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The Rentel Project Phases

The realization of an offshore wind farm is the result of a long and complex process that can be split up into three main phases:

1. Development and contracting

2. Manufacturing and installation

3. Operations and maintenance







otary projects

1. Development and contracting phase

During the development phase, a large number of studies are executed in order to complete the design, conclude the necessary contracts and financing, as well as reduce or mitigate the risks:

  • Concession applications: determine concession area and environmental conditions, prepare a preliminary business plan, identify contractors, project execution plans, etc.
  • Feasibility study: wind simulations, wind farm layouts, risk assessments, business modelling, consulting, and discussions with authorities.
  • Permitting: environmental impact assessments, submitting permit requests and obtaining all required permits from nautical, aviation, federal and regional authorities.
  • Design: the foundations and offshore transformer station are unique to the project, geophysical and geotechnical surveys and the design itself will be certified by an independent accredited certified body.
  • Contracting: preparing and submitting tenders to contractors, negotiating offers and contracts, conclude the contracts and continuously work on interface management and risk mitigation.
  • Financing: obtaining net assets, the required support from the shareholders and arrange the external funding of the project together with commercial banks and multilaterals.

2. Manufacturing and installation phase

The manufacturing & installation phase is split up as follows:

  1. Preparation of the seabed
  2. Manufacturing and installation of the foundations
  3. Manufacturing and installation of wind turbines
  4. Building and installation of the offshore transformer station
  5. Manufacturing and laying of infield, export & land cables

Step 1: Preparation of the seabed

Prior to installing the foundation towers, the seabed needs to be levelled out and a gravel bed needs to belaid for stabilization purposes.

Step 2: Monopile foundations

The wind turbines at sea need to be installed on top the foundations. For the Rentel offshore wind farm, we will use monopiles. Monopiles are long cylindrical steel tubes driven deep into the seabed by a hydraulic piling hammer. The monopile is constructed onshore and then transported to the desired location by means of specialized installation vessels. Once delivered, a crane lifts the monopile and puts it into a special frame so that the monopile can be hammered into the seabed. The monopile foundations weigh up to 1.250 Ton and have an 8 metre diameter. 

On top of the monopile foundations, a ‘Transition Piece’ will be installed. This is the yellow part of the foundation that is visible above the water. On top of this transition piece, the tower of the wind turbine will be installed and fixed. This transition piece allows maintenance technicians brought there by specialized vessels to enter the turbine. One transition piece weighs up to 320 Ton and has a 6 metre diameter.


Step 3: Wind turbines

The Rentel wind farm comprises 42 D7 Siemens turbines. These wind turbines have a nominal capacity of
7 MW which can be increased to 7.35 MW, environmental conditions permitting. Each blade has a length of 75 meter (1,5 times the length of an Olympic swimming pool!) and the total rotor diameter is 154 m.
The turbines’ tip height is 196 meters above Lowest Astronomical Tide, making them the tallest offshore wind turbines in Belgium. The total weight of the rotor, nacelle and tower comes in at approximately 950 tons, which is the equivalent weight of 75 double-decker buses!

The wind turbine parts will be manufactured in many different places and brought to Siemens’ facilities to be assembled. Upon assembly, the different parts of the wind turbine will be moved from their production facilities to the pre-assembly harbour in Ostend.

In the pre-assembly harbour, the 3 parts of the tower will be assembled. At the REBO site, in the harbour of Ostend, the blades, towers and nacelles of the wind turbines will be loaded on the jack-up installation vessel ‘Apollo’, provided by GeoSea. Apollo's 800 ton main crane loads the various components onto its 2000 m² deck (approximately a third of a football field) so that they can be sea-fastened before being transported.

At the offshore site, the DP-2 vessel will position herself precisely and jack-up next to a foundation that is ready for wind turbine installation. Following final preparatory works, first the tower with nacelle is set in place, then the three single blades are mounted one after the other. In good weather conditions, a complete wind turbine should be installed in less than 24 hours!

Once the wind turbine has been installed, the technical crews will connect the wind turbine to the power grid.

Step 4: Offshore Transformer Platform

The offshore transformer platform (or substation) collects and exports the power generated by turbines through specialized submarine power cables. Known as ‘the heart of the wind farm’, it is an essential component of each offshore wind farm. Its main functions are: stabilizing and maximizing the voltage of power generated offshore, transforming of the produced energy from 33kV up to 220kV high voltage, conducting the electricity to the main land.

The Rentel advanced transformer platform weighs around 1.100 Ton. The platform consists of a large steel building with 4 deck levels equipped with all kind of auxiliary systems specifically designed and built to fit the conditions of the North Sea. The bottom of the topside rests around 20 m above the sea level, on a 1.850 Ton heavy monopile foundation, which has been fixed 40 m below the seabed level in order to survive extreme sea state conditions over the entire lifetime of the project.


Step 5: Infield, export and land cables

The electricity produced by the Rentel offshore wind farm needs to be brought to shore in order to feed it into the National Grid for distribution to Belgian electricity consumers.









In order to transport the wind energy transformed into electricity, three types of cable are needed:

>Infield Cable

Within the wind farm, a number of ’inter-array’ cables link the different wind turbines to each other and to the offshore substation where the electricity is transformed from 33kV up to 220kV.


>Export Cable

From the substation, the electricity is taken ashore by an export cable. The 40 km long export cable connects the offshore substation to the onshore national grid in Zeebrugge, known as the Stevin substation.

The export cable with a diameter of as much as 30 cm weighs about 90 kg per running meter. After all, these cables have to be able to transfer large amounts of energy to the national grid and the Belgian electricity consumers. The export cable is designed and built specifically to meet the conditions along the cable route, taking into account thermal conductivity of the seabed. The cable is buried in the sea bed.

>Land Cable

Close to the landfall, the export cables will connect with onshore cables which will be buried between the coast and a new substation known as the Stevin substation. It's through these land cables that the energy finally joins the onshore network.


3. Operations and maintenance

During the operational phase of the offshore wind farm, which lasts 20 to 22 years, a large number of things need to be monitored and optimized:

  • Monitoring and maintenance of turbines, foundations and electrical infrastructure
  • Environmental monitoring
  • Production and capacity optimization

Of course, a wind farm needs to be managed with the utmost caution and attention to detail. All maintenance efforts are preventive or corrective and safeguard the level of efficiency of the wind farm.

Nevertheless, the most important is of course the production and delivery of green and clean energy to the Belgian market!