Offshore Wind Turbine Foundations Research Articles

Experimental Study Of The Wave Field Around A Monopile Due To Moderate Steepness Irregular Incident Waves

Most offshore wind turbine foundations are of monopile type and need regular marine operations to be conducted nearby during the lifetime of the turbine. Proper wave estimation around the monopile is needed to assure the safety of the operations. The trend of increasing monopile's diameter also adds the need to investigate the wave field around the increased monopile diameter for the marine operation analysis. Experimental campaigns of wave field around a monopile in irregular incident waves with different steepness are reported in this study to extend the available experiments which focused mostly on the runup of cylindrical structures. The wave field at two radial positions and six angular positions around the monopile is investigated. The results are processed to obtain the Linear Transfer Function of the wave field around the monopile and the Exceedance Probability of the wave height and crest of the waves around the monopile. For the tested conditions, the linear solution accurately captures the frequency domain characteristics and the exceedance probability of wave height but fails to predict the crest exceedance probability.

Tidal currents-induced scour development around pile foundations: Effects of flow velocity hydrograph

There is a lack of research in the existing literature regarding the scour around foundations for offshore wind turbines under tidal currents, which primarily relies on laboratory experiments with simplified flow velocity hydrographs like square tidal currents. To improve the prediction accuracy of scour development under tidal currents, a series of experiments were conducted to investigate the local scour process around a pile foundation under two typical tidal velocity hydrographs (sinusoidal and square tidal currents) in a specially designed fluid-structure-soil coupling flume. The results demonstrate that reciprocating tidal currents lead to a continuously evolving process of sediment erosion and backfilling around the pile. Although the shapes of the scour holes are similar between sinusoidal and square tidal currents, there are significant differences in the evolving process of the scour depth, presenting a short-platform shape and serrated shape, respectively. A consistent relationship is found between the dimensionless scour depth and the dimensionless effective flow work (DFW) under both tidal and unidirectional currents. An equivalent velocity expression for sinusoidal and square tidal currents is proposed and verified using existing experimental data. Furthermore, an empirical expression for the scour depth reduction coefficient between square tidal currents and unidirectional currents is proposed. These outcomes not only establish a theoretical approach for simplifying tidal currents hydrographs in laboratory experiments, but also provide practical guidance for assessing the tidal currents-induced scour development around pile foundations for in-situ offshore wind turbines.

Unidirectional horizontal cyclic behaviors of the monopile-wheel composite foundation in sand

The monopile-wheel composite foundation is an innovative type of offshore wind turbine (OWT) foundation with good bearing capacity. The foundation is subjected to cyclic loads from wind, waves, and tides, so it is necessary to study its horizontal cyclic characteristics. By introducing cyclic degradation constitutive and Rayleigh damping parameters, numerical models are generated and validated by comparing with laboratory experiment results. The laws of horizontal accumulated displacement of the composite foundation are then explored through changes in the cycle amplitude, number, wheel diameter, height of loading point and pre-vertical load. The main findings are that the composite foundation is more likely to reach a stable displacement under constant amplitude cyclic loading compared to that of monopile. Under transient dynamic response (74 % of the horizontal ultimate bearing capacity), accumulated displacement will increase rapidly regardless of the foundation form. The composite foundation has smaller hysteresis loop area and slower accumulation rate of bending moment. The soil pressure weakening of the monopile mainly occurs in front of the pile, while that of the composite foundation mainly occurs under the wheel. After cyclic loading, the V–H envelopes for the composite foundation contract unevenly inwards. Above these, prediction methods for the cumulative displacement, maximum bending moment and V–H envelopes for composite foundation considering cyclic degradation are proposed.

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.

Tilt adjustment strategy and applicability of four-bucket jacket foundation for offshore wind turbines

With the rapid development of offshore wind turbine industry, the development of sea areas will gradually move from offshore to deep sea. In this process, multi-bucket jacket foundation will become a highly potential infrastructure solution in the water depth of about 50 m due to its unique advantages. The closed space is formed inside the bucket foundation during the penetration process, so the suction sinking method can be used for penetration, the accurate grasp of the critical suction during the penetration process of the four-bucket jacket foundation determines whether the penetration process can be carried out smoothly. At the same time, appropriate tilt adjustment strategies need to be adopted promptly after the inclination occurs during the penetration process. In this paper, the tilt adjustment technique in the penetration process of the four-bucket jacket foundation was taken as the research object. Firstly, the model test was used to study the critical suction of the foundation at three different penetration depths, and the test results are compared with the theoretical calculation results. Subsequently, foundation on the above research results, the applicability of the tilt adjustment strategy when the tilt angles of 1°, 2° and 3° occurred at three penetration depths was analyzed, which provides technical guidance for the development of practical projects.

Fatigue damage analysis method of offshore wind turbine foundation

The systems of offshore wind turbines are established in a complex marine environment, and are subjected to all kinds of ocean loadings such as waves, currents and wind for a long time. It is found that offshore wind turbine foundations are seldom damaged due to the problem of strength and stability under normal operation conditions. Most of the damages are caused by the fatigue damage under long-term loads. Aimed to this problem, the monopile foundation of offshore wind turbine is analyzed as an example in this study. The fatigue damage is analyzed systematically using the method of time-domain, combining the rain-flow counting method, S–N curve with linear and nonlinear fatigue damage models. In addition, a modified nonlinear fatigue damage model is proposed based on the Corten-Dolan and Manson-Halford nonlinear fatigue damage models, which can consider the modification of load interactions and the effect of loading sequence. In this way, the short-term fatigue damage value of offshore wind turbine foundation can be obtained. On the basis, the long-term probability distribution function of wind speed is obtained according to the actual monitoring results of wind load, and the fatigue damage value of the offshore wind turbine foundation in the design life can be obtained. Finally, a complete fatigue damage analysis method of offshore wind turbines is proposed, which is applicable to both short-term fatigue damage and long-term fatigue damage.

New hybrid foundation solutions for offshore wind turbines

This paper reviews latest developments of substructures for offshore wind turbines focusing on investigations and applications of hybrid foundations. Model tests and numerical analyses were used to simulate the loading of hybrid piles in sand. The results of pile-soil interaction were investigated to confirm the changes in soil stiffness around the hybrid monopile head. The mechanism and factors affecting the change in lateral stiffness of the hybrid foundation were explained by analysing p–y curves for M+H loading conditions in sand. Based on this research, a new shape of p–y curves for hybrid monopiles was established and a method for determining key parameters was proposed. The effectiveness of new p–y curves was verified by comparing back-calculated results with those from numerical simulations. The conducted tests confirmed that the hybrid monopile displacement is 30–50% smaller when compared to a standard monopile with similar dimensions. The gained experiences can be useful for designers and researchers to enhance the design of foundations for offshore wind turbines.

FDEM analysis of deep rock mass failure and its impact on horizontal bearing capacity in offshore wind turbine piles

Rock-socketed single piles are integral components of offshore wind turbine foundations in rock-based sea areas. Traditional numerical models have been employed to analyze the failure modes and lateral interactions between piles and the rock medium. However, these do not simulate the decline in the horizontal bearing capacity due to rock breakage, posing potential safety risks. In this study, the combined finite-discrete element method (FDEM) was used to simulate the deformation process of submarine rock masses transitioning from an intact to a fractured state. A notable innovation in the proposed approach is the consideration of the effects of the rock mass fracture and pile-rock cohesion on the load-bearing behavior of single-pile foundations. The FDEM model was validated through full-scale lateral load tests. The results showed a distinctive sequence in failure mechanisms, with the tensile failure of the rock mass around the pile at significant depths preceding the compression failure. Local failure at the contact interface significantly reduced the horizontal bearing capacity—a phenomenon that is inadequately represented by conventional finite element models. Furthermore, the cohesion element embedding method was introduced and used to simulate the influence of the tensile properties inherent in grouting materials on the failure strength and mode of the pile-rock interface. This study highlighted the importance of considering the nuanced effects of rock failure in the design of horizontally loaded piles. Failure to do so could lead to nonconservative designs and compromise the safety and efficacy of offshore wind turbine foundations. By pushing the boundaries of numerical modeling with the FDEM, the study aims to provide a more accurate and reliable framework for the design and assessment of rock-socketed single piles for offshore wind energy applications.

A Study on the Scour Surrounding the Fixed Foundation of an Offshore Wind Turbine under Complex Waves, Tidal Currents, and Pile Vibration Conditions

Scouring around the fixed foundations of offshore wind turbines (OWTs) remains a pressing concern within the offshore wind industry, significantly jeopardizing OWT safety. Despite previous efforts, the current understanding of this phenomenon remains incomplete, as various factors related to offshore wind farms and OWT operations can exert intricate influences. To further explore this matter, this paper undertakes a new study to further understand the interplay between monopile vibrations, tidal currents, and sea waves, elucidating their combined impact on scour surrounding an OWT’s foundation. It was found that, besides pile vibration amplitude and frequency, the pile vibration direction can also notably influence the scour around the monopile. Specifically, the most pronounced scour mostly transpires when the monopile undergoes vibrations along the 135° direction relative to the inflow direction. The mildest scour mostly occurs when the pile vibrates along the 90° direction. Additionally, this study reveals that different types of waves, indicated by the ratio of water depth, H, to wavelength, L, also engender varying scour around the monopile. The influence of waves on scour increases with the decrease in the H/L ratio, implying that OWT foundations are more threatened by shallow water waves than by deep water waves.

Experimental study on axial compression fatigue of grouted connection in jacket offshore wind turbines

The grouted connection (GC) of the offshore wind turbine (OWT) jacket foundation mainly bears axial loads, the wind and wave load in the working environment make the structure prone to fatigue problems. The construction environment of the OWT foundation is relatively harsh, so there will be vertical and horizontal errors during the construction process. In this paper, the fatigue test of 9 specimens were designed to analyze the influence of vertical and horizontal errors, water ingression and other factors on the axial fatigue performance of the GC section. The static ultimate bearing capacity test was carried out to determine the load ranges of the fatigue test. Results of fatigue tests indicate that the fatigue performance of the GC considerably degenerates with the load eccentricity, especially at the end sections of the inner and outer tubes. Water ingression has a significant negative influence on the fatigue performance of the GC section. The presence of water contributes to the propagation and development of cracks in the grout layer. Load range, vertical and horizontal errors and grout material have negligible effect on the fatigue performance of the structure.

Sand deformation behavior around modified suction caissons during lateral loading and analytical method obtaining lateral bearing capacity

The modified suction caisson (MSC) is an innovative offshore wind turbine foundation. Model tests and numerical simulations were carried out to explore the sand deformation behavior around the MSC under lateral loading. The vertical displacements fitting method and the level bubble method were proposed obtaining the sand deformation range. Influences of the vertical load, loading eccentricity and external structure sizes were investigated. It was found that the shape of the sand surface deformation range in front of the caisson can be fitted by ellipse functions. In the limit state, the sand surface deformation ranges around both the MSC and the regular suction caisson (RSC) decrease with increasing loading eccentricity and vertical load. Besides, the sand surface deformation range in front of the MSC increases with increasing the external structure height. Three-dimensional shape of the sand deformation range in front of the MSC is dominated by the external structure height and the rotation center embedded depth. Based on the three-dimensional sand deformation behavior and limit equilibrium method, equations estimating the lateral bearing capacity of the MSC were proposed. The bearing capacities of MSCs obtained by using the proposed equations agree well with the test and numerical simulation results.

Behavior Analysis of a Bucket Foundation with an Offshore Wind Turbine during the In-Water Sinking Process

To enhance the safety of the in-water sinking operation for an integrated system, including a bucket foundation (BF), tower, and rotor nacelle assembly (RNA), in complex marine environmental conditions, a model test of in-water sinking for an offshore wind turbine and bucket foundation (OWT–BF) is conducted. The motion behavior of the OWT–BF and the sling tensions during the in-water sinking process are investigated, and the numerical method is validated through test results. The results demonstrate a positive correlation between the wave height and motion response of the OWT–BF. The most critical stage of the in-water sinking operation occurs when the top cover of the bucket is fully submerged, resulting from the substantial cross-sectional difference between the bucket base and the transition section. Furthermore, the closer the OWT–BF is to the seabed, the less it is affected by waves in terms of motion response. It is advisable to conduct the in-water sinking operation of the OWT–BF in sea states with wave heights below 1.5 m. Simultaneously, slings can efficiently control the motion response of the OWT–BF, thereby enhancing the safety of the sinking operation.

Parametric study of catenary mooring system on floating wind turbine foundation dynamic response

The ability of the floating offshore wind turbine foundation to withstand harsh wind and wave conditions is intrinsically linked to the meticulousness employed in the design of the mooring system. Therefore, it is vital to conduct research to reveal the variations in sensitivity of new mooring materials parameters and the response characteristics of the foundation in the presence of intricate operational circumstances. A fully coupled time-domain dynamic analysis is performed via the commercial software AQWA to examine the VolturnUS-S semi-submersible foundation, specifically engineered to support the IEA 15 MW offshore reference wind turbine. The numerical simulation results indicate that synthetic fibers, specifically Aramid and high-modulus polyethylene, can replace the steel wire as mooring materials. At the same time, the mooring arrangement, length, diameter, and fairlead position influence foundation motions and mooring tensions, bringing valuable insight into the fluctuation patterns of these responses.

Scour Protection Measures for Offshore Wind Turbines: A Systematic Literature Review on Recent Developments

Offshore wind energy is considered as one of the most promising resources of clean and renewable energy to replace fossil fuels. Additionally, its cost is expected to be lower than onshore wind energy as the technology matures. Offshore wind turbines (OWTs) normally operate in harsh ocean environments, which could impact their structural integrity. Scour erosion around foundations of OWTs can substantially change the overall stiffness of these structures and shorten their lifetime. Currently, there are a limited number of studies on countermeasures and their engineering requirements for decreasing the scouring effect; this is due to their different hydraulic circumstances, such as their stability, reliability, and resistance capacity. To this end, advancements in scour protection measures in the offshore energy sector are evaluated in this paper through a thorough and critical review following the PRISMA systematic literature mapping approach. This includes 68 papers on scour protection and over 30 scour protection designs for various types of wind turbine foundations. Here, we aim to provide an overview of the latest scouring protection measures and their comprehensive assessment, as well as their prospects and future challenges. The findings of this study will provide key insights into scour protection measures for OWTs and will subsequently contribute to the future growth of the offshore renewable energy sector.

Experimental Investigation on the Motion Characteristics of Air-Floating Tripod Bucket Foundation during Free Floating

In recent years, multi-bucket foundations have been studied and gradually adopted in engineering practices as a novel foundation for offshore wind turbines within a range of water depth of 30 to 50 m. This study investigated the motion characteristics of air-floating tripod bucket foundation (AFTBF) through a series of experiments during free air-floating. The experimental results show that the surge force appears to be the most important factor influencing pitch moment and motion, whether it is a change in water depth or a draft for AFTBF. The maximum amplitudes of surge acceleration and pitch angle show a trend of increasing with narrower spacing and decreasing with wider spacing, while the heave acceleration shows an opposite trend. The added mass and damping of heave motion for AFTBF increase with shallower water due to the increasing pressure difference between the inside and bottom of bucket foundation. The shallower the water depth and the larger the draft, the longer the resonance period of heave.

Scour Effect on the Lateral Bearing Behaviour of Monopiles Considering Different Slenderness Ratios

Scour leads to the loss of soil around monopile foundations for offshore wind turbines, which affects their structural safety. In this paper, the effect of scour on the lateral behaviour of monopiles was extensively investigated using finite element analysis, and calibration and comparison were undertaken using centrifuge tests. Piles with three slenderness ratios, i.e., 3, 5 and 8, were studied by keeping the diameter constant and varying the embedment length. Three scour types (local narrow, local wide and global) and four scour depths (0.5D, 1D, 1.5D and 2D; D signifies the pile diameter) were considered in this investigation. The results indicate that the lateral resistance of the pile is the greatest in the case of local narrow scour, followed by that in the cases of local wide scour and global scour. When the scour depth is larger than 1D, the influence of the scour type on the pile lateral bearing behaviour is insignificant. The influence of the scour type and scour depth on the pile lateral bearing behaviour is broadly similar for piles with slenderness ratios of 3, 5 and 8. However, the piles featured with smaller embedment lengths show a larger decrease rate in their lateral capacity, which means the effect of scour should cause more concern on small slenderness ratio monopiles.

Offshore wind farm foundations as artificial reefs: The devil is in the detail

Climate change and global biodiversity loss call for clean energy production systems with minimised ecological impacts. Offshore wind energy production will become one of the main uses of global marine spaces within next decades. Offshore wind turbine foundations can function as artificial reefs but it is unknown if these capabilities apply to different foundation types. We collected field data on Atlantic cod (Gadus morhua), a species under pressure in the southern North Sea, around three foundation types to assess these capabilities. Catch rates showed that monopile foundations with rock protection on the seabed were able to attract significantly more fish than monopile foundations with sandbag protection and jacket foundations. Fish densities varied on small scales meaning that reef effects were spatially restricted. This implies that offshore wind energy production can be used as tool to combine climate change mitigation with local biodiversity conservation but that a consideration of the wind farm design is required.

Simplified modelling of full-strain-range non-linearity of cyclically loaded undrained clays

Constitutive modelling of cyclically loaded undrained clay is of significant importance for various branches of geotechnical engineering exemplified by offshore wind turbine (OWT) foundation design and prevention and mitigation of earthquake hazards. Fine-grained soils can display non-linear stress-strain relations from small (10-5) to relatively large (10-1) strain levels. This full-strain-range non-linearity can remarkably affect the cyclic response. Effective stress-based constitutive models have achieved great success in modelling clayey soils, whereas they can be overly complex for practicing engineers, in particular, when considering non-monotonic loading and full-strain-range non-linearity. This paper explores the possibility of modelling, in simplified manner, the full-strain-range nonlinearity of cyclically loaded undrained clay, which can directly utilize outcomes of in situ site exploration and routine laboratory tests. For this purpose, we idealize soils under undrained conditions as single-phase materials governed by total stress. Considering that most current total stress-based models are proposed for metals and may be limited to capture the non-linear stress-strain relations of soil, a novel generalized non-linear (GNL) hardening law is proposed that can describe versatile stress-strain relations of undrained clays. Bounding surface and a mapping rule considering update of projection centre are introduced to reflect the influences of maximum past stress history and recent stress history, respectively, on the stress-strain nonlinearity. The simplified constitutive model is first validated at the element level by simulating monotonic and cyclic loading laboratory tests on undrained clays. Later, the proposed soil model is applied to the finite element analyses of OWT pile foundation subjected to cyclic lateral loading.

Residual strength of corroded ring-stiffened cylinder structures under external hydrostatic pressure

Corrosion damage to submarine pressure hulls is a crucial consideration during the structural design phase because it diminishes the structural strength of a submarine, reducing its operational depth and posing a severe risk to the life of the crew. This necessitates using a validated analytical method to estimate the residual strength and assess the safety of corrosion-affected pressure hulls. This paper presents an experimental and numerical investigation of the residual strength of corroded ring-stiffened cylinders, which are typical components of submarine pressure hulls, the main legs of semi-submersibles, and Floating Offshore Wind Turbine (FOWT) foundations. Four steel ring-stiffened cylinder models were fabricated: two remained intact, whereas the other two were artificially corroded by machining. Hydrostatic collapse tests were conducted in a pressure chamber. To evaluate the effect of initial shape imperfections caused by welding fabrication on the strength of the structure, the initial shape imperfections were measured using a three-dimensional coordinate measuring machine. The experimental results illustrate a significant reduction in the residual strengths of the two damaged models compared with those of the intact models. The collapse processes simulated using the Abaqus software package are presented, demonstrating a close agreement between the experimental results and numerical predictions.

Single pass laser vacuum welding of thick steel plates using electromagnetic support

The increasing demand for renewable energy produced by offshore wind turbines goes along with an increased demand in the production of offshore wind turbine foundations, so called “monopiles”, which are made by joining thick metal sheets. The industrial standard of multi-layer submerged arc welding (SAW) for joining of thick metal sheets is the current bottleneck in the production of monopiles. A possible increase in productivity by the implementation of high-power laser welding in a newly developed mobile vacuum chamber (MoVac) and an electromagnetic root support is the subject of this study. Single run butt welds are performed in flat position on S355 mild steel of thicknesses up to 80 mm using a disc laser system with 1030 nm wavelength and a maximum output of 60 kW. The laser optic is fixed on the MoVac-System which is held and manipulated by an articulated robot.