New Cable Connection Between Offshore Platforms


For the first time, two sister platforms in an offshore wind farm will be connected by a bridge system with the aid of a new flexible high-voltage cable with a conductor cross-section of 800 mm2, which increases the guaranteed energy feed-in for the wind farm operator. Pfisterer has received a major contract for this pilot project within the framework of the connection of the DolWin3 converter station. The electrical connector manufacturer already possesses the pre-qualification for the Feltoflex connecting cable with a conductor cross-section of 800 mm². The onshore work to equip the converter station with medium-voltage cable systems has already begun, and completion of the HVDC offshore connection is planned by 2017.

In the southwestern area of the North Sea, the DolWin gamma converter platform with a capacity of 900 MW is currently being constructed as the third connection within the DolWin cluster. Offshore converter platforms are normally connected directly to the mainland by means of a submarine cable. So far, any connections between platforms have been a rarity. This solution, which significantly raises the in-feed guarantee, is now being actively promoted with DolWin gamma. To be able to switch over between the converter stations when necessary, the two sister platforms — DolWin gamma and DolWin alpha — are sited close together, 80 kilometers offshore, and are linked by means of a bridge with a powerful high-voltage cable.

Unique Offshore Cable Routing

Pfisterer will lay all the high-voltage and medium-voltage cable systems within the platform, as well as the cables on the connecting bridge. This major contract with a value of almost 6 million euros is being carried out as a turnkey project. The configuration also features a feasibility study, which includes the dimensioning and calculation of current carrying capacity. 

“One particular challenge is laying the 4.5-kilometer-long, high-flexibility Feltoflex HV cable,” said Vukasin Basara, project manager for HV cable projects at Pfisterer. “For this purpose, we have developed an extraordinary concept. The cable is routed from the upper deck of the DolWin alpha platform at a height of 35 meters by means of a rising ladder rack and a bridge of about 30 meters in length, to the DolWin gamma platform.”

High Flexibility in Rough Seas

The connection between the platforms by means of a highly flexible 155 kV Feltoflex cable will have an overall length of 250 meters, therefore, it will be the world’s longest connection between two offshore platforms. The cable, comprising approximately 4,000 individual strands of wire, is insulated with a special mixture of high modulus ethylene propylene rubber (HEPR) instead of cross-linked polyethylene (VPE). The increased flexibility of this cable is a definite advantage in high winds. This allows the platforms to move by up to 800 mm relative to each other without causing any mechanical damage to the cable.

Tested and Safe

To ensure the construction of a cable connection with a power rating of 155 kV and the extraordinarily large cross-section of 800 mm² was initially possible, Pfisterer was included at the project development stage in 2011. In the course of prequalification, the company conducted numerous preliminary investigations for its solid-insulated Connex pluggable connection system. This successfully passed the contact aging test according to ICE 61238-1, as well as the electrical tests specified in IEC 60840 Section 12.4.

MV Installation Started

The platform is currently being constructed in the dockyard at RostockWarnemünde.

The start of the onshore installation work in the medium-voltage range also marks the start of the execution phase for Pfisterer. The internal cable connections between the switchgear and the transformers will first be installed with MV cable — including the connections to the joint compartments with HV cable — before the offshore cabling stage. In the next stage, the 4,500-meter-long HV cables will be installed, as well as the Connex connection system and Connex HV joints in various sizes. 

“The DolWin3 order is consolidating our position as the leading manufacturer of cable assemblies in the offshore wind energy sector, as well as our expertise and ability to offer complete integrated systems,” said Eduardo Santana, director of sales for cable accessories and systems at Pfisterer. 

Source: Pfisterer

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