By Ritesh Gupta 

Operating offshore wind turbines that have been erected to date in Europe have mainly relied on usage of monopiles. But considering what the offshore wind industry needs, especially in the context of deeper waters and bigger turbines set to feature in European offshore wind projects, it is not surprising to find new and innovative structures from foundation specialists today.

Wind Energy Update (WEU) Survey: Average Rating for Anticipated 5 Year Market Share

Going forward gravity base foundations (GBS) are expected to make steady progress.

As explained in the WEU Offshore Foundations Report 2013, the key concept behind GBS resides in the large lumped mass necessary to anchor and maintain the turbine stability throughout its life and support all environmental conditions. The caisson typically rests on the seabed, necessitating limited preparation work to the site. Since this concept relies solely on the dead weight of the foundation, the caisson which constitutes the main body of the base is typically filled in situ with ballast stones, concrete or other high density aggregate.

Such foundations have featured, mainly for shallow water wind farms and particularly in the Baltic Sea.

Scope

The industry needs several types of offshore foundations, says Niels Brix, vice president, business development, Seatower. He asserts that monopiles have proven to be a viable technology up until around 30m, but above this water depth, it makes a lot of sense to look at the cost curves and to consider queries regarding loads etc.

“Steel is not inexpensive and above 30m monopiles are not a proven technology as the loads from larger turbines would compromise this type of foundation,” says Brix. “[Seen from this perspective] I feel confident that GBS will obtain a significant market share in deepwater projects.”

At the same time, it is also highlighted that in case of GBS, their benefit on the supply chain and potential for cost reduction will require further innovation until they start affecting the share of existing offerings in the next decade or so.

Advantages

Referring to installation, Verena Clarissa Gohs, marketing manager at Strabag Offshore Wind, says a significant advantage for the offshore works is that the GBF can be installed without using any driven piles. As well as avoiding the need for piling, the introduction of buoyant concrete gravity base foundations also removes the need for specialist vessels, as these so-called “self-installing” designs can be towed to site using standard tugs.

Brix explains that overall the GBS are easier and much cheaper installed than known steel strutures. Primarily because one does not need expensive jack up vessels, offshore cranes or hammers.

Two main differences are worth highlighting:

• First there are some GBS concepts using a special build vessel to install them. And then there are self-buoyant structures, which float to the installation site using a towing vessel. The installed price will differ. This is due to the cost of such vessels and the lead time getting the vessel to the market.

• Second the speed you can erect an offshore windfarm using self-buoyant structures should be noted as it paves way for significant savings. GBS can be beneficial for a project owner considering the fact that adding more towing vessels you can speed up the construction a lot.

In terms of acceptance, the WEU Offshore Foundations Report 2013 mentions that after the Middelgrunden, Rødsand 1, Lillgrund and Rødsand 2 wind farms in the Baltic Sea, the Belgian Thornton Bank I wind farm is the only project to have used concrete gravity base foundations in the North Sea. Six foundation structures, each up to 3,000 tonnes, were lifted into the water then transported to site and installed using the sheerleg crane Rambiz. This solution proved to be relatively expensive for the specific location, and a jacket foundation has been selected for the next two phases of the project.

Brix says that some of the cost overruns in the installation processes offshore can be avoided using GBS.

“If we look at the challenges in some of the executed sites till date, it is a point that piling and drilling causes added cost - also more than expected up front. Using GBS these costs are avoided and in addition for some concepts dredging is avoided or minimised. This is all counting to a more rapid and easy installation process,” he says.

Game Changer

Seatower’s solution addresses the need for flexibility in the production set up.

The underlining factor is the production of foundations on the quay side, not needing a CAPEX-heavy factory but a simple construction site to carry out the civil works.

“Also, we are not bound to one location as our plan is to manufacture locally. Our design is unique, as it is a relatively light weight gravity hybrid-structure, which will be loaded out smoothly and therefore again also allows a more rapid installation proces as we float out the foundations using towing vessels. Not needing any special purpose vessels or cranes for the transportation and installation proces we can plan the installation very effectively. This is a game changer as large scale projects will benefit from both a low CAPEX compared with today’s buildings of jackets,” says Brix.

As far as realistic expectations are concerned, a section of the industry believes that the cost effectiveness of the gravity base and suction bucket foundations is not seen as being very probable in the near future.

On this, Brix says, “The near future is just to come. Water depths of more than 30 meters will soon be the norm for many large scale offshore projects. Such projects are perfectly suitable for Seatower foundations out-beating the prices of jackets and ensuring a much smoother manufacturing and installation proces. Look at the oil and gas installations on deeper water and count in the need of a supply chain supporting faster delivery times and local content.”

GBS and gravity/ steel hybrid foundations have featured in deep water installations in the oil and gas industry for decades. The technology is as such known and proven. It is therefore a natural evolvement for the offshore wind industry to focus on GBS for larger turbines in the case for deeper water projects as this drives down the cost of electricity.

Source: Seatower’s Cranefree foundations