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Abstract This study compares the dynamic behaviour of a conventional and a compact gearbox for the DTU 10 MW wind turbine supported on a monopile offshore structure. The conventional gearbox configuration is composed of two planetary epicyclic stages and one parallel stage, while the compact gearbox configuration consists of a fixed planetary stage and a differential compound epicyclic stage. The design methodology for these two gearboxes is described, and the final gearbox specifications show a lighter weight and smaller volume for the compact gearbox design compared to the conventional one. Computational gearbox models are established using the multi‐body system dynamic analysis method. A decoupled approach is employed for the gearbox load effect analysis. Comparisons of the dynamic behaviour between these two gearboxes are conducted under pure torque load cases, tangential pin position error conditions and non‐torque load cases. The results demonstrate that the compact gearbox has better dynamic performance under different torque load cases and is more robust to withstand the effects of manufacturing errors and rotor non‐torque loads compared to the conventional gearbox. It is believed that the proposed compact gearbox concept is promising and would be a good alternative for multi‐megawatt floating wind turbines, although challenging with respect to the design and operation complexity.

Abstract This paper deals with numerical modeling of the hydraulic blade pitch actuator and its effect on the dynamic responses of a floating spar‐type wind turbine under valve fault conditions. A spar‐type floating wind turbine concept is modeled and simulated using an aero‐hydro‐servo‐elastic simulation tool (Simo‐Riflex [SR]). Because the blade pitch system has the highest failure rate, a numerical model of the hydraulic blade pitch actuator with/without valve faults is developed and linked to SR to study the effects of faults on global responses of the spar‐type floating wind turbine for different faults, fault magnitudes, and environmental conditions. The consequence of valve faults in the pitch actuator is that the blade cannot be pitched to the desired angle, so there may be a delay in the response due to excessive friction and the wrong voltage, or slit lock may cause runaway blade pitch. A short circuit may cause the blade to get stuck at a particular pitch angle. These faults contribute to rotor imbalance, which result in different effects on the turbine structure and the platform motions. The proposed method for combining global and hydraulic actuator models is demonstrated in case studies with stochastic wind and wave conditions and different types of valve faults.

The probability density of crest amplitude and duration that exceeds a given level is used in many theoretical and practical problems in engineering that are subjected to fluctuating loads such as wind and wave loads. The presently available joint distributions of amplitude and period are limited to excursion through a mean-level or to describe the asymptotic behavior of high level excursions. This paper extends the knowledge by presenting a theoretical derivation of probability of wave exceedance amplitude and duration for a stationary narrow-band Gaussian process. A density function is suggested that has the salient feature to depend only on the three lowest spectral moments m0, m1, and m2 and desired level of exceedance, H. It does not require any condition on the autocorrelation function. This paper shows how increase in H, increases the correlation between excursion periods and amplitude. This paper also shows that how the accuracy of the proposed joint distribution relates to spectral width parameter, v, and that accuracy increases for higher levels of H, especially for a spectrum describing a physical phenomenon such as a sea state spectrum. It was demonstrated that the marginal distribution of amplitude is Rayleigh distributed, as expected, and that the marginal distribution of excursion duration works for asymptotic and non-asymptotic levels. Results demonstrate that the established distribution fits well with ideal narrow-band Gaussian processes as well as the sea states at three European sites - in the Atlantic Ocean and the North Sea. The suggested model is found to be a good replacement for the existing empirical distributions.

With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years, there is an interest in investigating the technological and economic feasibility of combining offshore wind turbines (WTs) with wave energy converters (WECs). In the EU FP7 MARINA Platform project, three floating combined concepts, namely the spar torus combination (STC), the semi-submersible flap combination (SFC) and the oscillating water column (OWC) array with a wind turbine, were selected and studied in detail by numerical and experimental methods. This paper summarizes the numerical modeling and analysis of the two concepts: STC and SFC, the model tests at a 1:50 scale under simultaneous wave and wind excitation, as well as the comparison between the numerical and experimental results. Both operational and survival wind and wave conditions were considered. The numerical analysis was based on a time-domain global model using potential flow theory for hydrodynamics and blade element momentum theory (for SFC) or simplified thrust force model (for STC) for aerodynamics. Different techniques for model testing of combined wind and wave concepts were discussed with focus on modeling of wind turbines by disk or redesigned small-scale rotor and modeling of power take-off (PTO) system for wave energy conversion by pneumatic damper or hydraulic rotary damper. In order to reduce the uncertainty due to scaling, the numerical analysis was performed at model scale and both the numerical and experimental results were then up-scaled to full scale for comparison. The comparison shows that the current numerical model can well predict the responses (motions, PTO forces, power production) of the combined concepts for most of the cases. However, the linear hydrodynamic model is not adequate for the STC concept in extreme wave conditions with the torus fixed to the spar at the mean water level for which the wave slamming on the torus occurs and this requires further investigation. Moreover, based on a preliminary comparison of the displacement, the PTO system as well as the wind and wave power production, the STC concept will have a lower cost of energy as compared to the SFC concept. However, the cost of energy of either the STC or the SFC concept is higher than that of a pure floating wind turbine with the same floater.

The gearbox is one of the most expensive components of the wind turbine system. In order to refine the design and hence increase the long-term reliability, there has been increasing interest in utilizing time domain simulations in the prediction of gearbox design loads. In this study, three problems in time domain based gear contact fatigue analysis under dynamic conditions are discussed: (1) the torque reversal problem under low wind speed conditions, (2) statistical uncertainty effects due to time domain simulations and (3) simplified long term contact fatigue analysis of the gear tooth under dynamic conditions. Several recommendations to deal with these issues are proposed based on analyses of the National Renewable Energy Laboratory’s 750 kW land-based Gearbox Reliability Collaborative wind turbine.

Because of scale effects and inappropriate hydrodynamic models, the nonlinear hydroelastic response of net cages used for fish-farming cannot be analyzed precisely with traditional model testing or combinations of finite element methods (FEMs) and load models. In this study, an innovative hybrid method is proposed to determine the hydroelastic response of full-scale floater-and-net systems more accurately. In this method, the net for the fish cage was vertically and peripherally divided into similar interconnected sections with different hydrodynamic parameters, which were assumed to be uniformly distributed over each section. A model of a typical section was subjected to various towing velocities, oscillation periods, and amplitudes in a towing tank to simulate the potential motions of all sections in the net under various currents, waves, and floater movements. By analyzing the measured hydrodynamic force from this test section, a hydrodynamic force database for a typical net section under various currents, waves, and floater motions was built. Finally, based on an FEM, the modified Morison equation and the hydrodynamic force database, the hydroelastic behavior of the full-scale fish cage was calculated with an iterative scheme. It is demonstrated that this hybrid method is able to produce correct hydroelastic response for both steady and oscillatory flows. The hydroelastic response of a two-dimensional example of a full-length net panel with steady currents and floater oscillations was studied in detail. [ABSTRACT FROM AUTHOR]

Autofrettage is a practical method for increasing the elastic carrying capacity and the fatigue life of thick-walled cylinders such as cannon and high-pressure tubular reactor. Many analytical and numerical solutions for determining the residual stress distribution in an autofrettaged tube have been reported. Iris still difficult to model the Bauchinger effect, which is dependent on the prior plasticity in an analytical solution. The reduced Young's modulus during unloading affects residual stress distribution. However until now this effect has not been considered in any analytical model. In this paper an autfrettage analytical solution considering Young's modulus and the reverse yield stress dependent on the prior plasticity, based on the actual tensile-compressive curve of the material and the von Mises yield criterion, has been proposed. New model incorporates the Bauschinger effect factor and the unloading modulus variation as a function of prior plastic strain, and hence of the radius. Thereafter it assumes a fixed nonlinear unloading profile. The comparison of predicted residual stress distribution by the present solution with that of fixed unloading curve model, and test results shows that the present solution gives accurate prediction of residual stress distribution of an autofrettaged tube. This analytical procedure for the cylinder permits an excellent representation of various pressure vessel steels. [ABSTRACT FROM AUTHOR]

Abstract  Fatigue cracks have been known to occur in welded ships for several decades. For large ocean-going ships wave-induced vibrations can, depending on trade and design, cause up to 50% of the fatigue damage. The vibrations may be due to springing and whipping effects. In this paper, we address the fatigue damage caused by wave-induced vibrations in a containership of newer design trading in the North Atlantic. The fatigue damage was obtained both experimentally and numerically. The experimental results were found from tests performed with a flexible model of the ship, while the numerical predictions were done using nonlinear hydroelastic strip theory. The measurements showed that the wave-induced vibrations contributed approximately 40% of the total fatigue damage. The numerical method predicted the wave frequency damage well, but was found to overestimate the total fatigue damage by 50%. This was mainly due to an overprediction of the wave-induced vibrations. The discrepancy is partly related to three-dimensional (3D) effects which are not included in the two-dimensional (2D) slamming calculation, and partly to an overprediction of the springing contribution. Moreover, the numerical method does not account for the steady wave due to forward speed. By using a simplified approach we show that high-frequency damage can be significantly reduced by including the steady wave for the relevant vessel, implying better agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract  Chains used in the offshore industry typically are exposed to continued high rates of corrosion, in part due to the working of the chain not permitting a build up of protective rusts. Herein, a procedure is described for estimating the corrosion loss of low-alloy steel chain under continued immersion corrosion conditions. The procedure is based on extensive recent investigations of the effects of water temperature, salinity, water velocity, and surface roughness on steel corrosion under field conditions. Since the working of the chain does not allow corrosion products (rust) to build up on the critical contact surfaces, erosion due to wear and abrasion can be estimated separately. Two example applications are given. [ABSTRACT FROM AUTHOR]

The theory and procedure established by Wu and Moan in 1996 and 2005 and Wu and Hermundstad in 2002 were applied to a high-speed transatlantic pentamaran containership. Nonlinear time-domain simulations of ship motions and load effects were carried out in different sea states. The simulated responses were validated against model tests with satisfactory results. The short-term probabilities of exceedance were estimated by using different stochastic analysis procedures. The long-term probabilities of exceedance were obtained based on the short-term results. These served as information about loading in a reliability-based design approach. The load effects in a semiprobabilistic design were also calculated at an appropriate probability of exceedance level. [ABSTRACT FROM AUTHOR]

Abstract A method for the prediction of slamming loads on ship hulls is presented and validated for a 20-knot, 120-m car carrier. A nonlinear strip theory is used to calculate the relative motions of ship and wave. The relative vertical and roll velocities for a slamming event are given as input to the slamming calculation program, which is based on a generalized two-dimensional Wagner formulation and solved by the boundary element method. The method is fast and robust. Model tests of a car carrier have been carried out in regular head, bow, and bow quartering waves of various heights. Slamming on two panels in the upper part of the bow flare has been studied. It has been found that the water pile-up around the bow due to the forward speed of the vessel significantly increases the slamming pressures. A simplified way of including this effect is presented. When the calculated slamming pressures are corrected for 3D effects, they compare well with the measured data. Since the effect of the wave elevation due to the forward speed and the effect of three-dimensional flow act in opposite directions, excluding both of them produced results that also agreed quite well with the experiments, especially for the most severe slamming events. [ABSTRACT FROM AUTHOR]