Stochastic dynamic response analysis of a floating vertical‐axis wind turbine with a semi‐submersible floater

Floating vertical‐axis wind turbines (FVAWTs) provide the potential for utilizing offshore wind resources in moderate and deep water because of their economical installation and maintenance. Therefore, it is important to assess the performance of the FVAWT concept. This paper presents a stochastic dynamic response analysis of a 5 MW FVAWT based on fully coupled nonlinear time domain simulations. The studied FVAWT, which is composed of a Darrieus rotor and a semi‐submersible floater, is subjected to various wind and wave conditions. The global motion, structural response and mooring line tension of the FVAWT are calculated using time domain simulations and studied based on statistical analysis and frequency‐domain analysis. The response of the FVAWT is compared under steady and turbulent wind conditions to investigate the effects of turbulent wind. The advantage of the FVAWT in reducing the 2P effect on the response is demonstrated by comparing the floating wind turbine with the equivalent land‐based wind turbine. Additionally, by comparing the behaviour of FVAWTs with flexible and rigid rotors, the effect of rotor flexibility is evaluated. Furthermore, the FVAWT is also investigated in the parked condition. The global motions and structural responses as a function of the azimuthal angle are studied. Finally, the dynamic response of the FVAWT in selected misaligned wind and wave conditions is analysed to determine the effects of wind‐wave misalignment on the dynamic response. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of floating sun gear in a wind turbine's planetary gearbox with geometrical imperfections

This paper addresses the effect of gear geometrical errors in wind turbine planetary gearboxes with a floating sun gear. Numerical simulations and experiments are employed throughout the study. A National Renewable Energy Laboratory 750 kW gearbox is modelled in a multibody environment and verified using the experimental data obtained from a dynamometer test. The gear geometrical errors, which are both assembly dependent and assembly independent, are described, and planet‐pin misalignment and eccentricity are selected as the two most influential and key errors for case studies. Various load cases involving errors in the floating and non‐floating sun gear designs are simulated, and the planet‐bearing reactions, gear vibrations, gear mesh loads and bearing fatigue lives are compared. All tests and simulations are performed at the rated wind speed. For errorless gears, the non‐floating sun gear design performs better in terms of gear load variation, whereas the upwind planet bearing has more damage. In the floating sun gear scenario, the planet misalignment is neutralized by changing the sun motion pattern and the planet gear's elastic deformation. The effects of gear profile modifications are also evaluated, revealing that profile modifications such as crowning improve the effects of misalignment. Copyright © 2014 John Wiley & Sons, Ltd.     

Reliability-based assessment of deteriorating ship structures operating in multiple sea loading climates

A reliability-based model for assessment of deteriorating ships subjected to multiple environmental conditions is established. Deterioration due to combined crack growth and corrosion in structural components of a ship hull is accounted for based on a fracture mechanics formulation. The model enables to take into account the corrosion-induced increased crack growth rate in two ways: (1) the increased stress range produced by the plate thinning (wastage) effect and (2) corrosion fatigue itself. Sensitivity studies are carried out to evaluate the effect of inspection updating on a production ship subjected to two different climate conditions. The hazard rate concept is adopted as a measure of reliability and emphasized throughout the different case studies.     

Effect of DGPS failures on dynamic positioning of mobile drilling units in the North Sea

Basic features of differential global positioning system (DGPS), and its operational configuration on dynamically positioned (DP) mobile offshore drilling units in the North Sea are described. Generic failure modes of DGPS are discussed, and a critical DGPS failure which has the potential to cause drive-off for mobile drilling units is identified. It is the simultaneous erroneous position data from two DGPS's. Barrier method is used to analyze this critical DGPS failure. Barrier elements to prevent this failure are identified. Deficiencies of each barrier element are revealed based on the incidents and operational experiences in the North Sea. Recommendations to strengthen these barrier elements, i.e. to prevent erroneous position data from DGPS, are proposed. These recommendations contribute to the safety of DP operations of mobile offshore drilling units.     

Numerical simulation of local ice loads in uniform and randomly varying ice conditions

The ice loading process has a clear stochastic nature due to variations in the ice conditions and in the icebreaking processes of ships. The statistical characteristics of local ice loads are typically studied on the basis of field measurements. In this paper, a numerical method was applied to simulate a ship moving forward in either uniform or randomly varying ice conditions, where the thickness and strength properties of the ice encountered by the ship were assumed to be constant or randomly generated using the Monte Carlo method. The purpose of this simulation is to show the origin of the statistical variation in ice loading, which is difficult to identify in field measurements. To validate the numerical results, an icebreaking tanker, MT Uikku, was then modeled in a simulation program, the ice loading process was stochastically reproduced and the calculated amplitude values of the ice-induced frame loads were compared with the field measurements.     

Assessment of emerging and traditional halogenated contaminants in Guillemot (Uria aalge) egg from North-Western Europe and the Baltic Sea

Persistent organic pollutants (POPs) are readily detected in biological samples at remote sites in the Arctic and sub-Arctic due to long-range transport from source areas. The aim of this study was to investigate the presence of POPs, polybrominated contaminants and their metabolites in guillemot (Uria aalge) eggs from Iceland, the Faroe Islands, Norway and Sweden to assess spatial trends of these compounds in the Arctic and sub-Arctic areas of Europe. Egg samples were extracted, and cleaned for chemical analysis. Concentrations of PCBs, 4,4′-DDE and β-HCH were an order of magnitude higher in eggs from the Baltic Proper compared to eggs from the North Atlantic. Concentrations of HCB were of the same magnitude at all sites, ranging from 160 to 520 ng/g fat. Concentration of BCPS was 100 times higher in eggs from the Baltic compared to eggs from the North Atlantic and seems therefore to be special regional problem. Concentrations of PBDEs were lower in eggs from the North Atlantic compared to eggs from the Baltic Proper but the difference was not as large as for PCBs and 4,4′-DDE. HBCDD showed the same spatial trend as PCBs, where the concentrations in eggs from the Baltic Proper were an order of magnitude higher than in eggs from the North Atlantic. OH-PCB and MeSO₂-PCB metabolites of PCBs, showed the same trend as the parent compounds while spatial trends of MeSO₂-DDE and OH-PBDEs, metabolites of 4,4′-DDE and PBDEs, respectively, differed from the trend of the parent compounds. This may be due to two factors; firstly, the limited ability of birds to metabolise DDT, and secondly, to natural production of OH-PBDE, respectively. Guillemot is suggested as a monitoring species for circumpolar monitoring.     

Probabilistic analysis of the uncertainty in the fatigue capacity of welded joints

This paper presents the results of a study of the uncertainty in the fatigue capacity (constant amplitude fatigue life) of welded steel joints, due to uncertainties related to geometrical and material parameters.An efficient method of probabilistic fracture mechanics analysis is described and applied. A linearelastic fracture mechanics model and the Paris-Erdogan law of crack propagation were adopted. Stressintensity factors were evaluated by employing an influence function method, which is very cost-effective. The main parameters were treated as stochastic variables. Data for weld and crack geometry of the non-load carrying fillet weld cruciform joint selected as the example joint in the study, were recorded from specimens. Other data were compiled from the literature. The uncertainties associated with the basic variables were transformed into a measure of uncertainty of the fatigue capacity by employing the Monte Carlo simulation technique. The relative contributions to the uncertainty in the fatigue capacity from the various factors were also compared.The $\text{S-N}$ data established analytically compared fairly well with test data obtained with 42 specimens. The probabilistic fracture mechanics analysis provided a sufficient sample of data to allow a test of analytical probability distributions to the fatigue life. The fit of two- and three-parameter lognormal and Weibull distributions was examined. Only the three-parameter lognormal pdf passed the chi-square test on the 5% confidence level.     

On design,modelling, and analysis of a 10‐MW medium‐speed drivetrain for offshore wind turbines

The design of a medium‐speed drivetrain for the Technical University of Denmark (DTU) 10‐MW reference offshore wind turbine is presented. A four‐point support drivetrain layout that is equipped with a gearbox with two planetary stages and one parallel stage is proposed. Then, the drivetrain components are designed based on design loads and criteria that are recommended in relevant international standards. Finally, an optimized drivetrain model is obtained via an iterative design process that minimizes the weight and volume. A high‐fidelity numerical model is established via the multibody system approach. Then, the developed drivetrain model is compared with the simplified model that was proposed by DTU, and the two models agree well. In addition, a drivetrain resonance evaluation is conducted based on the Campbell diagrams and the modal energy distribution. Detailed parameters for the drivetrain design and dynamic modelling are provided to support the reproduction of the drivetrain model. A decoupled approach, which consists of global aero‐hydro‐servo‐elastic analysis and local drivetrain analysis, is used to determine the drivetrain dynamic response. The 20‐year fatigue damages of gears and bearings are calculated based on the stress or load duration distributions, the Palmgren‐Miner linear accumulative damage hypothesis, and long‐term environmental condition distributions. Then, an inspection priority map is established based on the failure ranking of the drivetrain components, which supports drivetrain inspection and maintenance assessment and further model optimization. The detailed modelling of the baseline drivetrain model provides a basis for benchmark studies and support for future research on multimegawatt offshore wind turbines.     

Direct load control of residential water heaters

In Norway there is a growing concern that electricity production and transmission may not meet the demand in peak-load situations. It is therefore important to evaluate the potential of different demand-side measures that may contribute to reduce peak load. This paper analyses data from an experiment where residential water heaters were automatically disconnected during peak periods of the day. A model of hourly electricity consumption is used to evaluate the effects on the load of the disconnections. The results indicate an average consumption reduction per household of approximately 0.5 kWh/h during disconnection, and an additional average increase in consumption the following hour, due to the payback effect, that may reach up to 0.28 kWh/h per household.