Gravity-based Foundation Research Articles

Impact Risk Assessment of Gravity-Based Foundation of Offshore Wind Turbine Under Tsunamis

As an important renewable engineering structure, offshore wind turbine foundation is threatened by tsunami bore. A three-dimensional numerical model of dam-break wave tsunami is established and validated based on computational fluid dynamics to study the impacting effects of gravity-based foundation (GBF) of offshore wind turbines under tsunamis. The impacting process of the small, middle, and large tsunami on the GBF is numerically simulated, and the load effects characteristics of the GBF of the wind turbine at different stages of the tsunami impact process are analyzed. The research results show that the GBF is subjected to two peak impact forces under tsunami-impacting action. The tsunami bore has a frontal impact on the GBF, and the force peak of the flow direction is the largest, which has the greatest impact on the foundation of the wind turbine. Specifically, the middle tsunami exerts a maximum frontal impact force of 16.8[Formula: see text]N, with a peak dynamic pressure of 678.2 Pa, and a maximum base moment of 1.24[Formula: see text]N[Formula: see text]⋅[Formula: see text]m. Besides, the variation on the base moment in the incident flow direction of the incoming is larger than that in the transverse direction. The research provides valuable insights into the tsunami-resistant GBF, offering recommendations and guidelines for ensuring the long-term stability of offshore wind turbines in tsunami-prone regions.

Excess pore water pressure response of soil inside the mini bucket embedded in saturated soil under seismic loading

This study investigates the behavior of suction bucket foundations during seismic loading induced soil liquefaction through shaking table experiments using both steel and 3D printed mini buckets. The pore water pressure response of the soil inside and around the mini buckets was monitored, and the results show that excess pore water pressure ratios greater than 1.0, indicating liquefaction occurred inside and around the buckets. In order to verify whether the soil plug liquefies under seismic loading, a floating ball experiments was specifically designed. The findings suggest that behavior of suction bucket foundations may differ from that of gravity-based foundations when experiencing soil liquefaction, owing to their unique semi-enclosed geometric structure and suction installation method. The appropriately weighted steel mini bucket has the advantage of being less prone settlement in the liquefied soil, making it suitable as a monitoring platform for soil liquefaction.

Experimental analysis of scour around an offshore wind gravity base foundation

Scour processes and corresponding scour protection measures have been extensively studied for vertical cylinders. Several formulations for estimating scour and determining the optimum size of rocks for scour protection are widely available in the literature. However, when studying other types of structures, the geometric variability of the structures, limits the application of available semiempirical formulations. Scour phenomena around complex structures have been investigated experimentally in reduced-scale wave-current basins. However, the number of tests focused on support typologies different from monopile structures is still limited, restricting the availability of semiempirical formulations focused on conceptual design of scour protection solutions or in the estimation of the free scour development.In this work, a better understanding of the scour processes around gravity-based foundations (GBFs) and the performance of different scour protection solutions is gained via experimental results obtained from a large-scale test programme (1:35 test scale), including combined wave and current tests. The experimental tests were divided into two main types: (1) free scour and (2) scour protection tests. The main conclusion was that the triggering of a horseshoe vortex is the main driver of scour processes together with the contraction of flow lines at the foundation contours. Since monopile semiempirical formulations are not suitable for scour prediction, a new semiempirical approach was obtained from the test results. Moreover, two distinct scour mitigation methods were analysed (rock protection and concrete mattresses). Based on the undisturbed mobility parameter, statistical and dynamic limits were defined considering the rock grading tested. Finally, for concrete mattresses, the design driver was identified, and the adaptability of these types of solutions to seabed changes was verified.

A comprehensive review on scour and scour protections for complex bottom-fixed offshore and marine renewable energy foundations

Due to the marine environment's complexity, optimising offshore foundations' design is still a major focus of the scientific and industrial community, since foundations represent a large portion of the total investment. Scour is one of the main causes that lead to ultimate, and service limit states, impacting design optimization and costs. Scour at monopile foundations has been extensively studied. However, advances in scour protections for hybrid structures, and other bottom-fixed foundations for offshore renewable energy harvesting technologies are often kept confidential by stakeholders, and knowledge gaps must be covered. Thus, the present paper provides a summary of the most recent physical and numerical studies related to scour and riprap scour protections for complex bottom-fixed foundations, such as jackets, tripods, gravity-based foundations (GBF), high-rise structure foundations (HRSF), and suction buckets, while covering other marine energy harvesting technologies as well, including wave energy converters (WEC) and tidal energy converters (TEC). This manuscript presents knowledge gaps, recent improvements, and potential studies on hybrid foundations – offshore wind turbine foundations combined with WECs or TECs. It is shown that offshore scour protection is a study field with a wide margin of development, but crucial for the future of the offshore renewable energy sector, thus pointing out key challenges and major opportunities for future research.

Geotechnical Preliminary Design of Onshore Wind Turbine Foundations

A simplified geotechnical design procedure for foundations of onshore wind turbines is often beneficial because it can provide the types and sizes of foundations required to perform financial feasibility analysis of a project, and can also be used for preliminary design. This study presents a simplified method for designing onshore gravity-based foundations based on limited required data, namely, loads acting on foundations, data provided by manufacturers, and soil properties. The detailed design issues that may be essential for the final design are also discussed. A flowchart of the design process is also presented to visualize the geotechnical design of such foundations. The required calculation procedures can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem explaining the design of onshore wind turbine foundations was presented to demonstrate the proposed calculation procedure. The data used for the calculations were obtained from the literature. The proposed flowchart provides a quick way for the preliminary design of foundations of onshore wind turbine foundations. The foundation system obtained from the preliminary geotechnical design was numerically modeled using Plaxis 3D software. The global stability of the foundation system under Ultimate Limit State (ULS) loads and soil pressure developed under the foundation were calculated. The soil pressure beneath the foundation under Serviceability Limit States (SLS) loads and the total settlement and rotation of the foundation were calculated. The results obtained from the Plaxis 3D software and those obtained from the preliminary design were in agreement.

Numerical Study of Turbulent Wake of Offshore Wind Turbines and Retention Time of Larval Dispersion

Offshore Wind Farm (OWF) foundations are considered to have a potential impact on the larval dispersion of benthic species. This study focused on OWFs’ impacts on larval dispersion, considering factors such as the foundation type, flow velocity, flow direction, and release type using numerical modelling. At the scale of monopile and gravity-based foundations, a combination of two numerical models was used: the Eulerian model (OpenFOAM), solving the 3D Navier–Stokes equations for computing the hydrodynamics, and the Lagrangian model (Ichthyop), solving the advection–diffusion equation for the larval dispersion simulations. The validation model tests were evaluated with experimental data as a first step of the study. Accurate results were achieved, yielding a Turbulent Kinetic Energy (TKE) Root-Mean-Squared Error (RMSE) in the range of 6.82–8.27 ×10−5kg/m·s2 within the refined mesh, with a coefficient of determination (R2) approaching unity. For the second phase, more-realistic simulations were modelled. Those simulations demonstrated turbulent wakes downstream of the foundations and horseshoe vortex formations near the bottom. A larval dispersion was simulated using passive particles’ motion. Vertical flumes in the wake with particles experiencing both upward and downward motions, impacting the fall velocities of the particles, were observed. The influence of gravity-based foundations might lead to a stepping-stone effect with a retention time of up to 9 min, potentially allowing the settlement of competent larvae. In a similar geometry with an angular spring tide velocity, 0.4% of particles were trapped.

The Integration of Tools for the Techno-Economic Evaluation of Fixed and Floating Tidal Energy Deployment in the Irish Sea

Marine renewable energy (MRE) development will be crucial to achieve worldwide energy decarbonization. In Europe, 1 GW and 40 GW of ocean energy are set to be developed by 2030 and 2050, respectively. Support is essential if wave and tidal stream arrays are to become more economically viable than they currently are. Four recently developed open-access software tools are used in this study to investigate the critical and expensive elements of potential demonstration and commercial scale tidal projects. The tools have been designed and built to assist users with array configurations, foundation and mooring (F&M) design, operation and maintenance (O&M) strategies, and techno-economic analysis. Demonstration of their use is performed in this study to model scenarios for 2 MW, 10 MW, 40 MW, and 100 MW tidal energy projects employing typical 500 kW fixed and 2 MW floating turbines at the West Anglesey Tidal Demonstration Zone in the Irish Sea. The following metrics are examined: the power output and wake losses of staggered and line configurations; the design and costs of simple gravity-based foundations, gravity-based anchors and the four-chain catenary mooring system of a single turbine; the mean O&M costs and farm availability over the project life; and the breakdown of levelized cost of energy (LCoE) for all eight scenarios to ultimately reveal minimum values of 173 EUR/MWh and 147 EUR/MWh for fixed and floating tidal energy technologies, respectively. The thorough analysis facilitated within these four tools to forecast realistic situations in a specific location can help users design a tidal energy project for an area with considerable potential for commercial scale projects, and thus assist the ocean energy community in promoting and nurturing the sector in the years and decades ahead.

Offshore construction using gravity-base foundations indicates no long-term impacts on dolphins and harbour porpoise

There are concerns about the short- and long-term effects on cetaceans from offshore construction using pile-driving. Gravity-base foundations and anchored floating turbines are alternative installation methods that may have less impact on cetaceans. In this study, we investigated the response of dolphins (Tursiops truncatus and Lagenorhynchus albirostris) and harbour porpoise (Phocoena phocoena) to the construction of wind turbines using gravity-base foundations off Blyth, Northumberland, United Kingdom using cetacean echolocation recorders (C-PODs). Data were collected at nine sites across 3 years (2016–2018) before, during and after construction. Generalised additive mixed models were used to investigate temporal, environmental, and anthropogenic drivers of dolphin and porpoise occurrence from 143,215 h (5967 days) of C-POD data. The models explained 27% and 30% of the deviance in dolphin and porpoise occurrence, respectively. Overall, the results showed no long-term effect on the dolphin occurrence from the construction of the gravity-base wind turbine array. In contrast, porpoise occurrence increased by 32% and 75%, respectively, in the years during and after construction, compared to the before-construction year. Other predictors of dolphin and porpoise occurrence included month, hour of day, tidal currents and vessel sonar activity. Our findings indicate that wind turbine installation using gravity-base foundations had no long-term effects on the occurrence of dolphins or porpoise and may represent an offshore construction methodology that is less impactful to dolphins and harbour porpoise than impact pile-driven turbine installation methods. These results are important for future offshore energy developments; however, further studies are recommended to investigate potential species and location variations.

Experimental measurements of wave-induced scour around a scaled gravity-based Oscillating Water Column Wave Energy Converter

Experimental measurements of wave scour are reported for a 1:20 scaled model of an operating 200 kW Oscillating Water Column (OWC) Wave Energy Converter (WEC) in King Island, Tasmania, Australia. Test cases include different wave conditions (Keulegan-Carpenter (KC) number range of 0.45-1.02) and multiple geometric changes to the device, which are compared to scour measurements taken in the field around the King Island OWC WEC device. The measurements therefore provide the first ever comparison of field data with a scaled laboratory model of scour around an OWC WEC.Results highlight that scour forms at the back corners of the device before undermining the structure with depths up to 64 mm (scour depth to structure width ratio, S/B, of 0.094), although the magnitude of this scour is shown to be reduced by 23% by curving the back corners. Scour at the front of the device is highly sensitive to the configuration of the front chamber door, with very different scour scenarios depending on whether the chamber door is open, closed, or removed entirely. Comparison with field data shows that undermining scour at the back of the device is reproduced in both settings, and this leads to settlement of the structure in both the laboratory and field. This suggests that the physical modelling forms an accurate representation of a real-world site, and therefore demonstrates scour processes with applicability to OWC WECs and other gravity-based bluff body structures such as caissons or gravity-based foundations.

Physical Modelling of Offshore Wind Turbine Foundations for TRL (Technology Readiness Level) Studies

Offshore wind turbines are a complex, dynamically sensitive structure due to their irregular mass and stiffness distribution, and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoons, hurricanes, earthquakes, sea-bed currents, and tsunami. Because offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occur during the design lifetime. Traditionally, engineers use conventional types of foundation systems, such as shallow gravity-based foundations (GBF), suction caissons, or slender piles or monopiles, based on prior experience with designing such foundations for the oil and gas industry. For offshore wind turbines, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse metocean and geological conditions. The paper, therefore, has the following aims: (a) provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project; and (c) provide examples of applications in scaled model tests.

Nanomaterials in Protection of Buildings and Infrastructure Elements in Highly Aggressive Marine Environments

The 2030 Agenda and Sustainable Development Goals (SDG) are both an engineering challenge and an opportunity. Clean energy (SDG 7), sustainable cities and communities (SDG 11), and climate action (SDG 13) represent an effort to manage, plan, and develop our buildings and infrastructure. The purpose of this study is to contribute to this challenge by analysing nanomaterials in marine environment structures, both urban and maritime. To do this, we have analyzed different regulations of concrete properties in various countries, defining the characteristics of the cement, coating, water/cement rating, and chloride effect; the difference in durability based on conventional reinforcements and nanomaterials; and use on highly sensitive elements, buildings in marine environments, rubble mound structures, crown walls, and gravity-based foundations for wind power facilities. Division into overhead, underwater, or splash zones entails the use of epoxy resins or silica fume matrices in percentages far below ten percent. Using the most exposed and unfavorable structures, conclusions of application to buildings are established based on the recommendations in maritime engineering most exposed to the actions of the waves. The study concludes with recommendations regarding the durability, increased lifespan, and use of new materials in infrastructure elements in highly adverse marine environments.

Understanding rock–steel interface properties for use in offshore applications

The properties of unbonded rock–steel interfaces and the characteristics that control this behaviour seem to be an under-researched area in terms of geotechnical application, for example in the design of gravity-based foundation systems or dead weight anchors and the interaction of pipelines on rock. Although basic guidance does exist for rock–rock interfaces or pipeline behaviour, this focuses on macro roughness, with little consideration of micro roughness, relative roughness of the surfaces or their strengths and hardness. Therefore, in order for design and understanding to develop in these areas, there is a need for basic interface friction parameters and understanding of the interface characteristics that control the strength of the interface, such that correct values can be used, but also so that the interface properties can be best manipulated to improve interface interaction. This paper presents interface friction angles for four types of rock sheared against steel interfaces of different roughness at a variety of normal stresses. The rocks themselves have a range of surface roughness, strength and hardness. The results of the testing programme are used to improve a simple analytical approach for predicting the shear strength of rock–steel interfaces that allows input of key controlling parameters.

Durability aspects in self-compacting siderurgical aggregate concrete

The steel industry generated approximately 18 Mt of electric arc furnace slags in 2016. It is estimated that construction generates 10% of man-made CO2 emissions. To mitigate these two problems, the valorisation of these slags by using them to manufacture siderurgical concrete is considered. As there is a large volume of these aggregates, which have good mechanical properties, it seems sensible to propose their valorisation, even though this implies a series of modifications in concrete properties such as better mechanical properties or higher density. These modifications in the concrete properties make it especially suitable for some concrete applications, such as its use in marine gravity structures, e.g. dams, breakwaters or Gravity-Based Foundation (GBF). In this research, the suitability of these siderurgical concretes for exposure to marine environments has been proved in laboratory and by keeping some reinforced concrete samples in a tidal race for 10 months. Special samples were made in order to analyse the chlorine penetration; truncated conical shaped test pieces were used to analyse the effect of different coating thicknesses on the protection of the reinforcement. After the concrete characterization, it was found that the designed concretes had an adequate durability for exposure to the marine environment.

Comparison of the dynamic responses of monopiles and gravity base foundations for offshore wind turbines in sand using centrifuge modelling

Monopiles and gravity base foundations (GBF) are two of the most commonly used foundations for offshore wind turbines. As resonance can cause damage and even failure of wind turbines, understanding the difference between the dynamic responses of monopiles and GBFs under free vibration is important. However there is little experimental data regarding their natural frequency, especially from model tests carried out at correct stress levels. This paper presents the results of novel monopile and GBF tests using a centrifuge to directly determine the natural frequency (fn) of the foundation-soil system. The natural frequencies of wind turbine monopiles and GBFs in centrifuge models were measured during harmonic loading using a piezo-actuator, with the results confirming that soil-structure interaction must be considered to obtain the system’s natural frequency as this frequency reduces substantially from fixed-base values. These results will contribute in preventing resonance induced damage in wind-turbines.

Inspection of a high-cycle accumulation model for sand based on recalculations of a full-scale test on a gravity base foundation for offshore wind turbines

Offshore wind turbines (OWT) are generally founded on piles or gravity base foundations. Up to now, there is a lack of validated and standardized analysis procedures for such foundations, especially for the serviceability limit state, i.e. for the prediction of accumulated deformations. In order to gain experience on the behaviour of a OWT gravity base foundation under typical offshore loading conditions, a full-scale test has been performed near the shore of the North Sea. The test foundation was subjected to about 1.5 million load cycles simulating several storm events. The foundation and the sandy subsoil have been extensively instrumented so that a large amount of measured data on the cyclic soil-structure interaction were collected. Meanwhile, a numerical procedure for the proof of the serviceability for OWT foundations was developed. The procedure is based on finite element (FE) calculations in combination with a high-cycle accumulation (HCA) model. The application of the HCA model in simulations of the full-scale test is described in this contribution. The essential simplification of the subsoil and the determination of the input parameters for the FE simulations are shown. The settlements, pore water pressures and soil pressures predicted by the HCA model are compared to the measurements.

Functional differences in trophic structure of offshore wind farm communities: A stable isotope study

The proliferation of offshore wind energy installations causes a local change in biodiversity because these structures become heavily colonised by large quantities of fouling fauna, attract large mobile crustaceans and fish, and alter the macrofaunal communities in the soft sediments surrounding the wind turbines. Here, we analysed the stable isotope signals (δ13C and δ15N) of the faunal communities associated with a wind turbine, its scour protection layer (SPL) and the surrounding soft sediments. We hypothesised that structural differences in community composition would be reflected in food web complexity and that resource partitioning could be one of the mechanisms contributing to the co-existence of such dense communities. Sampling was conducted at a gravity-based foundation in the Belgian part of the North Sea, where both sessile and mobile organisms were collected along the depth gradient of the turbine, the SPL and the surrounding soft substrate. The results indicated that the structural differences of the communities are reflected in the food web complexity as indicated by the trophic niche size, the trophic diversity and the redundancy of the communities along the depth gradient. Higher food web complexity was associated with zones where high accumulation of organic matter occurs (soft substrate and SPL). Low food web complexity was observed in depth zones that are dominated by sessile suspension-feeding organisms (intertidal and Mytilus zone). The high trophic diversity and low redundancy observed within the trophic clusters of the soft substrate and the Metridium zone indicated that resource partitioning can be a mechanism allowing the co-existence of large densities of a wide variety of species.

The effects of slab geometries and wave directions on the steep wave-induced soil response and liquefaction around gravity-based offshore foundations

ABSTRACT Gravity-based offshore foundations generally consist of a bottom slab and one or more cylindrical shafts on top of it. The slab geometry can strongly affect the surrounding flow pattern, dynamic wave pressure distribution and further momentary soil liquefaction. In the present study, the effects of the slab geometry and the incoming wave angle to the foundations on the surrounding soil response are investigated. Gravity-based foundations with bottom slabs of cylindrical shape and hexagonal prismatic shape are considered. For the hexagonal foundation, two different incoming wave angles, i.e. propagating to the hexagon corner and the edge, are simulated. The waves are fully nonlinear steep non-breaking waves. The wave-structure interaction is validated against an existing experiment. Soil consolidation behaviour underneath the foundations is investigated. It is found that both the slab geometry and the incoming wave angle can affect the distributions of pore pressure and momentary liquefaction depth in the surrounding soil.

Cone-shaped hollow flexible reinforced concrete foundation (CHFRF) – Innovative for mountain wind turbines

This paper presents an innovative type of mountain wind turbine foundation, namely, the cone-shaped hollow flexible reinforced concrete foundation (CHFRF). It consists of a top plate, a base plate and a side wall that are made of reinforced concrete. The cavity of the CHFRF is filled with rubble and soil directly from the excavation for the CHFRF, which means that it can absorb the spoil. A rubber layer is placed beneath the CHFRF to increase the foundation flexibility to resist cyclic and dynamic loadings and to increase the bearing capacity. The great advantages of the CHFRF are the reduction in the usage of concrete and steel and the protection of the vegetation around the wind turbine, compared with conventional mountain wind turbine foundations that are solid structures. It is verified through model tests and a numerical simulation that the CHFRF can provide higher lateral bearing capacity in comparison to the regular circular gravity-based foundation under the same foundation diameter and height, and that the bearing capacity is increased by approximately 33.5% accordingly. It is also found that the rubber layer can effectively reduce the accumulated rotation of the CHFRF under cyclic loading. The accumulated rotation of the CHFRF with a rubber layer having a thickness of 4 mm is decreased by about 50% compared to that of the CHFRF with a rubber layer having a thickness of 2 mm. In addition, the volume of concrete used for the CHFRF is only one-fifth of that used for the circular gravity-based foundation. Therefore, the CHFRF outperforms regular mountain wind turbine foundations.

Vibration characteristics of offshore wind turbine tower with gravity-based foundation under wave excitation

In this study, vibration characteristics of offshore wind turbine tower (WTT) with gravity-based foundation (GBF) are identified from dynamic responses under wave-induced excitations. The following approaches are implemented to achieve the objective. Firstly, the operational modal analysis methods such as frequency domain decomposition (FDD) and stochastic subspace identification (SSI) are selected to estimate modal parameters from output-only dynamic responses. Secondly, a GBF WTT model composed of superstructure, substructure and foundation is simulated as a case study by using a structural analysis program, MIDAS FEA. Thirdly, wave pressures acting on the WTT structure are established by nonlinear regular waves which are simulated from a computational fluid software, Flow 3D. Wave-induced acceleration responses of the target structure are analyzed by applying the simulated wave pressures to the GBF WTT model. Finally, modal parameters such as natural frequencies and mode shapes are estimated from the output-only acceleration responses and compared with the results from free vibration analysis. The effect of wave height and period on modal parameter extraction is also investigated for the mode identification of the GBF WTT.

Gravity-Based Foundations in the Offshore Wind Sector

In recent years, the offshore wind industry has seen an important boost that is expected to continue in the coming years. In order for the offshore wind industry to achieve adequate development, it is essential to solve some existing uncertainties, some of which relate to foundations. These foundations are important for this type of project. As foundations represent approximately 35% of the total cost of an offshore wind project, it is essential that they receive special attention. There are different types of foundations that are used in the offshore wind industry. The most common types are steel monopiles, gravity-based structures (GBS), tripods, and jackets. However, there are some other types, such as suction caissons, tripiles, etc. For high water depths, the alternative to the previously mentioned foundations is the use of floating supports. Some offshore wind installations currently in operation have GBS-type foundations (also known as GBF: Gravity-based foundation). Although this typology has not been widely used until now, there is research that has highlighted its advantages over other types of foundation for both small and large water depth sites. There are no doubts over the importance of GBS. In fact, the offshore wind industry is trying to introduce improvements so as to turn GBF into a competitive foundation alternative, suitable for the widest ranges of water depth. The present article deals with GBS foundations. The article begins with the current state of the field, including not only the concepts of GBS constructed so far, but also other concepts that are in a less mature state of development. Furthermore, we also present a classification of this type of structure based on the GBS of offshore wind facilities that are currently in operation, as well as some reflections on future GBS alternatives.