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.