Rentel offshore wind farm

Challenge

The Rentel offshore wind farm is a 309 MW offshore wind concession located approximately 34 km from Zeebrugge in the Belgian North Sea. The project was developed by Rentel NV, a consortium of eight Belgian shareholders, including DEME Group, Otary Offshore Energy and Elicio NV and represents a total investment of EUR 1.1 billion. 
Rentel wind farm comprises 42 Siemens turbines of 7.35 MW and supplies CO2-friendly electricity to approximately 300,000 households. The wind turbines are installed on monopile foundations and connected through 33 kV inter array cables to an offshore transformer station (OTS). The substation is connected by a 220kV submarine cable to the onshore substation of Stevin in Zeebrugge. Water depths range between 26 m and 36 m below LAT. 

Our solution

 

DEME Offshore, Scaldis and Dredging International - worked together to perform the successful construction of Rentel.
The scope included the responsibility for the design of the scour protection, foundations and inter-array cables; for the transport and installation of the turbines and for the offshore substation. The scour protection is a two-layer system: the filter layer was installed prior to the foundation installation and the armour layer after the pull-in and termination of the cables. Additionally, the Group was responsible for design works, surveys, obstacle removal, sea trials for dredging works, design engineering and execution of crossings, cable burial by pre-trenching and backfilling, as well as beach pulling. 

The foundations for these large turbines comprise a monopile (MP), with a weight up to 1,250 tonnes and a length up to 83 metres, and a transition piece (TP) on top with a length of 22 metres and a weight of 325 tonnes. The MP and TP are connected by a conical grouted connection. With a height of 183 m, the turbines are the largest wind turbines so far in the Belgian North Sea.
DEME’s Heavy Lift Jack-up Vessel ‘Innovation’ installed the foundations. DEME Offshore faced several major challenges during the installation including the handling and manipulation of the heavy and large MPs and the soil conditions were not ideal for the jacking operations and pile hammering. A single, big bubble curtain was used to reduce the underwater noise level during piling operations. 
 

Each of the three sections of the inter-array cables were delivered in one length and cut on site. The transport and installation of these cables was carried out by DEME Offshore and the OTS foundation was transported and installed by ‘Innovation’. The 1,200-tonne substation for the wind farm offshore was handled by Scaldis’ Heavy Lift Vessel ‘Rambiz’, while the 42 wind turbines were installed by A2Sea’s jack-up installation vessel ‘Sea Installer’. DEME’s vessel ‘Flintstone’ was deployed for the cable installation works, while the fallpipe vessel ‘Rollingstone’ executed post-trenching and rock placement works. 
With the goal to reduce cable installation time and costs, DEME ensures that all of its cable laying and trenching equipment is deployed as efficiently as possible. The cable burial works at Rentel were optimised with the CBT 1100 trenching tool. 

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