Gravity Based Support Structures

Article published on Wednesday, 05 June 2013

The gravity type support structure is normally a concrete based structure which can be constructed with or without small steel or concrete skirts. The ballast required to anchor the foundation consists of sand, iron ore or rock filled into the base of the structure with adjustments in the designed base width to suit the soil conditions. The design will include a central steel or concrete shaft for transition to the wind turbine tower. The structure requires a flat base and for most locations will require some form for scour protection which is determined during detailed design stage.

Supporters argue that concrete costs are less volatile than other commodities such as steel. Furthermore concrete is durable in the marine environment leading to lower maintenance requirements. Where a dedicated mass production facility can be set up it may deliver productivity benefits and reduce costs.

In general, gravity foundations are designed with the objective of avoiding tensile loads (pulling/lifting) between the bottom of the support structure and the seabed. This is achieved by providing sufficient dead loads such that the structure maintains its stability in all environmental conditions solely by means of its own gravity. Gravity foundations are usually competitive when the environmental loads are relatively modest and the “natural” dead load is significant or when additional ballast can relatively easily be provided at a modest cost.

Gravity based structures can be designed to float to site using only tugs for transport making it a suitable option where the installation of the support structure cannot be performed by a heavy lift vessel or other special offshore installation vessels..

In the installation, the following equipment is generally needed: • A large floating crane • A barge that can transport and possibly store a number of the foundations on-board • A tugboat or group of tugs that can either the barge or both barge and crane into position in order to set the foundation in the right location

Some of the wind farms where this type of foundation has been installed are Rødsand 2 (Denmark); Vindeby (Denmark-first offshore wind farm in the world); Kårehamn (Sweden), which was completed in 2013.

Gravity Based Support Structure

Gravity Based Support Structure

Variations of gravity foundations

Crane Free Gravity Base (Seatower)

The developers of the Fécamp offshore wind farm have chosen to demonstrate and install Seatower’s patented Cranefree Gravity®foundation design. The Fécamp demonstrator project is the final step in Seatower’s objective to move the foundation design to full-scale manufacturing and deployment. Norwegian based Seatower will manage the installation project, including the demonstration of the entirely new “float-out-and-sink” offshore installation method. This means that an offshore wind foundation will be installed using only regular towing vessels. Seatower’s CEO Petter Karal has stated that Seatower’s technology will ease and speed up the construction of offshore wind farms at a time when cutting costs and risks is necessary to help the sector progress to more challenging project sites. He claims that Cranefree Gravity® foundations are generally more cost-efficient, quicker to install and less risky than current methods that use steel foundation designs. Importantly, they also allow for local construction, which provides welcome economic activities to the coastal communities close to the wind farms.

The Fécamp demonstration project, which is scheduled for early 2015, will showcase the unique technology’s game-changing installation advantages to developers of offshore wind farms across Europe. It can be deployed on project sites that are in deep waters i.e. more than 30m. In addition, construction management can run foundation installation programmes all year round, uninterrupted by poor weather. The process can easily be made quicker by increasing the number of towing vessels.

The Fécamp foundation will be pulled around 13km by a towing vessel before being positioned and subsequently sunk in a controlled manner by gradually letting in seawater. It will be outfitted with equipment that will measure wind and other parameters on the site, as well as to provide data for future optimisation of the technology. Eiffage, an established manufacturer of conventional, steel based foundations for the offshore wind sector, will fabricate the demonstrator foundation at the port of Le Havre.

Seatower Crane free gravity based foundations

Seatower Crane Free Gravity Foundation

Gravitas Gravity Base (Arup/Costain/Hochtief)

This gravity-base foundation can be installed in deeper waters with a self-installation system, which avoids the need for specialist marine equipment. The main characteristics of these foundations according to the proponents are:

• Reinforced concrete, non-piled, ballasted gravity structure • Caters for water depths up to 60m • Suits larger turbine sizes up to an anticipated 8MW • The design minimises the need for seabed preparation by accommodating existing seabed slopes and surface sediments • Designs can be standardised across individual or multiple sites to enable cost optimisation • Offers life cycle carbon footprint benefits through material selection

Gravitas Based Support Structure

Gravitas Based Support Structure

The proponents also promote that these concrete bases are configured for rapid construction using readily available construction skills. The construction takes place onshore and does not require deep water (10m draft) and the existing supply chain capabilities can be leveraged to enable projects to start immediately.

During the installation, the foundations are self-buoyant for ease of deployment to the wind farm location. Standard tugs are used to install the foundations and the sea-borne noise and vibration is minimised. Scour protection is designed for minimum maintenance over the design life in the wind farm.

GBF® gravity base (Vinci Construction UK/Freyssinet International)

This concrete gravity foundation is suitable for a large range of water depths: 20-55m many sea bed soil conditions and turbine sizes of between 3-10MW. This solution is assembled onshore.

The construction of the foundation, installation and pre-testing of the tower and turbine take place onshore. The transport and installation at the offshore site is carried out using a purpose-designed Transport and Installation Barge (TIB). The TIB is towed to the wind farm site, where it is ballasted to lower the wind turbine into position on the prepared seabed. Once the positioning has been verified, the TIB releases the gravity base, moves away and is deballasted, ready for its return journey back to the quayside.

According to the proponents installation is quick and it requires a low maintenance.

Custom made vessel - the wind turbine and support structure is transported to site in one piece

Custom made vessel - the wind turbine and support structure is transported to site in one piece

Consortia of Skanska, Smit Marine Projects and Grontmij

This gravity support structure is based on a caisson structure and according to the proponents it has been designed taking into account the construction and installation processes, and the main characteristics are:

• Flexible and scalable to suit multiple sites and metocean conditions, various water depths up to 60m and different turbine loads • Durable, long lifespan of 50 years – with low maintenance requirements • Self-buoyant structure eliminates need for specialist vessels • No access required onto the foundation during the tow or water ballasting operations • Turbine loads carried by the central shaft providing flexibility • Environmental benefits: use of recycled materials, low carbon footprint, eco-friendly design and installation minimises the noise and vibration impact on marine life

The proponents add that all aspects of the construction process uses readily available materials, labour and supply chain and that mass production logistics are employed using standard techniques; including the pre-fabrication of component parts.

All offshore construction activity is through SMIT Marine Projects using their own vessels for deployment and seabed preparation equipment for use in water depths of 60m.

The advantages of the proposed system are that only water ballast is required after installation to resist a summer storm and the sand fill operation is decoupled from the weather restricted operation. The decommissioning process is straightforward as the foundation structure is engineered to be re-floated.

Skanska, Smit Marine Projects and Grontmij gravity base foundation

Skanska, Smit Marine Projects and Grontmij gravity base foundation

Vici Ventus Concrete Gravity Foundation (CGF)

The main features of the structure given by the proponents are:

• For water depths up to 100m + • Firm or soft soil • Optimal stiffness and dynamic characteristics • Not fatigue sensitive • Maintenance free • Long design life, 100 years + • Can support larger turbines in the future • No piling or anchors

The structure consists of a self-floating and stable structure with a buoyant shaft for floating stability. The assembly and hook up are made onshore. Afterwards, the complete turbine is towed to site and the installation is by ballasting only. Normally is does not required seabed preparation or offshore crane vessels. The installation has a short duration and robust. The maintenance is practically non-existent and the removal is simple by reverse installation.

• Case example for 40m water depth: • Based on North Sea conditions, air-gap 20m • Concrete structure designed with sufficient floating stability to carry complete turbine (5MW) from inshore building site to installation site • Base Slab diameter 40m • Shaft diameter 24 – 6m • Towing draft 35m • Net concrete volume 2,500m3 • Solid ballast 13,000t (inshore and offshore)

Vici Ventus gravity base

Vici Ventus gravity base

STRABAG gravity base

STRABAG offers a serial system for the complete offshore production, making it almost independent from the effects of weather and rough seas.

The STRABAG terminals are quays with industrial plants and a suitable pier for the onshore mass production mass production of fully operational offshore wind turbine generators. There, the gravity base foundations are produced using pre-stressed concrete, and the wind turbines are fully assembled. The terminals are equipped with a complex rail and hoisting system that allows production to be done right at the shipping location.

The proponent cites that it provides:

• Industrial plant for large-scale production • High level of automation: assembly line procedure • Flexible adjustment of capacities • Complete pre-assembly onshore • Can be used in water depths to approx 50m • Solid and proven pre-stressed concrete construction technique • No pile-driving, small footprint • Minimises damage to ships on collision • Can be completely disassembled

With an extremely powerful transport system, Strabag believes that it can reliably take the entire unit to its operating location and install it on the sea bed. STRABAG terminals are located in logistically ideal locations, in close proximity to suppliers of towers, nacelles and rotor blades.

For the installation, an innovative floating crane was developed that was specially designed for the requirements of gravity base foundations. The STRABAG Carrier was designed to take complete systems, weighing more than 8,000t, directly from the pier and to transport them to the wind farms. The system is then installed offshore.

• A multifunctional, mobile offshore transport and installation unit • Minimisation of the offshore work • Reduction of offshore risks • Largely independent of weather and swell through robust offshore engineering • Eco-friendly installation

STRABAG gravity base

STRABAG gravity base

References

Offshore Standard: design of Offshore Wind Turbine Structures, DNV-OSJ101, Det Norske veritas (2010) Project UpWind Foundation Types - E.On Gravitas Offshore GBF® Gravity Base Consortia of Skanska, Smit Marine Projects and Grontmij Vici Ventus Concrete Gravity Foundation (CGF) STRABAG

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