Floating wind is developing rapidly, aiming to achieve reliable and cost competitive solutions. Shared mooring systems, with fewer mooring lines and anchors per floating wind turbine unit, are one possible solution to reduce costs. Most research on shared mooring systems focuses on intact configurations, and there has been limited focus on degraded states, such as accidental line breaks. The interconnected nature of shared systems requires evaluation of the consequences of accidental limit state (ALS) at early design stages, increasing the need for efficient evaluation tools.

Existing rules and regulations do not specifically address ALS design of shared systems, and there are no clear guidelines regarding power production in damaged states or a definition of system failure. It is evident that free drift must be avoided as well as progressive failure of several components, and that a system failure will require full shutdown of all units. This work aims to present a definition of system failure for shared mooring systems, as well as a suggestion for power production strategy in damaged states.

A low-fidelity static design methodology is developed for early-stage evaluation of the consequences of accidental line break in shared mooring systems. The evaluation criteria consider system response, profitability, and system reliability. Initial assessment of the power cable utilisation is included, based on top-end motion input and a combination of power cable axial tension and curvature. The method aims to speed up the early-stage design procedure and to examine the influence of different power production strategies on system response in damaged states.

The numerical model is built up of rigid body substructures with simplified representations of the tower and wind turbine. The mooring system is represented by finite element lines and fixed anchor points. Environmental forces are applied as static loads, neglecting dynamic effects as well as the change in forces due to structural response (such as yaw rotation). System response is assessed by considering floater motions, tension in intact mooring lines and cable utilisation. The relationship between top-end motions and cable axial tension and curvature is approximated using pre-scribed motion analyses.

This work will specifically review: (1) how reliability analyses can be applied in early-stage design of shared mooring systems and power cable layouts, (2) applicability of low-fidelity design methods for evaluation of different shared mooring system and power cable layouts, (3) development of guidelines for power production strategies in damaged states, including the expected impact on system reliability and profitability, and (4) a sensitivity study on how low-fidelity input affects reliability analyses results.

The rapid expansion of offshore wind energy has brought renewed attention to floating offshore wind turbines (FOWTs), though investor interest remains variable due to uncertainties in high-capital projects. Mooring systems are critical to FOWT design, and unlike traditional oil and gas platforms, they are subjected to unique loading histories influenced by turbine operation. Accurate fatigue assessment of mooring chains is essential for ensuring structural integrity and extending service life.
Over the past decade, several joint industry projects have conducted full-scale fatigue tests on both new and used mooring chains to refine existing assessment methodologies. While these efforts have yielded valuable insights, full-scale testing under realistic operational loads presents significant challenges. This paper reviews key findings from recent full-scale fatigue testing campaigns, identifies major limitations—particularly the reliance on constant amplitude loading—and discusses potential solutions.
Current testing practices often overlook the cumulative impact of low-amplitude cycles, which can become increasingly significant as mooring lines age. Moreover, design codes typically apply conservative safety factors to compensate for these limitations, introducing further uncertainty. Despite the critical role of fatigue in mooring performance, regulatory guidelines do not mandate physical fatigue testing, relying instead on S–N curve data derived from conventional offshore applications. This approach may not adequately capture the complex fatigue behavior of FOWT moorings.
To address these gaps, this paper proposes the development of enhanced fatigue assessment frameworks that are specifically tailored to the operational realities of floating offshore wind systems. These should incorporate variable amplitude loading, account for low amplitude cycles, and reflect the complex service conditions mooring chains endure. Advancing testing protocols and updating design standards are essential steps toward reducing uncertainty and ensuring the long-term reliability and cost-effectiveness of floating offshore wind technologies.

While there is an ambition to largely increase offshore wind turbine capacity over the coming decades, technological advancements are needed to enable this. While bottom-fixed turbines are suitable at shallow water depths, floating solutions are required for deeper waters. Floating wind turbines require a flexible mooring system to ensure that design requirements are fulfilled. This includes limiting the maximum offset of the platform to ensure the integrity of the power cable, and provide sufficient capacity against extreme loads and fatigue failure in the mooring lines.

Fulfilling these requirements becomes increasingly challenging as the water depth decreases. Consequently, there is a gap where the water depth is too deep for bottom-fixed turbines, while conventional mooring systems will introduce large dynamic loads in the mooring lines. Nylon fibre ropes are more flexible than polyester fibre ropes and steel chains. This flexibility has been shown to reduce extreme loads and fatigue damage accumulation, which can allow for reduced mooring line and chain dimensions and more sustainable designs.

However, the mechanical properties of nylon ropes will depend on temperature, load-history and water uptake. Over the last two years, the NYMOOR project has studied the properties of the nylon yarns used in mooring ropes. Testing on yarn level requires smaller equipment and less time than sub-rope testing, and more tests can be performed under a larger variation of conditions compared. Testing on yarn level will identify material properties, while testing on sub-rope level will yield the combined material and structural rope properties.

Three series of tests have been performed so far in the project, namely break load tests, creep tests, and stiffness tests. All tests have been performed on submerged yarn and at varying temperatures, with the lowest temperature tested being 4 degrees Celsius (above the glass transition temperature). To the authors’ knowledge, this is the first systematic study on the temperature-dependent properties of wet nylon yarns, which is the relevant properties to study for mooring applications. In general, wet yarn show a reduction in strength compared to dry yarn, and their strength increases as temperature decreases. Creep tests resulted in temperature-tension-dependent creep rates for unused yarn, and recovery strain rates post creep were also found. In addition, selected samples were re-tested after a relaxation period to investigate load-history dependent creep rates. In general, the strain and recovery rates were found to increase with increasing temperature. For the stiffness test, an increase in both the slow (quasi-static) stiffness and fast (dynamic) stiffness was observed as the temperature decreased. The temperature effect on stiffness was found to be highest at low load levels.

Moorsafe, the strongest anchor ever.

Background:

Mooring systems are a critical cost driver and safety factor for floating wind turbines and other offshore structures. There is a need for innovative anchoring technologies that combine reliable safe high holding capacity, efficient installation, cost efficiency, and reduced environmental impact.

Objective:

This paper precents the development and performance evaluation of the Moorsafe anchor. Olav Dale, a Norwegian citizen, is the originator of the unique Moorsafe anchor design technology designed to deliver high holding capacity relative to its own weight,
reducing costs and enhancing safety, while simplifying handling and minimizing seabed disturbance.

Method:

Field tests in Stockholm archipelago 2005-07-28 under DNV supervision have been conducted with Moorsafe prototype anchors ranging from 7.5 kg to 76 kg. Performance was assessed in terms of holding capacity under different load angles and comparison with conventional anchors. Case studies where Moorsafe anchors replaced traditional concrete block moorings are also presented.

Results:

The DNV test report 40000566-2 shows that Moorsafe anchors performed up to 123 times their own weight in holding capacity, with over twice the performance of market-leading anchors in equivalent conditions. In a very early stage 2000-06-26 Moorsafe was also tested against Stevpris anchors in Vryhof´s test facilities in Rotterdam, and Moorsafe outperformed Stevpris with more than twice the holding capacity. Even in comparison with suction anchors
Moorsafe in most sediments will perform more than twice the holding capacity and more than halving the anchoring costs.
Moorsafe is truly a disruptive anchor design, outperforming all other anchors by double the holding capacity which of cource is very hard to belive, explaining why most experts think "it must be to
good to be true". Probably that´s why Moorsafe has never got the attention and the funding needed to succed in scaling up to offshore
size anchors. In spite of all the market headwinds Moorsafe anchors
(30kg and 72kg) have been sold in more than 4000 units and has proven its superior holding power in a variety of soil conditions.
Moorsafe, stable and safe design has proven it can withstand up to 45 degree sideway and uplift forces. http://moorsafe.com.
The Swedish Traffic Authorities should anchor a floating road bridge
with 22 pcs of 20 tons concrete blocks = 440 ton, but we did the job with 22 pcs of 72 kg Moorsafe anchors.
The Swedish Coast Guard has chosen the 30 kg Moorsafe anchor for storm anchoring of offshore oil booms amid the superior holding power and easy installation and retrieval.

Conclution:

The Moorsafe anchor demonstrates a strong potential as the most cost-effective and sustainable anchoring solution for floating wind turbines. Its high strength-to-weight-ratio, resistance to multidirectional loads, and reduced material and CO2 footprint may
significantly contribute to scaling offshore renewable energy.
Further work will explore upscaling of the technology for full-scale
floating wind turbines and long-term performance in offshore conditions under cyclic loading.

Thank you very much for closely studying this abstract.
Best regards
Olav Dale

The growth of Floating Offshore Wind (FOW) as a key player in the energy transition is driving a sharp increase in the number of mooring lines. However, the high failure rate of mooring systems, particularly due to fatigue damage in the mooring chain, remains a critical industry concern. Consequently, understanding Out-of-Plane Bending (OPB) and In-Plane Bending (IPB) phenomena has become increasingly important. Previous studies have identified OPB/IPB mechanisms in large-diameter offshore grade chains, but have not reproduced realistic dynamic offshore conditions. Prior experiments applied static displacements to a chain link in test benches, without representing catenary dynamics due to the short chain length in the test benches. Because bending strain was forcibly induced rather than generated by dynamic motion, it remains unclear how floater motion contributes to chain bending. To address these limitations, the authors previously conducted a scaled tank experiment using a 4 mm chain that successfully measured OPB strain under fully dynamic catenary motion (presented at EERA DeepWind 2025).
Building on the previous work, the present study extends the investigation by incorporating both vertical and horizontal motions applied to the mooring connection point and introducing an additional chain size. The experiment uses 4 mm and 6 mm chains with expanded instrumentation to examine (i) scale effects between the two diameters, (ii) IPB behavior, (iii) OPB/IPB response under horizontal motion, (iv) OPB strain distribution within a single link, and (v) strain decay along downstream links. The results provide new insights of OPB and IPB phenomena due to the dynamic response of mooring chains, and verify the previously observed correlations of bending strain with mooring tension, acceleration, and displacement of the applied motion.
The experimental correlations support the identification of OPB/IPB mechanisms as bending stresses caused by (i) inertia from floater motion and (ii) catenary shape changes due to displacement of the mooring connection point. The floater motion generates inertia forces through interlink contact that load the bending stress to the chain link, while mooring displacement alters the catenary configuration. Until interlink sliding occurs, the chain link deforms locally to accommodate the change in catenary shape. Based on these mechanisms, a novel formula was developed to calculate bending stress. For component (i), the strain distribution within a single link revealed that bending moments arise from both floater motion and the dynamic lag of the downstream catenary; therefore, the previous formula was modified to include both effects. For component (ii), a new expression was proposed by establishing a force equilibrium during chain deformation. The calculated OPB strain showed good agreement with the measured values, while IPB stress trends were captured with slightly larger deviations between calculation and the measurement.
The proposed formula enables practical application by leveraging parameters available from existing numerical simulations, such as OrcaFlex. Using these simulation inputs, a calculation model was established to estimate time-series OPB and IPB stresses, enabling fatigue-life assessment based on floater motion. The study presents the experimental setup, key measurement results, analytical stress model, and finally, discussions of the limitations of the scaled experiment.