ACCURATE LEVELLING OF WIND TURBINES: Accurate levelling using hydraulic tools

While the industry is looking at ways to assemble a turbine onshore so it can be installed at sea in one operation, at the moment the common method still is to install it in several stages at sea. The foundations are installed first and followed by the tower and the WTG.

Whether the foundations are monopiles, tri-piles or jackets; accurate foundation levelling is critical to the functionality and longevity of an offshore wind turbine. Levelling can be executed controlled and accurately with high pressure hydraulic tools.

Offshore WIND asked Enerpac, a specialist company in this field, to explain the use of high pressure hydraulic tools capable of ensuring that the levelling is perfectly and accurately executed.

Wind turbines installed in water depths of around 25 meters have, until now, normally used a regular monopile foundation structure due to their ease of installation. The monopile supports the tower either directly or through a transition piece. A transition piece is placed on the top of the monopile with a flange for connecting and levelling the tower. In deeper waters, soft soil and with heavier turbines, a tri-pile or jacket foundation is preferred. We will use the installation of a traditional tri-pile foundation wind farm as a case study to explain the levelling process.

Accuracy adds up

Tri-pile foundations consists of three pylons and one transition piece for stabilization. On top of the foundation a 90m long tower is placed. Depending on the seabed conditions the piles are driven 30 to 45m into the sea bed using a guide frame to assist positioning and maintaining a vertical drive. Their position is accurately determined using a satellite controlled navigation system. The height of these piles above the water surface are also automatically determined to an accuracy of 3 to 4cm. Above the water surface, these piles are connected to each other by the transition piece on which the turbine tower stands. The weight of the rotor is approximately 180t that of the nacelle 280t and the tower had a weight of 160 t. Adding this total weight of 620t up to the foundation piles of 400t and a 500t transition piece, the total weight of an average wind turbine is approximately 1,520t.

It goes without saying that when a transition piece is not levelled correctly, a minor deviation leads up to cumulative deviation at the top of the wind turbine. Therefore a wind turbine needs to be exactly level to ensure generation of a maximum yield, and to be able to withstand the harsh conditions of the open sea.

The connection flanges are not always straight and manual millimeter correction of any flange deviation is not possible. Levelling of the 500t transition piece can be executed using high pressure hydraulic equipment. Synchronous levelling is a key factor during this operation as it is the most efficient and safest method.

Synchronous levelling with hydraulic cylinders

One method of carrying out synchronous levelling makes use of 700 bar high pressure hydraulic cylinders. These cylinders can be controlled manually or using Programmable Logic Control (PLC) driven systems. This is the most commonly used levelling solution operates high tonnage hydraulic cylinders with a multi-functional synchronous lifting EVO-system for levelling the supporting transition pieces quickly and accurately.

In standard tri-pile wind farm levelling projects, three cylinders are used, each with a capacity of 100t, mounted equally spaced around each pile. Each foundation therefore has nine cylinders with a total lifting capacity of 900t. The cylinders are connected to the computer of our EVO-system, a multifunctional synchronous lifting system, which finally levelled the supporting transition piece with a single push of the button. To achieve this, the cylinders first lifted the supporting transition piece by approximately 20mm and then levelled it to an accuracy of 1mm from that starting position. Time is saved because the process did not have to be stopped after any incremental value, caused by lifting or lowering at slow speeds, and then checked, manually re-measured and corrected. The installation process is controlled by the software, using stroke sensors and an inclination meter. Once the supporting transition piece is level, it will be locked in place together

with the piles by a 5m high grout caching, with the concrete being poured into the hollow space to make a 13cm thick ring against the wall of both the supporting transition piece and the piles. Welding or bolting is avoided in this part of the installation, because the concrete casing handles all the stresses.

Mechanical actuators and torque power

Another method to level the transition piece on a monopile is using spindles. The spindles can replace cylinders in the levelling process. They are manually controlled by hydraulic torque wrenches. The torque wrenches will move the actuators up and down. Using spindles or cylinders are perfect to operate within a three-point setup. The six levelling points of the transition piece are divided in three parts; two adjacent points are levelled simultaneously.

The three sets are levelled one at a time, and step by step. The operator uses the prescribed pattern, going around the transition piece multiple times to level and to check the accuracy and locking of the actuators. This way the levelling will be safe and accurately executed.

Safe because small levelling adjustments are made at one stroke, and accurately because the torque wrenches, connected to an electric or air driven pump, have a constant torque output providing high accuracy across the full stroke.

Powerful Solutions. Global Force

There is a wide range of high pressure hydraulic tools, equipment and integrated solutions to provide solutions for many large scale offshore wind related projects. These include lifting and skidding heavy structures, levelling of wind turbines, and bolting the segments of wind turbines together.

Companies such as Enerpac have the extensive knowledge required to provide suitable heavy lifting solutions for the demanding needs of the widespread offshore industries, and are able to provide customers with the right high-pressure hydraulic tools and solutions to work on many challenging operations.

With thanks to Richard Verhoeff, Integrated Solutions Specialist at Enerpac

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