Large-diameter monopiles serve as foundations for offshore wind turbines (OWT), and the diameters are now up to 10 meters. These monopiles exhibit lower embedded length-to-diameter (L/D) ratios compared to the conventional monopile that is widely employed in offshore oil and gas platforms. They undergo rotation when subjected to lateral loading like rigid or semi-rigid bodies. This distinct geometry necessitates a different approach to describe the soil reaction that is induced when a lateral load is applied. Previous research introduced a 1D model incorporating four soil spring components to represent four aspects of soil reaction namely lateral soil reaction, distributed moment, base shear force, and base moment. However, questions have arisen regarding the contributions of base components, specifically the base shear force and base moment, to maintaining monopile stability. In response, a series of monotonic loading tests on monopiles in dry sand is conducted to assess the contributions of base shear force and base moment to monopile stability. Following this assessment of base components, parametric analyses are carried out to investigate the effect of pile diameter (D), the L/D ratio, load eccentricity (e), and sand relative density on monopile responses under cyclic and monotonic lateral loading. In this study, the SANISAND-MS material model is employed within 3D Finite Element (FE) software to model ratcheting during cyclic lateral loading. Finally, an investigation is conducted with the aim of constructing a 0D model to represent base shear force and base displacement responses under both monotonic and cyclic lateral loading. ... Read more

Offshore wind energy's rapid expansion underscores the need for accurate and efficient methods to analyze the behavior of monopile foundations supporting wind turbines. While three-dimensional (3D) analyses provide comprehensive insights, their computational demands are significant. As an alternative, one-dimensional (1D) models with spring elements to simulate the interaction between the structure and the surrounding soil, offer efficiency and simplicity. Realistic soil behavior, characterized by elastoplasticity, necessitates proper calibration of the spring models in 1D analysis.
This thesis addresses the challenge of soil-monopile interaction analysis, specifically focusing on the monopile response under lateral static monotonic loading. The research commences by highlighting the development imperatives in monopile-founded offshore wind turbines. The first phase involves calibrating elastic springs through a comprehensive review of existing literature. This calibration accounts for variations in spring stiffness along the monopile's length. Subsequently, the study progresses to elastoplastic soil modelling, adopting a linear elastic perfectly plastic approach and employing only lateral shaft springs. Acknowledging the limitations of linear elastic perfectly plastic p-y response, new material models, namely a bilinear and an exponential model, are examined. A parametric analysis encompassing various monopile geometries and lateral load eccentricities is conducted. An optimization routine refines the bilinear and exponential model parameters to closely match 3D responses. The results demonstrate satisfactory agreement for the analyzed high L/D monopiles, yielding valuable insights and conclusions. However, the low L/D monopiles exhibit a less successful match, primarily attributed to the absence of rotational shaft springs in the analysis.
Furthermore, empirical design processes for applying the bilinear and exponential models are outlined. These processes are founded on the relationships between the model parameters and the length-to-diameter (L/D) ratio as well as the eccentricity-to-diameter (e/D) ratio. The study highlights the applicability of the bilinear model across various soil conditions, monopile geometries and lateral load eccentricities. In contrast, the exponential model's efficacy is constrained by the examined L/D ratios, warranting further analyses for expanded application.
In conclusion, this thesis presents a systematic transition from elastic to elastoplastic modelling for soil-monopile interaction analysis under static monotonic loading. The proposed bilinear and exponential models enhance the accuracy of 1D simulations, facilitating efficient design and analysis of monopile-founded offshore wind turbines. These methodologies contribute to the advancement of sustainable offshore wind energy, catering to diverse soil conditions and design scenarios. ... Read more

To support offshore wind turbines (OWT), monopiles are currently the most frequently used foundation method. These monopiles are open-ended steel tubes with a diameter of 5 - 12 m, which are most often installed in the seabed by means of hydraulic impact hammer ingusually mounted on a specialised ship. This method is produces high noise levels and releases shock waves into the water, which can damage the sea life’s hearing or even outright kill them. Additional measures can be taken to reduce noise emissions and adhere to strict environmental regulations written for marine biology protection, but they are costly and slow down projects significantly. There are other installation methods which have the potential to be more silent but are not well understood yet from a noise generation, drive ability, and lateral behaviour point of view. These are aspects that will be investigated within the SIMOX Joint Industry Project (JIP). One of these methods is vibratory installation.
A monopile used as a foundation for an OWT will experience a multitude of loads during its service life, the most important of which are the lateral loads. While the behaviour of impact hammered piles under lateral loading has been researched extensively, the behaviour of vibratory driven piles is still relatively unknown. The influence of different parameters used during installation (frequency, penetration speed)and other conditions (wall thickness and soil conditions) on that behaviour must be understood to be able to accurately predict how a vibrated monopile will react to both cyclic and monotonic loading.

The present research explores the behaviour of monopiles under lateral loading and the impact different installation parameters have on it. To achieve this, a laboratory testing campaign was carried out with model piles. The purpose of these tests is to produce qualitative results to be used in following field-testing campaigns within SIMOX. Piles were installed with differing installation parameters in sand beds with different density. These piles were then subjected to initial monotonic loading, followed by cyclic loading, and then monotonic loading again. The data obtained during the experiments was then interpreted and analysed. By comparing the results as well as measurements taken during and after installation, conclusions are made concerning installation parameters and other factors that may play a role on the lateral behaviour of monopiles.

The interpretation focused mainly on pile head displacement during initial lateral loading depending on penetration speed and frequency. The loading tests have shown that impact hammered piles under went lower displacements than vibrated piles after being loaded laterally. When solely considering vibrated piles, piles with a larger wall thickness showed lower displacements in general than thin-walled piles.

Frequency and penetration speed were found to play a role in the lateral behaviour of monopiles. In the experiments considered for this thesis, it seemed in dense sand crane-controlled piles showed lower displacements than free-hanging piles. In medium dense sand, lower penetration speed led to lower displacement during loading, but more research is needed on this topic to be able to formulate clear conclusions on the exact role of each installation parameters. The experiments also show an interesting phenomen on regarding measured soil elevation that might link compaction around the monopile to lower lateral displacements. The difference in elevation before and after installation seemed to correlate with lateral displacements. In general, piles with larger compaction around the pile displaced less during initial loading.

In conclusion, this paper provides a range of observations regarding the impact of installation parameters and other conditions such as wall thickness and sand density on the behaviour of monopiles under lateral loading, as well as offering a comparison with impact hammered piles. Recommendations and suggestions are given for further research and for the field testing experiments, so that the analysis made here may be used to predict the lateral behaviour of vibrated monopiles more accurately. ... Read more

Quay walls are often designed with Finite Element models (FE models) to take into account the complex soil-structure interaction and highly non-linear soil behavior. However, the effect of temperature variations is uncertain if it is taken into account in the design of the quay walls at the Port of Rotterdam.

Nowadays, new quay walls are often equipped with sensors that collect information about their behavior. These quay walls are known as smart quay walls. The measurement data of smart quay walls could be used to validate FE models and reduce parameter uncertainties. This could lead to an optimization of the functionality of the quay walls.

Smart quay walls have been observed to show much higher strain levels in the anchors during summer compared to the winter period. According to strain records, differences of up to 10% and 20% seem to be present, which is quite high. The objective of this thesis is to verify the effect of temperature on anchor force in quay walls using the data of smart quay walls.

The data analysis that took place analyzed data from five different quay walls (HHTT, SIF, EMO, Brammen terminal, Brittanniëhaven). Deformations, strains, anchor forces, groundwater levels and temperatures are some of the measurements that were investigated in order to understand the quay wall reaction to the seasonal temperature fluctuation effect. The most useful measurement data proved to be the deformations of the combi walls and the anchor forces in the MV-piles.

This research will eventually highlight the results of the extensive data analysis from different smart quay walls, while it will further prove that quay walls are affected by the effect of seasonal temperature fluctuation. The gain is that with the available data, it is verified that the wall is moving back and forth depending on the season. However, the deformations are minor compared to the deformations of the dredging period.

After data analysis, a FE model was set up to predict the deformations and anchor forces of the quay wall during seasonal temperature fluctuations. For the parameter determination, CPTs, later research projects, design records and triaxial tests were used. Regarding the FE model, the case that was used is the HES Hartel Tank Terminal (HHTT-quay), which is a smart quay wall in the port of Rotterdam. HHTT quay wall was selected as the most well monitored quay wall regarding the needs of this research. Moreover, the HHTT-quay consists of sections with and without a relieving platform. Both types were considered in this thesis.

Then comes the validation of the FE model with the measurement data. Moreover, having a FE model in PLAXIS 2D which can realistically model the cycle heating effects, could be used both to estimate deformations due to climate change effect, as well as the anchor forces leading to better quay wall design for the future.

As with all of the cycle effects, heating and cooling of the quay wall could cause deformations that after many years of operation of a quay wall could lead to excessive deformations. Additionally, increasing temperature will cause higher temperature fluctuations, which means larger cycles. Therefore, further research with more cycles, better quality data and a FEM that could calculate the cycle heating effects is crucial to a better understanding of the cycle phenomenon. ... Read more

At the end of 2019, the European Union (EU) put forward the European Green Deal to facilitate the technological progress necessary to achieve CO2-neutrality by 2050. Such a monumental achievement would require massive investments in infrastructure for the harvesting, storage and the transnational transportation of green energy. To date, the more mature of the scalable (cf. to hydroelectric) green-energy resources is offshore wind, with joint academic and industry efforts allocated to reduce its capital expenditure. Approximately 13-37% of the required investment for offshore wind farms is currently expended on the design, manufacturing, and installation of the substructure. Further reduction in the cost of offshore wind can be achieved by addressing the main technical challenges associated with the predominant offshore wind foundation, i.e., the monopile. The main challenges typically relate to its lifetime operations, namely, (i) the identification of the wind turbine's fundamental frequencies, which are strongly dependent on the monopile-soil interaction, (ii) and the prediction of the lifetime foundation tilt, but also the current installation technology (impact driving); the current norm in the offshore industry. In particular, impact driving is associated with (i) long installation times, especially in the presence of competent soils, (ii) excessive use of construction material (steel) to avoid pile damage under many hammer blows, and (iii) costly underwater noise mitigation measures to reduce noise the levels of installation-borne noise emissions harmful to marine life.

In an attempt to accelerate the growth of offshore wind, the Netherlands, country of origin of this study, has supported several research initiatives to reduce the engineering and manufacturing costs for the prevalent offshore wind foundation in the country (the monopile). This study elaborates upon the experimental findings of two major research projects, namely the DISSTINCT (2014-2018) and the Gentle Driving of Piles (2018-2022) projects, each designed to address specific technical uncertainties associated with the foundation concept. The DISSTINCT project (launched in 2014) aimed to improve the understanding of the natural frequency of installed monopiles as well as the engineering procedures used in the identification thereof. By conducting experiments at full scale on a monopile installed in the IJsselmeer lake in the Netherlands, the experimental campaign produced invaluable data on the dynamic response of monopiles during small amplitude lateral vibrations. Later, the GDP project (launched in 2018) was designed to propose, engineer, and demonstrate a novel monopile installation procedure, foreseen to alleviate most of the aforementioned installation-related challenges; the Gentle Driving of Piles (GDP) method. Moreover, the project would provide answers to questions concerning the long-term response of (mono)piles in sandy soils, relative to the installation method. For these reasons, an extensive experimental campaign was conducted in the port of Rotterdam (Maasvlakte II), where a total of 9 piles were driven into the sandy Maasvlakte soil via different driving procedures, namely with the established impact hammering, the traditional axial vibro-driving, and the new GDP method. Subsequently, the cyclic lateral performance for four of these piles (which were heavily instrumented), was evaluated via an elaborate 82.000 load cycle (≈42 hours) loading programme of slow (0.1 Hz) high amplitude, and fast (0.1 - 4 Hz) low amplitude cyclic force applied to the (mono)piles' head.

This study elaborates and builds upon experimental findings from the above-mentioned test campaigns. These measurements were first carefully examined, and later interpreted using a variety of modelling tools (both 1D and 3D FE modelling) formulated and adapted to meet the particular geotechnical and loading challenges of the examined fieldwork. Enabled by the diversity of the field and numerical work performed, this study addresses a number of engineering challenges and knowledge gaps related to the design of monopiles, namely i) their post-installation resonance frequency, ii) the long-term response to environmental loading, and iii) the impact of the installation method on the long-term operations. In particular, 3D FE modelling was adopted to successfully simulate the dynamic response of the examined monopile in the DISSTINCT project. The modelling efforts enabled the interpretation of the field test measurements, and in turn, inspired confidence in the suitability of available simulation tools to identify the resonance frequencies of monopile foundations, and accurately calculate dynamic soil-monopile interactions. For the interpretation of the GDP field test data, 1D FE modelling was employed. In the field, the elaborate lateral loading programme returned a fairly complex cyclic pile response, with pronounced differences in the performance of piles installed by different installation methods. The particular geotechnical conditions at the GDP site, i.e., site inhomogeneity and the 4 m deep unsaturated topsoil, prevented the direct comparison of the installation methods. This was later achieved through the formulation of a cyclic soil reaction p-y model able to simulate soil ratcheting and gapping effects. The results provided rich insights into the impact of relevant installation effects on the cyclic pile response on many loading cycles and indicated that the GDP-installed piles performed excellent overall in lateral cyclic loading. ... Read more

New offshore wind farms consisting of monopile-founded Offshore Wind Turbines (OWTs) are to be built in earthquake-prone areas. To design the monopile foundation and to accurately define the dynamic response of an OWT, the soil-monopile-superstructure interaction should be modelled properly. Due to the fact that the Three-Dimensional (3D) Finite Element (FE) analyses are complex and computationally expensive, research is focused on One-Dimensional (1D) FE models, in which the soil-monopile interaction is traditionally described via distributed translational springs representing the soil lateral load. Nevertheless, the increase of monopile diameter, followed by the monopile length-to-diameter ratio (L/D) decrease, implies the contribution of additional resistant components such as distributed moment, base shear and moment.

This thesis examines the 3D mechanisms to be accounted for in the 1D FE modelling of the soil-monopile-superstructure seismic response in case of a single-phased, linear visco-elastic soil layer. Both 3D and 1D FE analyses are conducted with the FE software OpenSees. The 1D analyses are simulated in two consecutive steps: first a Site Response Analysis is performed, next the recorded displacements over the soil layer depth are applied to the spring supports and the dynamic interaction of the system is simulated. Three different monopiles are considered with L/D equal to 26, 9 and 5. Two superstructures are examined, which are modelled as Single-Degree-of-Freedom systems. Distributed translational springs are assigned to the slender monopile (L/D=26), while for the stubbier monopiles the contribution of distributed rotational springs is examined as well. Lastly, the effect of considering the base moment and shear is also examined.

The stiffness of the soil reaction curves is calibrated by applying a monotonic lateral load and moment at the pile head, in case of the translational and rotational springs, respectively. The spring stiffness values are assumed uniform along the monopile length. As a next step, the dynamic response of the calibrated 1D models is examined in steady-state conditions, under the action of mono-harmonic excitation, and compared to the 3D results. Ultimately, the seismic response of the 1D models is examined in case of two earthquake excitations with different frequency contents.

In case of the monopiles with L/D=9 and 5, it is concluded that the use of monotonically-calibrated distributed translational and rotational springs provides a good match between 3D and 1D regarding the monopile head and superstructure response under seismic loading. Nevertheless, these 1D FE models cannot predict the base moment, for which a base rotational spring should be employed. In case of the stubbier monopile, with L/D=5, the base shear seems to positively affect the moment profile as well. Lastly, regarding the monopile with L/D=26, the employment of translational springs alone seems sufficient for the accurate prediction of the seismic response; however, the hereby monotonically-calibrated distributed translational springs result in a mismatch between 3D and 1D.
... Read more

In the low-lying parts of The Netherlands challenging ground conditions are often encountered. Thick layers of soft material such as peat and clay are present and pile foundations into deeper sand layers are required when objects are constructed that have to transfer significant loads to the subsurface without inducing settlement (differences). This also counts for pile foundations of wind turbines which, apart from compression and tension loads also are subjected to horizontal loading. The horizontal translational stiffness is an important and depending on the wind turbine manufacturer governing aspect in the required dimensions of the pile foundation (number and type of piles). In order to improve this horizontal translational stiffness, the foundation designer usually opts for the use of battered (raked) piles. The basic principle is that the axial stiffness of the pile will, in this way, contribute to the horizontal stiffness of the foundation as a whole. The verification of wind turbine requirements usually involve the use of generally accepted analytical 2D methods, simplifying the more complex 3D effect, to model the interaction between soil-pile and the group effect for closely spaced piles. Although the analytical model generally provides good results, investments can reduce significantly when only a small amount of piles can be saved upon. Also the effort in the field is reduced when using vertical piles... ... Read more

Offshore pipelines are considered the arteries of the offshore oil and gas industry and transport hydrocarbon products as well as other fluids. During the installation process of an offshore pipeline, the pipeline is most often located in a trench and covered with backfill material. The burial of the pipeline ensures onbottom stability as well as mechanical protection. With a Trailer Suction Hopper Dredger (TSHD) sand can be deposited in the trench, thereby covering the pipeline in a relatively controlled manner. One major risks associated with this backfilling method is the risk of vertical upward displacement of the pipeline during the backfilling process. The vertical upward movement of the pipe is referred to as pipeline flotation and may result in unprotected or damaged pipes. With the rising use of small diameter and lighter pipelines this risk has become more prevalent.

Pipeline flotation is induced by augmented buoyancy which originates from the presence of the water-sand mixture around the pipeline. The particle concentration of the water-sand mixture and the embedment rate of the pipe are leading in the assessment of the buoyancy force acting on the pipeline. The weight of the pipeline is the major force counteracting buoyancy force. In addition, a friction force, resulting from the contact between the pipe and the new formed sand layer around the pipe, counteracts the buoyancy force.

The aim of this research is to develop and validate a simplified numerical model for the analysis of offshore pipeline flotation during sand backfilling with a TSHD. The numerical model has been validated against the small-scale physical experiments of Eikhout (2021). The small-scale physical experiments have been developed and used by Yang (2020) and Eikhout (2021) to simulate a simplified sand backfilling process.

The numerical model is developed to simulate the sedimentation process and has been developed in the finite element software COMSOL Multiphysics. The model is capable of effectively simulating the simplified backfilling process from the small-scale experiments by Eikhout (2021). Moreover, the sedimentation model is able to simulate the inflow of material over time in the physical domain. The sedimentation process has been modelled with a convection equation in which the settling velocity of the water-sand mixture is described with the hindered settling formulation proposed by Metha (1986). The hindered settling formulation describes the velocity of the suspension as a function of its local particle concentration. The numerical data has been processed and used in a force balance which is able to predict the occurrence or absence of pipeline flotation.

The parameters in the hindered settling formulation as well as the numerical settings have been described from a theoretical perspective as well as their practical impact on the numerical solution. After validation against the small-scale physical experiments the numerical model has been used to simulate a more practical scenario. In addition a simplified, spreadsheet friendly, calculation method is proposed. ... Read more

For supporting Offshore Wind Turbines (OWT), monopiles are currently the most common foundations. The role of a foundation is to transfer safely the loading to the ground. The wind and the wave loads are considered cyclic because they repetitively apply on the OWT. The North Sea is sand dominated in many areas. During cyclic loading, permanent strains develop in the surrounding soil while the soil stiffness and strength are irreversibly affected. Through time, the accumulation of strains can lead to the soil failure. Thus, assessing the behaviour and stability of monopiles under cyclic loading is essential. To model the response of monopiles under lateral loading, the traditional design procedure is the use of p-y curves that express the lateral soil resistance in function of the pile deflection. The p-y curves are nowadays recommended to be calibrated on FE models. The Stiffness Degradation Method (SDM) of Achmus et al. (2009) is a numerical strategy that assesses the behaviour of a monopile under cyclic loading. The method estimates the cyclically degraded stiffness based on the results of a static analysis. The soil stiffness is degraded based on a semi-empirical power law that accounts for the number of loading cycles, the stresses in the soil after the static analysis and two model parameters calibrated on cyclic triaxial tests. The SDM was successfully implemented in PLAXIS 3D via a practical routine coded in Python and the use of soil clusters around the pile. The soil stiffness is degraded by updating the soil material within the clusters. The study model was verified by comparing results with the published reference system of Kuo (2008) for two piles with embedded length to pile diameter ratios of 2.7 and 5.3. The results indicate that the study model provides a stiffer pile-soil response than the reference model because the soil stiffness is overestimated. The degraded stiffness overestimation is attributed to the initial stiffness mismatch and the use of soil clusters. The impact on the short pile is greater than on the long pile because the short pile opposes less resistance to the loading and is thus more affected by the stiffness difference. The study model was validated against three 1-g pile tests for homogeneous uniform and multi-layered dense sand. The numerical results are in agreement with the test data. In the absence of cyclic triaxial tests, the two model parameters were directly calibrated on the pile head displacement of the experiment. The two model parameters have a significant impact on the stiffness degradation. Thus, model parameters from literature were classified from the highest to the smallest estimation of pile lateral displacement. The results of codified and published approaches (DNV-GL-0126; Duhrkop, 2009; Garnier, 2013) were compared with the results of the study model. The study model and the method of Garnier (2013) are in agreement. They both account for the loading amplitude, the number of cycles and the pile geometry. The codified procedure and the method of Duhrkop (2009) estimate higher lateral displacement compared to the study model. Finally, the 1D model was successfully calibrated with the highest displacement estimate of the study model. With this procedure, the 1D model accounts for the number of cycles, the pile geometry and the loading amplitude. The study model provides a less conservative approach for determining the pile lateral displacement under cyclic loading. The calibration of the 1D model on the pile deflection curves of the study model is a promising procedure which will require further research. ... Read more

Wind energy is now a popular competitor among other energy sources all around the world. The offshore wind industry has progressed in recent years, with larger wind turbines being installed in deeper oceans. The construction of such large-scale wind farms necessitates more modern foundation design technologies to increase operational safety while also lowering total structure set-up costs. The environmental load applied to offshore piles are of great complexity. Currents, wind, waves, and even earthquakes are very common dynamic loads in an offshore loading environment. Of course, when a wind turbine is working normally, it also has significant operation loads. The design of offshore wind turbine support structures often involves some universal criteria, e.g., the pull-out capacity of jacket structures on piles. Wind turbine foundation capacity is determined by the qualities of the offshore soil as well as the properties of support structure configurations. Therefore, it is necessary to take account of the potential cyclic impacts of soil-structural interaction to guarantee dependable responses of the wind turbine structure. This thesis aims at evaluating soil−structure interaction of offshore wind turbine foundations under cyclic loading, with emphasis on the tension capacity of axially loaded displacement piles, under different load conditions (cyclic-to-average ratios) on Fontainebleau NE34 sand in France. A newly developed constitutive soil model SANISAND-MS (2018) is applied to model sand stress-strain evolution. In this thesis, the soil is assumed a homogeneous linear elastoplastic material for the sake of simplicity. The SANISAND-MS constitutive model used in this thesis can capture sand ratcheting after considerable cyclic loading cycles. Furthermore, drained and undrained compression triaxial tests performed at DTU GEO−Lab were used to calibrate the model parameters of the constitutive model for Fontainebleau NE34 sand. The finite element model adopted here is built in an open-sourced platform, the OpenSees. The Small-strain approach is adopted in the finite element modelling part. The pile is simplified as a wished-in place which does not include the installation effect and the time effect after the installation and before cyclic tests. Finally, the modelling results are compared to the experiment results recorded by Tsuha et al. (2012). Clear stable, metastable, and unstable response types are recognized in the model results. However, the initial stress state of the sand at the soil-pile interface differs a lot compared to the experiment results. This is the consequence of not including the pile installation effects in the finite element modelling. Recommendations are given to use large-strain soil modelling techniques to include the pile installation process. ... Read more

Technology improvements and growing maturity of the offshore wind industry have resulted in significant cost reductions and rise in demand. More can be achieved by focusing on improving the understanding of key design areas of an offshore wind turbine (OWT). Since fatigue is one of the main design criteria for offshore structures and little is known about fatigue cumulative development in time under operating conditions, it formed the basis of this thesis. However, fatigue is a complex phenomenon that is dependent on numerous interlinked parameters, such as damping, loading type, and stiffness of support structure (SUS). For the purpose, data analysis was performed on measurements which were gathered over a period of four years from a monopile-supported OWT site. The measurement data included information about the environmental and operational conditions of the OWT; and strain, acceleration and inclinometer readings from its SUS. Use of different data acquisition systems during the measurement campaign inherently required significant efforts to synchronise and preprocess raw data to appropriate state for data analysis. Time-dependent lag was identified via computation of cross-correlation sequences between data segment pairings, and the lag was successfully corrected. With the use of a standardized rainflow cycle counting algorithm, strain-derived moment time series were converted to constant amplitude events, which could be used to derive damage equivalent bending moments, M-N curves and fatigue damage accumulation frequency spectra. Short-term damage equivalent bending moment (STEL) was revealed to have a linear dependency with turbulence intensity when the OWT was in the run-up operational state. Additionally, the fatigue damage was demonstrated to be higher for downstream turbines due to wake effects. Also, fatigue life consumption of OWT SUS increases above rated wind speed conditions despite decrease in load magnitude due to increase in SUS response frequency. The latter was confirmed with fatigue damage accumulation frequency spectra, which revealed that the importance of high frequency band (1 – 5 Hz) increases in conjunction with wind speed at above rated wind speed conditions. However, most of the overall fatigue damage is accumulated at the low frequency band, which associated with wind and wave loading, first SUS bending modes, 1P and 3P rotor harmonics. ... Read more

Offshore pipelines represent major items in the oil and gas industry nowadays. These submarine pipelines are usually covered with backfill for protection and a well-known method for this is sand backfilling with a trailing suction hopper dredger (TSHD). Although pipeline installation with a TSHD is a developed technique, there are still some challenges regarding uplift movements of the pipeline during the backfill procedure. This action is called pipeline flotation, which is a frequently occurring problem during the sand backfilling process and the costs for repair measures afterwards are very expensive. Accurate modelling of the sand backfilling process over a pipeline can lead to significant cost reductions by optimising the sand backfilling technique. Pipeline flotation is induced by the development of upward buoyancy forces from the sand-water mixture on the pipeline. If the weight of the pipeline is not sufficient enough, it will experience uplift movements during the backfilling process. The purpose of this research is to develop a better understanding of the mechanism of pipeline flotation. Due to a lack of available field data, it is anticipated that a set of laboratory experiments will provide a better insight into the parameters involved in pipeline flotation during sand backfilling. This research builds on the study Yang (2020), in which a small-scale set-up and 2D calculation model are designed to perform experiments concerning pipeline flotation during sand backfilling. A total amount of 50 experiments is performed with Geba Weiss sand in four different stages: with horizontal discharge, vertically upward discharge, elevated initial pipe positions and increased pipe specific weights. The set of experiments have provided better insight into physical processes that influence pipeline flotation during sand backfilling. The discharge of the sand-water mixture is dominated by its sedimentation and dispersion, so the model is based on empirical equations and the sedimentation theory. The main conclusion regarding the static force balance on the pipe is that the degree of pipe embedment influences the magnitude of an additional frictional force working downwards on the pipe body. A new approach for assessing this frictional downward force, due to the presence of sand that has deposited, is described and added to the model. In the small-scale experiments, this influence plays a significant role in pipe flotation triggering. An undesirable result from the small-scale experiments are the additional hydrodynamic effects from the fluid in the experimental tank, which are induced by the geometrical constraints. A pipe flotation limit was found in the parameter of sand volume concentration in the surroundings of the pipe. This flotation limit was defined at a domain concentration of approximately 7.5% for the experiments with a pipe specific gravity of 1.03. The boundary of flotation for heavier pipes was not achieved in this research, but evidence points towards a significantly higher flotation limit. The discharge flow rate and the total discharged sand volume are the key factors for development of domain concentration in the small-scale experiments. With the acquired data as a strong foundation, additional research regarding pipe flotation during sand backfilling is recommended. As there is still a strong variation in several important input parameters in the small-scale experiments, a Computational Fluid Dynamics (CFD) analysis is suggested to relate the domain concentration to the input parameters at every moment in time. Additional 3D experiments on a larger scale are also recommended to optimise the sand backfilling process and validate the CFD analysis. From this analysis, a prediction can be made regarding the boundary of pipeline flotation on the seabed when discharging with a TSHD. Moreover, additional research regarding the stress and drainage conditions underneath the pipe is proposed to validate the cause and accuracy of the relatively high friction coefficients. Finally, a model could be built to examine the friction force generated on the pipe due to its embedment. With this method, the suggested approach of the frictional resistance due to pipe embedment could be further analysed. ... Read more

Soil exhibits hysteretic damping. A commonly used implementation of this type of damping are the Masing rules. They consist of a loading and an unloading branch, dened as a piecewise function and having a nonsmooth character. This paper presents a framework how steady-state solutions of the motion of a soil column, with nonlinear hysteretic damping, can be obtained by using a variant of the Harmonic Balance Method (HBM), the Alternating Frequency/Time Harmonic Balance Method (AFTHBM), applied on a discretised soil column (a lattice system). The theoretical background of the method is presented, as well as its application to a soil column with both shear strain-dependent stiness and damping. Results show that the AFTHBM is an efficient method for obtaining steady-state results for nonsmooth nonlinear behaviour, which is in this paper presented by simulations for nonlinear media. The results of the AFTHBM are sensitive to time sampling and convergence tolerances; nonetheless, if these parameters are properly chosen, the application of AFTHBM leads to good results. ... Read more

Ground improvement in the form of soil compaction plays a important part in reclamation projects. The development of the Cofra Roller Compaction (CRC), a non-circular impact roller, has proven to be valuable in these projects. However, the heterogeneity of the subsoil causes locally a non-uniform degree of compaction. Traditional compaction control tests are limited in measuring depth, expensive and cause time delay. Therefore, the Continuous Compaction Control (CCC) and Continuous Impact Response (CIR)method were developed
in order to provide more real-time information of the compaction based on the response of the drum of the roller. The aim of this study is to develop a semi-empirical energy model which is based on the contact forces of the roller-soil interface as most CCC systems, but also uses field test data as was given in the CIR system to validate this model. The relevant parameters needed for this model were obtained from the field test conducted for the HES Hartel Tank Terminal project in Rotterdam. These included the impact acceleration, the cone resistance, the in situ density, the dynamic modulus and the dynamic plate load test velocity. Two methods are considered in this thesis and both aim to reproduce the measured values from the dynamic plate load test during the field test. The first method considers the acceleration signals and includes double numerical integration of these signals to obtain the displacement, while the other considers modelling the roller as a dynamic plate load test and obtaining the displacement from solving a 2-DOF spring-mass-damper system.
However, after analysis of the motion of the roller, it was observed that due to the non-circular shape of the roller, a wedge effect was created where horizontal shearing forces caused loosening of the soil. This inhibited soil compaction up to 0.5 m depth. The impact acceleration signals were thus not representative of the soil compaction. Nonetheless, the DPL-Soil model was proven to be successful in correlating the soil settlement to the dynamic modulus. This study considers a silty sand, so further research should be carried out to obtain correlations for various soils. In order to develop the semi-empirical energy model, it is thus recommended to capture an accurate acceleration response. This can be done by placing accelerometers at a minimum depth of 0.5 m, replacing the 8G accelerometer with e.g. 16G accelerometer and increasing the sampling rate to at least 1000 Hz. In order to filter out the soil variability, a field test with the roller should be performed on a homogeneous sand without fines. Correlations can then be drawn again for the same field tests performed in this thesis. Finite Element Modelling (FEM) could be used to model the interaction between the non-circular shape of the lobe, the rolling motion and the soil. This might form a better correction method for the acceleration signals than those explained in this thesis. Low frequency geophones can be used to measure the velocity directly. This because low frequency data of the accelerometer should be removed and the roller works on a low frequency. The load imparted to the ground could also be measured directly by burying earth pressure cells at a minimum depth of 0.5m and at various depths to get a more accurate representation of the pressure distribution through the soil layers. By using other numerical integration methods such as Simpson’s 3/8 rule and Boole’s rule, the numerical accuracy of the displacement response of the roller could also be improved. ... Read more

Offshore wind energy is considered a necessary renewable energy resource, that may stimulate the transition from fossil fuels. Following the successful development in Western Europe, offshore wind is quickly gaining momentum in the Asia-Pacific region. At variance with North Sea-based offshore wind turbines, structures installed in the Asia-Pacific region are prevalently exposed to typhoons, giving rise to severe wind speeds and, consequently, extreme waves. Such conditions have become design driving for support structures. Considering that the response of the support structure due to these extreme waves is dependent on soil stiffness, a state-of-the-art foundation model accounting for non-linear, hysteretic soil-monopile behaviour is included in integrated time-domain simulations. Besides considering load-dependent hysteretic damping, the foundation model accounts accurately for the unloading-reloading stiffness. This multi-directional macro-element model has been primarily developed and verified for fatigue limit state analyses. In this thesis, the results of additional 3D finite element verification analyses are presented to identify potential model limitations under ultimate limit state conditions. With regard to different geotechnical and loading scenarios, it is observed that the macro-element model satisfactory predicts load-dependent stiffness and damping, even for the extreme load levels relevant to the Asia-Pacific region. To capture the offshore wind turbine dynamic response to extreme loading, time-domain analyses are performed with two foundation models: 1) the current industry standard based on non-linear elastic API p-y curves, and 2) the non-linear elasto-plastic macro-element model. These models are calibrated against the API p-y curves and also against load-displacement curves from 3D finite element analyses. From the models calibrated against the API standard, the effect of accounting for the load-dependent stiffness and damping on the response at interface for extreme load cases is determined. A reduction of the moment at interface level is observed, due to an improved soil stiffness and damping estimation. Further, as the API p-y curves do not account for the correct initial stiffness, the response at interface level is additionally evaluated with the macro-element model calibrated to 3D finite element analyses. The results show a further decrease of the response, that may be attributed to the (initial) stiffer response of the monopile at mudline from 3D finite element analyses. One of the recommendations is to numerically evaluate the contribution of hysteretic damping with regards to the system damping. Therefore, the validity of the often-used linear damping estimation strategy is investigated for the non-linear system. The interference term in the response at mudline has shown to cause a phase difference, with respect to similar response that does not account for the interference term. The applicability of the logarithmic decrement method that is currently used for system damping estimation is therefore questioned. To evaluate this further, it is suggested to perform additional studies that account for a more adequate representation of the response spectrum. ... Read more

Since the exploitation of wind as a renewable energy resource, jack-up vessels equipped with more than three independent legs are increasingly employed to transport and install the components of offshore wind turbines. By lowering the movable legs the vessel is able to elevate the hull from sea water level. In elevated position the vessel provides a stable platform to perform installation activities. The legs are equipped with spudcans which serve as foundation of the vessel. The elevating process consists of a preload phase to ensure sufficient capacity to withstand operational and possible storm conditions. The preloading of four-legged jack-ups is performed by alternately applying vertical loads on diagonally opposite leg pairs, up to achieving a stable condition in which nearly constant load levels can be held by each leg. The aim of this research is to develop a 3D model to asses the preload duration of the jack-up vessel Aeolus in cohesive soil. The viscous behaviour of cohesive soil, like clay, influence the acting leg load during preload of the jack-up vessel. The shear strength of clay is a function of strain rate meaning that resistance increases due to viscous effects with increasing penetration rate. During spudcan penetration the shearing resistance is high but will reduce significantly when penetration is stopped as the viscoplastic resistance diminishes. Together with the onset of isotach soil behaviour this causes the loads to redistribute between the legs occurs. In this study it is assumed that sufficient preloading is achieved when the leg load reduction is limited to 400 ton / 15 min. To satisfy this criterionmultiple load cycles of each leg pair are performed. Site specific geotechnical data and information on the structural stiffness of the Aeolus have been available for this research and allowed for an accurate analysis of the processes during the preload procedure. The Soft Soil Creep (SSC) model is used as constitutive model and accounts for viscous effects by formulating irreversible strains by means of viscoplasticity. The soil at the project site is classified and the constitutive model is calibrated based upon the available soil test results. The structural behaviour of the vessel is captured via a simplified beam configuration representing the deck structure and legs, the stiffness of the beams is verified using the results from a so-called predrive analysis. The extension of the legs is established bymeans of negatively pre-stressed node-to-node anchors. Simulations of a single spudcan penetrating at various depths and penetration rates are performed to identify the extent of viscous strain rate effects fromthe results. With the developed 3D model Small Deformation Finite Element analyses of the preload procedure are performed. The leg loads and penetrations are monitored and compared to jacking data fromthe actual project site. The processes in the soil and structure are analysed and the influence on the preload procedure and preload duration is identified. For both type of simulations six different case-calculations are performed addressing variation in the initial spudcan depth, the OCR, the penetration rate, the permeability and the type of preload procedure. The simulations indicated that the penetration of a spudcan influences the penetration of an adjacent spudcan, this reciprocal influence of the spudcans emphasizes the importance of onemodel comprising all spudcans in the same 3D soil domain. The developed model slightly overestimates the spudcan penetration and underestimates the total preload duration. Simulations of the overshooting preload procedure and an alternative preload procedure are performed with the FE model. For the soil conditions used in this research, both the overshooting and the alternative procedure are effective in reducing the number of preload cycles to satisfy the preload criterion. Compared to the normal preload procedure, it is expected the overshooting procedure improves the preload duration. For the alternative procedure however, the duration of a preload cycle increases significantly and consequently the procedure does not improve the preload duration. Using a lower spudcan penetration rate during the normal preload procedure is also effective in reducing the number of preload cycles but significantly increases the elapsed time to complete the preload procedure. The above conclusions have been made on the basis of the model results, which is calibrated for the soil conditions at the specific project site. ... Read more

This work presents a numerical study on the dynamic response of monopile-supported offshore wind turbines (OWTs) under seismic loading conditions. For this purpose the realistic design ofa 8 MW OWT is considered as ideally located in a layered, non-liqueable site off the coast of Japan. This choice allowed to employ available site characterisation and seismic records from the well-known Japanese KiK-net. Modelling highlights are reported about the development of a dynamic, fully 3D FE model, in which the soil is described as a water-saturated elasto-plastic medium. The well-established SANISAND bounding surface formulation is adopted to reproduce the hydro-mechanical cyclic response of coarse-grained materials at the site considered, with model parameters derived via back-analysis of available seismic records. The seismic performance of the OWT is studied in relation to real seismic input of different intensity, accounting for the effect of combined horizontal and vertical components. Although in a non-liqueable site, pore pressure effects are clearly visible in the simulation results and affect the observed interaction between soil and foundation. The likely coexistence of seismic and SLS wind/wave loading is also considered for completeness. The numerical study leads to some interesting conclusions regarding the interaction between monopile head stiffness and seismic/cyclic soil response, and its impact on the motion and strength mobilisation along the OWT structure. In particular, some fundamental dierences between the dynamics induced by wind/wave loading and seismic input are put in evidence.
More generally, it is also shown how existing analysis approaches developed in the context of earthquake geotechnical engineering prove still suitable to support modern offshore wind developments in seismically active regions. ... Read more

As the demand for renewable energy is constantly rising, more offshore wind farms are being built at sea to meet European targets towards sustainable energy transition. Van Oord’s offshore installation vessel Aeolus was purposely-built to transport and install foundations of offshore wind turbines. To ensure that the vessel remains competitive in a market that is changing rapidly, Van Oord decided to upgrade this vessel to expand its operational capabilities to work on soft soil seabed conditions.

Equipped with moveable legs, the mobile jack-up vessel is able to elevate its hull out of the water and provide a stable work platform. Prior to each installation, the soil is preloaded by penetrating the spudcan footings into the soil under the self-weight of the structure, followed by progressively pumping sea water into the ballast tanks until the target preload is reached. Offshore industry guidelines follow the conventional bearing capacity theory to assess jack-up procedures. However, these conventional procedures are inadequate in incorporating all relevant mechanisms that contribute to the preloading behaviour in cohesive soils. The effect of consolidation around the spudcan footings on the soil strength and the viscous strain rate-dependent soil strength are two important aspects which are poorly described in the current guidelines. Using numerical modelling, a better understanding of these mechanisms is aimed for in order to evaluate spudcan bearing capacity in cohesive soils in a more detailed design.

An axisymmetric finite element model of the spudcan foundation is constructed in PLAXIS 2D to assess the effects of consolidation and rate-dependence of soil strength on the use of different preloading strategies. As spudcan penetration is relatively fast and clay permeability is relatively low, consolidation effects are found to be insignificant during the preloading process. Contrary, viscous strain rate effects dominate the soil strength of clay in predominantly undrained conditions. Higher shear strengths are found for higher spudcan penetration rates. During the standard jacking process, when the target preload value is reached and the spudcan is stopped, a reduction of the leg load occurs due to decreasing viscous resistances, which must actively be brought back to the target preload value. In clay soils, this leads to further penetrations and multiple preload cycles are required before the spudcan response is stabilised. In terms of preloading duration, a rather slow spudcan penetration rate is therefore beneficial when using the standard jacking procedure.

Alternatively, a different jacking procedure is assessed numerically, which adapts for these viscous strain rate effects by overshooting the target preload value by 10-15%, in order to reach the stable spudcan response more quickly. In this way, the spudcan is brought to the depth, which would have been reached by the creep-like additional penetrations in the iterative preload cycles under the target preload value. Once the load is reduced from the overshoot to the target preload value, it is likely that the preload can be sustained without additional penetrations and preload cycles. The overshooting jacking procedure reduces the required preloading time considerably for both spudcan penetration rates. The largest time savings are gained for the largest penetration rates. However, using this method, larger leg penetration predictions compared to the standard jacking procedure are observed for decreasing spudcan penetration rates. That is because in the standard jacking procedure, less creep-like additional penetrations are present when using a slow spudcan penetration rate. The overshooting jacking procedure can thus be considered beneficial for relatively fast spudcan penetration rates, both in term of preloading duration and spudcan penetrations.
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