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As the floating offshore wind industry matures, it has become increasingly important for researchers to determine the next-generation materials and processes that will allow platforms to be deployed in intermediate (50–85 m) water depths, which challenge the efficiency of traditional catenary chain mooring systems and fixed-bottom jacket structures. One such technology, synthetic ropes, has in recent years come to the forefront of this effort. A significant challenge of designing synthetic rope moorings is capturing the complex physics of the materials, which exhibit viscoelastic and nonlinear elastic properties. Currently, numerical tools for modeling the dynamic behavior of floating offshore wind turbines (FOWTs) are limited to mooring materials that lack these strain rate-dependent properties and have a linear tension–strain response. In this article, a mooring modeling module, moordyn, which operates within the popular FOWT design and analysis program, openfast, was modified to allow for nonlinear elastic mooring materials to address one of these shortcomings in the numerical tools. Simulations from the modified openfast tool were then compared with 1:52-scale test data for a 6 MW FOWT semi-submersible platform in 55 m of water subjected to representative design load cases. A strong correlation between the simulations and test data was observed.

As the wind industry develops larger turbines for offshore deployment, the problems with station keeping systems are exacerbated. As turbines increase in size, so do the loads on the turbine. Meanwhile, many offshore sites available for leasing occur in intermediate water depths (55–85 m), which will appear ever smaller relative to the increasing platform size of floating offshore wind turbines. This complicates the process of designing mooring systems for these larger systems and emphasizes the importance of having a good methodology for automating this process. In this paper, a routine is developed that will map objectives for a multi-objective genetic algorithm to obtain mooring radius-lowest cost designs over a range of radii simultaneously. This work will implement and expand on a tiered-constraint evaluation scheme that was developed in the previous work by West et al. [Modelling 2, 728–752 (2021)]. New components and constraints are added to the optimization problem to allow the optimizer to find realistically deployable designs with reasonably accurate cost estimates. These techniques will then be used to find the most economic mooring designs for a 15-MW floating offshore wind turbine with a hybrid mooring system.

Innovative floating offshore wind turbine (FOWT) platforms that deviate from the conventional semi-submersible, spar, and tension leg platforms (TLP) have become increasingly common due to the need to tap into the high wind energy potential located in deeper waters. One example is the hanging-mass concept, in which a suspended counterweight stabilizes a positively buoyant floater. This work presents a two-dimensional, nonlinear, multi-body model used to assess the influence of the counterweight mass and the suspension line stiffness on the system’s global performance, using linear stability analysis and time-domain simulations to conduct a parametric study. For example, the counterweight mass has a strong influence on the amplitude of rotational degrees-of-freedom. Corresponding natural periods may occur within the linear wave energy range for suitable counterweight sizes due to this strong influence leading to undesirable motions. High-frequency multi-body modes are also dependent on both the line stiffness and counterweight mass, which may result in high relative motion amplitudes and slack lines in certain conditions. Finally, the parametric study results contribute to preliminary hanging-mass FOWT design recommendations.

Floating offshore wind turbine (FOWT) hull technologies are evolving rapidly with many technically viable designs. However, a commercially dominant architecture has yet to emerge. Early hull designs including semisubmersible, spar, and tension leg platforms were largely derived from offshore oil and gas technologies, but recent developments in the commercial application and optimization of FOWTs have resulted in a number of unique, FOWT-specific hull configurations. One hull design of interest includes the application of a moonpool to aid in mitigating platform motion in the presence of waves. A version of this annular hull has been deployed in France and Japan. In this paper, a 6-MW version of an annular hull is studied through experimental model testing and numerical analysis. The primary portion of this work involves testing a 1/100th-scale model in the Harold Alfond Wind Wave Ocean Engineering Laboratory at the University of Maine. A secondary component of this work investigates the capability of ANSYS aqwa, a typical commercial hydrodynamic software, to recreate the wave-induced motion of a FOWT hull containing a moonpool. An additional secondary component of this study compares the wave-only performance of the annular hull to experimental data obtained for the DeepCwind semisubmersible, spar, and tension leg platform to provide context for the measured response. The results obtained show that ANSYS aqwa can adequately predict the gross response of the annular hull motion and that the moonpool design tested often exhibits greater motion than the systems tested during the DeepCwind campaign.

As part of the process of deploying new floating offshore wind turbines, scale model testing is carried out to de-risk and verify the design of novel foundation concepts. This paper describes the testing of a 1:43 Froude-scaled model of the TetraSpar Demo floating wind turbine prototype that shall be installed at the Metcentre test facility, Norway. The TetraSpar floating foundation concept consists of a floater tetrahedral structure comprising of braces connected together through pinned connections, and a triangular keel structure suspended below the floater by six suspension lines. A description of the experimental setup and program at the Alfond W2 Ocean Engineering Lab at University of Maine is given. The objective of the test campaign was to validate the initial design, and contribute to the development of the final demonstrator design and numerical models. The nonlinear hydrodynamic characteristics of the design are illustrated experimentally and the keel suspension system is shown to satisfy design criteria.

The design of a 1/15th geometrically scaled wave tank model of the Azura™ commercial-scale wave energy device is presented. The objectives of the wave tank tests, conducted at the University of Maine Harlod Alfond Wind/Wave Ocean Engineering Lab (W2), included verification of the Azura’s energy capture in irregular waves, evaluation of performance in survival wave conditions, and testing of two advanced control algorithms. Due to the difficulty in properly Froude Scaling a hydraulic system, the model used a direct-drive rotary motor/generator power takeoff (PTO), with the dynamics of the hydraulic PTO included via a hardware-in-the-loop simulation. This PTO implementation led to additional design requirements being imposed on the model drivetrain. In addition to the model PTO design, the instrumentation design, structural design, and test plans are presented. The resulting model and PTO achieved a high level of controllability, and accurately emulated the dynamics of the hydraulic PTO of the full-scale Azura prototype.

Ocean engineers are often tasked with developing innovative, low-cost floating offshore wind turbines as the industry moves toward deeper waters to access regions with higher offshore wind potential. One example is the hanging-mass concept, in which a suspended counterweight stabilizes a positively buoyant floater; a hybrid design that combines a spar's performance with the maneuverability of a semi-submersible. Simultaneously, using elastic cables instead of a rigid support structure to support a low center of mass presents the opportunity for cost savings. This work presents experimental results from tests conducted on a 1/48-scale model of a conceptual hanging-mass floating offshore wind turbine design. Compared with traditional platform designs, the hanging-mass's response amplitude operators (RAOs) show a generally better response in heave and pitch, as the natural periods occur well outside the wave energy range, unlike the other platforms examined. Comparison between the line tension RAOs for systems with different suspension line stiffness indicate that softer systems experience larger tension fluctuations at low periods while the inverse is true for stiff systems. Finally, response spectra show that high-frequency modes describing relative motion between the two bodies are excited by irregular waves, likely due to sum-frequency second-order effects.

As the offshore wind industry moves towards deeper waters, developers are adapting classical floating platform designs to optimize performance and cost. One such concept is a multi-body system consisting of a counterweight suspended from a buoyant hull using flexible cables. If the suspension lines remain in tension, the system may be assumed to behave as a rigid body. However, if a suspension line loses tension, relative motion between the bodies will occur which may destabilize the system. In this work, the hanging-mass floating offshore wind turbine is idealized as a forced spring pendulum. A parametric study is conducted by varying the line stiffness and counterweight mass to determine their effect on the dynamic line tension and global response of the system.

Floating offshore wind turbines (FOWTs) over the past decade have been targeted as a solution to reducing dependence on fossil fuel. At this point the hulls of FOWTs have been a huge point of emphasis for the research community. FOWT mooring systems, however, have recently started to garner more attention. One solution that has gained traction to reduce cost is mooring the turbine with a synthetic mooring system as opposed to the traditional chain catenary system. Currently there is guidance provided for designing synthetic mooring systems, but it is more geared to the needs of the offshore oil and gas industry, and often leads to conservative designs. This work investigates the stiffness models recommended by the design guides and their influence on FOWT global response.

This paper presents wind speed measurements collected at 40m to 200m above sea-level to support the New England Aqua Ventus I 12 MW Floating Offshore Wind Farm to be located 17km offshore the Northeast United States. The high-altitude wind speed data are unique and represent some of the first measurements made offshore in this part of the country which is actively being developed for offshore wind. Multiple LiDAR measurements were made using a DeepCLiDAR floating buoy and LiDARs located on land on a nearby island. The LiDARs compared favorably thereby confirming the LiDAR buoy measurements. Wind speed shear profiles are presented. The measurements are compared against industry standard mesoscale model outputs and offshore design codes including the American Bureau of Shipping, American Petroleum Institute, and DNV-GL guides. Significant variation in the vertical wind speed profile occurs throughout the year. This variation is not currently addressed in offshore wind design standards which typically recommend the use of only a few values for wind shear in operational and extreme conditions. The mean wind shears recorded were also higher than industry recommended values. Additionally, turbulence measurements made from the LiDAR, although not widely accepted in the scientific community, are presented and compared against industry guidelines.

A variable-scale model wind turbine has been developed by the Advanced Structures and Composites Center for testing scale-model floating offshore wind turbines. This model has been designed to be lightweight with a robust individual blade pitch mechanism. Froude number similitude is used to develop scaling relationships, while specialized blades have been designed to produce representative aerodynamic forces despite mismatched Reynolds numbers. Numerical simulations show that the model turbine is able to match the scaled aerodynamic thrust of commercial wind turbines by altering blade pitch and maintaining Froude number and tip-speed-ratio similitude. This turbine has the capability to accurately simulate commercial turbines of varying sizes in complex loading conditions with the additional capability to implement and test new control algorithms.

Design parameters needed for the development of Maine׳s offshore wind resource are calculated using Gulf of Maine buoy data. Extreme values of the significant wave height and mean associated peak period, eight minute average wind speed, and five second average gust wind speed are estimated using a Peaks Over Threshold (POT) extreme value estimation technique. Wind shear coefficients and gust factors are also estimated. Seven to thirty-two years of buoy data at five locations near areas of possible interest to Maine׳s offshore wind industry were examined. Buoy data was obtained from the University of Maine and the National Oceanic Atmospheric Administration. Predictions of extreme significant wave heights in this study compare well with past shipboard and hindcast predictions for open water sites. Sites closer to shore show larger differences from past predictions due to the large areal scale of the earlier studies. Extreme wind speeds and shear coefficients compare well with coastal onshore wind speeds published by the American Society of Civil Engineers.

Over the past 6 years, the University of Maine (UMaine) has been an active contributor in research and scale-model testing of floating offshore wind turbines (FOWTs). This paper serves to share the evolution of UMaine’s scale-model testing pedigree by exploring the various test campaigns at a high level, culminating with the design validation of the VolturnUS floating platform. These model test campaigns have each provided key insights into the behavior of FOWT platforms as well as improving the ability to perform model tests of FOWTs. In 2011, the UMaine-led DeepCwind Consortium carried out 1/50-scale model tests of a generic tension leg platform (TLP), a semi-submersible (semi), and a spar-buoy (spar) floating platform at the Maritime Research Institute Netherlands (MARIN) test facility. The designs were Froude-scaled and supported a scaled version of the 5-MW National Renewable Energy Laboratory (NREL) offshore research turbine. Data from these tests has been used extensively for numerical simulation validation efforts using NREL’s computer-aided engineering software FAST and laid the foundation for UMaine’s design efforts on VolturnUS. In 2013, UMaine conducted another test campaign at MARIN using the original semi-submersible from 2011 with an improved turbine as well as a 1:50-scale model of the VolturnUS concrete semi-submersible design. The improved DeepCwind semi-submersible data is currently being utilized in the validation of a large number of other analysis codes as part of the International Energy Agency’s OC5 project. In 2015, UMaine opened its own Wind/Wave test facility, the Alfond Wind/Wave Ocean Engineering Laboratory (W2). Utilizing this new facility, UMaine tested the 1:50-scale model DeepCwind semi-submersible, repeating the tests from MARIN, to validate the experimental equipment and procedures as well as demonstrate the capability of the W2. In 2016 UMaine carried out testing of a 1:52-scale model of the 100% design of the VolturnUS with a 6-MW topside as a final design validation to support the US Department of Energy-supported, full-scale Aqua Ventus demonstration project scheduled to be connected to the grid in 2019. A newly designed 6-MW scale model turbine was used in this test and the performance-matched turbine design methodology is described. Selected results from the test campaign and preliminary numerical comparisons are discussed as well as key lessons learned from the model test campaigns are presented.

Increased interest in floating offshore wind turbine (FOWT) technology has stimulated the development of new testing facilities and techniques to support the design of these structures. This paper presents some specific challenges of testing FOWTs and how these requirements have driven the development of wind/wave basins capable of providing both high quality wind and wave environments necessary to test these structures. Previous experience by the University of Maine (UMaine) and others as part of the DeepCwind Consortium in testing 1/50th-scale FOWTs at the Maritime Research Institute Netherlands (MARIN) Offshore Basin has supported the development of UMaine's Harold Alfond W2 Ocean Engineering Laboratory (W2), which includes a movable, open-jet wind machine specifically designed to provide a high quality wind field for testing FOWTs. A calibration testing campaign of this newly constructed facility will recreate a subset of the tests performed at MARIN. The new test campaign will contain system identification, wind turbine performance characterization, wave only, wind only, and combined wind/wave test cases utilizing both regular and irregular wave specifications. This paper will focus on the initial phase of this campaign which includes select system identification and regular wave tests. The test specimen is the same used in the original test program performed at MARIN and consists of the 1/50th-scale model semi-submersible along with the 1/50th-scale model version of the National Renewable Energy Laboratory (NREL) 5MW Reference Wind Turbine. Results from the new test campaign are studied and compared with the data from the 2011 MARIN test. Differences in data collection methods, basin geometry, model specifics and test methods were expected and are discussed for this initial phase. In particular, differences in the basin geometry necessitate development of a truncated mooring system to emulate the response of the full mooring system used at MARIN. The implications of utilizing this method are discussed as well as additional areas of future work to be completed in subsequent phases of the validation test campaign.

This paper presents an overview of the successful conclusion of 18 months of testing the first grid-connected floating offshore wind turbine prototype in the Americas. The prototype, called VolturnUS 1:8, was installed off Castine, Maine, USA. The prototype is a 1:8 scale prototype and serves to de-risk the deployment of a full-scale 6MW turbine. VolturnUS utilizes innovations in materials, construction, and deployment technologies such as a concrete semi-submersible hull and an advanced composite tower to reduce the costs of offshore wind. The prototype unit was designed following the American Bureau of Shipping (ABS) “Guide for Building and Classing Floating Offshore Wind Turbine Installations”. Froude scaling was used in designing the 1:8-scale VolturnUS prototype so that the motions of the prototype in the relatively protected site represent those of the full-scale unit in an open site farther offshore. During the past year, a comprehensive instrumentation package monitored key performance characteristics of the platform during operational, extreme, and survival storm conditions. Data collected include: wind speed, turbine power, rotor angular frequency, blade pitch, torque, acceleration; tower bending moment, 6 DOF accelerations at tower top and base, mooring line tensions, and wave elevation at the platform. During the past year the prototype has experienced many environments representative of scaled ABS design conditions including operational wind and sea-states, 50-year sea states and 500-year survival sea states. This large data set provides a unique view of a near full-scale floating wind turbine subjected to its prescribed environmental conditions. Inspections of the concrete hull following removal provided confirmation of material durability. Marine growth measurements provide data for future design efforts.

In an effort to harness the abundant offshore wind resource over deepwater, the development of numerical design tools for floating offshore wind turbines (FOWTs) has progressed steadily in recent years. However, at present, a validated model capable of completely coupling the full elastodynamic response between the mooring system, floating support structure, turbine tower and the wind turbine is not commercially available. The University of Maine has developed a new FOWT design, VolturnUS, which utilizes a concrete semi-submersible hull. For the VolturnUS design effort a number of numerical models were developed to analyze the system’s global performance. This paper presents the results of a validation study conducted to quantify the accuracy and suitability of a subset of these models for use in the design of the VolturnUS FOWT. Validation was conducted via comparisons of numerical model results to test data obtained from a 1:50 scale model testing campaign conducted by the University of Maine at the Maritime Research Institute, Netherlands offshore basin. The validation study evaluated the performance and capabilities of the numerical models over a range of design conditions. Emphasis was placed on design load cases (DLCs), which were found to govern the design of the FOWT. The DLCs follow the American Bureau of Shipping’s (ABS) Guide for Building and Classing Floating Offshore Wind Turbines. Through this method of model validation this work sought to quantify the numerical models’ accuracy, highlight their limitations, justify design assumptions, and identify areas requiring further development in the field of FOWT numerical modeling.

A new floating wind turbine platform design called VolturnUS developed by the University of Maine uses innovations in materials, construction, and deployment technologies such as a concrete semisubmersible hull and a composite tower to reduce the costs of offshore wind. These novel characteristics require research and development prior to full-scale construction. This paper presents a unique offshore model testing effort aimed at derisking full-scale commercial projects by providing scaled global motion data, allowing for testing of materials representative of the full-scale system, and demonstrating full-scale construction and deployment methods. A 1:8-scale model of a 6 MW semisubmersible floating wind turbine was deployed offshore Castine, ME, in June 2013. The model includes a fully operational commercial 20 kW wind turbine and was the first grid-connected offshore wind turbine in the U.S. The testing effort includes careful selection of the offshore test site, the commercial wind turbine that produces the correct aerodynamic thrust given the wind conditions at the test site, scaling methods, model design, and construction. A suitable test site was identified that produced scaled design load cases (DLCs) prescribed by the American Bureau of Shipping (ABS) Guide for Building and Classing Floating Offshore Wind Turbines. A turbine with a small rotor diameter was selected because it produces the correct thrust load given the wind conditions at the test site. Some representative data from the test are provided in this paper. Model test data are compared directly to full-scale design predictions made using coupled aeroelastic/hydrodynamic software. Scaled VolturnUS performance data during DLCs show excellent agreement with full-scale predictive models. Model test data are also compared directly without scaling against a numerical representation of the 1:8-scale physical model for the purposes of numerical code validation. The numerical model results compare favorably with data collected from the physical model.

A methodology for the design and optimization of a composite wind turbine tower for use on a floating offshore platform is presented. A composite turbine tower on a floating offshore platform not only has the potential to reduce maintenance and upkeep costs associated with the use of steel offshore but also has potential to reduce the tower mass and subsequently the support platform mass. The optimization problem is formulated to obtain a turbine tower that meets all strength and serviceability criteria and minimizes the tower mass. The optimization and design process link a number of dynamic analyses and finite element routines using a genetic algorithm. This work documents the optimization and design software and illustrates its use in various case studies for a 6 MW floating wind turbine system. Case studies include the optimization of a steel tower as a comparison to a composite material tower and the use of a cored, sandwich panel composite tower versus a solid composite tower. The results demonstrat...

In May of 2013 the VolturnUS 1:8 floating semi-submersible wind turbine was successfully deployed off the coast of Castine, Maine, making the unit the first grid connected offshore turbine in the United States. The VolturnUS 1:8 structure features a 20 kW turbine, a post-tensioned and reinforced concrete semi-submersible base and a fiber reinforced plastic (FRP) tower (E-glass and polyester resin). The VolturnUS 1:8 structure is a geometrically 1:8-scale of a 6 MW floating turbine design and is used to demonstrate the feasibility of both the concrete base and FRP tower and validate the performance of the structure in a scaled environment. Data collected from the deployed 1:8-scale structure will be used for modeling and simulating the behavior of the system at full-scale. The effort was led by the University of Maine’s Advanced Structures and Composites Center (UMaine) and a consortium of industry partners, including FRP manufacturer Ershigs, Inc. An overview of the process and methodology used in the analysis, design and testing of the 1:8 scale FRP floating wind turbine tower is presented. The use of an FRP tower on a floating wind turbine platform offers the benefits of reduced tower mass and maintenance requirements and has the potential to further reduce hull mass by lowering the global center of gravity of the structure. An FRP tower for use on the UMaine semi-submersible concrete VolturnUS 1:8 platform was developed that meets all strength and serviceability criteria and is robust enough to withstand the loading from both wind and waves. An overview of the tower loads analysis and FAST modeling, tower structural design, structural proof testing and preliminary analysis of performance are presented. The VolturnUS 1:8 wind turbine tower is the first time FRP materials have been used in an offshore wind tower application. Further, the methodologies and procedures that were developed in the design of the pilot-scale tower are directly applicable to the design and analysis of composite wind turbine towers at the full-scale level. These “lessons learned” are already in use as Ershigs and UMaine work to design a full-scale composite tower over 80 meters tall for use on the VolturnUS platform with a 6MW wind turbine. The results of the 1:8-scale program demonstrate the successful use of an FRP wind turbine tower on a floating platform and highlights the potential for the use of an FRP tower at the full-scale (6 MW) level.

Scale-model wave basin testing is often employed in the development and validation of large-scale offshore vessels and structures by the oil and gas, military, and marine industries. A basin-model test requires less time, resources, and risk than a full-scale test, while providing real and accurate data for numerical simulator validation. As the development of floating wind turbine technology progresses in order to capture the vast deep-water wind energy resource, it is clear that model testing will be essential for the economical and efficient advancement of this technology. However, the scale model testing of floating wind turbines requires accurate simulation of the wind and wave environments, structural flexibility, and wind turbine aerodynamics and thus requires a comprehensive scaling methodology. This paper presents a unified methodology for Froude scale model testing of floating wind turbines under combined wind and wave loading. First, an overview of the scaling relationships employed for the environment, floater, and wind turbine are presented. Afterward, a discussion is presented concerning suggested methods for manufacturing a high-quality, low-turbulence Froude scale wind environment in a wave basin to facilitate simultaneous application of wind and waves to the model. Subsequently, the difficulties of scaling the highly Reynolds number–dependent wind turbine aerodynamics is presented in addition to methods for tailoring the turbine and wind characteristics to best emulate the full-scale condition. Lastly, the scaling methodology is demonstrated using results from 1/50th-scale floating wind turbine testing performed at the Maritime Research Institute Netherlands (MARIN) Offshore Basin. The model test campaign investigated the response of the 126 -m rotor diameter National Renewable Energy Lab (NREL) horizontal axis wind turbine atop three floating platforms: a tension-leg platform, a spar-buoy, and a semisubmersible. The results highlight the methodology's strengths and weaknesses for simulating full-scale global response of floating wind turbine systems.