Habib J. Dagher

Experimental Comparison of Three Floating Wind Turbine Concepts

Beyond many of the Earth’s coasts exist a vast deepwater wind resource that can be tapped to provide substantial amounts of clean, renewable energy. However, much of this resource resides in waters deeper than 60 m where current fixed bottom wind turbine technology is no longer economically viable. As a result, many are looking to floating wind turbines as a means of harnessing this deepwater offshore wind resource. The preferred floating platform technology for this application, however, is currently up for debate. To begin the process of assessing the relative advantages of various platform concepts for floating wind turbines, 1/50 th scale model tests in a wind/wave basin were performed at MARIN (Maritime Research Institute Netherlands) of three floating wind turbine concepts. The Froude scaled tests simulated the behavior of the 126 m rotor diameter NREL (National Renewable Energy Lab) 5 MW, horizontal axis Reference Wind Turbine attached via a flexible tower in turn to three distinct platforms, these being a tension leg-platform, a spar-buoy and a semi-submersible. A large number of tests were performed ranging from simple free-decay tests to complex operating conditions with irregular sea states and dynamic winds. The high-quality wind environments, unique to these tests, were realized in the offshore basin via a novel wind machine which exhibited low swirl and turbulence intensity in the flow field. Recorded data from the floating wind turbine models include rotor torque and position, tower top and base forces and moments, mooring line tensions, six-axis platform motions and accelerations at key locations on the nacelle, tower, and platform. A comprehensive overview of the test program, including basic system identification results, is covered in an associated paper in this conference. In this paper, the results of a comprehensive data analysis are presented which illuminate the unique coupled system behavior of the three floating wind turbines subjected to combined wind and wave environments. The relative performance of each of the three systems is discussed with an emphasis placed on global motions, flexible tower dynamics and mooring system response. The results demonstrate the unique advantages and disadvantages of each floating wind turbine platform.

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

A new floating wind turbine platform design called VolturnUS developed by the University of Maine uses innovations in materials, construction, and deployment technologies such as a concrete semisubmersible hull and a composite tower to reduce the costs of offshore wind. These novel characteristics require research and development prior to full-scale construction. This paper presents a unique offshore model testing effort aimed at derisking full-scale commercial projects by providing scaled global motion data, allowing for testing of materials representative of the full-scale system, and demonstrating full-scale construction and deployment methods. A 1:8-scale model of a 6 MW semisubmersible floating wind turbine was deployed offshore Castine, ME, in June 2013. The model includes a fully operational commercial 20 kW wind turbine and was the first gridconnected offshore wind turbine in the U.S. The testing effort includes careful selection of the offshore test site, the commercial wind turbine that produces the correct aerodynamic thrust given the wind conditions at the test site, scaling methods, model design, and construction. A suitable test site was identified that produced scaled design load cases (DLCs) prescribed by the American Bureau of Shipping (ABS) Guide for Building and Classing Floating Offshore Wind Turbines. A turbine with a small rotor diameter was selected because it produces the correct thrust load given the wind conditions at the test site. Some representative data from the test are provided in this paper. Model test data are compared directly to full-scale design predictions made using coupled aeroelastic/ hydrodynamic software. Scaled VolturnUS performance data during DLCs show excellent agreement with full-scale predictive models. Model test data are also compared directly without scaling against a numerical representation of the 1:8-scale physical model for the purposes of numerical code validation. The numerical model results compare favorably with data collected from the physical model. [DOI: 10.1115/1.4030381]

Advanced Fiber-Reinforced Polymer-Wood Composites in Transportation Applications

Six wood-fiber-reinforced polymer (FRP) composite projects have been conducted over the past decade with three types of technologies developed at the University of Maine. The three featured technologies are (a) tension-reinforced glulam beams with preconsolidated E-glass FRP (GFRP) panels, (b) tension-reinforced glulam beams and panels with wet-impregnated E-glass fabrics, and (c) stress-laminated lumber using GFRP tendons. Through these six demonstration projects, it has been shown that properly designed wood-FRP composites are structurally feasible, durable, and cost-effective in selected applications.

Probabilistic Finite Element Analysis of Modified ASTM D3039 Tension Test for Marine Grade Polymer Matrix Composites

The variability of tensile mechanical properties of a polymer matrix composite material with woven fabric reinforcement is studied using both experimental work and numerical simulations. Four E-glass/vinyl ester composite plates were fabricated using the vacuum-assisted resin transfer molding (VARTM) by a US Navy contractor. The materials and process selected are representative of Marine grade composites typically used by the US Navy. Standard and modified D3039 tensile coupons were obtained from the plates and the laboratory results were compared with those of a 3D probabilistic finite element analysis (FEA). In the probabilistic FEA model, elastic properties, strength parameters, and geometric properties of the woven fabric E-glass/vinyl ester coupons were considered as random fields, and generated using Monte Carlo simulations. The study evaluates the effects of spatial correlation, finite element size, probability distribution functions (PDF) types, and failure criteria on statistical strength properties of the [(0w/90f)/(0f/90 w)]2s tension coupons. Comparisons of experimental and probabilistic FEA results provide useful information on how to assign mean, COV, and PDF of material properties to individual finite elements within a mesh. The results are relevant in developing design properties for these composites.

DURABILITY OF COMPOSITE REINFORCEMENT FOR TIMBER BRIDGES

Fiber-reinforced polymeric (FRP) composites are materials that are increasing in use in civil engineering applications. Despite the excellent mechanical properties and corrosion resistance offered by these organic matrix materials, their susceptibility to the synergistic effects of stress and environmental weathering hinders their widespread acceptance in civil engineering. The durability of a specific formulation of wood-compatible, pultruded, E-glass–phenolic composite is characterized. This composite is unique because its layered structure and void content make it compatible with standard structural wood adhesives. The durability of this wood-compatible FRP reinforcement cannot be directly determined from published work on the durability of E-glass composites because of its unique design. A durability test matrix was generated according to specifications and test standards from the International Conference of Building Officials Evaluation Service, Inc., and from the California Department of Transportation. Physical and mechanical properties that were used as indicators of degradation mechanisms and that applied to the bridge environment included tensile behavior, interlaminar shear strength, void content, and glass-transition temperature. Environmental testing involved exposure to various storage media, such as moisture, saline solutions, and calcium carbonate, followed by mechanical testing. Other exposure treatments included dry heat, cyclic freeze-thaw, accelerated weathering, and natural weathering. In addition to the strength-retention determination after environmental conditioning, control and exposed specimens were examined visually with optical and scanning electron microscopy to determine surface changes and their effect on failure and fracture modes.

ReLAM: Nonlinear Probabilistic Model for the Analysis of Reinforced Glulam Beams in Bending

AbstractThe University of Maine, in conjunction with Willamette Industries, Georgia-Pacific, Strongwell, and APA—the Engineered Wood Association, tested 90 fiber-reinforced polymer (FRP)-reinforced glue-laminated wood (glulam) beams. This study showed that a FRP reinforcement ratio of 3% in tension can increase glulam-allowable bending stress (Fb) by more than 100%. Furthermore, this physical testing was used to verify a nonlinear probabilistic computer model for reinforced glulam, called the reinforced laminated (ReLAM) model. ReLAM uses moment-curvature (M-Φ) analysis and Monte Carlo simulation to predict the strength and stiffness of a population of reinforced glulams, requiring as input distributions of the lamstock long-span flatwise-bending modulus of elasticity (E), ultimate tensile stress, and ultimate compressive stress. ReLAM output lists the reinforced glulam modulus of elasticity (MOE) and Fb in terms of the gross cross-sectional dimensions, providing beam properties in the same fashion as the...

Size effects on the bending strength of fiber-reinforced polymer matrix composites

This article investigates the size effects associated with length and width, on the bending strength of marine grade polymer matrix laminated composites. A new expression for predicting the strength size effect was developed and was verified using both laboratory testing and probabilistic finite element analysis (FEA). Woven [0/45/—45/0]5sf E-glass/vinyl ester material specimens with different lengths and widths and same thickness were evaluated to predict the effect of specimen size on the bending strength. In the probabilistic FEA models, elastic properties (E11, E22, G12, υ12), and strength parameters (F1t, F1c, F2t , F2c, F6, ε1t, ε 1c, ε2t, ε2c, ε12) of the woven fabric E-glass/vinyl ester ASTM D6272 coupons were considered as random fields, and generated using Monte Carlo simulations. The relationship between the bending strength and specimen size of the [0/45/—45/0]5sf lay-up was also evaluated experimentally. Thirty-two material specimens were prepared and tested according to ASTM D6272 using combinations of two span-to-thickness ratios (32 : 1 and 16 : 1), and two width-to-thickness ratios (3 : 1 and 1.5 : 1).

Probabilistic Finite Element Analysis of ASTM D6641 Compression Test for Marine Grade Polymer Matrix Composites

The variability of compressive mechanical properties of a polymer matrix composite material with woven fabric reinforcement is studied using both experimental work and numerical simulations. Four E-glass/vinyl ester composite plates were fabricated using the vacuum-assisted resin transfer molding (VARTM) by a US Navy contractor. The materials and process selected are representative of marine grade composites typically used by the US Navy. Standard ASTM D6641 coupons were obtained from the plates and the laboratory results were compared with those of a 3D probabilistic finite element analysis (FEA). In the probabilistic FEA model, elastic properties, strength parameters and geometric properties of the woven fabric E-glass/vinyl ester coupons were considered as random fields, and generated using Monte Carlo simulations. The study evaluates the effects of spatial correlation, finite element size, probability distribution functions (PDF) types and failure criteria on statistical strength properties of the [0/90]2sf compression coupons. Comparisons of experimental and probabilistic FEA results provide useful information on how to assign mean, Coefficient of variance (COV) and PDF of material properties to individual finite elements within a mesh. The results are relevant in selecting proper test methods and developing design properties for these composites.

Structural Performance of Wood Plastic Composite Sheet Piling

This paper describes the structural performance of an innovative wood plastic composite (WPC) sheet piling. Tensile coupon tests were performed to determine tensile strength and modulus over a range of strain rates for both WPC and specimens cut from commercially available vinyl sheet piles. Full scale four-point bending tests were conducted up to failure on 20 sets of joined pairs of WPC Z-piles with four different span lengths. For comparison, bending and coupon-level tension tests were also conducted on commercially available vinyl Z-piles and coupons cut from them. The coupon-level tests indicated that WPC had on average a 14% greater modulus than the vinyl specimens in the initial linear range, defined as between 10 and 40% of the respective material’s ultimate tensile strength. However, if the modulus is computed over the same stress range of 10–40% of the WPC ultimate tensile strength, both materials have a similar modulus. The vinyl had a tensile strength 3.6 times that of the WPC. During the bend...

Methodology for optimizing composite towers for use on floating wind turbines

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