Model Test of a 1:8-Scale Floating Wind Turbine Offshore in the Gulf of Maine¹

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. An offshore model test at an intermediate-scale with a wind turbine smaller than 1 MW can derisk the development of a commercial-scale VolturnUS system by providing scaled global motion data, allowing for testing of the same material systems used in the commercial system, and demonstrating full-scale construction and deployment methods. Such a test requires careful selection of the test site, wind turbine, scaling methods, model design, and construction to obtain meaningful data.

Model tests of offshore structures are routinely conducted in the offshore oil and gas sectors in wave basin facilities [1]. A basin model test offers a reduced risk and cost venue to accurately evaluate the floating system's dynamic characteristics. Scale model tests of floating wind turbines in a wave basin have been completed by several groups. Past wave basin test campaigns include Principle Power, Inc., U.S. [2], Hydro Oil & Energy, Norway [3], WindSea AS, Norway,² and the University of Maine [4,5]. These experiments aimed at validating dynamic global performance of various floating wind turbine platform types. These past efforts provide guidance for conducting scale model tests of floating wind turbines but do not consider the same materials, construction methods, or deployment methods as the full-scale system.

Several intermediate-scale floating wind turbines have been deployed offshore or are in planning stages throughout the world, although few data have been published [6]. There are currently six known intermediate-scale testing efforts of floating wind turbines by commercial and public entities. BlueH³ deployed a 100 kW tension-leg platform off the Italian coast. Floating Power Plant A/S deployed its Poseidon wind/wave prototype with three 11 kW turbines [7]. Sway conducted a 1:5-scale test of a tension-leg spar design off the Norwegian Coast.⁴ Kyoto University deployed a 1:2-scale 100 kW spar.⁵ A 1:4-scale testing program is planned by NORCOWE in Norway,⁶ and a 1 kW vertical axis turbine supported on top of a spar is planned by the Deepwind Consortium in Europe.⁷ These programs indicate the interest in conducting intermediate-scale tests prior to commercial-scale projects as a step in the development process for floating offshore wind technology.

This paper presents a unique offshore Froude-scale test program for an intermediate-scale offshore wind turbine off the coast of Maine, U.S. planned for eighteen months starting in June 2013. This testing program includes the selection of a suitable test site to apply correctly Froude-scaled wave loads and the selection of a commercial turbine that with the wind speed at the site, applies a Froude-scaled thrust load. The instrumentation plan required to measure environmental inputs and model response and the inclusion of design and construction methods representative of those used for a full-scale system are presented. The usefulness and applicability of the testing results are then discussed. The testing effort was developed to meet several objectives:

1. (1)

   Validate the VolturnUS design behavior at close to full-scale by conducting a Froude-scale test representative of a 6 MW floating turbine deployed far offshore in the Gulf of Maine.

2. (2)

   Design, test, and demonstrate advanced material systems, construction techniques, and deployment methods for the VolturnUS concept.

3. (3)

   Collect data for validation of coupled aeroelastic/hydrodynamic numerical models for floating offshore wind turbines.

4. (4)

   Develop deep water offshore wind testing capabilities, procedures, and methods.

The testing was performed near Castine Harbor, ME, as shown in Fig. 1. The model is a 1:8-geometric scale prototype of a 6 MW turbine platform system to be deployed in the Gulf of Maine far offshore. The turbine has been selected to apply a correctly Froude-scaled thrust load to the unit. Because the wind speeds are greater than those required by Froude scaling, a smaller diameter turbine rotor is needed to apply the correct thrust force. The 1:8-model is shown in Fig. 2. The concrete hull and tower were designed following the ABS Guide for Building and Classing Floating Offshore Wind Turbines [8]. The turbine and platform were constructed onshore and towed to site fully outfitted to demonstrate full-scale construction and deployment methods. A comprehensive instrumentation package monitors key performance characteristics of the waves, wind, current, and platform response to verify the full-scale design behavior and coupled aeroelastic-hydrodynamic modeling software.

The model is a 1:8-scale prototype of a 6 MW floating turbine deployed approximately 23 km offshore Maine. A scale factor, λ, of eight was chosen based on the expected wave conditions at the test site, the selected commercial turbine thrust loading and wind speed at the site, and practical constructability constraints associated with incorporating materials representative of the full-scale system. Froude-scale basin model testing techniques commonly used for offshore structures such as oil and gas and offshore structures have been adapted for this testing program [1]. Particulars of Froude-scaled testing of floating wind turbines have been incorporated following previous work which highlight the necessity of properly scaling the thrust perpendicular to the rotor plane, which is the dominant wind turbine loading on the floating structure [4]. The scaling parameters are shown in Table 1. Unique challenges exist to maintain these parameters in an intermediate-scale offshore test with competing requirements. For example, the site wind and wave conditions must occur with the correct magnitude, temporal characteristics, and joint occurrence. Similarly, a scale turbine is needed to apply the correctly scaled thrust force given the wind conditions at the test site. For the hull, the mass and dynamic characteristics must be scaled properly while also being constructed of appropriate full-scale construction materials and being deployed using representative full-scale deployment methods.

The VolturnUS 1:8-scale model was designed to be constructed, assembled, and deployed using similar materials and techniques as the full-scale 6 MW design. The full-scale design is made of concrete can be fully constructed and assembled dockside. After assembly, the entire structure is towed at a shallow transit draft with a single standard tug boat for mooring and electrical umbilical hook up at its final installation site.

Table 2 lists the gross properties of the model and the full-scale system. A comparison between the ideal target scale factor and the actual achieved scale factor of the model is presented. The scale factor compares well with the target scale values for the key parameters. Some dissimilitude for the turbine exists due to the competing goals of the test program. A commercial pitch regulated 20 kW turbine modified to produce 12 kW was used for the test. At this reduced power, the turbine applies the desired peak scaled thrust force at the rated wind speed. The power, turbine Cp, and rotor diameter are not perfectly scaled but were acceptable for the test given that the thrust load is the primary wind turbine forcing on the platform [4,5].

The platform also successfully incorporated the same materials as used at full-scale. The concrete hull design, composite tower design, and construction process replicate the full-scale system. The concrete hull is constructed of 15 concrete members and representative of the full-scale design and construction process. The connections, thicknesses, and reinforcements of the concrete hull are scaled. The only difference from the full-scale system is that the 1:8-scale model can be fully dismantled for shipping over the road on a truck. The 1:8-scale composite tower is constructed of the same reinforcements and resins as a full-scale tower. Both the tower and concrete hull underwent structural qualification testing to confirm structural designs prior to fabrication. The assembly of the hull, tower, and turbine all took place on land at the Cianbro Modular Manufacturing Facility in Brewer, ME. The model uses three catenary chain moorings anchored to the sea bed with drag embedment anchors. The average water depth at the test site is matches the target scaled depth as well. Figure 3 shows the completed unit.

The deployment of the prototype took place on June 2, 2013. The tow route began in Brewer, ME and proceeded down the Penobscot River to Castine, ME. The route was approximately 50 km (28 nm), and took about 11.5 hrs. The entire unit, fully assembled was towed with a single tug boat and an assistance vessel in such a fashion as to replicate the tow configuration employed in the deployment of a full-scale 6 MW VolturnUS. An image of the VolturnUS being towed is shown in Fig. 4.

The tow-out, which was to occur at approximately 3.7 km per hour, was expected to produce a tow line tension of 32 kN as predicted following guidance per DNV-RP-C205 [9]. During the actual tow-out, the line tension measured with an onboard load cell was very close to the calculations as shown in Fig. 5. This data confirmed the design tow line load prediction methods used for VolturnUS system. The measured tow line mean force is about 4 kN higher (10% greater) than the predicted value. This difference could be due to uncertainty in the speed of the vessel as this was recorded directly from the boat instrumentation. Another reason for the difference could be due to uncertainty in tidal and river current behavior during the tow-out. These were not directly measured. Considering these differences and unknowns, the predicted versus measured tow-line force compares well and helps to confirm the design tow-line load prediction methods used for VolturnUS.

Toward the end of the tow, the model encountered waves approximately 1.5 m in height (equivalent to 12 m full-scale). Despite this, the tow-out occurred without incident. Although not a desirable tow-out condition at full-scale, the exercise provides evidence that the stability and towing configuration of VolturnUS during tow-out is sufficiently robust. Upon arrival in Castine, the unit was hooked to preinstalled chain moorings connected to preset drag anchors.

Selection of a suitable test site for the offshore 1:8 Froude-scale model test requires careful treatment. The test site must generate the correct combinations of wind/turbine thrust and scaled waves in order to simulate the behavior of the full-scale 6 MW VolturnUS farther offshore. Full-scale design conditions for floating offshore wind turbines specified by the ABS Guide for Building and Classing Floating Offshore Wind Turbine Installations were estimated from metocean data obtained far offshore Maine and represent the desired full-scale design environments [10,11]. With the desired environmental conditions in hand, the Castine site was then confirmed to produce the desired environments by deploying a buoy at the Castine site.

There exists over 12 years of wind and wave buoy data at the proposed 6 MW full-scale design deployment site off Monhegan Island, Maine the location of the full-scale system [12]. Buoy data collected includes significant wave height and peak period, wave energy spectral parameters and 10 mins average wind speeds at 4 m above sea level. Using this data, full-scale DLCs were developed for the Monhegan site for a 6 MW floating wind turbine. Several critical ABS DLCs are shown along with the desired 1:8-scale values in Table 3. DLC 1.2 is an operational type loading condition used for fatigue analysis. DLC 1.6 is an extreme turbine operating condition with an associated 50-yr return period significant wave height concurrent with the operating wind speed. The table shows a critical subset of the DLC 1.6 case, where the turbine is operating at peak thrust with a 50-yr return period significant wave height and wave period associated with the rated wind speed. DLC 6.1 is another extreme case, where the turbine is shut down and the unit is subjected to a larger 50-yr significant wave height independent of wind speed. The last case shown is an example station-keeping survival load case (SLC), where the turbine is subjected to a 500-yr return period wave event with the turbine parked. These four load cases have been found to control the design of the full-scale VolturnUS platform and were used to identify the proper test site.

Numerous test sites capable of reproducing these load cases in the Gulf of Maine were considered for this 1:8-scale test. Castine, ME was selected after an analysis of data obtained from an instrumented metocean buoy deployed in the winter of 2012. The buoy data, including wind, wave, and current measurements confirmed that the site could readily produce the desired ABS scaled DLCs with a high probability. Figure 6 shows a scatter plot of the measured peak period and significant wave height at Castine. The scatter data represents the measured buoy data from the model test site in Castine. The red trend line is a power law fit of the full-scale peak period v. significant wave height relationship at the full-scale 6 MW test site scaled by the scale factors in Table 1 which represents the mean operational wave conditions required for DLC 1.2. 95% confidence intervals are also included for the trend lines. Also included on this figure are the peak periods and significant wave heights of DLC 1.6, 6.1, and the SLC. Noting the overlap of the trend line with the measured data at Castine, there is a high probability of experiencing the desired wave conditions during the deployment. The larger scaled extreme wave conditions, DLC 6.1 and SLC, have a lower probability of occurrence as evidenced by the limited data for larger waves collected during this short buoy deployment. The peak period as a function of the significant wave height, Hs, is also provided. This relationship was developed from the buoy record at the full-scale project site.

In addition to maintaining the correct wave peak period and significant wave height joint occurrence, the correct wave spectrum shape is also highly desirable. Figure 7 shows two images as an example of the wave spectral characteristics measured during the VolturnUS deployment by a wave staff mounted on the unit corrected for the motions of the hull using accelerometer data. The top image is a time record of wave elevation over a 1 hr period with a significant wave height of 1.6 m and a peak period of 5.2 s. This condition is representative of the SLC conditions. Further, the computed spectrum is very close to the desired scaled 500-yr survival case. The bottom image shows the measured wave spectrum along with a superimposed scaled 500-yr JONSWAP spectrum showing good agreement.

With the wave characteristics at the site found to be acceptable, the test site's ability to produce scaled turbine thrust loads in conjunction with the associated significant wave height is now presented. For the scale test to be meaningful, the wind turbine thrust not only needs to be scaled properly, but also must occur with an appropriate sea-state (significant wave height and peak period) representative of a full-scale design condition. To confirm that the site can produce turbine thrust and sea-states in the correct proportions with a high probability, the scatter plot, Fig. 8, was created. This scatter plot shows two sets of data. The desired scaled conditions are shown as small gray dots and the expected prototype conditions are shows as larger blue dots. The desired scaled conditions are representative of various DLC 1.2 type conditions and were obtained using 10 mins average wind speed and significant wave height data obtained from the buoy deployed at the full-scale site farther offshore. The wind speed data were extrapolated to hub height using a power law wind shear model with an exponent of 0.14 [13] and then used to estimate the 10 mins average 6 MW full-scale turbine thrust load thereby generating a turbine thrust v. significant wave height scatter diagram for the full-scale system. The 10 mins thrust was estimated using fast software for a representative 6 MW turbine for steady winds [14]. The thrust and significant wave height data were then scaled down using Froude scaling laws to generate the desired scaled conditions. The expected 1:8-scale prototype thrust and significant wave height scatter data were obtained using the same method except that the buoy data gathered at Castine during the winter of 2012 and 12 kW turbine thrust v. wind speed relationship was used. Also shown as a red diamond is the ABS DLC 1.6 case, which is an extreme load case. The maximum turbine thrust for both desired and expected is about 1.37 kN based on the operating characteristics of the turbine.

The overlap of the two sets of data shows that the site frequently produces turbine thrusts with the correctly matched significant waves. Further, extreme cases like DLC 1.6 are also likely to happen. This is an advantage of an intermediate-scale approach as many extreme return period events can be witnessed in short amount of time as compared to a full-scale deployment.

Several tests were completed to confirm the model's hydrostatic and hydrodynamic characteristics dockside prior to deployment. As a confirmation of the static stability characteristics of the 1:8-scale model, a fully assembled unmoored heave and pitch hydrostatic static stiffness test was completed by adding known weights on the structure and measuring the displacements. The results are shown in Fig. 9 and are compared to the estimated response. The testing confirmed the expected properties of the model. The heave measured has some variability due to local waves created by wind and small craft nearby during the test. This issue was not experienced in the pitch stiffness test.

As a confirmation of the dynamic characteristics of the model, a combined heave/pitch decay test was completed using a crane pulling and then releasing from one of the outside columns. Onboard instrumentation measured pitch and heave motions and corrected for angular displacement. Figure 10 shows a power spectral density plot created using the free decay data. The pitch and heave natural frequencies matched closely the estimated scaled values of 0.16 Hz and 0.12 Hz, respectively, as predicted using fast [14]. Hydrodynamic damping of the system was determined based on this data for use in a quadratic damping model discussed later in this paper.

Preliminary installed testing results are presented in this section as well as an overview of the onboard instrumentation systems. Figure 11 outlines the types and locations of sensors onboard the model, as well as the remote sensors. Data collected include wind speed and direction, wave height, wind turbine power and rotor speed, platform angular position, platform translational and rotational accelerations, tower loads, platform loads and mooring line loads. Sample rates vary for the instruments and range from 10 to over 60 Hz depending on purpose. For example, the inertial measurement unit used for measuring the motions of the platform are sampled at 60 Hz while turbine power output is 10 Hz. In total, over 60 channels of data are being collected from the floating wind turbine. A Lord microstrain 3DM-GX3-45 inertial measurement unit was used to measure the motions of the platform. The device follows a right-handed coordinate system with positive surge defined as the centroid of the turbine tower to Leg C and heave along the central axis of the turbine tower, positive upward. Also shown are the labels for the three legs of the platform as well as the coordinate system utilized for expressing the acceleration results. Further, an instrumented buoy provides a separate wave height measurement, wind speed, atmospheric, and current profiles from the surface to the seafloor. The buoy was used as a means of quality checking the data measured directly from the VolturnUS floating platform. This comprehensive set of data permits careful comparisons between predicted performance and measured performance, and as noted earlier, can also be used to accurately predict performance for a full-scale 6 MW VolturnUS floating wind turbine located farther offshore.

A Fourier analysis of select data from the tower bottom inertial measurement unit is now presented for an operational type event (i.e., DLC 1.2), where the turbine was producing power in a sea state of 0.52 m which is equivalent to a 4.2 m full-scale significant wave height. Data considered herein include angular rotations for which a Fourier analysis is performed to demonstrate that the data sets are capturing the expected motion behavior of the VolturnUS floating wind turbine system. The results are shown in Fig. 12. As depicted in the figures, the expected behavior observed includes slowly varying motions associated with second-order difference-frequency wave diffraction forces, motion in the wave energy frequency range and wind turbine excitation. The pitch motion is higher than the roll because the primary loading is in the sway direction. As the wave direction is not aligned perfectly with the sway direction, the asymmetry in the wave loading causes some appreciable yaw motion. With regard to the wind turbine excitations observed, specifically the once-per-revolution (1P) turbine excitation is present.

During this testing campaign several storm events occurred that produced the desired extreme scaled conditions required by the ABS guide. This intermediate-scale test offers a unique opportunity to study floating turbine behavior when exercised to design limits at near full-scale. In this section, one such event, occurring on November 1, 2013 is discussed. On this day the model experienced a scaled DLC 1.6 which considers the turbine operating at rated wind speed, applying the peak overturning moment to the platform while experiencing 50-yr waves. Key maximum recorded measurements collected during the event are shown in Table 4. The maximum recorded wave height and peak thrust of the turbine were within 3% of desired DLC 1.6 scaled conditions. Also shown is the predicted behavior from a coupled aeroelastic/hydrodynamic simulation of the full-scale system subjected to ABS DLC 1.6. The floating platform performed markedly well and exhibited accelerations very close to the predicted results used for design of the full-scale system. Direct comparisons of key parameters of the scale model test data to the numerical models of the full-scale system show very favorable results and give confidence for full-scale implementation. These favorable direct comparisons of the model to full-scale predictions also indicate that the model test program is performing as intended and produced model basin quality test data. Figure 13 shows the prototype in this extreme environment.

Validation of the results of coupled aeroelastic-hydrodynamic numerical modeling software fast [14] against experimental performance data collected is now presented. Detailed validation studies are presented by Allen et al. [15]. fast is a coupled aeroelastic and hydrodynamic model for simulation of floating wind turbine global performance and has been validated through comparisons with tank testing data [16–19]. The date of the event used for this model correlation was November 27, 2013, from 12:51:54 pm to 1:51:58 pm. The mean and maximum wind speeds were 15.4 m/s and 23.6 m/s, respectively. The significant wave height was 1/6. The peak period was 5.2 s and the maximum wave height was 2.6 m which is equivalent to a scaled 500-yr event for the 6 MW VolturnUS. The simulated time was 1 hr. To facilitate comparison between the data and model, a custom version of fast was utilized that computes wave diffraction forces from the measured wave elevation time series record collected during this event. The standard version of fast can only simulate regular waves or random waves based on a user defined spectra. Wind speed and direction were also input directly. This approach allowed for a reasonable modeling approximation of the measured environment and followed the work of Coulling et al. [16]. Figure 14 shows comparison between the data and fast for the tower base sway acceleration. Here, the sway acceleration and pitch angular acceleration are compared because the waves and wind came primarily from this direction. The numerical model results compare well with the measured performance capturing the maximum amplitude and frequency of the acceleration. Similarly, tower base roll acceleration is shown in Fig. 15 again confirming the ability of the numerical code to accurately predict the peak performance of the VolturnUS system. Table 5 lists maximum recorded values for select variables. The other directions not reported were very small and are omitted from the comparisons. The fast model compares very well with measured performance for these directions and further improvements are expected as a more refined study is underway.

A unique offshore intermediate-scale model testing approach has been developed and successfully produced scale global performance data as well as demonstrated the full-scale design, materials, construction techniques, and deployment methods. Scale model test techniques developed for floating wind turbine testing in wave basins were adapted for this offshore test. The Castine test site was selected to readily produce scale ABS DLCs representative of the full-scale project site far offshore in the Gulf of Maine. The ability of the test site to produce these conditions was confirmed through a comparison of measured metocean data collected at the model test site with far offshore data scaled by the appropriate factors. At 1:8-scale, this test site reliably produces scale extreme wind/ wave events (e.g., 50-yr return period) in a short period of time offering a unique venue for understanding floating turbine behavior in extreme events at near full-scale. The model was designed and constructed using full-scale structural materials, arrangements, and methods while maintaining desired hydrostatic and dynamic characteristics confirmed through dockside testing. A commercial grid-connected turbine was used and modified to apply the desired scaled dominant wind forcing, thrust. The wind turbine was the first grid-connected offshore wind turbine in the U.S. Over 60 channels of instrumentation were onboard the model. A scale mooring system consisting of three chain catenaries and drag anchors was implemented.

Since the experiment has been carefully crafted to match the scale system properties and environmental loads, the generated data can be used to predict the behavior of a full-scale 6 MW VolturnUS. This extensive data set has confirmed design calculations establishing confidence in the principles of the VolturnUS design methodology as well as the concrete hull and composite tower structural designs. The VolturnUS 1:8-scale floating turbine has been successfully subjected in service to scaled 50-yr extreme ABS design conditions. This experiment represents a major milestone in the design of the VolturnUS technology and floating turbines in general as there is little data for floating wind turbines subjected to their extreme design conditions in a real ocean environment. Based on current observations, the 1:8-scale VolturnUS exhibits responses in line with coupled aeroelastic-hydrodynamic model design predictions and helps to derisk the design and construction of a full-scale floating offshore wind turbines utilizing the VolturnUS platform technology. Numerical codes for offshore floating wind turbines have also been partially validated as part of this work providing, another benchmark for development and understanding of floating wind turbines. Numerical models of the physical model show good agreement.

In summary, the major lessons learned from this experiment include the following: (1) the test plan and procedures including site selection, prototype design, and instrumentation design allowed for a successful Froude-scale model test in a real offshore environment, (2) numerical design codes for floating offshore wind turbines are suitable for modeling a near full-scale floating turbine subjected to extreme design environments in a really offshore environment, and (3) an intermediate Froude-scale test can not only produce scaled motion data useful for predicting full-scale response but also serve as a proving ground for new materials, construction and deployment methods. This testing approach significantly derisks new technology and could be directly applicable to other new floating structure development efforts. Additional data from this test will be released through publishing in future peer reviewed journals and conference proceedings.

The authors would like to acknowledge the financial support of the U.S. Department of Energy EERE Grant No. DE-EE0003278.001, the National Science Foundation Partnership for Innovation Grant No. IIP-0917974, the State of Maine, the University of Maine, and the support of the members of the DeepCwind Consortium including Cianbro, Ershigs, and Vryhof Anchors.

ABS =

American Bureau of Shipping

Cp =

turbine power coefficient

DLC =

design load case

Hs =

significant wave height

IMU =

inertial measurement unit

SLC =

survival load case

λ =

scale factor