A Unique Challenge

Floating offshore wind will provide access to stronger and more consistent wind speeds when compared to fixed turbines. Floating wind farms will be located in shallow water depths in contrast to traditional offshore oil and gas platforms. The effects of the environmental conditions are typically more sever at these shallow water depths. 

Platform stability is critical for the turbines power production efficiency. This is less of an issue for fixed-bottom turbines as they are not affected by waves, wind or sea currents. Floating structures in deep waters will allow for longer mooring lines, that provide the required compliance.

The combination of floating offshore wind farms and shallow water depths present a unique challenge. To moor a single wind turbine, longer lines could be implemented to dampen the higher dynamic loads on the platform. However, this would result in a larger mooring footprint. This becomes more problematic when introducing multiple floating wind turbines due to space limitations.

A Portfolio of Stiffness Characteristics

Mooring lines in a floating wind farm need a compact footprint. They must also have the necessary dampening characteristics for peak loads. Therefore, to develop a floating offshore wind farm mooring solution that operates efficiently, the stiffness properties must be considered. MoorLine by Bekaert, is available in a range of materials and offers a portfolio of stiffness characteristics.

The mooring line material is an important factor with regard to the stiffness properties within the system. Polyester is a tried and tested solution in deep water mooring applications, as a result the stiffness properties are well understood. High-modulus polyethylene (HMPE) has higher stiffness properties compared to polyester, therefore makes it an interesting material for tension leg platforms (TLP). Nylon has also had an extensive offshore track record, although in much harsher applications such as single point mooring (SPM).

The Most Cost-Effective Method of Mooring for Floating Offshore Wind Farms

Compared to polyester and chain, nylon increases the compliance of the mooring system. The higher compliance enables a dampening effect in the mooring line which reduces the magnitude of peak loads in dynamic scenarios.

This reduction of loads can provide the option of two major benefits. Either, an improved lifetime of all the components as fatigue lifespan is increased. Or, being able to reduce the size of all mooring components, due to the lower requirements on tensile strength.

In 2017, Bekaert, together with 8 European organisations took part in a joint industry project (JIP). The JIP was funded by the Scottish Government and administered by the Carbon Trust under the Marine Renewables Commercialisation Fund (MRCF). The objective was to test and qualify new nylon components for permanent mooring applications.

A testing programme was carried out, confirming promising results for the long-term effectiveness of a nylon-based mooring system. The tests also confirmed the potential cost savings of nylon when compared to chain-based mooring systems at a commercial scale.

Due to the traditional use of nylon in more harsh applications, there is a negative perception of the materials fatigue properties. However, the combination of coated nylon fibres with an optimised rope design, have shown an improvement for long term applications. As the relevant fatigue and abrasion failure modes have been minimised.

A taut-leg configuration, utilising MoorLine Nylon could be the most cost-effective method of mooring for floating offshore wind farms. Bekaert continue to work closely with class societies. In order to demonstrate that nylon ropes are suitable for permanent mooring applications. Providing an additional material solution to the Bekaert MoorLine toolbox.

The design of the mooring system will heavily depend upon the type of floating foundations and location of the farm. For your next demonstration or full-scale project, Bekaert are ready to assist in your mooring requirements.