In order to design offshore wind turbines, an engineer must understand the environmental loads that are imposed on the structure. This chapter describes the wind, wave, current and ice loading phenomena and how to translate the environmental characteristics to design loads against which the structure must be designed.

To meet growing energy demands, the Kyoto protocol and the much desired diversification of supply, wind energy has become a mainstream source of energy in the EU. Cost wise it is already competing with gas fired electricity. In the last decade wind moved offshore to accommodate even more wind power. The offshore wind resource is more abundant and of a better quality, resulting in higher electricity output. On the other hand, the cost of installing turbines offshore is higher than onshore. To improve the cost-effectiveness of offshore wind, the risks involved must be known and mitigated and the critical design parameters must be optimised. From an engineering point of view, these requirements can be met through the following steps: - understand the basics of offshore wind turbines - apply lessons learned from previous projects - improve design tools. This thesis focuses on the design of the support structure. First, the basics of offshore engineering and of wind energy technology are summarized, specifically focused on the support structure design. Then, an overview is given of four actual offshore wind farm designs and their details. The design methods were compared mutually and with a design of a typical offshore oil platform. For most of the design steps, the methodology is consistent. Only the fatigue damage assessment is done differently for each individual project. Fatigue assessment in offshore engineering is done in the frequency domain. This method can be applied because the wave loads can be effectively linearized. The advantages of the frequency domain method are the clarity of presentation of intermediate results and the final outcome as well as the speed of calculation. The offshore wind industry standard (both onshore and offshore) is to use time domain simulations, which enables taking all non-linearities of the turbine operation into account. A disadvantage of this for the design of support structures is that offshore contractors lack both the aerodynamic knowledge and knowledge of the turbine details to use the full time domain simulation method to calculate the total fatigue damage. In this thesis a frequency domain method is developed to solve this problem. An interface between turbine manufacturer and offshore contractor is created that avoids the need to transfer commercially sensitive turbine details. The offshore contractor can further optimise the support structures with the software packages he normally uses. The frequency domain method is tested for the Blyth offshore wind turbines, for which a validated computer model and on-site measurements were available. Further, the method is applied to a design for the Dutch offshore wind farm to be erected at Egmond in 2006. In both cases, the frequency domain method works very well and gives results that compare well with time domain results. The computer time required to perform a fatigue calculation has been reduced from several hours in the time domain to less than 2 minutes in the frequency domain. This high speed of calculation opens possibilities for parameter variations to check the sensitivity of design choices and for optimisation of every structure within the wind farm. This has the potential to significantly reduce cost and risk. A key issue in the accuracy of the method is the effect of the aerodynamic damping of the operating turbine on support structure dynamics. Several calculation methods for this damping have been tested and have shown to give reasonable results. More work is needed to more accurately pinpoint the magnitude of this aerodynamic damping. The frequency domain method is currently being implemented in the software of an offshore contractor while other companies have already shown interest.

Abstract Float-over installations offer opportunities to install heavy topsides in remote areas beyond the lifting capacity of available crane vessels, minimizing offshore hook-up and improving the overall project schedule. The topside is placed on a barge or heavy transport vessel, maneuvered in between the slot of the pre-installed substructure. By ballasting the vessel in combination with a hydraulic lowering system the topside is lowered onto the substructure. During the installation procedure large horizontal loads are transferred onto the substructure. These are often the determining load conditions for the design of substructure and foundation. Furthermore, environmental conditions at locations such as West-Africa do not permit year around floatover installation, and even in the favorable season daily fluctuations may result in delays in the installation process due to weather down time. The Ampelmann is a platform on six hydraulic cylinders, which is capable of compensating the motions of a vessel in six degrees of freedom. The platform is invented for marine personnel transfer and a prototype is currently being constructed. By placing the topside on multiple Ampelmann systems, the motions of the vessel can be compensated, resulting in an increase of the available weather windows for installation. Because the vessel can move freely under the topside, no horizontal loads will be transferred onto the substructure. This significantly decreases the design criteria for the substructure and temporary provisions required during the mating procedure. The topside will be placed on four Ampelmann systems with a central passive cylinder and four passive supports. The passive cylinders can carry the static load of the topside resulting in a reduction of the required dynamic power. The fact that topside is not moving with the vessel has an influence on the motion characteristics of the vessel, which is simulated and taken into account. The required dynamic power of the system is determined resulting in the technical feasibility of this new installation method. By comparing the cost and benefits of the Ampelmann float-over system with conventional float-over systems, an assessment is made of the financial feasibility. Introduction Installation of integrated topsides with the use of a crane vessel is restricted due to the maximum lifting capacity and availability of these vessels at certain geographical locations. Installation of the topside in separate modules relaxes the requirements for these heavy lift vessels, but hook up and commissioning time increase resulting in higher cost, particularly in remote areas such as West-Africa. An alternative to using a heavy lift vessel is the float-over method. When using the float-over method, the topside is placed on a barge or heavy transport vessel and maneuvered in between the slot of a pre-installed substructure. The vessel is then ballasted until the load of the topside is transferred onto the substructure. In locations exposed to wind seas or ocean swells, hydraulic jacks are used. These jacks can be retracted in a short period of time, reducing the exposure to impact loads before, and creating an air gap after the load transfer to safely remove the vessel from the jacket slot and eliminating the chance that the vessel impacts the topsides. In West-Africa, where many float-over installations have been and will be performed, the environmental conditions do not permit year around float-over installation. Even in the favorable season the SSW swell waves, which are generated by storms in the South Atlantic region, may occasionally result in large vessel motions, seriously affecting the number of available weather windows to perform the op

L'invention concerne un navire (1) avec une plate-forme de compensation de mouvement (4), ladite plate-forme disposant d'au moins un transporteur (6) pour supporter, deplacer et/ou transferer une charge, d'actionneurs (5) pour deplacer le transporteur par rapport au navire, de preference suivant six degres de liberte, d'un systeme de commande pour operer les actionneurs (5) et de capteurs de mouvement pour mesurer les mouvements du navire (1) par rapport a au moins un element se trouvant dans l'environnement, lesdites mesures etant exploitees comme entrees par le systeme de commande, au moins un element (9) de pression au moins partiellement passif etant dispose pour appliquer, en cours d'utilisation, une pression au transporteur afin de supporter au moins partiellement celui-ci.

Currently it is standard practice to use Airy linear wave theory combined with Morison's formula for the calculation of fatigue loads for offshore wind turbines. However, offshore wind turbines are typically placed in relatively shallow water depths of 5-25 m where linear wave theory has limited accuracy and where ideally waves generated with the Navier-Stokes approach should be used. This article examines the differences in fatigue for some representative offshore wind turbines that are found if first-order, second-order and fully non-linear waves are used. The offshore wind turbines near Blyth are located in an area where non-linear wave effects are common. Measurements of these waves from the OWTES project are used to compare the different wave models with the real world in spectral form. Some attention is paid to whether the shape of a higher-order wave height spectrum (modified JONSWAP) corresponds to reality for other places in the North Sea, and which values for the drag and inertia coefficients should be used. Copyright (C) 2004 John Wiley Sons, Ltd.

Wind turbines for electricity production have two seemingly opposing constraints; they need to be structural secure yet of low cost. To meet the first constraint, it would be an obvious choice to design a stiff structure of consequently large mass but this would drive up the cost. By reducing the mass a more cost effective turbine can be realized. However, such lightweight structures are by definition more flexible. To design a cost effective flexible system, thorough understanding of the dynamics is essential. This paper reviews the theoretical basics of the dynamic design options and applies these to realistic situations, including offshore machines under wave action. The wind energy converter and the support structure form an integrated dynamic system that must be developed in mutual interdependency and close co-operation. This paper provides a contribution to this integration process by extending the design approach initiated in the Opti-OWECS study [1] and the work of Kühn [2].

SUMMARY Aerodynamic damping can have a large impact on the lifetime of the support structure of an offshore wind turbine since it can reduce fatigue significantly. When performing fatigue calculations in the frequency domain, as proposed in another paper to this conference, the aerodynamic damping ratio is required as an input parameter. A theoretic approach that was developed for constant rotor speed turbines becomes inaccurate when applied to variable speed turbines. Simulation results give a good lead to the aerodynamic damping ratio, but the results can be influenced by the modelling of the control system. In an evaluation, both theory and simulation results were compared to the aerodynamic damping ratio needed in the frequency domain method to match the response spectra of time domain simulations. Both approaches gave comparable approximations of the damping under rated wind speed. At wind speeds above rated, the "real" damping resulted to be much higher than the estimated ones. Given the importance of damping, further investigation of this subject is firmly recommended. In the design of offshore wind turbine support structures, dynamics play a major role. This means that fatigue can be a design driver, making fatigue damage assessment a critical design check throughout the design process. At present, the driving dynamic excitation sources for fatigue, wind and waves, are modelled in complex time domain simulation programs to incorporate all non-linear features. The drawback of this method is that a full model of the turbine needs to be available to the offshore contractor; a prerequisite hardly ever met in the initial design stages, sometimes not even during final design. Furthermore, a large number of environmental states need to be simulated, which means that the lifetime fatigue check of one support structure may require 24 hours of calculating. This means that design optimisation is slow and far from ideal. At the Delft University of Technology, this defect in the design methodology of offshore wind turbines has been subject of study over the last few years. When designing traditional (oil & gas) offshore structures, the dynamic response of structures to wave induced loads for fatigue damage assessment is always done in the frequency domain. Although wave-structure interaction is also non-linear, the linearised frequency domain method does give highly accurate results with the benefit of calculation speed: a check only takes a minute or two giving the designer all freedom to optimise the structure. If this method could also be applied for the design of the support structures of offshore wind turbines, offshore contractors only need to extend their current methods of design. The method devised by the Delft University of Technology is presented in another paper to this conference (1). The basic approach is to uncouple the support structure and the turbine at the yaw bearing. Both systems are modelled in different programs. The turbine is simulated in the traditional time domain, where for each wind speed class and turbulence, a long simulation will result in a transfer function between turbulent wind and tower top load. In a finite element program, the support structure is modelled. By applying a tower top load and wave loading respectively for the frequency range of interest, two transfer functions are acquired: for tower top and for wave loading. The combination of turbulent wind and random wave spectra with the transfer functions will then result in stress response spectra for locations along the support structure that can be further analysed to find the fatigue damage. This method works because wind and wave excitation can be seen as independent phenomena. The only dependency is the aerodynamic damping. To incorporate this effect, where tower top motion induces opposing aerodynamic response loads, effectively damping the very tower top motion, the aerodynamic damping can be incorporated as additional structural damping in the finite element model of the support structure. This paper describes the nature of aerodynamic damping and different approaches of determining its magnitude. 2 AERODYNAMIC DAMPING FOR CONSTANT SPEED TURBINES The basics of aerodynamic damping can be illustrated rather easily by considering a tower top in motion. When the tower top is moving forward, the blades experience a small increase of wind speed and will respond to it aerodynamically. The response is such that an extra aerodynamic force will counteract the tower top motion, so the eventual excursion of the tower top due to the induced tower top velocity will be less. When the tower top moves backward the aerodynamic force decreases, again reducing the tower top motions. As this effect is linked to the velocity term in the equation of motion, it is comparable to damping, hence the term aerodynamic damping.

SUMMARY The Ampelmann is a new ship-based access system for offshore wind turbines. It uses a motion platform to cancel out the ship deck motions for easy access. The motion system is actively controlled by software that is constantly fed by data from a motion sensor. The drawback of this system is that either computational or component failures can lead to injuries or even casualties. This paper studies the safety philosophies behind the design of control systems in cars, aircrafts and medical devices in order to develop a safety design philosophy for the Ampelmann. 1 INTRODUCTION The Ampelmann system In order to provide safe ship-based access to offshore wind turbines, the Delft University of Technology is currently developing a system named "Ampelmann". This system enables safe transfer of personnel and goods by providing a motionless transfer deck on a vessel. This deck is mounted on top of a Stewart platform, a mechanism (often used for flight simulators) that can provide motions in all six degrees of freedom using six extendible legs. The Stewart platform is fixed on the ship deck. To keep the transfer deck motionless, a sensor is constantly measuring the motions of the ship deck. With the use of custom-made software, the legs of the Stewart platform are controlled in such a way that all ship motions are continuously compensated, thereby creating a stable and motionless transfer deck. Personnel and goods can then safely be moved from this transfer deck to the turbine's platform. An artist's impression of a transfer from the Ampelmann to a wind turbine is given in figures 1 and 2. Proof-of-concept In the beginning of 2005, the basic concept of the Ampelmann system was successfully demonstrated at the Delft University of Technology after two months of intensive research and development. This resulted in a proof-of-concept by means of a perfectly working scale model Ampelmann. The system consisted of basically three parts: a motion sensor, a motion system and the control software. The motion sensor used in this system was an Octans III as shown in figure 3. This device contains three gyrocompasses and three accelerometers to provide the translations and rotations in all six degrees of freedom. A Micro Motion System as illustrated in figure 4 was used as the motion system. This is a small-size Stewart platform with a neutral height of 67.9 cm. Finally, in-house software named Ampelmann Converter (see figure 5) …

SUMMARY Near shore locations for offshore wind turbines are becoming scarce. Therefore it is necessary to move into deeper waters resulting in an increase of the amount of steel needed for longer foundation piles. This paper investigates a method to determine a so called subsoil design window in which an optimal preliminary design of the monopile support structure is determined with respect to the natural frequency, the soil stiffness and the penetration depth. The API 2000 recommended p-y method will be studied and a literature study was performed on scale effects resulting from extrapolation of the p-y method to larger diameters. Also the effects of high cyclic loading on the degradation of the pile-soil stiffness ratio is studied. Finally, a parametric study is performed in order to determine the influences of the diameter on the penetration depth, the soil stiffness and the natural frequency of the support structure. This paper is part of a larger study in which the soil stiffness, the natural frequency and penetration depth can be displayed in 3D graphs, displaying the windows of design. These figures give a range of possible design directions not in principle based on turbine data, but on the in situ combination of soil and possible structure options. They can function as a communication portal between foundation design engineers and wind turbine engineers/manufacturers.