possible to open the dry dock and load-out the foundation. For this it was necessary the use of a conventional tug boat (50Tn) and a vessel to guide the foundation.
The foundation was then ready to move to the next position, near the rip-rap, where the tower was to be assembled. It was necessary to wait for a very high tide, so this movement was done mid-August 2016.
Finally, the dry-dock was filled with soil again, in order to restore the port to its previous condition.
The telescopic tower has been designed to be fabricated using precast concrete pieces. Every precast concrete piece requires moulds or formworks to be constructed. Esteyco previously developed a method to build onshore concrete towers which has been used for this project, adapted to the necessities of its telescopic configuration.
The telescopic technology requires an improved geometry of the pieces, reducing the tolerances, to allow a tower lift without issues. The construction of these moulds has taken place in the workshop of a Spanish company called Coalca, which has already built for Esteyco the moulds for the full-scale onshore testing telescopic tower constructed and tested in Daganzo, Spain. The workshop is near Madrid and the pieces were sent in containers to the Canary Islands. This capability of being shipped is very important for future construction of tower series, since the moulds for a wind farm could be reused in the next one, reducing costs and improving in the learning curve.
The construction yard of the pieces has been placed close to the port, to simplify the transport of the concrete pieces, since some of them are 33 m long and weigh 110 t.
Esteyco has already fabricated concrete pieces with this system, using the experience obtained in the installation of a considerable number of towers in Spain, Brazil, Poland, and USA.
After the float-out, the foundation was placed near the quay, and ballasted to lay on top of a previously prepared bed, at a height of -3.50 m. Just after, the dry dock was filled and restored to its previous condition. The assembly of the tower was made from the pier, with a LR 1600 crane. These are the steps followed:
The telescopic configuration of the tower brings down the centre of gravity during the towed self-floating transport, allowing the bottom platform to temporarily act as a self-stable floating barge over which the complete system can be pre-assembled in-shore at low draft, low height and effectively towed to the site
The telescopic tower system consists of tubular tower levels which are preassembled on the harbour in a “folded” telescopic configuration over the floating platform,The tower is divided into 3 segments of 3 to 6 precast concrete pieces each. These segments are named T0, T1 and T2 and they refer to the bottom, middle and upper sections of the tower, respectively, which are, at the same time, the outer, middle and inner sections when the tower is folded
Once the complete folded unit is preassembled onshore, conventionally towed to the site and ballasted to rest on the seabed, the tower can be lifted to its final position by means of cables and heavy-lift strand jacks which are reused to lift one tower level after the other. These are installed on top of the tower bottom section, and the operation is complete from that level (all sections). The estimated lifting rate is of (8-12m/Hour).
Esteyco and Gamesa have been working together in the design and construction process of the prototype, revising all the stages from assembly to service life and complying with the requirements from the turbine and the substructure.
Gamesa is responsible for the supply, reception and storage of the complete wind turbine parts at Port of Arinaga. The turbine components arrived at the end of March 2017 and were stored on a warehouse near Arinaga harbour:
The WTG installation and pre-commissioning is scheduled for next February 2018, after the STAP is installed.
This installation process requires using the crane LR1600, since the height for installing the blades does not allow any other crane.
The STAP (steel adapter piece) has been installed on January 24th. The piece serves as transition between the substructure and the WTG. ESTEYCO has a large background designing this type of pieces for several different turbines for many manufacturers, since each of the 400+ towers already installed have required one. Its length varies from 3 to 10 meters, depending on the Hub Height that has to be reached and the equipment that has to be placed inside.
In this case, the STAP is 9,22 m high, and will host two platforms with all the cabinets that the turbine requires for control and safety. It has an external diameter of 3,75 m, with an average thickness of 19 mm.
The main goal of the STAP is to provide a flat surface to place the turbine (yaw section). The tolerances are very demanding to guarantee an even support between both surfaces and avoid too much stress.
The STAP design verification is included in the certification process with TÜV SÜD. This design was made by ESTEYCO in collaboration with GAMESA, which provided the design of the upper flange, following previous designs for this type of turbine.
Several tests have been carried out with a scale prototype to test the feasibility of the project at the IHC (Institute of Environmental Hydraulics of Cantabria)
During the months of July and August 2017, the tank testing campaign on the El Pardo channel (CEHIPAR) was carried out for the validation of the transport and installation process of the GBS platform designed by ESTEYCO, The main tests carried out were the following:
Two ballasting & monitoring tests have been performed to test the naval behaviour of the concrete structure and the correct operation of the ballasting system. Those tests proved the stability of the foundation, provided a better understanding of the behaviour of the ballast system and served to get to know how to work with tug boats.
After the dock was flooded and the breakwater was removed, the foundation was towed outside, where a mayor draft was available. For this towing operation, the foundation was fixed with cables to three winches placed in the ground, to assure that the movement of the foundation was controlled and to allow the tugboat to pull with its full power to test the strength of the anchorages of the base.
Once in position, the foundation ballasting is carried out, filling the cells of water up to 1,8 meters, increasing the total weight of the foundation in 1.250 Tn. The ballast system proved a magnificent behaviour, having the foundation ballasted in the foreseen time and keeping it levelled during the entire process.
With the foundation placed in the placed on the new position near the rip-rap, the 2nd step of construction operations are developed. These operations consist of the assembly of precast slabs and the in-situ concreting of the upper slab and the assembly of the telescopic tower by positioning with a heavy-lift crane.
The second test was performed when the tower was completely built. This operation consisted in floating the whole structure, extracting water from the cells. The tilting of the structure was monitored to check that the weight of the tower and the auxiliary means did not affect the stability. Also, it was confirmed that the draft was in the expected range and that it was possible to move the structure with a high tide.
A final test was carried out few days later to check the behaviour of the foundation regarding the stability and the resistance to an intense pulling force from the tug boat.
In order to test the stability, a differential water level was placed in the cells, to test the structure and contrast the tilting angle with the calculations. At the same time, the tug boat was pulling with a force close to the maximum allowed, to verify the structural capacity of the anchorages to different angles and to induce another tilting angle to the structure.
On November 29th 2017, the lifting tests of the telescopic tower were successfully completed. The complete hoisting of the concrete tower of more than 90 meters was tested and it was demonstrated that it complied with all the technical requirements.
The telescopic system has been tested several times, with different purposes:
The purpose was to check the system particularly regarding 3 main aspects: