Research project examines mooring design for floating offshore wind
Floating offshore wind (FOW) is viewed as a viable solution at water depths exceeding 50 to 60 m (164 to 197 ft) with abundant wind resources. By mid-century, DNV predicts that 2% of the world’s electricity demand can be supplied by cost-efficient and dependable FOW.1 As a fast-evolving technology, it has the potential for less foundation material, shortened installation cycle and decommissioning, and additional wind power generation. However, to fully take up the potential, floating wind needs to overcome its major challenges, costs, and confidence.
New risks can be managed by using off-the-shelf bottom fixed turbines and well-known oil and gas technology for floaters.
In the mooring system for the support structure, short chains with synthetic fiber ropes can reduce cost and improve station-keeping. Keeping the risk for mooring failure at an acceptable level is one of many key issues to assure confidence.
Assure confidence in mooring systems
In today’s challenging oil field and FOW developments and operations, out-of-plane bending (OPB) fatigue of the chains in the upper sections of mooring systems is one crucial example that requires attention and prevention. OPB can potentially add significant fatigue stresses to the links, creating increased risk of failure in the mooring line.
Failures due to this phenomenon first occurred in 2002.2,3 The most notable was the loading buoy installed at the Girassol oil field in block 17 offshore Angola in 1,350 m (4,429 ft) of water.
Although the mooring system had been designed according to offshore industry standards, the premature rupture of mooring chains – after only 235 days of service – was caused by bending fatigue of the first free chain link inside the chain hawse. This resulted in three anchor legs breaking almost simultaneously. It was discovered that high pretension levels, combined with mooring chain motions, caused interlink rotations that generated significant OPB fatigue loading.
As a result, a novel method was created to estimate fatigue in mooring chains, with redesign of both the top chain segment and hawse connection, as well as the installation of a new chain-connecting arm.4
Collaboration and cross learning
To improve understanding of the fatigue mechanism and propose design recommendations, a joint industry project (JIP) involving 28 companies ran from 2007 to 2013. Based on testing chains up to 146 mm (5.7 in.) in diameter, the JIP led to the development of the current fatigue design guidelines for mooring chains.
To help decrease the weight and cost of OPB mitigation equipment on large diameter mooring chain systems, DNV has developed a test rig at its laboratory in Høvik, Norway, to conduct OPB stiffness identification. This rig can apply tensions up to 350 metric tons (386 tons) and interlink rotations in the range of ±3 degrees in up to 190-mm (7.5-in.) chain.
Using this rig, two test projects have been performed to study the OPB behavior of large mooring chains – beyond the tested range considered by the OPB JIP – in both wet and dry conditions and for both small and large interlink angles.5 The goal was to:
• Understand chain OPB physics for large diameter chains
• Measure interlink stiffness and maximum sliding moments
• Provide data for validation of finite-element models.
As OPB fatigue mitigation measures are complex, heavy, and expensive, design optimization greatly benefits from dedicated testing. This is particularly important with a growing trend toward the use of larger mooring chains, beyond the parameters considered during the guidelines’ development, in demanding sea state conditions.
This is an example of a key area for cross learning and collaboration between the oil and gas industry and the burgeoning floating wind sector. Not only will these studies enhance understanding of OPB, the findings are also useful for the design and importantly, the material selection of floating wind mooring systems, thereby reducing the risk to the turbines.
Synthetic fiber ropes
Since the 1990s when the oil and gas industry moved into deeper waters, polyester (polyethylene terephthalate) has become the material of choice for permanent mooring applications. Petrobras was the first to incorporate the high efficiency polyester fiber ropes to its deepwater mooring systems. Now considered a mature technology, the ropes have been used in catenary and taut configurations to moor drilling and production units as well as storage and offloading facilities.6
Through the years there have been significant advancements in the synthetic fiber rope technology. DNV has also developed Standards and Recommended Practices, partly based on JIPs and experience from in-house testing of large ropes for deepwater developments. For instance, DNV’s Syrope JIP improved methods and analysis tools for the design of synthetic fiber rope mooring systems. The initiative focused on the interpretation of change-in-length performance with input data from extensive lab-testing of rope.
In many circumstances, to moor a floating wind turbine safely and securely, the ability to tailor mooring elasticity through synthetic fiber material selection provides substantial advantages compared to designing with steel. The polyester ropes preferred in the oil and gas industry may under some circumstances also be too stiff for mooring floating wind turbines. Also, the elasticity of nylon may be a preferred option to dampen the dynamic loads. There is very limited experience in the industry regarding the performance characteristics of nylon for long-term mooring. Therefore, qualification and testing of nylon needs to be done.
The use of synthetic ropes has several other advantages. Notably, a very low weight in the water imposes lower vertical loads compared with chains, which reduces floater structure costs. Another advantage is the smaller mooring footprint in a taut mooring system.
DNV is currently working closely with the FOW sector to demonstrate the fitness of nylon rope for robust, long-term floating wind turbine moorings. It is also transferring the extensive experience with synthetics from oil and gas to investigate cost-effective handling usage. At the DNV Technology center for offshore mooring and lifting in Bergen, Norway, the synthetic mooring rope characteristics are being tested to provide confidence in solutions.
The independent energy expert and assurance provider has to date performed several test projects of nylon mooring ropes, as well as ongoing qualification of nylon for long-term mooring applications.
The future is floating
Floating wind turbines are technically feasible, and the commercialization outlook is positive. This decade alone will see rapid progress from demonstration projects to commercial-scale deployments.
According to the EU Blue Economy Report 2021, FOW has a power generation potential of 4 million megawatts (MW) of energy in EU waters. Currently, just 62 MW of capacity has been installed. The report asserts that 3 million MW of capacity could be installed in areas with a water depth of more than 100 m (328 ft), giving the potential to open up new markets for wind energy in the Atlantic Ocean, Mediterranean Sea, and Black Sea.7
As the number of projects increases, new wind regimes will be exploited and turbine manufacturers will continue to develop a range of models, mooring systems, and materials for varying wind and buoyancy conditions. Key to the uptake of floating offshore wind potential is the need to accelerate delivery of safe, sustainable, and cost-effective solutions to strengthen confidence in this burgeoning technology. Addressing innovative mooring solutions and using existing knowledge on failure modes and prevention will play a major role.
References
1. https://www.dnv.com/news/floating-wind-power-to-grow-2000-fold-by-2050-but-more-comprehensive-standards-and-risk-management-required-report-192001
2. Bhattacharjee, S., Angevine, D., Majhi, S., & Smith, D., “Permanent Mooring Reliability & Mooring Risk Management Plan (MRMP): A Practical Strategy to Manage Operational Risk”, Offshore Technology Conference, Houston, Texas, 2015. doi:10.4043/25841-MS
3. Ma, K. T., Duggal, A., Smedley, P., L Hostis, D., & Shu, H. “A Historical Review on Integrity Issues of Permanent Mooring Systems”, OTC 24025, Offshore Technology Conference, Houston, Texas, 2013
4. Jean, P. & Goessens, K. & Hostis, D., “Failure of Chains by Bending on Deepwater Mooring Systems”, 58. 10.4043/17238-MS Offshore Technology Conference, Houston, Texas, 2005
5. Barros, P., Carlberg, I. S., Høgsæt., Karimi, M. R., Braun, J., Gooijer, E., & Vargas, P. “Out-of-plane bending (OPB) test of large diameter mooring chains”, OMAE2020-18805, OMAE 2020, Fort Lauderdale, Florida, 2020
6. Rossi, R., Del Vecchio, C. J. M., and Gonçalves, R. C. F. “Fiber Moorings, Recent Experiences and Research: Moorings with Polyester Ropes in Petrobras: Experience and the Evolution of Life Cycle Management” OTC-20845-MS, Offshore Technology Conference, Houston, Texas, 2010
7. https://ec.europa.eu/oceans-and-fisheries/system/files/2021-05/the-eu-blue-economy-report-2021_en.pdf
