Mooring System Research Articles

Advances in Research for Mechanical Characteristics of Vertically Loaded Anchors for Offshore Platforms under Cyclic Loads

Vertically loaded anchors (VLAs) are widely used as mooring foundations in marine environments. Their working conditions typically involve deep-sea seabed, frequently subjected to cyclic loads induced by wind, waves, and currents. Therefore, understanding the mechanical properties of VLAs under cyclic loading is essential for ensuring the safety of mooring systems. This paper summarizes the current research status on the mechanical properties of VLAs under cyclic loading, analyzing the mechanisms by which cyclic loads affect these properties. Additionally, it reviews and summarizes the research methods applied to studying VLAs under cyclic loading, discussing the issues inherent in various methodologies. Finally, it provides an outlook on future research into VLAs under cyclic loading, laying the groundwork for subsequent studies on the bearing mechanisms of novel VLAs, such as the double-plate vertically loaded anchor (DVLA), under cyclic loading.

A theoretical framework for robust implementation of in situ measurements of ocean currents and waves in dynamics of mooring systems

We develop an approximated method to solve analytically the equations of motion that describe mooring line dynamics in a one-dimensional model. For the first time, we derive integral closed-form expressions to compute dynamic properties of mooring lines subject to ocean currents and waves of arbitrary time and spatial dependence, in terms of modified Bessel functions. This is done by decomposing the mooring line in three regions where different approximations and mathematical techniques of solution are carried out. Our analytical results provide a robust framework to simulate and analyze extreme realistic oceanic events when data from in situ ocean observation systems are available, regardless of the resolution or coarseness of subsurface measurements and even for long acquisition times. In order to prove the advantages of this approach, we have processed data from two stations in the National Data Buoy Center of the National Oceanic and Atmospheric Administration. From simulations with ocean currents data, we have gained insights into the coupling of the spatial modulation of ocean currents with the characteristic wavelengths of elastic lines. From simulations with ocean waves data, we have defined a scheme to analyze wave data and identify the contribution of each subset of frequency peaks to the net fluctuation of mooring line tension. This could be useful for classification of irregular waves based on their impact on mooring line tension. The development of better tools that integrate theoretical and experimental findings is necessary for the assessment of marine structures under the environmental conditions associated with climate change.

Offshore geotechnical challenges of the energy transition

Offshore wind is the most mature of the offshore renewable energy technologies and has a significant role to play in the energy transition. 2000 GW of offshore wind capacity is anticipated globally by 2050 in order meet the targets of the Paris Agreement; 35 times the current installed capacity. The pace and scale of offshore wind ambitions to support the energy transition present a range of challenges for the offshore geotechnical sector and the broader offshore wind sector. Challenges extend across the life-cycle of projects from marine spatial planning, site investigation, design, manufacturing, installation, operation and decommissioning, and across the supply chain regarding availability of raw materials for foundations, anchors and mooring systems, vessels and equipment for site investigation and installation, and trained geotechnical personnel. This paper identifies five key challenges and sets out the necessary shifts in technology, culture and practice in geotechnical engineering to achieve the ambitious targets to deliver offshore wind at the pace and scale required for the energy transition. The paper closes with a reflection on the consequence of delaying or not meeting net-zero targets, and thus identifying the urgency for these shifts in technology, culture and practice to be developed and adopted.

Insights into different marine aquaculture infrastructures from a life cycle perspective

Aquaculture facilities represent an often-neglected process in environmental impact studies. This study focus on the environmental impact assessment of alternative net materials in Mediterranean marine aquaculture. A Life Cycle Assessment was conducted using primary and secondary data from specific databases and literature. Three baseline scenarios were compared: copper alloy net cages with 100 % of recycled material (CAN100), 75 % of recycled material (CAN75), and polyethylene net (PEN) System boundaries include manufacturing and disposal of cages, nets, and mooring system. The use and emissions of antifouling paints and CAN were considered. Sensitivity analysis of the most impacting sub-processes and Uncertainty analysis were also conducted. The use of CAN is advantageous in terms of environmental impact, but only considering a complete recyclability of the net at the end of its service life. Moreover, when considering a reduced service life of the PEN due to the detrimental effect of biofouling, the advantage of the CAN is even more evident. To counteract the negative effect of biofouling, copper-based antifouling paints are generally used in marine aquaculture. These products are a main environmental hotspot in PEN systems. Therefore, a higher consumption of such products could determine an environmental burden shifting from CAN to PEN ones. So far, CAN are not widespread in the aquaculture industry, mainly due to the high cost of initial investment compared to traditional PEN. Considering operational and environmental advantages, CAN cages could represent an affordable and resilient solution for aquaculture enhancing environmental, economic, and social performances of this industry.

Coupled analysis of floating offshore wind turbines with new mooring systems by CFD method

This paper presents a fully coupled aero-hydro-mooring numerical model of the National Renewable Energy Laboratory's (NREL) 5 MW OC4 semi-submersible Floating Offshore Wind Turbine (FOWT). The model's accuracy was validated through comparisons with existing experimental and numerical data, utilizing OpenFOAM, a Computational Fluid Dynamics (CFD) software. The key contributions of this study include demonstrating the model's precision in predicting aerodynamic performance, motion responses, and mooring system dynamics under wind and wave conditions. Additionally, a comparative analysis is conducted between a Conventional mooring system (CMS) and a New mooring system (NMS). The results show that the NMS offers improved stability in surge and heave motions, crucial for operational effectiveness and resilience. Furthermore, the NMS demonstrates a more efficient stress distribution and also reduced tensions in the mooring lines, contributing to the long-term durability and safety of the structure. In terms of aerodynamic performance, the differences between the two mooring systems are limited due to the combined surge and pitch motion phase difference. Future research will focus on optimizing these phase differences to enhance performance and refine the cost-efficiency of the mooring system.

The impact mechanisms of intermediate water depth on the coupled dynamic responses of large-capacity floating wind turbines

ABSTRACT The currently planned floating wind farms in China mostly have water depths in the intermediate range. Therefore, investigating the impact mechanisms of intermediate water depths on the coupled dynamic responses of floating wind turbines is of significant importance. In comparison to deep water, mooring systems in intermediate water depths are more prone to experiencing catenary shape loss and stiffness. The contribution of difference-frequency effects increases with decreasing water depth, resulting in a more substantial impact on low-frequency responses. The Full Quadratic Transfer Function (QTF) method accurately captures low-frequency responses. Placing clump weights in the bottom section of the mooring line effectively restricts the displacement range of floating turbines while maintaining average fairlead tensions at levels similar to the other two forms. On this basis, focusing on the most optimal mooring system performance, experiments were conducted to validate the coupled dynamic responses of floating wind turbines in intermediate water depths.

Dynamic performance analysis of grid mooring system for gravity cages

ABSTRACT Gravity cage model is utilised to assess the influence of different mooring variables, (buoy buoyancy, frame line length and depth) on the dynamic performance and mooring tension of its grid mooring system. Numerical model's validity is confirmed using Morison equation and lumped mass method, followed by an analysis of how these variables impact cage motion and mooring tension under diverse environmental conditions. The findings indicate that enlarging the frame line length can equalise tension distribution within the mooring system and minimise peak tension. Optimum buoyancy is pivotal for maintaining equilibrium between maximum tension and cage displacement under wave-load conditions. When waves and currents work together, insufficient buoyancy and excessively deep frame lines may cause significant gaps between the cage and water surface, potentially leading to fish escape. Finally, further research comparing the tension distribution of each anchor line in a 2×1 cage group with the cages’ displacement.

Study on the mooring systems attaching clump weights and heavy chains for improving the typhoon resistance of floating offshore wind turbines

The southeastern coastal region of China is the most favorable area for offshore wind resources in the country, but typhoon impacts constrain the development of floating wind turbines in this region. Therefore, ensuring reliable and efficient mooring performance is crucial due to significant safety concerns presented by dynamic platform motions and mooring line tensions encountered during typhoon conditions. In this study, two mooring models were proposed to investigate the impact of clump weights or heavy chains on enhancing the typhoon resistance of floating offshore wind turbines. The 10-MW OO-Star semi-submersible platform was used as the reference structure. The aero-hydro-servo-elastic coupled simulations using OpenFAST were conducted for the selected environmental condition. The surge motion and fairlead tension were compared in both time-domain and frequency-domain for different mooring configurations. The results demonstrate that adding clump weights and heavy chains effectively limits the surge motion and tension fluctuations. Additionally, five installation positions were proposed and compared. Furthermore, a comparison between clump weights with heavy chains was conducted. The performance of a mooring system can be improved in the typhoon condition by incorporating optimal clump weights or heavy chains.

Numerical Study on Hydroelastic Responses of Submersible High-Density Polyethylene Circular Seaweed Platforms Held by Single-Point Mooring System and Buoys

Numerical Study on Hydroelastic Responses of Submersible High-Density Polyethylene Circular Seaweed Platforms Held by Single-Point Mooring System and Buoys

PVSails: Harnessing Innovation With Vertical Bifacial PV Modules in Floating Photovoltaic Systems

AbstractIn the context of offshore floating photovoltaic systems (FPVs), this paper explores the use of bifacial photovoltaic modules installed in the vertical position. The energy harvested from the rear face of vertically configured bifacial PV modules compensates for the reduced production at the front face of the module, and this demonstrates the potential of bifacial technology for offshore applications. By comparison, most existing horizontally tilted bifacial FPV systems gain only a small benefit in production from the rear face of the module due to the minimum radiation received, and what also must be taken into consideration is the negative effect of significant soiling owing to the low tilt angle of the PV modules. Hence, to overcome these drawbacks, we have developed the innovative “PVSail” concept, which explores the deployment of vertical FPV systems on floats, buoys, or poles/minipiles. Floating vertical bifacial PV systems (VBPVs) have huge potential to harness all the energy generation capabilities enhance by reflected light, especially from snow‐covered surfaces in northern regions. Our analysis considers a patented mooring and vertical PV system that allows the VBPV structure to align with the prevailing wind direction to shed wind loads, and our numerical analysis explores the potential of VBPV applied to Catania in Italy and Nigg Bay in the United Kingdom. Our analysis study has revealed that across an azimuth angle range (0°–180°), vertical bifacial modules experience roughly a 9% decrease in energy yield at Catania and about a 5% energy yield gain in higher latitude regions like Nigg Bay. Additionally, increasing the latitude of the installation location of VBPV reduces the energy yield sensitivity to the orientation, that is, azimuth angle. The PVSail concept opens the door to novel deployment possibilities in offshore renewable energy projects.

Seasonal variations in the contribution of zooplankton fecal pellets to the particulate organic carbon fluxes over the slopes of the Pacific Arctic region

As part of the Korea Arctic Mooring System (KAMS), sequential sediment traps were deployed at KAMS1 over the East Siberian Sea slope (∼115 and ∼335 m) and at KAMS2 over the Chukchi Sea slope (325 m) to collect sinking particles from August 2017 to August 2019. Fecal pellet carbon (FPC) fluxes and their contribution to the particulate organic carbon (POC) fluxes were measured to assess the role of zooplankton fecal pellets in the biological carbon pump at both sites. FPC fluxes increased at the onset of an under-ice bloom and peaked during the melt period at both sites in 2018. At KAMS1, a remarkable increase in FPC fluxes reflected the enhanced grazing of large copepods during the anomalously productive spring and summer of 2018, however their contributions to the POC fluxes mostly remained <10%. At KAMS2, relatively low FPC fluxes during the under-ice bloom suggested the export of a larger proportion of pellets produced by small copepods. Sustained FPC fluxes from January to May 2018 at KAMS2 contributed up to 24% of the POC fluxes, possibly resulting from pellet production by overwintering copepods grazing on particles laterally transported into the region in the presence of ice. These results indicate that despite their limited contribution to the POC fluxes, FPC fluxes varied with food availability, zooplankton community structure, and hydrographic conditions over the East Siberian and Chukchi Sea slopes.

Investigation of mooring breakage impact on dynamic responses of a 15 MW floating offshore wind turbine

The stability and integrity of the mooring system are some of critical factors affecting the safety and performance of floating offshore wind turbines (FOWTs). For this reason, it is necessary to investigate the dynamic responses of the rotor, platform, and the remaining cables of the FOWT subjected to mooring breakages. This is because a mooring breakage significantly increases the risk of damage to the FOWT, especially for a nonredundant mooring system. This study has analyzed the platform motions and mooring tension of a 15 MW FOWT, where each offset column connects to two and three mooring lines to enhance the redundancy of the mooring system. The fully coupled simulations of the FOWTs under mooring breakage scenarios are examined using the well-validated numerical framework, OpenF2A, to consider wind, wave and current loading combinations. The result reveals that the breakage of a single mooring has a minor impact on the aerodynamic performance and aeroelastic response of the FOWT for both mooring system configurations. Notably, the platform experiences significant surge and sway when the upwind mooring breaks, leading to a sharp increase in tension of the remaining mooring lines positioned in the same direction. Moreover, the occurrence of snap load events is another factor resulting in the abrupt increase in the mooring tension. However, the maximum tension in the remaining mooring lines has not exceeded the threshold of breaking stress for both redundant mooring systems. The mooring configuration with two catenary cables connected to each column is suggested for the station-keeping system of the 15 MW FOWT considering the dynamic behavior and manufacture cost.

Verification and validation of a coupled CFD–FEM approach with overset structured grid through free decay and regular wave tests

This paper presents a comprehensive verification and validation study applied to the hydrodynamic analysis of a semi-submersible Floating Offshore Wind Turbine (FOWT) platform. The numerical simulation is conducted using a coupled Computational Fluid Dynamics-Finite Element Method (CFD–FEM) approach. The CFD program incorporates an overset grid interpolation method, facilitating a more systematic grid refinement in the estimation of discretization uncertainties. A mooring analysis program, based on the FEM scheme, is integrated with the CFD program to simulate the dynamic responses of mooring system. The necessity of the nonlinear mooring model is demonstrated by comparing solutions with a comparative test using a linear mooring model. The verification study, based on a least-squares-based extrapolation method, quantifies the spatial and temporal discretization errors of the numerical model. The numerical solutions based on the baseline model are further validated against the model test measurements. A pitch free decay model test and a regular wave model test conducted within the OC5 project are utilized for the verification and validation study. Results from both tests demonstrate the reliability and accuracy of the coupled method.

Dynamic analysis of an improved shared moorings for floating offshore wind farm

Shared mooring lines have demonstrated their effectiveness in reducing the installation costs of floating offshore wind farms by minimizing the requirement for multiple anchors. However, this innovation has also brought complex restoring properties and a greater tendency towards resonance. To tackle this challenge, this research presents a novel approach for shared mooring systems, implemented in a 2 × 2 orthogonal wind farm arrangement featuring 5 MW wind turbines installed on the Barge platform. Through the application of FAST and AQWA, a comparative analysis is conducted juxtaposing the original mooring (OM) and three new mooring models, namely buoy mooring (BM), clump weight mooring (WM) and buoy clump weight mooring (BWM). A comprehensive examination is carried out to assess the impact of on platform and mooring characteristics. The results show that the BM can effectively reduce the platform's surge and sway response, and that the mooring tension and laid length decrease as the net buoyancy increases. The WM increases the response amplitude at the yaw direction. The BWM with link buoys helps to improve the dynamic response of the platform and increases the length of the mooring laid. This analysis provides valuable insights for the design of floating wind farms incorporating shared mooring systems.

Shared mooring system designs and cost estimates for wave energy arrays

For floating renewable energy devices to become more cost-efficient and commercially scalable, their mooring system designs must be low-cost and suited for large-scale installations. Large arrays of floating devices, such as wave energy converters (WECs), will likely be designed with an individual mooring system for each device in the array. However, new mooring technology advancements provide options to use shared mooring lines to connect adjacent devices to one another, reducing the total number of anchors in the array, thereby reducing material use and cost. This paper explores the design, modeling, and cost analysis of shared mooring systems for various sizes of arrays consisting of heaving oscillating water column (OWC) WECs. Shared mooring systems for WEC arrays sized in 2×N and N×N grid layouts are designed to meet the relevant design standards, checking their performance with a nonlinear time-domain dynamic simulation, and the costs of each are calculated and compared. Several assumptions are taken in the design process to produce efficient results, providing a preliminary optimization for guidance on design decisions rather than a full, detailed design analysis. Mooring system costs per WEC were found to decrease as the number of WECs in the array increase, up to certain array sizes. The 2 × 3 array had the lowest mooring system cost per WEC out of all arrays considered, with a 60% cost reduction relative to using individual mooring systems. The 3 × 3 and 4 × 4 arrays achieved a 50% cost per WEC reduction. In addition to these significant cost reductions, the shared mooring system designs can provide advantages through smaller mooring system footprints, lower installation times, and less seabed disturbance.

Hydrodynamic analysis of three-module semi-submersible platform and its mooring design

The floating platform consisting of three interconnected semi-submersible modules deployed in shallow water with severe environmental conditions will be subject to very high environmental loads during storms and its position keeping will become a huge challenge. Therefore, it is important to investigate the hydrodynamic characteristics of the platform and design a robust mooring system to meet different demands during the service period of the platform. In this study, the hydrodynamic and motion analyses of the three-module semi-submersible platform were conducted by using both numerical simulation and experimental tests. In addition, a distinctive X-shaped mooring system was designed, which is characterized by relatively low stiffness in the direction of beam sea in which the environmental loads are the highest, and better load sharing among different mooring lines so that the extreme tension is curbed. Model tests were performed in a wave basin to acquire the platform motions and mooring tensions, and comparisons were made between the measurements and numerical predictions. The results of the study verify the feasibility of the three-module connected platform and its mooring system.

Analysis of floating platform-mooring system-pile-soil interactions under wave loadings: A case study of the triangle-shaped semi-submersible platform

The floating platform-mooring system-pile-soil interactions under wave loadings represent a complex yet understudied aspect of floating offshore wind turbines (FOWTs). This paper presents the development and application of an Incompressible Material Point Method-Finite Element Method-Material Point Method (IMPM-FEM-MPM) model to simulate these interactions for a triangle-shaped semi-submersible platform (Tri-SSP), which serves as the foundation of China's first deep-sea FOWT, named ‘Fuyao.’ The proposed model is validated by studying the hydrodynamic responses of the OC4-DeepCwind platform. Under regular waves, increasing wave heights lead to heightened motion amplitudes, with the Tri-SSP exhibiting distinct phase differences between surge, pitch, and heave motions. Anchor Group 1 experiences significant peak tensions, ranging from 2.26 MN to 4.47 MN, which influence soil displacement, shear forces and bending moments along the pile shaft, emphasizing the impact of wave loading on pile-soil interactions. Under 1-h irregular waves, tension in Anchor Group 1 reaches extreme levels, with tensions up to 8 MN observed, underscoring the critical role of mooring systems in mitigating wave-induced forces. The calculated maximum stress within the pile indicates the structural integrity of the DH36 steel pile under these conditions. Comparisons of hydrodynamic responses, both with and without pile-soil interactions, show minimal discrepancies in motion amplitudes, indicating a minor influence on the dynamics of the platform and mooring system. These findings contribute to the understanding of complex interactions in deep-sea FOWTs, informing design and operational strategies for enhanced stability and performance.

Influence of Grease Contamination on Acoustic Emission Monitoring of Low-speed Roller Bearings

Large-scale low-speed rolling element bearings form crucial connections in offshore installations such as heavy-lifting cranes, single point mooring systems, and wind turbines. Due to the stochastic nature of wind and waves, the applied loads on these bearings are hardly predictable. Furthermore, the remote nature of the offshore environment requires a high standard of operational safety and reliability, reinforcing the need for advanced inspection and monitoring methods. Acoustic emission (AE) is a continuous monitoring technique for condition assessment of low-speed bearings, as it may detect the stress waves associated with developing degradation within the rolling elements. This paper presents an investigation on the influence of grease contamination on acoustic emission (AE) monitoring of low-speed bearings. It discusses several experiments that have been conducted in various regimes of particle contaminated lubrication, and proposes a strategy for condition monitoring. The presented experiments have been conducted with differing bearing geometries in multiple testing environments. The contamination particles have been both naturally developed and artificially introduced. The results total of 9 experimental cases are discussed and compared. All experiments follow the same data-acquisition principles, utilising arrays of three AE transducers sensitive in a range between 40-580 kHz. Data is recorded while operating the bearing at low speed under load. Abrasive wear of the rolling elements and crushing of the particles are concluded to be the main sources of AE due to particle contamination. Processing consists of waveform cross-correlation clustering and feature analysis. The results suggest that hard abrasive particles make significant contribution to the AE signals, which is associated with abrasive wear of the rollers and raceways. Comparisons with the contribution of softer and finer particles are presented as well.

Experimental modelling of a novel concrete-based 15-MW spar wind turbine

Physical model testing is a crucial step in the validation process of floating offshore wind concepts that can be used to contribute to the worldwide decarbonization objectives. It is also important to understand the coupled performance of all the sub-systems that conform to a floating wind concept: wind turbines, mooring systems or floating platform hydrodynamic responses. Few experimental tests had been conducted, and no real-time hybrid modelling had been performed in these tests. In this paper, an open-source experimental dataset is presented, and the dynamics of a concrete spar-based floating concept withstanding the IEA 15-MW Reference Wind Turbine are investigated. An experiment on a 1:55 scale concrete-based spar (WindCrete) at IHCantabria's ocean basin was developed by means of the hardware-in-the-loop technique under the COREWIND EU-H2020 project. The available data consist of the hydrodynamic characterizations and seakeeping tests with a total of 57 available datasets. The proper behaviour of the WindCrete platform and the designed mooring system is confirmed by the maximum motion values and accelerations below the established acceptance criteria defined. Moreover, this test program identifies the importance of the wind loads over the dynamic performance of the concept.

Generalized Quasi-Static Mooring System Modeling with Analytic Jacobians

This paper presents a generalized and efficient method for quasi-static analysis of mooring systems, including complex scenarios such as when shared mooring lines interconnect multiple floating wind or wave energy devices. While quasi-static mooring models are well established, most published formulations are focused on specific applications, and no publicly available implementations provide efficient handling of large mooring system networks. The present formulation addresses these gaps by: (1) formulating solutions for edge cases not typically supported by quasi-static models; (2) creating a fully generalized model structure such that any combination of mooring lines, point masses, and floating bodies can be assembled; and (3) deriving analytic expressions for the system Jacobians (stiffness matrices) so that systems with many degrees of freedom can be solved efficiently. These techniques form the theory basis of MoorPy, an open-source mooring analysis library. The model is demonstrated on nine scenarios of increasing complexity with features of interest for offshore renewable energy applications. When compared with steady-state results from a lumped-mass dynamic model, the results show that the quasi-static formulation accurately calculates profiles and tensions and that its analytic approach provides more efficient and reliable computation of system stiffness matrices than finite-differencing methods. These results verify the accuracy of the MoorPy model.