Effects of a passive tuned mass damper on blade root impacts during the offshore mating process

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 (Hs); however, it decreases with increasing wind-wave misalignment and with shifts in the wave spectral peak period (Tp) 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.Aerospace Manufacturing Technologie

Numerical investigation of rain droplet impact on offshore wind turbine blades under different rainfall conditions: A parametric study

The leading edge of a fiber composite wind turbine blade (WTB) is prone to erosion damages due to repeated rain droplet impact during its service life. Such damages are critical to the blade's aerodynamic as well as structural performance, ultimately resulting in substantial repair costs. An effective design of a coating material for WTB is necessary and its analysis must include variables associated with erosive rain droplets such as (1) droplet diameter, (2) impact velocity, and (3) droplet impact angle. The present paper develops and validates a coupled fluid structure interaction (FSI) computational model for simulating rain droplet impact on WTBs, where the structure domain is modelled using conventional finite element method (FEM) and the fluid domain is modelled using smooth particle hydrodynamics (SPH). The 3D numerical model, developed in LS-DYNA, is validated with published experimental results. Further, a parametric study is considered to understand the effects of varying droplet size, impact angles and impact velocities on the impact responses of the leading edge coating system subjected to different rainfall conditions. The rainfall conditions considered for the analysis correspond to four different rainfall intensities (I) – light rainfall (2mm/hr), moderate rainfall (10mm/hr), heavy rainfall (25mm/hr), and very heavy rainfall (50mm/hr). The results show that the impact responses on the coating system increase with increasing droplet size and increasing droplet impact angle with maximum impulses, stresses and damages developed for normal impingement (90°). Also, the effects of droplet impact angles in the range of 50° to 90° are found critical for rainfall intensities representing very heavy rainfall conditions (I>25mm/hr). The results of the peak contact forces and impulses for the above combination of variables used in the numerical study are found in satisfactory agreement with analytical formulations developed through published experiments. Finally, repetitive rain droplet impact analyses are considered and number of impacts required for onset of erosion damages are found to increase by more than seven times upon reducing impact velocities from 140 m/s to 80 m/s for very heavy rainfall conditions (I>25mm/hr). The present study is expected to deliver a validated numerical model that can contribute towards enhancing the erosive capacity of a WTB.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic

Effects of Wind-Wave Misalignment on a Wind Turbine Blade Mating Process: Impact Velocities, Blade Root Damages and Structural SafetyAssessment

Most wind turbine blades are assembled piece-by-piece onto the hub of a monopile-type offshore wind turbine using jack-up crane vessels. Despite the stable foundation of the lifting cranes, the mating process exhibits substantial relative responses amidst blade root and hub. These relative motions are combined effects of wave-induced monopile motions and wind-induced blade root motions, which can cause impact loads at the blade root’s guide pin in the course of alignment procedure. Environmental parameters including the wind-wave misalignments play an important role for the safety of the installation tasks and govern the impact scenarios. The present study investigates the effects of wind-wave misalignments on the blade root mating process on a monopile-type offshore wind turbine. The dynamic responses including the impact velocities between root and hub in selected wind-wave misalignment conditions are investigated using multibody simulations. Furthermore, based on a finite element study, different impact-induced failure modes at the blade root for sideways and head-on impact scenarios, developed due to wind-wave misalignment conditions, are investigated. Finally, based on extreme value analyses of critical responses, safe domain for the mating task under different wind-wave misalignments is compared. The results show that although misaligned wind-wave conditions develop substantial relative motions between root and hub, aligned wind-wave conditions induce largest impact velocities and develop critical failure modes at a relatively low threshold velocity of impact.Aerospace Manufacturing Technologie

Sea state estimation based on vessel motion responses: Improved smoothness and robustness using Bezier surface and L1 optimization: Improved smoothness and robustness using Bézier surface and L1 optimization

Floating structures oscillate in waves, where these wave-induced motions may be critical for various marine operations. An important consideration is thereby given to the sea states at the planning and operating stages for an offshore project. The most important information extracted from a sea state is the directional wave spectrum, indicating wave direction, significant wave height, and wave spectrum peak period. Among several available methods of measuring and estimating the directional wave spectrum, the wave buoy analogy technique based on vessel motion responses is an in situ and almost real-time solution without extra costs of devices. If the forms of the wave spectra are not predefined in the estimation, the method is called a nonparametric approach. Its most remarkable advantage is the flexible form, but the smoothness should be regulated. After the discrete Fourier transform has been applied to the measured vessel motions, smoothing is necessary. However, this process results in disturbed vessel cross-spectra and a lowpass characteristic of the windowing function. This paper presents a nonparametric approach for directional wave spectrum estimation based on vessel motion responses. It introduces novel smoothness constraints using Bézier surface and includes a more robust estimate using L1 optimization. Both techniques are applied to the wave buoy analogy for the first time. Numerical simulations are conducted to verify the proposed algorithm.Aerospace Manufacturing Technologie

Model-free anti-swing control of complex-shaped payload with offshore floating cranes and a large number of lift wires

Being one of the most commonly used offshore operations, offshore lifting operations become increasingly challenging due to the gradually growing size and weight of payloads. The research on automatic control in lifting operations, e.g., anti-swing control and heave compensation, only considers simple-shaped payloads, such as lumped-mass rigid points. However, the sizes and orientations of many structures cannot be neglected. To lift heavy and large-scale payloads, larger and higher cranes are required. Alternatively, it is possible to share the total loads by enhancing the number of lift wires that may limit the tension on each lift wire. However, the complicated configuration introduces significant complexity into the design of the automatic anti-swing algorithm, especially to the control allocation module. This paper performs a preliminary study on the anti-swing control of a complex-shaped suspended payload lift using a floating crane vessel and a large number of lift wires. Inspired by the knowledge of inverse dynamics and range-based localization, a general model-free anti-swing control scheme is proposed. The controller has a simple form without considering state–space equations, but it can reduce the pendular payload motion regardless of the detailed system configuration. An offshore wind turbine tower–nacelle–rotor preassembly installation using floating crane vessel is adopted as a case study to verify the performance of the proposed control strategy.Aerospace Manufacturing Technologie

Minimum leading edge protection application length to combat rain-induced erosion of wind turbine blades

Leading edge erosion (LEE) repairs of wind turbine blades (WTBs) involve infield application of leading edge protection (LEP) solutions. The industry is currently aiming to use factory based LEP coatings that can applied to the WTBs before they are shipped out for installation. However, one of the main challenges related to these solutions is the choice of a minimum LEP application length to be applied in the spanwise direction of the WTBs. Generally, coating suppliers apply 10–20 m of LEP onto the blades starting from the tip of the blade using the “rule of thumb”, and no studies in the literature exist that stipulate how these LEP lengths can be calculated. In this study, we extend the scope of a recently developed long-term probabilistic framework to determine the minimum LEP application length required for WTBs to combat rain-induced erosion. A parametric study is performed where different wind turbines with varying power ratings of 2.1 MW to 15 MW at different Dutch sites ranging from inland to coastal are considered. The results of the study show that the LEP application length is sensitive to the choice of the site, as well as the turbine attributes. Further, LEP lengths for WTBs are found to be the highest for turbines installed at coastal sites and turbines with higher power ratings. A detailed investigation is further performed to check the sensitivity of the LEP application length with the wind turbine parameters. The results of the study are expected to provide guidelines to the industry for efficient repair strategies for WTBs.Aerospace Manufacturing Technologie

Leading edge erosion of wind turbine blades: Effects of blade surface curvature on rain droplet impingement kinematics

The issue of leading edge erosion (LEE) of wind turbine blades (WTBs) is a complex problem that reduces the aerodynamic efficiency of blades, and affects the overall cost of energy. Several research efforts are being made at the moment to counter erosion of WTBs such as-testing of advanced coating materials together with development of high-fidelity computational models. However, the majority of these studies assume the coated surfaces as flat, while the surface curvature and the shape of the aerofoil at the blade's leading-edge exposed to such rain fields is neglected. The present study questions the assumption of a flat surface, in the context of LEE of WTBs, and provides guidelines for erosion modelling. The critical parameters associated with rain droplet impingement kinematics on leading edge are compared for blade impact with (a) flat surface assumptions together with (b) the effects of the blade's surface curvature. A parametric study is performed which includes WTBs of varying sizes and power ratings ranging from 750 KW to 10 MW, different positions along the blade length, and different rain droplet radii ranging from 0.1 mm to 5 mm for a land based wind turbine operating at rated wind speed. It is found in the study that droplet impingement kinematics are influenced by the surface curvature at the leading edge, the effect of which is significant for representing erosion at the blade tip for smaller blades, and for exposure to rainfall intensity with larger rain droplet size. A master curve describing the threshold level along the blade length is established for various WTBs and rainfall conditions, where flat surface approximation of the surface yields noticeable error and violates the impingement process. The results of the study are expected to aid the modeller in developing advanced numerical models for LEE for WTBs.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic

Bondline thickness effects on damage tolerance of adhesive joints subjected to localized impact damages: Application to leading edge of wind turbine blades

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.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic

A review of impact loads on composite wind turbine blades: Impact threats and classification

A fiber-reinforced composite wind turbine blade (WTB) is exposed to numerous impact threats during its service life causing damages that can be detrimental to its structural integrity. Currently, impact loads are not considered during blade design, so high safety factors are introduced, which result in a conservative design. However, as wind turbine blades become stiffer and lighter and health monitoring systems become more sophisticated, the design process is shifting toward damage-tolerant approaches. The design philosophy accepts damages to the structure, but it also requires that the damaged blade still meet structural and functional requirements. This design procedure requires a comprehensive understanding of different impact threats and their characteristics, which is currently unavailable in the public domain. This paper is a first attempt to review the impact loads on composite wind turbine blades. The aim of the current paper is to (a) identify different sources of impact threats on wind turbine blades during different stages of their service life, (b) describe their qualitative (causes and vulnerable regions) as well as quantitative characteristics (size, mass, and velocity of impactor), and to (c) provide modeling guidelines by comparing these impact threats using five different criteria - (i) relative deformability of projectile and wind turbine blade, (ii) impact velocity, (iii) kinetic energy of impact, (iv) repeatability of impacts and (v) nature of the impact. The review paper will be of special interest to researchers working on wind turbine blades and will serve as a baseline report for designing damage-tolerant blades. Recommendations are also provided for future research.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerospace Manufacturing Technologie

A computational framework for coating fatigue analysis of wind turbine blades due to rain erosion

The rain-induced fatigue damage in the wind turbine blade coating has attracted increasing attention owing to significant repair and maintenance costs. The present paper develops an improved computational framework for analyzing the wind turbine blade coating fatigue induced by rain erosion. The paper first presents an extended stochastic rain field simulation model that considers different raindrop shapes (spherical, flat, and spindle), raindrop sizes, impact angles, and impact speeds. The influence of these raindrop characteristics on the impact stress of the blade coating is investigated by a smoothed particle hydrodynamics approach. To address the expensive computational time, a stress interpolation method is proposed to calculate the impact stress of all raindrops in a random rain event. Furthermore, coating fatigue analysis is performed by including the fatigue crack initiation in the incubation period and the fatigue crack propagation in the mass-loss-rate increasing period due to raindrop impact. Finally, the proposed computational framework is verified by comparing the estimated fatigue life with those obtained in literature. The results from the study show that by incorporating the statistics of rainfall data, the proposed framework could be used to calculate the expected fatigue life of the blade coating due to rain erosion.Aerospace Manufacturing Technologie