Mooring Line Loads Research Articles

Volcano tsunamis and their effects on moored vessel safety: the 2022 Tonga event

Abstract. The explosion of the Hunga Tonga–Hunga Ha'apai volcano on 15 January 2022 (Tonga 2022) was the origin of a volcano-meteorological tsunami (VMT) recorded worldwide. At a distance exceeding 10 000 km from the volcano and 15 h after its eruption, the moorings of a ship in the port of La Pampilla, Callao (Peru), failed, releasing over 11 000 barrels of crude oil. This study delves into the profound implications of the Tonga 2022 event, investigating whether it could have led to the breaking of the mooring system. We conducted a comprehensive analysis of this significant event, examining the frequency content of the time series recorded at tide gauges, DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys, and barometers in the southern Pacific Ocean. Our findings revealed that the maximum energy of the spectra corresponds to the 120 min wave period off the coast of Peru, with the arrival time of these waves coinciding with the time of the accident in the port. We used a Boussinesq model to simulate the propagation of the volcano-meteorological tsunami from the source to the port in Peru to study the impact of those waves on the mooring system. We used the synthetic tsunami recorded in the port as input for the model that simulates mooring line loads based on the ship's degrees of freedom. The results suggest that the 120 min wave triggered by the VMT could significantly increase mooring stresses due to the resulting hydrodynamic effects, exceeding the minimum breaking load (MBL). We conclude that the propagation of the long wave period generated by the VMT caused overstresses in moored lines that triggered accidents in port environments. This event showed the need to prepare tsunami early warning systems and port authorities for detecting and managing VMTs induced by atmospheric acoustic waves. The work provides new insights into the far-reaching impacts of the Tonga 2022 tsunami.

Innovations in Offshore Wind: Reviewing Current Status and Future Prospects with a Parametric Analysis of Helical Pile Performance for Anchoring Mooring Lines

This study examines the current status and future potential of the offshore wind sector. Offshore wind is pivotal in transitioning to a low-carbon society and meeting rising energy demands, despite being capital-intensive. The industry aims to develop larger-scale wind farms in deeper ocean locations, with projections indicating significant cost reductions. To explore deeper ocean areas, specialized foundations like floating platforms moored to the seabed are required. This study proposes helical piles anchored in the seabed as a method to secure mooring lines. Using Plaxis 3D, a parametric examination was conducted on helical piles with two plates: one fixed at the pile’s toe and the other varying in position between 0.5 and 13 m from the seabed surface. Load inclination angles (0, 20, 40, and 60 degrees) were used to simulate mooring line loads. Results indicate the optimal Zh/Z ratios for maintaining load-bearing capacity and stability: 0.12 (10 mm movements), 0.22 (25 mm), and 0.26 (50 mm) for small shaft diameters; and 0.34 (10 mm), 0.38 (25 mm), and 0.46 (50 mm) for large shaft diameters. These findings highlight the importance of specific load inclination angles based on shaft diameter and allowable movement for effective performance.

A quasi-static mooring analysis approach for berthed ships

The existing literature extensively explores the environmental loads experienced by moored ships, a process known as mooring analysis. In order to model this analysis realistically, it is necessary to consider a number of variables, including ships' equations of motion, response amplitude operators, and mooring equipment capabilities. However, port authorities may lack comprehensive ship data. This study introduces a quasi-static equilibrium-state search method for berthed ships. It needs just basic ship particulars data and mooring arrangement data of berth and ship. The method enables the determination of crucial information such as mooring line load distribution, rope tension, rope failure and equilibrium ship position. Its versatility across ships and ports makes it noteworthy. Various analyses conducted on the method produce results with mean absolute percentage error values ranging from 0.57% to 2.23%, which are deemed significant and meaningful. Furthermore, the mathematical model of the method, which is currently considered to have three degrees of freedom, is open to improvement. The method has the potential to contribute significantly to various aspects, including the tail effect, the pretension effect, and the analysis of ropes with different stiffness values and capabilities.

Data-driven forecasting of FOWT dynamics and load time series using lidar inflow measurements

This study focuses on forecasting the fairlead tension and floater dynamics time series of floating offshore wind turbines (FOWTs) using a data-driven approach that incorporates onboard sensor measurements and lidar inflow data. Sensors on FOWTs can provide data on turbine dynamics, such as rotational and translational movements, and load metrics like mooring line loads. However, these sensors are limited to current state measurements and do not provide future signal projections. In this research, we investigate a data-driven forecasting methodology using a simulated dataset. This dataset encompasses FOWT responses to diverse environmental conditions and the associated lidar measurements. Utilizing a Long Short-Term Memory (LSTM) sequence-to-sequence model, this study forecasts the time series of fairlead tension, surge, and pitch for forecasting horizons of 20, 40, and 60 seconds, considering lidar ranges from 100m to 500m. The performance of these forecasting models is benchmarked against a simple persistence model. The results indicate that incorporating lidar inflow measurements significantly improves the forecasts of fairlead tensions and platform motions. The enhancement for pitch motion forecasts is observed across all forecasting horizons. For fairlead tension and surge motion, the enhancement is observed for the longer horizons of 40s and 60s. These findings underscore the value of lidar data in accurate forecasting and emphasize the need to account for the interplay between lidar range, wind speed, and forecasting horizon to achieve optimal forecast accuracy.

Dynamic seabed-anchor capacity enhancements for taut-moored floating offshore wind

Decarbonisation of global energy supply to meet Net Zero targets by 2050 requires rapid expansion of offshore wind [4]. Much of this growth will come from floating offshore wind (FOW) technology where seabeds are less congested, high energy wind resources are located and conventional fixed offshore wind is not practical [1]. The required scale of FOW requires a step change in mooring and anchoring technology compared to existing hydrocarbon solutions. New, efficient and reliable mooring and anchoring systems are essential to economically deliver the necessary FOW [1]. Taut mooring arrangements can be attractive for FOW turbines as they require less length and lighter synthetic mooring line than traditional chain catenary mooring arrangements. However, taut moorings transmit significantly higher mean and peak tensions to the anchor compared to catenary moorings. It is therefore important to fully quantify the capacity available from anchors during typical FOW load conditions, including dynamic effects.
 This study focuses on how a numerical anchor macro model, Ancmac [6], can be used to capture and quantify dynamic anchor-seabed capacity benefits such as from seabed added mass, [5] and viscous soil strength effects. These effects can enhance the dynamic anchor capacity and are not typically considered in anchor design. Typical mooring-floater fluid-structure interaction analyses also model the connection of the mooring lines to the seabed as a fixed pin connection and so seabed-anchor-mooring interactions are not typically considered. Ancmac can replace this fixed pin connection node at the seabed as it uses mechanical analogue components (e.g. spring-viscous-slider and mass) to simply and practically link forces on the anchor with anchor and chain movements in the time domain to determine the seabed response and the current available anchor capacity. This study presents an example case where Ancmac is used to model the response of an embedded plate anchor that is subjected to a high amplitude, short period (= 6 and 10 s) high mean load event (Figure 1a). The anchor loads , , are derived from applying a 1-50 year storm design loading event on a 15MW FOW turbine [3] with a taut mooring line configuration composed of high modulus synthetic polyester rope [8].
 Results show that if dynamic seabed benefits are not considered (purple line in Figure 1a), then a static anchor capacity of = 4.15 MN and corresponding anchor size of = 11.13 m2 is required (for an anchor buried in slightly overconsolidated soft clay =30 kPa and bearing capacity factor =12.42). If beneficial dynamic seabed enhancements are considered (shown by red lines ), then a lower initial static anchor capacity can be adopted =3.13 MN, which corresponds to a 25% decrease in the required anchor size (=8.41 m2). During the design loading event, as the applied anchor load, increases above the available static capacity, towards the maximum applied value, the anchor begins to move (Figure 1b) generating dynamic resistance from mobilising the non-linear viscous slider and added mass resistance components. The resistance in the viscous slider component is based on a model for the change in undrained strength with increasing equivalent strain rate (=, where is anchor diameter), as shown in Figure 1c. The resistance from the viscous slider component reaches a maximum at the time marked slightly after the peak of the applied mooring line loads, at time , as shown in Figure 1a-c. Reducing the period of the applied load (=10 s to 6 s) increased the added mass resistance as the anchor is subjected to higher accelerations. As a result, the anchor experienced significantly smaller (50% less) maximum displacement and reduced velocities. This comparison is also evident in Figure 1d, which compares the contributions of resistance forces from the spring-viscous-slider and added mass components.
 Ancmac can also capture the long-term enhancements in seabed strength and anchor capacity as a time-varying function of the life-cycle of seasonally varying, operational applied loads. This could, for example, further reduce the required anchor sizes, as can increase from beneficial long-term seabed consolidation effects [7]-[10]. These short and long-term seabed benefits that occur over the range of different loading timescales, and result in enhanced anchor capacities for loads that are relevant to FOW taut mooring configurations, will be further discussed during the presentation.

A comparison of extreme mooring loads and response of a spar-buoy wind turbine using conditional waves

Three-hour random sea-state evaluations for ultimate limit state design of floating offshore wind installations are time-consuming and prevent the use of high-fidelity numerical methods from being implemented. In this work, three conditional waves, Most-Likely Wave (MLW), Most-Likely Response Wave (MLRW) and Conditional Random Response Wave (CRRW), are experimentally investigated as alternative means to generate statistical extreme responses in surge and mooring line loads of a spar-buoy substructure, with shorter time-series duration compared to a 3-hour JONSWAP sea-state. MLW and MLRW are found to generate the greatest mooring line loads, far greater than the statistical expected maximum from a 3-hour random sea-state. However, MLW is uncorrelated with the substructure’s linear transfer function of response, and the MLRW includes no memory effects from a random background sea, indicating possible conservatism in the associated load magnitudes. CRRW is a MLRW embedded within a random background and is found to excite a similar mooring load as the 3-hour sea-state, with memory effects included. CRRW could thus be an appropriate method of generating statistically appropriate load responses within a reduced time-series. However, nonlinear effects on the mooring are not accounted for in any of the methods and will be included in future work.

Hydrodynamic Response of Mocean Wave Energy Converter in Extreme Waves

The design of moored floating wave energy converters (WECs) must take into account extreme responses and mooring line loads in order to ensure their survival and continued wave power generation in the ocean environment. This study focuses on Mocean Energy's hinged raft WEC and aims to provide a comprehensive understanding of its hydrodynamic characteristics in survival wave conditions. To achieve this, a physical model study was conducted on a Froude scale of 1 in 50 at the FloWave Ocean Energy Research Facility, University of Edinburgh.
 The experiments involved the use of NewWaves focusing of crest and trough at the model hinge location, as well as long irregular waves. Motion responses of the fore and aft bodies of the WEC were measured using a Qualisys camera, and single component load cells were used to measure the forces in the 3-point catenary mooring line. The hydrodynamic characteristics of the WEC were evaluated in terms of response amplitude operators and non-dimensional mooring line loads.
 Results indicate that the fore and aft bodies of the WEC exhibit similar motion responses, except for the pitch motion. The aft body has a pitch response 2 to 3 times higher than the fore body. Concerning the moorings, the wave load on the mooring line in line with the wave direction was found to be higher than the other two mooring lines which were arranged at an angle to the wave direction.
 In this paper, a brief discussion of the model set-up, parameters, test procedure, analysis of results, and discussion will be reported. The results will provide insight into the behaviour of the Mocean device in survival wave conditions and will aid with the determination of appropriate design parameters for optimal performance and survival in the ocean environment.

Fatigue Life Assessment for Wave Energy Converter Mooring Lines under Realistic Wave Climates

Despite the untapped resource stored in ocean waves, none of the suggested wave energy converters (WECs) has yet demonstrated economical viability. This viability lies in two counter-productive aspects: enhancing energy absorption and generation, and reduction of the loads in the critical elements such as mooring lines and dynamic cables. Both objectives are counter-productive in the sense that the former implies enhancing the motion, while the latter requires reducing this motion. Hence, a trade-off between the two objectives must be found.
 To that end, the accurate lifetime estimation of the most critical components is crucial, which depends on the fatigue and extreme loads. The latter depends on extreme events, which are highly nonlinear and need to be characterised by fully-viscous fully nonlinear numerical models. In contrast, fatigue effects cause cracks in the material, which appear due to cyclic loading and, thus, computationally more efficient numerical models are required in order to study all the relevant loading conditions within the operating region.
 The present study will evaluate different methodologies to assess the impact of fatigue loads in WEC mooring lines, estimating the lifetime of the mooring lines under realistic wave climates. The method follows the following steps:
 
 Mooring line tension is evaluated via hydrodynamic simulations under different realistic resource conditions,
 Statistical characteristics of the load cycles are extracted from the time history,
 The damage (D) corresponding to each load cycle condition is computed,
 The lifetime (L) is estimated by combining realistic resource conditions and the damage corresponding to each condition
 
 In the present study, this approach will be applied to different mooring line configurations (including wire ropes and chains), materials (stud and studless) and dimensions (length and diameter of mooring lines), as shown in Figure 1 for stud and studless chains as a function of the diameter.
 The lifetime estimation will be performed in different locations across the European coast, analysing the impact of resource conditions on fatigue lifetime. In addition, the most relevant design parameters will be identified.

Assessment of nylon versus polyester ropes for mooring of floating wind turbines

With the development of floating wind turbines comes the need for mooring systems suitable for shallow water depths. Compliance becomes challenging for mooring systems composed of chain and wire rope. Polyester ropes can improve compliance, but the even lower stiffness of nylon may further reduce mooring line loads. However, fatigue lifetime of nylon ropes has been limiting. This paper presents results of stiffness and fatigue tests on long lay polyester and nylon ropes and evaluates the performance of two mooring systems for a floating wind turbine. The fibre ropes’ complex stiffness behaviour is modelled by the Syrope model, with mooring analyses based on fully coupled time-domain simulations. Nylon ropes reduce the dynamic loads in the mooring system, particularly wave frequency loads. This improves the fatigue lifetime of all components, allowing a reduction of the chain cross-section area by ∼40% compared to the polyester-based mooring system. The long lay nylon ropes show excellent fatigue properties. With the reduced loading, the lifetime is longer for the nylon ropes than the polyester ropes. Ultimate loads are also reduced when using nylon, allowing reduction of the breaking load by ∼30%. This gives a thinner and lighter rope than the equivalent polyester rope.

Methodology for load estimation in mooring chains using a fuzzy genetic model

Mooring lines load prediction typically requires data from several sensors and high computational power. In this paper, a soft computing methodology is proposed to estimate the load on mooring chains based on fuzzy logic that uses only some of the natural frequencies of these systems. Two fuzzy inference systems are constructed and optimized by genetic algorithm. The performance of triangular and Gaussian membership functions in modelling the natural frequencies and their behavior in the optimization process are compared. The accuracy of this methodology is validated by experimental data obtained from a test bench. In addition, the robustness of the fuzzy inference system in dealing with uncertainties is studied using statistical parameters. Furthermore, a novel normalization method is proposed to generalize the fuzzy inference systems and eliminate the training process. The results indicate that the load on the chain can be estimated with high precision, and the proposed methodology is suitable for monitoring loads in mooring lines.

Dynamic Response of a SPAR-Type Floating Wind Turbine Foundation with Taut Mooring System

Compared with the traditional catenary or semi-taut mooring lines, the taut mooring system is more advantageous in many aspects, such as reduction of mooring line loads, erosion and fatigue damage during the powering productions of the floating wind turbines. This paper presents a taut mooring system made of synthetic fiber mooring lines, which can experience large elongations for a spar-type floating wind turbine. A finite element method (FEM)-based tensile mooring line model is proposed to study the mooring statics and dynamics of the floating wind turbine. A time domain modelling method coupled with the developed mooring line model is adopted to study the dynamics of a spar-type floating wind turbine foundation moored by the taut mooring system under regular waves. A systematic dynamic response and structural analysis are conducted based on variations in the mooring length and pretension. Additionally, comparative performance analyses are investigated for two mooring configurations with different numbers of mooring lines: two-point and three-point taut mooring system. It is found that factors, such as mooring length, pretension and the number of mooring lines, have significant impact on the in-plane and out-of-plane motion responses of the foundation.

Dynamics of two floating wind turbines with shared anchor and mooring lines

Floating wind farms present the opportunity to harvest wind resources located in deep water sites. Shared mooring designs can contribute in making floating wind energy more cost-competitive, and it is important to understand the new system dynamics that arise. We are presenting here HAWC2Farm, an extension of HAWC2 that can model multiple wind turbines with shared mooring lines. We apply the new modeling capabilities to simulate two 15 MW floating wind turbines on spar floaters with shared mooring lines. We consider two different sites and we identify and compare the natural frequencies and mode shapes of the shared mooring designs with those of an individual moored turbine. Furthermore, we investigate the influence of design parameters on the systems’ natural frequencies and we show that it is possible for a shared mooring design to achieve similar characteristics as a single turbine design. Finally, we test the response of the shared mooring design in steady wind and regular waves and find that the surge displacement of the upstream turbine and its mooring line loads are considerably larger compared to the single turbine case.

Experimental study on a breaking-enforcing floating breakwater

Floating breakwaters are moored structures that attenuate wave energy through a combination of reflection and dissipation. Studies into floating breakwaters have been generally restricted to optimising the attenuation performance. This study presents a novel floating breakwater type that was developed to have good attenuation performance while keeping wave drift loads as small as possible. The floating breakwater was designed as a submerged parabolic beach that enforces wave energy dissipation through breaking. The design was tested in a 3D shallow-water wave basin in captive and moored setups for regular and irregular wave conditions. Results are presented in terms of attenuation performance, motions, and (mooring) loads. The results show that the breaking of waves improves the attenuation performance of the floater in captive setup. However, in moored setup, the attenuation performance was dominated by diffraction and radiation of the wave field, with breaking being of secondary importance. This shows that breaking-enforcing floating breakwaters have potential, but require a high vertical hydrostatic and/or mooring stiffness in order to enforce intense breaking. Mean wave drift loads on the object showed significant difference between breaking and non-breaking waves in both setups, with breaking waves leading to lower normalized loads. This is attributed to breaking-induced set-up and set-down of the water level. As a result, the new breakwater design has a more favourable balance between wave attenuation and drift loads than common (i.e., box-, pontoon-, or mat-type) floating breakwater designs. Tests with varying surface roughness showed that floating breakwaters may benefit from dual-use functions that naturally increase the roughness (e.g., shellfish, vegetation), which have a marginal effect on the attenuation performance, but increase the added mass and hydrodynamic damping and as such, reduce mooring line loads.

Model testing of a floating wave energy converter with an internal U-shaped oscillating water column

The development of wave energy converters is centred on the combination of two factors: cost-effectiveness of energy extraction and system survivability on extreme sea conditions. Despite advances in numerical modelling, wave tank model testing still presents the most reliable option to evaluate these two factors. This paper presents an experimental study on the hydrodynamic responses of the UGEN, a wave energy converter consisting of an asymmetric floater with an internal U-shaped tank partially filled with water. The device absorbs energy through the oscillating-water-column (OWC) motion inside the U-shaped tank, induced by the wave action on the floater. The experimental testing of a bottom-moored 1:24th-scale model was performed at the COAST Laboratory, University of Plymouth, UK, considering regular and irregular wave conditions. The wave tank test measurements report six-degree-of-freedom rigid-body motions, mean drift forces, OWC motion, structural stresses and mooring loads. Results present the characterization of the energy absorption at the internal OWC induced by the floater’s sway, heave and roll. The occurrence of low-cycle auto-parametric resonance under certain wave conditions was detected and induced large roll motions, which affected power extraction and increased mooring line loads, particularly for large wave amplitudes.

Numerical modeling of keying of vertically installed plate anchor in sand

Pile driven plate anchors are typically oriented vertically during installation. To function properly in a mooring system they must be rotated, or keyed, to an angle normal to the mooring line load. Keying involves reduction in anchor embedment, with concomitant reduced load capacity, so realistic prediction of embedment loss during keying is essential to design. This study investigates the kinematic behavior and load capacity of a vertically installed strip plate anchor embedded in uniform cohesionless soil using large deformation finite element (LDFE) analysis. Embedment loss during keying is evaluated as a function of load eccentricity, load angle, and plate thickness. The model indicates that the plate angle at which maximum pullout resistance occurs increases with increasing eccentricity. The finite element simulations also show the embedment loss during keying decreases with increasing load eccentricity. Loss of anchor embedment increases with increasing pullout angle. The model also predicts that the loss in anchor embedment increases with decreasing anchor thickness, especially when eccentricity is small. The simulations show anchor during keying was essentially independent of the soil elastic modulus to have virtually no effect on keying behavior for shallow anchors, but elastic effects can be significant for deeply embedded anchors.

Compact floating wave energy converters arrays: Mooring loads and survivability through scale physical modelling

The current cost of electricity has been preventing the technological progress of wave energy converters. The use of compact wave energy converters arrays is seen as a technological breakthrough for reducing these costs. These array configurations aim to maximize the energy extracted per unit area of marine space and to promote the sharing of components and installation, operation and maintenance tasks. Mooring lines are fundamental components in these systems and represent a considerable portion of the project cost. Reducing the number of mooring lines through the application of arrays with inter-body connections has a high potential for cost reduction. In this paper, we present the experimental study of different configurations of a five-device array of spar-buoy oscillating-water-column wave energy converters in a wave basin, focusing on the analysis of the devices motion and the mooring line loads. The study compares the performance of a single isolated device, an array with independently-moored devices and three arrays with inter-body connections, with different levels of connectivity in the mooring arrangement. Tests were carried out for moderate and extreme wave conditions. Results show good performance for all configurations when subjected to moderate sea conditions. Under extreme sea conditions, high peak tensions were observed in the lines of all array configurations. Particularly large values were detected in the inter-body lines, caused by their full extension. Based on these results, guidelines for the design of mooring systems for compact wave energy converter arrays are presented.

Evaluating Mooring Line Test Procedures through the Application of a Round Robin Test Approach

Innovation in materials and test protocols, as well as physical and numerical investigations, is required to address the technical challenges arising due to the novel application of components from conventional industries to the marine renewable energy (MRE) industry. Synthetic fibre ropes, widely used for offshore station-keeping, have potential application in the MRE industry to reduce peak mooring line loads. This paper presents the results of a physical characterisation study of a novel hybrid polyester-polyolefin rope for MRE mooring applications through a round robin testing (RRT) approach at two test facilities. The RRT was performed using standard guidelines for offshore mooring lines and the results are verified through the numerical modelling of the rope tensile behaviour. The physical testing provides quantifiable margins for the strength and stiffness properties of the hybrid rope, increases confidence in the test protocols and assesses facility-specific influences on test outcomes. The results indicate that the adopted guidance is suitable for rope testing in mooring applications and there is good agreement between stiffness characterisation at both facilities. Additionally, the numerical model provides a satisfactory prediction of the rope tensile behaviour and it can be used for further parametric studies.

Second-order resonant motions of a deep-draft semi-submersible under extreme irregular wave excitation

Deep-draft semi-submersible platforms may present significant second-order resonant motions in severe sea states. Evaluating the slow-drift motions of such floating systems is important from the initial stages of their designs considering not only the offset requirement and mooring line load but also the air-gap responses beneath the lower deck. Theories and corresponding applications for these second-order resonant motions have been investigated for decades, but the problem has not been solved completely considering the requirements of a model that is sufficiently accurate and expedient. In this study, high-quality numerical simulations and model tests were conducted. Numerical calculations were carried out with the applications of full quadratic transfer function (QTF) matrices by means of commercial boundary element method (BEM) code. Comparisons show perfect agreement between the wave-frequency components obtained from both the model tests and the numerical calculations. Furthermore, the results also indicate that the second-order resonant motions can be to some extent estimated numerically by the second-order potential theory and that deviations can still be observed compared with those motions obtained by the model tests.

Large-scale testing of a hydraulic non-linear mooring system for floating offshore wind turbines

The mooring system has been recognised as a key area of expense that needs to be addressed to improve the cost competitiveness of floating offshore wind turbines. The devices installed to date have generally adopted designs from the oil and gas industry using heavy mooring materials, providing the required safety margins but with a significant degree of conservatism. Recent interest in the usage of lighter and more compliant mooring materials has shown that they have the potential to reduce peak line loads, which would in-turn reduce costs. However, the lack of operational experience with such materials has limited their adoption in a risk averse industry. This paper reports on the large-scale physical testing of a hydraulic-based mooring component with non-linear stiffness characteristics. The performance of the device is characterised in a laboratory both statically and dynamically, as well as in conditions representative of operating in a sea state using a combined physical and numerical modelling approach. The results show that the dynamic stiffness of the component is a function of load history and hydraulic pre-charge pressure, while the inclusion of the device as part of the OC4 semi-submersible floating wind platform can reduce the peak mooring line loads by up to 9%. Beyond the physical test results, the calculations suggest that the peak load reduction in the modelled scenarios could be as much as 40% if the device can be scaled further. The paper supports the adoption of innovative mooring systems through dedicated component and performance testing.

A physical model approach to nonlinear vertical accelerations and mooring loads of an offshore aquaculture cage induced by wave-structure interactions

A grand aim of aquaculturists is to investigate the interactions between the oceanic environment and aquaculture structures, in order to make the structures more durable to loads caused by waves and current. To that end, we investigate the effects of extreme wave conditions by leveraging a physical model approach. We assume that the floating collar of the cage moves with the wave in heave, leading to extreme conditions. Thus, the wavelength is much larger than the size of the floating collar model and the wave frequency is much lower than the natural period of heave. For the floating collar model without netting, under wave-only conditions, the first- and second-harmonic components of the vertical acceleration are proportional to the wave amplitude ζa, and square of the wave amplitude ζa2, respectively. The third- and fourth-harmonic components exhibit oscillations at non-dimensional wave number νa≈0.055 due to the change of free surface. For the floating cage model under combined waves and current conditions, the first-harmonic component is more likely proportional to ζa, while the second-harmonic component shows erratic behavior. The effects of mooring loads in front and aft mooring lines are also assessed.