Mooring Chain Research Articles

Parametric Sensitivity Analysis of Mooring Chains of a Floating Offshore Wind Turbine in Shallow Water

Floating offshore wind turbines (FOWTs) are severely restricted in numerous sea areas due to challenges from the strong nonlinear characteristics of mooring chains in shallow water (less than 50 m). Therefore, this paper introduces a design method of mooring chains of a FOWT at a water depth of 44 m and carries out a parametric sensitivity analysis on length, nominal diameter, and clump weights of mooring chains. The results of the study found that compared with the mooring chains in deep water, the mooring chains in shallow water show obvious nonlinear characteristics in mooring tension, the lying section of the mooring chain on the seabed, and the mooring chain spatial angle, which brings great risk to the safe operation of FOWTs. The change in the nominal diameter of the mooring chain has a certain impact on the dynamic characteristics of a FOWT, but it is not as significant as that from the change in the length of the mooring chain. In addition, a mooring chain in shallow water is prone to the slack–taut phenomenon; thus, this paper puts forward an optimization investigation using clump weights at the suspension section of the mooring chain, which improved the performance of the mooring chain significantly.

Analysis of mooring performance and layout parameters of multi-segment mooring system for a 15 MW floating wind turbine

IntroductionFloating wind power is the important path for the development of offshore wind energy, and the performance of the mooring system of floating wind turbines (FOWTs) significantly affects their economic viability, safety, and sustainability.MethodsThis paper systematically analyses the positioning performance, mooring line extreme loads, and fatigue response of a FOWT equipped with both single segment and multi-segment mooring systems, based on the IEA 15 MW large turbine and a floating platform. The hydrodynamic performance of the floating platform is calculated, and the platform’s motion-sensitive directions are analysed through Response Amplitude Operators (RAOs). The natural periods of the platform are validated by free decay tests. The six degrees of freedom (DOFs) motion response and the mooring line peak tensions are analysed under normal and extreme conditions.ResultsThe results show that both mooring systems provide good motion performance and stable tilt angles for the platform. Under ALS (single-line failure) condition, the multi-segment mooring system demonstrates a notable capacity to resist impact loads, with comparatively minor fluctuations in mooring line tension. In the multi-segment system, fatigue damage primarily occurs in the upper mooring chain, with damage approximately 4.5 times greater than that of the bottom chain over a 1-year period. The effects of mooring line spread angles and lengths on performance are also analysed. The results indicate that the mooring line spread angle has slight impact on platform motion response and mooring line tension, while mooring line length significantly affects the extreme tension of the lines.DiscussionThe findings of this study can provide some references in the design of mooring systems for future FOWTs.

Dynamic response analysis of a floating bridge structure under combined actions from seismic loading and waves at different incident angles

Abstract This paper investigates the complex coupled response of a cross-sea floating bridge under the combined actions of seismic loading and wave at different incident angles. The bridge structure is modeled using the finite element method, considering the interaction between the girder, pier, pontoon, and mooring chains. Wave forces are calculated based on potential theory, and an added mass approach is applied to simulate the hydrodynamic forces induced by the earthquake. Simulations were conducted using a one-year random wave at three different incident angles of 0o, 45o, and 90o, combined with three-dimensional seismic loading of varying magnitudes: 0.3 g, 0.5 g, and 1 g. The simulation results indicate that the direction of the incident wave significantly influences the bridge's response. The coupling effect between wave and seismic forces on the floating bridge's response is not simply additive. Generally, when the earthquake magnitude is low, the wave loading significantly impacts the bridge's dynamics. However, as the earthquake magnitude increases, the bridge's response becomes dominated by the seismic forces, rendering the influence of wave forces negligible.

MULTIAXIAL AND UNIAXIAL FATIGUE ANALYSIS OF A MOORING CHAINS FOR AN OFFSHORE FLOATING WIND TURBINE

This paper presents the fatigue study (uniaxial fatigue and multiaxial fatigue) of mooring chains for a floating wind turbine (FWT). The operating principle of a floating offshore wind turbine (FOWT) and general information on uniaxial and multiaxial fatigue are presented, together with a reminder of the standards governing fatigue in the offshore sector. Uniaxial fatigue damage and service life calculations are carried out for various loading conditions and sea states. The finite element method is carried out using ANSYS software in order to estimate the best mapping of stresses and strains on the structure of the mooring chain. Finally, a simulation was also carried out using usingMatlab software to calculate the damage and multiaxial fatigue life. Comparison of the two analyses shows that uniaxial fatigue analysis minimises long-term damage and that multiaxial fatigue should be preferred for estimating the durability of offshore structures under extreme loading.

Effect of Surface Roughness on Corrosion Resistance of Mooring Chains for Offshore Floating Photovoltaics

Effect of Surface Roughness on Corrosion Resistance of Mooring Chains for Offshore Floating Photovoltaics

Investigation of motion response suppression characteristics in floating platforms equipped with novel spiral side plates

AbstractTo enhance the stability of floating wind turbine platforms, this study combines the structural characteristics of helical side plates and proposes the installation of serrated helical side plates on Spar wind turbine platforms. Based on potential flow theory, the present study employs blade element momentum theory and radiation‐diffraction theory. Upon establishing a dynamic data link library, wind‐wave coupling was implemented, and the dynamic response characteristics of platforms with different helical side plates were compared. The results indicate that in the frequency domain analysis, the platform with serrated helical side plates demonstrates reduced sensitivity to waves, with a particularly notable increase in added mass in the heave direction, suggesting excellent hydrodynamic performance. In the time domain analysis, improvements in the platform's surge and heave stability performances were observed, measuring 10.99% and 10.64%, respectively, in extreme conditions. Owing to the unique features of the serrated structure, the tension in the mooring chains on the wave‐facing side is reduced, thereby effectively lowering the fatigue load on the mooring chains.

Investigation on local mooring stresses of floating offshore wind turbines considering mooring chain geometrical and material nonlinearity

As the renewable energy industry moves towards deep water, mooring chains play an irreplaceable role in position keeping. The geometrical and material nonlinearity of the mooring chains significantly affects moored platform responses and mooring responses. To investigate local mooring stress evolution under wave loads, a simulation method was proposed, combining hydrodynamic loads derived from fully coupled simulations of floating offshore wind turbines (FOWTs). The simulation method, embedding the lumped mass mooring model, was validated by comparing the experimental data from OC4 semi-submersible wind turbines. Their reasonable segment numbers and the mooring structural damping ratio were also discussed. Based on the validated simulation technology, a coupled 3D-mooring analysis was performed, replacing linear mooring stiffness suggested in FAST tutorials with nonlinear mooring stiffness calculated by axial stretching the real-sized 3D chains. The nonlinear mooring stiffness is assumed, taking into account detailed geometry, nonlinear material properties, and contact behavior between chains. The nominal diameter of the real-sized 3D-chains was determined, so that the chains had the same volume per unit length as the validated case, ensuring a similar Morison's force on the mooring line. Then, the local mooring stresses were investigated by loading the derived nonlinear fairlead tension onto both sides of real-sized 3D chains in an explicit finite element software. The evolution of local mooring stresses under design wave loads with different wave headings of 0, 30, 60° was compared and discussed. As wave direction increases, this study indicated that the amplitude of local maximum principal stress decreases and its phase leads. Additionally, its magnitude is mainly influenced by axial tension force and contact behavior.

Corrosion-fatigue damage identification in submerged mooring chain links using remote acoustic emission monitoring

This paper investigates the feasibility of detection, localisation, and monitoring of corrosion-fatigue damage in mooring chain links using remote Acoustic Emission (AE) technique in submerged conditions. A large-scale experiment was conducted on a studless R4 chain retrieved after about two decades of operation offshore. Ultrasound signals were continuously measured using fixed and movable arrays of AE transducers placed on perpendicular planes in the water tank enclosing the chain. The AE parameters extracted from the measured signals have been analysed. AE sources were successfully localised on the 3D geometry of the chain links. The results suggest that damage growth can be detected and localised using non-contact underwater AE transducers.

Corrosion susceptibility of mooring chain steel under cyclic stress

Corrosion susceptibility of mooring chain steel under cyclic stress

Underwater Image Transmission Method and Realization Based on Inductively Coupled Mooring Chain

Underwater Image Transmission Method and Realization Based on Inductively Coupled Mooring Chain

Reliability analysis of mooring chains for floating offshore wind turbines

Reliability analysis of mooring chains for floating offshore wind turbines

Non-Contact Acoustic Emission Monitoring of Corrosion in Mooring Chain Links

Mooring chains are key parts of floating energy production units, such as floating wind turbines, photovoltaic islands, and floating production-storage-offloading units (FPSOs). These structures are predominantly subject to corrosion and fatigue. Detailed integrity assessment of mooring chains can be challenging due to difficult access, complex geometry, surface conditions (often with the presence of corrosion pits), and weather conditions. Additionally, the presence of marine growth on the surface of the chain links often requires the need of surface cleaning to obtain a detailed assessment of the structural integrity. This paper highlights an experimental investigation of monitoring of corrosion-induced damage in mooring chain links by means of non-contact Acoustic Emission (AE) technique. Accelerated corrosion experiments were performed on a large-scale mooring chain sample recovered from the field. A dedicated experimental set-up was designed to apply accelerated corrosion on two chain links submerged in natural sea-water. An immersion tank, i.e. an instrumented 1000x1000x1200mm3 water tank, was used to submerge the chain links in natural sea-water. The corrosion process was accelerated by means of anodic polarization. Ultrasound signals were continuously measured using two arrays of underwater AE transducers (in the frequency range between 50-450 kHz) placed on two perpendicular planes at a fixed distance from the chain links. Corrosion-induced AE sources were localized on the 3D geometry of the tested links. The variation and evolution of AE parameters extracted from the measured signals were analyzed as a function of testing time. The results of the accelerated corrosion experiment suggest that corrosion-induced ultrasound signals can be detected and monitored by means of non-contact AE. The results of this study may be used for characterization of corrosion-induced AE signals in mooring chain links.

Reliability‐based resilience assessment method for mooring systems under mooring failure

AbstractOffshore floating structures rely heavily on their mooring systems, which can be disrupted by various events during long‐term operation. These could lead to a mooring failure, affecting the usual operations of the structure or even causing more severe hazards. Resilience provides a comprehensive evaluation of how the mooring system performs after a disaster, which is key to optimizing the structural design and operational safety. In this paper, we develop a general and user‐friendly method to quantitatively assess the resilience of mooring systems under mooring failure. We use the reliability index to represent the performance of the mooring system. We then derive its RV, ACI, and RCI, which are based on a system performance curve and reliability analysis. We also consider the effects of climate change and the corrosion of the mooring chain. These factors can significantly affect environmental loads, structural performance, and the recovery process. Moreover, an illustrative example is provided that guides us through the methodology. The proposed method is applied to assess the resilience of a certain mooring system in the South China Sea over a 30‐year service life under different failure scenarios. Our results indicate that overlooking climate change in the design and operation of the mooring system can lead to a significant overestimation of its reliability index and resilience value.

Delayed fracture behavior of ultra-high-strength mooring chain steel evaluated by potentiostatic hydrogen-charging combined with SSRT

The influence of hydrogen charging potentials on the hydrogen embrittlement susceptibility of R6 ultra-high strength mooring chain steel was investigated via constant potential hydrogen charging slow strain rate tensile tests combined with thermal desorption analysis. The results reveal that hydrogen charging leads to a 38.94% decrease in elongation, while the impact on tensile strength is relatively minor. Furthermore, the specimens experienced intergranular cracking at the critical potential of −1150 mV, with the size of the brittle region increasing as the negative charging potential becomes more negative. And, hydrogen atoms can cause local embrittlement of materials and increase KAM value.

Assessing the costs and benefits of dynamically positioned floating wind turbines to enable expanded deployment

In addition to technical and economic challenges, the offshore wind industry must also contend with environmental and socio-political concerns that constrain development. Here, the viability of dynamically positioned floating offshore wind platforms that could ameliorate these constraints is analyzed. Such a design can unlock deeper, less-contentious waters that are farther from shore and reduce environmental interactions in the ocean, thus expanding and improving deployment opportunities. While prior studies on this application only focused on energy performance, the resulting cost of energy and benefits have not yet been quantified. Although prior studies found that the energy-performance of this strategy does not show promise, it is possible that a full understanding of the costs and benefits will reveal a decision space where it makes sense. To address this gap in the literature, the cost penalty of this application compared to moored turbines is estimated here. A physical engineering model is used to compute the environmental loads on a floating offshore wind turbine to estimate the expected energy needed by the station-keeping thrusters and one potential environmental benefit—eliminated mooring chains. The models were validated using marine engineering simulation software. Dynamic positioning realizes a cost penalty of at least 30 U.S. dollars/megawatt-hour while eliminating up to 900 km of mooring chains for a 1-gigawatt array. Up to 3 megawatt-hours of stored energy are needed to station-keep during continuous low winds. Furthermore, dynamic positioning is more viable for turbines of intermediate size (8–10 MW) rather than larger turbines (15 MW). The results show that aerodynamic loads on the turbine fall sharply beyond the rated wind speed, operational and design improvements may further improve the viability of this application, and the energy performance of this application continually improves with added thrusters although the cost of energy does not.

Effect of Sulphate-Reducing Bacteria Activity on the Performance of Thermally Sprayed Aluminium and Polyurethane Coatings

In the present work, we studied whether the exposure of synthetic seawater with anaerobic sulphate-reducing bacteria (SRB) on some steel samples generates a bacterial biofilm in their surfaces. Bare steel belonging to a mooring chain as well as two coating systems applied on the steel surface were studied: polyurethane (PU) and thermally sprayed aluminium (TSA) with and without an epoxy-based sealant. After 30 days of immersion in SRB-inoculated synthetic seawater, a bacterial count was attained, and the samples were observed using scanning electron microscopy (SEM) and locally analysed using X-ray scattered energy spectroscopy (EDS). A biofilm developed on every tested surface (continuous or in the form of pustules), with evidence of metabolic activity of the SRB. Finally, a mechanism of degradation for TSA in the presence of SRB is proposed for environments with a high concentration of bacteria.

Relationship between chain axial resistance and confining stress for South China Sea carbonate sand: an element test

For mooring systems, the relationship between chain axial resistance and confining stress affects the tension transfer of embedded mooring chain, and an effective width parameter Et was adopted to reflect the complex geometry of the chain. However, the relationship in carbonate sand is fully understood. Designed as an element test, monotonic and cyclic loading tests were conducted to investigate the variation of chain axial resistance with confining stress under different conditions in South China Sea carbonate sand. Peak effective width parameter and secant coefficient were particularly analyzed to describe the gradual mobilization. The results show that the Et value in South China Sea carbonate sand has a mean increase of 15% compared with Pingtan quartz sand, which makes the chain harder to pull into the carbonate sand. The irregular particle shape may contribute to the higher axial resistance. In cyclic tests, the peak post-cyclic Et-m is larger than that of monotonic test under lower confining stress but is smaller under higher stress. This paper provides some references to chain profile prediction in the carbonate sand.

An Out-of-Plane Bending Fatigue Assessment Approach for Offshore Mooring Chains Considering the Real-Time Updating of Interlink Bending Stiffness

Fatigue failure caused by frequent tension and bending loads is a crucial safety concern for mooring chains used on floating structures in the oil and gas industry. The bending effect for a chain’s fatigue is usually not considered by existing fatigue analysis methods, and even if it is considered, existing commercial software can only calculate the constant interlink bending stiffness without consideration of the stiffness variations due to tension and friction. To address this issue, this study develops an in-plane bending (IPB)/out-of-plane bending (OPB) fatigue assessment approach for offshore mooring chains considering the time-varying nonlinear interlink bending stiffness. Initially, the mooring tension and IPB/OPB angles are calculated by an in-house code MeCAP (multi-element coupled analysis program), considering the time-varying interlink stiffness between the chain links. The fatigue life of mooring chains for pure tension–tension (TT) and OPB combined fatigue at different hotspot locations (A, B, and C) is calculated by MeCAP-fatigue, a newly-developed module in MeCAP. Based on the analysis model, a comparative study is implemented to investigate the effects of interlink stiffness on fatigue damage. The differences and advantages of the combined fatigue calculation method over the pure TT fatigue assessment method are discussed. The results illustrate that combined fatigue would be underestimated significantly if zero interlink stiffness (hinge joint assumption) is applied or even constant interlink stiffness (calculated by mooring pretension), while pure TT fatigue will be largely unaffected. Moreover, compared to pure TT fatigue, the OPB combined fatigue assessment method, considering the effects of OPB/IPB and the time-varying properties of the connection between chain links, could evaluate fatigue life more comprehensively.

Bidirectional vibration control for fully-coupled floating offshore wind turbines

The vast and stable wind resources present in deep waters have made deep-sea floating wind power the mainstream trend for future development. However, the operating environment of offshore floating wind turbines is complex and variable. The joint action of wind and waves causes significant platform motion and turbine vibration, reducing power generation efficiency, causing structural damage, and shortening the service life of turbines. Hence, this study focuses on the NREL 5 MW DeepCwind semi-submersible wind turbine and establishes a fully-coupled floating wind turbine model utilizing the SIMPACK multi-body dynamics software. The model includes aero-, hydro-, flexible body structure, mooring, control, and transmission chain components. 3D pendulum tuned mass dampers (3D-PTMD) and bilinear tuned mass dampers (2TMDs) are designed to control the bidirectional vibration causes due to wind-wave misalignment in the platform motion and nacelle displacement of the floating wind turbine. The research results show that: For the roll and pitch of the platform, there is only one platform roll or pitch main frequency, and the amplitude at the main frequency can be effectively reduced by setting the frequency of 2TMDs near the main frequency, thereby suppressing the vibration of the platform; For the displacement of the nacelle, in addition to the main frequency of platform roll or pitch, there is also a secondary frequency of first-order bending of the tower, 2TMDs corresponding to the main frequency can effectively reduce the amplitude at the main frequency, and 3D-PTMD corresponding to the secondary frequency can effectively reduce the amplitude at the secondary frequency, it is better to configure 2TMDs alone than 3D-PTMD alone, and it is best to configure both at the same time.

Effect of boriding on tribocorrosion behaviour of HSLA offshore mooring chain steel

This study explores the transformative potential of boriding to enhance the tribocorrosion resistance of mooring chain steel for offshore environments. The boride layers formed at various temperatures (800 °C, 900 °C, and 1000 °C) for 1 h, revealing high hardness (17–21 GPa). X-ray diffraction confirmed dual-phase FeB and Fe2B borides, contributing to superior mechanical properties. Energy-dispersive X-ray spectroscopy showed clear phase boundaries. Potentiodynamic data demonstrated improved corrosion and tribocorrosion resistance, particularly at higher boriding temperatures. Mechanical effects played a pivotal role in tribocorrosion behaviour, emphasizing the need to optimize boriding process temperatures. Electrochemical impedance spectroscopy highlights the 900 °C-1 h sample as offering excellent corrosion resistance. This study underscores boriding's potential to enhance the tribocorrosion resistance of offshore mooring chain steel.