António Sarmento - Academia.edu (original) (raw)

Papers by António Sarmento

The present paper reports a numerical study concerning the modeling and geometric optimization of... more The present paper reports a numerical study concerning the modeling and geometric optimization of a two body heave oscillating point absorber wave energy converter (WEC) using the force reacting principle. The device work principle is based on a surface body (or floater) force-reacting against a submerged body, as the excitation force of both, the floater and the submerged body, are in phase opposition. The relative motion between the two bodies generates electrical power through a linear power take off system (PTO). From the hydrodynamic point of view, the main purpose of the concept is to emphasize the radiation capabilities of the body placed under the free surface, as it has both excitation force components (diffraction and Froude Krylov) in phase. Therefore, the excitation force (and thus the hydrodynamic damping coefficient) is higher in the case of a submerged volume variation heave oscillating body (like the AWS wave energy converter) when compared to a floating buoy with th...

The present study aims at assessing the key spectral parameters of the incident wave on a fixed o... more The present study aims at assessing the key spectral parameters of the incident wave on a fixed oscillating water column (OWC) device, based on the air pressure measurements inside the chamber and on the numerical hydrodynamic coefficients of the device. The methodology is based on the equation of continuity of the air in the time-domain and linear decomposition of the air flow in the usual terms of radiation and diffraction flows. By applying the Fast Fourier Transform, the time-domain equation is transposed to the respective frequency-domain equation. This methodology was applied to the 400 kW OWC power plant, on the Island of Pico, Azores, which has been monitored since 2005 by the Wave Energy Centre. The numerical hydrodynamic coefficients obtained by the 3D radiation-diffraction boundary element code, AQUADYN-OWC, were used in this study. No measurements of the incident wave in front of the plant are available; therefore the results obtained for a set of records are compared wi...

The 400-kW Pico OWC plant built in 1995-1999 is a bottom-mounted shoreline structure, equipped wi... more The 400-kW Pico OWC plant built in 1995-1999 is a bottom-mounted shoreline structure, equipped with a horizontalaxis, Wells turbine-generator set. Main refurbishment actions through 2004-2006 included the complete replacement of the degraded electrical equipment and restorations of the existing mechanical components. Full-scale monitoring included the analysis of instantaneous values of the water free-surface elevation inside the chamber, the air pressure in the chamber and turbine duct, the rotational speed of the turbine and the electrical power output. This paper describes the plant monitoring during the full-scale demonstration from September 2005 to October 2006 and discusses the data and results obtained so far.

Engineering Failure Analysis, 2015

The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes le... more The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes lead to stall in the turbine and stators' blades, which represents an unintended action as it creates potentially damaging conditions to the mechanical components, plus reducing the efficiency of the takeoff system. This work focuses on the cracking damage caused to the guide vanes of the Pico Island Wave Power Plant due to the loading from turbulent air flow and consequent vibrations, as a result of turbine stall. Comprehensive analysis of the design and manufacturing phases, as well as to the constitutive metal properties (AISI 316L stainless steel), including the consequences of welding, were carried out. Modal testing of the blades was performed. Strain gauges and pressure sensors were used to evaluate the loading profile in real-sea working conditions and different suggestions were made in order to postpone and prevent crack nucleation and propagation. The appearance of cracks seems to have been caused by mistakes taken at both design and manufacturing phases, eliminating from the guide vanes the ability to endure the aggressive loads which they were subjected to. The lack of an active control strategy on the power plant resulted in consistent turbine stall, thus contributing to the excessive loadings subjected to the blades.

An oscillating water column (OWC) wave-energy device is simulated in a 2D numerical wave tank. Th... more An oscillating water column (OWC) wave-energy device is simulated in a 2D numerical wave tank. The fluid flow modelling is fully nonlinear. A new wave generation by spinning dipole is proposed; when coupled to a robust wave absorption technique, it permits long-term simulations without energy accumulation in the flume. The turbine power takeoff mechanism is driven by a self-adaptive controller. The technique initially developed for active piston wave absorbers is applied successfully to OWC power plants. It is based on a Kalman filter frequency tracking algorithm. This control compares favorably with more conventional open-loop systems.

Volume 8: Ocean Renewable Energy, 2013

Several methods have been proposed in the literature to find a state-space model for the wave-rad... more Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, we compared four methods, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against those derived from the numerical code WAMIT. A new state-space module was implemented within FAST, an offshore wind turbine computer-aided engineering tool, and we compared the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the new state-space module.

Applied Ocean Research, 2020

The paper describes the basic studies, design, construction and operation of a shoreline OWC wave... more The paper describes the basic studies, design, construction and operation of a shoreline OWC wave power pilot plant on the island of Pico, Azores, Portugal. Construction began in 1996. The concrete structure of the chamber (square planform with inside dimensions of × 12 m 12 m at mean water level) was built in-situ on rocky bottom (about 8 m water depth), spanning a small natural harbour (gully). The plant was equipped with a horizontalaxis Wells turbine-generator set rated 400 kW. The plant was fully automated in order to supply electrical energy to the island grid on a permanent basis. It became operational in 1999. The paper describes the life of the plant during the following almost twenty years, including frequent repairs, modifications and the use of the plant as supplier of energy to the island grid, and as an infrastructure for research and training.

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 2, 2005

This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where... more This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where the same problem was studied in the frequency domain. It concerns the same sphere, connected to the seabed by a tension line (single point moored), that oscillates with respect to the vertical direction in the plane of wave propagation. The pulsating nature of the sphere is the basic physical phenomenon that allows the use of this model as a simulation of a floating wave energy converter. The hydrodynamic coefficients and diffraction forces presented in Linton (1991) and Lopes and Sarmento (2002) for a submerged sphere are used. The equation of motion in the angular direction is solved in the time domain without any assumption about its output, allowing comparisons with the previously obtained results.

Green Energy and Technology(Virtual Series)

... (2003), where validation exer-cises are ... It is emphasised that the outputs of the frequenc... more ... (2003), where validation exer-cises are ... It is emphasised that the outputs of the frequency domain code are extensively used as inputs in the time domain simulation also developed by ... A piston will displace twice as much water as a flap, with the same stroke, so the wave will be ...

Volume 8: Ocean Renewable Energy, 2013

ABSTRACT This paper presents a verification exercise with three different codes for floating offs... more ABSTRACT This paper presents a verification exercise with three different codes for floating offshore wind turbine modeling: FAST, S4WT and SIMPACK. The comparison showed good agreement in most of the results, and the main differences identified can largely be traced back to the different physical models used by the three simulation softwares. A detailed analysis of the wind turbine loads and motions is also included. FAST offered a greater computational efficiency compared with the other two softwares. Nevertheless, if one is interested in more detailed loads on blades, exact blade deflection predictions in bending and torsion, elastic effects of the floating platform etc., the more detailed codes S4WT and SIMPACK are beneficial.

Volume 9B: Ocean Renewable Energy, 2014

This paper focuses on the analysis of a floating wind turbine under multidirectional wave loading... more This paper focuses on the analysis of a floating wind turbine under multidirectional wave loading. Special attention is given to the different methods used to synthesize the multidirectional sea state. This analysis includes the double-sum and single-sum methods, as well as an equal-energy discretization of the directional spectrum. These three methods are compared in detail, including the ergodicity of the solution obtained. From the analysis, the equal-energy method proved to be the most computationally efficient while still retaining the ergodicity of the solution. This method was chosen to be implemented in the numerical code FAST. Preliminary results on the influence of these wave loads on a floating wind turbine showed significant additional roll and sway motion of the platform.

Proc. 8th EWTEC, 2009

Future large-scale implementation of wave energy converts (WECs) will introduce an anthropogenic ... more Future large-scale implementation of wave energy converts (WECs) will introduce an anthropogenic activity in the ocean which may contribute to underwater noise. The Ocean houses several marine species with acoustic sensibility; consequently the potential impact of the underwater noise needs to be addressed. At present, there are no acoustic impact studies based on acquired data. The WEAM project (Wave Energy Acoustic Monitoring) aims at developing an underwater noise monitoring plan for WECs. The development of an acoustic monitoring plan must consider the sound propagation in the ocean, identify noise sources, understand the operational characteristics and select adequate instrumentation. Any monitoring strategy must involve in-situ measurements. However, the vast distances which sound travels within the ocean, can make in-situ measurements covering the entire area of interest, impracticable. This difficulty can be partially overcome through acoustic numerical modelling. This paper presents a synthetic study, on the application of acoustic forward modelling and the evaluation of the impact of noise produced by wave energy devices on marine mammals using criteria based on audiograms of dolphins, or other species. The idea is to illustrate the application of that methodology, and to show to what extent it allows for estimating distances of impacts due to acoustic noise.

Applied Ocean Research, 2003

It is shown that due to power takeoff losses, optimal control provides maximum energy absorption,... more It is shown that due to power takeoff losses, optimal control provides maximum energy absorption, but not maximum energy production. A new reactive control criterion in the frequency-domain is deduced assuming constant power takeoff efficiency, respectively, in the power feeding and power absorption parts of the wave cycle. If applied in the time-domain, this criterion requires the incident wave to be predicted some time into the future. Whilst the OWC type of Wave Energy Converters (WEC) is presented in the paper, the extension to WECs of the floating body type is also considered. Illustrative numerical results for a two-dimensional OWC of simple geometry are presented, which include the performance of this device in three wave spectra with increasing demands of active control for improved energy production. Linear hydrodynamic theory is considered throughout the paper.

Ocean Engineering, 1990

The paper deals with phase control as a method of increasing the energy absorption by oscillating... more The paper deals with phase control as a method of increasing the energy absorption by oscillating water column (OWC) devices, from regular as well as from irregular waves. The power takeoff machine considered is a modified version of thc self-rectifying axial-flow Wells air turbine, whose rotor blades are of variable setting angle; this allows the air pressure and flow rate to be controlled independently from each other. Results oi numerical simulations arc presented for three different control strategies applied to energy absorption from irregular waves by an OWC device of simple, two-dimensional geometry. Experimental data from a turbine model arc used in the simulation.

The present paper reports a numerical study concerning the modeling and geometric optimization of... more The present paper reports a numerical study concerning the modeling and geometric optimization of a two body heave oscillating point absorber wave energy converter (WEC) using the force reacting principle. The device work principle is based on a surface body (or floater) force-reacting against a submerged body, as the excitation force of both, the floater and the submerged body, are in phase opposition. The relative motion between the two bodies generates electrical power through a linear power take off system (PTO). From the hydrodynamic point of view, the main purpose of the concept is to emphasize the radiation capabilities of the body placed under the free surface, as it has both excitation force components (diffraction and Froude Krylov) in phase. Therefore, the excitation force (and thus the hydrodynamic damping coefficient) is higher in the case of a submerged volume variation heave oscillating body (like the AWS wave energy converter) when compared to a floating buoy with th...

The present study aims at assessing the key spectral parameters of the incident wave on a fixed o... more The present study aims at assessing the key spectral parameters of the incident wave on a fixed oscillating water column (OWC) device, based on the air pressure measurements inside the chamber and on the numerical hydrodynamic coefficients of the device. The methodology is based on the equation of continuity of the air in the time-domain and linear decomposition of the air flow in the usual terms of radiation and diffraction flows. By applying the Fast Fourier Transform, the time-domain equation is transposed to the respective frequency-domain equation. This methodology was applied to the 400 kW OWC power plant, on the Island of Pico, Azores, which has been monitored since 2005 by the Wave Energy Centre. The numerical hydrodynamic coefficients obtained by the 3D radiation-diffraction boundary element code, AQUADYN-OWC, were used in this study. No measurements of the incident wave in front of the plant are available; therefore the results obtained for a set of records are compared wi...

The 400-kW Pico OWC plant built in 1995-1999 is a bottom-mounted shoreline structure, equipped wi... more The 400-kW Pico OWC plant built in 1995-1999 is a bottom-mounted shoreline structure, equipped with a horizontalaxis, Wells turbine-generator set. Main refurbishment actions through 2004-2006 included the complete replacement of the degraded electrical equipment and restorations of the existing mechanical components. Full-scale monitoring included the analysis of instantaneous values of the water free-surface elevation inside the chamber, the air pressure in the chamber and turbine duct, the rotational speed of the turbine and the electrical power output. This paper describes the plant monitoring during the full-scale demonstration from September 2005 to October 2006 and discusses the data and results obtained so far.

Engineering Failure Analysis, 2015

The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes le... more The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes lead to stall in the turbine and stators' blades, which represents an unintended action as it creates potentially damaging conditions to the mechanical components, plus reducing the efficiency of the takeoff system. This work focuses on the cracking damage caused to the guide vanes of the Pico Island Wave Power Plant due to the loading from turbulent air flow and consequent vibrations, as a result of turbine stall. Comprehensive analysis of the design and manufacturing phases, as well as to the constitutive metal properties (AISI 316L stainless steel), including the consequences of welding, were carried out. Modal testing of the blades was performed. Strain gauges and pressure sensors were used to evaluate the loading profile in real-sea working conditions and different suggestions were made in order to postpone and prevent crack nucleation and propagation. The appearance of cracks seems to have been caused by mistakes taken at both design and manufacturing phases, eliminating from the guide vanes the ability to endure the aggressive loads which they were subjected to. The lack of an active control strategy on the power plant resulted in consistent turbine stall, thus contributing to the excessive loadings subjected to the blades.

An oscillating water column (OWC) wave-energy device is simulated in a 2D numerical wave tank. Th... more An oscillating water column (OWC) wave-energy device is simulated in a 2D numerical wave tank. The fluid flow modelling is fully nonlinear. A new wave generation by spinning dipole is proposed; when coupled to a robust wave absorption technique, it permits long-term simulations without energy accumulation in the flume. The turbine power takeoff mechanism is driven by a self-adaptive controller. The technique initially developed for active piston wave absorbers is applied successfully to OWC power plants. It is based on a Kalman filter frequency tracking algorithm. This control compares favorably with more conventional open-loop systems.

Volume 8: Ocean Renewable Energy, 2013

Several methods have been proposed in the literature to find a state-space model for the wave-rad... more Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, we compared four methods, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against those derived from the numerical code WAMIT. A new state-space module was implemented within FAST, an offshore wind turbine computer-aided engineering tool, and we compared the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the new state-space module.

Applied Ocean Research, 2020

The paper describes the basic studies, design, construction and operation of a shoreline OWC wave... more The paper describes the basic studies, design, construction and operation of a shoreline OWC wave power pilot plant on the island of Pico, Azores, Portugal. Construction began in 1996. The concrete structure of the chamber (square planform with inside dimensions of × 12 m 12 m at mean water level) was built in-situ on rocky bottom (about 8 m water depth), spanning a small natural harbour (gully). The plant was equipped with a horizontalaxis Wells turbine-generator set rated 400 kW. The plant was fully automated in order to supply electrical energy to the island grid on a permanent basis. It became operational in 1999. The paper describes the life of the plant during the following almost twenty years, including frequent repairs, modifications and the use of the plant as supplier of energy to the island grid, and as an infrastructure for research and training.

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 2, 2005

This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where... more This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where the same problem was studied in the frequency domain. It concerns the same sphere, connected to the seabed by a tension line (single point moored), that oscillates with respect to the vertical direction in the plane of wave propagation. The pulsating nature of the sphere is the basic physical phenomenon that allows the use of this model as a simulation of a floating wave energy converter. The hydrodynamic coefficients and diffraction forces presented in Linton (1991) and Lopes and Sarmento (2002) for a submerged sphere are used. The equation of motion in the angular direction is solved in the time domain without any assumption about its output, allowing comparisons with the previously obtained results.

Green Energy and Technology(Virtual Series)

... (2003), where validation exer-cises are ... It is emphasised that the outputs of the frequenc... more ... (2003), where validation exer-cises are ... It is emphasised that the outputs of the frequency domain code are extensively used as inputs in the time domain simulation also developed by ... A piston will displace twice as much water as a flap, with the same stroke, so the wave will be ...

Volume 8: Ocean Renewable Energy, 2013

ABSTRACT This paper presents a verification exercise with three different codes for floating offs... more ABSTRACT This paper presents a verification exercise with three different codes for floating offshore wind turbine modeling: FAST, S4WT and SIMPACK. The comparison showed good agreement in most of the results, and the main differences identified can largely be traced back to the different physical models used by the three simulation softwares. A detailed analysis of the wind turbine loads and motions is also included. FAST offered a greater computational efficiency compared with the other two softwares. Nevertheless, if one is interested in more detailed loads on blades, exact blade deflection predictions in bending and torsion, elastic effects of the floating platform etc., the more detailed codes S4WT and SIMPACK are beneficial.

Volume 9B: Ocean Renewable Energy, 2014

This paper focuses on the analysis of a floating wind turbine under multidirectional wave loading... more This paper focuses on the analysis of a floating wind turbine under multidirectional wave loading. Special attention is given to the different methods used to synthesize the multidirectional sea state. This analysis includes the double-sum and single-sum methods, as well as an equal-energy discretization of the directional spectrum. These three methods are compared in detail, including the ergodicity of the solution obtained. From the analysis, the equal-energy method proved to be the most computationally efficient while still retaining the ergodicity of the solution. This method was chosen to be implemented in the numerical code FAST. Preliminary results on the influence of these wave loads on a floating wind turbine showed significant additional roll and sway motion of the platform.

Proc. 8th EWTEC, 2009

Future large-scale implementation of wave energy converts (WECs) will introduce an anthropogenic ... more Future large-scale implementation of wave energy converts (WECs) will introduce an anthropogenic activity in the ocean which may contribute to underwater noise. The Ocean houses several marine species with acoustic sensibility; consequently the potential impact of the underwater noise needs to be addressed. At present, there are no acoustic impact studies based on acquired data. The WEAM project (Wave Energy Acoustic Monitoring) aims at developing an underwater noise monitoring plan for WECs. The development of an acoustic monitoring plan must consider the sound propagation in the ocean, identify noise sources, understand the operational characteristics and select adequate instrumentation. Any monitoring strategy must involve in-situ measurements. However, the vast distances which sound travels within the ocean, can make in-situ measurements covering the entire area of interest, impracticable. This difficulty can be partially overcome through acoustic numerical modelling. This paper presents a synthetic study, on the application of acoustic forward modelling and the evaluation of the impact of noise produced by wave energy devices on marine mammals using criteria based on audiograms of dolphins, or other species. The idea is to illustrate the application of that methodology, and to show to what extent it allows for estimating distances of impacts due to acoustic noise.

Applied Ocean Research, 2003

It is shown that due to power takeoff losses, optimal control provides maximum energy absorption,... more It is shown that due to power takeoff losses, optimal control provides maximum energy absorption, but not maximum energy production. A new reactive control criterion in the frequency-domain is deduced assuming constant power takeoff efficiency, respectively, in the power feeding and power absorption parts of the wave cycle. If applied in the time-domain, this criterion requires the incident wave to be predicted some time into the future. Whilst the OWC type of Wave Energy Converters (WEC) is presented in the paper, the extension to WECs of the floating body type is also considered. Illustrative numerical results for a two-dimensional OWC of simple geometry are presented, which include the performance of this device in three wave spectra with increasing demands of active control for improved energy production. Linear hydrodynamic theory is considered throughout the paper.

Ocean Engineering, 1990

The paper deals with phase control as a method of increasing the energy absorption by oscillating... more The paper deals with phase control as a method of increasing the energy absorption by oscillating water column (OWC) devices, from regular as well as from irregular waves. The power takeoff machine considered is a modified version of thc self-rectifying axial-flow Wells air turbine, whose rotor blades are of variable setting angle; this allows the air pressure and flow rate to be controlled independently from each other. Results oi numerical simulations arc presented for three different control strategies applied to energy absorption from irregular waves by an OWC device of simple, two-dimensional geometry. Experimental data from a turbine model arc used in the simulation.