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Why trust cast iron for marine renewable energy projects

Ballast has been widely deployed recently by various marine renewable energy installations. In particular, gravity-based structures and floating wind turbine technologies require high-quality mooring systems to restrict their movements, and make installation and maintenance easier. Cast iron ballast responds to all these requirements and can be easily implemented into different designs with various functions.

Applications of Ballast in the Marine Renewable Energy Industry

Gravity-base ballast

Gravity-based structures are a quite common type of tidal turbine foundation, kept in place by the additional ballast. When these foundations are made of concrete, a massive supporting structure is needed leading to high material and logistic cost. On the contrary, cast iron is much more denser, stronger and harder than concrete and results in a cost-effective solution, far less prone to impact and strains. Since this is a fast developing sector, all involved solutions should be dynamic and easily adaptable to the various designs. Cast iron ballast can vary in shape and weight and can reach up to 20 tons.

A good example is SABELLA, (lead image above), which was placed in the strong tidal currents of the waters of the Fromveur passage, near Ushant in northern France to produce energy by capturing the kinetic motion of the tides. The French tidal turbine manufacturer chose cast iron ballast to stabilize its solution because of the following advantages:

  • A ballast base can be placed on no matter what kind of a seabed – hard or soft.
  • In-depth geological site investigations are not needed.
  • The simple design of the ballast base doesn’t require a frequent maintenance.
  • Cast iron ensures a highly reliable protection against erosion and other coating damages.
  • Utilizing cast iron ballast costs far less than a drilled foundation.
  • Installation is much safer since the ballast base doesn’t accommodate human operators and can be quickly dropped to the seabed with a heavy-duty machine.
  • Cast iron ballast is made of recycled steel. The carbon assessment of this process is positive and many studies have confirmed its environmental neutrality.
  • Cast iron could be used as well inside foundations, which needs a static ballast system. Here the density of the material coupled with the modularability of the solution can contribute to the reduction of the foundation size.
  • Cast iron optimises the efficiency and costs of the anchors, especially those of suction piles whose diameter can be reduced by the implementation of a specific amount of cast iron ballast at their top.

Underwater cables

Since cables are deployed in almost any single step of an offshore project, their protection is one of the most important requirements for the sustainability of the project. The protecting and ballasting shells made of cast iron are one of the most effective and convenient solutions right now at the market. (See image for example)

FMGC, a French foundry, is providing a solution combining the protecting, the stabilizing and the bend restrictor aspects of the shells in one product. The hydrodynamic design of the protecting and ballasting shells results in an increase in the bend radius to get closer to the cable and the undersea relief. This solution has been designed to ensure easier and faster installation, inducing a decrease of installation costs. Cast iron shells can be used in the protection and ballasting of underwater electricity, transmissions and telecommunications cables, but also widely for underwater conducts.

The specific design and the material choice of cast iron contributed in this case to an important cost reduction due to:

  • The elimination of the need for other stabilization systems such as concrete mattress or rock dumping.
  • The limitation of the potential damages on the cable ensuring an important protection and an optimal stabilisation thanks to the higher thickness of the shells compared to other shells available on the market.

Because of the critical importance of the protecting and ballasting shells, most manufacturers study the mechanical properties and the product behaviour in operational conditions. FMGC for example has already achieved numerical and mechanical tests on different scales proving the reliability of the cast iron ballast integrated into the shells and the resistance of its entire range of products. But to go even further in its analysis, the company is conducting some tests in the SEM-REV site. This is an offshore test site with a dedicated team based at Le Croisic and it plays an essential role in the development of a new industrial value chain in France.

The use of cast iron for offshore ballasting shells ensures cable’s protection, stability, strength and best possible performance thanks to the mechanical properties of the material able to withstand the harshness of marine environments.

Clump Weights

 

Clump weights are another ballast solution made of cast iron (image right) and especially designed to offset the vertical forces against the anchor and restrict the movements of an offshore wind floating structure or a floating oil and gas rigs. Holding firmly in place floating turbines in a highly reliable and cost-effective way is a common challenge for the growing renewable sector. If the offshore mooring is reliably designed and manufactured, the installation and the maintenance of the entire device are significantly eased with dedicated components.

The cast iron clump weights are available in two models:

  • The “distributed” configuration is a set of medium-sized clump weights, distributed over a segment of the mooring lines near the touch down point. This configuration allows the reduction of the mooring line lengths and thus the associated costs while keeping an optimal effectiveness.
  • The “mutualized” configuration consists of one clump weight, attached to one specific point of the anchor line. This configuration neutralizes the impact of wind and wave on the anchoring line and leads to a reduction of the tensions applied on the mooring lines.

Thanks to their high-density, cast iron clump weights ensure a compact solution that contributes to the mooring dynamic and effectiveness by ensuring the restoring forces, limiting the applied tensions, but also the vertical loads and uplifts. This highly resistant and reliable solution contributes to the ability of the floaters to handle the solicitations induced by its environment (wind, waves…)

Turbine designs and associated floaters keep varying in size and shapes to accommodate improved functionalities. Their installation gets even more expensive and the deployment and the operation of these technologies require the implementation of robust and proven technology in order to guarantee investors’ confidence and reliable returns. Clump weights are designed to be positioned on the mooring lines in order to ensure the restoring forces without resorting to expensive solutions or longer mooring lines length.

A good example here to mention is Floatgen – France’s 1st floating offshore wind turbine. Indeed, to limit the tension on the innovative floating mooring lines, clump weights have been especially designed and manufactured for the project. These are the 1st clump weights in France that have been ever used in the installation of a floating offshore wind turbine. The solution has significantly contributing to the floater performances though an optimised mooring system.

Why choose cast iron?

As discussed above, cast iron features a number of advantages for offshore ballasts. The right dimensioning, eliminating any oversize and the high-density of the material significantly minimize projects cost. Time cycle, dock and offshore operations, ships and storage spaces are all optimized. Moreover, cast iron solutions are fully recyclable at the end of the project.

As confirmed by the IN VIVO ENVIRONMENT research (please notice that the study is available only in French; please contact us in case you need further information) on the exact impact of gray cast iron on the marine environment:

  • «FGS and FGL cast iron behave in a similar way when corrosion occurs in marine environment.»
  • «Cast Iron corrosion is harmless for marine environment due to the low quantities of waste leached.»
  • «The corrosion rate of cast iron is relatively low, around 0.25 millimeter per year.»

In times of climate challenges and ongoing debates, the marine renewable energy sector is one of the most promising industrial segments. To thrive, the market needs innovative solutions, created by experienced engineers with innovative ideas. Cast iron offshore ballast offers a simple but highly efficient solution, helping professionals to overcome the challenges with greater safety and higher output at reasonable cost.

Marine renewable energy projects are discussed at POWERGEN International, set to hit New Orleans on November 19-21, 2019. Check out the event here.