Evolution of Offshore Foundations

By Dario Mulazzani, DAVI Product & Market Specialist Wind Energy & Heavy-Duty Division.

Located in such a dynamic and extremely powerful element as the sea, foundations represent one of the main elements of any offshore wind farm, accounting for over one-fourth of the total equipment cost. As they must support the wind turbines, absorbing all the forces and loads and providing a safe and stable base, defining the right typology of foundation can have a huge impact both on the economic and technical sides.

Offshore wind farms can typically be placed in a few selected locations only. There are two main conditions limiting the range of use of this type of installations:

• The depth of the seabed: greater depths associate higher costs and the use of more complex and specialized technologies, something that is reflected in the final investment of the wind farm, reducing its profitability. Closed seas, located within continental platforms, have an average depth significantly lower than oceans and open seas, being more convenient for this type of project.
• The distance to coast: in order to have a higher-intensity wind and less impact on the landscape and the coast, in the location of this type of facilities, it is sought to distance them as far as possible from the coastline, since this also allows the use of turbines of greater nominal size and power. However, this usually leads to an increase in depth.

The development of new technologies, manufacturing and construction procedures, the increase in the size of wind farms and turbines together with higher society’s awareness of visual and environmental impacts, have led to the displacement of these installations further from the coastline than ever before.

Depending on the depth of the seabed, as well as the surrounding conditions, different foundation solutions can be used:

• Gravity-based structure (GBS): concrete-based structure which can be constructed with or without small steel or concrete skirts. The base width can be adjusted to suit the actual soil conditions. The proposed design includes a central steel or concrete shaft for transition to the wind turbine tower.
• Monopile: the monopile support structure is a simple design made by steel tubes and cones on top of which sits the tower, typically through a transition piece. The monopile continues down into the soil.
• Tripod: three-leg structure made of cylindrical steel tubes. The central steel shaft of the tripod makes the transition to the wind turbine tower. The base width and pile penetration depth can be adjusted to suit the actual environmental and soil conditions.
• Jacket: structures typically used in O&G sector but optimized for the Offshore Wind Farms. It has a Transition Piece platform on top and the main structure is made of legs, braces and pin piles to anchor the complete structure to the sea bed. It can have four or three legs.
• Floating: The floating support structure consists of a floating platform and a platform anchoring system. The platform has a transition piece on top of which the tower is installed.

There is however a typology that stands out above the others, both in frequency and range of use: the monopiles, being selected in more than 60% of the worldwide offshore wind installations. Monopiles are deployed in the majority of depths and environments due to main characteristics, as follows:

• Simplicity: a simple and easily standardized design allows it to be manufactured in series without the need for high-end 3D cutting and welding technology, shortening construction and installation times and, therefore, reducing costs.
• Adaptability: linked to the previous point. Its simple design makes it possible to adapt to different installation site characteristics, avoiding the need for a large amount of field data and hence providing a competitive solution in both shallow and deep waters.

In order to adapt to the needs of new – more efficient and demanding – wind farms, monopiles design has evolved over the years. Its main dimensions, diameter, thickness and length have steadily increased to be able to support higher and heavier – more powerful – turbines installed at greater depths.

It has been observed that both the length and the diameter of the piles have evolved proportionally to the power of the turbines, maintaining constant ratios which can be summarized as follows:

• Monopile length [m] ≈ 14.5 ∙ Turbine Power [MW]
• Monopile diameter [m] ≈ 1.5 ∙ Turbine Power [MW]

For instance, for a 6-7 MW turbine (being the most mature capacity to date), monopiles are typically in the 90-100m length range with diameters and thickness at bottom sections of 9m and 90-100mm respectively for an overall weight often exceeding 1500 ton.

As mentioned, offshore installation allows for the deployment of larger turbines (both in terms of capacity and physical size), mostly due to constraints related to onshore transportation of the relevant heavy and expansive equipment: this is desirable as a wind farm comprised of larger turbines will necessarily require lower investment and maintenance cost per installed capacity, maximizing the project profitability.

It is hence likely that offshore wind turbines capacity will keep increasing with some manufacturers already bringing to the market 12-14MW turbines (such as the GE Haliade-X, currently being tested onshore). Although the aforementioned ratios are expected to decrease due to physical limitations, the so called XXL monopiles will weigh up to 3000 tons with diameters and thickness at bottom sections ranging from 12-14m and 120-140mm respectively.

Other foundation typologies are expected to follow a similar increasing trend.

All types of off-shore foundations are comprised of large plates jointed together and rolled into cans and cones; such workpieces are then assembled and welded into large structures which, in the case of Monopile Foundations, can reach up to 120m in length. With plates thickness often exceeding 140mm and increasingly large diameters (>12m) it becomes critical to minimize the amount of circumferential welding required to grow the overall length, often leading to the selection of increasingly large plates (up to 4.5m in width, with weights often exceeding 150 tons each).

Thus, fabricators engaged in the manufacturing of such heavy-duty off-shore foundations will need to face and overcome two main manufacturing constraints, as follows:

• increasing ratio between forming diameter and plate thickness (D/T): due to very large conical and cylindrical sections diameter, high D/T leads to workpiece instability; if not properly managed, such instability generates undesirable deformations depleting the overall rolling accuracy
• large diameter conical sections with angle up to 14°: commonly used to transition between monopiles bottom and top sections (having increasingly different diameters), the resulting cone developments are heavily unbalanced with a side and longitudinal footprint often exceeding 15m and 30m respectively for a 12m diameter.

For all these reasons, the fabrication of offshore foundations is certainly the most challenging and time-consuming rolling operation.

Fabricators are hence in need of a technology partner able to deliver high performance and reliable rolling solutions, capable of sustaining the demanding serial production characteristics of this Industry by increasing rolling accuracy, output and operator’s safety while decreasing downtimes, floor-to-floor processing time and manpower requirements.

Since 2007, when DAVI Wind Energy Division was created, DAVI has been leading the Market thanks to its cutting-edge technology developed working along the most accredited project developers and towers and foundations manufacturers.

For instance, to better serve the Off-shore Wind Energy Sector, dedicated heavy-duty feeding and handling equipment are continuously being developed and updated to ensure that Fabricators entering the DAVI World are provided with the most advanced hardware allowing them to maintain competitiveness in a fast changing and dynamic environment, requiring more and more challenging workpieces to be manufactured every year.

To date DAVI is already the technology partner selected by most major players, with over 300 installations dedicated to the Wind Energy Industry worldwide and approximately 60% of the overall Wind Energy rolling machines market share.

Don’t miss out, stay ahead with DAVI!

Note: The opinions, beliefs, and viewpoints expressed in this article do not necessarily reflect the opinions of Offshore WIND.