Taut Mooring Research Articles

Hydrodynamic performance characteristics of small-scale in-situ buoys with critical motion requirements

As the accuracy of high-precision oceanographic measurement instruments increases, the operational requirements for buoy motion become critical. This study focuses on the small-scale in-situ buoy, optimizing buoy configuration to ensure stability and performance. Aiming at the special structure which consists of a large floating column and several small tube connectors, a method for evaluating the hydrodynamic performance is proposed which combines potential flow theory with Morison equation. And the method validity is confirmed using experimental data. The performances analysis of different type buoy with catenary or taut mooring system are developed. According to the critical motion requirements for the long-term stability, the cylindrical type buoy equipped with a catenary mooring system is regarded as the most stable and economical choice, exhibiting a surge motion response of 4.07 m, a heave motion response of 2.89 m, and a pitch motion response of 9.37°. Furthermore, the anchor chain acts as an elastic damper which can mitigate the transient impact of the environmental loads, greatly limiting the amplitude of the longitudinal and heaving motions. And the maximum mooring line tension is 249.28 kN, which is significantly lower than other schemes. The evaluation method offers a technical support for engineering applications.

Model tests on chain-soil interaction of anchor pile for taut mooring system through transparent sand

The pile response of taut mooring systems considering chain-soil interactions remains unclear. In this study, model tests on the performance of anchor piles and the surrounding soil displacement field were conducted using transparent sand. Cyclic motion was imposed at the top end of the chain. The effects of pad eye location (z), motion velocity (v), motion amplitude (A), and amplitude increment (Ai) are discussed. In the constant-amplitude tests, the soil displacement caused by chain motion was apparent near the dip-down point, and the soil displacement increased with z and A. The maximum chain tension decreased owing to chain-soil interactions. The maximum tension with z = 0.55L and 0.67L (where L is the pile length) increased 9% and 15%, respectively, compared with that with z = 0.40L, and the maximum tensions at A = 80 and 120 mm decreased 15% and 29%, respectively, of that at A = 40 mm. In the incremental-amplitude tests, soil displacement was evident around the dip-down point and pile. The maximum tension during each cycle increased almost linearly with the number of cycles before pile failure, and the maximum tension during the entire test increased with z and v.

Assessing the impacts of wakes on floating wind farms with shared anchors

This paper examines wake effects for floating wind farms with shared anchors. Three 20-turbine farms are examined: a grid-formation baseline with no shared anchors, a farm based on 3-line anchors, and a farm based on 6-line anchors, governed by a wind turbine spacing of 8 rotor diameters. The IEA 15 MW reference turbine on the UMaine semisubmersible platform was used with a taut mooring system in deep- water depths representative of U.S. west coast lease areas. A steady-state wake model showed that, when evaluating a sweep of wind headings, the baseline design had the lowest wake losses, with a value of 11.7 %, followed by the 6-line at 12.9% and the 3-line at 13.8%. Dynamic simulations were run in FAST.Farm to analyse the effects of wakes on mean anchor loads for wind headings that showed significant wake losses. The baseline farm showed the largest anchor load reductions due to wake effects (up to 16%), followed by the 3-line farm (up to 8%), and the 6-line farm, which showed relatively consistent load magnitudes on the 6-line anchors across all headings.

Design and comparative analysis of alternative mooring systems for offshore floating photovoltaics arrays in ultra-shallow water with significant tidal range

Large-scale floating photovoltaics (FPV) are garnering increased attention as a sustainable solution for renewable energy production and efficient utilization of ocean spaces. These FPV arrays are predominantly deployed in shallow water environments, which present significant challenges in mooring system design due to substantial tidal fluctuations. In such settings, mooring systems may become slack and lose functionality at low tide or risk damage from excessive stiffness at high tide. The primary aim of this study is to develop a mooring system tailored to FPV arrays, capable of withstanding extensive tidal variations without causing damage to the floating modules due to high mooring line tension. Employing HydroD, a hydrodynamic analysis tool based on potential flow theory, alongside OrcaFlex, a time-domain coupled analysis tool predicated on the lumped mass method, we conducted a comparative assessment of six distinct mooring configurations. These configurations, varying in mooring line materials and components, were evaluated from static, dynamic, and fatigue analysis standpoints across a range of tidal levels. The results indicate that taut mooring systems integrated with buoys or clumps consistently exhibit superior performance in all test scenarios. Accordingly, these systems are highly recommended for securing FPV arrays in shallow water environments characterized by severe tidal variations, offering optimal stability and resilience.

Dynamic responses of a semi-submersible integrated wind-energy-aquaculture structure under regular and irregular waves

Offshore aquaculture has expanded significantly, and offshore wind energy is more abundant and stable than it is on land. The integration of aquaculture facilities and offshore wind-turbine technology to build a comprehensive platform has both scientific and theoretical significance. To further investigate an integrated wind-energy and aquaculture structure and its dynamic response characteristics, we performed physical model experiments to investigate the motion response properties of the new wind-energy-aquaculture integrated structure. Regular and irregular wave experiments were conducted on a wave flume. The motion responses and mooring line tensions of the platform modules were obtained. Under regular wave conditions, the three degree-of-freedom motion response of the model under two different mooring configurations exhibit the similar trend as the wave height and wave frequency increased. The heave responses under different mooring configurations were generally consistent, whereas the surge responses exhibited low-frequency characteristics. The mooring line tensions of the taut mooring configuration changed significantly with the wave parameters, whereas those with catenary mooring exhibited minor changes. Under irregular wave conditions, structures with catenary mooring systems exhibited smaller motion responses, thereby maintaining their stability.

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.

Numerical and experimental studies of the effects of WEC motion on a combined wind-wave energy platform

The development of eco-friendly carbon-free marine energy is actively underway to alleviate the impact of global climate change due to excessive emission of carbon dioxide. To reduce the variability of energy production according to marine environment conditions and maximize energy production, the need for research on a suitable ocean energy platform is increasing. Therefore, various studies on motion control of ocean energy platforms are being conducted.
 In this study, the motion response of a combined wind-wave energy platform equipped with multiple wave energy converters (WEC) capable of controlling the motion of a floating offshore wind turbine (FOWT) was numerically analysed and verified through a wave tank experiment. The combined energy platform with multiple WECs attached to the FOWT not only produces stable energy by controlling the motion response of the FOWT with the movement of the WEC, but also produces wave energy by the relative motion of the WEC.
 The FOWT is a spar platform with taut moorings, and WEC is a moving cylinder hinged to the FOWT. The motion reduction effect of the FOWT by the movement of the WEC was investigated. Numerical analysis used the ANSYS AQWA program, which is a potential flow-based hydrodynamic program. To consider the restraining force of the FOWT due to the motion of the WEC, the multi-body dynamics theory was applied. The accuracy of the numerical model was improved by applying mooring dynamics, weakly nonlinear Froude-Krylov force, and nonlinear hydrostatic force. The experimental model was fabricated on a scale of 1/100 of the numerical model and was performed in a two-dimensional mini wave tank. To prevent the sidewall effect of the tank, the experiment was conducted only for the incident waves with various periods under the head sea conditions.
 The numerical analysis results were in good agreement with the experimental results. In particular, the pitch of FOWT decreased significantly near the pitch resonance period because of the motion of WEC. The change in motion characteristics of the combined energy platform according to the movement of the WEC under various wave conditions was analysed.

On the state-of-the-art of CFD simulations for wave energy converters within the open-source numerical framework of DualSPHysics

There are currently several types of devices capable of harnessing wave energy, exploiting a broad variety of physical transformation processes. These devices – known as Wave Energy Converters (WECs) – are developed to maximize their power output. However, there are still uncertainties about their response and survivability to loads induced by adverse environmental conditions, with a consequent increase of the Levelized Cost of Energy (LCOE), which prevents in fact their commercial diffusion. As evidenced by a large body of research, marine renewable energy devices need to have more robust design practices. To address this issue, we propose the CFD-based DualSPHysics toolbox as a support in the design stages. DualSPHysics is high-fidelity software inherently suited to numerically address most challenges posed by multiphysics simulations, which are required to reliably predict WEC response in situations well beyond operational conditions. It should be noted that WECs, generally, may be connected to the seabed and comprise mechanical systems named Power Take-Offs (PTO) used to convert the energy from waves into electricity or other usable energies. To reproduce these features, DualSPHysics benefits from coupling with the multiphysics library Project Chrono and the dynamic mooring model Moordyn+. In this work, the augmented DualSPHysics framework is utilised to simulate a range of very different types of WECs with a variety of elements, such as catenary connections, taut mooring lines, or linear and nonlinear PTO actuators. Version 5.2 of the open-source licensed code was recently released, making the numerical framework publicly available as one unit. This work aims to provide a numerical review of past applications, and to demonstrate how the same open-source code is able to simulate very different technologies.
 
 Specifically, this paper proposes routine modeling and validation procedures using the SPH-based solver DualSPHysics applied to five different WEC types: i) a moored point absorber (PA); ii) an oscillating wave surge converter (OWSC); iii) a floating OWSC (so called FOSWEC); iv) a wave energy hyperbaric converter (WEHC); and v) a multi-body attenuator (so called Multi-float M4). For each device listed above, we provide validation proof against physical model data for various components of the floater(s) and PTO related quantities, performed under specific sea conditions that aim to challenge their survivability. Within the scope of this research, we present the WEC response with respect to the degrees of freedom that really matter for each of the floatings due to hydrodynamic interactions (i.e., heave, surge, and pitch), along with quantities more intimately connected to the anchoring systems (e.g., line tension) or the mechanical apparatus (e.g., end-stopper force). The quality of the results, the discussion built upon them and the demonstrated solver exploitability to a wide range of WECs show that one software model can run all cases using the exact same methodology, which is of great value for the marine energy R&D community. Finally, we discuss future research objectives, which include the implementation of automation to apply open control systems and possible applications to subsets of WEC farm arrays and other floating energy harnessing devices.

Optimal Design of a Submerged Tidal Device for Low Current Environment

The tidal current power is one of the reliable renewable ocean energy resources that can be deployed at the region having ocean current. The feasible current speed for the application of tidal device is known to be more than 2m/s. As there are many islands in Asia where the current speed is below than the feasible speed, the submerged device that can produce the power at lower current speed with easy deployment and retrieve is introduce. To simplify the installation and O&M operations, the 4-point taut mooring system is applied that can also secure the designated position. In this study, a HAT (horizontal axis turbine) of 5-kW submerged tidal device is designed and analyzed for a low current region that consist of two buoyancy tanks, a yaw control strut, and a diffuser-type brimmed duct. To analyze the most stable condition, 9 different cases of width and height of the device and 8 different mooring arrangements (total 72 different cases) are investigated both by frequency and time domains. The results show that the dynamic behavior that mainly affects the turbine performance decreases with lower strut height. However, it was observed that the device width has little effect on the dynamic response of the device. The device dynamic motion decreases with decreasing azimuth angle and increasing hang-off angle of the mooring configuration. The optimal design configuration that can produce the maximum power is found that of 4.42 m width, 2 m height, 30° azimuth angle and 60° hang-off angle. According to API RP 2K code, the mooring system is also designed in both intact and damaged conditions. The proposed tidal device could be applied for the low current speed areas with easy implementation and maintenance.

Numerical investigations on the installation of dynamically installed anchors with attached mooring line

Dynamically installed anchors (DIAs) are considered to be one of the most efficient mooring solutions for taut mooring systems. During their installation, DIAs are released from a certain height above the seabed and reach high velocities (up to 35 m/s) before impacting the seabed. However, a potential issue with DIA installation is that the anchor may not remain completely vertical during descent, resulting in the anchor impacting the seabed at an oblique angle and a significant reduction in holding capacity. To address this issue, this paper proposes a new numerical model for DIAs with attached mooring lines, which can consider the forces acting on anchors and mooring lines. A total of 176 numerical simulations were conducted under different initial configurations of mooring lines, and 128 simulations were performed, taking into account the influence of the horizontal motion of Anchor Handling Vessels (AHV). By comparing the velocity, embedment depth and tilting angle, possible combinations of the mooring line configurations in situ were back-analyzed. The simulations results show that the presence of the mooring line significantly reduces the final penetration depth of the anchor, and that the proposed installation model accurately simulates the change process of velocity and displacement during installation. Additionally, when DIAs impact the seabed at an inclined state, the tilting angle will increase due to soil resistance on the anchor tip. Finally, the initial state of the DIA at release is influenced by the horizontal motion of the AHV, which can influence the tilting mechanism of the DIA.

Analysis on the split absorber integrated with taut-moored floating turbine

A new wave energy converter is proposed in this paper, consisting of three split heave point absorbers, combined with a taut-moored floating turbine. It is adapted to the waves in the China Sea area, which are characterized by short periods and small amplitudes. Based on a series of physical model tests in regular, irregular, and extreme waves, the hydrodynamic performance of the integrated device is systematically investigated under different damping forces and incident wave directions. The experimental results reveal that the split point absorber presents new hydrodynamic characteristics and that the wave energy capture efficiency of the new device is greatly improved for the short-period waves in low sea states. Moreover, due to the out-of-phase heave motion, as well as the induced shallow water effect, the submerged platform makes a contribution to improving the energy capture efficiency of the split floater, particularly pronounced in the case of a high damping force in the power takeoff system. Under the condition of incident wave direction being coincident with the horizontal projection of mooring lines, the energies of pitch motion and mooring force of the integrated system are increased as a result of the high-frequency oscillation, which needs to be solved by further optimizing the taut mooring system.

On the mooring methodology of heaving point absorber arrays

This paper aims at developing a mooring methodology for floating point absorber arrays. To date, the literature gives no clear indications of the mooring influence on the potential energy conversion of a floating wave energy converter. Hence, this study targets to provide general insights into the dynamics of a floating wave energy converter under the influence of moorings. These are gained by conducting an experimental study on the motion response of a wave energy converter array in regular wave conditions using three different mooring systems: a taut line mooring, a vertical tension leg mooring, and a conventional slack catenary mooring. Results show that the potential energy conversion is strongly influenced by the choice of the mooring system and the incident wavelength. In intermediate wavelengths, a taut mooring system leads to a 8% increase in energy conversion potential. In contrast, the tension-leg moored array exceeds the potentially converted energy of the catenary moored array in wavelengths significantly longer than the wave energy converter’s length by over 80%. The results of this study provide an insight into the site-specific design of point absorber arrays and the best choice of the mooring system to increase the potential power output.

Numerical modelling of plate anchors under sustained load: The enhancement of capacity from consolidation

Embedded plate anchors can be used to moor floating offshore facilities. In taut mooring systems, a sustained tension carried by the anchors affects their long-term capacity due to consolidation effects in the soil surrounding the anchor. The resulting gain in capacity provides a potential basis for more efficient anchoring system design. To quantify this effect, and to understand the underlying mechanism, a systematic numerical study has been performed, to capture the effect of sustained loading and consolidation on soil failure mechanisms around an embedded plate anchor. The modified Cam clay model is used, to capture the effect of consolidation on soil strength, and also to faithfully model the process by which a water-filled gap forms under the plate, due to a loss of contact between the plate and the soil, with seepage into the gap zone. A critical observation is the load transfer process when such a gap forms beneath the plate after the applied tension causes the effective stress to fall to zero. The formation of this gap is shown to depend, in a systematic way, on the sustained tension, or preload, the soil strength ratio, the relative sharing of the anchor load between compression above the plate and tension below, as well as the embedded depth of the anchor. Simple relationships that capture these mechanisms are provided, linking the net change in soil strength above and below the anchor to the change in undrained capacity. As an example, it is shown that in soft normally-consolidated clays, a sustained preload of 60 % of the initial capacity, can create a 25 % gain in capacity, at which point the factor of safety is raised from 1/0.6 = 1.67 to 2. This gain is only available for anchors embedded by more than twice their diameter. At shallower embedment, the loss of strength in the unloaded soil beneath the anchor outweighs the gain in strength in the smaller zone of soil above the anchor, and the consolidation process causes a net weakening.

Coupled aero-hydro-servo-elastic analysis of 10MW TLB floating offshore wind turbine

This paper presents a fully coupled aero-hydro-servo elastic analysis of a 10MW offshore wind turbine supported by a tension leg buoy (TLB) platform, with a taut mooring system. The study investigates the TLB's dynamic response characteristics under the northern North Sea environmental conditions with water depths of 110 metres, comparing the performance of steel, polyester, and nylon mooring line configurations. Innovating floating wind turbines requires a cost-effective system that achieves reliable performance in operational and survival conditions. The innovative system should compete with other existing FOWT types and fixed-bottom offshore wind turbines in terms of LCOE. The dynamic responses of the relatively less complex TLB platform in terms of construction and installation showed small motion and accelerations for all available mooring materials from the current supply chain, enabling the wind turbine to be installed without significant modifications to their control systems. The mooring materials' elasticity is essential in the system achieving motion response.

Wave-induced structural response analysis of the supporting frames for multiconnected offshore floating photovoltaic units installed in the inner harbor

In this study, a hydrodynamics-based structural response analysis procedure of supporting frames for multiconnected offshore floating photovoltaics (FPVs) is suggested. Based on the suggested simulation methodology, the dynamic behavioral characteristics of the system were investigated. First, rational mooring types for FPVs installed in the inner harbor were studied based on the analysis results. Subsequently, extensive parametric studies have been conducted to determine a rational design strategy for dynamic response mitigation. In this study, the effects of the initial configuration and axial stiffness of taut mooring lines, arrangement angles of FPV systems, and floater dimensions on the static and dynamic responses of supporting frames under irregular waves in the inner harbor were studied. The corner module in the multiconnected system showed critical responses among others, the investigation of which is required for safety evaluation. Furthermore, it was observed that the mooring stiffness governed the overall stiffness of the system, which was proportional to the mitigation of the dynamic responses. Finally, the unsupported length of the frame, which was defined by the floater size, was found to be the key parameter for the mitigation of structural responses.

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.

Anchor loads for shallow water mooring of a 15 MW floating wind turbine—Part II: Synthetic and novel mooring systems

Novel and synthetic mooring tethers can enable tuning of a floating wind turbine’s mooring system stiffness behaviour, eliciting specific desirable response and load characteristics and potential cost reductions. This paper presents the results of detailed coupled dynamic simulations considering the IEA 15MW turbine and VolturnUS-S semi-submersible reference platform deployed in the Celtic Sea. The influence of synthetic and novel mooring tethers on anchor loads and platform dynamics in shallow waters is evaluated. Through comparison to results published in Part I, this study highlights that replacing chain with polyester ropes or novel mooring tethers effectively reduces the peak anchor loads by up to 84% and 75% respectively for the smallest footprints (293m), and up to 80% and 59% respectively for the largest footprints (838m). However, this results in significantly increased platform excursions, at times exceeding the 25m allowable limits. The weight of the compliant novel mooring reduces the excursions whilst maintaining lower loads profiles. The excursions can be further minimised by deploying taut moored systems, requiring the anchor to sustain higher loads including, importantly, vertical loads. This paper informs mooring and anchor design optimisation for large-scale floating wind turbines while also advising future research into taut moorings for floating wind turbines.

A semiparametric Bayesian method with birth-death Markov Chain Monte Carlo algorithm for extreme mooring tension analysis

A series of 1: 50 model tests have been conducted to study the dynamics of a taut mooring system and two hybrid mooring systems attached to a deep-water semi-submersible in the 100-year wave condition of the South China Sea. Three typical environmental conditions were tested, including head sea, beam sea and quartering sea. The mixture Gamma- Generalized Pareto distribution (GPD) model is applied to study short term extreme dynamic mooring tensions based on the measured data, the results are compared with the estimations from mixture Gamma and two-parameter Weibull models. A birth-death Markov chain Monte Carlo (MCMC) sampling procedure is developed to fit the parameters of a mixture Gamma- GPD via Bayesian inference, and an extensive discussion of the de-clustering procedure, parameters of the prior distribution for threshold and MCMC iteration steps on Bayesian inference are carried out. It is found that the Weibull model underestimates the most probable largest dynamic mooring tensions. On the contrary, the mixture Gamma model overestimates the extreme dynamic mooring tension significantly. The mixture Gamma- GPD can present good estimations of extreme dynamic mooring tension, and its performance is robust to the sample definition, MCMC iteration steps and parameters of threshold prior distribution.

Effect of the multi-segment mooring system by buoy and clump weights on the dynamic motions of the floating platform

In this paper, two hybrid mooring system is proposed to improve the performance of the dynamic motions of the floating platform. The concepts are a multi-segment arrangement of the catenary and taut mooring lines, including buoy and clump weight. Two mooring concepts named MSM-BSW and MSM-BMW were designed for a 5 MW OC4-DeepCwind semi-submersible floating wind turbine. These models' mooring forces and platform response were compared to the main model (OC4) under regular and irregular wave conditions. The dynamic response of the platform is performed utilizing both Morrison theory and three-dimensional radiation/diffraction theory based on the boundary element method (BEM) in ANSYS-AQWA software. Adding buoy, increase the horizontal mooring line angle at the fairlead, resultantly on one hand's mooring stiffness in surge and pitch increased so the MSM-BSW model could reduce surge and pitch motions. On the other hand, the buoy can reduce the mooring suspended weight, leading to an increase in the heave platform response. In the case of MSM-BMW, more clump weights increased the mooring line stiffness in the heave direction. In regular and irregular wave conditions, numerical results indicate that the change of natural periods in both MSM-BSW and MSM-BMW case studies leads to the low-frequency resonance having a significant difference from OC4. In contrast, the wave frequency resonance of the three concepts is similar.

Comparison of hydrodynamic performance of floating breakwater with taut, slack, and hybrid mooring systems: An SPH-based preliminary investigation

A tautly moored floating breakwater exhibits favorable wave attenuation performance at a specific sea level, and a slackly moored floating breakwater is more adaptable to tidal variation. To bring their merits into full play, a hybrid mooring system composed of seaward taut ropes and leeward slack chains is proposed. Then, a Smoothed Particle Hydrodynamics-based numerical model is built to compare the wave transmission and reflection coefficients, mooring forces, and motion response of a floating breakwater with taut, slack, and hybrid mooring systems, respectively. The results show that, at low tide, a tautly moored floating breakwater has the best wave attenuation performance. A hybridly moored floating breakwater comes second, but its mooring forces are smaller. At high tide, in the shorter-wave regime, the taut mooring system still leads to the best wave attenuation performance, while in the longer-wave regime, a hybrid mooring system becomes the best. Notwithstanding the respective dominant regimes of the taut and hybrid mooring systems at high tide, the hybrid mooring system is always subjected to smaller mooring forces. It is hoped that this applied research can shed light on the engineering design of the mooring system of the floating breakwater.