Fugro Seacore Ltd successfully installed five 500kW wind turbines 4km off the south western coast of Gotland in the Baltic, Sweden. Each three bladed turbine was installed on to a 2.1m diameter steel monopile foundation grouted into a 2.25m diameter socket drilled 10m into the seabed.
All the works on the 5 turbines were undertaken from Fugro Seacore’s Deep Diver Jack-up barge. To create the socket a 2.25m internal diameter conductor / guide tube was used to ensure the verticality of the socket. This conductor was temporarily connected to a sacrificial casing shoe section using a subsea gripper can. The assembly was lowered onto the seabed whilst being held laterally at the top by the piling gates of the JUB. Fugro Seacore’s 30t T8 reverse circulation hydraulic rotary drill was then lifted onto the top of the conductor and the downhole drill equipment installed through the conductor to create the required 2.25m diameter by 10m deep socket. This socket was drilled through the mudstone and limestone rock layers. As the drill bit initially progressed through the weaker top layers of rock, the sacrificial casing shoe and the conductor followed closely behind until it’s toe seated in the competent rock. This eliminated any potential loose material from the upper zone entering the socket or creating hole collapse. Once the socket was complete the sub sea gripper was released from the sacrificial shoe leaving the shoe was proud of the seabed and ready to accept the monopile foundation.
Fugro Seacore then floated out the buoyant monopile which was towed 8km from their onshore base at Burgvik harbour. The JUB was then lowered to sea level to enable the gate to act as a guide for the monopole. The pile was then buoyant lifted by the onboard crane into the vertical and traversed into the piling gate. Once in the gate and in position over the socket, it was slowly lowered down into the socket whilst water was pumped in to alleviate the buoyancy. In its final position the monopile protruded about 3m above sea level. Grout was then pumped down through fixed grout lines within monopile to fill the annulus between the pile and the socket.
After all five monopile foundations were installed, Deep Diver returned to Burgvik to be re-mobilised with a bespoke turbine rotor assembly system, along with extending the legs by a further 11m resulting in a length of 45m to enable erection of the tower sections, nacelles and rotors.
Once back on site the Ice Shield was installed around the monopile along with the base and access platform. The transformer and control box was then placed on the platform and the 15m lower tower section was positioned over these units and bolted to the base. The upper tower section was then lifted and bolted to the lower section.
The height of the JUB was then raised a further 9m to provide a 21m air gap between the underside of the barge and the sea. The 21.5t Nacelle was then installed on to the upper tower section with a further bolted connection. The three blades of the rotor were fitted to their hub which was attached to the assembly system on the stern of the Jack-up. To complete the installation the complete rotor assembly was lifted and installed onto the Nacelle.