This paper presents results of a hydrodynamic model test of a monopile modeling the NREL 5 MW offshore wind turbine. This paper mainly introduces the test process of the monopile offshore wind turbine under the action of shallow water breaking waves and provides an analysis of the relevant test data. This experiment was carried out in the State Key Laboratory of coastal and offshore Engineering department of the Dalian University of Technology. The monopile model used in the experiment was the NREL 5 MW offshore wind turbine on a 1:80, scale. The physical model is rigid and made of steel. Breaking waves were examined. The wave run-up measurements of seven points are presented, and the experimental data are further processed and analyzed. INTRODUCTION Compared with traditional energy, wind energy has many advantages: pollution-free, renewable and low risk. According to relevant studies, the amount of wind energy on earth is extremely abundant, and it can completely meet the human demand for electricity. China has abundant wind energy resources on land, especially in Inner Mongolia, coastal areas and Xinjiang, however, the offshore wind energy reserves are more abundant. The development of offshore wind power is an inevitable trend in the development of international wind power and China's wind power; however, considerable development is required. Currently, monopile foundations are widely used in shallow and medium water depths where offshore wind farms are widely developed because of its low cost, simple installation and lack of seabed preparation required. According to statistics of major offshore wind farms in China, a monopile foundation accounts for 46% of wind farns. With the development of offshore wind power construction, monopile foundation type accounts for an increasing proportion of offshore wind farms (Bi, 2019). The offshore turbine system is a strongly coupled system, which bears various loads such as aerodynamics, hydrodynamics and control loads. Wave load is one of the key environmental loads in the design of offshore turbine foundations, and it is also an important part of the turbine coupling calculation. The structures installed in shallow water regions with sloping bottoms are exposed to many nonlinear wave interactions such as breaking waves, and shoaling. Among these interactions, forces from breaking waves are a major concern in the design of structures installed in such regions. As the wave propagates along the sloping bottom, due to the interaction of the waves with the bottom, the wave particle velocities become larger than the phase speed. At a certain critical point, the wave becomes unstable and breaks, dissipating a large amount of wave energy in the form of turbulent kinetic energy. The breaking wave front possesses large particle velocities and kinetic energy, and imparts a significant impact force on the structure. As the wave-breaking phenomenon is highly nonlinear, an analytical approach towards the problem is cumbersome (Liu et al. 2018).

With the size of the wind turbine and water depth being continually increasing, offshore wind turbine (OWT) installation has become a major challenge for the development of offshore wind energy. The traditional OWT installation methods either use a crane vessel to achieve a single lift installation of the integrated wind turbine onto the pre-installed foundation, or use a jack-up vessel to conduct modular lift installations for the different pieces of OWTs such as the tower, nacelle, and blades. The crane vessel method is limited by the lift capacity and height of the crane vessel, and is sensitive to the environmental excitations. The modular lift method by jack-ups generally entails a large number of offshore lifts and a protracted period for offshore hook-up and commissioning, resulting low installation efficiency and high costs. In addition, the working water depth of the jack-up installation vessels in the market is generally limited to 35 meters. The increasing interests in exploiting offshore wind energy in deeper water may bring new challenges to the installation of OWTs. Therefore, many novel installation methods have been proposed by practitioners and academia. This paper presents a novel integrated mating method based on a ship-type installation vessel with the capacity of transporting 4 integrated wind turbines on one route. By this method, the blades, rotor, nacelle and tower are assembled as one piece at the shipyard, which is then loaded out onto the installation vessel. During transportation, the 4 integrated turbines are horizontally placed on the 8-legged frame mounted on the ship deck. The installation of the integrated turbines involves anchoring and prepositioning of the installation vessel, erection of the integrated turbine by the hydraulic jacking system and mating of the tower stabbing cone with the preinstalled foundation. This installation approach is an analogy to the float-over deck installation method for the offshore platforms. A case study of offshore wind turbine installation onto a pre-installed monopile foundation by the new installation method is conducted in this paper. By using ANSYS-AQWA, numerical simulations are carried out to investigate the hydrodynamic characteristics of the installation vessel and the nonlinear dynamics arising from the mating operations. In addition, parametric studies are performed to study the effects of the motion control systems such as the mooring system, fender system and shock absorber within the monopile, on the vessel motions and mating forces.

In this study, an unresolved CFD-DEM was used to investigate a fluid flow and a behavior of sediment particles around a monopile. In order to consider the interaction between particles on the seabed and a current, an improved CFD-DEM solver was implemented within the OpenFOAM framework by proposing a void fraction method based on the kernel function. To validate computational methods, a settling velocity of a single particle, an angle of repose and an incipient motion of particles were simulated and compared with the existing experimental data. Finally, a scour around the monopole was predicted and discussed. INTRODUCTION In relatively shallow water, most offshore wind turbines are based on monopile foundations as bottom-fixed structures. Theses bottom-fixed foundations installed on an erodible seabed are exposed to scour, which may lead to structural failure. Therefore, it is essential to understand how the hydrodynamic environments affects the foundation and the interaction between flow, structure and seabed (Sumer, 2014). In particular, the scour can be defined as the phenomenon that the seabed particles around the foundation structure are transported due to the interaction of the fluid flow and the structure. The scour is a threat to the stability of the structure exposed to currents and waves. In order to alleviate the scour problem, many studies have been conducted experimentally (Dargahi, 1989, Whitehouse, 1998; Sumer and Fredsoe, 2002). In particular, most of the work has been done on scour phenomenon with monopile foundations. It led to various empirical formulas and methods to predict scouring depth and extension (Matutano et al., 2013). On the other hand, the computation can be employed as an alternative tool to study the scour process around structure (Pang et al., 2016). Recently, several computational methods have been developed to estimate the equilibrium scour depth. The scour depth and extent were estimated based on the bed shear stress exerted by the flow field in the Eulerian-based approach (Park et al., 2017). However, the single-phase model usually was not able to consider interparticle interactions in the scour around structures. To deal with this problem, many studies have adopted a two-phase model (Yeganeh-Bakhtiary et al., 2011; Hajivalie et al., 2012). These models can be divided into two types, depending on the method used. One is the so-called Euler-Euler two phase model, which treats the fluid and sediment phases as separate continuous mediums. The other calculate the particle motion individually using a discrete element method (DEM). The advantage of this method is that it can analyze a large amount of particles based on a simple collision model and analyze the exact behavior of the particles. Thus, to consider the interaction between the fluid flow and the seabed soil, a CFD and DEM coupling method is needed. Recently, studies on the sediment transport using the CFD-DEM coupling method have been carried out (Schmeeckle, 2014; Sun and Xiao, 2016), and even the scouring around a submarine pipeline was performed to investigate the flow behavior and particle motions (Yeganeh-Bakhtiary et al., 2013; Zhang et al., 2015). However, there have been only a few studies to realize the scour phenomenon around the monopile using CFD-DEM coupling method.

This study focuses on the analysis of the higher harmonic wave moments (around the seabed) on a vertical cylinder under the action of focused wave groups. The moment is known to be more nonlinear than the horizontal wave force; however, it is not very much investigated in the literature due to the difficulty of measuring accurately the mudline moment. We analysed the carefully measured wave loads from the tests in the Kelvin tank in the University of Strathclyde where a four-phase method is employed to extract the harmonic wave loads. The mudline moment shows a ‘Stokes-like’ underlying harmonic structure similar to the horizontal force. An approximation model is established to estimate the harmonic moment from the linear moment component. The model requires the nonlinear horizontal force coefficients and the moment arm of each harmonic. The moment arm for each higher harmonic is found from the measured forces and moments. The approximation model is demonstrated to be successful from both the measured data and numerical simulation. INTRODUCTION Large waves are usually expected during the operation of offshore wind turbines which are typically supported by bottom-mounted monopiles in relatively shallow waters. The typical Keulegan-Carpenter ( KC ) number for offshore wind turbine monopiles implies that inertia loading will dominate and viscous drag forces can be neglected. For non-breaking waves the dominant load will be at the same frequency as the incoming waves and can be well captured by a standard linear calculation. However, there will be higher harmonics to the loading by large waves and these can make up a significant part of the magnitude of the total loads. The higher harmonic loads will tend to act near the free surface and therefore have a higher moment arm than the linear loads – and hence increase moment loading on the foundations. However, not very much work was published in the literature that focuses on the investigation of the nonlinear moment on a vertical circular cylinder. In a large amount of the work on the higher harmonic loads, researchers mostly focus on the horizontal wave force. This is partial because, importantly the higher harmonic force will potentially act at around the natural frequency for which an offshore wind turbine is designed for (see Kallehave et al. 2015). This is of obvious concern for structural and geotechnical design. This resonance is coupled with the well-known problem associated with column-supported offshore structures which is the ’ringing’ occurring at a substantially higher frequency than the dominant wave frequency. The ringing is known to be caused by nonlinear extreme waves exciting transient response at the structural resonant modes. The higher-harmonic ringing loads on a vertical surface-piecing cylinder had been observed in the offshore field and measured in laboratory experiments. The observations had been reported in a number of experimental studies (Davies et al., 1994; Krokstad and Stansberg, 1995; Chaplin et al., 1997).

ABSTRACT Offshore wind power has a huge potential growth as a kind of renewable energy. Today one of the most common concepts for wind turbine foundations is the very large-diameter monopile. The conventional method for assessment of steel-pipe pile lateral performance is the application of p-y curves recommended in API RP 2GEO. However, the applicability of the API method for large-diameter monopile needs to be further studied. 3D finite element analyses are performed for the behaviour of the laterally loaded large-diameter monopiles. A work hardening soil constitutive model for saturated clay is used in these analyses, which has been calibrated with the results of stress-strain soil behaviour. Based on this model, parametric studies for different slenderness ratios have been developed for the pile lateral bearing capacity and p-y curves. The deformation features of piles are gradually transformed from flexible piles to rigid piles with the decrease of the slenderness ratio. Through comparing the numerical results with analytical results obtained by API method, the scope of application of existing p-y curves method is ensured. INTRODUCTION Offshore wind energy produced by wind turbines has become a promising source of renewable energy and will grow enormously in the coming years. Although the costs of installing a wind turbine in offshore wind farms are 30% – 50% higher than onshore wind farms, they encounter higher average wind speeds and save landing resource. Meanwhile, the development of offshore wind energy has encountered a series of challenges, one is determining the option of a foundation type and the design principle of the foundation. Up to now, five typical types of the foundations have been proposed: gravity foundations, monopile foundations, jacket structures, suction caissons and floating systems (Lombardi et al., 2013). The advantages of offshore steel piles are the simplicity of manufactures and installations as well as the widely successful experience in offshore gas and petroleum engineering field. Thus, monopiles are still the most common support structure of offshore wind turbines (>75%), typically used in water depths ranging from 15m and 35 m (Doherty and Gavin, 2011; Aissa et al., 2018; Damgaard et al., 2014).

ABSTRACT When an offshore foundation is exposed to waves and currents, local scour could be developed around a pile and even lead to a structural failure. Therefore, understand and prediction for the scour due to sediment transport around foundations are important in the engineering design. In this study, flow and scour around a monopile foundation exposed to a current were investigated by using the computational fluid dynamics (CFD) and discrete element method (DEM) coupling method. The open source computation fluid dynamics library, OpenFOAM, and a sediment transport library were coupled in the OpenFOAM platform. The results of simulations regarding the incipient motion of the particle were presented. The flow fields and sediment transport around the monopile were simulated. The scour depth development was simulated and compared with existing experimental data. INTRODUCTION Most of offshore wind turbine farms constructed under 60 m depth are installed as bottom-fixed structures. It is important to install a foundation structure that can stably support the load of the superstructure in a bottom-fixed offshore wind turbine. Bottom-fixed offshore wind turbines consider several design factors depending on the seabed ground and marine environment. Among them, a scour can be defined as the phenomenon that the seabed particles around the foundation structure are transported due to the interaction of the fluid flow and the structure. The scour is a cause of deterioration of the stability of the structure which must withstand the large turnover moment acting on the turbine. As the operation period of bottom-fixed offshore wind turbines increase, the researches on the scour problem have been done (Whitehouse, 1998; Sumer and Fredsoe, 2002). In particular, experimental and numerical studies on the scour around the monopile, which is the simplest foundation of bottom-fixed structures, have been performed (Dargahi, 1989; Pang et al., 2016). Park et al. (2017) predicted the scour using the bed shear stress by CFD. However, the Eulerian-based CFD approach does not sufficiently take into account the influence of the soil. For the soil transportation simulation, the DEM approach is used. Cundall and Strack (1979) was first presented the basic concept of the discrete element method, which simplified the collision between particles using the spring-dashpot model. In the DEM, small size overlap between the particles is allowed, and the behavior of the particles is analyzed by the repulsive force generated by the particle overlap. The advantage of this method is that it can analyze a large amount of particles based on a simple collision model and analyze the exact behavior of the particles. Thus, to consider the interaction between the fluid flow and the soil, a CFD and DEM coupling method is needed. Recently, studies on the sediment transport using the CFD-DEM coupling method have been carried out (Schmeeckle, 2014; Sun and Xiao, 2016), but there have been only a few studies to realize the scour phenomenon.

ABSTRACT The present work highlights differences between two soil models (the classical p - y curves and a FEM based super element model) and two hydrodynamic force models (the classical Morison approach and the more complex Rainey slender body force models). The paper highlights the differences between the models, focusing on the effect on the total fatigue experimented by the rotor on a reduced load matrix. A monopile for a 13.2 MW rotor was designed in the stiff-soft region and fully aeroservo-elastic simulations were run using the aero-servo-hydro-elastic tool 3DFloat. Results show conservative prediction of loads given by the PY curves, leading to a possible conservative design and increase of the overall amount of steel used. As for the hydrodynamic loads models, the Rainey and the Morison model gave identical results when provided with linear wave kinematics. INTRODUCTION The wind industry is steadily growing in Europe. In 2017 nearly 3,148 MW of net additional capacity was installed, corresponding to 560 new Offshore Wind Turbines (OWTs) across 17 wind parks (WindEurope, 2018). At the same time, following the steady growth of the wind energy market, a steep decrease in Levelized Cost of Energy (LCoE) has been achieved through developments in wind turbine technology and in particular through the increase of the rotor size. Today's wind turbines have larger rotors, higher hub heights, longer blades and considerably increased rated power capacity. LM blades, a Danish company, has recently announced the world's longest blade: 88.4 m long and specifically designed for Adwen's AD 8–180 (Adwen, 2018) wind turbine model, with 8MW nominal capacity and a 180 meter rotor diameter. General Electric is planning a new 12 MW rotor Haliade-X (GE, 2018) which will stand 260 m tall and have 107 m long blades. Simultaneously, the research community has been putting consistent effort on large rotors (e.g., AVATAR (Schepers, 2017) INNWIND (Jensen et al., 2017) SANDIA (Griffith and Richards, 2014) with the Sandia 13.2 MW SNL-03 100 m blade developed by Griffith and Richards (2014) being the largest well documented concept currently present in the literature.

ABSTRACT The lateral load response of a monopile is often modelled by p - y curves. However, the traditional formulation of p - y curves is unable to describe the behavior of liquefied soil. This paper presents a numerical investigation of how to develop p - y curves for monopiles installed in liquefiable soil. A monopile is modelled in the 3D finite element software package PLAXIS 3D, where a soil model able to capture liquefied soil behavior is applied. The numerical results are used to extract relevant data to assess liquefaction in the soil. Based on this information, p - y curves are generated for the liquefied soil. INTRODUCTION Most offshore wind farms are located relatively close to the coastlines in Northern Europe, where the seismic activity is generally low. However, the interest in offshore energy has emerged in areas with high seismic activity, such as in East Asia, which requires a foundation design that is able to withstand the effects of seismic loading. The seismic load from e.g. an earthquake is cyclic by nature. This type of loading can lead to generation of excess pore pressure in the soil. In loose (contractive) soils, the generated excess pore pressure may reduce the effective stress level to zero. Hence, the soil strength and stiffness are lost. This is the liquefaction phenomena (Andersen 2009, 2015, Nielsen et al. 2013, Nielsen 2016), which can be critical for the foundation (Japan National Committee on eathquake Engineering 1964, Hsein Juang et al. 2005, Cubrinovski 2013). The most used foundation concept for offshore wind turbines is the monopile foundation concept, as illustrated in Figure 1. One of the governing design criteria for a monopile is the rotation of the foundation. To assess the rotation, the lateral displacement at seabed must be evaluated. The lateral displacement of a monopile is typically calculated using p - y curves, where the soil-structure stiffness is modelled using springs. These p - y curves have been developed to describe the monotonic load-displacement behavior. A typical p - y curve for cohesionless soil has an upward convex shape (Det Norske Veritas 2011), where the tangential stiffness of the soil decreases as the pile displaces towards the soil. Different design practices (Ashford et al. 2011) suggest to apply a multiplier ( m p ) to the p - y expression for the non-liquefied soil to assess the effect of liquefaction by simply decreasing the stiffness and ultimate resistance of the soil. Nevertheless, laboratory triaxial tests show that the stress-strain behavior of post-liquefaction monotonic loaded soil follows an upward concave shape (Rouholamin et al. 2017), where the stiffness increases as the strains increase. Standard p - y formulations will therefore not give a good representation of the post-liquefaction soil behavior. In a design where there is a risk of liquefaction, the engineer therefore requires a representation of the soil-structure interaction, which better captures the liquefaction in comparison to what is given by standard non-liquefied p - y curves.

ABSTRACT The foundation stiffness under operational loads is a crucial aspect for the design of monopile foundations, as key design issues, structural natural eigenfrequencies as well as the structural fatigue damage are significantly affected. According to measurement results, the current calculation methods lead to an underestimation of the actual existing foundation stiffness. Concerning this matter, this paper presents numerical simulations to derive foundation stiffnesses. Furthermore, the damping of the system is investigated from the calculated load-displacement and moment-rotation curves in order to determine the hysteretic damping ratios. INTRODUCTION Monopile foundations are currently a widely used foundation concept in water depths up to 40 m. For such depths and the current generation of wind turbines with rated energy output > 8 MW, pile diameters of 8 m and more are necessary to fulfill the design requirements. A crucial requirement for monopiles with such diameters regards the stiffness under operational loading, which strongly affects the eigenfrequency of the overall structure. For the usual "soft-stiff" design, it must be ensured that the eigenfrequency lies sufficiently above the 1P excitation frequency resulting from the rotational frequency of the wind turbine in order to avoid resonance effects and associated high fatigue loads. It is common practice in the design of monopiles to calculate the bearing behavior with a subgrade reaction model, in which the soil is represented by springs with non-linear load-displacement relationships (Fig. 1). This approach is called p-y method and stated in the current Offshore Guidelines (API 2014, DNVGL 2018). For piles in sandy soils, based on investigations of Reese et al. (1974) and Murchison et O'Neill (1984), a hyperbolic tangent function is used to describe the p-y relationship. The p u -value and the initial stiffness of the p-y curve E py characterize the p-y curve. These values are depth-dependent and calculated from the angle of internal friction φ' and the buoyant unit weight γ' of the sandy soil. The method was originally developed and calibrated for flexible and small diameter piles. For the application to large-diameter monopiles, a modification of this method is necessary in order to account for the bearing behavior which is more similar to the behavior of a rigid pile.

ABSTRACT The future energy demand necessitates the exploration of all potential energy sources both onshore and offshore. Global trend has shifted towards offshore energy, which can be obtained from either carbon intensive or renewable options, hence requiring structures such as rigs, platforms, and monopiles. Most of these structures adopt easily installable construction techniques, where lower foundation need to be connected with the super structure by mean of grouted composite joints. Generally, these composite connections have exterior sleeve, interior pile and infill grout. Being located in remote offshore conditions, connections can experience considerable adverse loading during their lifetimes. Degradations were reported inside similar connections, which were installed in last three decades. Besides, grouting in the offshore sites may often be proven difficult, which eventually leads to reduced capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs in the future. This study aims at characterizing the nature of crack generation in grouted connections and thereby identifying the potential of repair using suitable repair material. Scaled grouted joints were manufactured using a novel mold, and connections were loaded under static load to visualize the main failure pattern. The failure mechanism and loading capacity are found compatible to previous results from earlier literature. Grouted connection was then repaired using cementitious injectable grout. The effectiveness of the repair system is also discussed. INTRODUCTION Current advancement in the modern world is dependent on energy. About 30% growth is expected in energy demand worldwide by the year 2040 (International Energy Agency 2014). Although, oil and gas (O&G) sector may well dominate the global primary energy supply for the rest of this century, energy security issues are directing countries towards renewable energy choices, which can reduce their dependency on fossil fuels as well as achieving sustainable energy future (Larsen & Petersen 2010). Hence, a future of sustainable energy demands substitution of fossil fuels by renewable energy sources around the world. The contribution of renewable energy sources in primary energy use, suggested by the New Policies Scenario, is raised to 18% in 2035 compared to 13% in 2011, where wind energy is expected to provide major share, limiting the rapid growth of traditional fossil fuels (International Energy Agency 2013). In fact, investments towards annual wind energy in 27 European Union member states (EU-27) will reach almost €20 billion with 60% towards offshore productions by 2030 (Krohn et al. 2009). At the end of 2015, Europes collective installed capacity reached 11027 MW across a total of 3230 wind turbines (Ho et al. 2016). Ho et al. also stated that there were 84 offshore wind farms, including sites under construction, in 11 European countries. Therefore, growing energy demand and advancement of technology lead to explore both onshore and offshore locations using wind structures, which are susceptible to adverse loading conditions and costly maintenance.

ABSTRACT The ice loading process has a clear stochastic nature due to variations in the ice conditions and in the ice-structure interaction processes of offshore wind turbine. In this paper, a numerical method was applied to simulate a monopile fixed-bottom and a spar-type floating wind turbine in either uniform or randomly varying ice conditions, where the thickness of the ice encountered by the spar were assumed to be constant or randomly generated. A theoretical distribution of the ice thickness based on the existing measurements reported in various literatures was formulated to investigate the response characteristics of the monopile wind turbine and spar wind turbine in such ice conditions. The effect of the coupling between the ice-induced and aerodynamic loads and responses for both operational and parked conditions of the rotor was studied. Moreover, the dynamic response of wind turbine in randomly varying ice was compared and verified with that of the wind turbine in constant ice. INTRODUCTION So far, more than 80% of the energy all over the world comes from fossil fuels. Excessive and improper use of fossil fuels has caused climate change and threatened human security and development. The Paris Agreement, which entered into force on 4 November, 2016, is a major step forward in the fight against global warming. Due to severe smog, forty Chinese cities reel under heavy air pollution. Air pollution becomes one of the key words in China in 2016 (PTI, 2016). Renewable energies play an important role for reducing greenhouse gas emissions, and thus in mitigating climate change. Offshore wind energy is recognized as one of the world's fastest growing renewable energy resources. By the end of 2015, totally 12,107 MW of offshore wind energy was installed around the world according to Global Wind Energy Council (GWEC) report (Fried, 2016). In Europe, 3230 turbines are now installed and grid-connected, making a cumulative total of 11,027 MW (Ho, 2016). However, governments outside of Europe have set ambitious targets for offshore wind and development is starting to take off in China, Japan, South Korea and the US. The 1.2 GW of capacity installed in Asia as of the end of 2015 was located China and mainly in Japan.

ABSTRACT In this work, advanced reliability assessment of OWT (offshore wind turbine) monopiles is proposed by combining reliability analysis method and SHM (structural health monitoring) / CM (condition monitoring) technology. A 3D (three-dimensional) parametric FEA (finite element analysis) model of OWT monopiles is developed, considering soil-structure interactions. A number of stochastic FEA simulations of OWT monopiles are performed, taking account of stochastic variables, such as wind loads, wave loads and soil properties. Multivariate regression is then used to post-process the FEA results, obtaining the performance functions expressed in terms of stochastic variables. After that, FORM (first order reliability method) is used to calculate the reliability index, evaluating the reliability of the OWT monopiles. In the presence of SHM/CM data, the reliability of monopile structures is reassessed and updated. The updated reliability index provides valuable information for decision making for inspection and maintenance of OWT monopiles. The application of the proposed advance reliability assessment method to a 45m-length OWT monopile is presented, showing great potential to reduce the OPEX (operating expenditure) of OWT monopiles by using the proposed method. INTRODUCTION Wind power is capable of providing a competitive solution to battle the energy crisis and global climate change, making it the most promising renewable energy resource. Currently, the vast majority of wind power are generated from onshore wind farms. However, the growth of onshore wind farms is limited to some extent by the visual pollution caused by large wind turbines and the limited available space to deploy onshore wind turbines. Compared to the land, there is more available space to deploy wind turbines at sea and the wind is stronger and steadier in offshore locations, driving wind industry move to offshore. According to European Wind Energy Association (EWEA, 2015), offshore wind in Europe is expected to reach 64.8 GW, supplying around 8.4% of total electricity demand in Europe in 2030. Representing around 80.1% of overall EU's installation in 2015 (Wilkes et al., 2016), Monopiles are currently the most widely used foundation for OWTs (offshore wind turbines), due to their ease of both manufacturing and installation. They are well suitable for water depths shallower than 30m (Maciel, 2010).

ABSTRACT The dynamic responses of monopiles induced by cyclic lateral loads, such as wind and wave, may change the lateral bearing behavior of the pile-soil system. A series of experiments were conducted in a wave- structure-soil interaction flume to identify the response features of monopiles under regular waves. The testing results indicate that the permanent displacement and cyclic amplitude of the fin pile is smaller than these of the monopile, and the aluminum monopile has the smallest response displacement. The wave load on the monopile with free-head is 4.99% lower than that on head-fixed monopile. The lateral stiffness of the monopile, fin pile and aluminum monopile, increases by 8.24%, 3.68% and 1.79%, respectively. The fin pile can be more efficient to resist the lateral load than the monopile with the same diameter. INTRODUCTION Monopiles are the one of the mostly-used types of the foundation for offshore marine structures, and their stability is affected by loadings in the harsh offshore environment. Because monopiles suffer from more complex loads compared with piles of onshore wind turbines, this effect which is important for engineering construction and maintenance is difficult to descript exactly in analytic method and needed to pay more attentions. The dynamic responses of monopiles induced by cyclic lateral loads, such as wind and wave, could change the lateral stiffness of the pile- soil system (Poulos, 1988, Levy et al., 2009). Many model tests reported that the rotation angle and displacement of the pile increased with increasing numbers of load cycles because of the soils degradations (Rosquoët et al., 2007; Pablo, 2011), and the cross-section optimization was an available method to improve the response of piles (Peng et al., 2011; Bienen et al., 2012). It is worthwhile to note that the most of exist model tests assumed that the cyclic point load could represent the wave load (a distributed load), and the responses of monopiles under real wave actions was relatively less in prior literature.

ABSTRACT In the present investigation the strength of a grouted connection of a Wind Turbine in offshore structures is studied numerically. The grouted connection joins the substructure with the foundation of the wind turbine and is subjected to a bending moment that simulates the effect of the wind on the structure. Grout connections have been built in order to transfer large moments due to the wind on the tower. In our analysis via the finite element method the stress state is analyzed in order to understand the behavior of grouted connections under static loading. In our study practical aspects such as constitutive formulations for the grout, mesh density, and steel/grout interaction are confronted. INTRODUCTION Until lately the foundation pile of most wind turbine (WT) structures was manufactured by junction of successive pieces of material weld together. Nowadays, the foundation pile of most wind turbines in offshore industry connects with the substructure with a grouted connection. Grouted connections have had extensive applications for the foundation of oil and gas platforms, where they have been used for main, skirt and cluster piles. A grouted joint is a structural connection formed by use of cementitious grout cast in an annulus formed between two concentric tubes with different diameters. The two cylindrical steel tubes, the transition piece (TP) and the monopole which are connected with a grouted connection (GC). This type of connection is being studied in this paper numerically. During the last years the offshore wind industry according to the behavior of the grouted connection did not result in an acceptable safety level. A lot of experimental, analytical and numerical investigations have been performed in order to confront the grout problem. A review of the experimental work concerning GCs was published by Dallyn et al. (2015) and Tziavos et al. (2016). The numerical modeling of grouted connection is an alternative to the experimental verification. Andersen and Petersen (2004) have carried out Finite Element Analyses (FEAs) in order to define rules for the design of GCs taking into consideration the experimentally tested models. Nielsen (2007) studied the steel-concrete interface and proposed practical modeling issues of FEA. Schaumann and Wilke (2007) proposed the application of shear keys in GCs in order to improve the transfer of load from the structure to the piles. Dedic (2009) investigated GCs in monopole WT subjected to horizontal load transfer analytically and numerically. Gjersoe et al. (2011) studied numerically the behavior between grout and steel, the degradation of grout compounds and proposed the use of packers. Prakhya et al. (2012) proposed a simple model for moment transfer and predict the multi stress state in GCs via the FEA. Lohning et al. (2013) tried to identify the mechanism of settlement of the TP using FEAs.

ABSTRACT This paper studies inverse kinematics to compute the joint parameters of 2-DOF rotary crane systems and the length of wire ropes that will cause a target structure to reach the desired position and orientation during collaborative tasks in offshore installation operation using two floating cranes. At first, connecting positions between wire ropes and target structure in a local coordinate system of the structure are described in a global coordinate system. The forward kinematic equations of two rotary crane systems are modeled with joint parameters of the crane systems and length of wire ropes. Inverse kinematics reverses these equations to determine the joint parameters of the crane systems making the wire ropes being perpendicular to the base or water plane. The connecting positions between wire ropes and the boom of each crane system are computed using this joint parameters, and the lengths of wire ropes considering their tensions are also computed using the balance equations of forces and moments acting on the structure and the wire ropes including gravitational forces and tensions. We consider convergence problems such as singularities, out-of-reach targets, etc. Furthermore, we also consider multiple solutions to achieve a desire pose and study how to choose an optimal one. Finally, we validate the inverse kinematics by applying it to monopile installation simulation using two floating cranes. INTRODUCTION Background A floating crane is almost essential for installing an offshore structure. Depending on the shape of the offshore structure and the environment of the installation area, some cases may require more than one floating crane. Collaborative tasks with two floating barges at an offshore site is quite a dangerous, so it is necessary to verify a safety using a simulation before real operation. To perform the simulation, the length of wire rope and the joint angle of crane according to the installation position and posture of the object structure are required. Currently, iteration work is necessary to confirm the correctness by calculating the joint angle of the crane and the wire length manually. Therefore, this study performs the inverse kinematic analysis to automatically calculate the rotation angle and wire length of the 2-axis rotary crane according to the position and attitude of the installation structure.

ABSTRACT Due to the lateral stiffness shortage of the monopile, and in order to improve the applicability of the monopile, a new wind turbine foundation type which is combining the jacket and monopile foundations is proposed. Based on the engineering example of offshore wind farm in Fujian Province, China, the main design parameters are considered. A 5 MW wind turbine foundation finite element model is established as the research object. Compared the results of monopile and the new composite foundation, conclusions are obtained. These findings will provide some reference for the new type foundation design. INTRODUCTION In recent years, the Chinese government on the development of clean energy, especially wind power gives a great policy support. China's wind power sector gained momentum due to the government's supportive policies. Sea wind is a permanent source which is inexhaustible and green new energy. Compared with onshore wind power, offshore wind power has many advantages, such as small land occupation, large wind speed, stable wind direction and little influence from surrounding buildings, etc.(Karadeniz et al., 2009) Commonly offshore wind turbine foundation type has gravity foundation, monopile foundation, tripod foundation, jacket foundation, floating foundation, suction caisson foundation and other types.(Westgate and DeJong, 2005) Monopile foundation as the most simple foundation structure is currently the most used wind turbines foundations for onshore and offshore wind farms.(Zaaijer, 2006) Its main advantages are the simplicity of design and manufacture, easy installation, and low costs.(Achmus et al., 2009) However, with the increase of water depth, the lateral rigidity of monopile becomes insufficient, and it can only be used for water depth within 30 m. Jacket foundation as another foundation for offshore wind turbines is more firm and stable than monopile foundation. It is suitable for 20 ~ 50 m water depth, but the costs for manufacture and installation are higher. (Jonkman et al., 2009)

ABSTRACT Offshore wind farms are regarded to be an important source of clear energy in recent years. The majority of offshore wind turbines (both current and planned) are founded on monopiles, they are open-ended steel pipe piles with diameters from 2.0 m to 8.0 m, which have soil coring inside. From the construction experience of existing offshore wind farm projects, the length of coring is nearly equal to the pile driving depth. In order to investigate the influence of soil coring on the horizontal bearing capacity of steel pipe pile with large diameter, a typical centrifuge model of an offshore wind monopile in sand was selected, which the diameter of prototype pile is 6.1m, the wall thickness is 0.12m, and the depth below soil is 31m with total length of 64m. Software of FLAC 3D was used to simulated loading procedure of model pile. In this research, different length of soil coring (0, 1/3, 1/2, 2/3 and 1 of pile driving depth) and different diameter (from 2m to 8m) were taken into consideration to explore the effect on lateral capacity of steel pipe pile. The results show that the lateral displacement of pile head decreases with the increase of length of soil coring. This decrease tendency would be more obvious with the increase of diameter of pile. Soil coring can be ignored to lateral capacity design of large-diameter piles. INTRODUCTION The offshore wind energy is a current major focus of the renewable energy policy, and becomes a cornerstone to China, which realize its ambitious target to generate 17 % of their energy from renewable sources by 2050. It is being generated at a tremendous pace, with the existing capacity of 10.2 GW by 2015 and predicted to be 150 GW by 2030. However, geological environment of seabed along the Chinese coast is complex, the soft soil layer is very thick, and supporting condition are much worse than that in Europe. The most common offshore wind farm foundation system is large-diameter hollow steel-driven pile. From recent offshore wind farm projects in China, pile diameters, D , of ranging from 2m to 8m, are now routinely used.

ABSTRACT Two common offshore wind turbine structures, the monopile type and the jacket type, subjected to ocean wave load were analyzed using finite element simulations. The FEM models were built in ABAQUS, and applied the load combinations consisting of the structural weight, the weight of wind turbine, the wind force, and the ocean wave force. In the analyses, we especially focused on the connection between the tower and the supporting structures, because wind force and ocean wave force may cause the damage to occur at the connections. In the model of the monopile structure, the grout connection to tie the upper steel tower and the lower steel pile is modeled by 3D cohesive elements. In the model of the jacket structure, the connection is a joint welded inclined members to tie the upper tower and the lower sleeves. Ocean wave forces acting on the structures are calculated based on Morison's equation, using the wave velocity and acceleration estimated by linear wave theorem and the site conditions of the wind farm in Taiwan. Both static and dynamic analyses with the distributed ocean wave force were carried out. Its effects on the stresses and deformation of two wind turbine structures were then investigated. We also discussed the possibility of using duplex stainless steel instead of mild carbon steel in the connection part of the jacket structure, because duplex steel has high yielding strength and excellent corrosion resistivity. INTRODUCTION Due to growing demands of clean energy, wind energy has become an important source of sustainable energy. Wind turbines have been installed offshore to accommodate even more wind power during the past decade in Europe. The first offshore wind farm in the world is located in Vindeby, Denmark in 1991 and the foundation of wind structures is the gravity base type. Currently, Taiwan Power Company is tentatively scheduling to build the first wind farm in the offshore region of Fangyuan, Changhua, which has the largest wind energy brought by the north-east monsoon in Taiwan.

ABSTRACT The paper examines key aspects of a shallow-water floater (FSO) motion response to forcing from landslide tsunamis using numerical models including: spatio-temporal characteristics of the hydrodynamic loading, mooring configuration for the FSO, and location/orientation of the FSO.The large, rapidly varying and spatially complex wave and flow patterns generated by the tsunami cause traditional mooring line arrangements to fail. A simple yet novel monopile mooring arrangement is successfully tested to reduce FSO motions and withstand the loads from the landslide tsunami events. The challenges associated with applying typical linearized deep-water motion response models in shallow-water are discussed. INTRODUCTION Hydrodynamic loads associated with tsunamis generated from subaerial and submarine landslides are significant hazards to floating structures. These impacts are particularly amplified in the shallow water nearshore areas due to wave shoaling. Limited depth, rapid variations (compared to tidal time scales) in the hydrodynamic parameters, wave nonlinearities and complex wave and flow patterns generated by interactions with the shoreline, and the infrastructure themselves pose challenges for numerical solutions to the floating structure motions and the loads experienced by the mooring systems. This paper presents the findings of numerical modeling studies conducted to assess, at a preliminary conceptual level, the motion response and associated loads of a permanently moored nearshore (shallow water) FSO (Floating Storage and Offloading) barge shaped hull, under landslide-induced tsunami waves. The FSO is located in a small inland cove within a larger fjord system. The fjord is approximately 3km wide near the cove entrance, varying between 200 to 300m deep, with steep side walls. The cove is roughly 1km × 1km in footprint (see Fig 1). Historical and hypothetical tsunamis generated by subaerial and submarine landslides along the cove and fjord walls are considered. Two different FSO locations within the cove are evaluated (Fig 1). In Layout 1 the FSO is located in the middle part of the cove, aligned such that the incident wave attack angle is roughly beam-wise (broadside). In Layout 2 the FSO is located on the right side of the cove and aligned such that the incident wave attack angle is quartering to head-on seas. In each configuration, the FSO horizontal center of gravity (CG) is located at about the -35m water depth contour.

ABSTRACT The load-bearing behavior of laterally loaded piles in soft clay is commonly determined by using the p-y method as recommended by the American Petroleum Institute (API). The recent application of these p-y curves for monopile foundations of offshore wind turbines necessitates their verification with regard to the large diameter and their response to small loads. Previous investigations from other researchers questioned the validity of the API approach for these conditions and recommend modified or completely new approaches. In this respect, the paper presents a comprehensive numerical study in order to assess a total of six static p-y approaches. It is concluded that none of the regarded approaches is generally valid for arbitrary soil conditions and pile dimensions. The findings from the investigation are finally used for the specification of requirements regarding a new, superior p-y approach. INTRODUCTION The intended expansion of the offshore wind energy in Europe requires for thousands of offshore wind turbines (OWT) to be installed in the North- and Baltic Sea. Herein, monopiles are currently the preferred foundation concept in water depths less than 40 m due to their cost-effectiveness regarding production and installation. A monopile (see Fig. 1) consists of a single steel pipe pile driven into the seabed. Induced by wind and wave actions, the monopile has to withstand large and discontinuous lateral forces H and bending moments M. The combination of large water depths and sizeable wind turbines requires for enormous monopile dimensions (D ≈ 7–8 m, L/D = 3.5–5). As common for the design of pile foundations, the ultimate limit state (ULS) and the serviceability limit state (SLS) design proof have to be fulfilled. The structural integrity of the OWT has to be verified by means of the ULS design proof whereby the degradation of the ultimate resistance due to cyclic loading has to be taken into account. For the SLS design proof, the accumulation of deformations due to long-term cyclic loading has to be considered when verifying the compliance with the comparatively strict deformation tolerances. Besides, the foundation stiffness due to operational loads, which rules the structural dynamic behavior, is often a design-driving aspect regarding the monopile diameter and the wall thickness. Herein, the calculations regarding the compliance with the tolerable bandwidth of eigenfrequencies (NFA) as well as the structural fatigue design (FLS) are based on the foundation stiffness due to relative small loads (Achmus, 2011). Hence, an optimized design of the mass-produced monopile foundations requires for an accurate prediction of the load-deformation behavior for large and small loads.