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Among several possible support structure types for offshore wind turbines, a full-height lattice tower is one design option. Advantages of this design are the smaller amount of steel used for the structure compared to other concepts, and the possibility to install the whole structure in one operation. Based on the complexity of dynamic loadings on the support structure by wind and wave as well as operational loads, an initial lattice tower design with constant member dimensions over the tower height shows a large optimization potential and can be optimized section by section. This paper presents basic considerations for an iterative optimization approach and identifies sensitivities for the optimization process of a full-height lattice tower. It was found that an analysis with constant member dimensions over the tower height gives an indication about the required dimensions for an optimized design.

A simple method, based in the frequency domain, was developed for calculating the dynamic response of a stall-regulated wind turbine. Emphasis is placed on two aspects of the method, which are necessary in order to obtain a reasonable linearization of behavior when the blades are stalled. First, the tangential (in-plane) component of turbulence is included, in addition to the axial component. Second, the linearized relationship between lift coefficient and angle-of-attack is adjusted to account for the effects of dynamic stall: separate linearizations are used for excitation and damping of vibration. A thorough comparison is made between linear and non-linear dynamic-stall methods, with the conclusion that the accuracy of the linear method depends upon the frequency and amplitude of oscillation. The linear dynamic-stall method is accurate at blade vibrational frequencies, but it can be inaccurate at frequencies in the vicinity of 1P or below, when the angle-of-attack oscillates with an amplitude of 3° or more. Load spectra of a Nordtank 500 turbine, calculated using the frequency-domain method, are compared with measurements. The frequency-domain method provides estimates of load spectra and aerodynamic damping (stability) that are useful for preliminary design and optimization, but the method lacks sufficient accuracy and generality to be used for certification. Copyright © 2011 John Wiley & Sons, Ltd.

The worldwide demand for renewable energy is increasing rapidly because of the climate problem, and also because oil resources are limited. Wind energy appears as a clean and good solution to cope with a great part of this energy demand. In Denmark for example, 20% of the electricity is produced from wind, and plans are towards reaching 50%. As space is becoming scarce for the installation of onshore wind turbines, offshore wind energy, when possible, seems as a good alternative. This work describes, for Europe and North America, the potential for offshore wind energy, the current status of this technology, and existing plans for the development of offshore wind parks. It also presents existing as well as promising new solutions for offshore wind energy.

Top tensioned strings and beams are often used in civil and marine applications. Typically these members have constant cross sections, and a pronounced, usually linear, tension variation, due to the effects of gravity. In this paper simple, approximate formulas for the natural frequency of such strings are derived, based on asymptotic techniques, while for the tensioned beam case approximate closed-form results are developed by the Wentzel–Kramers–Brillouin method. Both derivations are shown in reasonable detail. While similar work is known for a beam with varying axial tension this is believed to be the first time that a single analytic expression is developed for the full length of the beam. A simple example in which the bottom tension is only 9% of the top tension is analyzed for cases with and without bending stiffness, and the solutions have been compared to the exact solution for the string case and to the results from three finite-element programs for the beam case. The accuracy was found to be ver...

A study on rotational effects and different stall delay models using a prescribed wake vortex scheme and NREL phase VI experiment data

In many engineering problems one deals with quantities that must be considered to be of a stochastic or random nature. This is true for the natural environment such as wind, waves, and earthquakes, which are the driving mechanism behind the loadings on a wide variety of land-based and offshore structures. From a design perspective it is important to determine the expected highest value of a stochastic process, and structural fatigue life. The relevant procedures are reasonably well established for processes that have narrow band spectra, but it is much less clear how to deal with non-narrow band cases. In this paper it is shown that the extremes of a Gaussian, non-narrow band process are asymptotically equal to the extremes calculated according to the narrow band formula. Also demonstrated is that fatigue estimates may, with good accuracy, be based on the narrow band formula unless the bandwidth becomes extremely large. These statements are illustrated by examples of a process with (1) a low pass box spectrum, and (2) a Pierson–Moskowitz wave amplitude spectrum. It is also shown that the bandwidth parameter may in some cases be counter-intuitive, as the Pierson–Moskowitz spectrum has a larger bandwidth parameter than the box spectrum, even when both cover the same frequency range.

The paper briefly outlines current design philosophies for floating bridges, with special reference to aspects that are deemed to be of interest in the context of very large floating structures. Since the design of submerged floating tunnels is done by people in the same milieu and deals with many of the same problems as in the design of floating bridges, while being in some respects more critical, some of the design philosophy of that subject is also included. The design philosophy and methodology for floating and submerged tunnel bridges draws heavily on Norwegian experiences in two large fields: offshore structures and conventional bridges.

In modern petrochemical plants the corporate and societal demands to plant safety and minimum environmental effects are high. These demands rise high performance requirements to the technical systems, specially the process control and safety systems including an effective operator support system with fault detection capability. The systems must have high reliability also against erroneous operations which may cause shutdown situations or quality deviations. PVC producing plants have high inherent hazard potentials. The risk can be minimised to an acceptable level with an integrated and well structured system consisting of a highly reliable basic process control system, a supervisory control system, an operator support system and a high integrity protective system. The PVC reaction studied in this paper is exothermic and non-linear, and open-loop unstable. This may cause stability problems with varying operational conditions. The size of modern PVC reactors is so big that two independent cooling systems are needed to keep the reaction under control. The parallel control of two such cooling systems is complicated. Many existing solutions have problems with oscillatory control and even stability problems, causing reduced safety margins. In this paper a system to solve the stability problem of parallel control is suggested. The philosophy of the solution is decoupling of the dynamic dependency of the parallel control loops. The system is as shown simulated on real process data, and it is installed on a Norsk Hydro PVC plant. When a reaction comes out of control, or rather when it shows a tendency to do so, an operator support system is needed to give the process operator an early warning so he can overrule the automatic control before a shut down situation occurs. Here is described a system for monitoring the heat of reaction in the autoclave. The system consists of a model based estimator. The process model is based on first principles. There are uncertainties in some parameters, specially the heat transfer and the measurement of the cooler difference temperature. The operator support system performs state and parameter estimation as well as fault detection, based on an extended Kalman filter. The system is tested on-line. In this paper results of simulations and on line estimates are presented.

The current American Petroleum Institute's recipe [API RP 2A WSD, Recommended practice for planning, designing and constructing fixed offshore platforms, working stress design. API, USA, 1993.] for calculation of hydrodynamic loads on offshore truss structures is compared with the corresponding North Sea Design Practice, as given by the rules of Det Norske Veritas. Most emphasis is put on the hydrodynamic coefficients and the estimation of design current as these issues are identified to be particularly critical. Use of the updated API (1993) recommendations in which the drag coefficient for roughened cylinders is increased from a minimum of 0·6 (API 1991) to 1·05 (API 1993) and where current is included, could lead to a general increase in the estimated load level on slender offshore structures [Petrauskas, C., Heideman, J.C. & Berek, E.P., Extreme wave force calculation procedure for the 20th edition of API RP 2A. OTC paper 7153, In Proc. OTC 1993, Houston, Texas, 1993, pp. 201–211]. The main emphasis with regard to the impact of the new API recommendations, however, is that a consistent approach is provided to the calculation of 100-yr directional loads. This includes taking into account the effect of marine growth on force coefficients, modifying the wave kinematics for directional spreading, and considering current blockage effects, conductor shielding effects, and joint occurrence of wave height and current (i.e., using the associated current as being representative of the current that would lead to the 100-yr load). It is concluded that a consistent approach, such as that underlying the new API RP 2A (1993) recipe, is preferable to the current North Sea Design Practice [Det Norske Veritas, Environmental conditions and environmental loads. DNV classification notes 30.5, 1001.] in this field, and thus that the North Sea Design Practice should be updated. This relates in particular to selection of hydrodynamic coefficients. Measurement programmes to obtain full scale global force data simultatneously with wave and current data are furthermore recommended.

The paper presents a numerical model based upon the Reynolds equations with a one-equation turbulence closure. A finite element formulation with a weak form of the method of characteristics is applied. Numerical results are presented for flow around a truncated cylinder in longitudinal oscillations, and for twodimensional stationary flow towards a fixed or oscillatory cylinder. Dynamic separation and vortex generation are predicted realistically. Hydrodynamic forces, Strouhal number and other relevant quantities are compared with measurements. Generally the numerical predictions are in good agreement with the experimental results.

The fatigue assessment of support structures is one of the most significant challenges in the design of offshore wind turbines (OWT). Fatigue analysis can be conducted in either the time-domain or the frequency-domain. The advantage of frequency-domain analysis is its time efficiency. This paper shows how the frequency domain method can be used to dimension lattice-type OWT towers such that they meet the fatigue criteria in the preliminary design stage. Two types of lattice towers, a three-legged and four-legged version, were redesigned in the fatigue limit state for the NREL 5 MW baseline wind turbine sited at a water depth of 35 m. The wall thickness of the members was chosen as the only variable and varied during the design process until the towers could survive for at least 20 years. In comparison with designs based upon ultimate strength, the mass of both types of towers increased no more than 30% when the fatigue limit state was considered. It is concluded that the lattice type structure requires only half as much material as its tubular counterpart. The three-legged tower is promising because of its simple geometry, even though it displayed a lower torsional stiffness than the four-legged tower. All the analyses in this paper were performed by an in-house FE code, intended for the early design stage of lattice towers. Once the optimum configuration is found in the early design stage, integrated time-domain analyses for the entire OWT system might be required to refine the design, taking all the nonlinear parameters into account.

An individual pitch controller (IPC) based on the multivariable Linear Quadratic Gaussian (LQG) concept is presented to reduce loads in megawatt-size wind turbines. Most turbines currently installed use collective pitch control to pitch the blades in order to limit the excess of wind power and to regulate the rotor speed above rated conditions. However, research has shown that IPC control is much more effective to reduce blade loads. Both collective and individual pitch control are implemented for the NREL 5 MW reference turbine. Simulation results are used to illustrate the advantage of the IPC approach, and its ability to reduce much of the flap-wise blade motion is demonstrated.

The present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time. Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV. Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated. The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests. Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.

In modern MW-size machines it has become a common practice to introduce controllers that provide active damping of turbine components to reduce blade, tower and drive-train loads, whilst optimising energy capture. However, as wind turbines become larger and more flexible, these controllers have to be designed with great care as the coupling between flexible modes increases and so does the potential to destabilise the turbine. The most direct method to address the above issues has been to exploit the pitch control capabilities. Individual Pitch Control (IPC) has been proposed many times over the last few years for load mitigation. Bearing this in mind, this paper investigates two different approaches to design a controller to pitch each blade individually in the wind turbine operating region III. The first one is a decentralised control algorithm and the second one is an H ∞ loop shaping design. A controllability analysis of the wind turbine is also included in the paper. The investigation is conducted based on the NREL 5MW benchmark wind turbine. Turbine modeling and control is conducted in FAST and Simulink.Copyright © 2011 by ASME

Offshore wind turbines are becoming more common due to the scarcity of suitable land sites. By going offshore, maintenance costs become one of the driving expenses. Hence more reliable components should be implemented on offshore wind turbines. The down-wind rotor configuration does not require as powerful yaw drive as the upwind rotor configuration to align with the wind direction. Thus the yaw system can be simpler with fewer components that can fail and require maintenance. This paper presents numeric simulation studies of how the tower shadow impacts the blades when they pass through the wake behind the tower. The work concentrates on bottom-fixed offshore wind turbines designed for the specifications of the NREL offshore 5-MW baseline wind turbine. The blade response has been compared for a full truss tower and a conventional tubular tower to show how the different tower shadows influence the blades. The blades on the more transparent truss tower experience less root flapwise moment fluctuations due to the weaker tower shadow. The simulations were performed by means of GH Bladed, version 3.82.Copyright © 2010 by ASME

A historical perspective on the performance of vertical and inclined flare structures is provided to illustrate the breadth of the problem and provide a basis to appreciate the engineering solutions to the issue of flow-induced vibrations.

Recent and historical literature regarding hydrodynamics has been reviewed, with offshore wind turbine support structures in mind. Under conditions of separated flow, several relevant phenomena have been noted which are not covered by the commonly-used Morison equation: 1. damping of structural vibration or slow-drift motion; 2. the interaction of structural vibration and vortex shedding; 3. loads near the free-surface; and, 4. burst motions, caused by impulse-like loading from steep waves. References have been given to books and articles that describe the phenomena in more detail. A form of the Morison equation is proposed which has separate empirical coefficients for each of the velocity and acceleration terms. The coefficients can be determined from existing test data with use of least-squares error minimization. A simplified form of the equation provides a means to obtain conservative bias on both the applied load (bias towards a high drag coefficient) and damping (bias towards a low drag coefficient). Further investigation into free-surface and burst motion (ringing) phenomena is recommended, considering a slender wind turbine monotower in 20 to 50 m water depth.Copyright © 2009 by ASME

A selective and incomplete review of the status of the development of offshore wind turbines is given. The main focus is on state of the art and emerging solutions, and emphasis is on solutions for deep water.

Wind turbines have grown tremendously during the last 20 years and it is of interest to investigate how large they can be expected to be. In the present paper several structural aspects have been studied in order to see how they develop as a function of size. It was found that as long as thrust and centrifugal forces dominate, then most aspects of the turbine are independent of scale, provided geometric similarity scaling is performed. The most important deviation from this rule is that selfweight of the blades results in stresses that grow linearly under geometric scaling. The effects of selfweight appear to become important for the largest machines currently available. (5MW)Copyright © 2007 by ASME

The paper describes how an existing computer code, VICoMo, see Moe & Arntsen (2002), has been extended to analyse Vortex Induced Vibrations (VIV) of SCR’s for situations in which the flow is neither in the plane of the riser, nor perpendicular to it. Various strategies were attempted in order to make the solution deal with flows at oblique angles. It was found that the best approach was to assume that the riser motions occurred in plane perpendicular to the flow, while motions in the flow direction were disregarded. One source of uncertainty is associated with the empirical database in VICoMo which is based on tests in which the flow was perpendicular to the riser elements. The timeseries from the experiment showed large temporal variations. Actually the average motions amplitudes at given points may be as low half of the maximum value. Also the largest amplitudes at different points on the experimental envelope on the riser will usually have been reached at quite different times. These features were not included in the theoretical model, and made direct comparisons between predictions and measurements difficult. We ended up comparing the envelopes of motion response. In several of the cases the predictions matched the experiments closely, for frequency, modeshape as well as amplitudes. In other cases the match was somewhat less satisfactory, however.Copyright © 2004 by ASME

The theory behind a computer programme for prediction of Vortex Induced Vibrations (VIV) of risers is summarised, and some experiences with its use are given. The programme in question is named VICoMo (VIV by Complex Modes) and is based on modal analysis by means of complex modes. The approach is iterative in nature because the added mass and damping forces depend nonlinearly on motion amplitude and frequency. Experiences with some runs to model a prototype Steel Caternary Riser (SCR) are summarised. A key element herein is the strategy for the choice of the parameters governing the iterations.Copyright © 2002 by ASME

A review has been made of some of the regular flow applications of the Morison formula, then some new material concerning the force coefficients in irregular wave situations has been presented The regular flow cases considered are fixed circular cylinders subjected to

At present the features of boundary layers near to the sea bed are not very well known. This applies to such matters as the thickness of the boundary layer in steady currents and the exact nature of the boundary layers in waves, and also the magnitude and shape of the local turbulence spectra, not to mention the sediment transport, associated with these causes. It can thus be seen that considerable uncertainty exists concerning the loading mechanism of pipeline spans. This paper deals with the determination of the natural frequencies of a pipeline in a free span. The basic problem has a strong bearing on the effect of the loading on the pipelines.

The sea surface and the wave loading are described as non-narrow band stochastic processes. The expected rate of occurrence of force maxima at any intensity is determined for force per unit length of pile as well as for the total force acting on the pile. Depending on the pile diameter, the force is taken to be given by the inertia term, i.e. to depend linearly upon particle acceleration, or to be given by the Morison formula in which a quadratic term in particle velocity is added to the inertia term. The peak distribution for the inertia case will be Rayleighian, and an asymptotic solution for the Morison-type loading is also presented. The expected frequency of maxima will be different for the different cases, and can in general be predicted from the wave-amplitude power spectrum. It is shown that the result is quite sensitive to the high-frequency tail of these spectra and doubt is raised about the suitability of today's “standard” wave spectra for force and response calculations.

An experimental investigation of the forces on cylinders in a steady current is reported. The damping of structures in calm water or currents is discussed in view of the experiments and it is concluded that the hydrodynamic damping based on Morisons formula may be strongly unconservative. Structures in waves are also discussed and the authors expect similar results for the case.