Technological Innovations

The Concept of Choice

As a tension-leg-platform (TLP) the GICON®-SOF has significant advantages compared to other floating substructure types like semi-submersibles or spar buoys. The semi-sub gains its stability from its outer dimensions, activating the water plane area to compensate the overturning moment caused by the turbine thrust. Thus, to support a 6 to 10 MW turbine a semi-sub would have huge outer dimension. This heavily decreases the number of possible manufacturing sites.

In contrast, a spar type floater is very slender but it needs a very great draught (~100m) to stabilize a 6 MW turbine. This makes manufacturing and transport & installation (T&I) in particular very difficult. Contrary to semi-subs and spars where catenary moorings are used, a TLP is attached to the seabed with taut mooring lines. The structure‘s buoyancy which is much greater than its weight is causing a strong upward directed force. By tensioning the moorings the entire structure becomes firmly braced, which causes a very stiff system resisting even the strongest weather conditions with minimum accelerations and deflections. Due to its inherent stability a TLP can be designed much smaller compared to a semi-sub or a spar.

GICON and its research partners have been developing the GICON®-SOF since 2009. In this process three key issues were identified that led to high costs. To tackle these issues an improved manufacturing and installation concept was developed to drastically reduce the levelized costs of energy (LCOE).

Semi-Submersible | Spar-Buoy | Tension-Leg-Platform | Source: Daniel Walia, Endowed Chair for Windenergy Technology, University of Rostock

1. Efficient Construction

Shipyard and dry-dock rent are important cost factors. These can be minimized by prefabricating the components and transporting them to a harbour close to the commissioning site. This leads to a drastically reduction of the building time and therefore short occupancy of the dry-dock. In addition prefabricating means a great flexibility with regard to the supply chain, as well as for possible assembly sites. Introducing prefabrication also makes the entire construction process less demanding on the site where it is carried out. Furthermore, the assembly of the GICON®-SOF does not rely on a dry- dock. It is also possible to be carried out on a floating pontoon or just a flat area of the harbour using a shiplift for the launch.

2. New Materials

99% of current offshore structures are conventionally build by welding elements of shipbuilding steel. However, welding operations are very time consuming and expensive.  For one single steel floating substructure for a 6 MW turbine at least four months of working would be necessary. When considering man hours, rent for the dock and raw materials, this floating substructure would cost 2500-3000 € per metric ton. For the GICON®-SOF instead, prestressed ultra-high performance concrete (UHPC) elements will be used, which leads to a significant reduction in production cost.

For the GICON®-SOF instead, prestressed Ultra-High-Performance-Concrete (UHPC) pipes will be used. UHPC has a very high density and therefore high bearing capacities. This means the GICON®-SOF can be build more efficiently which leads to lighter designs. Prestressing is a proven method in bridge engineering. By this approach a prestressing force is applied to the concrete element, causing a high compression within the concrete matrix. This ensures, that during the entire lifetime of the structure, no tensile stresses occur within the concrete material. As concrete can bear very high compression but only very low tensile stresses prestressing is a great method to improve the bearing capacity and life time of members made from this material. The prefabricated concrete elements have a cost of  450-500€ per metric ton which is much more cost efficient than welded steel structures.

The buoyancy bodies, which are placed at the 4 corners of the GICON®-SOF, are made out of concrete shell elements. These elements are well known and have been proven in tunnel engineering over decades where they are designed to last at least 100 years. This includes also the sealing which can bear mountain water pressures of more than 1*10⁶Pa.

These proven technologies are perfectly suitable for the application in the GICON®-SOF resisting even the hardest offshore conditions during its entire lifetime.

Laboratory test of a prestressed UHPC pipe | Source: Daniel Walia, Endowed Chair for Windenergy Technology, University of Rostock

3. Eased Installation Process

A drawback of a conventional TLP is that it is not stable until it is braced by the moorings. Thus, expensive special operation vessels are necessary for the transportation (from the harbour to the installation site) and its installation. Furthermore, conventional TLPs require the seabed anchors to bear very high vertically directed pulling forces, which means a demanding task to geotechnics and anchor designers. This also makes the installation very difficult and expensive.

By incorporating a lowerable gravity anchor, which will be ballasted for the installation, the GICON®-SOF solves the above mentioned flaws of a conventional TLP. The anchor acts as a barge while towing the entire platform including the wind turbine from the harbour to the installation site. This enables a very easy transport for which only simple tug boats are necessary. Once arrived at the commissioning site the anchor will be ballasted and lowered to the seabed. Then the moorings will be tightened, completing the installation. Furthermore, the gravity anchor makes costly preliminary work for the installation of conventional driven or drilled anchors obsolete.

The GICON®-SOF combines the advantages of a semi-sub (self-stabilising and easy to transport) with them of a TLP (small outer dimensions).

Installation Steps | Source: Daniel Walia, Endowed Chair for Windenergy Technology, University of Rostock