Mooring System Research Articles

A passively self-adjusting floating wind farm layout to increase the energy production: a sensitivity analysis

Offshore wind farm layout design techniques and methodologies currently focus on fixed bottom offshore wind turbines. As more floating wind farm (FWFs) are planned, new methodologies for FWF layout design and optimization are required to consider the different attributes between fixed bottom and floating offshore wind turbines (FOWTs). One main difference is the ability of FOWTs to move in the horizontal plane (surge and sway motions). In our work in [1], we showed that the motions of the FOWTs in the horizontal plane represent an opportunity which can be used to increase the FWF’s annual energy production (AEP). We can passively control the motions of the FOWTs in a FWF according to the incoming wind direction, moving the downwind turbines out of the wake of upwind ones. Since the horizontal motions of a FOWT are governed by the mooring system design (MSD) attached to it, this passive control can be done by designing a customized mooring system (MS) for every FOWT in the farm. In this work we build on our work in [1], as we carry out a sensitivity analysis to check the effect of the farm size and the wind rose on the methodology we introduced. The results show that the percentage increase of energy production due to passively relocating the FOWT is sensitive to the FWF size. Very small FWFs will gain less energy by relocating the FOWTs. Moreover, we show that even with a more uni-directional wind rose relocating the FOWTs in a FWF remains beneficial.

Rational Simplification of High Fidelity Wind Turbine Models Used for Mooring Analysis

This study investigates the use of simplified turbine modelling in analysis of a soft mooring system (horizontal plane periods > 250 s). Two simplified turbine models are developed, tested and compared to a high fidelity turbine model in a case study of the INO WINDMOOR 12 MW FOWT with a soft mooring system. The chosen simplified modelling strategies are a drag disc model, a de-coupled approach where the aerodynamic loads are pre-calculated in a fixed turbine simulation and a model with reduced degrees-of-freedom. Corrections to the pre-calculated force model is also developed and compared. The results show that the pre-calculated force approach gives better comparisons to the full model than the drag disc model, and that applying a controller correction to account for the wind turbine controller’s response to floater motions improves the predicted response.

Research on movement law of the deployment process for submerged mooring system

Safe deployment is the premise of submerged mooring system before it is put into use. In this paper, on the basis of analyzing the deployment process of the full ocean depth submerged mooring system, a calculation model of the laying process is established, and the influence law of the laying movement of the submerged mooring system, under two working conditions of deployment in the deep sea and deployment in the shallow sea, is analyzed and studied. The relevant results have certain guiding significance for the design and engineering application of the submerged mooring system.

Data-driven modeling of hydrodynamic loading using NARX and harmonic probing

Optimizing floating wind turbines and their mooring systems requires validated computational models that predict wave-frequency and low-frequency hydrodynamic loads. Low-frequency loads are crucial for determining extreme offsets and tension in mooring lines and are generally described by a quadratic transfer function. The quadratic transfer function, obtained with numerical tools, accurately predicts low-frequency loads in mild sea states. However, since the existing numerical methods are based on potential and perturbation theory, they generally fail to accurately predict low-frequency loads in moderate-to-extreme sea states where current, viscous, and beyond-second-order potential effects become significant. Developing a procedure for empirical transfer function estimation is, therefore, necessary to overcome these limitations. This paper describes an existing framework for estimating any higher-order transfer functions from experimental data. The framework employs a nonlinear auto-regressive model based on Kriging to establish a causal input/output relationship between the wave-elevation and hydrodynamic force histories exerted on the floater. Then, higher-order transfer functions are extracted using harmonic probing. The procedure was validated by estimating the linear surge transfer function of the INO WINDMOOR 12 MW floater using synthetic data. The data-driven results showed an excellent agreement with the theoretically computed transfer function.

Second-order wave loads on floating wind parks with shared mooring

Wave loads on lattices of marine structures, such as wind parks with shared mooring systems, are considered. Modal excitation induced by second-order low-frequency wave loads is derived, leading to modal quadratic transfer functions. The consequence for the modal loads, of applying Newman’s approximation is investigated.

Towards fully autonomous floating offshore wind farm operation & maintenance

Wind energy is one of the most versatile and promising sustainable energy solutions. Wind energy can be harvested in both onshore and offshore environments. Due to the environmental and societal impacts of onshore wind farms, the focus on developing offshore wind farms has been steadily increasing. For deep-water areas, floating wind turbine solutions have been developed in the past two decades. Each wind turbine is mounted on a floating structure and connected to a mooring system. While the floating wind turbine technology enables electricity generation in water depths where fixed-foundation turbines are not feasible, operation and maintenance (O&M) have become serious issues. This paper investigates the challenges and potential enabling technologies for the development of autonomous floating offshore wind farms. In particular, the paper explores the potential utilization of information and communication technology (ICT) and robotics toward fully autonomous floating offshore wind farm O&M, aiming for cost reduction and improved operational safety. The presented solutions cover fundamental aspects in floating platform design, remote operation in the form of digital twins, autonomous underwater robots and surface vehicles, and eco-friendly energy storage.

Interaction between regular waves and floating breakwater with protruding plates: Laboratory experiments and SPH simulations

Overseas construction often adopts floating breakwaters as working platforms or wave attenuation. Laboratory and numerical investigations of the hydrodynamics during the interaction between regular waves and floating breakwater with protruding plates were conducted. By coupling DualSPHysics and MoorDyn, the motion of the floating structure under the mooring system is realized. The accuracy of the coupling model is verified by comparing the free surface elevation and floating motion. The effects of protruding types and plate length on the performance of the floating breakwater are analyzed. The results show that the reflection and dissipation coefficients increase with the increase of plate length. Under medium-long period waves, the wave attenuation of floating breakwater with protruding plates is weak. Horizontal protruding plates are more capable of restraining sway than vertical plates. Under the large draft, the horizontal protruding plates are more effective in suppressing the roll.

Investigation of Low-Frequency Pitch Motion Characteristics for KRISO Standard Offshore Structure (K-Semi) Moored with a Truncation Mooring System

In this study, a model test was carried out to evaluate the station-keeping performance of Korea Research Institute of the Ships and Ocean Engineering (KRISO) standard semi-submersible, K-Semi in the Deep Ocean Engineering Basin (DOEB). The test was performed using a 1/50 scaled model with 12 mooring lines. The water depth was set to 3.2 m using a moveable bottom structure and truncated mooring lines. The dynamic behavior of the K-Semi was examined using a free decay test and regular wave as well as irregular wave tests of 1800. Large surge, heave and pitch due to drift motion from second-order effect were observed. The reason for the excessive low-frequency pitch motion is attributed to the restoring force and moment related to the mooring system as excessive surge occurred. The numerical model of K-Semi with the truncated mooring system was tuned and calibrated using model test results such as free decay and regular wave tests. Through numerical analysis, the motion characteristics of the K-Semi were compared with irregular model test results. In the case of a semi-submersible using a mooring system, it is observed that excessive pitch motion due to vertical restoring force and moment may occur when large horizontal displacement occurs. The effect of low-frequency pitch motion related to surge motion is an important factor in upwell estimation as well as second-order motion in referred DNV-GL OTG13.

Diagnosis of the health status of mooring systems for floating offshore wind turbines using autoencoders

Floating offshore wind turbines (FOWTs) show promise in terms of energy production, availability, and sustainability, but remain unprofitable due to high maintenance costs. This work proposes a deep learning algorithm to detect mooring line degradation and failure by monitoring the dynamic response of the publicly available DeepCWind OC4 semi-submersible platform. This study implements an autoencoder capable of predicting multiple forms of damage occurring at once, with various levels of severity. Given the scarcity of real data, simulations performed in OpenFAST, recreating both healthy and damaged mooring systems, are used to train and validate the algorithm. The novelty of the proposed approach consists of using a set of key statistical metrics describing the platform’s displacements and rotations as input layer for the autoencoder. The statistics of the responses are calculated at 33-minute-long sea states under a broad spectrum of metocean and wind conditions. An autoencoder is trained using these parameters to discover that the proposed algorithm is capable of detecting mild anomalies caused by biofouling and anchor displacements, with correlation coefficients up to 98.51% and 99.16%, respectively. These results are encouraging for the continuous health monitoring of FOWT mooring systems using easily measurable quantities to plan preventive maintenance actions adequately.

SeaFisher—A Submersible High-Density Polyethylene Offshore Fish Pen

Moving offshore for fish farming poses challenges due to the more energetic sea environment. In this paper, a novel offshore fish pen design named SeaFisher has been proposed. The SeaFisher comprises modular cubic pens that are assembled to form a large 2 × n array offshore fish pen. Its frame structure is made from HDPE, making it flexible and durable against the harsh sea environment. Specially tailored connection brackets and connector pods are designed to assemble bundles of HDPE pipes forming the SeaFisher structure. The SeaFisher is moored using a single point mooring to minimize environmental and collision loads, and for improved waste dispersal. More importantly, the SeaFisher possesses ballast tubes positioned on the top surface to allow it to submerge to a desired water depth to dodge the strong surface waves during severe weather events. This paper presents the engineering design details and hydroelastic analysis of the SeaFisher. Based on a hydrostatic analysis, suitable materials were chosen for the various components of the SeaFisher, and the components were appropriately sized up. By using the software AquaSim v.2.17.3, the SeaFisher’s hydroelastic responses under different sea-state conditions were investigated. It is found that the designed SeaFisher structure and mooring system are adequate with respect to strength and stiffness for the considered sea-state conditions of up to 8 m significant wave height and 0.8 m/s current speed. It is expected that the SeaFisher will be a game changer for offshore fish farming due to its cost-effectiveness and ability to survive in severe storms.

Coupled analysis between catenary mooring and VLFS with structural hydroelasticity in waves

The rapid growth of marine renewables has led to the development of very large floating structures (VLFS) that are designed to operate in deep seas. It is significant to understand the mechanism of the coupled effects between deformable VLFS and catenary mooring system. This paper presents a time-domain hydro-elastic-moored model developed by integrating a quasi-static mooring module into a fully coupled Computational Fluid Dynamics (CFD) - discrete-module-beam (DMB) approach. The model is used to investigate the coupled effects between structural hydroelasticity and loose-type mooring systems on a deformable VLFS in waves. The mooring and hydroelasticity codes are validated separately and show favourable agreement with other numerical and experimental results. Then the coupled effects between the mooring system and structural hydroelasticity are evaluated by assigning various design parameters, i.e., VLFS structural stiffness and mooring stiffness. The numerical results, including dynamic motions, longitudinal vertical bending moments (VBMs) and mooring tension forces are presented and analysed. These results can be used to design a VLFS with mooring in medium-deep sea, and help with the conventional mooring design for a less-stiffness VLFS due to hydroelastic response.

Control synthesis via Impedance-Matching in panchromatic conditions: a generalised framework for moored systems

This study focuses on addressing the challenge of integrating the tangled mathematical model of the mooring system into an effective control synthesis. The presented synthesis framework utilises the impedance-matching technique to achieve the desired controller performance by adapting the control parameters to align with the dynamic characteristics of the moored wave energy device. By leveraging this technique, the simulation framework provides a means to effectively incorporate the intricate mooring dynamics into the control synthesis process. Furthermore, this paper aims to delve into the concept of defining a representative control action by examining the input-exciting force of the feedback-controlled system. Through a straightforward case study, the authors demonstrate the significant impact of the mooring on the system dynamics and underscore the applicability of the proposed simulation framework. Moreover, the paper verifies the importance of considering the controlled system’s exciting input when addressing control synthesis, particularly in panchromatic conditions.

Sensitivity analysis of the cage volume and mooring forces for a gravity cage subjected to current and waves

A numerical model is developed to calculate the mooring forces and cage volumes for a gravity cage system. The collar, nets, bottom ring and mooring system are all modelled by line elements due to the slender structure. The effect of structural parameters on the cage capacity and mooring forces is evaluated in a sensitivity analysis, to provide knowledge in the design phase of a fish cage. Heaver sinkers and nets with smaller solidity are preferred to prevent the volume reduction of fish cages but they lead to a larger mooring force or an increase in the probability of fish escaping. Linear wave theory is applied. Short waves increase the mean value of the mooring force and higher waves result in a larger amplitude of the mooring force. It is also found that short waves can change the shape of the nets while the shape of the cage subjected to long waves almost keeps constant as the one under uniform flow, which obtains a good agreement as previous experiments.

Dynamic Responses of a 1:5-Scale Ocean Current Energy Converter

The objective of this study is to numerically evaluate the dynamic responses of the Floating Kuroshio Turbine (FKT)—an ocean current energy converter—installed for field testing. In autumn 2022, the FKT was installed and tested at a 1:5 scale in the open sea area, located 1 kilometre off the LiuQiu Island coast, Taiwan. Because of unfavourable site conditions, the mooring and power cable systems for the FKT had to be customarily designed for the testing conditions. An investigation of the dynamic motions and structural responses of the mooring and cable systems was performed prior to physical testing to ensure the integrity of the testing campaign. The numerical analyses were performed using the commercial software DNV SIMA, where the prospect of the surfaced FKT and the mooring entanglement were evaluated and compared. This paper details the modelling process and the evaluation of the dynamics of the entire FKT system. Finally, the challenges and potential improvements for numerical assessments of a testing-oriented offshore marine energy system are identified and discussed.

SIMULATIONS OF EXTREME WAVE LOAD ON AN OSCILLATING WATER COLUMN WAVE ENERGY CONVERTER

Extreme load analysis is an essential step in the design of structural and mooring systems for wave energy converters (WEC). The current study aims to evaluate the structural loads on the MARMOK Oscillating Water Column (OWC) WEC under both regular and irregular wave conditions, as well as to evaluate the effectiveness of its station keeping (mooring) systems. The project employs the open source computational fluid dynamics simulation software OpenFOAM to model the fully moored floating system. The WEC device hydrodynamics are validated against laboratory data for free decay in heave and pitch motions. In addition, the station keeping model is validated against mooring tension static offset tests and benchmarked against experimental data in irregular waves. An elegant method of numerically recreating irregular wave inflow conditions from empirical measurements is shown which allows for consistent comparison between the model and experimental results. The calibrated numerical model is then employed to study the full-scale system’s responses. Preliminary results of structural loadings on the fixed and floating spar buoy (with one mooring configuration as sample) are presented in the paper.

Hydrodynamic studies of a 15 MW semi-submersible FOWT to assess the suitability of the inclusion of a damper system

Floating Offshore Wind Turbines (FOWT) can exploit the high energy density found in the offshore environment, with turbines now reaching up to 15 MW in size. At the same time, however, the energetic environment and the massive size of the device present significant challenges in the motion stabilization and mooring system. To overcome these challenges, a tuned mass damper (TMD) has been considered for integration in the FOWT for peak motion reduction. This paper investigates the baseline responses including motion, dynamic response, and tensile loading of the mooring line for a 15MW FOWT on a semi-submersible platform without TMD to identify the damageable motion and the impacts of the TMD on the motion response under wave-wind environmental loadings. The comprehensive analysis is conducted in a package for the dynamic analysis of offshore marine systems, named as Orcaflex. The dynamic and motion characteristics of the 15MW FOWT are analysed and compared under different environmental parameters. The wave and wind parameters are quantified by the 20-years statistical data of the Celtic Sea including both operational and extreme conditions (with a 50-year return period). Subsequently, the key parameters of TMD are investigated by configuring different combinations of mass, damping coefficients and stiffnesses. The preliminary results of the study show that the TMD system can successfully mitigate extreme motion characteristics, however this is strongly dependent on damping properties. Unsuitable TMD designs may increase the motion responses of FOWT and the tensile loading on the mooring line. Therefore, the TMD properties have to be adjusted based onsite environmental conditions). With this consideration, an active TMD with changeable damping properties will be conducted in future research.

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.

Review of TEAMER Awards for WEC-Sim Support

Testing Expertise and Access for Marine Energy Research (TEAMER) is a U.S. Department of Energy Water Power Technologies Office sponsored program, overseen by the Pacific Ocean Energy Trust, which aims to advance the state of marine energy technologies. The program connects technology developers with experts at U.S. facilities, including numerical modeling and analysis facilities, like WEC-Sim. The WEC-Sim facility is supported by the WEC-Sim development team at Sandia National Laboratories and the National Renewable Energy Laboratory. WEC-Sim (Wave Energy Converter SIMulator) is an open-source software for simulating wave energy converters. WEC-Sim can model the multi-body dynamics of devices comprised of bodies, joints, power take-off systems, and mooring systems. Since TEAMER’s first round of support in 2020, there have been eighteen TEAMER awards focused on numerical model development in WEC-Sim. TEAMER awards for WEC-Sim support have modeled a wide range of wave energy converter archetypes, including point absorbers, attenuators, oscillating water columns, and many other novel architectures. A wide variety of studies have been conducted, leading to important insights for TEAMER partners and software improvements for WECSim. This article highlights several successful WEC-Sim TEAMER awards. The awards described herein include TEAMER recipients Ocean Motion Technologies, AquaHarmonics, iProTech, East Carolina University, Virginia Tech, Maiden Wave Energy, and the University of Massachusetts Dartmouth. The awards of these seven partners contain a wide range of investigations and cover the creation of baseline hydrodynamic models, PTO modeling, geometry optimization in both boundary element methods and WECSim, and model tuning and validation.

Dynamic response of floating offshore renewable energy devices: Sensitivity to mooring rope stiffness

The offshore renewable energy sector has seen a rise in floating devices, all of which require mooring and anchoring systems. Synthetic ropes have emerged as a promising technology for cost reduction in this system. However, characterising the behaviour of these materials, which exhibit complex non-linear, visco- elastic and plastic structural properties, presents challenges. Numerical modelling and tank testing are the available tools for developers to overcome these challenges, however, there is a lack of guidelines for test facilities regarding the design of tank-scale mooring systems. The present work focuses on the numerical design of a typical semi-taut mooring system using synthetic materials suitable for future-generation floating offshore wind turbines. A coupled time-domain hydrodynamic model was employed to explore the dynamic sensitivity of the device to changes in mooring rope stiffness. The results demonstrate that changes in line axial stiffness have a greater impact on platform surge and mooring line tension than on heave and pitch responses. These findings establish preliminary margins for target stiffness values, which are valuable for selecting mooring materials for scaled tank test models. Although the case study was floating wind, the results have broader applicability to wider floating marine energy device design.

Experimental Study of the Nonlinear Behaviour of Deep-Sea Mooring Polyester Fibre Ropes

Abstract Mooring ropes are essential components of ships and offshore floating structures and they are subjected to cyclic axial loads. This study investigates the evolution of the full-cycle stiffness of fibre polyester ropes under long-term static and dynamic loading. First, the static stiffness characteristics of the ropes, including the rope elongation properties at different stages, shrinkage rates, and creep coefficients after an idle period, are examined under static loads; an empirical formula for static stiffness is established. Second, the dynamic stiffness characteristics of the ropes are investigated under cyclic loads that are typical of platform production operations. The stabilities of the structure under different tensions are compared; the effects of mean tension, tension amplitude, and load cycle on the dynamic stiffness of the ropes are analysed and an empirical formula is established to predict the dynamic stiffness during the engineering design phase. The results of this study can be helpful for the rational design of deep-sea taut-leg mooring systems because they present the evolution of the full-cycle stiffness characteristics of mooring ropes.