Taut Mooring Systems 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.

Numerical Analysis of Catenary and Taut Mooring System Combination Effects on Heave and Pitch Motion Responses of a Semi-submersible Offshore Platform

Numerical Analysis of Catenary and Taut Mooring System Combination Effects on Heave and Pitch Motion Responses of a Semi-submersible Offshore Platform

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.

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.

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.

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.

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.

Model Test of the Pullout Bearing Capacity of End-Bearing Torpedo Anchors

Torpedo anchors (TAs) are regarded as one of the most efficient mooring solutions for taut mooring systems, and end-bearing TAs are a new type of TA that primarily relies on end-bearing plates at the tail to generate a pile-end resistance to improve their pulloutbearing capacity (P). Therefore, the estimation of the pullout capacity of the end-bearing TA is vital for the design of offshore floating facilities. In this study, pullout model tests were conducted to investigate the P of conventional and end-bearing TAs and examine the effects of factors such as the embedment depth (h), the relative density (Dr), the pullout angle (α), and the area (A) of the bearing plates on P. The test results show that, under oblique pullout loading, the P on the conventional TA increased slowly as displacement increased, while there was a peak on the load–displacement curve of each end-bearing TA with a relatively large A. The end-bearing TAs considerably outperformed the conventional TAs in terms of the P. In addition, increasing h, Dr, and A significantly increased the P of the end-bearing TAs. However, increasing h and Dr slightly decreased the ability of the bearing plates to increase the P of the end-bearing TAs. These research results can provide a guideline for TA installation in deep-sea engineering.

Fatigue Life Prediction of Mooring Line on Indonesian Tsunami Early Warning Systems (Ina-TEWS) Buoy

Ina-Buoy Gen 3.1 is one of the application of the program that has been developed by BPPT (Agency for the Assessment and Application of Technology) in order to reduce the risk of a tsunami disaster in Indonesia. This Indonesian Buoy for Tsunami Early Warning System (INATEWS - Buoy) consists of a tsunami buoy, an ocean bottom unit (OBU), a satellite and a ground station. This paper discuss about calculation of fatigue life due to wave loads for the mooring line that used to hold the buoy position in the sea. A taut mooring system is applied to the buoy until sinker at seabed. Calculation of operational tension at mooring line obtained using numerical simulation fully licensed software Orcaflex ver 11 owned by Centre Technology for Maritime Industrial Engineering (PTRIM), BPPT with wave input of Hs=2.5m and Tp=10.5sec. The depth of sea is varied at 2088m (Sunda Strait), 3563m (Cilacap Sea) and 4082m (Bali Strait). By knowing those tension forces, stress at time history on the mooring line can be obtained. This stress time history is then applied to Palmgren – Miner formula that based on SN-Curve of the mooring line material to predict the life of the mooring line. By applying a safety factor of 2.5 it is found that the fatigue life of the mooring line at water depth of 2088m in Sunda Strait is 6,9 years, then, at water depth of 3563 m in Cilacap Sea is 15,8 years, finally at water depth of 4082 m in Bali Strait is 10 years.

Effect of Failure Mode of Taut Mooring System on the Dynamic Response of A Semi-Submersible Platform

Effect of Failure Mode of Taut Mooring System on the Dynamic Response of A Semi-Submersible Platform

Experimental study of the influence of the rope material on mooring fatigue damage and point absorber response

The present work investigates the influence of mooring material on the hydrodynamic responses of a point absorber by experimental tests. Three taut mooring systems are considered with different materials, namely wire, nylon and polyester. In order to study the influence of mooring material on point absorber hydrodynamic responses, the other factors that may influence system dynamics are kept as unchanged as possible. During the model tests, the pretension of the three mooring systems are set to be similar and the parameters of wave conditions are identical. Decay tests are conducted to determine the system natural periods and linear damping coefficients in different modes. Totally seventeen regular wave cases and two irregular wave conditions are tested to study the motion responses of the buoy as well as mooring dynamics. The response amplitude operators of motion responses and mooring tensions are obtained by the regular wave model tests. Three wideband spectral methods as well as two simplified spectral methods are applied to study mooring fatigue damage in the random wave conditions, and the results are compared with the estimations of rainflow counting approach. It is shown that the α0.75 method provides the most precise estimations of fatigue damage in most mooring cases among spectral methods discussed this study.

Time-domain analysis of a bean-shaped multi-body floating wave energy converter with a hydraulic power take-off using WEC-Sim

A novel floating wave energy converter (BFWEC) with multiple ‘bean shape’ floats and a hydraulic power-take-off system (HPTO) is presented. The overall setup consists of a central buoy (C·B.) connected to a set of floats around it using a lever arm and is position restrained using a taut-mooring system. Three configurations of BFWEC are proposed with four, six, and eight number of floats. A detailed numerical analysis is carried out by modeling the device in a time-domain using an open-source code: WEC-Sim (Wave Energy Converter Simulator), assuming potential wave theory (PWT). By incorporating HPTO to all the three configurations, simulations are performed in monochromatic waves. Appropriate parameters of HPTO components for each configuration are estimated for the maximum capture width of BFWEC. The effect of the HPTO damping coefficient in maximizing the output of BFWEC is also assessed. Despite the dimensions of BFWEC tailored to the shallow-intermediate Indian waters, the device is simulated with extreme wave characteristics to test its conversion characteristics under the wave conditions away from the resonance.

Design and comparative analysis of alternative mooring systems for floating wind turbines in shallow water with emphasis on ultimate limit state design

Floating wind turbines represent a cost-efficient energy solution in deep water where bottom-fixed wind turbine becomes excessively expensive. However, mooring design is quite challenging for all shallow water depths including the transition water depth between bottom-fixed and floating wind turbines, in the order of 50–80 m, for which floating concepts might become more cost-effective than bottom-fixed ones. In this paper, mooring system design for floating wind turbine in shallow water are studied considering both catenary and taut mooring systems. Seven mooring concepts designed for a 5 MW semi-submersible floating wind turbine at 50 m water depth are compared with the purpose to identify solutions that are structurally reliable and economically attractive. The concepts are made of different mooring line materials (chain and synthetic fibre rope), mooring components (clump weight and buoy) and anchors (drag embedment anchor and suction anchor). Based on the latest experimental data, the nonlinear tension-dependent stiffness of synthetic fibre rope are described with an improved numerical model. Performance of the seven mooring concepts are compared with respect to mooring line characteristics, motion response amplitude operator, utilization factor considering the ultimate limit state design and cost etc. Six mooring design concepts are finally recommended for future assessment regarding the application of floating wind turbines in shallow water say 50 m and deeper.