Germany: New cable connection between offshore platforms

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 started, 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 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 six million euros is being carried out as a turnkey project. The configuration also includes a feasibility study, including dimensioning and calculation of current carrying capacity. “One particular challenge is laying the 4.5-kilometer-long, high-flexibility Feltoflex HV cable. 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,” explains Vukasin Basara, Project Manager for HV Cable Projects at Pfisterer.

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 and therefore 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 possible in the first place, Pfisterer was included at the project development stage back 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 Rostock-Warnemünde. 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 then 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," explains Eduardo Santana, Director Sales Cable Accessories & Systems at Pfisterer.