Offshore Switchyard (OSY) design connecting multiple wind farms to the Belgian grid




Peter Busch Nielsen, Ramboll Oil & Gas

Peter Busch Nielsen

M.Sc., civil and structural eng., Head of Department, Structures, Denmark
T: +45 5161 7238

In December 2016, Ramboll was awarded the detailed design contract for the offshore switchyard (OSY) for Elia’s Modular Offshore Grid in Belgian waters. The OSY platform will act as an offshore electricity hub, which will transmit the electricity ashore from the wind farms Rentel, Northwester 2, Mermaid and Seastar. The wind farms are estimated to provide approximately 1 GW of renewable energy to the Belgian grid, which will be transmitted through two 220 kV export cables located on the OSY platform and one additional export cable from the Rentel wind farm. The modular offshore grid type will allow the connection and commissioning of the wind farms to the OSY platform in multiple steps and over a period of time. The contract with Elia included the detailed design of the entire substation, covering both jacket and complete topside design based on an existing conceptual design.

Complete topside and substructure design

The substation is designed for carrying equipment for high-voltage switchgear equipment and other facilities such as diesel generators, batteries and control panels for the operation of the switchyard. The platform consists of a steel topside installed on a four-legged steel jacket supported by piles. The main way of access will be by helicopter, and two additional boat landings are provided for alternative means of access. The topside structure is designed as a relatively flexible structure without braces and the walls consist of light non-load carrying wall panels. This means that the overall structural stability is ensured by the four primary columns and a system of moment rigid joints. Furthermore, the structural deck configurations are stacked with the secondary structure, located on top of the primary structure. The reason for this topside structural concept is to facilitate a streamlined fabrication process. The water depth at the substation location is approximately 34.0 meters.

The topside was planned with the following main components:

  • One HV GIS (Gas Insulated Switchgear)
  • Control & communication rooms (SCADA)
  • LV & utility room
  • Diesel power rooms
  • Public room

A cost-effective structural design through-leg piles

A lean layout of the jacket combined with through-leg pile features ensures robustness and cost efficiency through the entire life-cycle. Pile make-up split into two pieces offers the advantage of reducing requirements in load capacity of the heavy lift vessel. The high resistance of legs to ship impact decreases the amount of steel in the bracing. The connection between piles and legs is represented by shim plates of “crown” shape, which are welded offshore after the pile driving installation is completed. 

First OSY platform in the North Sea

The OSY platform will be the first of its kind in the North Sea. The platform will enable stepwise connection of the four wind farms, which will be constructed and commissioned according to their own dedicated schedules. The OSY platform will thereby be able to receive and transmit the power from the four wind farms regardless of their time of construction. This will reduce the amount of cables on the seabed and reduce the fabrication and maintenance costs. 

In the long term, the OSY platform is planned to be connected to an international platform shared by the neighbouring countries. This will enable the transmission of power across borders in the event of insufficient wind in some of the wind farm locations. 



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