Mooring Line Tension Research Articles

Influence of turbulence models on the dynamic response of a semi-submersible floating offshore wind platform

With the aim of evaluating the effects of the selection of the wind turbulence model on the dynamic response of a semi-submersible floater supporting the DTU 10 MW RWT, we ran aero-hydro-servo-elastic simulations employing the OASIS code developed at IHCantabria. For each of the 4 sea states selected, apart from the reference case with only wave loads, we analysed 30 different realizations with coupled wind and wave loads for each of the 7 full-field turbulent winds generated by the turbulence simulator TurbSim: Kaimal and von Kármán spectral models with three different turbulence classes each and the API turbulence model.From the resulting platform motions and mooring line dynamics, it can be concluded that the selection of the wind turbulence model is not trivial. At U0=5.83 m/s, the obtained results show a high dependence on the turbulence class. However, as the wind speed increases and the wind turbine achieves the rated speed, the impact of the turbulence intensity is drastically reduced, and the design tension is dominated by the thrust force and thus by the vertical wind speed profile. The highest design tension of the windward mooring lines is obtained with the API model at U0=8.75 m/s (at least 357 Tm).

Experimental investigation on dynamic responses of a spar-type offshore floating wind turbine and its mooring system behaviour

This study investigated the dynamic behaviour of a spar-type offshore floating wind turbine (OFWT) analysed under various wave loads by conducting an experiment campaign. Both the characteristic of the spar platform itself and the spar with realistic moorings have been investigated. A particular focus of the study was the motions of the mooring lines, which were measured at a series of discrete points along one line using an underwater motion capture system, in parallel with tension measurement at the anchor points. The main objective of this study is to provide experimental measured data to validate numerical software on the research of mooring line motions.A free decay test has been carried out to confirm the spar platform motions natural frequencies and damping characteristics. The platform was then subjected to a range of regular and irregular waves to obtain the motion characteristics of the platform and the mooring lines. For the mooring line motions, which has rarely been discussed in published research, a non-linear snatching phenomenon has been observed at the tank for some wave frequencies. This non-linear behaviour shows that snatching leads to the high instantaneous mooring line tensions and platform accelerations could cause the failure of the mooring lines in the real structure.

Validation and application of nonlinear hydrodynamics from CFD in an engineering model of a semi-submersible floating wind turbine

Nonlinear hydrodynamics play a significant role in accurate prediction of the dynamic responses of floating wind turbines (FWTs), especially near the resonance frequencies. This study investigates the use of computational fluid dynamics (CFD) simulations to improve an engineering model (based on potential flow theory with Morison-type drag) by modifying the second-order difference-frequency quadratic transfer functions (QTFs) and frequency-dependent added mass and damping for a semi-submersible FWT. The results from the original and modified engineering models are compared to experimental data from decay tests and irregular wave tests. In general, the CFD results based on forced oscillation tests suggest increasing the frequency-depending added mass and damping at low frequencies compared to first order potential flow theory. The modified engineering model predicts natural periods close to the experimental results in decay tests (within 5%), and the underprediction of the damping is reduced compared to the original engineering model. The motions, mooring line tensions and tower-base loads in the low-frequency response to an irregular wave are underestimated using the original engineering model. The additional linear damping increases this underestimation, while the modified QTFs based on CFD simulations of a fixed floater in bichromatic waves result in larger difference-frequency wave loads. The combined modifications give improved agreement with experimental data in terms of damage equivalent loads for the mooring lines and tower base.

New system design for the cultivation of extractive species at exposed sites - Part 2: Experimental modelling in waves and currents

Aquaculture is projected to be a major supplier of marine proteins to large parts of the global population. This includes bivalves, which have a high potential to offset protein deficits, as they are highly adaptable to varying water temperature, salinity, desiccation, and oxygen conditions. This work is part of a two-piece contribution on novel marine aquaculture technology and details physical laboratory tests of a new cultivation system for bivalve farming called “Shellfish Tower”. The tested 1:20 model consists of a rectangular cage (2 × 2 m prototype scale) with a central buoyancy element and a height of 2 – 4 m. Testing was done in a current flume as well as a wave basin for current velocities between 0.4 – 2.2 m/s and wave heights of 1.6 to 5.0 m with periods between 5 to 14 s. The tests were conducted to prove the feasibility and functionality of this aquaculture system, which is usable for the collection and cultivation of mussel spat as well as for the grow-out of oysters, scallops, and seaweed in marine environments. Tests carried out in a current flume revealed that drag coefficients decrease with increasing current velocities, and range from Cd=0.5 to 2.5, while the mooring inclination increases from 12° to 84° with increasing flow velocity, which is highly dependant on the buoyancy related pretension. The examination of the mooring line tensions recorded in a wave basin showed that the largest values of snap-induced tension were up to 10 times that of the semi-static tension. The maximum-recorded tension on the system was 48 kN for a single and 89 kN for a double configuration, compared to non-snap tension values, which were in the range of 6 – 10 kN. The insights gathered in this study will inform the future design of aquaculture systems in high-energy environments and allow for an integration into numerical models.

Hydrodynamic Performance Improvement of Double-Row Floating Breakwaters by Changing the Cross-Sectional Geometry

This paper is presented to develop the hydrodynamic performance of double-row floating breakwater (FBW) by changing cross-sectional geometry in the high wave periods. The ANSYS-AQWA software is employed for the present calculations, which is a potential-based boundary element method (BEM). The rectangular moored pontoons in the single- and double-row types are selected, and the results of the wave transmission coefficient and response amplitude operator (RAO) are presented and compared. The numerical results showed good agreement with experimental data at different wavelengths, wave height, and the distance between double-row FBWs. Then, the performance results of FBWs for five shapes (rectangular, π-shaped, plus-shaped, triangular-shaped, and box-shaped) in the wave transmission coefficient, RAO, and mooring line tension are presented and compared to each other. The results showed that the plus-shaped FBW has a better performance in reducing wave transmission than other shapes. In waves with long periods, the performance of π-shaped, triangular-shaped, and box-shaped FBWs is reduced, and the rectangular FBW loses its efficiency. Overall, the plus-shaped FBW has preferable performance regarding RAO response, mooring tension, and wave transmission.

Investigation on mooring breakage effects of a 5 MW barge-type floating offshore wind turbine using F2A

Stability and integrity of the station-keeping system are vital to the safety and performance of a floating offshore wind turbine (FOWT). Failure of a mooring line significantly increases the risk of damage to the FOWT. Consequently, it is necessary to investigate the impacts of a mooring breakage on dynamic responses of the rotor, platform and the remaining mooring lines of a FOWT. In order to address the associated research needs of a FOWT subjected to mooring failures, this study has analyzed and predicted the transient behaviors of a 5 MW barge-type FOWT under rated and extreme conditions using a coupling framework based on FAST and AQWA (F2A) when a mooring is subjected to a sudden breakage. It is found that the mooring breakages have a minor impact on the aerodynamic performance and aero-elastic responses of the FOWT, although a notable yaw-deviation of the rotor is caused. The platform sway and yaw motions are significantly influenced by the breakage of an upwind mooring line. Thus, the tension in the remaining adjacent mooring line is increased by as much as 156% and 41.6% under the examined rated and extreme conditions, respectively. The emergency shutdown following the mooring line breakage decreases the platform drift and yaw motions, while the platform pitch motion is enhanced by the shutdown measure due to the absence of aerodynamic damping. In addition, significant reductions in the tensions of upwind mooring lines that suffer the most severe tension are achieved. The shutdown measure is beneficial in ensuring the operational safety of the FOWT subjected to a sudden breakage of a mooring line.

Design and Automated Optimization of an Internal Turret Mooring System in the Frequency and Time Domain

A cost-effective mooring system design has been emphasized for traditional offshore industry applications and in the design of floating offshore wind turbines. The industry consensus regarding mooring system design is mainly inhibited by previous project experience. The design of the mooring system also requires a significant number of design cycles. To take aim at these challenges, this paper studies the application of an optimization algorithm to the Floating Production Storage and Offloading (FPSO) mooring system design with an internal turret system at deep-water locations. The goal is to minimize mooring system costs by satisfying constraints, and an objective function is defined as the minimum weight of the mooring system. Anchor loads, a floating body offset and mooring line tensions are defined as constraints. In the process of optimization, the mooring system is analyzed in terms of the frequency domain and time domain, and global and local optimization algorithms are also deployed towards reaching the optimum solution. Three cases are studied with the same initial conditions. The global and local optimization algorithms successfully find a feasible mooring system by reducing the mooring system cost by up to 52%.

Structural responses of large-sized floating wind turbine with consideration of mooring-line dynamics based on coupled FEM simulations

ABSTRACT The dynamic response of a 5 MW floating wind turbine is examined using the coupled finite element simulations, and the dynamic effects of the catenary is considered, while the coupling between flexible components are included. The restoring performance of the mooring-line is analysed based on the vector equations and numerical simulations. The stiffness hysteresis and the influence of the catenary dynamics on the restoring performance are studied. Then the structural responses undergoing wind and wave loads, are examined. Our results show that the mooring-line tension significantly rise due to catenary dynamics, and the snap tension gets around three times larger. The mooring-line stiffness presents a hysteresis character, owing to the fluid/structural damping, which becomes more obvious with the increase of motion frequency/amplitude. Moreover, the structural response gets smaller principally because of the hysteresis effect. The spar displacement and the tower root stress become respectively 18.4% and 32.7% smaller.

Small-Scale Study of Mooring Line Tension Thresholds Based on Impulsive Load Analysis during Big Floating Structure Operation and Commissioning

Marine wind energy business competitiveness is strongly related to offshore substructures and their logistics. With the aim of avoiding oil and gas methodologies to reduce installation costs, some designs make use of multi-floater systems linked by cables. Optimization of these systems in search of larger operational windows increments the likelihood of snap-load event occurrence. This work describes the analysis of cable load during physical model simulation of a DEMOGRAVI3 installation procedure applying statistical and signal-processing methods. The authors describe a simple methodology to define the maximum load threshold for a given mooring line set-up in order to avoid snap loads.

The Effect of Additional Subsea Floater on Mooring Line Tension and Motion of CBM (Conventional Buoy Mooring) Ø 4.5 m

Single point mooring of Conventional Buoy Mooring (CBM) is an offshore mooring structure where the mooring line attached from bow of ship to CBM. Lack buoyancy on CBM needed Additional subsea floaters to increase buoyancy of CBM. Time domain simulation on Moses software version 11 used to calculate the effect of the variation number of subsea floaters to chain tension and CBM Motion. The result is a single configuration subsea floater has higher tension and extreme motion than multiple configuration subsea floaters.

Simulation of Mooring Line Operation in Ship Mooring

In order to simulate the process of ship mooring more accurately, catenary equations are used to model mooring lines. Use different catenary models to simulate different phases of ship mooring operation. Establish a basic elastic catenary equation of mooring line by using the micro mechanical equilibriums theory. Then, a double-catenary model of heaving line and cable with a point load, imitating the effect of an eye splice, is developed based on existing catenary models in the literature. By calculating the mooring line tension, analyze whether the tension exceeds the safety load. The simulator is verified through simulations. A set of case studies is used to verify the effectiveness and practicability of the model. Integrate and realize the three-dimensional visualization of mooring line operation in ship mooring operation. The established model can guide mooring operation of ships.

Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

A Study on the Effectiveness of the Heading Control on the Mooring Line Tension and Position Offset for an Arctic Floating Structure under Complex Environmental Loads

Even though interest in developing the Arctic region is increasing continuously, the standard procedure to be used to analyze the station-keeping performance of a floater considering ice loads has not been established yet. In this paper, the effectiveness of heading control with a dynamic positioning system is analyzed to evaluate the improvement of the performance of the station-keeping system in the ice conditions. Complex environmental loads with ice-induced forces were generated and applied to a ship type floater with dynamic positioning and mooring systems. Three-hour time-domain simulations were conducted for the two different station-keeping systems with mooring only and mooring with a dynamic positioning system. Position offsets and mooring line tensions for the two scenarios were compared with maximum values and most probable maxima (MPM) values. The results of the simulation showed that the heading control can reduce 8.2% of MPM values for the mooring lines and improve the station-keeping performance by about 16.3%. The validity of the station-keeping system that was designed was confirmed, and it is expected that the specification of mooring lines can be relaxed with the heading control.

Effect of design configuration on structural response of longline aquaculture in waves

The submerged deep-water longline aquaculture facility is known as a typical, flexible multi-degree-of-freedom offshore structure. Effects of layout concepts of the dropper weight, the upper buoy pattern and the incident wave angle on longline aquaculture systems have been studied by means of numerical modellings and conducting scaled model experiments. The results have showed that the dropper weight and the incident wave angle have significant influence on the motion and loading of the raft-like system. The maximum displacement of the main line is negatively correlated with the dropper weight. The maximum tension of the main line, the downstream mooring line, the upper whip lines and the dropper linesincreases with the increase of the dropper weight. The tension of the upstream mooring line decreases slightly with the increase of the dropper weight. The impact of the upper buoy pattern on the motion and loading of the system is so insignificant as to be ignored. The maximum tension of the mooring lines and the displacement of the long line are positively correlated with structure orientation within the angle range of 0 ° to 60 ° and show negative correlation within the structure orientation of 60 ° to 90 °. The impact of structure orientation on the main line, the upper whip lines and the dropper lines is not so obvious. This study is expected to benefit the design and safety assessment of the longline aquaculture facility with similar structures and environment conditions in this study.

Probabilistic fatigue surrogate model of bimodal tension process for a semi-submersible platform

Mooring-line tension typically involves a bimodal process in which a wave-frequency component and a low-frequency component are induced by dynamic wave loading. The non-Gaussian characteristics make fast frequency domain estimation inaccurate. Although the time-domain fatigue damage is believed to be the most reliable method, it is computationally expensive; thus, in the present study, surrogate models including artificial neural network and kriging models are constructed for fatigue-damage prediction of the bimodal tension process to improve the accuracy and efficiency compared with the simple frequency- and time-domain approaches. A parametric study is conducted to investigate the spectrum parameters and correction factor for the bimodal tension process, and the fatigue damage under arbitrary wave conditions is evaluated using interpolation techniques for long-term fatigue analysis. The fatigue failure probability of mooring lines under dynamic environmental loads is calculated using the surrogate models and various spectral fatigue techniques. The results indicate that with a finite database of time-domain fatigue damage, the proposed surrogate models-based approach can provide a significantly more accurate assessment of the fatigue failure probability than spectral-based approaches.

Influence of Clumps-Weighted Moorings on a Spar Buoy Offshore Wind Turbine

The spar buoy platform for offshore wind turbines is the most utilized type and the OC3 Hywind system design is largely used in research. This system is usually moored with three catenary cables with 120° between each other. Adding clump weights to the mooring lines has an influence on the platform response and on the mooring line tension. However, the optimal choice for their position and weight is still an open issue, especially considering the multitude of sea states the platform can be exposed to. In this study, therefore, an analysis on the influence of two such variables on the platform response and on the mooring line tension is presented. FAST by the National Renewable Energy Laboratory (NREL) is used to perform time domain simulations and Response Amplitude Operators are adopted as the main indicators of the clump weights effects. Results show that the clump weight mass is not as influential as the position, which turns out to be optimal, especially for the Surge degree of freedom, when closest to the platform.

Experimental Research on A New Type of Floating Breakwater for Wave-Absorbing and Energy-Capturing

To avoid the damage caused by big wind and wave in cage culture, and to solve the problem of energy supply faced by automatic breeding equipment, a new type of floating breakwater, named as Savonius double buoy breakwater (SDBB), is proposed in the paper. The floating breakwater is composed of HDPE cylindrical double buoys and horizontal axis Savonius rotors, and has the functions of wave-absorbing and energy-capturing. Based on the linear wave theory and energy conservation law, the Fourier Transform was applied to separate the two-dimensional wave frequency domain, and the energy captured by the rotors and absorbed by the floating breakwater were calculated. Experiments were conducted in a two-dimensional wave-making flume, and the transmitted waves at different wave heights and periods, the tension of mooring lines, and the rotational torque exerted on the Savonius rotor were measured. A series of performance comparison tests were also performed on the new floating breakwater and the traditional double-floating breakwater. Results show that the new floating breakwater is better than the traditional one in terms of reducing wave transmittance, and the combination of the floating breakwater with Savonius rotors can provide for marine aquaculture equipments with green power supply to a certain degree of self-sufficiency.

Numerical investigations on load transfer of mooring line considering chain–seabed dynamic interaction

Taut or semi-taut mooring lines are often used in deep-sea area to connect the floating structures to the anchors in the seabed. In previous studies, the quasi-static analysis method was adopted to assess the load transfer of the mooring line. However, the inertia effect of mooring line and chain–seabed dynamic interaction cannot be neglected in practice. In this paper, the soil strain rate effect was incorporated in the chain–seabed dynamic analyses, and the load transfer mechanism of mooring line was investigated based on the proposed model. The numerical model was firstly verified with the quasi-static analysis results and experimental data under dynamic condition, and then the contributions of different forces to the variations of line tension were evaluated. It is found that two factors (inertia force and soil resistance) have a great influence on the mooring line tension variation. The inertial effect is dominated under high-frequency condition, and the maximum line tension at the fairlead is larger than that under static condition. However, the tension at the pad-eye under dynamic condition always has a certain increase. The reason is that partial reverse soil resistance under dynamic condition increases the tension at the pad-eye.

An Experimental Study on the Motion of the Floater Moored near Port in Waves Generated by a Ship

In the past, various research on the effects of waves generated by ships has been investigated. The most noticeable effect of the waves generated by a passing ship is the increase of the hydrodynamic forces and the unwanted large motion of the moored ship and high mooring forces that occur. Thus, it is crucial to investigate the effect of the waves generated by the passing ship near port on the motion of the moored ship and the tension of the mooring lines. A model test was performed with virtual ship-generated waves in a square tank at CWNU (Changwon National University). The IMU (Inertial Measurement Unit) and Optical-based system were used to measure the 6DOF (Six Degrees of Freedom) motion of the moored floater. Additionally the tension of mooring lines were measured by the tension gauges. The effects of the wave direction and wave height generated by the virtual ship-generated waves on the motion of the moored floater were analyzed.

Seabed trenching near the mooring anchor: History cases and numerical studies

The floating platform with taut or semi-taut mooring systems is the main option for oil and gas exploitations in deep sea. However, it has been reported that the reciprocating motion of the mooring line tends to induce the seabed trench near the anchor, which consequently reduces the anchor capacity. In this paper, two parts of work have been carried out to enhance the understanding of the seabed trench. Firstly, the dimensions of reported trenches in the field as well as their formation conditions are summarized, and four feature points are proposed to describe the trench profile. Secondly, a numerical method considering mooring line dynamics to predict the final trench profiles is proposed, which contains two simulation modules, i.e. mooring line dynamics and seabed trenching. The method is verified using the data from the field and centrifuge tests. The influences of the mooring line tension and soil strength on trench profiles are comprehensively investigated further. The results show that obvious trenches are likely to form when the amplitude of mooring line tension variation is large and the soil shear strength is low. The main impact of the formed trenches is that the loading direction at the pad-eye tends to be horizontal.