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E. Kementzetzidis

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3 records found

From cyclic sand ratcheting to tilt accumulation of offshore monopiles

3D FE modelling using SANISAND-MS

Journal article - H. Liu, E. Kementzetzidis, J. A. Abell, F. Pisano

Serviceability criteria for offshore monopiles include the estimation of long-term, permanent tilt under repeated operational loads. In the lack of well-established analysis methods, experimental and numerical research has been carried out in the last decade to support the fundam ...

Cyclic behaviour of laterally loaded (mono)piles in sand

With emphasis on pile driving effects

Doctoral thesis - E. Kementzetzidis

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.@en

Gentle Driving of Piles at a Sandy Site Combining Axial and Torsional Vibrations

Quantifying the Influence of Pile Installation Method on Lateral Behavior

Journal article - E. Kementzetzidis, F. Pisano, A. Tsetas, A. Metrikine, A. Metrikine

Gentle driving of piles (GDP) is a new technology for the vibratory installation of tubular (mono) piles that aims to achieve both efficient installation and low noise emission by combining axial and torsional vibrations. To provide a preliminary demonstration of the GDP concept, ...

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Feasibility of using a Helical Pile Foundation System to support a 15MW FOWT using a Tension Leg Mooring System

A study into the performance of a helical pile anchoring system for a TLP

Master thesis - W.C.I. van Tuyll van Serooskerken, J.S. Hoving, E. Kementzetzidis, D. Mus, A. Metrikine

This thesis explores the feasibility of using a helical pile foundation with a tension leg mooring system to support a 15MW floating offshore wind turbine and assesses its financial viability compared to alternative anchor concepts. The study commenced with an extensive literature review, drawing from a diverse range of sources, including reviews, offshore guidelines, research papers, and expert interviews. The literature review was conducted to enhance the understanding of the use of helical piles in the offshore industry, encompassing their fundamental characteristics, current applications, and potential contributions. For this investigation, a Tension Leg Platform (TLP) featuring a 15 MW wind turbine, engineered by Heerema Engineering Solutions (HES), was employed. Time-domain simulations, accounting for environmental conditions, were carried out using the OrcaFlex software. These simulations led to the determination of the mooring line tensions, with the maximum value identified as the design load case. During the helical pile geometry optimization process, which focused on maximizing uplift capacity, it was established that a single helical pile in dense sand can achieve a maximum uplift capacity of 7.91 MN, while taking into account geotechnical, structural, and installation constraints. As a result, considering the significant uplift capacity demands of TLPs, it is essential to employ helical pile grouping, necessitating a minimum of four helical piles. Consequently, the design of the helical pile group anchors confirms their ability to meet the uplift and lateral capacity requirements of TLPs. Furthermore, the exploration of variations in equipment, steel strength, helical pile geometry, and soil conditions has yielded promising results. It is important to note that achieving the required installation depth for these group anchors involves a significant force and torque during the installation process, presenting potential challenges. Consequently, the development of equipment capable of meeting these installation requirements is vital for ensuring the feasibility of these helical pile group anchors. The performance of helical pile group anchors was analysed from economic, technical, and environmental perspectives, with a comparison to suction and driven pile anchor concepts. The economic evaluation showed that the estimated total costs for the 4-pile helical group anchor, utilizing a singlepile installation method, are notably higher when factoring in the costs related to developing helical pile equipment, making it less financially attractive compared to the developed suction and driven pile anchor concepts. However, when excluding these equipment costs, the 4-pile helical group anchor ranked as the second-most financially attractive option, regardless of the installation technique, with only the single driven pile anchor being less expensive. Notably, it was discovered that, in a scenario where structural, geotechnical, and installation constraints are disregarded, the single pile helical anchor emerges as the most financially attractive among all anchor types, even surpassing the single pile driven anchor. This suggests that overcoming challenges related to scaling up helical pile dimensions, like enhancing their structural integrity, and reducing installation requirements through innovative designs, could make the use of fewer piles in a helical pile group anchor a feasible choice. In addition to these quantifiable factors, helical piles offer the advantage of a low-noise installation method, which becomes increasingly important due to the growing noise disturbance legislation in certain regions. Furthermore, their adaptability to various ground conditions through flexible installation techniques makes them a convenient choice for sites with limited access or specific inclination requirements...

Lateral Response of Monopiles in Sand under Monotonic and Cyclic Loads

A 3D Finite Element Investigation

Master thesis - F. Firman Habib, F. Pisano, L. Flessati, E. Kementzetzidis, Haoyuan Liu, Kuen-Wei (Wayne) Wu, Naoual El Kanfoudi

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 ...

Optimization of the installation sequence of an Offshore Wind Farm Monopile Installation Template

By improving the foundation design

Master thesis - M.D. Bosma, A. Metrikine, J.S. Hoving, E. Kementzetzidis, M. Velduizen, J. ter Braak

Heerema Marine Contractors is assigned to execute the installation of 64 monopiles for the He Dreiht Offshore Wind Farm provided by Energie Baden-Württemberg (EnBW). The installation of the monopiles is done by means of a template structure. This template is specifically designed for this project and is installed on the seabed temporarily. The initial design of the template includes large mudmats to ensure the required stability during the full monopile installation sequence. Analysis shows that this initial template design does not perform well with regard to the template installation sequence. Additionally, when lifting the template through the wave zone, the large horizontal surface area of the mudmats yields significant hydrodynamic forces acting on the template in vertical direction. . Therefore, the main objective of this thesis is to generate and develop alternative foundation concepts to optimize the template installation sequence. The foundation concepts considered in this research are a mudmat, suction bucket, push-in pile and helical pile foundation concept. Using the mudmat as a foundation concept yields a foundation area of 924 m^2 in order to ensure stability on the seabed, independent of the soil type. The suction bucket foundation concepts results in a horizontal foundation area of 390 m^2 for sand, or 117 m^2 for a clay seabed. The push-in pile and helical pile foundation concepts developed for a sand seabed, result in an area of 306 m^2 and 319 m^2. For a clay seabed respectively a foundation area of 75 m^2 and 88 m^2 is required. The operability of each foundation concept is determined by means of a hydrodynamic analysis of the template when lifting it through the wave zone. The operability is the amount of time the template can be lowered through the wave zone, expressed in percentage based on 24 hours. A simplified method stated by DNV is used to determine the hydrodynamic loads acting on the foundation in heave direction. For the considered homogeneous sand and clay seabed, the mudmat foundation concept proved to have the lowest operability. The suction bucket concept scored second, and the push-in pile and helical pile foundation concept showed the best performance on operability for both types of seabed. To evaluate which concept yields the most optimal template installation sequence, a multi-criteria analysis (MCA) is performed. This MCA also takes the installation time, construction costs and the risk of damage into account as criteria. Taking all these criteria into account, it is obtained that the mudmat foundation concept does provide the best performance when considering the template installation sequence, based on the assumptions stated throughout this thesis. Even though the mudmat foundation concept has the lowest performance of all foundation concepts when lifting the template through the wave zone, this concept overall performs better than the alternative foundation concept due to a short installation time on the seabed, low construction costs and low damage risks. The suction bucket foundation concept shows an average score on all criteria. This resulted in the second preferred option. The push-in pile and helical pile foundation concepts show a good operability, however, these concepts score low on the construction costs, installation time and damage sensitivity. As a result, these concepts show the lowest performance considering the template installation sequence.

Is under-base filling necessary?

Assessing behaviour of partially installed suction caisson

Master thesis - A. Sharma, Kenneth Gavin, M.A. Cabrera, Weiyuan Zhang, Erik ter Horst, E. Kementzetzidis

In the past decade, suction caissons have emerged as a preferred offshore foundation solution for wind turbines due to their silent installation process and potential for recyclability. However, there has been growing speculation regarding the necessity of under base filling, which involves filling the gap between the top plate of the suction caisson and the seabed. Some experts have suggested that under certain conditions, this under base filling may not be required at all. Furthermore, concerns have been raised about the efficacy of under base filling in achieving full contact between the top plate and the seabed, as it has been observed that gaps may persist even after the filling is applied. Consequently, doubts have been cast on the overall need for under base filling. However, there is limited research focused on understanding the behavior of water plugs in the absence of under base filling, at different loading conditions ( Compression , tension , cyclic etc.). This knowledge gap motivates this thesis study, which aims to investigate the behavior of water plugs specifically in dense sand samples, as sand is considered more critical compared to clay in terms of its variability in drainage conditions that can influence the foundation’s performance. To achieve this, a series of centrifuge tests were conducted on suction caissons that were partially installed and some fully installed. The results of the experiments shed light on the role of under base filling in different loading scenarios. Under monotonic compressive loading at higher rates, it was observed that under base filling played no significant role in the load transfer . Both the caissons with and without under base filling exhibited similar load transfer mechanisms, indicating that filling the gap may not be necessary in such loading conditions. Additionally, under tension loading, it was found that under base filling had little to no effect on the development of tensile capacity. By expanding our understanding of the necessity and effectiveness of under base filling, this study contributes to the ongoing discussion surrounding suction caisson design and installation practices for offshore wind turbine foundations.

Dynamic modelling of offshore monopile decommissioning

A study into the first moments of complete removal of monopiles

Master thesis - D.J.M. Meijer, A. Metrikine, E. Kementzetzidis, B.C. Ummels

The offshore wind sector in Europe has grown significantly over the past 20 years. Following this surge in number of installations, the first generation of turbines is reaching its end-of-life (EOL) phase and thought needs to be put into end-of-life strategies. As the market for lifetime extension is still in its infancy and re- powering is economically unappealing, decommissioning is the most obvious solution. The tower, Rotor Nacelle Assembly (RNA) and electrical cables will most likely be removed by reverse-installation methods, as well as the Transition Piece (TP). Removal of the monopile foundation however, is considerably more diffi- cult due to the large embedded length of the pile in the soil. The common way of decommissioning such piles in industry (mostly the Oil & Gas (O&G) industry) is to cut them at or below the seabed and leave the remaining stumps in place. This research investigates more sustainable methods that remove the foundation completely, as this is more in line with the modern circular economy and regulations, i.e., OSPAR, are getting more stringent, focusing on complete removal of all offshore structures in the future. To remove the entire monopile foundation using only a blunt uplift force, tremendous amounts of force are required. It is thus desired to lower this extraction force by some technique to reduce soil resistance. This research investigates the possibility of reducing soil resistance surrounding monopiles by utilizing pitch and heave vessel motions. Such motions can induce an harmonic force on the monopile during the first mo- ments of lifting. The hypothesis is that low frequency oscillations of about 0.1 Hz can induce plastic strain accumulation of the soil, resulting in permanent displacement of the pile. This hypothesis is tested by con- structing two models, one describing vessel motion in MATLAB, and the other describing the pile-soil inter- action under cyclic loading, which is modelled in the open source software OpenSees. An extensive market study is performed to find a representative maximum set of dimensions of the first generation of monopiles installed offshore. Monopiles with a grouted connection to the transition piece are considered as first gener- ation monopiles, and it is these monopiles that are of interest to the study because they need to be removed the first. The vessel considered in this research is the Pioneering Spirit and the crane its Jacket Lift System (JLS), due its large lifting capacity. After delineating to the monopile size and the vessel, assumptions are made regarding the extraction process. A cranemaster is added to the system to avoid slack in the lines and a connection tool between the crane wire cables and the monopile is selected, assuming a rigid connection between the two. The results indicate that applying a low frequency cyclic load to a monopile can result in the accumulation of plastic strain of the soil and, consequently, to larger monopile displacements than in monotonic load cases of the same magnitude. In the simulations performed in this research, this effect, called cyclic degradation, is observed to be more intense in sands than in clays. It is observed that plastic strain is largest when large forces are applied to the model, hence it is desired to maximize the combination of tension and amplitude of the applied cyclic force. The effect of cyclic degradation however, is stronger if the forcing frequency is increased to a value near the resonance frequency of the pile-soil system. This value lies in the region of 4 to 8 Hz, depending on the force input characteristics because of the non-linear behavior of soils. Achieving fre- quency multiplication of the input force has been considered in both active and passive ways. It is concluded that only active frequency multiplication may deliver the needed cyclic degradation, since passive frequency multiplication diminishes the amplitude of the cyclic force, driving up the resonance frequency of the pile- soil system even further. The applicability of the pile-soil model, and thus these conclusions, is only valid during the first moments of lifting, when displacements are relatively small. The results obtained in this thesis are the result of a relatively conservative soil dynamics model and a non- conservative hydrodynamical model. The reliability of the results could be further improved if the two models are combined in a co-simulation, where at every time-step the pile displacement and vessel motion are deter- mined. Additionally, expanding the OpenSees model so that accurate representations of large pile displace- ments are included, will allow for calculation of longer simulations, where the pile is completely removed from the soil. The hydrodynamic model can be improved by using an entire sea-state as input.

First steps into the calibration chamber pile tests with glauconitic sands

On the behaviour of the sand fraction from glauconitic sand of the Kattendijk formation in Antwerp, Belgium

Master thesis - A. Rodriguez Piedrabuena, Kenneth Gavin, E. Kementzetzidis, M. Mento, Jort van Wijk

The Global Wind Energy Council (GWEC) forecasts an accelerated increase in wind installations as a result of the aggressive climate change and green energy objectives set by the majority of countries. As a consequence, new regions worldwide that were previously thought to be unsuitable for the construction of offshore wind farms due to the soil characteristics are planned to be explored. This is the case of glauconite sands, which include glauconite pellets that are very susceptible to crushing. The crushing transforms the coarse-grained sand into a fine-grained soil, changing the geotechnical characteristics of the soil. This poses particular geotechnical challenges, since the soil-structure interaction between the pile being driven and the sand changes, the side friction increases with depth and the prediction of blow counts in drivability analyses is underpredicted. In addition to this, the glauconitic sand influences the response in cone penetration tests (CPTs), characterised by high cone tip resistance (qc) and sleeve frictions (fs), leading to high friction ratios (Rf) and resulting in misleading soil classifications. IQIP, as a company specialised in installation and foundation projects internationally that provides innovative and high-quality equipment and solutions for construction projects related to Offshore Wind industry, is interested in better understanding the mechanical behaviour of glauconite sand during pile installation due to the risks posed by this sand, e.g. premature pile installation refusal. In order to understand this soil that poses geotechnical challenges, laboratory test programs should be designed in order to capture the soil behaviour and physical modelling is a cost effective and valuable tool. This research aims to obtain detailed information on the soil response of glauconitic sand in saturated conditions during pile installation with a new sample preparation method for glauconitic sand containing fines. It will investigate the feasibility of cone penetration testing, cyclic loading, pile driving, as well as the analysis of the shear band and grain crushing around the instrumentation penetrated in the sample in a calibration chamber (CC). Based on the results from the three performed tests it could be concluded that the preparation method led to a good procedure to prepare repeatable and homogeneous samples under fully saturated conditions, as well to a representative response to pile penetration. With this, it is possible to perform further research on the sand fraction of glauconitic sands using the same sample preparation method and testing equipment.

Uplift behavior of offshore shallow foundations during retrieval

An experimental study on the pressure differences that occur during uplift of mud-mats of pre-piling templates

Master thesis - B.M.I. Rietema, Kenneth Gavin, P. van der Male, E. Kementzetzidis

Offshore structures are often founded on mud-mat foundations which prevent the structure from settling excessively. Pressure differences are generated under mud-mats during removal from the seabed. These differences lead to a force that resists the uplift. A literature study on b ...

Optimising Vibrojet® Performance for Offshore Installation

Using 2D Lab Tests and Particle Recognition Software

Master thesis - J.W. Kerkhof, E. Kementzetzidis, J.S. Hoving, A. Metrikine

Design of a constant diameter retractable Jet-ring with inside-out jetting combined with a Vibro-hammer

Master thesis - D.H.J. de Jong, J.S. Hoving, E. Kementzetzidis

Reducing noise pollution during offshore monopile installation is crucial for marine life. Current installation methods create too much noise or do not have the certainty of reaching the required depths. By using a Vibrohammer combined with a Jet-ring this problem could be solved. A Jet-ring is a tool to reduce the sand induced inside shaft friction by fluidizing the inside of the monopile during installation. To make this concept a viable option, the Jet-ring should be retrievable as a reduction in cost but also in materials. The aim of this thesis is to design a retrievable Jet-ring for a variable-diameter monopile without changing the diameter of the Jet-ring. For this goal to succeed the focus will be on the possibility of using inside-out jetting. This design would be an addition to the Vibro-jet® of GBM Works, which is not retrievable yet. Keeping the diameter of the Jet-ring constant keeps the design simple, reducing the chance of failure inside the monopile where the Jet-ring has to withstand harsh conditions under a lot of water and sand. After creating a complete design of the retrievable Jet-ring, an experiment was designed to prove the concept. The designed concept is a Jet-ring which is small enough to fit through all sections of the monopile. In order to reduce the shaft friction from a distance, the nozzles are pointed outwards. This way the nozzles first have to erode a layer of sand before reaching the monopile wall. For comparison, 4 different water inlet concepts were tested, including the designed Jet-ring. All additional concepts were chosen to confirm and compare certain aspects of the Jet-ring. The test setup included several cameras, a flow and a pressure meter. The concepts were placed at the bottom of the tank with a sand bed above it during the experiment. A repeatable experiment was created by keeping the relative density equal which was done by keeping the starting bed height constant combined with an equal weight of sand at every experiment. At the top of the sand bed, a bolt was placed which would fall through when the sand was fluidized. After execution, the height of the sand bed was measured when the bolt fell through. In addition to the bed height, the duration until fluidization was measured. For all concepts, three different flow rates were executed minimally three times per flow rate to reduce outliers. The processed results can be divided into two categories: visual and numerical results. The visual results contained the flow patterns and other mechanisms helping to fluidize the sand bed. By looking at the flow patterns, explanations were found for the numerical results. The numerical results are the bed height and duration until fluidization. Looking at the results, the importance of having pressure behind the flow was shown. Comparing the base case, with a homogeneous water inlet, to the other concepts jetting with ten nozzles showed that fewer nozzles with a higher pressure resulted in faster fluidization. Furthermore, the importance of prolonging the erosion mechanism is shown. A concept directly jetting into the side of the tank lost a lot of energy in the flow resulting in almost double the fluidization time. While the energy dissipation against the wall of the tank is substantial, energy dissipation due to erosion seems relatively small. This was concluded by comparing a concept that jetted from the centre of the tank and a concept that jetted from the side. The last conclusion makes the designed tool using inside-out jetting a viable option for the retrievable Jet-ring. While the results are promising, some remarks should be made on the experiment and the design. While congestion by sand could occur during the experiments the results still give a relevant view on the capability of the concept to reduce the inside shaft friction of the monopile because the same mechanisms such as erosion and fluidization are used to penetrate the sand during the congestion as during monopile installation. Furthermore, because of the smaller diameter of the tank, the complete inside was fluidized. This could be not the case during monopile installation as sand could accumulate at the centre of the monopile. However, this possibly accumulating sand is not sure to influence the shaft friction as the sand against the monopile wall that induces the shaft friction is fluidized or weakened. These additional studies could strengthen and prove the concept of inside-out jetting further in the future.

Seismic response of monopile foundations for offshore wind turbines: from 3D to 1D modelling of soil-foundation interaction

Master thesis - D. Delavinia, F. Pisano, A. Tsouvalas, E. Kementzetzidis, S. Panagoulias

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.

Identification of Fatigue and Resonance Frequency Influencing Parameters of an Offshore Wind Turbine via Data Analysis of Real Measurements

Master thesis - M. Laanes, F. Pisano, E. Kementzetzidis, Stavros Panagoulias, Alexandros Iliopoulos, Willem Geert Versteijlen, E. Lourens, A. Metrikine

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.

3D FE seismic analysis of a monopile-supported offshore wind turbine in a non-liqueable soil deposit

Master thesis - V. VĂCĂREANU, F. Pisano, A. Tsouvalas, Kenneth Gavin, E. Kementzetzidis

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 ...

Reliability Based Design Optimization on Quay Walls by re-calculating partial factors

Master thesis - P. Zakynthinos, L. Flessati, E. Kementzetzidis, A.A. Roubos, A.P. van den Eijnden, C.J.W. Habets

In the years to come, the Netherlands will face a substantial challenge as over 1,500 kilometers of aging quay walls and sheet pile walls approach the end of their technical lifespan. Infrastructure managers anticipate that the necessary replacements will necessitate investments amounting to billions of euros. Moreover, this task carries a significant environmental footprint, notably in terms of CO2 emissions. The construction work required for these replacements will also result in disruptions and reduced accessibility, inconveniencing users. This study addresses two pivotal aspects. Firstly, it focuses on enhancing the design aspects of new structures and optimizing costs, with a specific focus exploring how these enhancements can ease the financial challenges faced by infrastructure managers. Secondly, it investigates the safety of existing structures and explores ways to maximize their loadbearing capacity while maintaining safety standards. The expected outcomes of this study promise improved design aspects, cost-efficiency, and enhanced safety measures. Quay walls can fail due to various mechanisms. This research investigates three primary causes: yielding of soil, yielding of quay wall and anchor yielding. Quay walls illustrate the complexities of soil-structure interaction. To address this, models were developed in both Plaxis and D-Sheet Piling. D-Sheet Piling was the preferred choice due to its computational speed. The reliability analysis was conducted with Probabilistic Toolkit. Considering the calculation methods, First Order Reliability Method (FORM) was employed, emphasizing in efficient computational results in contrast to the Monte-Carlo approach. In the first aspect, the partial factors were recalculated and compared them with the existing EC partial factor approach. To optimize the current design methodology, the retaining height of the structure was adjusted based on its reliability index. Additionally, the maximum anchor force required was re-evaluated for the structure. This procedure has been conducted for two scenarios, considering and not considering model uncertainty. Furthermore, an analysis was conducted to understand how altering the retaining height can lead to reduced steel usage, subsequently impacting costs and CO2 emissions. In the second aspect, it was pursued to enhance the structure’s performance by introducing a factor "n" across four distinct scenarios: 1. Simultaneously increasing all loads. 2. Increasing the surcharge loads on the terrain. 3. Increasing the bollard load. 4. Raising the final excavation level in front of the quay wall. While this study aligns with the extensive body of research in the field of civil engineering, It seeks to offer a new and sustainable approach on understanding quay wall design, focusing specifically on the designers’ viewpoint. Through the exploration of innovative design frameworks and approaches, this research seeks to make a valuable contribution to the long-term sustainability of quay wall structures. It aims to redefine our approach to accessibility and safety in these crucial structures. The comprehensive investigations conducted throughout this study provide an enhanced comprehension of quay wall design, reliability, and the optimization of performance.

Soil-Monopile Interaction: from Elastic to Elastoplastic soil reaction modelling under Quasi-Static Monotonic Loading

Master thesis - A. MAKRYKOSTA, F. Pisano, A. Tsouvalas, E. Kementzetzidis, S. Panagoulias

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.

Workability study for going-on-location of jack-up vessels

Master thesis - G.F. Holland, P. van der Male, E. Kementzetzidis, P. Naaijen

This thesis focuses on the going-on-location (GoL) operation of jack-up vessels in the offshore wind energy industry. The GoL process involves the transition of a jack-up vessel from a free-floating state to a state where it’s elevated above the waterline and pinned to the seabed. The workability of these vessels is significantly affected by the impact loads experienced during the GoL process, particularly when interacting with stiff seabeds and non-negligible sea states. In current methodologies, certain parameters like wave height and wave period set the limitations to when GoL process can proceed. For safety reasons, the GoL process is halted if these conditions are exceeded. Instead of solely depending on external factors like wave height an wave period, the focus transitions towards how the vessel dynamics relate to the impact forces it encounters during the GoL. To address this limitation, a comprehensive framework has been developed that combines hydrodynamic, structural, and soil-spudcan interaction elements to evaluate impact forces during the GoL process. The framework offers a flexible and case-specific configuration. It allows for easy modifications, integration, and replacement of components and input parameters. This case-specific arrangement is advantageous due to the wide range of jack-up vessels and environmental variations. In model implementations adhering to the framework, the jack-up vessel is represented as a multi-body structure, in contrast to the conventional rigid-body representation often employed. Within the multibody approach the spudcans, the legs, and the vessel are described by separate bodies each with its own properties. The primary focus of this research is on the dynamic soil-spudcan interaction process, which has not been extensively covered in existing standards. The soil-spudcan interaction model is to determine the instantaneous force acting on the spudcan as it contacts the seabed during GoL. By integrating elasto-plastic soil behavior into the soil-spudcan interaction element, the model encompasses descriptions of soil resistance to spudcan penetration and lateral displacement, taking into account memory and potentially stateful characteristics. A simulation model, adhering to the framework, has been developed, integrating hydromechanical, structural, and soil-spudcan interaction submodels within the Orcaflex environment. Three distinct simulation scenarios are examined: an undisturbed vessel (free-floating), a disturbed vessel (full GoL), and a pinned vessel (elevated jack-up). The disturbed vessel scenario, which includes a full GoL process, has exhibited consistency in both undisturbed vessel simulations, where the vessel is the free-floating stage, and in pinned vessel simulations, where the vessel is in the pinned stage. The impact phase is situated between these two boundary cases, and the framework effectively represents simulation models within its scope. In addition, simulations with varying sea states are performed for regular and irregular sea states. Simulations involving varying regular wave patterns suggest that the maximum downward spudcan velocity (DSV) is a critical parameter influencing the magnitude of impact forces on the spudcans. For irregular waves, the simulations indicate that the maximum impact forces are more closely related to the pinned vessel scenario, as the maximum impact occurs towards the end of the impact phase. In conclusion, this thesis has effectively described the behavior of jack-up vessels during the impact phase of the GoL process. For any model utilizing the framework, the GoL process can be simulated, and the results can be analyzed to assess workability. Furthermore, the study proposes a potential correlation between vessel dynamics and maximal impact forces, a relationship that could potentially guide on-board decision-making processes. The enhanced understanding of the interaction between the spudcan and the seabed, along with the comprehensive framework, contributes to improving the decision-making process for executing the GoL operation of jack-up vessels in the offshore wind energy industry.

Soil – Anchor Interaction of a Floating Offshore Wind Turbine under Cyclic Load

Master thesis - G. Christopoulos, L. Flessati, E. Kementzetzidis, G. Lavidas, Apostolos Bougioukos, Feike Savenije

The rapid growth of Floating Offshore Wind (FOW) has spurred intensive research across various aspects of the floating system. A standard practice in mooring system design within the industry is that anchors remain fixed on the seabed. In contrast, plate-type anchors exhibit mobi ...

Investigation of the effects of nonlinear soil-structure interaction for a jacket-founded offshore wind turbine

Master thesis - M.H. Dirkzwager, A. Tsouvalas, E. Kementzetzidis, S. Panagoulias, Stavros Panagoulias, V.S. van den Bosch, C. De Winter

Offshore wind energy is expanding rapidly as governments aim for net-zero emissions, with monopiles and jackets being the primary foundation methods for offshore wind turbines (OWTs). Supply chains, heavier turbines and deeper waters influence the efficiency of jacket foundations. This research considers a jacket-founded OWT. The development of OWTs has sparked its expansion to building sites with a proclivity to earthquakes. Such loading involves soil-structure interaction of which more needs to be known to improve accurate computational modelling. Current practice of Siemens Gamesa Renewable Energy (SGRE) is to perform aeroelastic simulations in their computational tool BHawC. The Foundation Designer delivers a foundation Superelement to maintain secrecy. The equation of motion for the jacket foundation can then be solved linearly with a reduced amount of unknowns. However, prevalence of nonlinearity in soil raises the question to what degree a linear model adequately captures the response. This study investigates the impact of soil nonlinearity on OWT dynamic behavior under seismic loads by comparing linear and nonlinear soil models. The analysis involves performing seismic simulations using multiple earthquakes. Another distinction is made through soil models with different characteristics. Nonlinearity is introduced to the soil stiffness and energy dissipation mechanism under cyclic loading. The investigated variations are a linear (elastic), geometrically nonlinear (nonlinear elastic) or both geometrically and physically nonlinear soil model (nonlinear plastic). The numerical model consists of a Rotor Nacelle Assembly (RNA), tower, transition piece, jacket, piles and soil springs. Beam elements are used for the tower, jacket and piles. The transition piece is simulated by stiffening the top jacket braces. The RNA is modelled using a lumped mass with rotational inertia. It is vertically eccentric to the tower top and connected with a rigid link. The earthquake is applied uniform over depth and only horizontal movements are considered. The findings underscore a difference in results between the linear and nonlinear models. The evaluated results from simulations consist of forces, displacements and dynamic characteristics of the structure. Also noted should be that the computational time of the linear model is significantly lower. The results found in models can differ greatly due to the loading spectrum with highly varying frequency peaks. Another factor is softening of the stiffness. The frequency domain of the elastic model results consists of narrow peaks at the system's natural frequencies. The peaks for the nonlinear elastic model are wider due to softening of the stiffness. For the structure used in this research, softening introduces coupled modes with greater displacements along the height of the structure. This makes it possible for the evaluated results to have higher values, even with less energy put into the system. The plastic models' peaks are of a width in between the elastic and nonlinear elastic model due to the combined use of isotropic hardening and nonlinear stiffness. When the model falls back on its initial stiffness upon unloading, the eigenfrequencies related to that stiffness become more pronounced. To match the occupancy of wider frequency peaks, loading and unloading should both happen nonlinearly. This can be achieved by using kinematic hardening instead of isotropic hardening. Plasticity generally reduces peak displacement and sectional moment values and nonlinear stiffness broadens the response frequency spectrum. Careful consideration of cyclic material behaviour, eigenfrequencies and loading characteristics are essential for a realistic model.