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QA

FAQ

This page contains the most frequently asked questions about our production and storage.

Wind energy is created by converting the movement of air (wind) into energy. In the past, we used windmills to grind grain or pump water. Today, wind energy is mainly known as the electricity generated by wind turbines, often on large wind farms.

A wind turbine converts wind into electricity. The wind makes the rotor blades (the blades) move. A generator converts this rotating movement into electricity.

We have been innovating and building the foundations for a sustainable future for the next generations for 75 years. We offer complete monopile foundation solutions for offshore wind turbines.

Monopiles are the most frequently used foundation for offshore wind turbines and comprise a steel tube up to 120 metres in length and 11 metres in diameter. Monopiles are mainly used in shallow to medium water depths.

A monopile’s production process starts with a large flat steel sheet. This is rolled to form a half shell or complete can, depending on the monopile’s size. One sheet can be enough for smaller diameters but two half shells are used for larger monopiles. In this case, we weld these two shells together using a so-called longitudinal seam. This is a straight weld seam that seals the shell to form a complete can.

After forming the can, we then join several cans together to reach the full monopile length. We do this by welding the cans together using circumferential seams, which are weld seams that connect the ends of the cans together. This welding is largely an automated process that uses a four-headed welding machine, enabling us to work fast and with precision.

As soon as the tube is complete, we conduct a thorough inspection to check that the weld seams are strong and are of high quality. The monopile is then given a protective coating to protect the steel from rust and corrosion in the saltwater environment. This ensures that the monopile lasts for many years, even in harsh offshore conditions.

Due to their huge size and weight, installing a monopile is a challenging process. A monopile can be up to 120 metres long and weigh a thousand tonnes. This demands powerful vessels and heavy-duty cranes to move the monopile to the right site.

Monopiles are traditionally installed using pile-driving, which involves driving the monopile into the seabed using a large hydraulic hammer. The pile-driving process starts by using a crane to lift the monopile and position it above the installation site. Once the monopile is suspended above the correct site, a hammer is used to drive it into the seabed. The energy required for pile-driving operations depends on the seabed conditions and the foundation’s diameter. Less power is needed for soft seabed types, while a harder seabed requires more energy.

One of the disadvantages of pile-driving is the underwater noise this produces. This noise can harm marine life, including fish and marine mammals. For that reason, alternative installation methods are increasingly being considered. Vibration is one of these alternatives. This involves using vibration to push the monopiles into the seabed, using a vibrating device placed on top of the pile. The monopile is lowered slowly, while the vibrations ensure that the pile sinks into the seabed under its own weight. Although vibration produces less noise, research is still required to determine the effects on monopile stability and on the seabed location in which they are placed.

The choice of installation method therefore depends on various factors, such as the seabed conditions, monopile size and the possible ecological impact. Whichever method is used, the goal remains the same: creating a firm foundation for the wind turbine so it can withstand the harsh offshore conditions for decades to come.

At the end of a wind farm’s service life (usually after 40 years), the monopiles must be removed from the sea. This is often a challenging process because the monopiles can be up to 30 or even 40 metres deep in the seabed. It is not always necessary to remove the entire monopile. For the safety of fishing and other maritime activities, a monopile usually needs to be cut to about 3 metres below the seabed.

Removing a monopile starts with exposing the top section of the pile. This can be done by digging away the seabed around the pile. A special saw, such as a diamond wire saw, is then used to saw off the monopile. This technique enables the monopile to be cut efficiently at the correct depth.

What happens with the part of the monopile that is still embedded in the seabed varies in each situation. Sometimes this is left in place if it is unlikely to cause any ecological or technical problems. A technique for complete monopile removal is currently being developed, with the main focus being on minimising ecological impact and maximising the reuse of materials.

The first offshore wind farms were constructed in the early 2000s and will be reaching the end of their service life in the coming years. A lot of research on decommissioning methods is therefore currently being conducted. Recycling plays a major role here, as a monopile’s steel is very suitable for reuse. Sif wants to play an active role in this, together with other market parties. Returning as much of the monopiles as possible to shore enables us to recycle the steel for new applications.

The cleaning and recycling process starts once a monopile has been removed from the sea. Monopiles are first cleaned thoroughly, removing all fouling, including barnacles and algae. We also remove the protective anti-rust coating. What remains is black steel.

This steel is cut into transportable sections for easy return to the steel plant. Here it is recycled. This old steel has a high recycling value, particularly in the production of green steel. Older steel can be blended with new steel, resulting in a more sustainable production process.

Sif aims to play a key role in this future recycling stream, with the manufacture of new, larger monopiles from old monopiles, in close cooperation with our customers and steel plants. This ensures that the materials retain their value and safeguards our contribution to a circular economy. Reusing high-quality raw materials means we can reduce the environmental impact of steel production as well as help build a more sustainable future.

A transition piece is the connection between a monopile and the wind turbine’s tower. It comprises a support tube and additional components, such as a platform. The connection can be made using concrete or bolts.

A traditional monopile has a separate Transition Piece (TP). This connects the monopile with the wind turbine’s tower. The TP is already equipped with secondary steel, such as a boat landing and external platform. These are the components that provide access to the turbine. After installing the offshore monopile, the TP is hoisted onto the monopile, where it is secured with bolts or cement. Part of the TP protrudes above the monopile and is known as the skirt. The wind turbine tower can then be placed on the TP.

A TP-Less monopile (also known as an extended monopile) has the tower mounted directly onto the monopile, without a TP in between. The monopile itself is extended for this. The boat landing and external platform are mounted separately onto the monopile after the offshore foundation has been installed. An internal platform is also installed in the monopile for hauling in the power cables and tightening the bolts between the monopile and the tower.

Because the monopiles are driven into the seabed with great force, these individual components cannot be assembled in the factory. They are fitted offshore following the monopile’s installation. A TP-Less monopile requires four crane hoists: one for the monopile, one for the boat landing, one for the external platform and one for the internal platform. Just two crane hoists are needed for traditional monopiles with a TP: one for the monopile and one for the TP. This means that installing the TP-Less method is often more complex.

A jacket is a supporting structure for wind turbines and is mostly used in deeper waters. It comprises multiple piles that are connected together. The four longest tubes are known as jacket legs and the piles that are driven into the seabed are known as pin piles.