Thomas Zambrano

Dynamic Modeling of Deepwater Offshore Wind Turbine Structures in Gulf of Mexico Storm Conditions

A Fourier spectrum based model of Gulf of Mexico storm conditions is applied to a 6 degree of freedom analytic simulation of a moored, floating offshore structure fitted with three rotary wind turbines. The resulting heave, surge, and sway motions are calculated using a Newtonian Runge-Kutta method. The angular motions of pitch, roll, and yaw are also calculated in this time-domain progression. The forces due to wind, waves, and mooring line tension are predicted as a function of time over a 4000 second interval. The WAMIT program is used to develop the wave forces on the platform. A constant force coefficient is used to estimate wind turbine loads. A TIMEFLOAT computer code calculates the motion of the system based on the various forces on the structure and the system’s inertia.Copyright

Design and Installation of a Tension Moored Wind Turbine

This paper describes the design of the floater and the mooring system for a small wind turbine. The engineering basis and the hydrodynamics calculations are described, as well as the installation and commissioning sequences.Copyright

Developing robotic swarms for ocean surface mapping

In this paper we describe a model for achieving collective motion in a swarm of robotic boats with the purpose of ocean surface mapping. We also present the implementation and experimental validation of the concept through sea trials with 16-24 robotic boats. The model uses only local neighbor-neighbor interaction and reacts to external information measured in an environmental property or internal information possessed by a leader. We model and demonstrate three kinds of the behaviors: size-tunable mapping, environment-responsive migration, and leader-responsive migration. We also present experimental results from field trials carried out in Catalina Harbor, Santa Catalina Island, California.

Redefining the target: Viewing the wave energy resource from a power take-off perspective

We consider the implications for the wave energy resource when viewed from the perspective of the power take-off mechanism. While globally the majority of wave energy is concentrated in low frequency swells, the dependence of power production on acceleration and velocity means an energy-harvesting device gathers more power from higher frequency waves.

Targeting the Wave Resource From the Power Take-Off Perspective

Frequency dependence in a power take-off mechanism means that power generated does not have a one-to-one correspondence with power available in the waves. Viewing wave power resources from the perspective of the power takeoff device leads us to question the assumption that waves carrying the maximum power are the best target for resource development. We explore this power take-off perspective for a heave device fitted with an electric generator and find that power is preferentially produced from higher frequency waves.While globally most wave power is concentrated in low frequency swells, when power production is dependent on velocity, as when driving an electrical generator, an energy-harvesting device gathers more power from higher frequency waves. We examine spectral data from the Pacific Ocean and the Gulf of Mexico and find that the peak frequency for heave velocity, and hence for power production, can be as much as twice the peak frequency of the resource power in the waves.Furthermore, Response Amplitude Operator (RAO) considerations demonstrate the need for small, ‘nimble’ devices to access the most easily available wave power. We compare the total energy capture from the empirical wave spectra using devices tuned to different natural frequencies and find the most energy is captured, regardless of geographic location, with devices that have natural frequencies around 0.2 Hz, far above the swell frequency.Copyright

Generating Power From Ocean Waves Using a Float With Excessive Buoyancy: Theory and Dynamic Model Results

We are exploring a new approach to ocean energy extraction through a device that we refer to as the NAF (an acronym for Non-Archimedean Float). The NAF is a fully submerged body with excess buoyancy; i.e., the mass of the body is far less than the mass of the water it displaces. When such a float is tethered beneath the ocean surface the buoyancy yields a large force vector in the direction perpendicular to the isobaric surfaces that parallel the water/air interface. The constant shifting of the wave troughs provides the opportunity for energy extraction using turbines affixed to the float. We are exploring the NAF concept because its simplicity results in many inherent benefits. The device has few moving parts, gathers energy from waves coming in any direction, and exists as a non-obtrusive, completely submerged installation. A numerical model of the NAF has been created to determine the dynamic behavior and power output for various configurations and under various wave conditions. The numerical model is set up to calculate the various forces experienced by the NAF float, and from these it calculates the velocity and position of the float through time series steps. The model effectively demonstrates which variables are important and how power output relates to NAF dimensions. One early finding from the model result relates to tuning the natural frequency of the NAF to match the natural frequency of the waves. The NAF moves like an inverted pendulum, and its natural frequency is primarily dependent on the length of the pendulum. Regardless of the actual float buoyancy, the 6 to 12 second periods that typify average wave conditions dictate that the NAF tether should be between 30-m and 60-m long. Also, a scale version of this novel energy device consisting of a float tethered beneath the ocean surface was deployed off the coast of southern California. The deployment yielded rich data sequences that are sufficient for comparison with a dynamic numerical model.Copyright