ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V010T00A001. doi:10.1115/OMAE2017-NS10.

This online compilation of papers from the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Current Energy — Analysis, Design and Operation

2017;():V010T09A001. doi:10.1115/OMAE2017-61544.

Flow Induced Motions (FIMs) of two tandem, rigid, circular cylinders with piecewise continuous restoring force are investigated for Reynolds number 24,000≤Re≤120,000 with damping, and restoring force function as parameters. Selective roughness is applied to enhance FIM and increase the hydrokinetic energy captured by the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter. Experimental results for amplitude response, frequency response, interactions between cylinders, energy harvesting, and efficiency are presented and discussed. All experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The nonlinear-spring, Converter can harness energy from flows as slow as 0.33 m/s with no upper limit. (2) The nonlinear-spring Converter has better performance at initial galloping than its linear-spring counterpart. (3) The FIM response is predominantly periodic for all nonlinear spring functions used. (4) The influence from the upstream cylinder is becoming more dominant as damping increases. (5) Optimal power harnessing is achieved by changing the linear viscous damping and tandem spacing L/D. (6) Close spacing ratio L/D = 1.57 has a positive impact on the harnessed power in VIV to galloping transition. (7) The interactions between two cylinders have a positive impact on the upstream cylinder regardless of the spacing and harness damping.

Commentary by Dr. Valentin Fuster
2017;():V010T09A002. doi:10.1115/OMAE2017-61569.

In recent decades, significant research and development has been invested in techniques that harvest renewable energy from an ocean environment. Starting from offshore wind energy we also see developments in devices that extract energy from sources such as waves and tides. Although a lot can be learned by transferring existing on- and offshore technology, a multitude of new challenges arouse. The offshore wind industry for example has struggled with wave induced loads on their wind turbine foundations structures, which resulted in intense nacelle and rotor vibrations. These vibrations have a significant impact a turbine’s lifetime. The specific problem of oscillating wave loads worsens for tidal energy turbines as no longer only the foundation but also the rotor itself is submerged and directly exposed to wave induced forces.

Therefore, the best possible determination of these loads is a key prerequisite for any holistic vibration and fatigue analysis. During operation, the turbine rotor will, depending on the actual tidal current velocity, rotate at different speeds whilst the waves propagate over it. The number of ratios between wave period Tw and rotational period of the rotor Tr is therefore infinite. This raises the question what load changes a revolving rotor blade experiences and which combination of Tw/Tr and wave encounter angle will generate the maximum loads per cycle. This study presents a comprehensive, general approach to identify maximum possible wave induced forces and moments on stationary and turning rotor blades, for any turbine design, position in the water column and orientation in the wave field. The procedure is exemplified for a generic 3-bladed horizontal axis turbine which is fixed to the seabed. The approach identifies loads depending on wave period and height, the period of rotor rotation and the wave encounter angle, utilising diffraction theory. Forces and moments are firstly calculated in a global coordinate system and subsequently transferred into a blade fixed coordinate system. This allows for an examination of the load changes as the blades rotate about the turbine axis. Thus, the worst-case scenarios in terms of load changes for each combination of wave parameters can be identified. Those maximum load cycle events are then combined to transfer functions, for loads and moments respectively. Transfer functions such as these will later allow for a quick identification of maximum load changes and cycle periods, depending on any given environmental condition. This information will help identifying structural loads and immanent fatigue and vibration issues during the actual turbine design process.

Commentary by Dr. Valentin Fuster
2017;():V010T09A003. doi:10.1115/OMAE2017-62068.

Marine hydrokinetic turbines typically operate in harsh, strongly dynamic conditions. All components of the turbine system must be extremely robust and able to withstand large and constantly varying loads; the long and relatively slender blades of marine turbines are especially vulnerable. Because of this, modern marine turbine blades are increasingly constructed from fiber reinforced polymer (FRP) composites. Composite materials provide superior strength- and stiffness-to-weight ratios and improved fatigue and corrosion resistance compared to traditional metallic alloys. Additionally, it is possible to tailor the anisotropic properties of FRP composites to create an adaptive pitch mechanism that will adjust the load on the turbine in order to improve system performance, especially in off-design or varying flow conditions. In this work, qualitative fundamentals of composite structures are discussed with regards to the design of experimental scale adaptive pitch blades. The load-deformation relationship of flume-scale adaptive composite blades are characterized experimentally under static loading conditions, and dynamic loading profiles during flume testing are reported. Two sets of adaptive composite blades are compared to neutral pitch composite and rigid aluminum designs. Experimental results show significant load adjustments induced through passive pitch adaptation, suggesting that adaptive pitch composite blades could be a valuable addition to marine hydrokinetic turbine technology.

Commentary by Dr. Valentin Fuster
2017;():V010T09A004. doi:10.1115/OMAE2017-62131.

Flow Induced Vibrations (FIV) are conventionally destructive and should be suppressed. Since 2006, the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan has been studying FIV of multiple cylinders to enhance their response for harnessing hydrokinetic power from ocean, river, and tidal currents. Interactions between multiple cylinders in FIV enable high power-to-volume ratio in a converter consisting of multiple oscillators of cylinders. This paper investigates experimentally the relation between oscillation patterns and frequency response of two cylinders in tandem. All experiments are conducted in the recirculating channel of the MRELab for 30,000<Re<120,000. Phase analysis reveals three dominant patterns of oscillation of two tandem cylinders by calculating the instantaneous phase difference between the two cylinders. This phase difference characterizes each major pattern. One is characterized by nearly 180° out of phase oscillations and one by small lead or lag of one cylinder over the other. In the third pattern, the instantaneous phase difference changes continuously from −180° to +180°. Using frequency spectra, oscillation characteristics of each cylinder are revealed in each flow speed range. Comparison of oscillation patterns and frequency spectra reveals that each oscillation pattern is related to a distinctly different frequency response.

Commentary by Dr. Valentin Fuster
2017;():V010T09A005. doi:10.1115/OMAE2017-62166.

Flow-induced vibrations of two elastically mounted circular cylinders in staggered arrangement were experimentally investigated. The Reynolds number range for all experiments (2.5×104<Re<1.2×105) was in the TrSL3 flow regime. The oscillator parameters selected were: mass ratio m* = 1.343, spring stiffness K = 250N/m, and damping ratio ζ = 0.02. The experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel in the MRELab of the University of Michigan. A closed-loop, virtual spring-damper system (Vck) was used to facilitate quick and accurate parameter setting. Based on the characteristics of the displacement response, five vibration patterns were identified and their corresponding regions in the parametric plane of the in-flow spacing (1.57<L/D<4.57) and transverse cylinder-spacing (0<T/D<2) were defined. The hydrodynamic forces and frequency characteristics of the vibration response are discussed as well.

Commentary by Dr. Valentin Fuster
2017;():V010T09A006. doi:10.1115/OMAE2017-62271.

Flow induced vibrations of two rough, rigid, tandem-cylinders on springs are investigated for power conversion for Reynolds number 30,000 ≤ Re ≤ 120,000. Passive turbulence control (PTC) in the form of roughness strips is employed to enhance FIV and increase the power harness efficiency of the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) converter. Numerical simulations are performed using two-dimensional, Unsteady Reynolds-Averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model. The center-to-center spacing ratio d / D of the two cylinders is set as 2.0 or 2.57 with mass ratio m* = 1.343 , damping ratio ζ = 0.26, and stiffness K = 1,200 N/m. Amplitude response, frequency response, interaction, energy harvesting, and conversion efficiency are presented and discussed. The main conclusions are: (1) In the VIV region at Re = 60,000, the amplitude response, frequency response, harnessed power, and power conversion efficiency of the upstream cylinder is the same for the two spacing ratios. Due to the shedding effect, the motion of the downstream cylinder for spacing ratio d/D = 2.0 is more severely suppressed than spacing ratio d/D = 2.57, which reduces the harnessed power and conversion efficiency for the downstream cylinder. (2) In the galloping region at Re = 110,000, due to the different impingement of the shed vortices on the downstream cylinder, the upstream cylinder harnesses more power and has higher energy conversion efficiency for spacing ratio d/D = 2.0 than d/D = 2.57.

Commentary by Dr. Valentin Fuster
2017;():V010T09A007. doi:10.1115/OMAE2017-62693.

Flow-induced vibration (FIV), primarily vortex-induced vibrations (VIV) and galloping have been used effectively to convert hydrokinetic energy to electricity in model-tests and field-tests by the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan. The developed device, called VIVACE (VIV for Aquatic Clean Energy), harnesses hydrokinetic energy from river and ocean flows. One of the methods used to improve its efficiency of harnessed power efficiency is Passive Turbulence Control (PTC). It is a turbulence stimulation method that has been used to alter FIV of a cylinder in a steady flow. FIV of elastically mounted cylinders with PTC differs from the oscillation of smooth cylinders in a similar configuration. Additional investigation of the FIV of two elastically mounted circular cylinders in staggered arrangement with a low mass ratio in the TrSL3 flow-regime is required and is contributed by this paper. A series of experimental studies on FIV of two PTC cylinders in staggered arrangement were carried out in the recirculating water channel of MRELab. The two cylinders were allowed to oscillate in the transverse direction to the oncoming fluid flow. Cylinders tested have, diameter D = 8.89cm, length L = 0.895m and mass ratio m* = 1.343. The Reynolds number was in the range of 2.5×104<Re<1.2×105, which is a subset of the TrSL3 flow-regime. The center-to-center longitudinal and transverse spacing distances were T/D = 2.57 and S/D = 1.0, respectively. The spring stiffness values were in the range of 400<K<1200N/m. The values of harnessing damping ratio tested were ζharness = 0.04, 0.12, 0.24. For the values tested, the experimental results indicate that the response of the 1st cylinder is similar to a single cylinder; however more complicated vibration of the 2nd cylinder is observed. In addition, the oscillation system of two cylinders with stiffer spring and higher ζharness could initiate total power harness at a larger flow velocity and harness much higher power. These findings are very meaningful and important for hydrokinetic energy conversion.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Ocean Renewable Energy — Regulatory and Environmental Considerations

2017;():V010T09A008. doi:10.1115/OMAE2017-61667.

The financial performance of a marine energy project is based on assumptions with significant uncertainty. To fully appraise the risk, potential investors require an understanding of the likelihood of deviations from the assumed most likely case for a project’s financial performance. A Monte Carlo Analysis (MCA) model with flexible user defined uncertainty definitions for all inputs is developed for this study. A realistic tidal energy project is used as a case study to compare the central, optimistic and pessimistic Levelised Cost of Energy (LCOE) and Internal Rate of Return (IRR) values derived using commonly used deterministic methods and the probabilistic MCA model. The improvement in decision support due to the probabilistic analysis is shown and the possibility for misinterpreting the deterministic results in highlighted. Two sensitivity analysis methods are employed to identify key risks and emphasise the need to use the most appropriate method for the type of analysis being conducted. Finally, the significance of some commonly ignored parameters is tested and shown to be important for accurately appraising the investment risk in a real project. Thus this paper provides guidance and tools to help investors make informed decisions with confidence.

Commentary by Dr. Valentin Fuster
2017;():V010T09A009. doi:10.1115/OMAE2017-62612.

This paper aims to make a contribution to assessing the viability of offshore wind power projects on the Galician coast. Several of the factors involved in these projects are studied, such as site selection and employed technologies regarding turbine and floating foundations. Estimated costs ‘ analysis and financial evaluation are performed for a chosen solution. Based on the conducted study, an offshore wind farm in Galicia may become valid in a prospect of an electricity tariff to the producer in line with other European countries. Furthermore, an expected decrease of costs of floating platforms once produced in series and of offshore technology as a whole, in addition to incentives, would make the investment much more attractive.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Ocean Renewable Energy — Thermal, Hybrid and Other Forms

2017;():V010T09A010. doi:10.1115/OMAE2017-61042.

Concept of the real-time hybrid model (ReaTHM® 2) testing framework for marine hybrid power plant is presented. The benefits and challenges with regard to using the model-scale power plant for the testing are explained. As a feasibility study of the methodology, tests are performed at the Hybrid Power Laboratory with a model-scale physical diesel-electric power plant. In this test, a load profile from onboard measurements from a ship is used as a numerical part of the system. In the model-scale power plant, the electrical part of the plant is used as an actuator to generate the load for the diesel engine. The traceability of the components and the total system to the given load profile is quantified in terms of time delay and tracking errors. For conclusion, the limitation of the test is analyzed and suggestions for improving the results are provided.

Commentary by Dr. Valentin Fuster
2017;():V010T09A011. doi:10.1115/OMAE2017-61401.

Despite various technical and operational improvements, shipping remains a contributor to global emissions of greenhouse gases, nitrogen oxides, and sulphur oxides, among others. As a part of its efforts to limit adverse health and environmental impacts of shipping, the International Maritime Organization (IMO) has enforced regulations to control these emissions. In addition, some countries, such as Norway have imposed additional regulations to control emissions further. Environmental regulations and concerns call for an improved environmental profile within shipping, which motivates this study.

Alternative fuels and power systems are required for a substantial reduction in emissions from shipping. Hydrogen and fuel cells are among the most promising solutions from an environmental perspective. A fuel cell is an electrochemical conversion device, which produces electricity through the reaction of an oxidant with hydrogen or another hydrogen-rich fuel, such as a hydrocarbon fuel. Since the electricity production does not entail fuel combustion, emissions are reduced substantially. When hydrogen gas is used as the fuel, only water is formed as the byproduct. In addition to emitting ultralow or zero emissions, fuel cells offer high energy conversion efficiency, low noise level, and low vibration.

The Norwegian energy system is based on electricity from renewable energy sources and mainly hydropower. Renewable energy output is strongly affected by the weather conditions, among other factors, and the supplies fluctuate accordingly. There is a need for a means to store and use the renewable energy surplus. In addition, from a technical point of view Norway still has potential to further develop hydro and wind power. Excess power can be used for production of hydrogen through water electrolysis, which in turn can fuel different means of transportation, such as shipping.

This paper aims at contributing to the research body on the use of hydrogen and fuel cells in shipping. First, a short introduction to hydrogen fuel and fuel cells is given. Then, an elaboration on pros and cons of powering vessels with fuel cells is presented. After providing an overview of current marine applications of fuel cells, the paper discusses potential vessels, which can benefit from this technology. Finally, the environmental benefits of using fuel cells are shown through a preliminary case study. Data from the Automatic Identification System (AIS) in the Norwegian waters is used for estimating operational profile of a vessel, its current emissions, and potential emission reduction by using hydrogen and fuel cells. The results of this study show the potential of hydrogen and fuel cells in reducing emissions of shipping and set forth the research gaps.

Commentary by Dr. Valentin Fuster
2017;():V010T09A012. doi:10.1115/OMAE2017-61752.

In this paper, load sharing curves are generated for marine systems with multiple gensets, where the goal is to reduce both gas emissions and fuel consumption. Initially, the average emissions and fuel consumption for each engine are calculated based on the specific emission and Specific Fuel Oil Consumption (SFOC) curves of each generator set (genset). An optimal nonlinear load sharing subject to gas emission and fuel consumption minimization is found for each engine. One result is that identical gensets should not have the same droop curve on the optimum condition, since it would lead to equal load sharing among them and a suboptimal configuration. Cases with two identical engines, two different engines, and multiple different engines were studied. The results show that by modifying the usually linear droop curve of engines, it is possible to reduce the fuel consumption and the gas emission, and it is also possible to fine tune the solution such that the fuel consumption or gas emissions are reduced.

Commentary by Dr. Valentin Fuster
2017;():V010T09A013. doi:10.1115/OMAE2017-62223.

Energy resources of offshore wind and ocean wave are clean, renewable and abundant. Various technologies have been developed to utilize the two kinds of energy separately. This paper presents the principle of an integrated generation unit for offshore wind power and ocean wave energy. The principle of the unit includes that: The wind rotor with retractable blades and the 3-DOF (degrees of freedom) mechanism with the hemispherical oscillating body are used to collect the irregular wind and wave power, respectively; The energy conversion devices (ECDs) are utilized to convert mechanical energy from both the wind rotor and the 3-DOF mechanism into hydraulic energy; The hydraulic energy is used to drive the hydraulic motors and electrical generators to produce electricity. Some analyses and experiments of the unit is conducted.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wave Energy — Analysis and Experimentation

2017;():V010T09A014. doi:10.1115/OMAE2017-61029.

In the paper, the integrated structure of a dual cylindrical caisson embodying an OWC device is designed. The seaside hemi-toroidal column of the dual cylinder caisson is an air chamber for water column oscillation while the leeside hemi-toroidal column of the dual cylinder is filled with sand for caisson stability. Experimental investigations were carried out on the performance of the OWC at different water depths, wave heights and wave periods, as well as on the layout of the caissons and the nozzle area ratio. Experimental results show that the water depth has significant effect on the wave energy conversion efficiency of dual cylindrical caisson breakwaters embodying an OWC. The new structure proposed as a breakwater or revetment structure especially arranged in a one-third convex arc has advantages in collecting more energy and creating a fine landscape environment.

Commentary by Dr. Valentin Fuster
2017;():V010T09A015. doi:10.1115/OMAE2017-61333.

In this paper, a detailed hydrodynamic analysis of the thrust generated by an oscillating hydrofoil is presented. The hydrofoil is mounted at the bow of a platform supply vessel as a means of auxiliary propulsion and, is vertically oscillating due to the ship’s heave and pitch motions in head-waves. Firstly, responses of the ship without hydrofoil are obtained using a 3-dimensional frequency-domain panel method. The responses of the ship with hydrofoil are then obtained by taking into account the extra forces and moments due to the presence of the foil. Secondly, three methods are used to calculate the average generated thrust, namely: a quasi-static method, a simplified frequency-domain method and, a dynamic stall method. All three methods are compared and appraised against numerical/experimental data available in the literature, for a similar hull form.

Commentary by Dr. Valentin Fuster
2017;():V010T09A016. doi:10.1115/OMAE2017-61478.

The Anaconda WEC belongs to a new generation of wave-energy converters that are currently on their way to reach a pre-commercial stage. It consists of a long rubber tube that is designed to float head to waves. The tube is filled with water and its stern is connected to a power take-off system (PTO). As a result of the interactions with the incoming ocean waves, the tube conveys internal pressure bulges whose intensity grows in the direction of the PTO. A pneumatic system is considered herein, in which electrical energy can be produced from a turbo-generator set.

The present research focuses on the performance assessment of a free-floating Anaconda model with air-flow PTO. The results of a series of tests with a 1:50 scale physical model in wave-tank are presented and discussed. In this model the pneumatic chamber connects with the atmosphere through an orifice plate. Several calibrated orifices of different diameters have been tested. The tests were undertaken in regular waves that translate to waves of 7 to 14 seconds in full-scale. Pressure in the pneumatic chamber and the water-column oscillations in the shaft were monitored. They provide estimates of the extracted power and energy capture efficiency of the system.

One of the aims of the study is to account for the effects of compressibility on the power output of the system, as well as properly assessing the impact of scale effects upon performance estimates. The results, presented as a function of the wave frequency, are ultimately used to predict prototype performance.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V010T09A017. doi:10.1115/OMAE2017-61522.

The paper presents a random vibration analysis of a U-Oscillating Water Column wave energy harvester (U-OWC). The U-OWC comprises a vertical duct on the wave beaten side, in addition to the elements of conventional OWCs. From a mathematical perspective, the U-OWC dynamic response is governed by a set of coupled non-linear differential equations with asymmetric matrices of mass, damping, and stiffness. In this work, an approximate analytical solution of the U-OWC equations of motion is sought by using the technique of statistical linearization. This technique allows pursuing rapid random vibration analyses via classical linear input-output relationships. The analysis is conducted by considering the case of the full-scale prototype in the port of Civitavecchia (Rome, Italy). The reliability of the proposed approach is assessed versus relevant Monte Carlo data. For this, realizations of sea states compatible with typical power spectral density functions of sea waves are employed. The performed analyses prove that the statistical linearization technique based approach is an efficient and reliable tool which may both circumvent the use of time-consuming Monte Carlo simulations, and be used for a variety of design optimization related parameter studies.

Commentary by Dr. Valentin Fuster
2017;():V010T09A018. doi:10.1115/OMAE2017-61537.

In the field of wave energy converter control, high fidelity numerical models have become the predominant tool for the development of accurate and comprehensive control strategies. In this study, a numerical model of a novel wave energy converter, employing a pneumatic power take-off, is created to provide a low-cost method for the development of a power-maximizing control strategy. Device components and associated architectures are developed in the time domain solvers Proteus DS and MATLAB/Simulink. These two codes are dynamically coupled at run time to produce a complete six degree of freedom, time domain simulation of the converter. Utilizing this numerical framework, a genetic algorithm optimization procedure is implemented to optimally select eight independent parameters governing the PTO geometry. Optimality is measured in terms of estimated annual energy production at a specific deployment location off the West Coast of Canada. The optimization exercise is one layer of PTO force control — the parameters selected are seen to provide significant improvements in the annual power output, while also smoothing the WEC power output on both a sea-state by sea-state and wave-by-wave basis.

Commentary by Dr. Valentin Fuster
2017;():V010T09A019. doi:10.1115/OMAE2017-61849.

This paper addresses experimental and numerical validation of power output efficiency about an approximate complex-conjugate control with considering the copper loss (ACL) method. A bottom-fixed point absorber type wave energy convertor (WEC) model was used for the experiments carried out at National Maritime Research Institute, Japan (NMRI). In order to model a power take-off (PTO) system constructed by a permanent magnet linear generator (PMLG), a liner shaft motor (LSM) was used for the model test. To investigate characteristics of the ACL method, the resistive load control (RLC) method and approximate complex-conjugate control (ACC) method were also tested by the WEC model. A simulation code based on WEC-Sim (Wave Energy Converter SIMulator) v2.0 written by MATLAB/Simulink, which is developed by collaboration works between the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (Sandia), was used for the validation. The simulated results in regular waves have good agreement with measured ones in terms of the float heave motion, the vertical force and the control input force. Through the experiments and numerical simulations in regular waves, the ACL method has advantages in high power production compared with the RLC and the ACC methods for the WEC model. In addition, the power output characteristics of the ACL method in irregular waves were checked experimentally and numerically.

Commentary by Dr. Valentin Fuster
2017;():V010T09A020. doi:10.1115/OMAE2017-61930.

Although linear theory is often used to analyse wave energy devices, it is in many cases too simplistic. Many wave energy converters (WECs) exceed the key linear theory assumption of small amplitudes of motion, and require the inclusion of non-linear forces. A common approach is to use a hybrid frequency-time domain model based on the Cummins equation with hydro-dynamic inputs coming from linear wave theory (Ref. [1]). Published experimental data is sparse (Ref. [2]) and the suitability for the broad variety of WEC technologies has yet to be proven.

This paper focuses on the challenges faced when attempting to validate a numerical model of a WEC using a variety of scaled physical tests in a waveflume. The technology used as a case study in this paper is a pitching WEC in close proximity to a fixed structure. Challenges are presented relating to waveflume effects and obtaining accurate physical input parameters to the numerical model.

Commentary by Dr. Valentin Fuster
2017;():V010T09A021. doi:10.1115/OMAE2017-62008.

This paper describes the validation process of a wave energy converter, the PB3, in support of achieving commercial ready status. The PB3 is a power and communication platform. It is a moored system which extracts useful electrical power from waves. The PB3 was developed for offshore remote autonomous applications such as the ones found in the oil and gas industry. Here, the results of full-scale ocean deployments of two PB3 PowerBuoys are reviewed in the context of final product performance validation, including power generation, and reliability. Furthermore, the Accelerated Life Testing of the PB3’s Power Takeoff (PTO) system is also discussed. A final Technology Readiness Level (TRL) is assessed based on API 17N TRL scale.

Topics: Oceans
Commentary by Dr. Valentin Fuster
2017;():V010T09A022. doi:10.1115/OMAE2017-62036.

The Resonant Wave Energy Converter 3 (REWEC3) is a wave energy converter belonging to the family of Oscillating Water Columns (OWCs). It comprises an oscillating water column and an air pocket connected to a turbine, as for traditional OWCs. In addition, it has a small vertical U-shaped duct used for connecting the water column to the open wave field. Because of this particular geometrical configuration, it is also known as U-Oscillating Water Column (U-OWC). During the past decade, small scale field experiments and theoretical analyses proved its potential for full scale applications. Currently, a full-scale prototype has been operating in the Port of Civitavecchia (Rome, Italy), where a REWEC3 was constructed within the context of a major port enlargement. This paper shows some results of the monitoring activity on a single chamber equipped with pressure gauges. The results show some initial energetic performances of the REWEC3 in wind-generated seas.

Commentary by Dr. Valentin Fuster
2017;():V010T09A023. doi:10.1115/OMAE2017-62139.

This study explores and verifies the generalized body-modes method for evaluating the structural loads on a wave energy converter (WEC). Historically, WEC design methodologies have focused primarily on accurately evaluating hydrodynamic loads, while methodologies for evaluating structural loads have yet to be fully considered and incorporated into the WEC design process. As wave energy technologies continue to advance, however, it has become increasingly evident that an accurate evaluation of the structural loads will enable an optimized structural design, as well as the potential utilization of composites and flexible materials, and hence reduce WEC costs. Although there are many computational fluid dynamics, structural analyses and fluid-structure-interaction (FSI) codes available, the application of these codes is typically too computationally intensive to be practical in the early stages of the WEC design process. The generalized body-modes method, however, is a reduced order, linearized, frequency-domain FSI approach, performed in conjunction with the linear hydrodynamic analysis, with computation times that could realistically be incorporated into the WEC design process.

The objective of this study is to verify the generalized body-modes approach in comparison to high-fidelity FSI simulations to accurately predict structural deflections and stress loads in a WEC. Two verification cases are considered, a free-floating barge and a fixed-bottom column. Details for both the generalized body-modes models and FSI models are first provided. Results for each of the models are then compared and discussed. Finally, based on the verification results obtained, future plans for incorporating the generalized body-modes method into the WEC simulation tool, WEC-Sim, and the overall WEC design process are considered.

Commentary by Dr. Valentin Fuster
2017;():V010T09A024. doi:10.1115/OMAE2017-62141.

Of interest in this study is the long-term response and performance of a two-body wave point absorber (“Reference Model 3”), which serves as a wave energy converter (WEC). In a previous study, the short-term uncertainty in this device’s response was studied for an extreme sea state. We now focus on the assessment of the long-term response of the device where we consider all possible sea states at a site of interest. We demonstrate how simulation tools may be used to evaluate the long-term response and consider key performance parameters of the WEC device, which are the heave and surge forces on the power take-off system and the power take-off extension. We employ environmental data at a designated deployment site in Northern California. Metocean information is generated using approximately 15 years of data from this site (National Data Buoy Center site no. 46022). For various sea states, a selected significant wave height and peak period are chosen to describe representative conditions. Then, using a public-domain simulation tool (Wave Energy Converter Simulator or WEC-Sim), we generate various short-term time-domain response measure for these sea states. Distribution fits to extreme response statistics are generated, for each bin that represents a cluster of sea states, using the open-source toolbox, WDRT (WEC Design Response Toolbox). Long-term distributions for each response variable of interest are estimated by weighting short-term distributions by the likelihood of the sea states; from these distributions, the 50-year response can be derived. The 50-year response is also estimated using an approximate but more efficient inverse reliability approach. Comparisons are made between the two approaches.

Commentary by Dr. Valentin Fuster
2017;():V010T09A025. doi:10.1115/OMAE2017-62675.

A detailed methodology was used to select the sea states tested in the final stage of the Wave Energy Prize (WEPrize), a public prize challenge sponsored by the U.S. Department of Energy [1]. The winner was selected based on two metrics: a threshold value expressing the benefit to effort ratio (ACE metric) and a second metric which included hydrodynamic performance-related quantities (HPQ). HPQ required additional sea states to query aspects of the techno-economic performance not addressed by ACE. Due to the nature of the WEPrize, limited time was allotted to each contestant for testing and thus a limitation on the total sea states was required. However, the applicability of these sea states was required to encompass seven deployment locations representative of the United States West Coast and Hawaii. A cluster analysis was applied to scatter diagrams in order to determine a subset of sea states that could be scaled to find the average annual power flux at each wave climate for the ACE metric. Four additional sea states were selected, including two highly energetic sea states and two bimodal sea states, to evaluate HPQ. These sea states offer a common experimental testing platform for performance in United States deployment climates.

Topics: Wave energy , Seas
Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wave Energy — Design and Optimization

2017;():V010T09A026. doi:10.1115/OMAE2017-61294.

Here, least-squares policy iteration, a reinforcement learning algorithm, is applied to the reactive control of a wave energy converter for the first time. Simulations of a linear point absorber are used for this analysis. The focus of this study is on the implementation of displacement constraints. The use of a penalty term is effective in teaching the controller to avoid the selection of combinations of the damping and stiffness coefficients that would result in excessive displacements in particular sea states. However, the controller can learn that the actions are bad only after trying them, as shown by the simulations. For this reason, a lower-level control scheme is proposed, which changes the sign of the controller force based on the magnitude of the float displacement and sign of its velocity. Its effectiveness is proven in both regular and irregular waves, although greater care is required for the determination of soft constraints.

Commentary by Dr. Valentin Fuster
2017;():V010T09A027. doi:10.1115/OMAE2017-61323.

It is expected that large farms of Wave Energy Converters (WECs) will be installed and as part of the consenting process it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the state-of-the-art of the methodologies to assess the impact a WEC farm has on the incoming wave field through the use of a coupling methodology. A Boundary Element Method (BEM) solver is used to obtain the near-field wave solution accounting for the wave-body interactions within the array of WECs and a Mild-Slope Equation (MSE) model is used to assess the wave transformation in the far-field. The near-field solution obtained from the BEM solver is described as an internal boundary condition in the MSE model and then propagated throughout the domain. The internal boundary condition is described by imposing the solution of the surface elevation in the area where the farm is located. The methodology is applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique is done first for a single flap and then for an array of 5 flaps. Finally, a mild-slope bathymetry and the influence of the changing depth on the wave transformation is assessed in order to prove the versatility of the method to be applied to real scenarios.

Commentary by Dr. Valentin Fuster
2017;():V010T09A028. doi:10.1115/OMAE2017-61612.

Experimental studies were carried out to investigate the possibility of utilization of auto-parametrically excited oscillation based on the Mathieu instability. For the simplicity, the subject is limited to the heaving motion of a spar-buoy type point absorber. The device consists of an inner cylinder and 12 outer cylinders. Each outer cylinder is equipped a movable floating column controlled by a ball screw mechanism and the buoyancy of the outer cylinders can be dynamically changed to induce the Mathieu instability. Based on results of the numerical simulation and the model tests, the possibility of utilization of auto-parametrically excited oscillation for WEC was shown.

Commentary by Dr. Valentin Fuster
2017;():V010T09A029. doi:10.1115/OMAE2017-61892.

In order to produce a large amount of electricity at a competitive cost, farms of Wave Energy Converters (WECs) will need to be deployed in the ocean. Due to hydrodynamic interaction between the devices, the geometric layout of the farm will influence the power production and affect the surrounding area around the WECs. Therefore it is essential to model both the near field effects and far field effects of the WEC farm. It is difficult, however, to model both, employing a single numerical model that offers the desired precision at a reasonable computational cost. The objective of this paper is to present a coupling methodology that will allow for the accurate modelling of both phenomena at a reasonably low computational cost. The one-way coupling proposed is between the Boundary Element Method (BEM) solver NEMOH, and the depth-averaged mild-slope wave propagation model, MILDwave. In the presented cases, NEMOH is used to resolve the near field effects whilst MILDwave is used to determine the far field effects.

Commentary by Dr. Valentin Fuster
2017;():V010T09A030. doi:10.1115/OMAE2017-61912.

A linear dynamic model for a wave energy converter (WEC) has been developed based on the results of experimental wave tank testing. Based on this model, a model predictive control (MPC) strategy has been designed and implemented. To assess the performance of this control strategy, a deployment environment off the coast of Newport, OR has been selected and the controller has been used to simulate the WEC response in a set of irregular sea states. To better understand the influence of model accuracy on control performance, an uncertainty analysis has been performed by varying the parameters of the model used for the design of the controller (i.e. the control model), while keeping the WEC dynamic model employed in these simulations (i.e. the plant model) unaltered. The results of this study indicate a relative low sensitivity of the MPC control strategy to uncertainties in the controller model for the specific case studied here.

Topics: Uncertainty
Commentary by Dr. Valentin Fuster
2017;():V010T09A031. doi:10.1115/OMAE2017-61917.

A study was performed to optimize the geometry of a point absorber style wave energy converter (WEC). An axisymmetric single-body device, moving in heave only, was considered. Design geometries, generated using a parametric definition, were optimized using genetic algorithms. Each geometry was analyzed using a boundary element model (BEM) tool to obtain corresponding frequency domain models. Based on these models, a pseudo-spectral method was applied to develop a control methodology for each geometry. The performance of each design was assessed using a Bretschneider sea state. The objective of optimization is to maximize harvested energy. In this preliminary investigation, a constraint is imposed on the the geometry to guarantee a linear dynamic model would be valid for all geometries generated by the optimization tool. Numerical results are presented for axisymmetric buoy shapes.

Commentary by Dr. Valentin Fuster
2017;():V010T09A032. doi:10.1115/OMAE2017-62136.

A wave energy converter (WEC) system has the potential to convert the wave energy resource directly into the high-pressure flow that is needed by the desalination system to permeate saltwater through the reverse-osmosis membrane to generate clean water. In this study, a wave-to-water numerical model was developed to investigate the potential use of a wave-powered desalination system (WPDS) for water production in the United States. The model was developed by coupling a time-domain radiation-and-diffraction-method-based numerical tool (WEC-Sim) for predicting the hydrodynamic performance of WECs with a solution-diffusion model that was used to simulate the reverse-osmosis process. To evaluate the feasibility of the WPDS, the wave-to-water numerical model was applied to simulate a desalination system that used an oscillating surge WEC device to pump seawater through the system. The annual water production was estimated based on the wave resource at a reference site on the coast of northern California to investigate the potential cost of water in that area, where the cost of water and electricity is high compared to other regions. In the scenario evaluated, for a 100-unit utility-scale array, the estimated levelized cost of energy for these WECs is about 3–6 times the U.S.’s current, unsubsidized electricity rates. However, with clean water as an end product and by directly producing pressurized water with WECs, rather than electricity as an intermediary, it is presently only 12% greater than typical water cost in California. This study suggests that a WEC array that produces water may be a viable, near-term solution to the nation’s water supply, and the niche application of the WPDS may also provide developers with new opportunities to further develop technologies that benefit both the electric and drinking water markets.

Commentary by Dr. Valentin Fuster
2017;():V010T09A033. doi:10.1115/OMAE2017-62174.

Preliminary conceptual design of a submerged wave energy converter (WEC) device in shallow water is presented. The WEC consists of a fully submerged, horizontal, flat plate that is restricted to vertical oscillations due to surface waves. Thin rails are used to guide the vertical oscillations and restrict the motion in all other directions. The vertical oscillation of the plate is converted to electricity by use of a direct drive power take-off (PTO) device located on the seafloor. The PTO is a linear generator consisting of a translator, directly linked to the plate by a solid shaft, and a stator. The plate oscillation is controlled by use of a spring, and by the damping effect of the PTO. The oscillations of the horizontal plate is determined by coupling the fluid governing equations with the equation of vertical motion of the horizontal plate that consists of sum of all vertically acting forces on the plate, including the vertical wave-induced force, the frictional force due to the guide rails, the spring force, and the damping force due to the PTO. The fluid flow is governed by use of the Level I Green-Naghdi equations. We also used the Navier-Stokes equations coupled with the volume of fluid method, solved through OpenFOAM. Comparison and discussion of the results from the two theoretical approaches are provided. The wave energy device has a very simple configuration, and the energy output is independent of the direction of incoming waves. Moreover, the entire device is fully submerged at all times and hence it is protected from the impact of breaking waves on the surface. It is concluded that the proposed wave energy device can be a reliable solution for wave energy harvesting in shallow to intermediate water, while minimizing the common challenges seen in many WEC devices.

Commentary by Dr. Valentin Fuster
2017;():V010T09A034. doi:10.1115/OMAE2017-62220.

A new class of Wave Energy Converter (WEC) is presented — the Floating Pendulum Dynamic Vibration Absorber (FPDVA). This concept offers significant design benefits to other WEC technology in the form of low cost installation and mechanical moving components located above the waterline only. The key elements of the FPDVA concept are highlighted. The performance of the concept is demonstrated through numerical modeling with calibration of the numerical models via physical tank testing. The Power Take Off (PTO) system is described, and the bench tests are presented. A discussion about the control systems required to operate the FPDVA system and the likely floating body mooring configurations are also presented. The technology has patent pending status. Future phased development of the technology is planned to progress its Technology Readiness Level (TRL) status from TRL 4 to TRL 9.

Commentary by Dr. Valentin Fuster
2017;():V010T09A035. doi:10.1115/OMAE2017-62575.

Model-Predictive Control (MPC) has shown its strong potential in maximizing energy extraction for Wave-Energy Converters (WECs) while handling hard constraints. As MPC can solve the optimization problem on-line, it can better account for state changes and reject disturbances from the harsh sea environment. Interests have arisen in applying MPC to an array of WECs, since researchers found that multiple small-size WECs are more economically viable than a single large-size WEC. However, the computational demand is known to be a primary concern for applying MPC in real-time, which can determine the feasibility of such a controller, particularly when it comes to controlling an array of absorbers.

In this paper, we construct a cost function and cast the problem into a Quadratic Programming (QP) with the machinery force being the “optimizer,” for which the convexity can be guaranteed by introducing a penalty term on the slew rate of the machinery force. The optimization problem can then be solved efficiently, and a feasible solution will be assured as the global optima. Constraints on the motion of the WEC and the machinery force will be taken into account. The current MPC will be compared to others existing in literature, including a nonlinear MPC [1] which has been applied in wave-tank tests. The effects of constraints on the control law and the absorbed power are investigated. Performances of the WEC are shown for both regular and irregular wave conditions. The current MPC is found to have good energy-capture capability and is able to broaden the band-width for capturing wave energy. The reactive power required by the PTO system is presented. The additional penalty term provides a tuning parameter, of which the effects on the MPC performance and the reactive power requirement are discussed.

Topics: Wave energy
Commentary by Dr. Valentin Fuster
2017;():V010T09A036. doi:10.1115/OMAE2017-62589.

While renewable energy is generally considered to be a well-researched field, wave energy converters (WECs) are still in early industrial stages, for example due to high costs, even though the potential of WECs in countries such as the UK is very high. Apart from the power plant location, the amount of power generated by a wave energy converter is also highly influenced by the efficiency of both the energy transfer from the wave to the plant’s generator and the power take-off (PTO) itself. Improving on any of these aspects therefore increases the power output and economic attractiveness. Based on a commercial development project by the NEMOS GmbH in Germany, this paper presents a more efficient means of connecting a floater and a rotary PTO based on a free traction mechanism consisting of a custom belt and matching pulley. In addition to regular longitudinal forces, the belt system can transfer transversal forces of up to 14 % of its pulling force and allowing run-in angles up to 8°. First tests show an average efficiency to 99.6 % in wet conditions. The paper lays out the theoretical background of the new design and discusses existing alternatives, before detailing the taken approach to design and optimization. The results are validated and compared to an existing rope design and a benchmark flat belt.

Commentary by Dr. Valentin Fuster
2017;():V010T09A037. doi:10.1115/OMAE2017-62651.

The Mutriku breakwater wave power plant is located in the Bay of Biscay, in Basque Country, Spain. The plant is based on the oscillating water column (OWC) principle and comprises 16 air chambers, each of them equipped with a Wells turbine coupled to an electrical generator with a rated power of 18.5 kW. The IDMEC/IST Wave Energy Group is developing a novel self-rectifying biradial turbine that aims to overcome several limitations of the Wells turbine, namely the sharp drop in efficiency above a critical flow rate. The new turbine is symmetrical with respect to a mid-plane perpendicular to the axis of rotation. The rotor is surrounded by a pair of radial-flow guide vane rows. Each guide vane row is connected to the rotor by an axisymmetric duct whose walls are flat discs. In the framework of the “OPERA” European H2020 Project, the new biradial turbine will be tested at Mutriku and later will be installed and tested on a floating OWC wave energy converter — the OCEANTEC Marmok-5’s — to be deployed at BiMEP demonstration site in September of 2017. The aim of the present paper is to perform critical comparisons of the performance of the new biradial and the Wells turbine that is presently installed at Mutriku. This is based on results from a time-domain numerical model. For the purpose, a new hydrodynamic frequency domain model of the power plant was developed using the well know WAMIT software package. This was used to build a time-domain model based on the Cummins approach.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wind Energy — Analysis and Operation

2017;():V010T09A038. doi:10.1115/OMAE2017-61062.

Aero-elasticity is an important issue for modern large scale offshore wind turbines with long slender blades. The behaviour of deformable turbine blades influences the structure stress and thus the sustainability of blades under large unsteady wind loads. In this paper, we present a fully coupled CFD/MultiBody Dynamics analysis tool to examine this problem. The fluid flow around the turbine is solved using a high-fidelity CFD method while the structural dynamics of flexible blades is predicted using an open source code MBDyn, in which the flexible blades are modelled via a series of beam elements. Firstly, a flexible cantilever beam is simulated to verify the developed tool. The NREL 5 MW offshore wind turbine is then studied with both rigid and flexible blades to analyse the aero-elastic influence on the wind turbine structural response and aerodynamic performance. Comparison is also made against the publicly available data.

Commentary by Dr. Valentin Fuster
2017;():V010T09A039. doi:10.1115/OMAE2017-61074.

This paper presents a preliminary development and validation of a high-order coupled time-domain simulation code DARwind for floating offshore wind turbine systems. In the code, unsteady Blade-Element-Momentum method with some corrections has been utilized to calculate aerodynamic loads. Combination of potential-flow theory and Morison“s equation are applied to calculate hydrodynamic loads. A quasi-static catenary mooring model is used to consider restoring forces from mooring lines. Kane“s dynamic equations and a high-order coupled model with mode superposition are proposed to model kinematics and structural dynamics of floating offshore wind turbine systems. Subsequently, the effectiveness of the code and its unique high-order coupled dynamic characteristics have been verified by code-to-code tests.

Commentary by Dr. Valentin Fuster
2017;():V010T09A040. doi:10.1115/OMAE2017-61203.

Depending on the environmental conditions, floating Horizontal Axis Wind Turbines (FHAWTs) may have a very unsteady behaviour. The wind inflow is unsteady and fluctuating in space and time. The floating platform has six Degrees of Freedom (DoFs) of movement. The aerodynamics of the rotor is subjected to many unsteady phenomena: dynamic inflow, stall, tower shadow and rotor/wake interactions. State-of-the-art aerodynamic models used for the design of wind turbines may not be accurate enough to model such systems at sea. For HAWTs, methods such as Blade Element Momentum (BEM) [1] have been widely used and validated for bottom fixed turbines. However, the motions of a floating system induce unsteady phenomena and interactions with its wake that are not accounted for in BEM codes [2]. Several research projects such as the OC3 [3], OC4 [4] and OC5 [5] projects focus on the simulation of FHAWTs.

To study the seakeeping of Floating Offshore Wind Turbines (FOWTs), it has been chosen to couple an unsteady free vortex wake aerodynamic solver (CACTUS) to a seakeeping code (InWave [6]). The free vortex wake theory assumes a potential flow but inherently models rotor/wake interactions and skewed rotor configurations. It shows a good compromise between accuracy and computational time.

A first code-to-code validation has been done with results from FAST [7]on the FHAWT OC3 test case [3] considering the NREL 5MW wind turbine on the OC3Hywind SPAR platform. The code-to-code validation includes hydrodynamics, moorings and control (in torque and blade pitch). It shows good agreement between the two codes for small amplitude motions, discrepancies arise for rougher sea conditions due to differences in the used aerodynamic models.

Commentary by Dr. Valentin Fuster
2017;():V010T09A041. doi:10.1115/OMAE2017-61204.

Numerical time- or frequency-domain techniques can be used to analyze motion responses of a floating structure in waves. Time-domain simulations of a linear transient or nonlinear system usually involve a convolution terms and are computationally demanding, and frequency-domain models are usually limited to steady-state responses. Recent research efforts have focused on improving model efficiency by approximating and replacing the convolution term in the time domain simulation. Contrary to existed techniques, this paper will utilize and extend a more novel method to the frequency response estimation of floating structures. This approach represents the convolution terms, which are associated with fluid memory effects, with a series of poles and corresponding residues in Laplace domain, based on the estimated frequency-dependent added mass and damping of the structure. The advantage of this approach is that the frequency-dependent motion equations in the time domain can then be transformed into Laplace domain without requiring Laplace-domain expressions of the added mass and damping. Two examples are employed to investigate the approach: The first is an analytical added mass and damping, which satisfies all the properties of convolution terms in time and frequency domains simultaneously. This demonstrates the accuracy of the new form of the retardation functions; secondly, a numerical six degrees of freedom model is employed to study its application to estimate the response of a floating structure. The key conclusions are: (1) the proposed pole-residue form can be used to consider the fluid memory effects; and (2) responses are in good agreement with traditional frequency-domain techniques.

Commentary by Dr. Valentin Fuster
2017;():V010T09A042. doi:10.1115/OMAE2017-61308.

This paper focuses on load reduction by implementing controllable trailing-edge flaps on an offshore wind turbine (OWT) supported on different fixed bottom structures in various water depths. The benchmark NREL 5-MW offshore horizontal axis wind turbine is used as a reference. This work utilizes the wind turbine simulation tool FAST with coupled stochastic aerodynamic-hydrodynamic analysis for obtaining the responses. The flap is controlled using an external dynamic link library through PID controller. Blade element momentum (BEM) theory and Morison equation are used to compute the aerodynamic and hydrodynamic loads, respectively. BEM theory is presently modified to account for unsteady effects of flaps along the blade span. Variation in force coefficients is shown due to unsteady effects of flaps. The present analysis results show reduction up to 8–29% in blade loads for the turbine with different support structures on implementing controllable trailing edge flaps. Also, an influence of blade load reduction on tower base and nacelle is shown. Tower loads are calculated considering aerodynamic and hydrodynamic loads individually. This study can form the basis for evaluating the performance for large-scale fixed offshore wind turbine rotors.

Commentary by Dr. Valentin Fuster
2017;():V010T09A043. doi:10.1115/OMAE2017-61339.

Fatigue assessment is a very important part in the design process of offshore wind turbine support structures subjected to wind and wave loads. Fully coupled time domain simulations due to wind and wave loads can potentially provide reliable fatigue predictions, however, it will take high computational effort to carry out fatigue analysis of the simultaneous wind and wave response of the support structure in time domain. For convenience and reducing computational efforts, a fast and practical method is proposed for predicting the fatigue life of offshore wind turbine jacket support structures. Wind induced fatigue is calculated in the time domain using ANSYS based on rainflow counting, and wave induced fatigue is computed in frequency domain using SACS based on a linear spectral analysis. Fatigue damage of X-joints and K-joints under combined environmental loads of wind and wave is estimated by using the proposed method. To verify the accuracy of the proposed formula, fatigue damage based on time domain rainflow cycle counting is calculated and can be considered as a reference. It is concluded that the proposed method provides reasonable fatigue damage predictions and can be adopted for evaluating the combined fatigue damage due to wind and wave loads in offshore wind turbine.

Commentary by Dr. Valentin Fuster
2017;():V010T09A044. doi:10.1115/OMAE2017-61446.

To achieve economically and technically viable floating support structures for large 10MW+ wind turbines, structural flexibility may increase to the extent that becomes relevant to incorporate along with the corresponding physical effects within aero-hydro-servo-elastic simulation tools. Previous work described a method for the inclusion of substructural flexibility of large-volume substructures, including wave-structure interactions through linear radiation-diffraction theory. Through an implementation in the HAWC2 simulation tool, it was shown that one may incorporate the effects of additional modes on substructure and wind turbine response as well as predict the excitation of substructure flexible modes. This work goes one step further and describes a method to calculate internal substructural stresses that includes dynamic effects. In dynamic calculations, the substructure flexibility is considered through a reduced set of modes, selected based on their relevance to the external load frequency range, and represented with a superelement. The implementation of this method in aeroelastic simulation tool HAWC2 and wave-structure analysis program WAMIT is described, highlighting the practical challenges. A case study of the DTU 10MW Reference Wind Turbine installed on the Triple Spar concept is presented to illustrate the method. The results show that the substructure flexible modes, global platform motion and wind turbine loads can influence sectional loads within the substructure.

Commentary by Dr. Valentin Fuster
2017;():V010T09A045. doi:10.1115/OMAE2017-61468.

The finite element model (FEM) of a pentapod offshore wind turbine (OWT) is established in the newly compiled FAST. The dynamic responses of the OWT are analyzed in detail. Further, a tuned mass damper as a passive control strategy is applied in order to reduce the OWT responses under seismic loads. The influence of the tuned mass damper (TMD) locations, mass and control frequencies on the reduction of OWT responses are investigated. A general configuration of TMD can effectively reduce the local and global responses to some degree, but due to the complexity of characteristics of the OWT structure and seismic waves, the single TMD can not obtain consistent controlling effects.

Commentary by Dr. Valentin Fuster
2017;():V010T09A046. doi:10.1115/OMAE2017-61512.

The rapid shrinkage of fossil fuel sources and contrary fast-growing energy needs of social, industrial and technological enhancements, necessitate the need of different approaches to exploit the various renewable energy sources. Among the several technological alternatives, wind energy is one of the most emerging prospective because of its renewable, sustainable and environment friendly nature, especially at its offshore locations. The recent growth of the offshore wind energy market has significantly increased the technological importance of the offshore vertical axis wind turbines, both as floating or fixed installations. Particularly, the class of lift-driven vertical axis wind turbines is very promising; however, the existing design and technology is not competent enough to meet the global need of offshore wind energy. In this context, the project AEROPITCH co-investigated by EOLFI, CORETI and IRPHE aims at the development of a robust and sophisticated offshore vertical axis wind turbine, which would bring decisive competitive advantage in the offshore wind energy market. In this paper, simulations have been performed on the various airfoils of NACA 4-series, 5-series and Selig profiles at different chord Reynolds numbers of 60000, 100000 and 140000 using double multiple streamtube model with tip loss correction. Based on the power coefficient, the best suitable airfoil S1046 has been selected for a 3-bladed vertical axis wind turbine. Besides the blade profile, the turbine design parameters such as aspect ratio and solidity ratio have also been investigated by varying the diameter and chord of the blade. Further, a series of wind tunnel experiments will be performed on the developed wind turbine, and the implementation of active pitch control in the developed turbine will be investigated in future research.

Commentary by Dr. Valentin Fuster
2017;():V010T09A047. doi:10.1115/OMAE2017-61525.

Dictated by the world’s escalating energy demands, offshore infrastructure is moving beyond the immediate continental shelf into deeper waters. Although the monopile solution has proven its reliability for many years, its feasibility in larger depths is questionable, or even limited, and multi-pod foundations, such as jacket structures, could be regarded as viable alternatives. Their main advantage, compared to the monopile alternative, is that they are able to sustain large lateral loads through axial stressing rather than bending at their supports (usually materialized using piles or suction caissons).

Recognizing this reality, the present study attempts to compare the performance of a conventional monopile system with that of a jacket foundation when taking into consideration extreme earthquake loading. Although safety fuses do exist to isolate the mechanical equipment from the direct effects of such loading, our focus in this study is on the irrecoverable deformation at the foundation level which, under circumstances, may render the turbine inoperable.

To this end, two foundation alternatives supporting an offshore wind turbine in the Mediterranean Sea are comparatively discussed: the conventional large diameter monopile and a jacket foundation supported by smaller piles or suction caissons. Results show that under expected operational loads the performance of the two systems is practically equivalent. However, extreme loading conditions may significantly alter the response and may, in some cases question the common practice.

Commentary by Dr. Valentin Fuster
2017;():V010T09A048. doi:10.1115/OMAE2017-61568.

Floating wind turbines (FWTs) are exposed to dynamic and cyclic environmental loads during their service life. Fatigue assessment has become an important aspect in the design phase of a FWT. A fracture mechanics (FM) based fatigue assessment was performed for the 12 points around the tower base of a 5 MW floating wind turbine supported on a spar platform. The aligned wind and waves are selected as environmental conditions for the fatigue assessment. The stress ranges on the wind turbine tower base are achieved through a rainflow counting method based on the results from the time-domain analysis using the FAST software. A comparison between fatigue lives predicted by the FM and S-N curves based approaches is made. The impact of the variation of initial crack depth, critical crack depth and stress concentration factors (SCFs) on the ratio of the fatigue life predicted by two approaches is investigated. The study shows that the fatigue life predicted by the FM based approach is more conservative than that predicted by the S-N curves based approach and also the fatigue life is highly sensitive to the material constant of Paris Law C and SCFs.

Commentary by Dr. Valentin Fuster
2017;():V010T09A049. doi:10.1115/OMAE2017-61587.

Nowadays there is increased interest to incorporate energy storage technologies with wind turbines to mitigate grid-related challenges resulting from the intermittent supply from large-scale offshore wind farms. This paper presents a new concept to integrate compressed air energy storage (CAES) in floating offshore wind turbine (FOWT) structures. The FOWT support structures will serve a dual purpose: to provide the necessary buoyancy to maintain the entire wind turbine afloat and stable under different met-ocean conditions and to act as a pressure vessel for compressed air energy storage on site. The proposed concept involves a hydro-pneumatic accumulator installed on the seabed to store pressurized deep sea water that is pneumatically connected to the floating support structure by means of an umbilical conduit. The present study investigates the technical feasibility of this concept when integrated in tension leg platforms (TLPs). The focus is on the impact of the additional floating platform weight resulting from the CAES on the dynamic response characteristics and loads when exposed to irregular waves. A simplified model for sizing the TLP hull for different energy storage capacities is initially presented. This is then used to evaluate the dynamic response of nine different TLP geometries when supporting the NREL1 5MW baseline wind turbine model. Numerical simulations are carried out using the marine engineering software tool ANSYS Aqwa©. The work provides an insight on how TLP structures supporting wind turbines may be optimised to facilitate the integration of the proposed CAES concept. It is shown that it is technically feasible to integrate CAES capacities on the order of Megawatt-Hours within TLP structures without compromising the stability of the floating system; although this would involve a substantial increase in the total structure weight.

Commentary by Dr. Valentin Fuster
2017;():V010T09A050. doi:10.1115/OMAE2017-61630.

Floating offshore wind offers a promising potential in the development of renewable energies. One of the key component of offshore wind farms projects is the inter-array power cable. Additionally, the dynamic behavior of floating support under offshore environmental conditions requires dynamic power cable structures which have to withstand both fatigue and extreme loads over its lifetime.

Both global and local analyses are required to properly design a dynamic power cable. Indeed, the axial, bending and torsion loads due to environmental conditions (waves and current) and the floating support motion are calculated from a global model by using a dedicated software. Then, the global loading has to be transferred to local sub-models in order to calculate stresses acting on the different components of the power cable.

This paper describes a 3D finite elements (FE) model dedicated to a detailed prediction of stresses in an armoured power cable. The loads under extreme environmental conditions are first evaluated from a global analysis. The local model, developed in a commercial FE software, uses periodic boundary conditions to reduce the computational costs and accurately model cables with constant curvatures in space. The model includes contact pressure and friction effects between all cable components, as well as potential lateral contacts between adjacent armour wires, and the radial stiffness of the cable core.

The applicative example, focusing on the amour layers, illustrates the potential of the model.

Commentary by Dr. Valentin Fuster
2017;():V010T09A051. doi:10.1115/OMAE2017-61686.

We present a multiscale approach to model a windfarm under real meteorological conditions. The multiscale model consists of a mesoscale atmospheric code coupled to a stochastic ocean wave model, a microscale model and a super-microscale model. The mesoscale model (with 2.5km × 2.5km horizontal resolution) forces the microscale model (with finer 100m × 100m horizontal resolution). The microscale model is capable of resolving surface variations both on wavy and complex terrain surfaces. Finally, the computed wind, temperature and turbulent kinetic energy from microscale model are used to provide boundary conditions to a super-microscale model, which has features to resolve turbine wakes using actuator line model. The three classes of models are validated using a very diverse array of observational data obtained from satellites, radiosondes and a wind tunnel. Towards the end, performance of an operational onshore wind farm under realistic meteorological conditions is evaluated.

Topics: Modeling , Wind farms
Commentary by Dr. Valentin Fuster
2017;():V010T09A052. doi:10.1115/OMAE2017-61688.

Most mesoscale models are developed with grid resolution in the range of kilometers. Therefore, they may require spatial averaging to analyze flow behavior over the domain of interest. In doing so, certain important features of sub-grid scales are lost. Moreover, spatial averaging on the governing equations results in additional terms known as dispersive fluxes. These fluxes are ignored in the analysis. The aim of this paper is to identify the significance of these fluxes for accurate assessment of flow fields related to wind farm applications. The research objectives are hence twofold: 1) to quantify the impact of wind turbines on MBL characteristics. 2) to account for the magnitude of dispersive fluxes arising from spatial averaging and make a comparison against the turbulent flux values. To conduct the numerical study the NREL 5MW reference wind turbine model is employed with a RANS approach using k-ε turbulence model. The results are presented concerning spatially averaged velocity, wake deficit behind the turbine, dispersive and turbulent fluxes.

Commentary by Dr. Valentin Fuster
2017;():V010T09A053. doi:10.1115/OMAE2017-61747.

This study evaluated, by time-domain simulations, the fatigue life of the jacket support structure of a 3.6 MW wind turbine operating in Fuhai Offshore Wind Farm. The long-term statistical environment was based on a preliminary site survey that served as the basis for a convergence study for an accurate fatigue life evaluation. The wave loads were determined by the Morison equation, executed via the in-house HydroCRest code, and the wind loads on the wind turbine rotor were calculated by an unsteady BEM method. A Finite Element model of the wind turbine was built using Beam elements. However, to reduce the time of computation, the hot spot stress evaluation combined FE-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Counting scheme and appropriate SN curves. Based on a preliminary sensitivity study of the fatigue damage prediction, an optimal load setting of 60-min short-term environmental conditions with one-second time steps was selected. After analysis, a sufficient fatigue strength was identified, but further calculations involving more extensive long-term data measurements are required in order to confirm these results. Finally, this study highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as opposed to the mean wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind and wave conditions.

Commentary by Dr. Valentin Fuster
2017;():V010T09A054. doi:10.1115/OMAE2017-61779.

In this paper, the original double symmetric cross section beam formulation in RIFLEX used to model the blades is compared against a newly implemented generalised beam formulation, allowing for eccentric mass, shear and elastic centres. The generalised beam formulation is first evaluated against an equivalent ABAQUS beam model (Using the generalised beam formulation implemented in ABAQUS) which consists of DTU 10MW RWT (reference wind turbine) blade in static conditions. A static displacement is applied to the tip, which is close to an operating load. The results appear very similar and ensure that the implementation is correct.

The extended beam formulation is afterwards used on the Land-based 10MW turbine from DTU with external controller. This case study aims at evaluating the effect of the newly implemented formulation on realistic, flexible structure. During the study, the blades were discretised using both the old and new formulation, and dynamic simulations were performed. The effect of the beam formulation was evaluated using several wind conditions that are thought to be characteristic of operating conditions. Results show slight difference between two formulations but could be more significant for next generation flexible blades.

Commentary by Dr. Valentin Fuster
2017;():V010T09A055. doi:10.1115/OMAE2017-61925.

This paper proposes a formulation for the assessment of the unsteady aerodynamics of floating offshore wind turbines, based on wind tunnel experiments through surge and pitch imposed motions on the 1/75 DTU 10 MW scale model. Rotor thrust and torque were analysed out of a set of different combinations of amplitudes and frequencies of the imposed mono-harmonic motion, following the idea of splitting these forces into steady and unsteady contributions, respectively through steady and unsteady aerodynamic coefficients. The latter were analysed, for different tip-speed ratios, both experimentally and numerically, with respect to a newly introduced parameter, the “wake reduced velocity”, which turned out to be effective in the description of the unsteady regime. Experimental results have shown good consistency of the formulation and put the basis for further studies on this topic, for the comprehension of this phenomenon and for the development of reduced-order models for control purposes, with the focus of the global system dynamics.

Commentary by Dr. Valentin Fuster
2017;():V010T09A056. doi:10.1115/OMAE2017-62039.

The main objective of this paper is to investigate the effects of hurricanes on low cycle fatigue of tower and blades in offshore wind turbines. For this purpose, first, recent observations on hurricane turbulence models were discussed. Second, the buffeting wind loads on the wind turbine structure were introduced. A new formulation was used to address unsteady wind forces on the tower. This new formulation was later used to modify NREL-FAST (Fatigue, Aerodynamics, Structures, and Turbulence) for the analysis. In the next step, according to importance of recent findings about hurricanes, hurricane wind and wave fields were simulated based on the Saffir-Simpson hurricane wind scale. Then, to investigate the effects of various turbulence models on the wind turbine structures, the modified NREL-FAST was used to analyze structure-wind-wave-soil interaction of the NREL-5 MW monopile wind turbine. Finally, the low cycle fatigue analysis was presented and discussed. Results for various hurricane turbulence models showed that by using quasi-steady analysis of the tower, the spectrum Model A and Model B resulted in average 53% lower and 12% higher damage index compared to the conventional Kaimal spectrum model respectively; however, by considering unsteady formulation on the tower, spectrum Model A and Model B resulted in average 96% and 24% lower blade root damage indices compared to the conventional Kaimal spectrum model respectively.

Commentary by Dr. Valentin Fuster
2017;():V010T09A057. doi:10.1115/OMAE2017-62077.

In order to run a fatigue analysis on a floating structure, it is common practice among ocean engineers to rely upon a large set of test cases, each with a unique set of environmental conditions. For a specific test site, the issue remains of how to obtain a limited set of environmental conditions for these test cases, sometimes known as bins, which can accurately recreate the conditions. When considering a floating offshore wind turbine, it is necessary to obtain a timeseries of not only the wave conditions, but also the wind conditions (and perhaps current, if possible). Thus, it is common to have greater than 5 dimensions in the time-series (e.g., significant wave height, wave period, wave direction, wind speed, wind direction, etc). The creation of bins in two dimensions is quite easily solved by creating an arbitrary grid and taking the mean of all the observations which fall in a specific cell. In higher dimensions, an N-dimensional cell is not easily visualized and so the resulting set of bins cannot easily be graphically represented. In this paper, an efficient, iterative algorithm is developed to convert N-dimensional metocean data into a set of discrete bins of arbitrary size. The algorithm works by setting a tolerance level on the number of observations that must be included in a cell in order to create a bin. If the population threshold is not met, the observations remain unbinned and another iteration is required. Generally, the population threshold can be a function of iteration number so that all observations will be binned. The algorithm can properly take into account extreme data by setting a tolerance level on the N-dimensional distance by which an observation can be included in a certain bin. A quality measure, q, is created to measure the level of representation of the original data by a set of bins, independent of the number of bins. Depending on the tolerance levels, the algorithm can be completed in seconds on a normal laptop for the available data set of 20 years with a 3-hour sampling rate. The observations and bins from a case study are shown as an example of how the bins can be created and visualized.

Commentary by Dr. Valentin Fuster
2017;():V010T09A058. doi:10.1115/OMAE2017-62620.

The coupled nonlinear motion and the influence of vortex induced loads on the motion of Spar type FOWT is studied based on the aero-hydro-vortex-mooring coupled model. The first order and second order difference-frequency wave loads are calculated based on 3D potential theory; the aerodynamic loads on the rotor are calculated based on blade element momentum theory; the vortex induced loads are simulated with CFD approach; the mooring forces are solved by the catenary theory and the nonlinearity of restoring forces are also considered. The nonlinear coupled model is set up and a numerical code is developed for solving the motion of OC3-Hywind Spar-type FOWT in time domain. The responses of FOWT are calculated under different load cases. It is shown that the amplitudes of sway and roll are excited by vortex excited force, and the roll amplitudes can reach to pitch value order in some circumstances. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to roll natural frequency, not only the roll but also the motions in other DOFs are increased and the super-harmonic resonance response occurs in heave motion.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wind Energy — Design and Simulations

2017;():V010T09A059. doi:10.1115/OMAE2017-61009.

Power production from the high energy density offshore wind has now emerged as a potential source of renewable energy for the future of mankind. While the installed global cumulative offshore wind capacity in 2014 was around 8 GW [1], almost all of this came from the shallow water sites (water depths equal to 25m or less) where typical bottom founded structures (such as: monopole, jacket, concrete gravity type, etc.) are used to support the turbines.

Today, most of the shallow water sites are exhausted, and the industry is looking for setting up turbines at the deeper water sites, where the existing bottom founded structures tend to become massive and expensive. On the other hand, the floating wind turbine concept, apart from being expensive, is more suitable for water depth of 100 m or more.

It is expected that in the near future, the industry will primarily focus on concepts that may extend the application range of the existing bottom founded structures towards the deeper waters.

In this paper, a novel support structure concept termed “Bottom supported tension leg tower” is presented, and its preliminary technical feasibility is checked for a monopile type structure for 50 m water depth. The concept can also be used with a jacket or a gravity type support structure. The potential of using the monopile based structure at 100m water depth is also briefly addressed.

Commentary by Dr. Valentin Fuster
2017;():V010T09A060. doi:10.1115/OMAE2017-61084.

In recent years, research on energy saving and emission reduction has been given much attention and the rise in the usage of renewable energy technologies has been remarkable. Through the use of wind energy, the goals of energy saving and emission reduction could be realized for ocean-going ships. Sail assisted technology is undoubtedly safe and reliable, but also is suitable for the current ship transportation. In order to ensure the safety of the ship and promote the full usage of clean energy at sea, a new type of ship propulsion-assisted wing sail was proposed innovatively. Focusing on the automatic and flexible structural features of the wing sail, this new type of collapsible wing sail is designed to increase the propulsion efficiency by enlarging the wing sail area in the transverse and vertical directions when the wind conditions get available. In the meanwhile, it can be folded up automatically in the poor weather condition and reduce the wind area in the transverse section and improve the sailing safety for ship. This new wing sail is composed of three parts: the main wing, the front wing and the back wing. Based on the technology of Computational Fluid Dynamics (CFD), the sail parameters such as gaps and rotation angles between different wings were investigated and the best setup parameters with excellent aerodynamic performance were fixed. Through the numerical simulation methods, the results of lift coefficients and drag coefficients for the new wing sail under different attack angles were obtained and also compared with the traditional arc-shaped rigid sail and variable-camber sail which was innovatively proposed by Qiao Li in 2015. From the viewpoint of the sailing performance of the vessel, our results demonstrate that this new type of wing sail has good aerodynamic performance and can reduce fuel costs for commercial vessels.

Topics: Wings
Commentary by Dr. Valentin Fuster
2017;():V010T09A061. doi:10.1115/OMAE2017-61121.

In this paper, integrated analyses performed in SIMA are compared against experimental results obtained using real-time hybrid model testing (ReaTHM®) carried out in the ocean basin facilities of MARINTEK in October 2015. The experimental data is from a 1:30 scaled model of a semi-submersible wind turbine. Coupled aero-hydro-servo-elastic simulations are performed in MARINTEK’s SIMA software. The present work extends previous results from Berthelsen et al. [1] by including a blade element/momentum (BEM) model for the rotor forces in SIMA and comparing the coupled responses of the system to the experimental results. The previously presented hydrodynamic model is also further developed, and the importance of second order loads (and applicability of approximate methods for their calculations) is examined. Low-frequency hydrodynamic excitation and damping are seen to be important, but these loads include a combination of viscous and potential forces. For the selected concept, the second order potential flow forces have limited effects on the responses.

Commentary by Dr. Valentin Fuster
2017;():V010T09A062. doi:10.1115/OMAE2017-61179.

The wind turbine design standards advise choosing one of two recommended turbulence models for load simulations of offshore wind turbines. The difference in fatigue loads for the two turbulence models is relatively small for bottom-fixed wind turbines, but some floating wind turbines show a higher sensitivity to the chosen turbulence model. In this study, the motions and mooring line fatigue damage of two semi-submersible floating wind turbines are investigated for three different wind speeds: 8 m/s, 14 m/s and 20 m/s, and three different wave states for each wind speed. For both concepts, the CSC 5 MW and the CSC 10 MW, the low-frequency surge response is important for the mooring line tension, and the simulations using the Kaimal turbulence model give the largest variation in tension at the surge eigenfrequency. However, using the Mann turbulence model in the load simulations give a higher response in the range of the blade passing frequency (3P). The CSC 10 MW has a higher aerodynamic thrust relative to the CSC 5 MW, and will therefore have a larger surge response at the lower frequencies than the CSC 5 MW. At the lowest wind speed, where the variation in mooring line tension at surge eigenfrequency is high, the fatigue damage is larger if the Kaimal turbulence model is applied to the load simulations. However, at the highest wind speed, using the Mann turbulence model in the simulations, give a higher mooring line fatigue damage.

Commentary by Dr. Valentin Fuster
2017;():V010T09A063. doi:10.1115/OMAE2017-61182.

In this paper two different approaches to calculate the wave impacts on a monopile are introduced and compared to model test results. Within the scope of the WiFi JIP (Joint Industry Project Wave Impacts on Fixed turbines) model tests at MARIN (Maritime Research Institute Netherlands) and at Deltares were conducted to investigate the effects of steep (and breaking) waves on the support structure of bottom fixed offshore wind turbines. Three different ways to generate breaking waves were used. At MARIN focused waves were generated to obtain breaking wave events. Two different methods to calculate slamming loads on monopiles are compared to the wave impacts measured. The results are analyzed in time history and load maxima plots. In addition the obtained curling factors of the focused waves are compared to irregular sea states recorded on a flat seabed and a sandwave. A good representation of the loads calculated by Wienke’s approach was observed for all waves investigated. However, to estimate the kinematics of focused breaking wave events more accurate methods than stream function are needed.

Topics: Waves , Turbines
Commentary by Dr. Valentin Fuster
2017;():V010T09A064. doi:10.1115/OMAE2017-61305.

The effects of operational wave loads and wind loads on offshore monopile wind turbines are well understood. For most sites, however, the water depth is such that steep and/or breaking waves will occur causing impulsive excitation of the monopile and consequently considerable stresses, displacements and accelerations in the monopile, tower and turbine.

At Belwind offshore wind farm (offshore Zeebrugge, Belgium) the waves and accelerations of a Vestas V90 3MW wind turbine have been monitored since November 2013, using wave radar and several accelerometers. During this period the wind turbine was exposed to several storms and experienced several wave impacts, resulting in vibrations in the monopile.

The measurements were compared with results from a numerical model for the flexible response of wind turbines due to steep waves. Previously this model was compared with scale model tests with satisfying results. The full-scale measurements provide an additional cross-check of the model.

The numerical model consists of a one-way coupling between a CFD model for wave loads and a simplified structural model based on mode shapes. An iterative wave calibration technique has been developed in the CFD model to ensure a good match between the simulated and measured incoming wave profile, obtained with the wave radar. This makes a deterministic comparison between simulations and measurements possible. This iteration is carried out in a 2D CFD domain (assuming long-crested waves) and is therefore relatively cheap. The calibrated numerical wave is then simulated in a 3D CFD domain including a (fixed) wind turbine. The resulting wave pressures on the turbine have been used to compute the modal excitation and subsequently the modal response of the wind turbine. The mode shapes have been estimated from the measured accelerations at the Belwind turbine. A grid refinement study was done to verify the results from the numerical model. The horizontal accelerations resulting from this one-way coupling are in fair agreement with the measured accelerations.

Commentary by Dr. Valentin Fuster
2017;():V010T09A065. doi:10.1115/OMAE2017-61408.

An innovative concept of harnessing wind energy is presented. The concept consists of a wind driven ship equipped with a hydro-generator that converts the kinetic energy of the water flow into electricity. The electricity is then converted into hydrogen by electrolysis. In the present study the use of a Flettner rotor is considered to propel the ship. A mathematical model of the hydrogen producing ship is developed based on existing data for high performance ship hulls and aerodynamic coefficients of existing Flettner rotors. The design is optimized with respect to the axial induction velocity through the water turbine disk. Results indicate that a 22m long vessel could produce 200 kW while a 80 m long vessel is able to generate 1 MW of mechanical power both for a true wind speed of 8 m/s.

Commentary by Dr. Valentin Fuster
2017;():V010T09A066. doi:10.1115/OMAE2017-61424.

A crucial problem regarding the offshore electricity generation is the levelized costs of energy (LCOE). This is an even larger problem for floating substructures for offshore wind turbines. This paper highlights a substructure for floating offshore wind turbines (FOWT) for a one step installation process. It deals with the parametric study of the TLP’s structure to gain hydrostatic and hydrodynamic stability during the transport and installation process of the TLP equipped with a 6 MW wind turbine. At first a hydrostatic analysis with the software tool MOSES (V.7.06.062) has been performed. Hydrodynamic simulations with ANSYS AQWA (V.17.2), based on 2D potential flow theory, have been conducted afterwards to get information about the motion behavior of the TLP in wind, current and waves.

Commentary by Dr. Valentin Fuster
2017;():V010T09A067. doi:10.1115/OMAE2017-61450.

The interest in floating offshore wind turbines (FOWT) has been growing substantially over the last decade and, after a number of prototypes deployed [1], the first offshore floating wind farms have been approved and are being developed. While a number of international research activities have been conducted on the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV2, OC4-Phase II3), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages [2]. Due to the substantial differences between HAWT and VAWT aerodynamics, the analyses on floating HAWT cannot be extended to floating VAWT systems.

The main aim of the present work is to compare the dynamic response of the FOWT system adopting two different mooring dynamics approaches. Two version of the in-house aero-hydro-mooring coupled model of dynamics for VAWT “FloVAWT” [3] are used: one which adopts a mooring quasi-static model, and solves the equations using an energetic approach [4], and a modified version of FloVAWT, which uses instead the lumped-mass mooring line model “MoorDyn” [5]. The floating VAWT system considered is based on a 5MW Darrieus type rotor supported by the OC4-Phase II3 semi-submersible.

The results for the considered metocean conditions show that MoorDyn approach estimate larger translational displacements of the platform, compared to the quasi-static rigid approach previously implemented in FloVAWT. As expected, the magnitudes of the forces along the lines are lower, being part of the energy employed for the elastic deformation of the cables. A systematic comparison of the differences between the two approaches is presented.

Commentary by Dr. Valentin Fuster
2017;():V010T09A068. doi:10.1115/OMAE2017-61451.

The design process for offshore wind turbines includes a fatigue life evaluation of the structure with the relevant environmental conditions at the specified wind farm location. Such analyses require long-term distributions of the environmental parameters including their correlation. In general, the significant wave height, wave peak period and mean wind speed are the most important parameters for describing offshore environmental conditions. However, due to the low side-to-side damping level of offshore bottom-fixed wind turbines, wave directions misaligned with the wind direction may excite low-damped vibrational modes. As a consequence, the accumulated fatigue damage in the wind turbine foundation may change, compared to collinear wind and waves. In the current work, an extension to the three-parameter environmental joint probability distribution is presented, with the resulting distribution being a function of the significant wave height, peak period of the total sea, mean wind speed and the wave directional offset compared to the mean wind heading i.e. the wind-wave misalignment. The sea states within a 1-year return period for Dogger Bank are presented, as well as the 10- and 50-year environmental contour lines and extreme wind-wave misalignment angles.

Commentary by Dr. Valentin Fuster
2017;():V010T09A069. doi:10.1115/OMAE2017-61495.

In the assessment of marine structures in shallow waters domain it is important to take into account the nonlinear (or non-Gaussian) nature of the irregular waves when predicting short and long-term responses of such structures. Other sources of nonlinearities in the response are also present due to some nonlinear effects such as: wet-dry surface effects, wind force on dry parts of the structure, drag term in Morison hydrodynamic force equation, etc. The estimation of the characteristic short-term extreme responses requires the extreme value analysis of a non-Gaussian stochastic process. There are many approaches available in literature which can be employed, such as: Hermite-based model, Weibull-fitting model, etc. In this paper two distinct Weibull fitting models (one based on the first two and other based on the first three moments of the response peaks sample) and Hermite-based models using both conventional and linear moments (L-moments) are investigated for the prediction of extreme short-term response of mono-column wind tower installed in a water depth of 20m and subject to wave, current and wind loading. The tower responses (load effects) time-histories are obtained by means of a time-domain finite element-based program using 3-D geometric nonlinear beam elements developed for the dynamic analysis of this type of structure. In this program, the nonlinear behavior of the irregular waves is modelled by means of the second order Sharma and Dean theory [1] and the wind forces are represented by a very simplified load model based on wind velocity simulated time-series and the obstruction area of the tower and blades.

Commentary by Dr. Valentin Fuster
2017;():V010T09A070. doi:10.1115/OMAE2017-61652.

A new floating foundation for multi-MW wind turbine is being developed within a collaboration between SBM Offshore and IFP Energies nouvelles. This inclined leg TLP, is made up of four immersed buoys and a bracing structure, making the floater transparent to wave excitation. The particular mooring arrangement gives the floater interesting motion properties since it creates a fixed point close to the nacelle, strongly reducing the motion at this elevation.

In order to validate the concept and the simulation strategy, a model test campaign has been carried out during three weeks in 2015 at MARIN’s offshore basin. The downscaling is performed according to the Froude law of similitude to maintain the hydrodynamic loadings and behavior. The tower bending natural period, the mooring stiffness, and the turbine rotation speed are also maintained in order to reproduce the relevant structural modes and check that no unexpected phenomena occur in the system during production or parked conditions.

The scale 1/50 was initially selected so that the MARIN Stock Wind Turbine (MSWT) can be used. This model wind turbine was designed by MARIN with low Reynolds blade airfoils to mimic the NREL 5 MW wind turbine, especially the thrust force. However because of mass distribution issues, the scale has to be changed from 1/50 to 1/40, at this scale only the thrust force and the rotation speed can be replicated.

First, a set of calibration tests are performed in the basin and simulated with Orcaflex™ and DeepLinesWind™ for a better understanding of the system and to validate independently the various components of the numerical models. Secondly, design parked and operational cases are conducted with wind, wave and current loadings for two floater orientations and two water depths.

The objective of this campaign is to validate the concept behavior as well as the simulation tools and methodologies. Hydrodynamic and structural models are very similar in both software and are checked with the calibration tests from the basin, whereas two strategies are implemented to model the aerodynamic contribution. The Simplified Coupled Simulations (SCS), performed with Orcaflex, use the aerodynamic forces recorded during the model tests to be imposed at tower top; the Fully Coupled Simulations (FCS), run with DeepLinesWind, use the aerodynamic loading computed with the BEM theory from the measured wind.

Commentary by Dr. Valentin Fuster
2017;():V010T09A071. doi:10.1115/OMAE2017-61654.

In the WiFi-JIP project, the impact of steep (and breaking) waves on a monopile support structure was studied. Observations during model tests showed that large tower top accelerations occur due to a slamming wave. Using experiments and simulations results, a new formulation of the design load for a slamming wave was developed. Instead of the embedded stream function, as applied in industry, the wave train is generated with the nonlinear potential flow code Oceanwave3D. On the wave train a set of conditions is applied to find the individual waves, that are closest to the prescribed breaking wave and most likely cause a slamming impact.

To study the effect of the new slamming load formulation on different sized offshore wind turbines, aero-hydroelastic simulations were performed on a classic 3MW wind turbine, a modern 4MW wind turbine and a future 10MW wind turbine. The simulations are performed with and without a slamming wave load. The slamming has a clear effect on the tower top acceleration. Accelerations due to the wave impact are highest for the 3MW model at the tower top and at 50m height. A serious tower top acceleration of almost 7m/s2 due to wave slamming is found for the 3MW turbine. This is an increase of 474% compared with the case of Morison wave loads only.

Commentary by Dr. Valentin Fuster
2017;():V010T09A072. doi:10.1115/OMAE2017-61655.

IFP Energies nouvelles (IFPEN) is involved for many years in various projects for the development of floating offshore wind turbines. The commercial deployment of such technologies is planned for 2020.

The present paper proposes a methodology for the numerical optimization of the inter array cable configuration. To illustrate the potential of such an optimization, results are presented for a case study with a specific floating foundation concept [1].

The optimization study performed aims to define the least expensive configuration satisfying mechanical constraints under extreme environmental conditions. The parameters to be optimized are the total length, the armoring, the stiffener geometry and the buoyancy modules. The insulated electrical conductors and overall sheath are not concerned by this optimization. The simulations are carried out using DeepLines™, a Finite Element software dedicated to simulate offshore floating structures in their marine environment. The optimization problem is solved using an IFPEN in-house tool, which integrates a state of the art derivative-free trust region optimization method extended to nonlinear constrained problems. The latter functionality is essential for this type of optimization problem where nonlinear constraints are introduced such as maximum tension, no compression, maximum curvature and elongation, and the aero-hydrodynamic simulation solver does not provide any gradient information.

The optimization tool is able to find various local feasible extrema thanks to a multi-start approach, which leads to several solutions of the cable configuration. The sensitivity to the choice of the initial point is demonstrated, illustrating the complexity of the feasible domain and the resulting difficulty in finding the global optimum configuration.

Commentary by Dr. Valentin Fuster
2017;():V010T09A073. doi:10.1115/OMAE2017-61676.

Experiments with both uni- and multidirectional wave realizations with a stiff pile subjected to extreme wave forces are considered. Differences in crest heights and force peaks resulting from directional spread waves are analysed. The wave realizations are reproduced numerically in the fully nonlinear wave model OceanWave3D. The numerical reproductions compare well to the experiments. Only for the largest forces significant differences are seen, which is due to a very simple breaking filter applied in OceanWave3D. In the wave spectra, the higher harmonics occur for smaller frequencies than the straight multiples of the peak frequency. Further, the higher harmonics of the multidirectional wave spectra contain less energy. Both effects can be explained by the second order wave theory. Finally, the computed wave kinematics are used to investigate the dynamic response of an offshore wind turbine. The excitation of the first natural frequency is largest for the unidirectional wave realizations, as the higher harmonics are largest for these realizations.

Commentary by Dr. Valentin Fuster
2017;():V010T09A074. doi:10.1115/OMAE2017-61689.

Recently, wind turbine has been developed from onshore area to offshore area because of more powerful available wind energy in ocean area and more distant and less harmful noise coming from turbine. As it is approaching toward deeper water depth, the dynamic response of the large floating wind turbine experiencing various environmental loads becomes more challenge. For examples, as the structural size gets larger, the dynamic interaction between the flexible bodies such as blades, tower and catenary mooring-lines become more profound, and the dynamic behaviors such as structural inertia and hydrodynamic force of the mooring-line get more obvious. In this paper, the dynamic response of a 5MW floating wind turbine undergoing different ocean waves is examined by our FEM approach in which the dynamic behaviors of the catenary mooring-line are involved and the integrated system including flexible multi-bodies such as blades, tower, spar platform and catenaries can be considered.

Firstly, the nonlinear dynamic model of the integrated wind turbine is developed. Different from the traditional static restoring force, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and the time in terms of vector equations. And, the modified finite element simulation is used to model a flexible and moving catenary of which the hydrodynamic load depending on the mooring-line’s motion is considered. Then, the nonlinear dynamic governing equations is numerically solved by using Newmark-Beta method.

Based on our numerical simulations, the influences of the dynamic behaviors of the catenary mooring-line on its restoring performance are presented. The dynamic responses of the floating wind turbine, e.g. the displacement of the spar and top tower and the dynamic tension of the catenary, undergoing various ocean waves, are examined. The dynamic coupling between different spar motions, i.e. surge and pitch, are discussed too. Our numerical results show: the dynamic behaviors of mooring-line may significantly increase the top tension, particularly, the peak-trough tension gap of snap tension may be more than 9 times larger than the quasi-static result. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the displacement of the spar significantly reduces by around 26%. Under regular wave condition, the coupling between the surge and pitch motions are not obvious; but under extreme condition, pitch motion may get about 20% smaller than that without consideration of the coupling between the surge and pitch motions.

Commentary by Dr. Valentin Fuster
2017;():V010T09A075. doi:10.1115/OMAE2017-61708.

A simulation study is performed to identify the key contributors to lifetime accumulated fatigue damage in the support-structure of a 10 MW offshore wind turbine placed on a monopile foundation in 30 m water depth. The relative contributions to fatigue damage from wind loads, wave loads, and wind/wave misalignment are investigated through time-domain analysis combined with long-term variations in environmental conditions. Results show that wave loads are the dominating cause of fatigue damage in the support structure, and that environmental condtions associated with misalignment angle > 45° are insignificant with regard to the lifetime accumulated fatigue damage. Further, the results are used to investigate the potential of event-based use of control strategies developed to reduce fatigue loads through active load mitigation. Investigations show that a large reduction in lifetime accumulated fatigue damage is possible, enabling load mitigation only in certain situations, thus limiting collateral effects such as increased power fluctuations, and wear and tear of pitch actuators and drive-train components.

Commentary by Dr. Valentin Fuster
2017;():V010T09A076. doi:10.1115/OMAE2017-61763.

This paper presents the numerical and experimental implementation of a 2 degrees-of-freedom (DoF) setup for simulating the surge and pitch motion of OC5 semi submersible floating offshore wind turbine, through the “hardware-in-the-loop” (HIL) approach during wind tunnel tests. This approach is hybrid since a real-time combination of computations and measurements are carried out during the experiments. This allows to separate the model tests of floating wind turbines into wave/ocean basin and wind tunnel tests, as it is currently done within the H2020/LIFES50+ project respectively at Marintek (Norway) and Politecnico di Milano (Italy), with the possibility of exploiting the advantages of each facility and overcoming the scaling issues and conflicts (e.g. Froude-Reynolds) that are emphasized when it comes to testing both wind and wave in a single test facility. In this paper the modelling approach and experimental implementation are presented, with a special focus on signals and data handling in the real-time HIL control system aimed at minimizing the effect of model/full scale discrepancies. Results are shown for free decays, regular and irregular sea states, showing promising results for the next 6-DoF system being finalized.

Commentary by Dr. Valentin Fuster
2017;():V010T09A077. doi:10.1115/OMAE2017-61798.

In this study, we assess the impact of different wave kinematics models on the dynamic response of a tension-leg-platform wind turbine. Aero-hydro-elastic simulations of the floating wind turbine are carried out employing linear, second-order, and fully nonlinear kinematics using the Morison equation for the hydrodynamic forcing. The wave kinematics are computed from either theoretical or measured signals of free-surface elevation. The numerical results from each model are compared to results from wave basin tests on a scaled prototype. The comparison shows that sub and superharmonic responses can be introduced by second-order and fully nonlinear wave kinematics. The response at the wave frequency range is better reproduced when kinematics are generated from the measured surface elevation. In the future, the numerical response may be further improved by replacing the global, constant damping coefficients in the model by a more detailed, customizable definition of the user-defined numerical damping.

Commentary by Dr. Valentin Fuster
2017;():V010T09A078. doi:10.1115/OMAE2017-61864.

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.

Commentary by Dr. Valentin Fuster
2017;():V010T09A079. doi:10.1115/OMAE2017-61944.

Floating Axis Wind Turbine is a concept of a floating vertical axis offshore wind turbine. In this design, a vertical axis turbine is directly mounted on a rotating spar buoy so that it does not require mechanical bearing supports of the heavy rotor. Multiple roller-generator units are on another small semi-sub float for extracting power from the rotating spar. A water tank model of 1/100 scale 5MW turbine and model power take-off units of about 1/20 scale are used for checking the concept. The results show the stability of the proposed turbine and demonstrates the function of roller-generator units.

Topics: Wind turbines
Commentary by Dr. Valentin Fuster
2017;():V010T09A080. doi:10.1115/OMAE2017-62038.

The dynamic response of floating offshore wind turbines is complex and requires numerous design iterations in order to converge at a cost-efficient hull shape with reduced responses to wind and waves. In this article, a framework is presented, which allows the optimization of design parameters with respect to user-defined criteria such as load reduction and material costs. The optimization uses a simplified nonlinear model of the floating wind turbine and a self-tuning model-based controller. The results are shown for a concrete three-column semi-submersible and a 10 MW wind turbine, for which a reduction of the fluctuating wind and wave loads is possible through the optimization. However, this happens at increased material costs for the platform due to voluminous heave plates or increased column spacing.

Commentary by Dr. Valentin Fuster
2017;():V010T09A081. doi:10.1115/OMAE2017-62040.

The objective of the Joint Industry Project Wave impact on Fixed foundations (WiFi JIP) was to increase the understanding of breaking and steep wave impact’s on fixed foundations of offshore wind turbines (OWT). The project was set-up as a Joint Industry Project (JIP) and in total 20 companies and research institutes participated in the project.

In this paper a summary of the complete WiFi JIP project will be presented. At the start of the project the state of the art design methods and guidelines were reviewed (WP1). Thereafter a jacket and a monopile foundation were designed using these state-of-the-art tools that were available at the start of the project. This effort has been reported in WP2 , where design computations were carried out using the embedded stream function approach for several sea states. In this WP Siemens, ECN and Ramboll also calculated the impact response of the monopile to surging and spilling type wave breakers with their engineering tools. In the next phase the designed foundations were tested in MARIN’s shallow water model basin. The foundation for the monopile was modelled as a rigid and flexible foundation. The foundations were tested in regular waves, irregular sea states and so called focused waves. During the model tests the wave heights, wave run-up, accelerations, impact pressures and loads on the foundation and boat landing were measured. The model test results were reported in WP3 and 7 and used as validation for WP9 and 10. WP4 delivered more understanding of realistic design conditions for areas typical for OWT, like the North Sea. Particular attention was paid to the probability of occurrence of breaking and steep waves and the associated slamming load. For this an extensive 5 week experimental program was performed from September to October 2013 in the wide wave-current flume at Deltares (Atlantic Basin). During these tests both waves and current were simulated and two bathymetries.

WP8 provided analyses of the performed full scale measurements on the response of a OWT. The full-scale measurements were done for a Vestas V90 3MW wind turbine in the Belwind windfarm which is located 46 km off the coast of Zeebrugge on the Bligh Bank. The CFD simulations performed in WP 9 showed that a good agreement is obtained between the CFD simulations and the model and full scale measurement.

In work package 10, an improved methodology was developed based on the outcome of the previous WP’s to model the breaking wave impact of plunging type breakers. In WP11 and 12 this new approach is applied on different case study’s by ECN.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V010T09A082. doi:10.1115/OMAE2017-62197.

In this paper, outline of the demonstration test for using suction anchor and polyester rope in a floating wind mast (that was converted from floating wind turbine) is presented. After marine soil investigations and specimen test for polyester rope, the basic design methodology for mooring line including polyester rope and suction anchor is presented. Considering the marine soil investigation result, the profile of the suction anchor was determined to be somewhat short and wide dimensions. Installation of the suction anchor was successfully made at both the test site and the targeted demonstration site. Here, a newly developed suction pump system was successfully used. Installation of polyester rope was also made successfully. However, we observed discrepancy in the suction pressure during the installation of suction anchor at the demonstration site, although we did not see the discrepancy at the trial test site.

Commentary by Dr. Valentin Fuster
2017;():V010T09A083. doi:10.1115/OMAE2017-62246.

In the present paper, the dynamic response of a spar buoy wind turbine under different wind and wave conditions is discussed. Physical model tests were performed at the Danish Hydraulic Institute (DHI) off-shore wave basin within the EU-Hydralab IV Integrated Infrastructure Initiative.

The OC3-Hywind spar buoy was taken as reference prototype. A spar buoy model, 1:40 Froude-scaled, was tested using long crested regular and irregular waves, orthogonal (0 degrees) and oblique (20 degrees) to the structure. Here the results concerning regular waves, with incidence orthogonal to the structure, are presented; the selected tests considered rotating and non-rotating blades. Measurements of displacements, rotations, accelerations, forces response of the floating structure and at the mooring lines were carried out. Based on the observed data, FAST wind turbine simulation tool, developed and maintained by the U.S. Department of Energy’s (DOE’s), National Renewable Energy Laboratory (NREL), was calibrated and verified. The numerical model takes into account the wave induced response and the effects of the mooring lines on the overall system.

The adopted spar buoy has three equally spaced mooring lines that were modelled as quasi-static taut or catenary lines through MAP++ (static module) and MoorDyn (dynamic module) in the FAST simulation tool. The tensions along the fairleads of the three mooring lines were examined.

At the end of the calibration procedure, the numerical model was successfully used to simulate the dynamic motions of the floating wind turbine under combinations of wind and sea states for the selected wave attacks.

All data from the DHI tests were converted to full scale using Froude scaling before being analyzed.

Commentary by Dr. Valentin Fuster
2017;():V010T09A084. doi:10.1115/OMAE2017-62452.

Offshore wind energy extraction has gathered momentum around the world due to its advantages over onshore wind farms at various fronts. The floating support system with vertical axis wind turbine might prove to be feasible concept in medium to deep waters. In this context, this paper addresses an investigation of hydrodynamic analysis of three column semi-submersible with Vertical Axis Wind Turbine (VAWT) in parked condition under regular and random waves. Free decay experiments were conducted for using scale model (1:75) in a laboratory wave basin at the Department of Ocean Engineering in Indian Institute of Technology Madras, India. Computational Fluid Dynamics (CFD) simulations were used to assess damping characteristics and validated with the experiments. Numerical simulations of hydrodynamic motion response of the floater were carried out using potential flow theory based commercial software (ANSYS AQWA). The damping values obtained from experiments were used in numerical simulations to obtain motion response and Response Amplitude Operator (RAO). The motion response obtained from the study was used to verify the suitability of the system for deployment in east and west coast of India.

Commentary by Dr. Valentin Fuster
2017;():V010T09A085. doi:10.1115/OMAE2017-62585.

The total wave load on a gravity based foundation for offshore wind turbines is influenced by the pore pressure from beneath the structure. The pore pressure is induced by the wave-structure-seabed interaction. Often the uplift force is included in a simplified way in the design of the gravity based foundation. This leads typically to very conservative designs in order to accommodate the uncertainties in the procedure. The experiments shall lead to better prediction models based on for instance CFD model’s with the direct calculation of pressure variations in the seabed and any erosion protection layer. Herewith, it will be possible to get a direct assessment of wave loads on the foundation, also under the seabed level. The study includes experiments as well as numerical analyses. A good agreement between the experimental results and the numerical analyses was found. In the numerical analyses, it was possible to investigate the effect of air content in the pores, which turned out to have an effect on the distribution of the pore pressure.

Commentary by Dr. Valentin Fuster
2017;():V010T09A086. doi:10.1115/OMAE2017-62625.

The VAWT (vertical axis wind turbine) has advantages in the development of large-scale offshore wind power. This paper presents a motion study of a 5 MW floating VAWT composed of the Φ type Darrieus wind turbine and a truss spar floating foundation with heave plates. The surge, heave and pitch motion equations considering the effects of retardation function of the floating VAWT were established and solved numerically. Several load cases were carried out to analyze the motion performances of the floating VAWT. The results show that the wind forces have minimal influence on the heave motions of the floating VAWT, while they obviously increase its surge and pitch mean displacements. For LC3, the surge, heave and pitch frequencies of the floating VAWT are dominated by the wave frequencies, and the 2P (twice-per-revolution) response of pitch motions is not significant. For LC4, the 2P response of pitch motions of the floating VAWT are more significant than LC4.

Commentary by Dr. Valentin Fuster

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