Reducing tower fatigue through blade back twist and active pitch-to-stall control strategy for a semi-submersible floating offshore wind turbine

The necessity of producing more electricity from renewable sources has been driven predominantly by the need to prevent irreversible climate chance. Currently, industry is looking towards floating offshore wind turbine solutions to form part of their future renewable portfolio. However, wind turbine loads are often increased when mounted on a floating rather than fixed platform. Negative damping must also be avoided to prevent tower oscillations. By presenting a turbine actively pitching-to-stall, the impact on the tower fore–aft bending moment of a blade with back twist towards feather as it approaches the tip was explored, utilizing the time domain FAST v8 simulation tool. The turbine was coupled to a floating semisubmersible platform, as this type of floater suffers from increased fore–aft oscillations of the tower, and therefore could benefit from this alternative control approach. Correlation between the responses of the blade’s flapwise bending moment and the tower base’s fore–aft moment was observed with this back-twisted pitch-to-stall blade. Negative damping was also avoided by utilizing a pitch-to-stall control strategy. At 13 and 18 m/s mean turbulent winds, a 20% and 5.8% increase in the tower axial fatigue life was achieved, respectively. Overall, it was shown that the proposed approach seems to be effective in diminishing detrimental oscillations of the power output and in enhancing the tower axial fatigue life

The Economics of Regulations on Hen Housing in California

Beginning January 1, 2015, conventional cage housing for egg-laying hens is scheduled to be prohibited in California. We consider the economic implications of the new hen housing regulations on the California shell egg industry. Our data show that egg production is more costly using noncage systems than conventional cages. The main result of the new regulations will be a drastic reduction in the number of eggs produced in California, a large increase in egg shipments from out of state, little if any change in hen housing for eggs consumed in California, and little change in egg prices in California.animal welfare regulation, hen housing, egg supply, egg prices, egg costs of production, Agribusiness, Agricultural and Food Policy, Agricultural Finance, Consumer/Household Economics, Farm Management, Livestock Production/Industries, Production Economics, Productivity Analysis, Q11, Q18,

The use of APS thermal barrier coatings in corrosive environments

Thermal barrier coatings (TBC) can be used to reduce the metal temperature of gas turbine blades enabling higher Cr alloys (lower strength) to be used when gas turbines are to be used in corrosive environments (where hot corrosion resistance is required). However, the TBC must also be resistant to the corrosive environment and remain attached to the blade. A 1000 h test to evaluate air plasma-sprayed (APS) TBC adhesion to a low-pressure plasma-sprayed CoNiCrAlY bond coat (with and without through thickness cracking) under hot corrosion conditions at 850 °C has been carried out. The APS TBC significantly reduced the hot corrosion rate of the CoNiCrAlY; however, delamination cracking occurred with a thinner thermally grown oxide than would be expected from isothermal and cyclic oxidation testing

Kinetics of duplex oxide growth on 9Cr steels exposed in CO2: application of dimensional metrology

Investigations into potentially extending the lives of UK advanced gas-cooled reactors have highlighted the need for improved understanding of the long-term oxidation and carburisation of 9Cr ferritic steels. These steels were used in evaporators and primary superheaters and as these are to be used beyond their original design lives, it is necessary to ensure that these degradation routes remain within acceptable levels. A dimensional metrology technique has been applied to archived autoclave samples to measure such damage. These samples had previously been exposed to a range of temperatures, pressures and gas chemistries representative of those experienced by 9Cr steels in CO2-rich AGR gases. Earlier sample assessments had focused on weight change measurements, but the dimensional technique enables measurement of duplex oxide thicknesses around samples and the extraction of related data from longer exposure times. These data also support estimation of the extent of both carbon and oxygen uptake

Stress corrosion of Ni-based superalloys

The development of gas turbines to increase fuel efficiency is resulting in progressively higher operating temperatures in the under platform regions of the blades. These regions have traditionally been considered low risk areas. However, higher metal temperatures combined with stresses and the deposition of contaminants from the cooling air system may result in complex degradation mechanisms. Static stress corrosion testing has been conducted on C-ring specimens at a range of stresses in a hot corrosion environment. Cracks were observed in C-rings after exposure times greater than 100 h. Scanning electron microscopy (SEM) systems were used to image cracks and characterise deposits to improve understanding of the mechanism. Finite element analysis (FEA) has been used to model the stress intensity under test conditions. CMSX-4 specimens subject to static stresses combined with hot corrosion demonstrated significant material degradation (crack initiation and propagation) suggesting a combined stress corrosion mechanism resulting in cracking

Simulating novel gas turbine conditions for materials assessment: cascade design and operation

Integrated gasification combined cycles can incorporate pre-combustion carbon capture. High-H2 syngas produces high H2O levels after combustion, potentially accelerating gas turbine component damage. Determining materials systems’ suitability for this novel environment requires exposures in representative environments. Thus, an existing 0.7 MW burner rig was modified to generate the combustion environment and incorporate a cascade of 15 air-cooled turbine blades. Computational fluid dynamic calculations using blade dimensions and flow requirements supported the cascade design and determined blade placement within the gas flow. Trials of the modified unit have shown that a simulated combusted H2-rich syngas composition was generated at gas temperatures ≤1440°C. A 1000 h exposure has been carried out with thermal barrier coated blades to demonstrate the operation of the unit

Impact of deposit recoat cycle length on hot corrosion of CMSX-4

Hot corrosion causes significant problems for both aerospace and power generation industries, where the combination of high temperature, corrosive gases, and contaminants severely limits component operating lifetimes in gas turbine hot gas streams. Multiple laboratory testing methodologies exist to study this hot corrosion, and these can be affected by a range of variables. This paper investigated the impact of varying deposit recoat cycle length when using the ‘deposit recoat’ testing method. CMSX-4 samples were exposed to simulated type II (pitting) hot corrosion conditions, with the same overall deposit load (averaged across the total exposure run), but different deposit recoat cycles. Post-exposure, samples underwent dimensional metrology analysis to compare metal loss resulting from different deposit recoat cycle lengths. Results for CMSX-4 suggest very small differences in corrosion losses, indicating CMSX-4 hot corrosion datasets obtained from deposit recoat experiments with different deposit recoat cycle lengths can be compared with confidence

Corrosion fatigue testing: the combined effect of stress and high temperature corrosion

A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism

Interaction of hot corrosion fatigue and load dwell periods on a nickel-base single crystal superalloy

The effects of type II hot corrosion on the fatigue resistance of turbine blade superalloys is of growing interest as gas turbine (GT) original equipment manufacturers (OEMs) strive to optimise the operational efficiencies and versatilities of GT systems. Hot corrosion fatigue has been observed in the under platform regions of first stage GT blades, this location is subject to both relatively high principal stresses and stress gradients, combined with temperatures up to those associated with type II hot corrosion (500–700 °C). The effect of the deposition flux of corrosive salt species and the tensile stress dwell period on the fatigue performance and resultant crack morphologies of single crystal (SC) superalloy CMSX-4 has been studied at 550 °C. Deposit recoat methodologies were applied to specimens that were cyclically fatigued with a load-controlled trapezoidal waveform. It was observed that introducing a longer dwell period increased the number of {1 0 0} crack initiations and reduced the fatigue life (load cycles to failure). Optical and SEM microscopy and EDX techniques were used to examine specimen fractography, and mechanisms of crack advance and propagation discussed

Impacts of temperature and surface finish upon steam oxidation of austenitic steel TP347HFG

The effects of the steam oxidation process on an austenitic steel (TP347HFG) exposed under isothermal conditions between 600 and 800 °C for up to 2500 h have been investigated. Samples with both as-received and ground surfaces have been exposed and the impact of surface finish on the oxidation process analysed using scanning electron microscopy with energy dispersive X-ray analysis. Exfoliated oxide flakes have also been examined to characterise their microstructures on fractured sections as well as external and spalled surfaces. Microscopic analyses demonstrated that ground surfaces possess better steam oxidation resistance than as-received surfaces due to their ability to form a more protective chromium-rich layer. The formation of regions of thicker multi-layered oxides was noted on both types of surface finish, covering large areas on as-received surfaces and only nodules on ground surfaces (spreading with increasing exposure temperature and time)