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Is nylon rope a better mooring systems option for Floating Offshore Wind?

Author: Yusuf Arikan 

Catenary, semi-taut and taut mooring configurations are usually considered for floating offshore wind turbines (FOWTs). Multiple environmental forces, such as wind, wave, current and turbine thrust, dictate mooring strength requirements. However, excessive forces can develop within the mooring and anchor system without flexibility. 

Synthetic ropes are required for taut mooring configurations to provide the flexibility required for a compliant mooring system. Synthetic ropes such as Aramid, HMPE (Dyneema), Polyester, and Nylon are commonly considered for mooring systems by the floating wind industry. Polyester and nylon are more suitable for taut mooring configurations due to their relatively higher flexibility. They also offer advantages such as reduced weight and improved installation efficiency. 

Methodology

2H, the independent Advanced Systems Engineering brand of Acteon, studied the impact of the rope selection on component sizing, such as chain diameter, and anchors. As a result, the impact on as-installed mooring and anchor costing was the following:

Both polyester and nylon axial stiffness are highly non-linear and dependent on

-         mooring mean loads
-         load amplitudes
-         loading frequency.

For polyester ropes, the effects of load amplitude and loading frequency are usually negligible and polyester axial stiffness can be bound by upper and lower bounds. However, for nylon ropes, the effects of load amplitudes and loading frequency are more dominant and cannot be neglected for mooring design. As a result, a non-linear hysteretic stiffness model is required to capture the complex nylon response characteristics. Because of its lower stiffness, nylon rope leads to lower dynamic loads, smaller mooring components, and substantially lower costs.

Watch video:

Results

In our simulations, the same level of compliance is achieved with three times less nylon length than polyester. The savings in material cost, environmental impact and installation time, are significant with a total 40-45% reduction in total cost.

Compared to polyester, which has been proven on floaters mooring systems for decades, nylon ropes will require further qualification testing. But already, simulations of nylon in a taut configuration appear favourable on a technical and economic standpoint.

2 Orcaflex simulation videos side by side
Smoother motions are achieved with nylon compared to polyester

In our simulations, the same level of compliance is achieved with three times less nylon length than polyester. The savings in material cost, environmental impact and installation time, are significant with a total 40-45% reduction in total cost.

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