Search

Abstract Wind turbine blades are exposed to numerous impact risks throughout their lifetimes. The impact risks range from bird collisions during operation to impacts with surrounding structures at the time of transportation and installation. Impact loads on the fibre composite blades can induce several complex, simultaneously interacting and visually undetectable damage modes and have a high potential to reduce the local and global blade stiffness. An assessment of such impact-induced damages is therefore necessary and usually involves high computational costs using numerical procedures, especially when analysing large composite components. To minimise this computational expense, different numerical impact modelling techniques are utilised, primarily shell-element-based approaches and multiscale-modelling-based global-local approaches. In this article, a comparison between (1) pure shell, (2) shell-to-solid coupling, and (3) submodelling finite element modelling techniques using Abaqus/Explicit is presented for a case where an impactor hits the leading edge of a blade. A high-fidelity local solid finite element model is developed for the leading edge of a DTU 10 MW blade at the region of impact and its stiffness is compared with baseline. A user material subroutine VUMAT for the intralaminar damage mode based on the Hashin failure criterion is formulated and then validated via an experiment from the literature. Finally, based on different numerical modelling techniques, impact investigations are performed, and the impact responses, damage to the blade and computational analysis durations are compared. It is found that the submodelling-based global-local approach is the most efficient analysis technique for this case, capturing failure modes including delamination, core crushing and local surface indentation in the blade. The findings of this study can be used to develop accurate and computationally efficient tools for modelling impact-induced damage to a blade. [ABSTRACT FROM AUTHOR]

Abstract For installing offshore wind turbines into deep waters, use of floating crane vessels is essential. One of the major challenges is their sensitivity to wave-induced vessel and crane tip motions, which can cause the impact of lifted components like blades and nacelle with nearby structures. The impact loads on fibre composite wind turbine blades are critical as several complex damage modes, capable of affecting the structural integrity, are developed. Planning of such installation tasks therefore requires response-based operational limits that consider impact loads on the blade along with their damage quantification. The research area considering the impact behaviour of the lifted blade is novel, and thus, the paper identifies vessel, blade and lifting parameters that determine impact/contact scenarios. Furthermore, for a case in which a lifted blade with its leading edge impacts the tower, a numerical modelling technique is presented in Abaqus/Explicit, and a comprehensive damage assessment of the blade and an investigation of the impact dynamics and energy evolution are performed. Sensitivity studies for two distinct blade designs and two different impact locations are considered. The results show that 7–20% of the impact energy is absorbed as damage in the blade, whereas the majority dissipates as rigid-body motions of the blade after the impact. The findings of the study highlight the requirement for advanced installation equipment, such as active tugger lines, to prevent successive impacts of wind turbine blades during installation. Highlights • Floating crane vessels are sensitive to wave-induced vessel and crane tip motions and can cause impact of the lifted wind turbine blade with surrounding structures. • The vessel, blade and lifting parameters which determine the impact scenarios are identified. • A comprehensive impact investigation is performed in Abaqus/Explicit for a case in which the blade impacts the preassembled turbine tower with its leading edge. • Sensitivity study for two distinct blade designs and two different impact locations are considered. • The results show that 7-20% of the impact energy is absorbed as damage in the blade, whereas the majority dissipates as rigid-body motions. [ABSTRACT FROM AUTHOR]

Unbalanced composite layups with bend-twist coupling show potential for aeroelastic tailoring in wind turbine blades. Before these materials can be implemented, their responses to long term cyclic loading must be considered. This paper studies the fatigue characteristics of an unbalanced glass-carbon hybrid laminate with a [45glass/-45glass/24carbon/24carbon]s layup. Flexural fatigue was performed at 7 different load magnitudes up to 1 × 106 cycles to characterize the failure modes and fatigue life of the composite. Stiffness degradation occurred on the tension side due to matrix cracking and small regions of delamination on the glass plies, whereas the failure mechanism of the laminate was by delamination between the glass and carbon. S-N curves were generated from experimental results and static finite element analyses (FEA) based on interlaminar shear stresses and were compared with laminates from previous literature. It was determined that the interlaminar stresses were influenced more so by the lower stiffness of the unbalanced layup than by the induced torsional deflections: leading to the conclusion that bend-twist coupling had little influence on flexural fatigue of glass-carbon hybrid composites. [ABSTRACT FROM AUTHOR]

The leading edges of wind turbine blades are adhesively bonded composite sections that are susceptible to impact loads during offshore installation. The impact loads can cause localized damages at the leading edges that necessitate damage tolerance assessment. However, owing to the complex material combinations together with varying bondline thicknesses along the leading edges, damage tolerance investigation of blades at full scale is challenging and costly. In the current paper, we design a coupon scale test procedure for investigating bondline thickness effects on damage tolerance of joints after being subjected to localized impact damages. Joints with bondline thicknesses (0.6 mm, 1.6 mm, and 2.6 mm) are subjected to varying level of impact energies (5 J, 10 J, and 15 J), and the dominant failure modes are identified together with analysis of impact kinematics. The damaged joints are further tested under tensile lap shear and their failure loads are compared to the intact values. The results show that for a given impact energy, the largest damage area was obtained for the thickest joint. In addition, the joints with the thinnest bondline thicknesses displayed the highest failure loads post impact, and therefore the greatest damage tolerance. For some of the thin joints, mechanical interlocking effects at the bondline interface increased the failure load of the joints by 20%. All in all, the coupon scale tests indicate no significant reduction in failure loads due to impact, hence contributing to the question of acceptable localized damage, i.e., damage tolerance with respect to static strength of the whole blade. [ABSTRACT FROM AUTHOR]

Highlights • Impact between the blade root and hub during an offshore mating task is studied. • The sideways impact between the guiding connection and hub is found critical. • Impact load causes severe bending and plastic deformation of the guide pin bolt. • Excessive bending of guide pin triggers the failure of the adjacent root laminate. • Guidelines are provided to aid onboard decision making after the impact events. • The study aims to derive response-based operational limits for the mating task. Abstract Single-blade installation is a popular method for installing blades on bottom-fixed offshore wind turbines. A jack-up crane vessel is often employed, and individual blades with their roots equipped with mechanical joints and bolted connections are lifted to the tower-top height and mated with a pre-assembled hub. The final mating phase is challenging and faces significant risks of impact. Due to relative motions between the blade and the hub, substantial impact forces may arise and lead to severe structural damages at root connections, thereby causing delays in the installation task. The present paper considers a realistic scenario of the mating process and investigates the consequences of such impact loads. Here, a single-blade model with tugger lines and a monopile model were established using a multi-body formulation, and relative velocities under collinear wave and wind conditions were obtained. A three-dimensional finite element model was developed for the blade root with T-bolt connections, and an impact investigation was performed for the case in which a guiding connection impacts the hub. The results show severe bending and plastic deformation of the guide pin bolt together with failure of the adjoining composite laminate at the root connection. Based on the type of damage obtained for the different environmental conditions considered, this paper also discusses its consequence on the installation tasks and suggests onboard decision making in case of an impact incident. The results of this study provide new insights regarding the mating phase and can be utilised to establish response-based operational limits. [ABSTRACT FROM AUTHOR]

Single-blade installation is a conventional method for installing blades on monopile-type offshore wind turbines. A jack-up crane vessel is commonly used, and individual blades are lifted to the tower top height and mated with the hub. The relative motions between the hub and blade root during the mating phase, partly due to wind-induced blade motion and partly due to wave-induced monopile motion, can induce substantial impact forces at the blade root. This can cause severe damage at the blade root connections and have a high potential to jeopardise the installation task. Mitigation measures are therefore required to limit the relative motion between the hub and the root during the mating process. In this article, we investigate the effects of a passive tuned mass damper (TMD) on the (1) impact velocities manifested between the blade root and hub during the mating phase and (2) its effect on the response-based limiting sea states. Time-domain multi-body simulations of an installation system characterising the mating operation with and without a TMD for collinear and misaligned wind and wave conditions have been performed, and the effectiveness of TMD for controlling the impact velocity is quantified. Furthermore, finite element analyses are performed to determine the threshold velocity of impact for a scenario in which a blade root with a guide pin suffers a sideways impact with the hub. It is found that the tuned mass damper can reduce the relative impact velocities by more than 40% and can substantially expand the allowable sea states and operability for the mating operation. Moreover, the effectiveness of TMD at reducing the impact velocity increases with increasing significant wave height (H s); however, it decreases with increasing wind-wave misalignment and with shifts in the wave spectral peak period (T p) away from the tuned frequency. The findings of the study can be utilised for planning safe and cost-efficient installation of latest-generation wind turbine blades. • The relative motions between the hub and blade root during the mating phase can jeopardize the offshore installation task. • Time-domain global response analysis is performed for an installation system with and without tuned mass damper (TMD). • In addition, effectiveness of TMD on the blade impact velocity during an offshore mating process is quantified. • Finite element analyses are conducted to establish allowable impact velocity and operational sea states are obtained. • TMD reduces the impact velocities by more than 40% and expand the limiting sea states and operability for the mating task. [ABSTRACT FROM AUTHOR]