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Over the last ten years, we have witnessed considerable changes in offshore wind farm development and execution. Farms of low power capacity close to shore, used used fixed platforms and high voltage AC (HVAC) as the means of power transmission. New farms of several hundred MW are now being designed, and the trend is to move to high voltage DC (HVDC) as the preferred form of power transmission. The next phase in the evolutionary process is to more further offshore, to floating platforms. This step change introduces many new challenges.
Main components of offshore support structures: a) monopile; b) jacket; c) gravity base; d) suction bucket (1 – tower, 2 – work platform, 3 – transition, 4 – intermediate platform, 5 – boat landing, 6 – external J tubes, 7 – scour protection, 8 – grouted connection, 9 – monopile, 10 – skirt, 11 – concrete layer, 12 – suction bucket, 13 – shaft, 14 – pile sleeve, 15 – mud mat) |
Typical offshore substation structure [Source: Bull. Pol. Ac.: Tech. 66(4) 2018}
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Main components of offshore support structures
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The topside is typically a box-shaped structure placed on top of the substructure that contains the electrical equipment included in the offshore substation, providing for some or all of the platform’s functions. The topside layout is mainly determined by:
1. Electrical substation layout. This layout results from the type of the power substation implemented in the offshore grid, the capacity of the offshore wind farm and electrical parameters (voltage, type of current (AC or DC) of the submarine transmission system). Topsides for high voltage DC (HVDC) substations are generally larger than their high voltage AC (HVAC) counterparts. 2. Additional requirements, which include:
3. Adopted solution for the support structure. Other factors that can impact the topside structure are topside port requirements due to the fact that the topside structure is made onshore and then transported to the installation location. The topside is composed of decks divided into rooms and halls with the lowest deck being the cable deck, whereas the topmost deck accommodates signaling equipment and a helipad (if present in design). Electrical equipment on OS offshore platforms is arranged in either of the following two ways:
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Racebank
Sandbank
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Electrical equipment of the offshore substation depends on the type of electrical current (AC – Alternating Current, or DC – Direct Current) used for the transmission of electrical energy. Due to the type of electrical current used, HVAC and HVDC offshore substations can be distinguished. The reasons for choosing HVDC instead of HVAC to transmit power from offshore to onshore systems are often numerous and complex. But in most cases the transmission distance to the shore is the key factor. Basic electrical equipment of HVAC substations includes:
Kriegers Flak offshore substation: a) lower deck; b) upper deck; 1 – transformer 220/33 kV, 2 – 220 kV GIS switchgear, 3 – 33 kV switchgear, 4 – Diesel generator, 5 – emergency switchgear, 6 – workshop and storage, 7 – SCADA, 8 – 0.4 kV switchgear, 9 – accumulator battery, 10 – social rooms, 11 – heat and ventilation |
Kriegers Flak offshore substation: lower deck
Kriegers Flak offshore substation: upper deck
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A large scale installation, such as DolWin 2 comprises the following main elements:
In the case of HVAC transmission, the offshore and onshore converter modules are omitted, and reactive compensation (reactors, STATCOM - static compensation added). The HVAC offshore substation would connect to the onshore substation (simplified manner)
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Overall Single Line Diagram
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The general trend is to build the offshore substation (OSS) and offshore converter platform (OCP) as a single installation. In such as case, we eliminate the HVAC cables between the two functional modules.
We can observe from the diagram, that the design incorporates a certain amount of redundancy. For example:
One of the key objectives is to test as early as possible, in controlled conditions. This will allow us weed out and resolve many small issues, and also prevent worker congestion in the final integration yard, and perhaps offshore. Areas of optimization include, the design and build of:
It can easily be recognized, that all monitoring, control and protection functions must be made available and tested for each module. By designing and building the modules for autonomous testing, we are also able to reduce the number of integrated cabling between modules. Cabling between modules is limited to power cabling, certain hard-wire cables, and network cabling. |
Offshore Substation / Converter Platform Single Line Diagram
Modularization will be key for offshore substations and converter platforms, in order to reduce delivery time, cost, and performance predictability, in an aggressive market. |
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We offer valuable expertise in a number of areas, however our primary focus is on Offshore Electrical Service Platform and Onshore Substation Engineering, Procurement and Construction within the Electrical, Instrument and Communications disciplines.
Command Center for all unmanned remote control and monitoring of turbines, Electrical Service Platforms, and Substations.
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Commissioning of Medium Voltage Switchgear. Similar experience in 35kV Hybrid switchgear, and Gas Insulated Switchgear up to 232kV
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Network Controllers, for Integrated Control Command and Safety (ICSS) for the electrical system.
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