Nested control co-design of a spar buoy horizontal-axis floating offshore wind turbine

ABSTRACT There has been little analytical study to clarify the relationships among the mooring chain configuration, the caisson padeye depth, the tensile load induced at the caisson padeye, and the horizontal load at the spar fairlead of a floating offshore wind turbine (FOWT). To elucidate the interaction within the integrated system comprising the suction caisson anchor, the mooring chain, and the spar buoy, a comprehensive analytical method of the mooring system was proposed using a three-dimensional displacement method. From the predicted results for the case study of the two suction caisson anchors with different padeye depths, it was found that (1) For the suction caisson anchor with the padeye set on the 2/3 embedded depth, pullout occurs at the horizontal load which is the 71% designed ultimate horizontal load on the fairlead. (2) For the suction caisson anchor with zero padeye depth, no pullout arises for the designed ultimate horizontal load.

Wave-powered water pump for upwelling in aquaculture: Numerical model and ocean test

Abstract In this work, numerical dynamic analysis of Single Point Wave Energy Converter (SPWEC) is conducted using the OrcaFlex software package. Maxsurf software package is used to create both the hydrostatic and hydrodynamic properties used in the OrcaFlex dynamic analysis. The numerical model replicates the WaveEl buoy WEC prototype project by Waves4Power which was held in Runde, Norway. The input data and model validation sources are elaborated with existing research. This work evaluates the use of a commercial software package OrcaFlex in wave energy converter modelling and analysis. The use of a 6D spar buoy in OrcaFlex can be used to simulate the motion of SPWEC. However, as the 6D spar buoy in OrcaFlex can only consider Morison’s equation, wave data adjustment should be made to keep the numerical model accurate and eliminate the possibility of numerical issues occurring. Unlike the past work which uses a heave plate to simulate the Power Take Off (PTO) device, this present work simulates the PTO by the use of winches within OrcaFlex. The numerical model is validated using the decay test which compares the resonant period and damping coefficient with existing research. The decay test undertaken in this project leads on model property with a 1.004 ratio of the damping coefficient and 0.9497 ratio of the resonant period compared with the existing research. Overall, regardless of the limitations, the results in this work can demonstrate that a 6D spar buoy and winches within commercial finite element software OrcaFlex is capable of representing the full-scale WEC and therefore supports this as a suitable methodology for marine renewable energy analysis especially floating wave energy converters such as SPWEC.

Higher wind velocities and lower wind shear are two motivations driving the development of floating offshore wind turbines (OWTs). However, such designs suffer from high expenses and complicated installation scenarios. Installation of offshore wind turbines is challenging due to the unpredictable nature of the environment and the technical complexities, especially at offshore sites. Mating of OWT on top of the pre-installed substructure is one of the critical stages of the installation operation. Grouted, welded, and bolted connections are utilized conventionally, but all have shortcomings. Welded and grouted connections suffer from fatigue forces, while a bolted connection requires minimal installation tolerances and sensitivity to impact forces. The design of a quick connection device (QCD) is expected to reduce the installation time, expand the operational weather window, and overcome the limitations of the earlier connection devices.The QCD described here comprises conic cross-sections, circular plates, and stiffeners connected to the floating substructure and OWT. This research uses a global model to estimate the relative velocities and displacements between the OWT and spar buoy. Furthermore, a local finite element model is developed to assess the influence of the impact forces and the design of the connection device. Implementing the hydrostatic stiffness of the floating spar within the impact simulations improved the simulation fidelity and reduced the impact damage. Different impact scenarios are performed, and the sensitivity of impact damage concerning the distribution of impact initiation points is assessed. Furthermore, an active control mechanism is used to reduce the relative motions between the installation vessel and the floating substructure. It is concluded that utilizing the anti-swing active control system minimizes the impact velocity and impact damage. This research can be extended by optimizing the design of the quick connection device.

A study on the annular cylindrical tuned liquid damper for dynamic control of spar buoy based measurement system

For floating wind turbines, one of the most interesting and challenging issues is that the movement of the rotor is strongly related to its floating platform, which results in corresponding variations in the wake characteristics of the turbine. Because the aerodynamic efficiency of the downstream turbines is affected by the wake characteristics, the power output will consequently vary depending on the different types of floating wind turbines and floating wind farms used. In this study, the rotor movement, wake characteristics, and corresponding wind farm power output are analyzed using a numerical method for three typical floating wind turbines: the semisubmersible type, spar buoy type, and tension leg platform type with a 5 MW configuration. A fixed-bottom monopile wind turbine is adopted as a benchmark. The simulation results show that of the three floating wind turbines, the rotor position and wake center are most dispersed in the case of the spar buoy type, and its wake also has the lowest impact on downstream wind turbines. Additionally, the power output of the corresponding spar buoy type wind farm is also the highest at different wind speeds, followed by the semisubmersible type, tension leg platform type, and then the fixed-bottom type. In particular, at low wind speeds, the wake effects differ significantly among the various types of wind turbines.

Effect of waves on the magnitude and direction of wind stress over the Ocean

Floating wind turbines have enormous potential in harnessing high wind speeds in deep ocean locations. Compared with bottom-fixed technology, the latter is limited by the depth to which it can be installed. These turbines are designed to be deployed in deep waters and rely on specialized floaters such as spar buoys, semi-submersibles, and tension leg platforms to provide structural support. These floaters are tethered to the seabed using mooring lines, which regulate their movements and maintain stability. However, the existing literature lacks in-depth studies that comprehensively analyze how waves and mooring lines impact the motion of a semi-submersible floater. To address this gap, computational fluid dynamics (CFD) simulations are conducted using the overset methodology to replicate the actual loads experienced by semi-submersible floaters accurately. The study accurately predicts the six degrees of freedom (DOF) of the platform’s motion. The mooring lines are modeled using static or moving boundaries, and their interaction with the floater surface has been modeled using contact forces. The approach can simulate the impact of waves and mooring lines on the floater’s motion. The study uses the specifications depicted in the OC5 semi-submersible platform with mooring in test conditions and compares it with available experimental data to validate the numerical model. Once validated, the model is used to explore the hydrodynamic behavior of the floating structure across a range of waves characterized by varying amplitudes, periods, and directions. Similarly, in the case of the mooring lines, variations in critical parameters such as stiffness, pre-tension, and free length are introduced. This systematic manipulation of parameters enables a comprehensive investigation into their respective impacts on the dynamic response and motion of the floating platform.

The installation of the present wind farms Hywind Scotland and Hywind Tampen are both carried out by towing the fully assembled wind turbine from the assembly site in the Norwegian fjords to the final offshore site. In the present study an alternative installation method is proposed where the fully assembled tower is transported to the site on the installation vessel and mounted onto the preinstalled floating substructure (a spar buoy). The paper presents a brief outline of the design process for the proposed concept and gives an overview of the work done to evaluate variations of the installation vessel and the proposed lifting mechanism. The paper is a summary of the results obtained by a project team in SFI MOVE addressing marine operations related to installation of floating offshore wind turbines.

Abstract. Floating offshore wind is widely considered to be a promising technology to harvest renewable energy in deep ocean waters and increase clean energy generation offshore. While evolving quickly from a technological point of view, floating offshore wind turbines (FOWTs) are challenging, as their performance and loads are governed by complex dynamics that are a result of the coupled influence of wind, waves, and currents on the structures. Many open challenges therefore still exist, especially from a modeling perspective. This study contributes to the understanding of the impact of modeling differences on FOWT loads by comparing three FOWT simulation codes, QBlade-Ocean, OpenFAST, and DeepLines Wind®, and three substructure designs, a semi-submersible, a spar buoy, and the two-part concept Hexafloat, in realistic environmental conditions. This extensive comparison represents one of the main outcomes of the Horizon 2020 project FLOATECH. In accordance with international standards for FOWT certification, multiple design situations are compared, including operation in normal power production and parked conditions. Results show that the compared codes agree well in the prediction of the system dynamics, regardless of the fidelity of the underlying modeling theories. However, some differences between the codes emerged in the analysis of fatigue loads, where, contrary to extreme loads, specific trends can be noted. With respect to QBlade-Ocean, OpenFAST was found to overestimate lifetime damage equivalent loads by up to 14 %. DeepLines Wind®, on the other hand, underestimated lifetime fatigue loads by up to 13.5 %. However, regardless of the model and FOWT design, differences in fatigue loads are larger for tower base loads than for blade root loads due to the larger influence substructure dynamics have on these loads.

Hurricane Epsilon (2020) was a late-season, category-3 tropical cyclone that underwent extratropical transition and became Extratropical Cyclone Epsilon on October 26. The upper ocean response to the passage of the storm was observed by three types of autonomous platforms: an eXpendable Spar buoy, an Air-Launched Autonomous Micro-Observer profiling float, and two Seagliders. Taken together, this array enabled the rare collection of contemporaneous observations of the upper ocean, air-sea interface, and atmospheric boundary layer before, during, and after the passage of the storm. The evidence presented suggests that Extratropical Cyclone Epsilon contributed to breaking down the residual North Atlantic summer stratification regime and accelerated the shift to the prolonged ocean cooling associated with winter. The synergistic capabilities of the observational array are significant for two reasons: (1) by enabling the comparison of complementary atmosphere and ocean observations, taken from different platforms, they permit a comprehensive approach to better understand how storm-induced momentum, heat, and moisture fluxes alter upper ocean structure, and (2) they demonstrate the ability of future, targeted deployments of similar observational arrays to assess the fidelity of coupled ocean-atmosphere-wave numerical prediction models.

Characterizing ramp events in floating offshore wind power through a fully coupled electrical-mechanical mathematical model

This study investigates the effect of horizontal low-frequency (LF) displacements and velocities on the responses of floating structures in irregular waves, with a focus on a deep-draft spar buoy. It employs a consistent second-order hydrodynamic model in the time domain and its two variations to assess the influence of LF motions. To accurately estimate LF velocities from total velocities, an improved wavelet low-pass filter is proposed and applied, overcoming challenges posed by one-sided data from simulations. For the examined spar buoy, the incorporation of LF displacements and velocities in the seakeeping analysis is deemed essential, as they are found to contribute to the reduction of slowly varying, and consequently, total surge and pitch responses. The study finds that the standard deviations of LF surge and pitch motions scale with significant wave height Hs as (Hs)b, with b close to 4/3 for surge motion, highlighting viscous damping as the dominating damping mechanism. For the storm seastate, third-order viscous drag loads may also have made a non-negligible contribution. Meanwhile, pitch motion exhibits b values around 1.1 for both low and storm seastates, indicating the significance of both wave-drift and viscous damping in LF pitch motion prediction. The effects of mooring stiffness are also discussed.

This paper presents a benchmarking study of four floating wind platform’ motion and dynamic tension responses to verify an innovative design with the intention of overall cost reduction of a durable, reliable, safe design. An aero-hydro-servo-elastic code is applied to benchmark a 10 MW tension leg buoy (TLB) floating wind turbine to the current leading technology types for floating offshore wind platforms, specifically spar buoy, Semi-submersible and tension leg platform (TLP) floating wind turbines. This study assumes that the platforms will deploy in the northern region of the North Sea, with a water depth of 110 m under various environmental conditions, including wind field descriptions covering uniform wind to fluctuating turbulent wind. The obtained dynamic response results showed low motion responses for the TLB platform for all design load cases. More specifically, the TLB surge and pitch motion responses are insignificant under both operational and survival conditions, allowing decreased spacing between individual wind turbines and increasing wind farms' total energy generation capacity. An additional benefit is that the wind turbine systems can be installed without significant pitch modification to the control system. The TLB platform is less complex which simplifies the construction process and has the potential for significant cost reductions.

Offshore wind has a great potential as a competitive source of renewable energy, especially in deep waters where wind speeds are more consistent and fewer restrictions apply for running large wind turbines. Previous analyses of Floating Offshore Wind Turbines (FOWTs) mostly considered obvious sources of loading: surface waves, currents, wind and mooring. However, in some deep-water locations, internal waves can occur and these have been shown to significantly affect floating structures. Since the hydrodynamic response of an FOWT governs the structure’s general stability, the aim of this research is to investigate the impact of sinusoidal internal waves on the platform motion of a free-floating SPAR-type cylinder. A potential flow model and Morison’s equation are applied numerically to calculate the forces acting on a free-floating cylinder in an oscillating flow. The commercial Finite Element Analysis software OrcaFlex is verified by the potential flow model of the oscillating flow and is then used to mimic sinusoidal internal waves acting on a free-floating cylinder in a stratified flow. Three different internal wave amplitudes and peak velocities are analysed, and the nine resulting cases are investigated for the oscillating and stratified flow each. It has been found that the pitch rotations of the SPAR cylinder were small (< 0.1°) in all cases and can likely be disregarded. The surge displacements of the free-floating cylinder were substantial in both oscillating and stratified flows, with maximum surge magnitudes of 423m and 120m, respectively. Therefore, significant additional mooring loads due to internal waves could be sustained by SPAR-type FOWTs.

I have only been to sea once with Doug Cato, as a graduate student. It did not end well. In 1996 the R/P FLIP, a manned spar buoy, was deployed off the coast of San Diego to test advanced passive acoustic tracking methods on baleen whales, with Doug along as an invited guest. He had long been interested in acoustic means of localizing whales and was one of the first scientists to suggest using relative differences between received levels on hydrophones (instead of just the relative timing) for fixing an animal call’s location [JASA 104(3), 1667–1678]. This idea is the fundamental basis behind “matched-field processing” (MFP), the technique behind the FLIP test. The trip was supposed to collect three weeks of data; instead, it collected 42 h. This presentation will explain the challenges of abandoning a vessel at sea, and why garbage scows are underappreciated. It will also explain my brief involvement in Mike Noad’s and Doug Cato’s HARC field experiment as part of another MFP demonstration, and why that did not end very well either. Interlaced with these tales of disaster is a review of how Doug’s thoughts on animal tracking have re-emerged under a variety of circumstances, especially in arctic bioacoustics.

In today’s energy scenario dominated by the need to develop new methods of generating electricity, energy-intensive infrastructures are a promising solution towards energy self-sufficiency and environmental sustainability. Floating offshore wind turbine represents one of the major solution to exploit renewable energies. Currently, the increase of offshore wind market opens up the possibility of integrating different technologies to take advantage of marine energy potential, in particular wave energy.&#x0D; This work presents a new wave-wind hybrid floating platform with three Oscillating Water Columns (OWC) integrated in a floating offshore wind spar buoy. The design methodology is described showing the size of geometric characteristics of OWCs and the size of the energy conversion system. The hybrid system is investigated by a time-domain model that integrates the wind turbine model, the hydrodynamics of the floater, the thermodynamics of the OWC air chambers, and the damping effect induced by the OWC air turbine. The thermo-aero-hydro coupled numerical framework is described to highlight the implementation of the thermodynamic model in WEC-Sim/MATLAB environment. Specifically, the water column dynamics are solved as pistonrigid body representation, enabling the time-domain analysis of the OWC dynamic behaviour. Impulse turbines are considered as OWC Power Take-Offs (PTO). The resource scenario of Mediterranean Sea is considered as case study, in terms of both wind and wave conditions. The analysis focuses on the power extraction capabilities of the OWCs and on the impact of OWCs on the productivity of the wind turbine. Further developments on multi-objective design tools and on optimal PTO control design aiming power maximisation could be conducted for hybrid energy platforms based on this established numerical framework.

Extreme load analysis is an essential step in the design of structural and mooring systems for wave energy converters (WEC). The current study aims to evaluate the structural loads on the MARMOK Oscillating Water Column (OWC) WEC under both regular and irregular wave conditions, as well as to evaluate the effectiveness of its station keeping (mooring) systems. The project employs the open source computational fluid dynamics simulation software OpenFOAM to model the fully moored floating system. The WEC device hydrodynamics are validated against laboratory data for free decay in heave and pitch motions. In addition, the station keeping model is validated against mooring tension static offset tests and benchmarked against experimental data in irregular waves. An elegant method of numerically recreating irregular wave inflow conditions from empirical measurements is shown which allows for consistent comparison between the model and experimental results. The calibrated numerical model is then employed to study the full-scale system’s responses. Preliminary results of structural loadings on the fixed and floating spar buoy (with one mooring configuration as sample) are presented in the paper.

This paper introduces a low-power off-grid oscillating water column wave energy converter with an internal battery bank. The research aims at the preliminary design and devising of the control strategy of a power electronics interface between the turbo generator and the battery bank. The converter comprises a spar buoy, a biradial turbine, a permanent magnet generator, a full-wave bridge rectifier, a braking chopper, a DC-to-DC step-down converter, and a lead-acid battery bank. The power-take-off system was modelled in Simulink/MATLAB, and its performance was assessed with steady-state simulations, considering a wave climate characteristic of Leixões, Portugal. The chamber pressure, the turbine, generator and rectifier performance were taken from experimental data sets. A simple battery model was derived from the manufacturer's datasheet. An ideal step-down DC-to-DC converter operating in discontinuous conduction mode regulates the battery charging current. This converter, in parallel with the braking chopper, adjusts the generator counter torque by regulating the current through the rectifier. Twelve system variables were recorded for selected pairs of input pressure and step-down converter design coefficient. The power at the rectifier's output terminals was mapped for the rotational speed and input pressure. The results show a system rating of 1.4 kW with 400 W of electrical power at 200 rad/s for the most frequent sea states. The range of the duty cycle, the inductance and the braking resistance were derived. Two closed-loop controllers were proposed for managing the step-down converter and the braking chopper. Their set points and saturation limits were derived from the simulation results.