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Wind turbine design

Our team of engineers collaborate to ensure full corrosion control throughout the entire life of the structure.

Turbine foundations

Offshore wind turbine foundations come in many shapes and sizes. The four most common designs are monopiles, jacket, tripod and gravity foundations. Which type of foundation and what size that is suitable depends on external factors such as wind speed, water depth, height of waves and seabed properties.

Offshore wind turbine tripod design
Offshore wind turbine tripod design

A corrosion protection strategy should be established early to avoid problems later.

Corrosion control challenges

Offshore wind turbines are typically located in shallow water with risk from high flow rates and seabed movement. Commonly applied cathodic protection designs can fall short under these conditions.

The corrosion protection design has to consider several issues, including the weight and manageability of anodes for offshore installation, options for mounting the anodes and hydrogen evolution in a sealed environment.

For a long time it was an accepted opinion that internal wind pile corrosion was not an issue, as this area is a confined space. If perceived as a sealed-off compartment, corrosion would automatically cease once all available oxygen in the column has been consumed. This assumption has been shown not to be fully valid as both sea water and oxygen have access to the inside of the monopile under certain conditions, for instance where large tidal variations exist. This may result in active corrosion. If not prevented, such corrosion will compromise the durability of the wind farm and reduce the service life.

Sacrificial anodes for wind turbine foundations

A contributing reason for internal corrosion is that specific micro-organisms in seawater, such as sulphur reducing bacteria (SRB) do not need oxygen to survive. If nutrients are sufficiently available they attack the metal from inside, resulting in local pitting. This failure mechanism is called bio-corrosion or microbial influenced corrosion (MIC).

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Scope of work

Design work undertaken by Cathwell would typically include the following:

  • Review of the corrosion protection philosophy and structure exposure.
  • Calculate the cathodic protection requirement.
  • Propose anode types and positions.
  • Document the work.
  • Indicate material price (and if requested, offer for the supply of cathodic protection material).

Engineer undertaking design on computers

Front-end engineering and design (FEED) will benefit from early project involvementAny documentation submitted by Cathwell will demonstrate the adequacy of the design, and will be sufficiently well organised and detailed to allow for third party verification, if requested.

Cathodic protection material can be offered by Cathwell in line with the result of the engineering work, if requested.

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Design codes

Cathodic protection of subsea structures is designed based on established codes, but also a good share of experience. If not otherwise requested, the specificiations set forth below will generally be considered as basis for the execution of conceptual and detailed cathodic protection design.

It should be noted, though, that the existing standards and codes which are widely used for the design of cathodic protection systems for offshore structures and monopile foundations have inadequacies in respect of the protection of internal (almost) closed compartments.

DNV-RP-0416: Recommended practice and guidance for design, construction and in-service inspection of corrosion protection systems for wind turbines.

DNV-OS-J101: Design of offshore wind turbine structures, providing principles, technical requirements and guidance for design, construction
and in-service inspection of offshore wind turbine structures.

DNV-RP-B401: Recommended practice for design, manufacture and installation of sacrificial anodes, primarily intended for CP of permanently installed offshore structures associated with the production of oil and gas.

NORSOK M-503: Cathodic protection requirements for cathodic protection design of submerged installations, pipelines, internally in vessels and tanks, and sea water containing compartments.

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Design input request

To undertake detail design of cathodic protection of subsea structures, the following input would generally be requested:

  • Drawings of the structure(s) to be cathodically protected, preferably in dwg format.
  • Type and extent of surface treatment and coating standard/category applied.
  • Intended location of the wind farm.
  • Requested design lifetime of the cathodic protection.
  • Cathodic protection specification to be applied, if any (e.g. DNV-RP-0416, NORSOK M-503 or DNV-RP-B401).

You may direct your input and request here.

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