Design basis considerations for the design of floating offshore wind turbines

The wind farm owner/operator must prepare a Design Basis to facilitate the design of floating offshore wind turbines. The Design Basis is crucial to ensure that the individual elements of the wind farm are designed according to the relevant standards and the actual site conditions. In case of under-design, systematic failures can occur across the wind turbines, which can result in progressive damage to the turbines of the wind farm. This paper focuses on the safety and overall economics, including limiting potential excessive costs of heavy maintenance caused by damage due to under-design. Thus, this paper highlights critical aspects of particular importance to be implemented in the Design Basis document. Meeting all required constraints for developing offshore wind farms in deep water may result in higher costs than initially anticipated. Nonetheless, a realistic cost estimation for all phases of the project, engineering, construction, transport, and installation on site, remains essential for all engineering projects, including those involving renewable energy.publishedVersio

Fatigue Analysis of Inter-Array Power Cables between Two Floating Offshore Wind Turbines Including a Simplified Method to Estimate Stress Factors

The use of floating offshore wind farms for electrical energy supply is expected to rise significantly over the coming years. Suspended inter-array power cables are a new design to connect floating offshore wind turbines (FOWTs) with shorter cable lengths than conventional setups. The present study investigates the fatigue life of a suspended power cable with attached buoys connecting two spar-type FOWTs. Typical environmental conditions for the North Sea are applied. The nonlinear bending behavior of the power cable is considered in the analysis. Fatigue assessment is performed using the numerical software OrcaFlex based on stress factors obtained from cross-section analysis. An effective method for obtaining the stress factors is proposed for early engineering design stages and compared with the finite element software UFLEX simulation results. The simplified method delivers similar results for axial tension loads and conservative results for bending loads compared with results obtained from the finite element software. Stress components resulting from curvature variation are identified as the main contributors to fatigue damage. The most critical locations along the power cable for fatigue life are close to the hang-off points

The Selhausen Minirhizotron Facilities: A Unique Set-Up to Investigate Subsoil Processes within the Soil-Plant Continuum

Climate change raises new challenges for agriculture. A comprehensive understanding of whole plant responses to a changing environment is the key to maintain yield and improve sustainable crop production. Although there are many projects approaching this challenge, most studies focus on the acquisition and analysis of above-ground field data. The subsoil processes involved in plant root growth and resource acquisition are rarely in focus, since very complex set-ups are required to obtain these data on field scale. Therefore, detailed measurement of the plant roots and the corresponding soil conditions are required. The minirhizotron facilities in Selhausen (Germany) are located within the TERENO-Selhausen test site in the lower Rhine valley. They enable non-invasive longer-term studies of the soil–plant continuum on two different soils in the same climate by offering a unique set-up to record above- and belowground information over entire crop growing seasons under various field conditions and agronomic treatments. Detailed information about soil water content, soil water potential, soil temperature and root development are collected with a high spatial and temporal resolution. Above-ground measurements, such as biomass, transpiration fluxes and assimilation rates are performed additionally. In recent years, continuous development and improvement of measurement technology and data analysis has facilitated the process, transfer and access to these data. Currently several dynamic and permanently installed sensors are used within the facilities. 7 m-long transparent tubes are horizontally located in several depths. An in-house developed RGB-camera system enables root imaging along the tubes in multiple directions. The images are analyzed with a deep neural network-based analysis pipeline that provides relevant root system traits, such as total root length and root length density. To obtain the spatial soil water content variations per depth, crosshole ground-penetrating radar (GPR) measurements are performed between the tubes. The derived permittivity and hence soil water content values show a clear spatial variation along the tubes and different behaviors for various plant and soil types. Recently, a novel analysis tool to derive the trend‑corrected spatial permittivity deviation was introduced, allowing an investigation of the GPR variability independently of static and dynamic influences.The ongoing measurements currently cover five years of wheat and maize trials, including water stress treatments, sowing density, planting time, and crop mixtures. Data collected in this study are available through the TERENO data portal and can be used to develop, calibrate, and validate models of the soil–plant continuum across different scales, including soil process, root development and root water uptake models, as well as model compilations, such as single-plant and multi-plant models. Further, the data can be of direct use for agronomists and ecologist

Estimating Soil Hydraulic Properties of the Soil-Plant-Root Zone Using Time-Lapse Horizontal Borehole Ground Penetrating Radar Data in a Sequential Hydrogeophysical Inversion Approach

The soil hydraulic parameters play a vital role in sustainable crop production. They are the governing factors for the spatialtemporal water distribution in the soil-plant-root-zone and therefore regulate the water availability for crops. Estimating these parameters, using standard invasive techniques is time-consuming and often only provides point-scale information. To non-invasively derive them at a field-plot scale, we combined the soil water content (SWC) information from a hydraulic model with the SWC distribution from time-lapse horizontal crosshole ground penetrating radar (GPR) measurements. This sequential hydrogeophysical inversion approach was applied to data of three wheat crop seasons of a rhizotron facility. The rhizotron facility contains of three plots with different surface water treatments: sheltered, natural and irrigated. This setup provides for each plot a unique data set, which includes root observations, GPR, and soil water potential (SWP) data, at six different depths, between 0.1 m - 1.2 m. Above-ground measurements include leaf area index, and consistent atmospheric conditions. The SWC was derived from the horizontal crosshole GPR measurements by analyzing the first arrival times of the electromagnetic waves along a pair of tubes. The results indicate, the SWC varies horizontally and vertically depending on weather conditions, soil properties, and root growth.In the next step, we established a one-dimensional hydrological model using the software HYDRUS 1D. It considers vegetation data, soil information, and atmospheric conditions. For the starting model, we used the soil hydraulic parameters derived by fitting the soil water retention curve to the SWP and the GPR SWC data, for all plots. For the sequential hydrogeophysical inversion, we combined the hydraulic model with the geophysical data set, for the natural plot. First, we used synthetic GPR data, to proof the concept of the inversion approach. Secondly, we applied the approach to field data.This study illustrates the feasibility of using time-lapse horizontal borehole GPR data to determine soil hydraulic parameters. These developments are essential to estimate the potential to use geophysical methods that enable non-destructive SWC-observation at the field-plot scale with a high spatial and temporal resolution

Estimating the effect of maize crops on time-lapse horizontal crosshole GPR data

Investigating soil, roots and their interaction is important to optimize agricultural practices like irrigation and fertilization and therefore increase the sustainability and productivity of crop production. In this study, we are combining two methods to examine non-invasively, characterize and monitor the soil-root zone throughout crop growing seasons: crosshole ground penetrating radar (GPR) and root-images within horizontal mini-rhizotrons. Over three maize crop growing seasons, we acquired in-situ time-lapse crosshole ground penetrating radar data and time-lapse root images, at two mini-rhizotron facilities in Selhausen, Germany. These facilities allow to horizontally measure data at six different depths, ranging between 0.1 m - 1.2 m and below three different plots with varying agricultural treatments, such as irrigation, sowing density, sowing date and cultivars. The GPR measurements result in the dielectric permittivity slices by applying standard ray-based analysis to zero-offset measurements along a pair of rhizotubes. Such horizontal permittivity slices can be linked to soil water content using petro physical relationships. Additionally, the root images provide a root fraction per image, which is derived by using a workflow combining state-of-the-art software tools, deep neural networks and automated feature extraction. The dielectric permittivity slices suggest a permittivity variation along the horizontal and vertical axes, depending on atmospheric conditions, soil properties, and root architecture. To quantify the influence of the roots on the spatial and temporal distribution of dielectric permittivity, we used statistical methods to reduce the impacting factors like soil heterogeneity, tube deviations and changing atmospheric conditions, which results in the spatial and temporal variability. For verification these permittivity variabilities are compared to the root fraction values. In general, using the spatial and temporal permittivity variations, we can detect the presence of roots and additionally recognize a varying influence of the roots over the duration of the crop growing season. Using these first results, we demonstrate that GPR can be applied to improve the characterization of the root-soil system related to maize plants. This could be the first step towards developing proxies e.g. for irrigation and fertilization applications using this non-invasive method

Using horizontal borehole GPR data to estimate the effect of maize plants on the spatial and temporal distribution of dielectric permittivity

Agro-ecosystems and their yield productivity are influenced by root water and nutrient uptake. This uptake depends on the crop root architecture and the soil water content distribution within the soil-root zone. Investigating this zone and its processes can help to optimize agricultural practices, like irrigation and fertilization and therefore helps to achieve the goal for sustainable crop production. Mini-rhizotrons have shown to be effective to non-invasively investigate the soil-root zone throughout crop growing seasons using horizontal rhizotubes installed at different depths in the subsurface. In this study, in-situ time-lapse crosshole ground penetrating radar measurements and root images were collected over three maize crop growing seasons at two mini-rhizotron facilities in Selhausen, Germany. These facilities allow to measure data at six different depths ranging between 0.1 m - 1.2 m and for three different plots with varying treatments. The dielectric permittivity was derived from the horizontal crosshole GPR measurements by using standard ray-based analysis along a pair of rhizotubes. Such horizontal permittivity slices can be linked to soil water content using petro-physical relationships. The root architecture is expressed as root length density and is derived from the images, using a workflow combining state-of-the-art software tools, deep neural networks and automated feature extraction. The results of the dielectric permittivity indicate horizontal and vertical variations, depending on weather conditions, soil properties, and root architecture. To quantify the impact of the roots on the spatial and temporal distribution of the dielectric permittivity, we used statistical methods to eliminate the effects of soil heterogeneity, tube deviations and daily evapotranspiration changes. Resulting in permittivity variation along the rhizotubes impacted by the presence of roots

Using horizontal borehole GPR data to estimate the effect of maize plants on the spatial and temporal distribution of dielectric permittivity

Agro-ecosystems and their yield productivity are influenced by root water and nutrient uptake. This uptake depends on the crop root architecture and the soil water content distribution within the soil-root zone. Investigating this zone and its processes can help to optimize agricultural practices, like irrigation and fertilization and therefore helps to achieve the goal for sustainable crop production. Mini-rhizotrons have shown to be effective to non-invasively investigate the soil-root zone throughout crop growing seasons using horizontal rhizotubes installed at different depths in the subsurface. In this study, in-situ time-lapse crosshole ground penetrating radar measurements and root images were collected over three maize crop growing seasons at two mini-rhizotron facilities in Selhausen, Germany. These facilities allow to measure data at six different depths ranging between 0.1 m - 1.2 m and for three different plots with varying treatments. The dielectric permittivity was derived from the horizontal crosshole GPR measurements by using standard ray-based analysis along a pair of rhizotubes. Such horizontal permittivity slices can be linked to soil water content using petro-physical relationships. The root architecture is expressed as root length density and is derived from the images, using a workflow combining state-of-the-art software tools, deep neural networks and automated feature extraction. The results of the dielectric permittivity indicate horizontal and vertical variations, depending on weather conditions, soil properties, and root architecture. To quantify the impact of the roots on the spatial and temporal distribution of the dielectric permittivity, we used statistical methods to eliminate the effects of soil heterogeneity, tube deviations and daily evapotranspiration changes. Resulting in permittivity variation along the rhizotubes impacted by the presence of roots

Linking horizontal crosshole GPR variability with root image information for maize crops

Abstract Non‐invasive imaging of processes within the soil–plant continuum, particularly root and soil water distributions, can help optimize agricultural practices such as irrigation and fertilization. In this study, in‐situ time‐lapse horizontal crosshole ground penetrating radar (GPR) measurements and root images were collected over three maize crop growing seasons at two minirhizotron facilities (Selhausen, Germany). Root development and GPR permittivity were monitored at six depths (0.1–1.2 m) for different treatments within two soil types. We processed these data in a new way that gave us the information of the “trend‐corrected spatial permittivity deviation of vegetated field,” allowing us to investigate whether the presence of roots increases the variability of GPR permittivity in the soil. This removed the main non‐root‐related influencing factors: static influences, such as soil heterogeneities and rhizotube deviations, and dynamic effects, such as seasonal moisture changes. This trend‐corrected spatial permittivity deviation showed a clear increase during the growing season, which could be linked with a similar increase in root volume fraction. Additionally, the corresponding probability density functions of the permittivity variability were derived and cross‐correlated with the root volume fraction, resulting in a coefficient of determination (R2) above 0.5 for 23 out of 46 correlation pairs. Although both facilities had different soil types and compaction levels, they had similar numbers of good correlations. A possible explanation for the observed correlation is that the presence of roots causes a redistribution of soil water, and therefore an increase in soil water variability

Wave and Wind Responses of a Very-Light FOWT with Guy-Wired-Supported Tower: Numerical and Experimental Studies

A floating offshore wind turbine (FOWT) concept with a guy-wire-supported tower was investigated to obtain motion results in waves considering its elastic model characteristics. The FOWT concept studied aims to reduce the construction costs by using a light-weight structure tensioned with guy wires and a downwind type. Wave tank experiments of an elastically similar segmented backbone model in the 1:60 scale were carried out to clarify the dynamic elastic response features of the structure. The experimental results were compared with numerical simulations obtained from NK-UTWind and WAMIT codes. The bending moment measured at the tower and pontoons had two peak values for different wave periods carried out. The short-wave period peak was due to sagging/hogging when the wavelength matched the floater length. The second peak was due to the large tower top acceleration, which caused a large bending moment at the tower base and pontoon to support the inertia force. The wind force was not significant to modify the FOWT response. The sensibility analysis in pontoons and tower rigidities confirmed the importance of the guy wires to support the inertia due to the waves and wind incidence. The new concept of a very-light FOWT with a guy-wire-supported tower may be an option for future FOWT developments