Gravity bases used to support wind turbines are not new and there have been a number of installations using these systems. In the main these are based on massive concrete structures that are floated out and then ballasted down onto a prepared sea bed using either sand or concrete as the long term ballast.

This is proven technology although there are question marks about the ability to remove such structures at the end of their useful life.

One of the appealing features of using gravity bases is that much of the construction work can be done ashore thus reducing the weather dependence of the installation, although the tower and turbine still have to be installed offshore. Now two new systems that are under trial have been developed that will allow the whole completed structure to be built at an onshore location and towed out ready for direct installation. The attraction of these systems is that they reduce the weather dependence of the installation operation as well as removing the requirement for the use of highly specialised jack-up installation vessels of which there is limited availability.

One of these new systems is the EU-funded ELISA Project which the designers claim can be completely pre-assembled and pre-commissioned in controlled harbour conditions. A fully operational 5MW prototype of the ELISA system has recently been installed at a location in the Canary Islands. This is claimed to be the first bottom-fixed offshore wind turbine completely installed without the need for heavy-lift vessels, the European Commission's Community Research and Development Information Service (CORDIS) reports.

"The ELISA technology is a pioneer in the development of completely self-installing offshore turbines," said project engineer Jose Serna. "The entire system is completely pre-assembled and pre-commissioned in controlled harbour conditions, enhancing the possibilities for industrialisation and the minimising risks related to offshore assembly work."

The ELISA 5MW prototype uses a gravity-based foundation, which essentially serves as a floating platform for the journey out to the installation site. From this foundation an automatically telescoping tower complete with the wind turbine in place is attached. "Each unit of the combined platform, tower and turbine is completely assembled onshore and it is then towed to its open-water site using conventional tugboats, where the platform is secured and the tower raised."

"Part of the motivation for this new development is that currently there are only three or four heavy-lift vessels in Europe capable of installing an 8MW turbine in waters deeper than 40 metres and Europe leads the way in comparison to other developed markets. In other words, this system could also be a key European export to such markets as the US and Japan." said Serna.

The telescopic configuration of the tower was designed to bring down the unit's centre of gravity, meaning that the platform serves as a self-stable floating barge from which the crew can pre-assemble the entire system inshore. "This vessel-free installation capacity is not only a source of large cost reductions, but also a way to support the clear trend towards larger offshore wind turbines, a key step towards improving a wind farm's cost of energy," said Serna.

Once towed into its final location, the platform is ballasted to rest on the seabed. Then, when secure, the tower is lifted to its final position via cables and conventional heavy-lift strand jacks. These jacks start by lifting one level of the tower, and then are reused to lift the next level, and the next, and so on until the tower is fully built. The jacks are supported by the one below, which also guides the hoisted tube as it rises in a self-installing procedure where the tower itself is the only supporting structure required and the whole procedure is carried out from a single access platform.

It is anticipated that the Elisa system will reduce costs by as much as 30-40% when compared to traditional solutions based on jackets or XL monopiles. It should also reduce maintenance costs as the researchers claim the system will improve the integrity of the structure and reduce operational costs. "As a matter of fact, the cost per MW of the prototype being developed is already below current market prices, despite all the investment required in auxiliary infrastructure with these costs dedicated to a single unit." says Serna

The ELISA system telescopes the tower up from the base while an alternative system being developed in Holland has part of the tower structure lowered below the base and this is then telescoped upwards as the base is lowered to the sea bed. MonoBaseWind who developed this system has contracted Dutch engineering company KCI for the basic engineering of the structural steel design of their self-installing Gravity Based Foundation (GBF) concept.

This self-installing GBF is specifically developed for wind turbines with a capacity of over 6MW which need to be installed in waters deeper than 40 metres. "The principle has been tested by computer simulation and by model testing at MARIN, and the next step is full scale testing," KCI said.

A demonstrator will be built in order for the self-installing GBP to be accepted for project financing. It is proposed that the site for this demonstrator unit will be the Borssele Kavel 5, which is specifically intended for testing new offshore wind technologies. The telescoping lower section of the tower hangs below the main base once the tow out has reached deeper water. In this deeper water which can be in a sheltered area the bottom section is lowered and the turbine attached to the top. Once completed in these sheltered waters the structure is towed to its site, the base lowered and the tower raised as its base touches bottom.

The MonoBaseWind is a cooperation between Orca Offshore, PB Consultants and Venture Counsels.

By Dag Pike